Abstract:
An electromechanical device is disclosed. The device includes a variable capacitor, and a switch circuit configured to pre-charge an input node with a pulse charge at said selected voltage level. The switch circuit includes only a first switch coupled to the variable capacitor and the first switch is configured to respond to an enable signal having a duration shorter than a mechanical time constant of the variable capacitor and the first switch is configured to apply the selected voltage level across the variable capacitor to cause the pulse charge to accumulate on the variable capacitor.

Description:
BACKGROUND  
       [0001]    This is a continuation in part of U.S. patent application Ser. No. 08/032,711, filed Mar. 17, 1993. 
         [0002]    This invention relates to visible spectrum (including ultra-violet and infrared) modulator arrays. 
         [0003]    Visible spectrum modulator arrays, such as backlit LCD computer screens, have arrays of electro-optical elements corresponding to pixels. Each element may be electronically controlled to alter light which is aimed to pass through the element. By controlling all of the elements of the array, black and white or, using appropriate elements, color images may be displayed. Non-backlit LCD arrays have similar properties but work on reflected light. These and other types of visible spectrum modulator arrays have a wide variety of other uses. 
     
    
     SUMMARY OF THE INVENTION 
       [0004]    In general, in one aspect, the invention features modulation of light in the visible spectrum using an array of modulation elements, and control circuitry connected to the array for controlling each of the modulation elements independently, each of the modulation elements having a surface which is caused to exhibit a predetermined impedance characteristic to particular frequencies of light. 
         [0005]    Implementations of the invention may include the following features. The surface may include antennas configured to interact with selected frequencies of light, or the surface may be a surface of an interference cavity. The impedance characteristic may be reflection of particular frequencies of light, or transmission of particular frequencies of light. Each of the modulation elements may be an interference cavity that is deformable to alter the cavity dimension. The interference cavity may include a pair of cavity walls (e.g., mirrors) separated by a cavity dimension. One of the mirrors may be a broadband mirror and the other of the mirrors may be a narrow band mirror. Or both of the mirrors may be narrow band mirrors, or both of the mirrors may be broad band, non-metallic mirrors. The cavity may have a cavity dimension that renders the cavity resonant with respect to light of the frequency defined by the spectral characteristics of the mirrors and intrinsic cavity spacing in an undeformed state. One of the mirrors may be a hybrid filter. One (or both) of the walls may be a dielectric material, a metallic material, or a composite dielectric/metallic material. The cavity may be deformable by virtue of a wall that is under tensile stress. The control circuitry may be connected for analog control of the impedance to light of each element. The analog control may be control of the degree of deformity of the deformable wall of the cavity. 
         [0006]    The predetermined impedance characteristic may include reflection of incident electromagnetic radiation in the visible spectrum, e.g., the proportion of incident electromagnetic radiation of a given frequency band that is, on average, reflected by each of the modulation elements. The modulation element may be responsive to a particular electrical condition to occupy either a state of higher reflectivity or a state of lower reflectivity, and the control circuitry may generate a stream of pulses having a duty cycle corresponding to the proportion of incident radiation that is reflected and places the modulation element in the higher state of reflectivity during each the pulse and in the lower state of reflectivity in the intervals between the pulses. The characteristic may include emission of electromagnetic radiation in the visible spectrum. The characteristic may include the amount of electromagnetic radiation in the visible spectrum that is emitted, on average, by the antennas. The characteristic may be incident electromagnetic radiation in the visible spectrum. The modulation elements may include three sub-elements each associated with one of three colors of the visible spectrum. The modulation element may be responsive to a particular electrical condition to occupy either a state of higher transmissivity or a state of lower transmissivity, and the control circuitry may generate a stream of pulses having a duty cycle corresponding to the proportion of incident radiation that is transmitted and places the modulation element in the higher state of transmissivity during each the pulse and in the lower state of transmissivity in the intervals between the pulses. The characteristic may include the proportion of incident electromagnetic radiation of a given frequency band that is, on average, transmitted by each of the modulation elements. 
         [0007]    The visible spectrum may include ultraviolet frequencies, or infrared frequencies. 
         [0008]    In general, in another aspect of the invention, the control circuitry may be connected to the array for controlling the amplitude of light delivered by each of the modulation elements independently by pulse code modulation. 
         [0009]    In general, in another aspect, the invention features a modulation element having a deformable portion held under tensile stress, and control circuitry connected to control the deformation of the deformable portion. 
         [0010]    Implementations of the invention may include the following features. The modulation element may be self-supporting, or held on separate supports. The deformable portion may be a rectangular membrane supported along two opposite edges by supports which are orthogonal to the membrane. The deformable portion, under one mode of control by the control circuitry, may be collapsed onto a wall of the cavity. The control circuitry controls the deformable portion by signals applied to the modulation element, and the deformation of the control portion may be subject to hysteresis with respect to signals applied by the control circuitry. 
         [0011]    In general, in another aspect, the invention features modulating light in the visible spectrum using a deformable modulation element having a deformation mechanism and an optical portion, the deformation mechanism and the optical portion independently imparting to the element respectively a controlled deformation characteristic and a controlled modulation characteristic. 
         [0012]    Implementations of the invention may include the following features. The deformation mechanism may be a flexible membrane held in tensile stress, and the optical portion may be formed on the flexible membrane. The optical portion may be a mirror. The mirror may have a narrow band, or a broad band, or include a hybrid filter. 
         [0013]    In general, in another aspect, the invention broadly features a non-metal deformable modulation element. 
         [0014]    In general, in another aspect, the invention features a process for making cavity-type modulation elements by forming a sandwich of two layers and a sacrificial layer between them, the sacrificial layer having a thickness related to the final cavity dimension, and using water or an oxygen based plasma to remove the sacrificial layer. 
         [0015]    Among the advantages of the invention are the following. 
         [0016]    Very high-resolution, full-color images are produced using relatively little power. The embodiment which senses the image incident on the array has relatively low noise. Their color response characteristics are tunable by selection of the dimensions of the antennas. The antenna or cavity embodiments are useful in portable, low power, full color displays, especially under high ambient light conditions. Phase controlled reflective embodiments are useful in passive light scanning such as optical disk readers without moving parts. The emissive embodiments also could be used as display devices especially in low-ambient-light conditions. 
         [0017]    Because of the dielectric materials used in some embodiments, the devices have the advantage of being extremely light efficient, making them especially appropriate for high intensity projection displays, and reducing or eliminating the need for backlighting in low ambient light applications. In addition, more accurate color representations are possible, as well as designs optimized for the IR and UV. Mechanical hysteresis precludes the need for active drivers, and this coupled with their geometric simplicity and monolithic nature brings defect losses down significantly. The devices are also exceptionally fast, low power, and non-polarizing. The fact that they can be reflective and/or transmissive enhances their flexibility. 
         [0018]    The process for fabrication as represented in some embodiments relies on benign chemicals, minimizing waste disposal problems, and facilitating the fabrication of devices on a variety of substrates (e.g., plastics or integrated circuits) using a larger variety of materials. Devices on plastic substrates have the potential of being extremely inexpensive. All of the manufacturing technologies used are mature, further reducing manufacturing costs. 
         [0019]    Other advantages and features of the invention will become apparent from the following description and from the claims. 
     
    
     
       DESCRIPTION  
         [0020]      FIG. 1  is a perspective view of a display device. 
           [0021]      FIG. 2  is a perspective schematic exploded view of a representative portion of the screen of  FIG. 1 . 
           [0022]      FIG. 3  is an enlarged top view of a tri-dipole of  FIG. 2 . 
           [0023]      FIG. 4  is a schematic view of a single dipole antenna of  FIG. 3 . 
           [0024]      FIG. 5  is a schematic perspective view, broken away, of a portion of the screen of  FIG. 1 . 
           [0025]      FIG. 6  is an enlarged top view of an individual tri-bus of  FIG. 2 . 
           [0026]      FIG. 7  is an enlarged perspective view of a representative portion of the screen of  FIG. 1 . 
           [0027]      FIG. 8  is a cross-sectional view along  8 - 8  of  FIG. 7 . 
           [0028]      FIG. 9  is a diagram of a portion of a control circuit of  FIG. 2 , and a corresponding dipole antenna of  FIG. 3 . 
           [0029]      FIGS. 10A ,  10 B,  10 C are representative graphs of the input voltage to the bias source of  FIG. 9 . 
           [0030]      FIG. 11  is a diagram of portions of the control modules for a row of pixels, 
           [0031]      FIG. 12  is a circuit diagram of an oscillator. 
           [0032]      FIG. 13  is a schematic diagram of a circuit module of  FIG. 2 , a corresponding dipole antenna of  FIG. 3 , and a graphical representation of the output of a binary counter. 
           [0033]      FIG. 14  is a circuit diagram of the pulse counter of  FIG. 13 . 
           [0034]      FIGS. 15 ,  16 ,  17 ,  18 , and  19  are top views of alternative dipole arrangements. 
           [0035]      FIGS. 20A through 20D  are perspective views of a cavity device. 
           [0036]      FIGS. 21A and 21B  are side views of the cavity device. 
           [0037]      FIGS. 22A through 22F  are graphs of frequency responses of the cavity device in different states. 
           [0038]      FIGS. 23A and 23B  are top and cutaway side views, respectively, of a display. 
           [0039]      FIGS. 23C and 23D  are top and cutaway side views, respectively, of another display. 
           [0040]      FIG. 24A  is a graph of an electromechanical response of the cavity device. 
           [0041]      FIG. 24B  is a graph of an addressing and modulation scheme for a display. 
           [0042]      FIGS. 25A through 25N  and  FIGS. 26A through 26K  are perspective views of the device during assembly. 
           [0043]      FIGS. 27A through 27C  are side views of dielectric mirrors. 
           [0044]      FIG. 27D  is a top view of a dielectric mirror. 
           [0045]      FIGS. 28A ,  28 B are perspective and top views of a linear tunable filter. 
           [0046]      FIGS. 29A ,  29 B are perspective and top views of a deformable mirror. 
       
    
    
       [0047]    Referring to  FIG. 1 , device  20  includes a screen  22  for displaying or sensing a high resolution color image (or a succession of color images) under control of power and control circuitry  26 . The image is made up of a densely packed rectangular array of tiny individual picture elements (pixels) each having a specific hue and brightness corresponding to the Part of the image represented by the pixel. The pixel density of the image depends on the fabrication process used but could be on the order of 100,000 pixels per square centimeter. 
         [0048]    Referring to  FIG. 2 , each pixel is generated by one so-called tri-dipole  30 . The boundary of each tri-dipole is 
         [0049]    T-shaped. The tri-dipoles are arranged in rows  32  in an interlocking fashion with the “Ts” of alternating tri-dipoles oriented in one direction and the “Ts” of intervening tri-dipoles along the same row oriented in the opposite direction. The rows together form a two-dimensional rectangular array of tri-dipoles (corresponding to the array of pixels) that are arranged on a first, external layer  34  of screen  22 . The array may be called an electrically alterable optical planar array, or a visible spectrum modulator array. 
         [0050]    On a second, internal layer  36  of screen  22  so-called tri-busses  38  (shown as T-shaped blocks in  FIG. 2 ) are arranged in an interlocking two-dimensional array  40  corresponding to the layout of the tri-dipoles on layer  34  above. Each tri-dipole  30  is connected to its corresponding tri-bus  38  by a multi-conductor link  42  running from layer  34  to layer  36  in a manner described below. 
         [0051]    On a third, base layer  44  of screen  22  a set of circuit modules  46  are arranged in a two-dimensional rectangular array corresponding to the layouts of the tri-dipoles and tri-busses. Each circuit module  46  is connected to its corresponding tri-bus  38  by a six-conductor link  48  running from layer  36  to layer  44  in a manner described below. 
         [0052]    Each circuit module  46  electronically controls the optical characteristics of all of the antennas of its corresponding tri-dipole  30  to generate the corresponding pixel of the image on screen  22 . Circuit modules  46  are connected, via conductors  50  running along layer  44 , to an edge of layer  44 . Wires  52  connect the conductors  50  to control and power circuitry  26  which coordinates all of the circuit modules  46  to  25  generate the entire image. 
         [0053]    Referring to  FIG. 3 , each tri-dipole  30  has three dipole sections  60 ,  62 ,  64 . The center points  59 ,  61 ,  63  of the three sections are arranged at 120 degree intervals about a point  65  at the center of tri-dipole  30 . Each section  60 ,  62 ,  64  consists of a column of dipole antennas  66 ,  68 ,  70 , respectively, only ten dipole antennas are shown in each section in  FIG. 3 , but the number could be larger or smaller and would depend on, e.g., the density with which control circuits  46  can be fabricated, the tradeoff between bandwidth and gain implied by the spacing of the antennas, and the resistive losses of the conductors that connect the antennas to the control circuit  46 . Only the two arms of each dipole antenna are exposed on layer  34 , as shown in  FIG. 3 . The dipole antennas of a given section all have the same dimensions corresponding to a particular resonant wavelength (color) assigned to that section. The resonant wavelengths for the three sections  60 ,  62 ,  64  are respectively 0.45 microns (blue), 0.53 microns (green), and 0.6 microns (red). 
         [0054]    Referring to  FIG. 4 , each dipole antenna  80  schematically includes two Ls  82 ,  84  respectively made up of bases  86 ,  88 , and arms  90 ,  92 . The bases of each antenna  80  are electrically connected to the corresponding circuit module  46 . The span (X) of arms  90 ,  92  is determined by the desired resonant wavelength of dipole antenna  80 ; for example, for a resonant wavelength of lambda, X would be lambda/2. Dipole antennas  66 ,  68 ,  70  have X dimensions of 0.225 microns (lambda 1 ,/2), 0.265 microns (lambda 2 /2), and 0.3 microns (lambda 3 ,/2), respectively. The effective length (Y) of bases  86 ,  88  from arms  90 ,  92  to circuit module  46  is also a function of the dipole antenna&#39;s resonant wavelength; for a resonant wavelength of lambda, Y is a multiple of lambda. 
         [0055]    Referring to  FIG. 5 , each of the bases  86 ,  88  physically is made up of four segments; (1) one of the conductors  96  of link  42 , (2) a portion  112  of tri-bus  38 , (3) a short connecting portion  124  of tri-bus  38 , and (4) one of the conductors  94  of link  48 , which together define a path (e.g., the path shown as dashed line  97 ) with an effective length of Y from the arm (e.g.,  92 ) to the circuit module  46 . 
         [0056]    The placement of link  42  perpendicular to the surface of layer  34  allows arms  90 ,  92  (formed on the surface of layer  34 ) to be spaced at an actual spacing Z that is closer than lambda/2, the minimum required effective Y dimension of bases  86 ,  88 . Spacing Z maybe chosen based on the bandwidth/gain tradeoff, and for example may be one quarter of the resonant wavelength for the dipole antennas of a given section (i.e., lambda/4, or 0.1125 microns (lambda 1 /4), 0.1325 microns (lambda 2 /4) and 0.15 microns (lambda 3 /4) for antennas  66 ,  68 ,  70 , respectively). 
         [0057]    Referring to  FIG. 6 , each tri-bus  38  is formed of aluminum on layer  36  and has three zigzag shaped bus pairs  100 ,  102 ,  104  for respectively connecting dipole antennas of the corresponding sections  60 ,  62 ,  64  of tri-dipole  30 . Bus pairs  100 ,  102 ,  104  are connected to individual dipole antennas  66 ,  68 ,  70  via conductors of link  42  ( FIG. 2 ) that are joined to the bus pairs at points, e.g.,  106 . 
         [0058]    Each bus pair  100 ,  102 ,  104  has two parallel buses  108 ,  110 . Bus  108  electrically connects together the arms of the dipole antenna  5  of the corresponding section and, independently, the related bus  110  electrically connects together the arms  92  of the dipole antennas of that same section. 
         [0059]    Points  106  delineate a series of fragments  112 ,  114 ,  116  on each of the three bus pairs  100 ,  102 ,  104 , respectively. Each fragment forms part of one or more of the bases  86 , or  88  and therefore contributes to the effective Y dimension. 
         [0060]    The lengths (Q) of fragments  112 ,  114 ,  116  are one-half of the resonant wavelengths (i.e. lambda/2) of the sections  60 ,  62 ,  64 , or 0.225 microns (lambda 1 ,/2), 0.265 microns (lambda 2 ,/2), and 0.3 microns (lambda 3 ,/2), respectively. 
         [0061]    The conductors of link  48  ( FIG. 2 ) are attached to tri-bus  38  at points  118 ,  120 ,  122  at the ends of buses  108 ,  110 . Between points  118 ,  120 ,  122  and the first points  106  on along buses  108 ,  110  are fragments  124 ,  126 ,  128 , which also form portions of the bases  86 ,  88  and are included to adjust the effective Y dimensions of those bases to be integer multiples of lambda/2. The lengths of the three fragments  124 ,  126 ,  128  are 0.1125 microns, 0.1525 microns, and 0.1875 microns, respectively. 
         [0062]    Referring to  FIG. 7 , each dipole antenna  80  is physically formed (of aluminum) on an insulating semiconductor (e.g. silicon dioxide of silicon nitride) substrate  130  (part of layer  34 ) by x-ray or electron beam lithography or other technique suitable for forming submicron-sized structures. 
         [0063]    Tri-busses  38  (not seen in  FIG. 7 ) are formed on the upper-side of a second insulating semiconductor substrate  132  (part of layer  36 ). Circuit modules  46  (not seen in  FIG. 7 ) are part of a third insulating semiconductor substrate  134  (part of layer  44 ) and are connected by conductors  50  to gold contact pads  136  (only one shown, not to scale) formed on the edge of substrate layer  134 . 
         [0064]    Referring to  FIG. 8 , circuit module  46  is formed in and on substrate  134  by any one of several monolithic processes. A section  138  of the substrate  134 , which has been previously coated with an insulating semiconductor oxide layer  140 , is repeatedly masked (whereby small windows are opened in the oxide layer, exposing the semiconductor beneath) and exposed to n and p dopants to form the desired circuit elements (not shown in detail in  FIG. 8 ). 
         [0065]    The individual circuit elements are connected to each other and to external contact pad  136  ( FIG. 7 ) by aluminum conductors  142 ,  50 , respectively. To form the connections, holes  144  are opened in oxide layer  140  and a sheet of aluminum is deposited, filling holes  144 . Using a masking technique similar to the one described above the unwanted aluminum is removed, leaving only conductors  142 ,  50 . 
         [0066]    Semiconductor substrate layer  132  is deposited directly on top of the remaining exposed oxide layer  140  and conductors  142 ,  50 . Holes  146  (one shown) (opened using a suitable lithographic technique) are channels for the electrical conductors  147  of links  48 , which connect tri-bus  38  and circuit module  46 . Tri-bus  38  is etched from a sheet of aluminum deposited onto the surface of layer  132 . The deposition process fills holes  146 , thereby forming the conductors of links  48 . 
         [0067]    Substrate layer  130  is deposited onto the surface of substrate layer  132  and tri-bus  38 . The arms of dipole antennas  80  are formed by depositing a sheet of aluminum onto the surface of layer  130  and etching away the unwanted metal. During the deposition process holes  148  are filled thereby forming the conductors  149  of links  42  between the arms of dipole antenna  80  and tri-bus  38 . 
         [0068]    The conductors  149  are the uppermost parts of bases  86 ,  88  ( FIG. 4 ) of dipole antennas  66 ,  68 ,  70 ; the lengths of conductors  149  together with the lengths of fragments  112 ,  114 ,  116  ( FIG. 6 ), the lengths of fragments  124 ,  126 ,  128 , and the lengths of the conductors  147  determine the effective Y dimension of bases  86 ,  88 . 
         [0069]    The length of the conductors  149  is determined by the thickness of the substrate  130  through which links  42  pass. Substrate  130  and links  42  are 0.05625 microns (i.e. lambda 1 ,/8) thick. This thickness is achieved by controlling the deposition rate of the semiconductor material as layer  130  is formed. 
         [0070]    The length of the conductors  149  is determined by the thickness of the substrate layer  132  through which they pass. This, layer and links  48  are therefore also 0.05625 microns  20  thick. 
         [0071]    The Y dimensions for the dipole antennas  66 ,  68 ,  70  of sections  60 ,  62 ,  64  therefore are as follows: 
         [0072]    (a) For section  60 , Y equals the sum of 0.05625 microns (length of the conductor in link  42 , lambda 1 /8)+n*0.225 microns (where 0.225 microns=lambda 2 /2, the length of a fragment  112 , and n=the number of fragments  112  in each base  86 ,  88  of the nth dipole antenna  66   n )+0.1125 microns (length of fragment  124 , lambda 1 /4)+0.05625 microns (length of the conductor in link  48 , lambda 1 /8), and that sum equals (n+1)*(lambda 1/2 ). 
         [0073]    (b) For section  62 , Y equals the sum of 0.05625 microns (length of link  42 , lambda 1 /8)+n*0.265 microns (where 0.265 microns=lambda 2 /2, the length of a fragment  114 , and n=the number of fragments  114  in each base  86 ,  88  of the nth dipole antenna  68   n )+0.1125 microns (length of fragment  126 , (lambda 1 /2)−(lambda 1 /4))+0.05625 microns (length of the conductor in link  48 , lambda 1 /8), and that sum equals (n+1)*(lambda 1 /2). 
         [0074]    (c) For section  64 , Y equals the sum of 0.05625 microns (length of conductor in link  42 , lambda 1 /8)+n*0.3 microns (where 0.3 microns−lambda 3 /2, the length of a fragment  116 , and n equals the number of fragments  116  in each base  86 ,  88  of the nth dipole antenna  70   n )+0.1875 microns (the length of fragment  128 , (lambda 3 /2)−(lambda 1 /4))+0.05625 microns (length of conductor in link  48 ), and that sum equals (n+1)*(lambda 3 /2). 
         [0075]    Referring again to  FIG. 1 , in some embodiments, the displayed image is not emitted from device  20  but is comprised of ambient light (or light from a source, not  25  shown) selectively reflected by the tri-dipoles  30  of screen  22 . 
         [0076]    In that case, each tri-dipole  30  receives ambient light having a broad spectrum of wavelengths and is controlled by the corresponding circuit module to reflect only that portion of the ambient light manifesting the hue and brightness of the desired corresponding pixel. 
         [0077]    The hue generated by tri-dipole  30  depends on the relative intensities of the light reflected by sections  60 ,  62 ,  64 . The overall brightness of that hue of light in turn depends on the absolute intensities of the light radiation reflected by sections  60 ,  62 ,  64 . Thus, both the hue and brightness of the light generated by tri-dipole  30  can be controlled by regulating the intensity of the light reflected by the dipole antennas in each section of the tri-dipole; this is done by controlling the reflectivity of each dipole antenna, i.e., the percentage of the light of the relevant wavelength for that dipole antenna which is reflected. 
         [0078]    The desired percentage is attained not by regulating the amount of light reflected at any given instant but by arranging for the antenna to be fully reflective in each of a series of spaced apart time slots, and otherwise non-reflective. Each dipole antenna, in conjunction with its circuit module, has only two possible states: either it reflects all of the light (at the antenna&#39;s resonant frequency), or it reflects none of that light. The intensity is regulated by controlling the percentage of total time occupied by the time slots in which the dipole antenna occupies the first state. 
         [0079]    Each dipole antenna is controlled to be reflective or not by controlling the impedance of the dipole antenna relative to the impedance of the medium (e.g., air) through which the light travels. If the medium has an effective impedance of zero, then the relationship of the reflectivity of the dipole antenna to zero (the controlled impedance of the dipole antenna) can be derived as follows. If we define a three-axis system x-y-z in which the x and y axes are in the plane of the array and the z axis is the axis of propagation of the incident and reflected waves, where z=0 is the surface of the array, then the incident plus reflected wave for z&lt;0 may be represented as: 
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         [0000]    where  E  (overbar) is the complex amplitude of the electric field of the sum of the transmitted wave and the reflected wave;  E   0  is the complex amplitude of the electric field of the transmitted  20  wave;  E   r , is the complex amplitude of the electric field of the reflective wave; x(hat) is the orientation of the electric field of the wave;  H  is the amplitude of the magnetic field; y(hat) is the orientation of the magnetic field; go is the permeability of free space; ε 0  is the permittivity of free space; k=ωsqrt[μ 0 ε 0 ] is the wavenumber; and η=sqrt [μ 0 /ε 0 ] is the impedance of free space. For z&gt;0 (i.e., within free space) only the transmitted wave exists and is represented by 
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         [0000]      E (overbar) is the complex amplitude the transmitted wave at z=0, k t =sqrt [με] is its wavenumber; η=sqrt [μ/ε] is the impedance of the medium, i.e. z&gt;0. Boundary conditions (z=0) for tangential electric fields are imposed on equations 1 and 2 and they are combined to yield, 
         [0000]        {circumflex over (x)}[E   0   +E   x   ]={circumflex over (x)}E   t    (5)
 
         [0000]    In the same way, continuity for tangential magnetic fields (z=0) at the boundary yields, 
         [0000]        ŷ (1/η 0 ) ( E   0   −E   R )= ŷ (1/η t ) E   t    (6)
 
         [0000]    Dividing equations 5 and 6 by  E   0 , and  E   0 /η 0  respectively gives the following two equations: 
         [0000]      1 +E   r   /E   0   =E   t   /E   0    (7)
 
         [0000]      1 −E   R   /E   0 =(η 0 /η t )( E   t   /E   0 )   (8)
 
         [0000]      E   r / E   0  is called Γ and is the complex reflection coefficient while E t /η 0 =T is called the complex transmission coefficient, and η t /η 0 =η n  is the normalized wave impedance. Solving for T and Γ yields 
         [0000]    
       
         
           
             
               
                 
                   
                     T 
                     _ 
                   
                   = 
                   
                     
                       
                         
                           E 
                           _ 
                         
                         t 
                       
                       
                         
                           E 
                           _ 
                         
                         0 
                       
                     
                     = 
                     
                       
                         2 
                         
                           1 
                           + 
                           
                             
                               η 
                               0 
                             
                             / 
                             
                               η 
                               t 
                             
                           
                         
                       
                       = 
                       
                         
                           2 
                            
                           
                             η 
                             n 
                           
                         
                         
                           
                             η 
                             n 
                           
                           + 
                           1 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
             
               
                 
                   
                     Γ 
                     _ 
                   
                   = 
                   
                     
                       
                         
                           E 
                           _ 
                         
                         R 
                       
                       
                         
                           E 
                           _ 
                         
                         0 
                       
                     
                     = 
                     
                       
                         
                           T 
                           _ 
                         
                         - 
                         1 
                       
                       = 
                       
                         
                           
                             η 
                             N 
                           
                           - 
                           1 
                         
                         
                           
                             η 
                             n 
                           
                           + 
                           1 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
         [0000]    For matched impedance values, η 0 =η n , the reflection coefficient is zero, and T=1 (i.e., no reflection), and in the case of a load at the boundary, a matched antenna, there is complete absorption. 
         [0080]    As η n  approaches zero or infinity, the reflection coefficient approaches plus or minus one, implying total reflection. 
         [0081]    Referring to  FIG. 9 , the impedance z L , of dipole antenna  80  is controlled by a variable resistance PIN diode  160  connected across bases  86 ,  88 . PIN line  162  to the output of a bias high voltage or a low voltage based on a line  168  from power and the output of bias source  164  is a high voltage, the resistance R of PIN diode  160  (and hence the impedance (E,) of the dipole antenna is zero causing full reflection; when the output of bias source  164  is a low voltage  98 , resistance R is set to a value such that the resulting impedance z@ is matched to z. (the impedance of the air surrounding the antenna), causing zero reflection. 
         [0082]    To generate an entire image on screen  20 , power and control circuitry  26  receives a video signal (e.g. a digitized standard RGB television signal) and uses conventional techniques to deliver corresponding signals to modules  46  which indicate the relative desired intensities of light reflected from all sections  60 ,  62 ,  64  of all of the tri-dipoles in the array at a given time. Circuit modules  46  use conventional techniques to deliver an appropriate stream of input control signal pulses to each bias source  164  on line  168 . 
         [0083]    The pulse stream on each line  168  has a duty cycle appropriate to achieve the proper percentages of reflectance for the three Sections of each tri-dipole. Referring to  FIGS. 10A ,  10 B, and  10 C, for example, pulse stream  170  has a period T and a 50% duty cycle. For the first 50% of each period T the input to bias source  164  is high and the Corresponding output of source  164  is a high voltage. During this portion of the cycle dipole antenna  80  will reflect all received light having the dipole antenna&#39;s resonant wavelength. For the second 50% of the cycle the output of source  164  will be low and dipole antennas  80  will absorb the received light. In  FIGS. 10B ,  10 C, pulse streams  172 ,  174  represent a 30% duty cycle and a 100% duty cycle respectively; with a 30% duty cycle the effective intensity of the light radiation of the dipole antennas of the section will be 30%; for a duty cycle of 100%, the effective intensity is 100%. 
         [0084]    For example, if a particular pixel of the image is to be brown, the relative intensities required of the three red, 25 green, and blue sections  60 ,  62 ,  64  may be, respectively, 30, 40, and 10. The input signals to the bias sources  164 , carried on lines  168 , would then have duty cycles, respectively, of 30%, 40%, and 10%. An adjacent pixel which is to be a brown of the same hue but greater brightness might require duty cycles of 45%, 60%, and 15%. 
         [0085]    Referring to  FIG. 11 , to accomplish the delivery of the pulse width modulated signals from circuitry  26  to the pixel circuit modules  46 , each circuit module  46  in the row includes storage  180 ,  182  for two bits. The bit  1  storage elements  180  of the modules  46  in the row are connected together to create one long shift register with the pulse width modulated signals being passed along the row data line  184  from pixel to pixel. If, for example, the period of the modulated signals is 1 millisecond and there are ten different intensity levels, then an entire string of bits (representing the on or off state of the respective pixels in the row during the succeeding 1/10 millisecond) is shifted down the row every 1/10 millisecond. At the end of the initial 1/10 millisecond all of the bits in elements  180  are shifted to the associated elements  182  by a strobe Pulse on strobe line  186 . The content of each element  182  is the input to the driver  188  for the appropriate one of the three colors of that pixel, which in turn drives the corresponding section  60 ,  62 ,  64  of the tri-dipole. The rate at which data is shifted along the shift registers is determined by the number of elements on a given row, the number of rows, the number of intensity levels, and the refresh rate of the entire array. 
         [0086]    In another embodiment, the light comprising the image is emitted by tri-dipoles  30  rather than being produced by reflected ambient light. In that case, each tri-dipole generates the light for a single pixel with a hue and brightness governed by the intensities of the light emitted by each of the three sections  60 ,  62 ,  64 . 
         [0087]    Each dipole antenna within a tri-dipole is caused to emit light at the resonant wavelength of that antenna by stimulating it using a signal whose frequency corresponds to the resonant wavelength. Thus, the sections  60 ,  62 ,  64  will emit blue (lambda 1 ), green (lambda 2 ), and red (lambda 3 ) light respectively when provided with signals whose frequencies equal, respectively, lambda 1 , lambda 2  and lambda 3 . 
         [0088]    For an idealized dipole, the current  I  and current density  J (overbar)(r′overbar) are described by 
         [0000]          I =jω q     (11)
 
         [0000]            J     (   r ′ )= {circumflex over (z)} I d{dot over (o)} (   r   ′)   (12)
 
         [0000]    where  q  is the charge density; z(hat) indicates the direction of the current (along the z-axis); ω is angular frequency; and d is the distance between ideal point charges representing the dipole. The vector potential  A (overbar) in polar coordinates is given by 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           A 
                           _ 
                           _ 
                         
                         = 
                         
                           
                             
                               r 
                               ^ 
                             
                              
                             
                               A 
                               r 
                             
                           
                           + 
                           
                             θ 
                              
                             
                                 
                             
                              
                             
                               
                                 A 
                                 _ 
                               
                               θ 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             ( 
                             
                               
                                 
                                   r 
                                   ^ 
                                 
                                  
                                 cos 
                                  
                                 
                                     
                                 
                                  
                                 θ 
                               
                               - 
                               
                                 
                                   θ 
                                   ^ 
                                 
                                  
                                 sin 
                                  
                                 
                                     
                                 
                                  
                                 θ 
                               
                             
                             ) 
                           
                            
                           
                             
                               
                                 μ 
                                 0 
                               
                                
                               
                                 I 
                                 _ 
                               
                                
                               d 
                             
                             
                               4 
                                
                               π 
                                
                               
                                   
                               
                                
                               r 
                             
                           
                            
                           
                              
                             
                               
                                 - 
                                 j 
                               
                                
                               
                                   
                               
                                
                               kr 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where θ represents the angle relative to the dipole; θ(hat) is the angular orientation of the wave; μ 0  is the permeability of free space; r is radius from the dipole; r(hat) is radial orientation of the wave; A r  is the radial component of the vector potential; A θ  is the angular component of the vector potential; and k is a factor which is used to represent sinusoidally varying waves. The H field is given by
   where φ is elevation, with respect to the dipole. The E field is given by,   The far-field equation is given by   
 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           H 
                           _ 
                           _ 
                         
                         = 
                         
                           ϕ 
                            
                           
                             
                               1 
                               
                                 
                                   μ 
                                   0 
                                 
                                  
                                 r 
                               
                             
                              
                             
                               [ 
                               
                                 
                                   δ 
                                   δθ 
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       r 
                                        
                                       
                                         
                                           A 
                                           _ 
                                         
                                         θ 
                                       
                                     
                                     - 
                                     
                                       
                                         δ 
                                         δθ 
                                       
                                        
                                       
                                         ( 
                                         
                                           
                                             A 
                                             _ 
                                           
                                           r 
                                         
                                         ) 
                                       
                                     
                                   
                                     
                                 
                               
                               ] 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           ϕ 
                            
                           
                             
                               j 
                                
                               
                                   
                               
                                
                               k 
                                
                               
                                 I 
                                 _ 
                               
                                
                               d 
                             
                             
                               4 
                                
                               π 
                                
                               
                                   
                               
                                
                               r 
                             
                           
                            
                           
                             
                                
                               
                                 
                                   - 
                                   j 
                                 
                                  
                                 
                                     
                                 
                                  
                                 kr 
                               
                             
                              
                             
                               [ 
                               
                                 1 
                                 + 
                                 
                                   1 
                                   
                                     j 
                                      
                                     
                                         
                                     
                                      
                                     kr 
                                   
                                 
                               
                               ] 
                             
                           
                            
                           sin 
                            
                           
                               
                           
                            
                           θ 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       
                         
                           E 
                           _ 
                           _ 
                         
                         = 
                           
                          
                         
                           
                             1 
                             
                               jωε 
                               0 
                             
                           
                            
                           
                             ∇ 
                             
                               × 
                               
                                 H 
                                 _ 
                                 _ 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               
                                 μ 
                                 0 
                               
                               / 
                               
                                 ε 
                                 0 
                               
                             
                           
                            
                           
                             
                               j 
                                
                               
                                   
                               
                                
                               k 
                                
                               
                                 I 
                                 _ 
                               
                                
                               d 
                             
                             
                               4 
                                
                               π 
                                
                               
                                   
                               
                                
                               r 
                             
                           
                            
                           
                             
                                
                               
                                 
                                   - 
                                   j 
                                 
                                  
                                 
                                     
                                 
                                  
                                 kr 
                               
                             
                              
                             
                               ( 
                               
                                 
                                   
                                     
                                       
                                         
                                           
                                             r 
                                             ^ 
                                           
                                            
                                           
                                             [ 
                                             
                                               
                                                 1 
                                                 
                                                   j 
                                                    
                                                   
                                                       
                                                   
                                                    
                                                   kr 
                                                 
                                               
                                               + 
                                               
                                                 
                                                   ( 
                                                   
                                                     1 
                                                     
                                                       j 
                                                        
                                                       
                                                           
                                                       
                                                        
                                                       kr 
                                                     
                                                   
                                                   ) 
                                                 
                                                 2 
                                               
                                             
                                             ] 
                                           
                                         
                                          
                                         2 
                                          
                                         cos 
                                          
                                         
                                             
                                         
                                          
                                         θ 
                                       
                                       + 
                                     
                                   
                                 
                                 
                                   
                                     
                                       
                                         
                                           θ 
                                           ^ 
                                         
                                          
                                         
                                           [ 
                                           
                                             1 
                                             + 
                                             
                                               1 
                                               
                                                 j 
                                                  
                                                 
                                                     
                                                 
                                                  
                                                 kr 
                                               
                                             
                                             + 
                                             
                                               
                                                 ( 
                                                 
                                                   1 
                                                   
                                                     j 
                                                      
                                                     
                                                         
                                                     
                                                      
                                                     kr 
                                                   
                                                 
                                                 ) 
                                               
                                               2 
                                             
                                           
                                           ] 
                                         
                                       
                                        
                                       sin 
                                        
                                       
                                           
                                       
                                        
                                       θ 
                                     
                                   
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       H 
                       _ 
                       _ 
                     
                     = 
                     
                       
                         ϕ 
                         ^ 
                       
                        
                       
                         
                           j 
                            
                           
                               
                           
                            
                           k 
                            
                           
                             I 
                             _ 
                           
                            
                           d 
                         
                         
                           4 
                            
                           π 
                            
                           
                               
                           
                            
                           r 
                         
                       
                        
                       
                          
                         
                           
                             - 
                             j 
                           
                            
                           
                               
                           
                            
                           kr 
                         
                       
                        
                       sin 
                        
                       
                           
                       
                        
                       θ 
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       E 
                       _ 
                       _ 
                     
                     = 
                     
                       
                         θ 
                         ^ 
                       
                        
                       
                         
                           
                             μ 
                             0 
                           
                           / 
                           
                             ε 
                             0 
                           
                         
                       
                        
                       
                         
                           j 
                            
                           
                               
                           
                            
                           k 
                            
                           
                             I 
                             _ 
                           
                            
                           d 
                         
                         
                           4 
                            
                           π 
                            
                           
                               
                           
                            
                           r 
                         
                       
                        
                       
                          
                         
                           
                             - 
                             j 
                           
                            
                           
                               
                           
                            
                           kr 
                         
                       
                        
                       sin 
                        
                       
                           
                       
                        
                       θ 
                     
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
     
         [0091]    Equation (16) describes the radiation pattern away from a dipole antenna at distances significantly greater than the wavelength of the emitted electromagnetic wave. It is a very broad radiation pattern providing a wide field of view at relevant distances. 
         [0092]    Referring to  FIG. 12 , the dipole antennas  66 ,  68 ,  70  of each section  60 ,  62 ,  64  are driven by signals (e.g., sinusoidal) with frequencies of 5×10 14  Hz, 5.6×10 14  Hz, and 6.6×10 14  Hz for red, green, and blue, respectively. These signals are supplied by three monolithic oscillators  200  (one shown) within circuit module  46 , each tuned to one of the three required frequencies. 
         [0093]    In circuit  200  (an a stable multivibrator), the center pair of coupled transistors  202 ,  204  are the primary active elements and will oscillate if the circuit admittance&#39;s are set appropriately. Diodes  206 ,  208 ,  210 ,  212  provide coupling capacitance&#39;s between the transistors and the inductors  214 ,  216  are used to tune the operating frequency. 
         [0094]    In a third embodiment, an image of the object is focused by a conventional lens (not shown in  FIG. 1 ) onto screen  22 , which then acts as an image sensor. The tri-dipoles of screen  22 , controlled by power and control circuitry  26 , generate electrical signals corresponding to pixels of the received image. The signals are then processed by a processor which, in conventional fashion, delivers a derived RGB video signal which can then be transmitted or stored. 
         [0095]    The signals generated for each tri-dipole are generated by the corresponding circuit module  46  and represent the hue and brightness of the light radiation received at that tri-dipole. 
         [0096]    Each section of tri-dipole  30  can only be used to measure light having the resonant wavelength of its respective dipole antennas, however, because most colors can be expressed as a combination of red, green, and blue, circuit module  46  can, by independently measuring the intensity of the light radiation received at each section  60 ,  62 ,  64 , derive a signal which specifies the hue and intensity of the received pixel. 
         [0097]    Referring to  FIG. 13 , dipole antenna  80  will absorb incident light radiation at its resonant wavelength when its reflection coefficient (Γ L ) is zero, which occurs when its controlled impedance (z L ) matches the impedance of the medium (z 0 ). In those circumstances, a voltage pulse is produced across the ends  308 ,  310  of dipole  80  for each incident photon. The relative magnitude of the light radiation received by each dipole antenna can thus be measured by counting the average number of pulses across ends  308 ,  310  over a given time period. 
         [0098]    In this embodiment, circuit module  46  includes a terminating load resistor  315  connected across ends  308 ,  310 . The controlled impedance of the combination of dipole antenna  80  and resistor  315 , described by the equations set forth below, is equal to z 0 . 
         [0099]    The voltage of the pulse across resistor  315  (created by an incident photon) is illustrated by the sine wave graph above register  15  and is described generally by the following equation 
         [0000]        V ( z )=   V +e   −jkz +Γ L   e   jkz    (17)
 
         [0000]    Because z L =z 0 , Γ L =0, and equation 17 simplifies to 
         [0000]        V ( z )=   V +e   −jkz    (18)
 
         [0100]    A pulse detector  318  amplifies and sharpens the resulting pulse to a square wave form as shown, which is then used as the clock (CLK) input  319  to a binary counter  320 . The output of the binary counter is sampled at a regular rate; collectively the samples form a digital signal representing the intensity of received light radiation over time. Each time counter  320  is sampled, it is reset to zero by a pulse on control line  322 , Counter  320  thus serves as a digital integrator that indicates how much light arrived in each one of a succession of equal length time periods. 
         [0101]    Referring to  FIG. 14 , in pulse detector  318  the pair of transistors  322 ,  324  serve as a high impedance differential stage whose output (representing the voltage difference between points  308 ,  310 ) is delivered to an amplifier  326 . Amplifier  326  serves as a high-bandwidth gain stage and delivers a single sided output pulse to a conditioning circuit  328  that converts slow rising pulses to square pulses  330  for driving counter  320 . 
         [0102]    In another embodiment, the array of tri-dipoles is operated as a phased array. The operation of phased arrays is discussed more fully in Amitay, et al.,  Theory and Analysis of Phased Array Antennas,  1972, incorporated herein by reference. By controlling the spacing of successive tri-dipoles across the array and the relative phases of their operation, wave cancellation or reinforcement can be used to control the direction in three dimensions and orientation of the radiation. Beams can thus be generated or scanned. In the case of an array used to sense incoming radiation, the array can be made more sensitive to radiation received from selected directions. 
         [0103]    Other embodiments are also possible. For example, referring to  FIG. 15 , each section of tri-dipole  400  array be a single dipole antenna  406 ,  407 ,  408 . The tri-dipole antennas are then arranged about a center Point  410  at 120 degree intervals in a radial pattern. Bases  411 ,  412 , as well as arms  414 ,  415 , of the dipole antennas, are all formed on the same surface. 
         [0104]    Referring to  FIG. 16 , each section may consist of multiple dipole antennas  406 ,  407 ,  408  connected by attaching the bases  411 ,  412  of each succeeding dipole antenna to the inner ends of arms  414 ,  415  of the preceding dipole antenna. Circuit modules  416  are formed on the surface of layer  413 . 
         [0105]    Referring to  FIG. 17 , a multi-dipole  430  could have five sections  432 ,  434 ,  436 ,  438 ,  440  composed of dipole antennas  442 ,  444 ,  446 ,  448 ,  450 , respectively. The dipole antennas of the different sections would have different resonant wavelengths. Other multi-dipoles might have any number of sections. 
         [0106]    The scanning of pixels could be done other than by pulse width modulation, for example, using charge coupled devices to shift packets of charge along the rows of pixels. 
         [0107]    Referring to  FIGS. 18 ,  19 , other arrangements of dipole antennas may be used in order to match the area required for the control circuit modules. 
         [0108]    Referring to  FIG. 18 , each section  470  of a tri-dipole in the reflective mode could be formed of a number of subsections (e.g.,  472 ) arranged in two rows  474  and a number of columns  47 . The antennas  478  in each subsection  472  are all served by a single PIN diode circuit  480  located at the peripheral edge of section  470  at the end of the subsection on the layer below the antenna layer. All circuits  480  for the entire Section  470  are in turn served by a single bias source  164  ( FIG. 9 ). This arrangement reduces the number of bias sources required for the entire array of tri-dipoles.  FIG. 19  shows an alternate arrangement in which there is but one row of subsections each served by a single PIN diode circuit at the end of the row. 
         [0109]    In order to reduce the number of conductors  50 , selected tri-dipoles could be used to receive control signals transmitted directly by light and to pass those control signals to the control circuits of nearby active tri-dipoles. 
         [0110]    The dipoles could be mono-dipoles comprised of only a single dipole antenna, all with the same resonant wavelength. 
         [0111]    Dipole antennas  470  could be randomly arranged on the surface of layer  472  of screen  22 . 
         [0112]    A different color regime, e.g. cyan-magenta-yellow, could be substituted for RGB. 
         [0113]    Spiral, biconical, slotted, and other antenna configurations could be substituted for the dipole configuration. 
         [0114]    The array could be three-dimensional. 
         [0115]    The successive tri-dipoles in the array can be oriented so that their respective antennas are orthogonal to each other to enable the array to interact with radiation of any arbitrary polarization. 
         [0116]    The PIN diodes could be replaced by other impedance controlling elements. Such elements might include quantum well transistors, superconducting junctions, or transistors based on vacuum microelectronics. Further improvement could be achieved by reducing the complexity of the third layer containing control circuitry. The electronics required to get control signals to the circuitry could be eliminated by the use of laser or electron beams to provide such signals. This would have the advantage of allowing for arrays of even higher density. 
         [0117]    The array could be fabricated on a transparent substrate, thus facilitating transmissive operation. 
         [0118]    In other embodiments, the antenna arrays alone (without control circuitry or connection buses) may be fabricated on one-half of a microfabricated interferometric cavity. The antenna array can be considered a frequency selective mirror whose spectral characteristics are controlled by the dimensions of the antennas. Such a cavity will transmit and reflect certain portions of incident electromagnetic radiation depending on (a) the dimensions of the cavity itself and (b) the frequency response of the mirrors. The behavior of interferometric cavities and dielectric mirrors is discussed more fully in Macleod, H. A.,  Thin Film Optical Filters,  1969, incorporated by reference. 
         [0119]    Referring to  FIG. 20   a , two example adjacent elements of a larger array of this kind include two cavities  498 ,  499  fabricated on a transparent substrate  500 . A layer  502 , the primary mirror/conductor, is comprised of a transparent conductive coating upon which a dielectric or metallic mirror has been fabricated. Insulating supports  504  hold up a second transparent conducting membrane  506 . Each array element has an antenna array  508  formed on the membrane  506 . The two structures,  506  and  508 , together comprise the secondary mirror/conductor. Conversely, the antenna array may be fabricated as part of the primary mirror/conductor. Secondary mirror/conductor  506 / 508  forms a flexible membrane, fabricated such that it is under tensile stress and thus parallel to the substrate, in the undriven state. 
         [0120]    Because layers  506  and  502  are parallel, radiation which enters any of the cavities from above or below the array can undergo multiple reflections within the cavity, resulting in optical interference. Depending on the dimensions of the antenna array, as explained above, the interference will determine its effective impedance, and thus its reflective and/or transmissive characteristics. Changing one of the dimensions, in this case the cavity height (i.e., the spacing between the inner walls of layers  502 ,  506 ), will alter the optical characteristics. The change in height is achieved by applying a voltage across the two layers at the cavity, which, due to electrostatic forces, causes layer  506  to collapse. Cavity  498  is shown collapsed (7 volts applied), while cavity  499  is shown uncollapsed (0 volts applied). 
         [0121]    In another embodiment,  FIG. 20   b , each cavity may be formed by a combination of dielectric or metallic mirrors on the two layers, and without the antennas formed on either layer. In this case the spectral characteristics of the mirror are determined by the nature and thickness(es) of the materials comprising it. 
         [0122]    In an alternative fabrication scheme,  FIG. 20   c , each cavity is fabricated using a simpler process which precludes the need for separately defined support pillars. Here, each secondary mirror/conductor,  506 , is formed in a U-shape with the legs attached to the primary layer; each secondary mirror/conductor thus is self-supporting. 
         [0123]    In yet another cheme,  FIG. 20   d , the cavity has been modified to alter it&#39;s mechanical behavior. In this version, a stiffening layer,  510 , has been added to limit deformation of the membrane while in the driven state. This assures that the two mirrors will remain parallel as a driving voltage is gradually increased. The resulting device can be driven in analog mode (e.g., cavity  511  may be driven by 5 volts to achieve partial deformation of the cavity) so that continuous variation of its spectral characteristics may be achieved. 
         [0124]    Referring to  FIGS. 21A and 21B , the modulation effect on incident radiation is shown. The binary modulation mode is shown in  FIG. 21A . In the undriven state (shown on the left) incident light  512  (the delta lambda represents a range of incident frequencies, e.g., the range of visible light) contains a spectral component which is at the resonant frequency of the device in the undriven state. Consequently this component (delta lamba n) is transmitted,  516 , and the remaining components (at nonresonant frequencies, delta lambda minus delta lamba n) are reflected,  514 . This operation is in the nature of the operation of a fabry-perot interference cavity. 
         [0125]    When the device is driven and the geometry altered to collapse (right side of figure), the resonant frequency of the device also changes. With the correct cavity dimensions, all of the incident light (delta lamba) is reflected. 
         [0126]      FIG. 21A  shows a binary mode of operation while  FIG. 21B  shows an analog mode, where a continuously variable voltage may be used to cause a continuously variable degree of translation of secondary mirror/conductor  506 . This provides a mechanism for continuous frequency selection within an operational range because the resonant frequency of the cavity can be varied continuously. In the left side of  FIG. 21A , the transmitted wavelengths are delta lambda n zero, while in the right hand side they are delta lambda n one. 
         [0127]    The equations which explain the performance of the cavity are set forth at the bottom of  FIG. 21B . Equation 1 defines the transmission T through a fabry-perot cavity. Ta, Tb, Ra, Rb are the transmittances and reflectances of the primary (a) and secondary (b) mirrors. Phi a and Phi b are the phase changes upon reflectance at the primary and secondary mirrors, respectively. Delta is the phase thickness. Equation 2 defines the phase thickness in terms of the cavity spacing ds, the index of refraction of the spacer ns, and the angle of incidence, theta s. Equation 3 shows that the transmission T becomes the transmission of the second mirror when the transmission of the first mirror approaches 0. 
         [0128]    There are a number of particular frequency response modes which would be useful in applying the invention to displays.  FIGS. 22A through 22F  illustrate some of the possibilities and relate to the equations of  FIG. 21B . These are idealized plots which illustrate transmission and reflectivity (T/R) of the cavity for wavelengths in the visible range in driven and undriven states for each of the driven and undriven response modes. The different modes are achieved by using different combinations of mirrors and cavity spacings. 
         [0129]    The spectral characteristics of the mirrors used can be referred to as broad-band and narrow-band. The mirrors are optimized for the visible range with a broad band mirror operating across the entire visible range (i.e., reflecting over a minimum range of 400 nm to 700 nm). Such a mirror is denoted in the stack formula 1.671¦0.775(ERS) 0.833M (ERS)¦1.671 where ERS denotes an equal ripple filter. The ERS filter has layers which are a quarter wavelength thick, and their refractive indices are n0=1.671, n1=1.986, n2=1.663, n3=2.122, n4=1.562, n5=2.240, n6=1.495, n7=2.316, n8=1.460, n9=2.350, n10=1.450. A narrow-band filter optimized for the color green would reflect only over the range of 500 nm to 570 nm, and transmit everywhere else. Such a filter is described by the stack formula 1¦C1 2C2 (3A/2 3B1 3A/2)2 (3A/2 3B 3A/2)6 (3A/2 3B1 3A/2)2¦1.52 where the refractive indices are nA=0.156, nC2=nB1=1.93, and nB=2.34. 
         [0130]    The cavity spacing (i.e., cavity height) in both driven and undriven states can be set to a predetermined value by the film thicknesses used in its construction. These two values determine whether a cavity is resonant or non-resonant. For a resonant cavity, the spacing is determined such that the undriven state coincides with the resonant peak of the narrower of the two mirrors. When a device is non-resonant, it must be driven in order for the device to become resonant. 
         [0131]    For example, if the undriven cavity spacing were 535 nm then, because this coincides with the center of the previously defined narrow-band mirror, there would be a transmission peak at this frequency. Peak spacing for this cavity is 267 nm so the other peaks, which would occur in the standard Fabry-Perot fall outside of range of the narrow band mirror. This would be considered a resonant cavity because the peak occurs in the undriven state. Driving the cavity so that the spacing were 480 nm would result in no transmission peak because all of the cavity peaks are outside the range of the narrow-band mirror. For all practical purposes the narrow-band mirror does not exist at this frequency and therefore the transmission peak disappears. 
         [0132]      FIG. 22A  shows a T/R plot of a cavity having broad band mirrors on both layers of the cavity. When undriven, this results in transmission/reflection peaks which occur at wavelengths which are approximately integral multiples of the cavity spacing. (the notation m delta lambda n in  FIG. 21A  denotes the fact that there may be a series of peaks.) This is classic fabry-perot behavior. In the driven state (shown to the right in  FIG. 22A ), the cavity resonance is shifted out of the visible range causing the device to act like a broadband mirror. 
         [0133]      FIG. 22B  shows a T/R plot for a cavity having one broad band and one narrow band mirror. This device has a resonant cavity, causing a transmission peak at the center of the narrow-band mirror&#39;s passband when the device is in the undriven state. Driving the device (right hand side of  FIG. 22B ) shifts the cavity resonance away from that of the narrow band mirror, and the device acts like a broadband mirror. 
         [0134]    In  FIG. 22C , the cavity is like that of  FIG. 22B , except the cavity is non-resonant which results in broadband mirror cavity behavior in the undriven state. When driven, the cavity spacing shifts into resonance, causing a transmission peak centered on the narrow-band mirror. 
         [0135]      FIG. 22D  shows the performance of a resonant cavity with two narrow-band mirrors. When undriven, there is a transmission peak centered on the mirrors&#39; response. Since the mirrors are narrow-band, the overall cavity response is that of a broad-band transmitter. Driving the device out of resonance (i.e. active) results in a reflective peak at the narrow-band center frequency. 
         [0136]    Like  FIG. 22D , the cavity of  FIG. 22E  has two narrow band mirrors, but it is a non-resonant cavity. Consequently its behavior is opposite that of the cavity portrayed in  FIG. 22D . 
         [0137]    Thus, when one of the mirrors is narrow banded, mirror a for example, the transmission approaches zero for frequencies outside its range. This is essentially the transmission of mirror b. When both of the mirrors are narrow banded, the transmission becomes a maximum outside the frequency range. In either case, the spurious peaks typical of a fabry-perot are avoided. The result is a device which can be described as a single mode resonant cavity. 
         [0138]    When both of the mirrors are narrow banded, then fabry-perot type behavior occurs only when the cavity spacing is correct. Making the mirrors narrow enough allows only a single peak to occur. Then it is unnecessary to be concerned about spurious peaks that might occur within the visible range. 
         [0139]      FIG. 22F  is for a cavity with a simpler design involving only a metallic mirror on one wall and a hybrid filter on the other wall. The hybrid filter is a combination of a narrow bandpass filter (outer surface) and an induced absorber (inner surface). The induced absorber is a simple structure which can consist of one or more dielectric or dielectric and metallic films. The function of the absorber is to cause incident light of a specified frequency range to be absorbed by a reflective surface (i.e. mirror). One such design can be achieved with a single film of refractive index n=1.65 and a thickness of 75.8 nm. The induced absorber only functions when it is in contact with the mirror, otherwise it is inconsequential. 
         [0140]    In the undriven state, the hybrid filter (a green centered narrow bandpass/induced absorber) reflects everything but the green light, which is unaffected by the induced absorber and subsequently reflected by the metallic mirror. Thus the overall cavity response is like that of a broad-band mirror. When in the driven state, the hybrid filter comes into contact with the metallic mirror. The absorber couples the green light into the mirror, and the result is an absorption peak at that wavelength. 
         [0141]    Referring to  FIG. 23A , a red 3×3 pixel (i.e., 9 cavities) subtractive mode display array based on the cavity device using the N-N (narrow band-narrow band) configuration of  FIG. 22D  is shown. The cavity pixels are formed at the intersections of primary mirror/conductors  602  and secondary mirror/conductors  604 . The display is fabricated on substrate  608  and driven via contact pads  606  connected to each conductor/mirror  604 . 
         [0142]    A full nine-pixel display comprises three replications of the array of  FIG. 23A  arranged on top of one another and fabricated on separate substrates or color planes  610 ,  612 ,  614 , as shown in  FIG. 23B . Each of the individual color planes interacts only with and reflects one color (e.g., red, green, or blue), while passing all other colors. This is done by selecting the mirror spectral characteristic and cavity spacing in each color plane appropriately. The color planes are physically aligned and electrically driven through the contact pads to produce full color images. The image would be viewed from below in  FIG. 23B . 
         [0143]    Referring to  FIGS. 23C and 23D , a single layer composite approach is shown. Such a device would be more complicated to fabricate (though the mirror designs are simpler) but may suffer from inferior resolution. In this case, all three colors reside on the same array  616 . Devices using either the B-B, B-N, N-N, or the M-F (B=broad band, N=narrow band, M=mirror, H-F=hybrid filter) configuration are used for this display. 
         [0144]    Either the three plane or the single plane approach may be used in either transmissive and reflective modes. Pixel size and overall display size can be altered to make the displays useful in many different display and spatial light modulator applications. These include direct view and projection displays, optical computing, holographic memory, and any other situation where a one or two dimensional modulator of light can be used. 
         [0145]    Because these structures depend on electrostatic attraction, which varies in an exponential fashion with cavity spacing, while the mechanical restoring force of the membrane varies linearly with cavity spacing, they exhibit hysteresis. As seen in  FIG. 24A , the straight line labelled membrane tension shows that the restoring force on the membrane varies inversely linearly with distance (i.e., cavity spacing). That is, the smaller the spacing, the stronger the mechanical force tending to restore the original, at rest spacing. On the other hand, electrical attraction between the two layers of the cavity (the curved line) varies exponentially with smaller spacing, that is, as the two layers get closer there is an exponential increase in the attractive force between them. This causes a hysteresis effect as follows. With a low driving voltage applied, the secondary mirror/conductor experiences a displacement towards the substrate until the force of restoration balances the electrical attraction. However if the voltage is raised past a point known as a collapse threshold, the force of restoration is completely overcome and the membrane is pressed tightly against the substrate. The voltage can then be lowered again to some degree without affecting the position of the membrane. Only if the voltage is then lowered significantly or eliminated will the membrane be released. Because the membrane is under tensile stress, it will then pull itself away from the substrate when the voltage is released. This hysteresis can be used to achieve matrix addressing of a two-dimensional array, as explained with reference to  FIG. 24B . 
         [0146]    The display can be addressed and brightness controlled using control pulse sequences in the driving voltage. Shown is a timing diagram for a 3×3 pixel array analogous to that shown in  FIG. 23A . During operation, a continuous series of −5 volts scanning pulses is applied to the rows (rows 1-3) of the pixel array in a sequential fashion, as seen in the charts labelled “Row”. The pulses appear at the same frequency on each of the rows but the pulses for different rows are staggered. These pulses are insufficient in their own right to cause the membrane to collapse. The columns (cols. 1-3) of the pixel array (see charts labelled “Col) are maintained at a bias voltage of 5 volts so that the nominal voltage across each unactivated pixel is 5 volts. At times when the scan pulses are delivered to that pixel, the nominal row and column potentials are 5 and −5 volts respectively, resulting in a cavity voltage of 10 volts. With a 10 volts potential applied to the row and a −5 volts potential to the column, the total voltage across the cavity becomes 15 volts which is sufficient to drive the secondary mirror/conductor into the collapsed state, where it will remain until the end of the scan when all of the column voltages are pulsed to zero. The three charts at the bottom of  FIG. 24B  show the on and off states of the three pixels identified there by row and column numbers. 
         [0147]    The intensity or brightness of a pixel may be varied by changing the fraction of the scan during which the pixel is activated. The scan cycle begins at  198  and ends at  199 . The frequency of the scan pulses is such that six pulses of a given row fall within the scan cycle, providing an opportunity to activate a pixel at any one of six times during each cycle. Once the pixel is activated it stays on until the end of the cycle. Therefore six different intensities are possible for each pixel. For the scan shown, pixel C1R1 is at full brightness, pixel C2R2 is at 4/6 brightness, and pixel C3R2 is at ⅙ brightness. All pixels are cleared at the end of the scan and the cycle begins again. Since these structures can be driven at frequencies as high as 50 kHz, this method of brightness control is practical. Assuming six brightness levels, there would be a possibility of more than 8 thousand row scans per second. 
         [0148]    Two processes for fabricating the arrays will be discussed; others may also be possible. 
         [0149]    Referring to  FIG. 25A , substrate  700  is first cleaned using standard procedures. The substrate may be of many different materials including silicon, plastic, mylar, or quartz. The primary requirement is that the material be able to support an optically smooth, though not necessarily flat, finish. A preferred material would likely be glass, which would permit both transmissive and reflective operation in the visible range. 
         [0150]    The substrate is then coated with the primary conductor/mirror layer(s)  702 . This can be achieved using a physical vapor deposition (PVD) method such as sputtering or e-beam evaporation. Other possible methods include chemical vapor deposition and molecular beam epitaxy. The dimensions and nature of the layer(s) depend on the specific configuration desired. Detailed examples are discussed below. 
         [0151]    Referring to  FIG. 25B , a photoresist  704  has been patterned on the primary conductor/mirror. The photoresist may be of a positive or negative type. The standard procedure for this step involves spinning of the resist, softbaking at 90 C., exposing through an appropriate mask, developing to produce the pattern, and hardbaking at 130 C. 
         [0152]    Referring to  FIG. 25C , the photoresist pattern is defined in the primary conductor/mirror by an etching process. This step can be achieved either by wet chemical means or by plasma or reactive ion etching (RIE). The choice of etching technique depends on the nature of the conductor/mirror. In the case of an aluminum conductor/mirror, chlorine gas may be used to perform the etch, with a standard chamber power of 100 watts producing an etch rate of 100 angstroms/min. Some mirror materials may resist RIE and in such cases a technique such as ion milling may be used. All RIE steps are performed at a pressure of 30 mtorr unless otherwise noted. All plasma etch steps are performed at a pressure of 100 mtorr unless otherwise noted. The photoresist is removed using standard solvents. 
         [0153]    Alternatively, the conductor/mirror may be defined using the well-known procedure called lift-off. This procedure is used to define a layer in a subsequent step and is described below. 
         [0154]    Referring to  FIG. 25B , support rail material  706 , has been deposited using one of the methods mentioned previously. This material should be an insulator, for example silicon dioxide or silicon nitride. The material should be deposited uniformly, and at a thickness equal to thickness of the spacer layer, which will be deposited later. This thickness should in general be at least a multiple of the wavelength of light of interest. A thickness of 0.5 microns would place such a device near the middle of the visible spectrum. 
         [0155]    Referring to  FIG. 25E , photoresist layer  708  is spun on and hardbaked. Since this layer will not be photolithographically defined, other polymeric materials may be used instead. The only requirement is that they dissolve in solvents such as acetone or methanol, and be able to withstand a vacuum. This is the first step in defining a lift-off stencil. 
         [0156]    Referring to  FIG. 25F , template layer  710  has been deposited using one of the methods of PVD. The layer is of silicon dioxide though other materials are possible. Ideally the material should be etched using a process which does not affect the underlying resist. Buffered Oxide Etch (BOE) which consists of Hydrofluoric acid diluted 7:1 with water can perform this step in 15 seconds. The layer need only be a thousand angstroms thick. 
         [0157]    In  FIG. 25G , photoresist layer  712  has been spun-on and patterned in a manner already discussed. 
         [0158]    Referring to  FIG. 25H , using a combination of BOE and RIE, the pattern of resist layer  711  has been transferred to layers  710  and  708 . In the first step, the BOE is used to etch through the silicon dioxide layer  710 . An oxygen plasma is used to etch through resist layer  708 , and to remove resist layer  711 . Plasma etching differs from RIE in that it tends to be less anisotropic, yielding profiles that are not purely vertical. 
         [0159]    Referring to  FIG. 25I , using an oxygen plasma, resist layer  708  has been slightly underetched in order to facilitate removal of the lift-off stencil. RIE using a carbon tetrafluoride chemistry (CF4/02 6:4) is then applied to etching through layer  706 . 
         [0160]    Referring to  FIG. 25J , spacer layer  712  is deposited using PVD techniques. This material can be any number of different compounds which can be deposited using this technique. There are two key requirements for such a material. The first is that the material be soluble in water but not in solvents such as acetone or methanol. Example materials include lithium fluoride, aluminum fluoride, and sodium chloride. The second is that it be deposited with extreme uniformity and thickness control. 
         [0161]    The former allows resulting structures to be underetched without damage by using water as the final etchant. Water is an extremely benign solvent, and makes possible the incorporation of many different mirror, conductor, and structural materials in the final device. 
         [0162]    The latter insures that subsequent layers conform to the variations of the substrate and therefore primary conductor/mirror. This parallelism is essential for the optical behavior of the resonant cavity devices. 
         [0163]    The spacer may also be composed of a polymeric material such as hardbaked photoresist or polyimide. To achieve the required thickness uniformity, a technique other than spinning must be used to deposit the polymer. Two such techniques include extrusion and capillary coating. The consequence of using such a spacer is that all subsequent process steps must be low temperature in nature to prevent outgassing and shrinkage of this layer. In this case, the spacer is ultimately removed using an oxygen plasma. 
         [0164]    The stencil is subsequently removed using an ultrasonic acetone bath and methanol rinse. This also removes or lifts off excess deposited spacer material and is what constitutes the final step of the lift-off process. 
         [0165]    Alternatively, by using negative photoresist and an oppositely polarized mask, a natural overhang may be produced via overexposure. The same may be accomplished with positive photoresist using a technique known as image-reversal. This would preclude the need to put down a sacrificial photoresist layer and a subsequent SiO2 layer. 
         [0166]    Referring to  FIG. 25K , secondary conductor/mirror layer(s) and support membrane ( 714 ) are deposited. The nature of the conductor/mirror is dependent on the application. The support membrane must have a tensile residual stress. Tensile stress is required for two reasons. First so that the resulting membranes will be flat and therefore parallel to the substrate in the quiescent state. Secondly, such structures have a tendency to stick when the membranes come in contact with the substrate. Sufficient tensile stress is required pull the membrane away when the drive voltage is reduced. 
         [0167]    The membrane must have the appropriate optical characteristics as well. For visible light this would mean transparency in the visible region. Silicon nitride is one candidate for this role for it can be deposited using plasma enhanced chemical vapor deposition (PECVD) with controlled stress. Other candidates include silicon dioxide, magnesium fluoride and calcium fluoride, all of which can be deposited using e-beam evaporation with a resulting tensile stress. 
         [0168]    In  FIG. 25L , photoresist layer  716  has been spun-on and patterned in a manner discussed above. 
         [0169]    In  FIG. 25M , using RIE or ion milling, layer(s)  714  have been etched according to the pattern of resist layer  716 . 
         [0170]    Referring to  FIG. 25N , the final etch is accomplished by placing the device in water for a period of time. The water is agitated, and when the devices are fully etched they are dried. 
         [0171]    One variation on this step involves the use of n-butyl alcohol to displace the water when the etch is finished. The devices are then placed in a chamber at approximately 32 degrees centigrade to cause the alcohol to freeze. After this step the devices are placed into a vacuum chamber where the air is then evacuated. This causes the alcohol to sublime and can reduce membrane sticking during the drying phase. 
         [0172]    Another alternative process has initial steps of assembly shown in  FIGS. 26A through 26C , analogous to those shown in  FIGS. 25A through 25C . 
         [0173]    Thereafter, in  FIG. 26D , photoresist or polymer layer  806  is spun on and a stencil layer,  208 , of silicon dioxide is deposited using PVD. This layer must be thicker than the spacer layer to be deposited. 
         [0174]    In  FIG. 26E , resist layer  810  has been spun-on and patterned using standard procedures. 
         [0175]    In  FIG. 26F , this step uses BOE and an oxygen plasma etch to define a lift-off stencil. 
         [0176]    In  FIG. 26G , the spacer material is chosen and deposited as described in  FIG. 25J . 
         [0177]    In  FIG. 26H , the stencil is subsequently removed using an ultrasonic acetone bath and methanol rinse. 
         [0178]    The step shown in  FIG. 26I  is analogous to that shown in  FIG. 25K . 
         [0179]    In  FIG. 26J , photoresist layer  814  has been spun-on and patterned. 
         [0180]    In  FIG. 26K , using RIE or ion milling, layer(s)  812  have been etched according to the pattern of resist layer  214 . 
         [0181]    The final etch is accomplished in a manner described above. 
         [0182]    All of the materials used for the mirrors must be deposited in such a way that their stress can be controlled. Ideally, they are deposited as “stress balanced” which means that the overall stress of the film or film stack is zero. Since the ultimate goal is that the support membrane conductor/mirror combination have an overall tensile stress, conductor/mirror films with compressive stress may be accommodated by having a high stress support membrane. The technique of ion assisted e-beam deposition (IABD) precludes the need for such accommodation. Using this technique, the residual stress of almost any film of interest may be controlled by bombarding the substrate with a stream of neutral ions during the deposition process. This make possible the total mechanical decoupling of the support material from the optical material. As a result, emphasis can be placed on depositing an optically neutral support membrane with ideal mechanical characteristics. In the same manner, a “mechanically neutral” (i.e. stressless) conductor/mirror can be deposited with ideal optical characteristic. 
         [0183]    Referring to  FIG. 27A , the simplest conductor/mirror configuration for an individual cavity is formed from a layer  900  that is either the substrate for the primary conductor/mirror, or the support membrane if this is the secondary. Layer  902  is a metallic film with a thickness on the order of several hundred angstroms. The film can be of aluminum, silver, or any number of metals, based on the spectral, and resistive properties as well as the ease with which the metal can be etched. 
         [0184]    The use of a metal as both mirror and conductor simplifies fabrication. Unfortunately the spectral characteristics of the metallic layer cannot be tailored, and the performance limits devices to very specific kinds of behavior. Layer  904  is an insulator and/or reflection enhancement film. This can be formed by oxidizing the metal, if aluminum is being used, in an oxygen plasma thus forming a thin layer of aluminum oxide. Alternatively, insulating layers or reflection enhancement layers may be deposited in a manner discussed before. Metallic mirrors must be somewhat transmissive and therefore no more than several hundred angstroms thick. Insulator films can have thicknesses from one hundred to several thousand angstroms. Their thickness is determined by the kind of voltages expected in driving the devices. 
         [0185]    Referring to  FIG. 27B , a more elaborate configuration is shown. This is a compound conductor/mirror, with layer  900  as the substrate or the support membrane. The conductor  906  is either a transparent film such as indium tin oxide (ITO), or a very thin metallic layer such as gold. Either can be deposited using suitable film deposition methods. Thicknesses for the ITO should be in the range of several thousand angstroms, and metallic conductors less than 100 angstroms.  908  is a multilayer dielectric stack comprising the mirror. Such a mirror consists of alternating dielectric films with differing indexes of refraction deposited by a suitable PVD process. By choosing films with appropriate thicknesses and indexes, mirrors with tailorable spectral characteristics can be fabricated as is well known in the art. In general, the thickness of the individual layers is one quarter the wavelength of the light of interest. 
         [0186]    Alternatively, these mirrors may be deposited using a technique known as codeposition. In this case, PVD is used to deposit two materials with different refractive indices simultaneously. Using computer control the refractive index of the resulting film can be varied continuously between those of either film. This deposition technique makes possible mirrors with virtually any spectral characteristic. 
         [0187]    The ability to design the characteristics of this mirror allow for devices with a greater variety of modes of operation. Unfortunately, because the conductive layer is not perfectly transparent, additional losses are incurred as a result. 
         [0188]    Referring to  FIGS. 27C and 27D , a dielectric mirror  908  is deposited directly on substrate  900 . Metallic conductor  902  and insulator  904  are deposited and patterned such that they form a border around the periphery of the mirror. Using this configuration, it is possible to provide drive voltages to the devices without compromising throughput since the conductor can be placed outside the active area of the device.  FIG. 27D  shows a planar view of this mirror configuration. 
         [0189]    Response times of this device may suffer as a result of decreased conductor area. 
         [0190]    Referring to  FIG. 28   a , a linear tunable filter is shown which has been fabricated using the process sequence defined above. The major difference is the nature of the mask used to define the self-supporting membrane, which is comprised of support  1006  and  1008 . The substrate,  1000 , is transparent in the frequency region of interest, and electrodes  1004  are used to drive the device. Dielectric mirror  1002  are defined separately to produce a configuration like that of  FIGS. 27   c ,  27   d . Three filters are shown though many more can be fabricated. Each filter  1010 ,  1012 , and  1014  is driven independently so that individual frequencies may be separated from an incident light beam. Such a device can find use in spectroscopic analysis, as a demultiplexor in a wavelength division multiplexed fiber optic communication system, a color printer, or any other application where independent frequency selection is required.  FIG. 28   b  is a top view of the structure. 
         [0191]    Referring to  FIGS. 29   a  and  29   b , a device known as a deformable mirror includes a self-supporting membrane  1102  fabricated on a substrate  1100 . When a potential is applied between actuator electrodes  1104  and conducting mirror  1106 , the surface of the mirror can be deformed in a controllable manner. Such a device can be used as a component in an adaptive optics system, or in any situation where control of an incident wavefront is required. 
         [0192]    Other embodiments are within the scope of the following claims.