Abstract:
A light modulator and a high speed spatial light modulator (230) with each pixel (231) made of stacked quarter wavelength layers (232, 234) of heterogeneous material. Each layer (232, 234) is composed of periodic quantum well structures whose optical constants can be strongly perturbed by bias on control electrodes (240, 242). The control electrodes (240, 242) act to either remove light absorbing electrons from the layer or to inject them into each layer. The effect is to produce either a highly relecting mirror or a highly absorbing structure. The spatial light modulator (230) is compatible with semiconductor processing technology. Also, a modulator invoking the Burstein effect in the form of a stack of p-n diodes is disclosed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to optical devices, and, more particularly, to light modulators of varying carrier densities in semiconductor materials. 
     Highly reflective mirrors made of alternating layers of non-absorbing materials are well known; see Jenkins and White, Fundamentals of Optics, ch 14 (McGraw Hill 1957), and FIG. 1A for a schematic perspective view of such a mirror and FIG. 1B for a graph of the reflectance. In such mirrors the two layer types have different optical constants (&#34;n&#34; is the index of refraction and &#34;k&#34; the attenuation so that ε=(n+ik) 2 ); and because there is a discontinuity in the optical constants at each layer interface, light which enters the mirror undergoes multiple reflections. If the optical thickness of each layer is chosen correctly (a quarter wavelength plus optional multiples of a half wavelength), the reflected rays will be in phase as illustrated in FIG. 1C and the mirror will have high reflectivity as illustrated in FIG. 1B. Narrow band reflectivity of 98% is routinely obtained in such multilayer structures. 
     To make such a multilayer mirror efficient, the optical absorption of each layer must be very small. Otherwise, significant optical absorption will take place within each layer and the subsequent multiple reflections within the mirror will further reduce the intensity of the internal light rays. 
     If the optical constants of the layers of such a multilayer mirror could be switched between absorbing and non-absorbing values, then a mirror of adjustable reflectivity (i.e. a light modulator) would result. But traditionally, the optical constants of a material can only be slightly adjusted by, for example, electric fields (the Pockels effect), which is insufficient to make a reasonable spatial light modulator. Thus it is a problem in the known multilayer mirrors to switch the optical constants of the layers. 
     Quantum well devices are known in various forms, heterostructure lasers being a good example. Quantum well heterostructure lasers rely on the discrete energy levels in the quantum wells to achieve high efficiency and typically consist of a few coupled quantum wells; see, generally, Sze, Physics of Semiconductor Devices, 729-730 (Wiley Interscience, 2d Ed 1981). High Electron Mobility Transistors (HEMTs) are another type of quantum well device and typically use only one half of a quantum well (a single heterojunction) but may include a stack of a few quantum wells. The HEMT properties arise from conduction parallel to the heterojunctions and in the quantum well conduction or valence subbands; the conduction carriers (electrons or holes) are isolated from their donors or acceptors and this isolation limits impurity scattering of the carriers. See, for example, T. Drummond et al. Electron Mobility in Single and Multiple Period Modulation-Doped (Al,Ga)As/GaAs Heterostructures, 53 J. Appl. Phys. 1023 (1982). Superlattices consist of many quantum wells so tightly coupled that the individual wells are not distinguishable, but rather the wells become analogous to atoms in a lattice. Consequently, superlattices behave more like new types of materials than as groups of coupled quantum wells; see, generally, L. Esaki et al, Superfine Structure of Semiconductors Grown by Molecular-Beam Epitaxy, CRC Critical Reviews in Solid State Sciences 195 (April 1976). Chemla et al, U.S. Pat. No. 4,525,687 and T. Wood et al, High-Speed Optical Modulation with GaAs/GaAlAs Quantum Wells in a p-i-n Diode Structure, 44 Appl. Phys. Lett. 16 (1984) describe a multiple quantum well device for light modulation: an applied electric field perturbs the exciton photon absorption resonances near the fundamental edge of direct gap semiconductors and provides the modulation; the use of quantum wells confines carriers and enhances the exciton binding energy. Further, the applied field modifies the envelope wave functions of carriers in the quantum wells and thus the confinement energies and the exciton binding energy. The net effect of the quantum wells is a pronounced absorption by exciton resonances, and these resonances have energies which are easily modifiable by an applied electric field. However, such resonance is extremely sharp and it is a problem to modulate a fairly broad band of light. 
     Resonant tunneling devices are the simplest quantum well devices that exhibit quantum confinement and coupling and were first investigated by L. Chang et al. 24 Appl. Phys. Lett. 593 (1974), who observed weak structure in the current-voltage characteristics of resonant tunneling diodes at low temperatures. More recently, Sollner et al. 43 Appl. Phys. Lett. 588 (1983), have observed large negative differential resistance in such devices (peak-to-valley ratios as large as six to one have been obtained), and Shewchuk et al, 46 Appl. Phys. Lett. 508 (1985) and M. Reed, to appear, have demonstrated room temperature resonant tunneling. However, resonant tunneling devices have little optical application and it is a problem to apply resonant tunneling to optical devices. 
     SUMMARY OF THE INVENTION 
     The present invention provides a light modulator that can be switched from optically reflecting to optically absorbing and devices such as spatial light modulators including an array of such modulators. The light modulator relies on tunneling injection and withdrawal of carriers in semiconductors to vary the absorption of incident light. The modulator may include a stack of quarter wavelength plates of alternating first and second materials, and in preferred embodiments each plate of the first material is itself a multilayer stack of quantum wells and tunneling barriers coupling the wells or superlattices. Optical switchability arises from injecting and withdrawing carriers to and from the wells: absorption requires excitation of carriers from the well levels or minibands to quasi-continuum levels or other minibands and provides a somewhat broadband modulation. Optionally, both the first and second material plates are made of multilayer stacks of quantum wells or superlattices. These modulators solve the problems of variation of optical constants and the sharpness of response. 
     In other preferred embodiments each first material plate is a heavily doped n +  type layer and each second material plate is a heavily doped p +  type layer so the stack of quarter wavelength plates is also a stack of p +  -n +  junctions. Switchability arises from bias variation of the junctions varying the depletion layer width and consequently the absorption of photons with energies near the bandgap energy. This leads to somewhat sharp photon energy response. 
     Further preferred embodiments use a single thin layer of a few quantum wells for carrier injection and withdrawal and this thin layer is separated from a mirror by a quarter wavelength plate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-C schematically illustrate stacked mirrors; 
     FIG. 2 is a schematic exploded view of a first preferred embodiment stacked modulator; 
     FIGS. 3A-B are a schematic perspective and conduction band edge views of a layer of the first preferred embodiment; 
     FIGS. 4A-B schematically illustrate transmission and absorption by a layer of the first preferred embodiment; 
     FIGS. 5A-C schematically illustrate injection and withdrawal of carriers to a layer of the first preferred embodiment; 
     FIG. 6 is a schematic exploded perspective view of a second preferred embodiment; 
     FIG. 7 is a conduction band edge diagram of a layer junction in the second preferred embodiment; 
     FIGS. 8A-C schematically illustrate injection and withdrawal of carriers to a layer of the second preferred embodiment; 
     FIGS. 9A-C are schematic perspective view and band diagrams of a third preferred embodiment; 
     FIGS. 10A-C illustrate in plan and perspective view lateral confinement in the preferred embodiments; 
     FIGS. 11A-B are schematic plan and cross sectional elevation views of a preferred embodiment spatial light modulator; 
     FIGS. 12A-C are schematic perspective view and band diagrams for a fifth preferred embodiment; 
     FIG. 13 is a schematic perspective view of a sixth preferred embodiment; 
     FIG. 14 is a graph of the dependence of reflectance upon layer thickness for the sixth preferred embodiment; 
     FIG. 15 is a schematic perspective view of a seventh preferred embodiment; and 
     FIG. 16 is a schematic prespective view of an eighth perferred embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first preferred embodiment stacked light modulator, generally denoted by reference numeral 30, is illustrated in schematic perspective exploded view in FIG. 2 and includes a stack of three layers 32 of active material, three layers 34 of transparent material, and surrounding material and electrodes, not shown for clarity. Active material 32 itself is composed of multiple layers of semiconductor material as suggested in the exploded portion of FIG. 2. The details of operation of modulator 30 are best understood after consideration of the properties of active material 32, which is itself a multilayer structure as suggested by the upper portion of FIG. 2. 
     First consider a single crystal of alternating layers of undoped Al x  Ga 1-x  As and undoped Al y  Ga 1-y  As with the layers all about 100 Å thick (the height and width may be in the order of a few microns in applications and do not significantly affect the optical operation) as illustrated schematically in FIG. 3A wherein the layers are labelled by their composition; either x or y. The conduction band edge along line B-B through the alternating layers is schematically illustrated in FIG. 3B for x=0.0,y=0.4; note the x layers form quantum wells with barrier heights (the conduction band discontinuity at the interfaces of the x and y layers) of about 0.36 eV (360 meV). Note that these numbers are derived by using the generally accepted partition of the bandgap difference between AlGaAs alloys into 60% appearing as a conduction band discontinuity and 40% as a valence band discontinuity. As will be apparent from the following discussion, the partition has no particular effect on the operation of modulator 30 beyond adjusting the numbers. 
     The operation of modulator 30 depends on transition rates of electrons between levels in the conduction band wells and tunneling rates, and thus some approximate quantitative analysis will be used as an explanation aid. However, the approximations used and the analysis performed should not be construed to be part of modulator 30. In particular, we shall use an approximation for an electron in the conduction ban for a direct current bias field by utilizing permanent magnets to provide the required biasing of the magnetostrictive elements. Features of the invention include the reduction of coil winding losses, reduction of wiring complexity and the elimination of coupling components which isolate the AC drive from the DC drive resulting in significant simplification of the power driver design. 
     SUMMARY OF THE INVENTION 
     The aforementioned problems of the prior art are overcome with other objects and advantages of permanent magnet biasing of magnetostrictive transducers which are provided by magnetic circuitry in accordance with the invention and utilizes permanent magnets which are magnetized to much higher pole strengths that are almost immune to depolarization by alternating flux fields. Samarium-cobalt magnets have these properties. In addition, the shape and relative orientation of the magnets determine the amount of polarizing flux density that may be uniformly distributed throughout the magnetostrictive bar. The cross-sectional area of the magnet ends is preferably the same as the cross-sectional area of ends of the bar so that the stray flux density is kept to a minimum thereby maximizing the uniformity of the flux density within the magnetostrictive bar. The magnets are mounted outside the coil that is used for alternating current energization of the magnetostrictive bar to minimize coupling coefficient losses from eddy currents and inductance leakage which would otherwise be present in greater amounts in the magnets if they were inside the coil. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned aspects and other features, objects, and advantages of the apparatus of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is an isometric view of a preferred embodiment of the magnetostrictive transducer of this invention; 
     FIG. 2 is a top view of another embodiment of the magnetostrictive transducer of this invention with biasing magnets on the interior portion of the transducer; and 
     FIG. 3 shows a different form of permanent magnet assembly on the interior portion of the magnetostrictive bars. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows an isometric view in partial cross-section and in partial exploded view of a preferred embodiment of a transducer 10 of this invention. The transducer 10 comprises radiating masses 11, magnetostrictive bars 12, permanent magnets 13, electrical coils 14, and stress wires 15. The magnetostrictive bars 12 are typically lengthwise grain oriented bars of the lanthanide series of materials of which Terfenol (Tb 0 .3 Dy 0 .7 Fe 2 ) is preferred. Each bar is electrically isolated by insulators 12&#39; from the adjacent bar 12 of the stack of bars 12&#39; in order to reduce the eddy current losses. Each stack of bars 12&#39; has its ends in contact with the corner blocks 16 so that the assembly of the stacks 12&#39; and the corner blocks 16 forms a square. Each stack of bars 12&#39; has an electrical coil or solenoid 14 surrounding it so that alternating current electrical energization of each coil produces an alternating driving field in each stack. The DC biasing flux density for each stack of bars 12&#39; is provided by a magnet 13. Each magnet 13 is adjacent to and outside each coil 14 surrounding each stack of bars 12&#39; which is to be provided with the DC bias magnetic field. The magnets have the property that they can be magnetized to high pole strengths and are almost immune to depolarization by alternating flux fields. Samarium-cobalt magnets have been found to be very satisfactory for producing the DC biasing magnetic flux required by the Terfenol rods 12. These magnets have recoil permeabilities close to that of air as do the Terfenol rods 12. Because of the low permeability of the rods 12, the magnets 13 have like-polarization ends adjacent to each other. The flux from the like-polarity ends of each magnet 13 oppose one another to assist in producing a return flux field on the exterior of the magnet. A portion of the exterior flux of each magnet passes through and along the length of the stack of magnetostrictive bars 12&#39; to the other end of each magnet where the flux path is completed through the magnet. The corner blocks 16 are fabricated from a nonmagnetic material, e.g., stainless steel. The length and height of the magnet 13 is preferably the same as the length and height of the stack of bars 12&#39;. The curved face 13&#34; of magnet 13 has been found to produce a more uniform field along the length of the stack 12&#39; than other configurations. The curved surface 13&#34; is preferably a portion of an elliptical surface. The surface 13&#39;&#34; of magnet 13 is flat and, as stated previously, adjacent to the electrical coil 14. It has been experimentally determined for a magnet configuration such as that shown in FIG. 1 that the magnetic flux density at the ends of the bars 12 of stack 12&#39; is about 50 percent greater than the magnetic flux density at the center of the bar. Optimally, the flux density should be constant throughout each bar 12. A non-constant flux density moves the operating point for each portion of the bar along the B-H curve for the magnetostrictive bar thereby reducing the maximum alternating current field (and hence the acoustic power output) which may be applied before saturation occurs. The length of the magnets 13 is preferably equal to the length of each of the bars 12 of a stack 12&#39; to obtain a most uniform longitudinal distribution of flux density throughout the bars 12 of stacks 12&#39;. 
     The magnets 13 are placed outside the coils 14 in order to reduce the eddy current losses in the magnet 13 produced by the AC field of the coils 14. The radiating masses 11 are attached to corner blocks 16 by screws 11&#39; which are threadedly engaged with holes 16&#39; in the corner blocks 16. The radiating masses 11 each have outer surfaces 11&#34; which form a quarter of a cylindrical surface so that when all four of said radiating masses 11 are attached to their respective corner blocks 16 the resulting transducer has a cylindrical form. Each radiating mass 11 is elastically connected to a neighboring mass 11 by a spring 17 which spans the gap 18 between the masses 11. The portion of the gap 18 between spring 17 and the exterior surface 11&#34; is filled with a water seal 19, typically a urethane, which together with a water proof top and bottom flexible cover (not shown) attached to the radiating masses 11 provides a transducer 10 which has a water-proof interior. The covers (not shown) have provision for a cable for supporting the transducer 10 and also for providing electrical access to the interior of the transducer 10. Stress wires 15 are attached by screws 15&#39; between the tops (and bottoms) of adjacent radiating masses 11 and parallel to the stacks of bars 12&#39; to provide compressive stress on the bars 12 and to form the assembly of the transducer 10. The need for compressive stress on the magnetostrictive bars 12 is well known to those skilled in the art, and the details of the use of stress wires 15 to provide this compressive stress is described in detail in U.S. Pat. No. 4,438,509 incorporated herein by reference and made a part hereof. As described in that patent, the tensioning of the stress wire 15 by rotatably attached screws 15&#39; threaded into the radiating masses 11 causes a compressive force on the bars 12 of each stack. The radiating masses 11 are typically of a nonmagnetic material such as aluminum which has the advantage of also being of low mass. The magnets 13 exert a repulsion force on each other and are forced against and held in place by the inner surface 11&#39;&#34; of the radiating means 11. 
     In operation, the transducer 10 has an alternating voltage applied to each of the coils 14. For unipolar operation of the transducer 10, i.e., where the radiating masses 11 move radially in phase with one another, the electrical coils 14 must be energized so that the AC magnetic flux direction is in phase for each stack of bars 12&#39; relative to the DC flux direction produced by magnets 13 in each stack of bars 12&#39;. Operation of the transducer 10 of FIG. 1 using permanent magnet DC flux biasing is slightly less efficient than that obtained when a direct current through the coil 14 is used to obtain optimum biasing because of the less uniform DC magnetic field produced by the magnets 13. 
     FIG. 2 is a top view of another preferred embodiment of a transducer 20 with permanent magnet biasing of the magnetostrictive bars 12. The transducer 20 of FIG. 2 is similar to that transducer 10 of FIG. 1 and the same numbers are utilized as in FIG. 1 to show corresponding parts of the transducer. The transducer 20 of FIG. 2 has, in addition to the elements shown in FIG. 1, a set of inner permanent magnets 22 of the same samarium-cobalt type as used in the transducer of FIG. 1. However, the magnets 22 are placed on the interior portion of the transducer within a nonmagnetic container 23 having at least four opposed walls 23&#39;. Typically, the container is of stainless steel. The container is slightly smaller than the inside perimeter formed by the electrical coils 14, but large enough to contain the magnets 22. Although the magnets 22 are shown in FIG. 2 as touching one another and spaced from the container 23, in actuality because of the opposite polarization of adjacent magnets 22, they will repell one another and be forced by the repulsion force to press against the sides of the container 23. Magnets 13, 22 on opposite sides of the same stack of bars 12&#39; have like-polarity ends adjacent to each other. 
     It is noted that geometrical constraints on the innermost magnets 22 require that they be shorter than the magnetostrictive bars 12. Inasmuch as the magnetic flux 24 produced by the outer magnets 13 produce greater flux density at the ends than at the center of the magnetostrictive bars 12, the shorter length of the inner magnets 22 helps to provide greater uniformity of flux density within the magnetostrictive bars 12 because the flux produced by the shorter magnets 22 will be greater near the center of the bars than at their extremities. Because each magnetostrictive bar 12 is under the influence of the magnetic field provided by the inner magnet 22 and the outer magnet 13, the magnetic flux of at least the inner magnets 22 may be reduced to provide a more uniform flux density in the magnetostrictive bar 12 which is approximately half of the saturation flux density of each bar 12. The lesser flux density from each magnet may also be accomplished by reducing the area of the ends 13&#39; and 22&#39; of the magnets 13, 22, respectively. Alternatively, the strength to which the permanent magnets 13, 22 are magnetized may be reduced and may differ in order to produce a greater uniformity of flux density along the length of the magnetostrictive bar 12. It is noted that, the inner magnets 22 also have their innermost faces 22&#34; of eliptical shape with the face 22&#39;&#34; next to coil 14 being flat. The magnets 13 and 22 have the elliptical surface only in the circumferential direction. 
     As noted earlier, the radiating masses 11, the permanent magnets 13 and the corner blocks 16 are in contact with one another when the screws 11&#39;, 15&#39; are tightened to form the transducers 10, 20 of FIGS. 1 and 2, respectively. Even after tightening screws 21, the gap 18 still exists in order to provide space for the changing circumference of the radiating masses 11 when they undergo sinusoidal radial expansion and contraction under the influence of the alternating current in coils 14. 
     FIG. 3 shows a top view of another structure 29 for obtaining DC magnetic biasing of the magnetostrictive rods 12. In FIG. 3, the permanent magnets 30 are trapezoidal and fit inside the container 23 as described earlier. The magnets are forced into the container 23 with like-polarity poles adjacent each other. Their mutual repulsion force causes them to be forced against the side walls of the container 23 and be maintained in that position. A typical flux line 31 produced by the trapezoidal magnets 30 is showh in FIG. 3. The uniformity of flux density in the magnetostrictive bars 12 produced by magnets 30 is sufficient to result in satisfactory operation of a transducer made using trapezoidal magnets 30 without the external magnets 13 of FIGS. 1 and 2. Greater uniformity of flux density in the magnetostrictive bars 12 of FIG. 3 may be obtained by adding permanent magnets 13 to the exterior surfaces of the coils 14, if desired. 
     Having described a preferred embodiment of the invention, it will now be apparent to one of skill in the art that other embodiments incorporating its concept may be used. For example, different shapes of permanent magnets may provide more uniform fields in the magnetostrictive bars. In addition, the invention may be applied to bias magnetostrictive bars in &#34;Tonpilz&#34; and other types of transducers which do not have the cylindrical form used in illustrating the preferred embodiments. It is felt, therefore, that this invention should not be limited to the disclosed embodiment, but rather should be limited only by the spirit and scope of the appended claims.