Abstract:
An improved plasma controlled millimeter wave (MMW) or microwave (μW) antenna is provided. A plasma of electrons and holes is photo-injected into a photoconducting wafer. A special distribution of plasma and a MMW/μW reflecting surface behind the wafer allows the antenna to be generated at low light intensities and a 180° phase shift (modulo 360°) to be applied to selected MMWs/μWs.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    The present invention claims priority under 35 USC §119 to provisional application Serial No. 60/265,681, filed on Feb. 2, 2001, the entire disclosure of which is incorporated herein by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention relates generally to a scanning antenna. More particularly, the present invention relates to a plasma controlled scanning antenna operable in the microwave (μW) or millimeter wave (MMW) bands, for example.  
         BACKGROUND OF THE INVENTION  
         [0003]    Scanning antennas are necessary to form and scan an electromagnetic beam. Historically, there have been generally two types of scanning antennas, either mechanically scanned or electronically scanned. Mechanically scanned antennas perform scanning by forming a fixed beam with the antenna and physically moving the antenna. Electronically scanned antennas have been based on phased arrays which often employ hundreds to thousands of phase shifters to individual elements or groups of elements.  
           [0004]    Mechanically scanned antennas are generally slower than desired and require precision hardware which is often expensive. Because mechanically scanned antennas rely on moving parts, reliability is an issue. Electronically scanned phased array antennas offer many advantages, but the large numbers of phase shifters make such systems costly.  
           [0005]    Accordingly, alternative scanning methods have been of recent interest. Generally, these alternative methods are motivated by a desire for higher performance at lower cost. For example, a non-mechanical scanning antenna, without phase shifters, has been developed and is based on a type of Fresnel zone plate. The antenna forms and steers a beam of millimeter wave or microwave radiation using a light-modulated photoconducting wafer. See, e.g., U.S. Pat. No. 5,159,486 to Webb, entitled “Instrumentation apparatus and methods utilizing photoconductors as light-modulated dielectrics”; U.S. Pat. No. 5,360,973 to Webb, entitled “Millimeter Wave Beam Deflector”; Webb et al., “Light-Controlled MMW Beam Scanner”, Proc. 1993 SBMO International Microwave Conference, Vol. II, Sao Paolo, Brazil, IEEE Cat. No. 93TH0555-3, p. 417; and Webb et al., “MMW Beam Scanner Controlled by Light”, Proc. Workshop on Millimeter-Wave Power Generation and Beam Control, Huntsville, Ala., Special Report RD-AS-944, U.S. Army Missile Command, 1993, p. 333, the entire disclosures of which are incorporated herein by reference.  
           [0006]    As another alternative, antennas have been developed which use at least two thin semiconductor reflecting plates (e.g., silicon) which are supported (e.g., on glass) and separated by a synthetic foam spacer of dielectric constant near one. There are, however, disadvantages associated with such technique. The use of two or more plates presents complications which require the spacing of the plates to be controlled. A synthetic foam spacer is fragile and easily damaged either mechanically or by temperature. The use of thin plates, especially in the case of silicon of about 50-200 μm in thickness, makes it difficult to achieve the required plasma density under photo-injection because of the effect of surface mediated recombination in the thin plates. See, e.g., U.S. Pat. Nos. 5,084,707, 5,585,812 and 5,736,966, each to Reits.  
           [0007]    Recently, antennas have been disclosed which use a single photoconducting plate, e.g. silicon, and a transparent millimeter wave reflector. See, e.g., Webb et al., “Photonically Controlled 2-D Scanning Antenna,” PSAA-8 Proceedings of the Eighth Annual DARPA Symposium on Photonic Systems for Antenna Applications, The Naval Postgraduate School, Monterey, Calif., Jan. 13-15, 1998 (available from DTIC No. AD-B233444); Webb et al., “Experiments on an Optically Controlled 2-D Scanning Antenna,” 1998 Antenna Applications Symposium, Allerton Park, Monticello, Ill., Sep. 16-18, 1998, p. 99; Webb et al., “Optically Controlled Millimeter Wave Antenna,” Proceedings International Topical Meeting on Microwave Photonics, Melbourne, Australia, Nov. 17-19,1999, p.275; and Webb et al., “Novel Photonically Controlled Antenna for MMW Communications,” Proceedings International Topical Meeting on Microwave Photonics MWP 2000, Oxford UK, Sep. 11-13, 2000, p. 97. However, there is no indication of optimum thickness of the photoconducting plate, the nature of the transparent millimeter wave reflector, or the MMW phase relations of the wafer which are desirable for best performance.  
           [0008]    In view of the aforementioned shortcomings associated with existing scanning antennas, there remains a strong need in the art for a further improved scanning antenna.  
         SUMMARY OF THE INVENTION  
         [0009]    An improved plasma controlled millimeter wave (MMW) or microwave (μW) antenna is provided in accordance with the present invention. A plasma of electrons and holes is photo-injected into a photoconducting wafer. A special distribution of plasma and a MMW/μW reflecting surface behind the wafer allows the antenna to be generated at low light intensities and a 180° phase shift (modulo 360°) to be applied to selected MMWs/μWs. The selected phase change produces superior performance over similar antennas without the phase change.  
           [0010]    As is known, Fresnel zone plates (FZP) are of two general types, blocking and phase correcting. The simplest form of FZP works by blocking radiation. Rays going through different parts of an aperture add in-phase or out-of-phase at a detection point. If those rays which add out of phase are blocked, then there is a large gain in received intensity. Generally the phase conditions which produce a large increase in power are present in a given direction and thus the FZP produces a beam of radiation in that direction.  
           [0011]    In previous transmissive-type antennas, a technique was used which involved a transient blocking FZP in which a spatially varying density of plasma of charge carriers, electrons and/or holes, was created by optical injection into a semiconductor or photoconductor wafer. The un-illuminated parts of the photoconductor with no plasma allow incident MMW from a feed behind the wafer to be transmitted through the wafer. In the illuminated regions, however, the photo-injected charge carriers alter the index of refraction of the wafer locally. At sufficient light intensity the plasma density was large enough to substantially block MMW in those local lighted regions; at large enough plasma density the plasma caused the transmitted MMW to asymptotically approach zero in magnitude. The wafer, modified by light in this way, is made to diffract incident radiation into a beam and thus comprised a transient FZP. Because the wafer responds rapidly to changes in optical injection, it is possible to change rapidly transient Fresnel diffractive conditions and thus rapidly change the beam direction.  
           [0012]    In accordance with an exemplary embodiment of the present invention, a MMW feed is positioned in front of the wafer and an optically transparent MMW reflecting surface (reflector) is positioned in close proximity to the back surface of the wafer. The reflector is designed to be highly reflecting to MMW but transmit visible or infrared light of a wavelength below the band gap of the wafer in order to photo-inject plasma. A controllable light source behind the reflector can be positioned close to the reflector to minimize the need for focusing optics for the light patterns. The wafer thickness is chosen to be nominally an odd integer multiple of the wavelength of the MMW in the wafer material. With this choice of parameters MMW incident on a lighted region of the wafer containing plasma will be phase shifted by nominally 180° from MMW incident on a dark region.  
           [0013]    These features of the present invention enable two advantageous modes of operation. One mode is an improved blocking FZP antenna, and the second mode is as a phase correcting FZP which uses all the incident MMW radiation. As a blocking FZP antenna, a low plasma density can be chosen which provides for the principle of destructive interference to be used to completely block the undesired out-of-phase MMW. With proper control of phase in the MMW this blockage can be made to be complete, not just asymptotically approaching zero, and at much lower plasma density than in previous designs. The fact that a lower plasma density is suitable for operation allows for much less light intensity and electrical power to be used.  
           [0014]    The second mode of operation, the phase correcting FZP, occurs at higher plasma density for the regions containing the out-of-phase rays. In this case when the plasma density created is large enough, the MMW are reflected from the front surface of the wafer. Because the wafer thickness is nominally an odd integer multiple of the wavelength of the MMW in the wafer material, the MMW reflected from the front surface of the wafer are given a 180° phase shift with respect to MMW in the dark regions which make a double pass through the wafer. In this way, the out-of-phase rays are given a 180° phase shift and thus constructively interfere in the beam. A large increase in beam power and antenna efficiency results.  
           [0015]    The present invention is described primarily in the context of an antenna designed to operate in the MMW band. However, it will be appreciated that the antenna may instead operate in other radio frequency (RF) bands such as the microwave (μW) band. For example, an antenna according to the present invention may be designed to operate anywhere in the range of 4 gigahertz (GHz) to 400 GHZ.  
           [0016]    According to one particular aspect of the invention, a plasma controlled reflector antenna is provided. The antenna includes a reflector configured to reflect radio frequency (RF) radiation having a frequency equal to that of an operating frequency of the antenna. In addition, the antenna includes a feed for illuminating the reflector with and/or receiving from the reflector RF radiation at the operating frequency to transmit/receive RF radiation. A Fresnel zone plate (FZP) wafer is also included adjacent the reflector and interposed between the reflector and the feed. The FZP wafer has a thickness substantially equal to n*λvac/(4*N), where n is an odd integer, λvac is the free space wavelength of RF radiation at the operating frequency, and N is the index of refraction of a material of which the wafer is made, in a non-plasma injected state. Furthermore, the antenna includes a controllable light source for projecting a controlled light pattern onto the FZP wafer to inject selectively plasma into regions of the FZP wafer illuminated by the light pattern, thereby creating regions in a plasma injected state and regions in a non-plasma injected state.  
           [0017]    To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 illustrates path length dependence and condition for constructive interference for rays between a source and a detection point;  
         [0019]    [0019]FIG. 2 is a schematic layout for a Fresnel-Kirchhoff solution for integrated amplitude Up for an antenna aperture, where Up at detection point P for MMW are emitted from S and passing through the aperture;  
         [0020]    [0020]FIGS. 3A and 3B represent gray scale plots of phase at a detection point plotted on the plane of an aperture where a ray went through the aperture, for a source at 76 mm from 146 mm effective diameter aperture, distant detector, 35 GHz radiation, for a detector 0° off axis, and 30° off axis, respectively;  
         [0021]    [0021]FIG. 4 illustrates a transmissive antenna configuration with back feed;  
         [0022]    [0022]FIG. 5 is an exploded view of a reflective antenna in accordance with the present invention, in which the front feed emits MMW which impinge on special thickness photoconducting wafer which has an optically transparent MMW mirror or reflector at the back surface; the MMW pass through the wafer, are reflected at the back surface and re-emerge at the front surface; a controllable light source projects a light pattern through the transparent reflector onto the wafer creating plasma; and the plasma forms the MMW into a beam;  
         [0023]    [0023]FIG. 6 is a cross-section of a reflective antenna in accordance with the present invention, with a n*λvac/(4*N) thick wafer and a 180° phase shift between in-phase zones without plasma and out-of-phase zones with plasma. Here n is an odd integer, n=1, 3, 5 . . . , λvac is the free space wavelength of the MMW radiation, and N is the index of refraction of the wafer material in the dark; assuming the ideal case, at low plasma density there is a 180° phase change on reflection at the back reflector and at high plasma density there is a 180° phase change on reflection at the front surface; the path length difference of n*λvac/(4*N) provides the desired overall phase shift of 180° (modulo 360°) between in-phase and out-of-phase zones;  
         [0024]    [0024]FIG. 7 illustrates the behavior of the magnitude of the reflectivity of an improved blocking FZP in accordance with the present invention; in this example, the calculation gives the reflectivity as a function of photo-injected plasma density of the wafer and reflector assembly of FIG. 6, assuming a semi-insulating silicon wafer of thickness 652 μm and 500 lines per inch metal mesh reflector of wire size 0.45×10 −3  in.; one will note the high reflectivity near 1 at lowest plasma density and the deep minimum in reflectivity near zero at a density of 4×10 14  cm −3 ;  
         [0025]    [0025]FIG. 8 illustrates the behavior of the magnitude of the reflectivity of an improved phase correcting FZP in accordance with the present invention; in this example, the calculation is for the reflectivity as a function of photo-injected plasma density of the wafer and reflector assembly of FIG. 6, assuming semi-insulating silicon wafer of thickness 640 μm and 500 lines per inch metal mesh reflector of wire size 0.45×10 −3  in; one will note the high reflectivity near 1 at lowest plasma density and the reflectivity rising to 0.9 at a plasma density of 2×10 16  cm −3 ; between the lowest and highest plasma density the phase of the reflectivity goes through a 180° phase change; and  
         [0026]    [0026]FIG. 9 illustrates the behavior of the phase of the reflectivity of an improved phase correcting FZP in accordance with the present invention; in this example, the same parameters of FIG. 8 were used; the wafer of thickness 640 μm ensures that the total phase change is 180° between the lowest plasma density on the left and the maximum plasma density of 2×10 16  cm −3  on the right; one will note that changes in the maximum of an order of magnitude produce only slight changes from the ideal 180° phase shift. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    The present invention will now be described in detail with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout.  
         [0028]    Transient Fresnel Zone Plates (FZPS)  
         [0029]    [0029]FIG. 1 shows MMW rays (r1+r2 &amp; r3+r4) from a source S passing through an aperture  12  to a detection point P. The Fresnel zone plate (FZP) conditions provide that all such rays arrive at the detection point in constructive interference.  
         [0030]    The FZP conditions can be understood with reference to FIG. 2 which shows an aperture  12  in an otherwise opaque screen. The amplitude of the radiation arriving at the detection point P, from a source S, is calculated by solving the Fresnel-Kirchhoff expression in the scalar amplitude approximation (see, e.g., G. R. Fowles, Introduction to Modem Optics, 2nd Ed., Dover Publ. New York, 1975.):  
         U   p     =       -       i                 k                   U   0                 -   ω                   t           4      π                ∫   ∫     aperture                                      k        (     r   +     r   ′       )             r                   r   ′              [       cos        (     n   ,   r     )       -     cos        (     n   ,     r   ′       )         ]               A                 U   p     =     integrated                 amplitude                 at                 P                           
 U P =integrated amplitude at P  
         [0031]    In general, rays arriving at P have a relative phase which depends on the point where they went through the aperture  12 . The phase at P is determined by the exponential in the Fresnel-Kirchhoff relation and depends on the positions of S and of P. FIGS. 3A and 3B show two examples of phase. FIG. 3A shows the relative phase at the detection point P of a ray plotted with a gray scale on the plane of the aperture at the point where the ray passes through the aperture  12 . In this case, the source point S and detection point P have been chosen to be collinear with the aperture  12 . In FIG. 3A, those rays with phase represented from white to gray are taken to be in-phase and, conversely, those rays with phase from gray to black are out-of-phase. If the out-of-phase rays are blocked, for example by a photo-injected plasma, then the only rays arriving at point P would be in-phase and a large increase in intensity at P would result.  
         [0032]    It is clear from FIG. 3A that the relative phase distribution displays the layout symmetry of S, P, and the aperture  12 . In particular, the relative phase depends on the angle that point P makes with the axis of the aperture  12 . If the relative position of P is moved off-axis, then the distribution of relative phase at point P is changed. FIG. 3B shows the phase distribution when point P is moved to 30° off-axis, all other parameters remaining the same. Therefore, it is evident to send a beam to this direction requires that a different ray distribution must be blocked. The two directions for point P in FIGS. 3A and 3B illustrate that to form a MMW beam in a specific direction it is desirable to inject a plasma selectively in a photoconductor. For photo-injection, light of wavelength below the band gap of the photoconductor is used. In order to send the beam in a new direction, the present invention is able to change the light pattern and thus the spatial distribution of the plasma. The present invention therefore utilizes a photoconducting material which is transmissive in the dark to MMW and responsive in the light, and a light source having a high degree of controllability.  
         [0033]    A variety of photoconducting materials can be used in accordance with the invention. These include elemental semiconductors such as silicon and germanium, or a member of the category of III-V and II-VI compound semiconductors. For a controllable light source, computer controlled light arrays composed of LEDs or solid state lasers can be used (see, e.g., U.S. Pat. No. 5,360,973). Alternatively, another type of light source could be a steered laser beam, for example. For an optically transparent MMW reflector, a fine metal mesh, a fine grid of conducting metal lines deposited on a transparent substrate, or a coating such as indium tin oxide on a glass substrate can be effective in accordance with the present invention.  
         [0034]    [0034]FIG. 4 illustrates a previously developed antenna architecture, generally designated  20 . The antenna  20  includes a MMW feed  22  which is behind an FZP wafer  24  and coupled to a MMW source  25 . The feed  22  transmits MMW radiation  26  toward the wafer  24 . A programmable light array  28  projects a light pattern onto the wafer  24  to form a Fresnel lens shaped plasma within the wafer  24 . The MMW radiation  26  which is not blocked by the pattern formed on the wafer  24  passes therethrough as radiated energy  30 . There are, however, performance limitations to such architecture. Such performance limitations are addressed herein by the antenna architecture of the present invention as will now be described more fully.  
         [0035]    New Reflective Antenna Architecture with Two Modes of Operation  
         [0036]    Referring now to FIG. 5, an antenna  50  having a reflective architecture is shown in accordance with the present invention. The antenna  50  includes a controllable light source  52  such as a computer controlled light array composed of LEDs or solid state lasers. Alternatively, the light source  52  could be a steered laser beam, for example. Moreover, it will be appreciated that any controllable light source emitting light of wavelength less than the band gap of the photoconducting material can be used. All are considered within the scope of the invention.  
         [0037]    The antenna  50  further includes a MMW reflector  54  positioned in front of the light source  52 . The reflector  54  is designed to allow the light from the light source  52  to pass therethrough, while serving to reflect incident MMW radiation. Exemplary constructions for the optically transparent MMW reflector  54  include a fine metal mesh, a fine grid of conducting metal lines deposited on a transparent substrate, or a coating such as indium tin oxide on a glass substrate. The thickness and spacing of the mesh or grid lines are selected so as to be effectively transparent at the higher optical frequencies of the light source  52 , while serving as a reflector at the lower MMW frequencies. For example, in an antenna  50  designed to operate at 35 Gigahertz (Ghz), the MMW reflector  54  may be made of a 500 lines per inch metal mesh of wire having a size of 0.45×10 3  inch. Of course, other sizes are possible and will depend on the operating frequency of the antenna  50 , etc., as will be appreciated by those having ordinary skill in the art.  
         [0038]    In addition, the antenna  50  includes an FZP wafer  56  positioned in front of and preferably immediately adjacent the MMW reflector  54 . As mentioned above, the wafer  56  is a photoconducting material which is transmissive in the dark to MMW, and is responsive in the light. A variety of photoconducting materials can be used as the wafer  56 . Such materials include, but are not limited to, elemental semiconductors such as silicon and germanium, or a member of the category of III-V and II-VI compound semiconductors.  
         [0039]    Finally, the antenna  50  includes an antenna feed  60  which is located in front of the wafer  56  at a distance FL corresponding to the desired focal length of the antenna  50 . The feed  60  may be a small MMW horn or the like, as will be appreciated. Alternatively, the feed  60  may be embodied by a small subreflector in the case of a Cassegrain or backfire-feed type construction, for example. The feed  60  is connected to a MMW source  61  in the case where the antenna  50  serves to transmit. In addition, or in the alternative, the feed  60  is connected to a MMW receiver (not shown) in the case where the antenna  50  serves to receive.  
         [0040]    In the case where the antenna  50  is a transmitting antenna, the feed  60  transmits MMW radiation  62  towards the wafer  56 . The controlled light source  52  projects a light pattern through the reflector  54  onto the back of the wafer  56 . The back surfaces of those regions of the wafer  56  which have been illuminated by the light source  52  have plasma photo-injected therein, and the plasma diffuses thru the wafer  56  towards the front surface. This causes the illuminated regions of the wafer  56  to reflect the MMW radiation  62  at the front face  64  of the wafer  56 . The regions of the wafer  56  which are not illuminated by the light source  52  do not include plasma. These non-illuminated regions therefore allow the MMW radiation  62  to pass through those sections of the wafer  56  to the MMW reflector  54  therebehind. The MMW radiation  62  is then reflected by the MMW reflector  54  and passes back through the wafer  56  towards the feed  60 .  
         [0041]    According to the preferred embodiment of the antenna  50 , the wafer  56  and reflector  54  satisfy certain specified conditions. A first condition is that the wafer  56  have a nominal thickness d that is an odd integral multiple of a quarter wavelength in the material, namely:  
           d=n*λvac /(4 *N )  
         [0042]    n=1, 3, 5, . . .  
         [0043]    N=index of refraction of wafer  56  in non-illuminated (dark) regions  
         [0044]    λvac=is the free space wavelength of the MMW radiation  62  at the operating frequency  
         [0045]    As is shown in FIG. 6, the controlled light source  52  may include a plurality of LEDs  70  arranged in an array. By selectively illuminating the LEDs  70 , heavy plasma density produces a 180° phase shift into the out-of-phase zones  72 . With respect to those regions where the LEDs  70  are not illuminated, low plasma density (or “in-phase”) zones  74  are provided. MMW radiation  76  which is incident on the high plasma density zones  72  incurs a 180° phase change on reflection at the front surface  64  of the wafer  56 . Comparatively, MMW radiation  78  which is incident on the low plasma density zones  74  incurs a 180° phase change on reflection at the MMW reflector  54 . The path length difference d=n*λvac/(4*N) provides the desired overall phase shift of 180° (modulo 360°) between in-phase and out-of-phase zones  74  and  72 , respectively.  
         [0046]    In order to maintain the proper phase relationships it is important that proper account of the dark state (i.e., low-plasma density state) refractive index of the wafer material, N, is taken into account in calculating the thickness d of the wafer  56 . For example, in the case of an operating frequency of 35 Ghz and a silicon wafer  56  with a dielectric constant of approximately e=11.7,  
           d=n*λvac /(4* N )  
         [0047]    N=sqrt e=3.42  
         [0048]    λ=0.857 cm  
         [0049]    n=1  
         [0050]    ∴d=626 μm  
         [0051]    It is also important that the MMW reflector  54  be in close proximity to the back surface of the wafer  56  as represented in FIG. 6. The afore-described configuration of the antenna  50  can be used as a blocking or phase correcting FZP, as will now be discussed.  
         [0052]    Blocking FZP with Low Plasma Density Mode of Operation  
         [0053]    [0053]FIG. 7 shows the calculated 35 GHz reflectivity as a function of plasma density of the antenna construction shown in FIG. 6. This calculation was done for a silicon wafer  56  which was 652 μm in thickness. The wafer  56  was assumed to be n-type of residual impurity 3.3×10 12  cm −3 . This impurity level is representative of a semi-insulating silicon material of resistivity 1000 Ω-cm. The index of refraction of the silicon as a function of carrier density was calculated using standard techniques. The MMW reflector  54  was composed of metal mesh 500 lines per inch and wire size 0.45×10 −3  located at the back surface of the wafer  56 . With plasma density increasing from zero it is seen that the reflectivity falls from near 1 (0.975) as plasma density increases and makes the wafer material slightly lossy. At plasma density of about 4.2×10 14  cm −3  the reflectivity is near 0 indicating a near perfect cancellation of reflections from the reflector  54  and the wafer front surface  64 .  
         [0054]    To achieve perfect cancellation it is appropriate to optimize the thickness d of the wafer  56  slightly from quarter wavelength. The thickness of the wafer  56  for a quarter wavelength at 35 GHz is 626 μm as shown above. However, there is a slight phase shift from the ideal 180° upon reflection at the reflector  54 . Accordingly, the thickness d of the wafer  56  was adjusted by 4 percent to 652 μm to give cancellation. The appropriate thickness adjustment may be determined empirically, for example, or via other means such a modeling techniques (See, e.g., M. Kohin et al., “Design of Transparent Conductive Coatings and Filters” in Infrared Thin Films, R. P. Shimshock Ed. Critical Reviews of Optical Science and Technology, Vol. CR 39).  
         [0055]    It is calculated that to achieve a photo-injected plasma density of 4×1 014 cm −3  requires a light intensity of only 7×10 −3  W/cm 2  or 7 mW/cm 2 . This is a modest light intensity. Bright sunlight, for example, has an intensity of order 100 mW/cm 2 . This calculation assumes a free carrier recombination time of 1000 μs which is realistic for carefully prepared silicon. To achieve a comparable level of blocking in a previous transmission mode antenna (see, e.g., U.S. Pat. No. 5,360,973) requires much higher plasma density. It is estimated that the plasma density in that case would have to be 3×10 15  cm −3  with a corresponding increase in light intensity of almost an order of magnitude.  
         [0056]    Thus the capability of operating at much lower light intensity reduces the power requirements on the light source  52  and the heating level on the wafer  56  which can be advantageous in low power applications.  
         [0057]    Phase Correcting FZP with High Plasma Density Mode of Operation  
         [0058]    The previously described blocking approach results in a loss of about 50% of the MMW amplitude from the beam. It is useful to estimate maximum efficiency or gain by noting that it can be shown that alternating in-phase and out-of-phase zones of FIG. 3 are of nearly equal area. If we suppress the dependence r and r′ then we can approximate the total electric field intensity as a sum over in-phase and out-of-phase zones where the zones are assumed to have equal areas:  
       E   =         ∫     i                 n                 phase                 zones               E       +       ∫     out                 of                 phase                 zones               E                 E   =           E   0          (     1     2      π       )              ∫       -   π     /   2       π   /   2              cos        (   θ   )                          θ           +         E   0          (     1     2      π       )              ∫     π   /   2         -   π     /   2              cos        (   θ   )                          θ                     E   =         (     1   π     )          E   0       +       (       -   1     π     )          E   0                 E   =       (     1   π     )          E   0                   after                 blocking                 out                 of                 phase                 rays               P   ∝         (     1   π     )     2          E   0   2         =     0.101        E   0   2                             
 
         [0059]    Accordingly the overall gain of the antenna  50  is nearly −10 dB and the efficiency is 10.1%. Thus, the approach of blocking MMW is a penalty to antenna efficiency. A more exact numerical solution of the Fresnel-Kirchhoff expression for efficiency confirms this result.  
         [0060]    However, as indicated in FIG. 6 if a uniform 180° phase shift is applied to the out-of-phase zones  72  rather than blocking them, then a large increase in maximum gain or efficiency would be produced:  
       E   =       (     2   π     )          E   0                   all                 zones                 contributing                 to                 beam               P   ∝         (     2   π     )     2          E   0   2         =     0.405        E   0   2                             
 
         [0061]    In that case, to the same approximation, the electric field would be doubled, the beam power increased by a factor of four, and the corresponding maximum efficiency to 40.5%. Once again, a more exact numerical solution of the Fresnel-Kirchhoff expression for efficiency confirms this result.  
         [0062]    [0062]FIG. 8 shows the 35 GHz reflectivity of a silicon wafer  56  and metal mesh reflector  54  as a function of plasma density. The same wafer  56  and metal mesh reflector  54  parameters were assumed as in FIG. 7 except that a wafer thickness was adjusted slightly to 640 μm was used. The reason for this slight adjustment of thickness is given below.  
         [0063]    In FIG. 8 it is seen that with plasma density increasing from zero, the reflectivity decreases from near 1 (0.975) as plasma density increases and makes the wafer material slightly lossy. At plasma density of about 4.2×10 14  cm −3  the reflectivity decreases to a minimum value of 0.05 indicating that not quite perfect cancellation of reflections from the reflector  54  and wafer front surface  64  can be achieved. With increasing plasma density the reflectivity increases reaching 0.9 at the highest density assumed of 2×10 16  cm 3 .  
         [0064]    The thickness of 640 μm represents a 2% adjustment from the quarter wavelength thickness at 35 GHz which is 626 μm. It is desirable to account for the slight phase shift upon reflection at the reflector  54 , and the phase shift at the front surface  64  at the highest plasma density light intensity used.  
         [0065]    [0065]FIG. 9 displays the phase of the reflectivity as a function of plasma density. With the wafer thickness of 640 μm, the total change in phase is exactly 180° from zero density to the highest density of 2×10 16  cm 3 . A higher or lower maximum plasma density produces only a slight penalty in phase shift from 180°. For example, at a plasma density of 2×10 15  cm −3  the shift in phase is changed by only 10°. Thus, the choice of maximum plasma density is not critical for the phase correcting FZP. However, the magnitude of the reflectivity, 0.9, is significantly higher at the higher plasma density.  
         [0066]    To produce a photo-injected plasma density of 2×10 16  cm −3  requires a light intensity of 0.3 W/cm 2  or 300 mW/cm 2 , once again assuming a free carrier recombination time of 1000 μs. At this light intensity, the change in phase of 180° between MMW radiation in the in-phase zones  74  and the out-of-phase zones  72  is achieved as given in FIG. 6, producing an efficiency which approaches the ideal for this configuration of 40.5%.  
         [0067]    Although the invention has been shown and described with respect to certain preferred embodiments, it is obvious that equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. For example, although the antenna  50  has been described primarily in the context of transmitting MMW radiation it will be appreciated that the antenna  50  may also operate as a receiving antenna for receiving MMW radiation. Moreover, although the antenna  50  is described as constituting a planar array of elements (e.g., light source, reflector, wafer, etc.), it will be appreciated that the elements may instead be curved or have some other shape without departing from the scope of the invention. Furthermore, although the antenna  50  is described primarily for operation in the MMW band, it will be appreciated that the antenna  50  could instead be designed to operate in other bands. The present invention includes all such equivalents and modifications, and is limited only by the scope of the following claims.