Relativistic semiconductor plasma wave frequency up-converter

A device that frequency up-shifts an impinging electromagnetic wave, facilitating signal pulse compression, consisting of a semiconductor block or waveguide with an optically-induced relativistic plasma wave which interacts with an applied or self generated electromagnetic signal.

BACKGROUND 
The present invention relates to the modification of an electromagnetic 
signal through interaction with an energy beam-induced moving plasma in a 
semiconductor. The modified signal is frequency up-shifted, producing a 
fast electromagnetic or electrical signal. The invented device can also 
generate fast electrical signals. 
A prior art method of producing fast electromagnetic signals utilizes a 
photoconductive switch. A semiconductor is placed between two contacts 
which are connected to a voltage source in the external circuit. The 
semiconductor behaves like an insulator until it is made to conduct with 
laser illumination of the proper wavelength. A fast pulse of laser energy 
will thus produce a fast electrical signal in the external circuit. One 
basic version of this switch gives an electrical signal amplitude that is 
approximately proportional to the optical energy deposited in the 
semiconductor. The other basic version uses semiconductors such as GaAs 
which can turn on completely when the illumination energy is above some 
threshold. The basic mechanism for this later version is referred to as 
optically initiated avalanche. Discussions of these types of switches can 
be found in the following U.S. Pat. Nos.: Davis 4,438,331; Ragle 
4,864,119; and Kim 5,028,971. The general advantage of photoconductive 
switches is their ability to produce fast, high amplitude pulses, for 
example, a 5 kV electrical pulse with a rise time of 100 picoseconds (ps). 
More generally, a laser illumination of the proper wavelength and energy 
between two separated conductors on a semiconductor substrate will 
electrically connect or "short" the two conductors. Thus, by turning on 
such illumination, it is possible to make an electrical connection for as 
long as the optical energy is applied. For a review of this technology see 
Lee, "Picosecond optics and microwave technology," IEEE Trans. Microwave 
Theory Tech., vol. MTT-38, pp. 596-607, 1990. 
A related technology uses adjustable location, stationary laser 
illumination on portions of a semiconductor waveguide in order to control 
the propagation velocity of the microwave signal. In this way, the 
application of laser light to the waveguide can be made to slow down the 
propagation of a microwave signal by changing certain characteristics of 
the waveguide, thus producing an optically controlled waveguide phase 
shifter. For a detailed treatment of this area see Cheung et al., 
"Optically controlled coplanar waveguide phase shifters," IEEE Trans. 
Microwave Theory Tech., vol. MTT-38, pp. 586-589, 1990, and Vaucher et 
al., "Theory of optically controlled millimeter-wave phase shifters," IEEE 
Trans. Microwave Theory Tech., vol. MTT-31, pp. 209-216, 1983. 
Another prior art way to produce fast microwave signals was recently 
reported by Savage et al. in "Frequency Upconversion of Electromagnetic 
Radiation upon Transmission into an Ionization Front," Physical Review 
Letters, vol. 68, Feb. 1992. This paper describes an experiment where an 
optically induced moving ionization front in a gaseous medium interacts 
with an impinging microwave signal, producing an up-shifted signal. Source 
radiation at 35 GHz was up-converted to 116 GHz when an ionizing laser 
pulse was propagated through a resonant microwave cavity. However, the 
tens of mJ of optical pulse energy used was inadequate to produce a true 
reflective plasma at microwave frequencies, giving up-shifts different 
than those predicted by a simple Doppler effect. The up-conversion under 
these conditions was rather inefficient, being less than 1% at 116 GHz. 
An ideal situation for efficient frequency up-conversion would be to 
produce an optically induced moving ionization front which is sufficiently 
dense to give complete reflection for an impinging electromagnetic signal. 
Such a case is analogous to the reflection of electromagnetic radiation 
from a moving mirror, which will give a Doppler shift dependent on the 
mirror's velocity. The up-conversion factor due to the relativistic 
Doppler effect in the rest frame of the observer for an ideal reflecting 
"front" moving toward the impinging electromagnetic radiation is given by 
equation 1 which is written and plotted in FIG. 1. The velocity of the 
electromagnetic radiation in the medium is c and the velocity of the 
moving reflecting front is v. As the velocity of the reflecting boundary 
becomes a significant fraction of the velocity of the electromagnetic 
radiation, very large up-shifts will occur. 
Because of the technological difficulty of producing fast laser pulses of 
sufficiently high energy to create a truly reflective ionization front in 
a gas, frequency up-conversion by the pure Doppler effect has not 
previously been achieved. The low efficiency up-conversion demonstrated by 
Savage et al. was due to a plasma-microwave interaction which is a 
superset of the Doppler effect. 
SUMMARY OF THE INVENTION 
The present invention uses energy beam illumination, preferably laser, to 
generate a moving conducting boundary in a semiconductor, preferably in a 
waveguide, which up-shifts an impinging electromagnetic signal via the 
relativistic Doppler effect. Each Fourier frequency component of the 
impinging signal is up-shifted similarly, giving temporal compression of 
the reflected signal when the interaction time of the optically-induced 
conducting boundary is as long as the duration of the original signal. For 
example, a gaussian microwave pulse of 1 ns duration travelling at 110.8 
the speed of an impinging photo-generated plasma "short" in a 
semiconductor produces an up-shift of 9 times as predicted by equation 1 
shown in FIG. 1, and translates into a temporal pulse compression to 0.11 
ns. Furthermore, since pulse energy is conserved, a narrowing of the pulse 
gives a concomitant increase in peak pulse amplitude of 9 times. 
The great advantage of using a semiconductor as the interacting medium is 
that the generation of laser or electron beam induced plasmas is highly 
efficient compared to a gas, which allows the production of moving plasma 
boundaries reflective to electromagnetic radiation with modest energy 
requirements. Also, the semiconductor can be conveniently used as a 
substrate for many common microwave or optical waveguide geometries, and 
allows for compact configurations, including microwave cavities. 
To determine the optical energy required in a semiconductor to produce 
adequate plasma densities, the relation of equation 2 is used, 
EQU n=(1-R).alpha.E(hc/.lambda.).sup.-1 [1-exp(-.alpha.z)] (2) 
where n is the photo-generated plasma density in cm.sup.-3, R is the 
reflection loss off the semiconductor, .alpha. is the absorption 
coefficient in cm.sup.-1, E is the optical energy density in J/cm.sup.2, 
hc/.lambda. is the photon energy, and z is the depth of the absorbing 
medium. Using parameters for the semiconductor GaAs, the absorption 
coefficient is 2000 cm.sup.-1 (equivalent to an absorption depth of 5 
.mu.m) at a wavelength of 877 nm, the material reflection loss is 30%, and 
the thickness is assumed to be 1 mm. For the coplanar strip waveguide, 
discussed in more detail later, useful reflections of microwave energy are 
achieved with plasma densities of 10.sup.15 cm.sup.-3. From equation 2, 
this corresponds to an optical energy density using the above parameters 
of only 162 nJ/cm.sup.2. Fast laser pulses in the tens of mJ range produce 
semiconductor plasma densities greater than 10.sup.18 cm.sup.-3. 
The generation of photocarriers in a semiconductor is an intrinsically fast 
process. The electronic transition energy from the valence to conduction 
band in semiconductors is a few electron volts, thus the production of an 
electron-hole pair in the material following the absorption of a photon 
can be as short as femtoseconds. Thus, photo-induced semiconductor plasma 
densities easily track the rise time of sub-picosecond laser pulses. 
The core of the invention is the generation in a semiconductor of a plasma 
moving at relativistic speeds dense enough and with a sharp enough 
boundary to reflect electromagnetic signals of frequencies ranging from 
radio through light. The impinging radiation can be propagating freely 
through any medium before entering the semiconductor or it can be 
channelled by any form of waveguide that will allow transmission into the 
semiconductor. The semiconductor itself can be formed as a waveguide. The 
plasma can be swept toward the impinging signal to achieve a Doppler 
reflection, or it can be swept from behind at a speed faster than the 
signal to compress the signal. 
Numerous geometries of semiconductor microwave waveguides can be used to 
accommodate an energy beam induced moving plasma, including, for example, 
the coplanar strip geometry in FIG. 2 and the microstrip geometries shown 
in FIGS. 3 & 4. In all of these geometries, the illumination can occur 
along the side (or top) of the waveguide, being swept across the guide to 
produce the moving plasma by deflection of an electron beam, by an 
electro-optic deflection of laser beam, or passively, through an optical 
element to the side of (or above) the waveguide such as a prism, which 
will control the effective sweep velocity for an incident distributed 
optical wavefront. The laser has a wavelength which maximizes the 
localized plasma density between the waveguide conductors. In the case of 
the coplanar strip (CPS) geometry shown in FIG. 2, the wavelength is 
chosen to give an optical absorption deep enough to "short-out" the field 
below and between the waveguide conductors 6, yet shallow enough to create 
a small volume of plasma between the conductors which effectively raises 
the density of photocarriers, thus increasing the reflection properties of 
that region to a microwave signal. An advantage of the coplanar strip 
geometry is that the relatively shallow absorption depths required 
translate into very high photocarrier densities since the incident optical 
energy is absorbed in a relatively small volume. 
In the case of the microstrip geometry illuminated from the side, the 
wavelength is chosen for a particular semiconductor to give an absorption 
depth sufficient to optically "connect" the upper and lower electrodes, 
giving again a reflective short. To facilitate this in the stripline 
configuration of FIG. 3, the upper electrode 18 is semi-transparent to the 
incident optical energy. 
Instead of illuminating the semiconductor from the side (or above), the 
laser illumination can be propagated in the same longitudinal direction as 
the electromagnetic signal as in FIG. 6 to produce a reflective boundary. 
For most applications, this is not the preferred embodiment since there is 
a strong tradeoff between the absorption of the semiconductor and the 
laser energy requirements. Though a higher material absorption will give a 
denser plasma for a given laser wavelength, too high an absorption will 
greatly attenuate the laser beam before it traverses the waveguide. 
One method of extracting the up-shifted "output" signal is accomplished 
with a directional coupler 42 as shown in FIG. 7. The burst of microwave 
energy to be up-shifted travels down a microwave waveguide from the 
source, passing through the coupler and impinging onto the optically 
generated plasma front. The directional coupler then routes the reflected 
and up-shifted signal into the output coaxial cable 44. Since the laser 
pulse is electronically triggered, the launching of the input signal is 
synchronized with the laser pulse to achieve the maximum time of 
interaction between the plasma and the input signal that the length of the 
semiconductor waveguide will allow. 
In an alternate microwave waveguide embodiment, the invention can be used 
to originally generate fast electrical signals, without the use of an 
external signal generator, by applying a biasing voltage across the 
waveguide and then sweeping the laser at high speed. The laser allows 
conduction at the point of illumination which, because the point is moving 
at relativistic speeds, produces a shock wave electrical signal. 
For a further treatment of the invention and its advantages, reference 
should be made to the subsequent descriptions as they relate to the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A key facet of the invention is the method of achieving a laser 
illumination sweep across the semiconductor waveguide to produce the 
moving plasma. A requirement of the laser sweep is that it be sufficiently 
fast to give relativistic velocities of the reflecting plasma front. One 
preferred way to achieve this is to use a prism 48 that is optically 
coupled to the semiconductor waveguide as shown in FIG. 8. A collimated 
laser beam 50 of height h is incident on a cylindrical lens 52 which 
focuses a "line" of laser illumination through prism onto the waveguide. 
Alternately, a collimated laser beam with sufficient power may be used 
without focusing. The diameter of the collimated laser beam is chosen such 
that the top portion of the beam just reaches the end of the waveguide 
after being refracted by the prism. In the case of the CPS waveguide 
geometry, this illumination line is focused into the conductive strip gap 
along the length of the waveguide. The cylindrical lens is tilted with 
respect to the incident laser beam in order to achieve the same focus 
along the length of the waveguide. In order to maximize the optical 
throughput, an anti-reflection coating 54 is used on the top surface of 
the prism, and to prevent total internal reflection at the interface 
between the prism and the semiconductor substrate 8, optical index 
matching material, preferably fluid, is used between these components. The 
index matching substance 56 must have a refractive index somewhere between 
the index of the prism and the semiconductor substrate for the laser 
wavelength used. 
The effective velocity of the optical sweep across the waveguide is the 
waveguide length divided by the time it takes the top "ray" of the line 
illumination to hit the far end of the waveguide measured from the time 
the bottom "ray" hits the near end of the waveguide. This effective 
velocity for a given length of waveguide is controlled by the prism angle 
.THETA..sub.1 in FIG. 8 and the prism's refractive index. The benefit of 
the prism is that it allows the sweep speed to be slowed down compared to 
the effective sweep speed that would occur if laser illumination were 
incident at an angle to the waveguide in the absence of a prism. 
Otherwise, v/c in equation 1 could be greater than unity, pushing the 
effect of the microwave-plasma interaction out of the regime of the 
Doppler effect and into the regime of shock wave formation. The 
propagation of the laser beam parallel to the waveguide surface at 50 
gives a greater degree of slow-down of the illumination sweep than if the 
laser beam were illuminating the prism at an angle, though either 
configuration can be used. 
A second preferred method of achieving relativistic sweep speeds uses 
electro-optic deflection of a laser beam across the waveguide. Using 
certain materials whose refractive index varies with applied electric 
field, it is possible to make devices which can deflect a laser beam with 
a sweep speed that depends on the rise time of the electrical signal being 
applied to the electro-optic material. The company ConOptics in Danbury, 
Conn. manufactures an off-the-shelf electro-optic beam deflector that 
gives deflections of 0.26 degrees or more for applied voltages of 3000 
volts. The speed at which the deflection occurs depends on the rise time 
of the applied electrical signal and the resistance-capacitance (RC) time 
constant of the device. ConOptics model 310A has a capacitance of 100 pF 
and is preferably driven with an off-the-shelf voltage pulser from Kentech 
Instruments Ltd. of South Moreton, England which is configured with an 
internal source impedance of under 1 ohm and provides peak voltages well 
in excess of 3000 volts. The RC time constant of 100 pF with one ohm is 
100 ps. Since this voltage pulser has a rise time of 100 ps, the time 
.tau.d for a complete deflection to occur is approximately 100 ps. 
Alternatively, the electro-optic beam deflector is configurable as a 
"travelling-wave" device, meaning that the electro-optic device will act 
like a transmission line to the applied voltage signal rather than a 
lumped element capacitor, thus allowing the deflection speed to equal the 
rise time of the applied electrical signal. 
Translating these known parameters into an effective sweep velocity is 
clarified by FIG. 9. The effective sweep velocity of the laser beam 58 
across the waveguide 60 of length y is given by equation 3, 
EQU V.sub.sweep =y/.tau.d=(l/.tau.d)tan(.beta.) (3) 
where .beta. is the maximum deflection angle, l is the distance from the 
electro-optic deflector to the waveguide, and .tau.d is the deflection 
time of the beam through the angle .beta.. Thus, the sweep velocity can be 
varied by simply changing the distance l and/or the electrical rise time 
of the applied voltage to the electro-optic deflection device 62. As an 
example, a .tau.d of 100 ps for .beta.=0.26 degrees, at a distance of l=2 
meters, translates into a sweep velocity of 9E7 m/s. The total scanning 
distance for these parameters corresponds to the waveguide length, which 
is 9 mm. 
An alternate method to sweep the laser illumination across the microwave 
waveguide uses phased-array optical beam steering. By controlling phase 
differences between different portions of the same laser beam and/or phase 
differences between individual lasers in an array of lasers, it is 
possible to steer the laser beam in the far field. An example of such 
phase control uses an array of optical waveguides whose relative phase 
relationships for a guided laser beam can be changed electro-optically or 
all-optically using the Kerr effect. Since the electro-optic effect and 
Kerr effect are very fast phenomena, the resulting rapid beam sweep 
velocity in the far field are suitable for this invention. An example of 
such a device is given in a recent paper by H. K. Chiang et al., "The 
Analysis of a Phased-Delayed Optical Two-State Switch," IEEE Photonics 
Technology Letters, vol. 4, p. 368, 1992. 
As an alternative to using a laser, the semiconductor plasma may be 
produced using electron beam illumination. The methods of sweeping 
electron beams are well known, and can be used to create a moving 
deflecting plasma in a semiconductor waveguide in a manner completely 
analogous to a photo-induced plasma wave. The similarities between these 
two types of plasma-inducing illuminations is paralleled by the 
similarities between optically controlled semiconductor switches and 
electron beam controlled semiconductor switches, and is exemplified in the 
paper by D. C. Stoudt et al., "The Recovery Behavior of Semi-Insulating 
GaAs in Electron-Beam-Controlled Switches," IEEE Transactions on Electron 
Devices, vol. 37, p. 2478, 1990. 
A preferred embodiment of the invention uses the so called "coplanar strip" 
(CPS) waveguide geometry shown in FIG. 2. The laser illumination front 2 
is depicted in a generic way----the medium above the waveguide may 
actually be a prism or could be air with the laser beam swept 
electro-optically. Placing a real conductive short across the conductive 
strips 6 in FIG. 2 will cause an impinging microwave signal to be 
reflected. Illuminating a region in the gap with laser light produces an 
optically generated electron-hole plasma which acts like a conducting 
short. By sweeping a fast laser pulse rapidly across the gap, a moving 
reflecting "short" or plasma front 4 is produced which up-shifts the 
impinging microwave signal. A configurational description of CPS waveguide 
design that allows extraction of the desired up-shifted signal follows. 
A block of semiconductor GaAs, 9 cm long and 0.6 mm thick is used as a 
waveguide substrate 8 as shown in FIG. 2. Gold conductive strips 6 of 2 
.mu.m thickness and 0.5 mm wide are interfaced to the bulk substrate 8 
through a doped n+ layer 10 that has been implanted on the substrate 
surface. Such preparation is common when making electrical contact with 
GaAs. The GaAs is preferably of a semi-insulating nature (commonly called 
SI-GaAs) in order to mitigate waveguide losses. A space between the strips 
of 48 .mu.m produces a coplanar strip waveguide characteristic impedance 
of 50 ohms, and a loss that is approximately 0.3 dB/cm at 10 GHz. The 
connector 12 is a common ssma type waveguide to coax connector mounted to 
a bulkhead 14 which allows the CPS waveguide to be connected to a coaxial 
cable. The inner and outer conductors of the ssma connector 12 are 
attached to the conductive strips 6 of the waveguide and are held in place 
by the bulkhead 14. The bulkhead is secured to the waveguide using epoxy. 
Referring to FIG. 7, a pulse generator such as the Kentech model ASG1 
pulser 38 produces a 0.4 ns full-width half-maximum (FWHM) gaussian 
voltage pulse which travels down a 50 ohm coaxial cable 40 through the 
broadband directional coupler 42. A suitable directional coupler such as 
model CWV-12R-33G made by Merrimac of West Caldwell, N.J. is used, which 
has a bandwidth range of 1-65 GHz. The pulse travels from the directional 
coupler via another 50 ohm coax 16, which feeds the input pulse into the 
coplanar waveguide through the attached ssma connector 12 that is mounted 
to the bulkhead 14. The pulse generator has an additional electrical 
output which is synchronized with the electrical pulses being generated. 
This additional output is used to trigger the production of laser pulses 
such that the beginning of the 0.4 ns input pulse impinges on the 
laser-generated plasma front at the far end of the CPS waveguide. Proper 
timing is achieved using cable delay lines to the laser trigger inputs, 
and/or variable digital delay instruments such as the Stanford Digital 
Delay DG-535. In this manner, the total interaction time of the impinging 
microwave pulse and moving plasma is maximized. This time is simply the 
waveguide length divided by the velocity of the sweeping laser 
illumination. The up-shifted and compressed reflected pulse travels out 
the CPS waveguide through the ssma connector and into the directional 
coupler 42, which routes the up-shifted output into the 50 ohm coaxial 
cable 44 and into the oscilloscope 46 for viewing. This entire process is 
repetitive, since the Kentech model generator operates at up to 1 kHz and 
the laser is easily triggered. The constraints on the maximum repetition 
frequency are the photoconductive carrier lifetime in the semiconductor 
and the maximum repetition rate of the pulse generator. The former depends 
on the semiconductor which for normal GaAs, is much less than 1 .mu.s. 
In alternative embodiments any electromagnetic signal source can be used in 
place of the specified pulse generator 38 in FIG. 7, including, for 
example, a photoconductive switch of the variety previously discussed. 
The preferred laser wavelength is 880 nm, near the GaAs bandgap. The laser 
pulse has a rise time in the picosecond range, thus producing a sharp 
plasma front in the GaAs. A significant reflection will occur for laser 
pulse energies as low as 200 nJ. Higher energies ensure that all frequency 
components of the input pulse are upshifted by the same factor, thus 
giving scale-invariant pulse compression with a concomitant increase in 
peak amplitude. 
The velocity of the microwave signal in the CPS geometry is c(e.sub.r 
').sup.-0.5, where c is the speed of light in a vacuum, and e.sub.r ' is 
substrate given roughly by the effective relative dielectric constant of 
the waveguide substrate given roughly by (e.sub.r +1)/2, where e.sub.r is 
the actual relative dielectric constant of the material. GaAs has an 
e.sub.r of 12.9, giving a signal velocity down the waveguide of about 
1.18E8 m/s. A 75 degree prism with a refractive index of 2.8 gives a laser 
illumination sweep of about 1.08E8 m/s across the waveguide. The resulting 
up-shift factor, as predicted from equation 1, is about 23, and manifests 
itself as a compressed pulse on the oscilloscope. In use, the oscilloscope 
is replaced by a desired load depending on the application. 
There are a number of alternative embodiments to the configuration of FIG. 
7. In a first alternative, the directional coupler 42 is replaced with an 
active optical switch for extracting the desired up-shifted output signal. 
Such a switch is shown in FIG. 10. The substrate 64 is a semiconductor, 
preferably GaAs, upon which are deposited conductive strips 66 on top and 
a metal ground plane 68 on the bottom, thus making microstrip waveguides. 
The connectors 74 are "ssma" type, the dimension g is 0.43 mm and the 
dimension h is 0.6 mm give a microstrip characteristic impedance of 50 
ohms. The regions indicated by shaded circles represent gaps in the 
conductive strips that are illuminated with laser light in order to 
electrically connect the various ports. The gap resistance between the 
conducting lines is 
EQU R.sub.gap =l.sub.gap.sup.2 /(Ne.mu.) (4) 
where l.sub.gap is the separation between conductors, e is the electron 
charge, .mu. is the dominant carrier mobility of the semiconductor, and N 
is the total number of photo-induced charge carriers and is proportional 
to the laser energy. Thus, the gap 70 is laser illuminated just long 
enough to allow passage of the input signal to be launched into the 
waveguide of FIG. 7, and then the gap 72 is laser illuminated to route the 
up-shifted output pulse toward the oscilloscope. Again, the lasers are 
synchronized with the pulse generator, allowing repetitive operation. 
An alternate configuration from FIG. 7 that avoids using a directional 
coupler or switch is shown in FIG. 11. The input signal 90 to be 
up-shifted is coupled into the CPS waveguide using a microwave waveguide 
that is connected between the signal source and the CPS. The waveguide 
could, for example, be a coaxial cable as referred to earlier which mates 
with an ssma type connector mounted and electrically connected to the CPS. 
Stationary laser illumination of the proper intensity, wavelength, and 
timing produces a conductive plasma 94 near the end of the CPS waveguide 
which reflects the impinging microwave signal back toward the beginning of 
the waveguide. A second, synchronized laser illumination 92 is then swept 
using one of the methods described earlier to produce a moving conductive 
plasma which reflects and up-shifts this microwave signal. The laser 
illumination that produced the conducting "short" 94 has since been 
extinguished so as to let pass the up-shifted output signal 98, which 
might be routed to some desired load via a coaxial cable electrically 
interfaced to the end of the CPS waveguide using standard connectors. The 
semiconductor substrate 96 must have a carrier recombination sufficiently 
fast to prevent a lingering plasma at the location of the initially 
created short 94 that would attenuate the output signal, preferably "low 
temperature grown" GaAs. For a 50 ohm CPS waveguide, the dimensions would 
be the same as in FIG. 2. A laser of 880 nm is used to illuminate the gap 
94. The concept of this configuration could be used with any waveguide 
geometry. 
Other alternative embodiments use different types of semiconductor 
waveguides. One such alternative uses a microstrip waveguide as shown in 
FIG. 3, again using either method of illumination sweep discussed 
previously. The upper conductive strip 18 is semi-transparent to allow 
laser illumination to pass through. The wavelength of the laser 
illumination, about 895 nm, is longer than for the CPS geometry since a 
greater absorption depth is required in the substrate 22 in order to get 
conduction between the upper conductive strip 18 and the metal ground 
plane 20. The upper conductive strip is made transparent by one of three 
alternative methods: a thin layer of deposited gold, a grid pattern, or by 
using a highly doped n+ layer which has a doping density sufficiently high 
that it acts like a conductor. Such n+ layers can be doped up to 
approximately 4E18 cm.sup.-3. Such transparent strips may, of course, be 
used with any of the waveguide geometries. The advantage of the microstrip 
geometry is an enhanced voltage standoff ability over the CPS design due 
to the wider conductor spacing. 
In another alternative waveguide, shown FIG. 4, instead of using a 
transparent or semi-transparent upper conductive strip, the laser 
illumination comes in from the side, between the metallization layers 24, 
into the semiconductor substrate 26. In order to accommodate laser 
illumination in this way, the lower metallization layer or ground plane is 
preferably made narrower than the geometry of FIG. 3. 
Another alternative waveguide geometry is the coplanar waveguide (CPW) 
shown in FIG. 5. The dimensions a, b, and c are chosen for the desired 
characteristic impedance, loss factor, dispersion, etc., using the 
parameters appropriate for the type of semiconductor substrate 32. The 
laser illumination is swept across both conductive strip gaps 28 in the 
usual manner described previously. Again, the conductive strips 30 and 31 
may be semi-transparent. 
Another alternative embodiment uses microstrip with laser illumination 34 
that propagates on the same axis that the impinging microwave signal 
travels as shown in FIG. 6. The wavelength of the laser is chosen to give 
adequate transmission through the length of the microstrip substrate 36 
while allowing enough absorption to produce a sufficiently high plasma 
density to create a reflecting front. This tradeoff renders this design 
much less desirable. However, using materials such as silicon and 
hydrogenated amorphous silicon at wavelengths between approximately 1.5 
and 2 um, it is possible to get absorption coefficients down below 0.4 
cm.sup.-1. This allows transmission through a 10 cm microstrip waveguide 
and provides enough absorption to produce an adequate photo-induced moving 
plasma for sufficiently high incident laser pulse energies. 
Another alternate embodiment uses the microstrip configuration of FIG. 12. 
A microwave input signal 110 is coupled into the microstrip in the usual 
way. The input microwave signal reflects off the open end of a gap with 
spacing l.sub.gap in the transparent or semi-transparent top conductor 104 
during a time when the stationary illumination 102 is not present. The 
reflected signal which now travels back toward the input is reflected 
again, this time by a moving conducting plasma front in the semiconductor 
created by a sweeping laser illumination 108 achieved using one of the 
methods previously described. The resulting up-shifted signal is routed to 
the end of the microstrip 105 using laser illumination 102 that 
effectively electrically shorts the gap. The up-shifted output signal 112 
is coupled out of the microstrip in the usual way using an ssma connector 
mounted and electrically connected to the end of the microstrip. A coaxial 
cable with a mating ssma connector then routes the up-shifted signal to a 
desired load. The microwave input signal and laser pulse illuminations are 
synchronized to give the timing required. Proper timing can be achieved 
using cable delay lines to the laser trigger inputs, and/or variable 
digital delay instruments such as the Stanford Digital Delay DG-535. 
As described previously, the top conductor in the microstrip is a highly 
doped layer deposited on the semiconductor substrate 100 which acts like a 
conductor, or a semi-transparent metallization deposited atop the highly 
doped layer. The resistance of the illuminated gap as a function of 
wavelength, spacing, and semiconductor characteristics is given by 
equation 4. 
A preferred embodiment of the above uses dimensions w=0.43 mm and h=0.6 mm 
in FIG. 12 to produce a microstrip characteristic impedance of 50 ohms for 
a semi-insulating GaAs substrate 100. The top conductors 104, 105 are a 
highly doped n+ region approximately 10 .mu.m thick, and the ground plane 
106 is a gold metallized region 5 .mu.m thick over a highly doped n+ 
layer. l.sub.gap =0.3 mm, and the laser illumination 102 in the gap has a 
wavelength of about 880 nm and an energy per pulse sufficient to give a 
gap resistance (equation 4) much smaller than 50 ohms. The duration of 
this laser pulse is as long as the temporal length of the up-shifted 
signal to be output. The swept laser pulse illumination has a wavelength 
of about 895 nm and has an energy per pulse which is hundreds of .mu.J or 
greater. Again, the higher the laser pulse energy, the denser the 
photo-generated plasma and the higher the reflection coefficient for 
microwave frequencies. 
A related alternate configuration using a coplanar strip (CPS) geometry is 
shown in FIG. 13. This geometry works in an exactly analogous manner to 
the previously described microstrip waveguide. Again, a microwave input 
signal 122 coupled into the CPS input travels down the waveguide and is 
reflected at the gap near the end of the CPS during a time when the gap is 
not illuminated. The reflected signal propagates back toward the CPS input 
where it is reflected by a moving plasma generated by laser illumination 
120 swept at a velocity v by one of the previously described methods. The 
gap illumination 116 is then applied to extract the up-shifted output 
signal 124. 
A preferred embodiment of the above uses a semi-insulating GaAs substrate 
114 with a highly doped n+layer upon which is deposited 2 .mu.m thick gold 
conductive strips 118. For a characteristic CPS impedance of 50 ohms, the 
dimensions are w=0.5 mm.times.=48 .mu.m, and h=0.6 mm. The gap spacing 
l.sub.gap is approximately 0.25 mm. The laser illumination in the gap has 
a wavelength of about 880 nm, and a pulse energy which gives an impedance 
much smaller than 50 ohms as given by equation 4. The swept laser pulse 
illumination is at a wavelength of about 880 nm, and will have an energy 
per pulse which produces a plasma density sufficient to act like a 
reflector to an impinging microwave signal. The resulting plasma density 
for a given optical energy density and semiconductor is given by equation 
2. 
The stationary laser illumination depicted in various configurations may be 
achieved using either an optical fiber coupled between the laser and the 
gap or an externally focused laser beam. 
Without the stationary laser illumination in the gaps of the configurations 
of FIGS. 12 and 13, some of the input signal will "leak" into the output 
through the effective gap capacitance. Since this leakage increases 
proportionally with frequency, the gap capacitance can be utilized as a 
high-pass filter to allow the up-shifted signal to be coupled to the 
output end of the waveguide. The general consideration for such a design 
is to make the gap spacing sufficiently large so the waveguide acts like a 
reflecting open at the gap, while keeping the gap spacing sufficiently 
small to give an effective gap capacitance which passes the up-shifted 
output signal. The effective gap capacitance for a microstrip is given by 
Bahl and Bhartra in chapter 2 of the book "Microwave Solid State Circuit 
Design," John Wiley & Sons, 1988. 
Again, the previously described concept is not limited for use with 
microstrip and CPS waveguides, but can be used with slotline waveguides, 
coplanar waveguides (CPW), geometries such as FIG. 4 where the 
illumination comes in from the side as described earlier, stripline 
waveguides, coaxial waveguides, etc. 
Another embodiment allows the production of shock waves using a microstrip 
waveguide geometry as shown in FIG. 14. A microwave signal 130 is input in 
the usual manner into the input of the microstrip. After the microwave 
signal begins travelling down the waveguide, laser illumination 128 is 
swept through the semi-transparent top conductor 126 at a velocity 
slightly greater than the velocity of travelling microwave. Since the 
photo-generated moving plasma pushes up against the travelling microwave 
signal with a greater velocity than the microwave signal itself, a shock 
wave results. The shock wave has higher frequency components than the 
original input signal, thus giving an up-shifted output signal 132. In 
order to choose the velocity of the laser illumination sweep, the 
microwave signal velocity is given by 
EQU v.sub.s =c (e.sub.eff).sup.-0.5, (5) 
where c is the speed of light in a vacuum and e.sub.eff is the effective 
dielectric constant for a microstrip which can be calculated for a given 
geometry using the formulas in chapter 2 of "Microwave Solid State Circuit 
Design," by Bahl and Bhartia, John Wiley & Sons, 1988. For a 50 ohm 
waveguide using a 0.6 mm thick (h) semi-insulating GaAs substrate with a 
gold ground plane 125 interfaced to the GaAs in the usual way, the top 
conductive strip in FIG. 14 is 0.43 mm wide. 
The coplanar strip (CPS) configuration of FIG. 15 works in a completely 
analogous manner to FIG. 14. A laser-induced moving plasma 140 sweeps 
across the gap between the conductive strips 136 and pushes up against a 
microwave signal 142 that was launched into the waveguide in the usual 
manner. The laser sweep velocity is again chosen to be slightly greater 
than the microwave signal velocity to produce an up-shifted shock wave 
output 144. The microwave signal velocity is given for a CPS waveguide in 
the book previously referenced. For a 50 ohm CPS waveguide on a GaAs 
substrate, the dimensions in FIG. 15 match those of FIG. 2. 
In yet another embodiment, a voltage is applied to the waveguide in order 
to produce a photoconductive signal upon application of a pulsed laser 
illumination. FIG. 16 depicts such a configuration using a microstrip 
waveguide with a semi-transparent top conductor 148. A voltage source 153, 
either DC or pulsed, of any voltage as appropriate for the application, is 
applied between the top conductor 148 and the ground plane 151 during the 
time the sweeping laser illumination 150 is incident on and through the 
top conductor and into the semiconductor substrate 146. The photocarriers 
generated between the top and bottom conductors are swept out into the 
external circuit by the applied voltage, producing a voltage pulse that 
travels down the waveguide at a speed characteristic of the waveguide, and 
with an electrical rise time that approximately tracks the rise time of 
the optical pulse. This voltage pulse propagates down the waveguide even 
if the laser illumination 150 hits the end of the waveguide without being 
swept. When the velocity of the swept laser illumination is faster than 
the characteristic velocity of the optically generated electrical pulse, 
the moving plasma overtakes the electrical signal, producing a shock wave 
output 152 that is up-shifted in frequency compared to the electrical 
signal that would have been produced if the laser illumination had not 
been swept. In order to choose the speed of the laser sweep, the 
characteristic velocity of a signal on the waveguide must be calculated 
using equation 5. For a 50 ohm microstrip, the dimensions for FIG. 14 are 
used. 
A completely analogous embodiment uses the CPS waveguide in the 
configuration of FIG. 17 with a voltage source 153 applied between the 
conductive strips 156. Again, the swept laser illumination 157 produces a 
photoconductive electrical signal propagating toward the end of the 
waveguide and followed by a moving reflective plasma which overtakes the 
electrical signal thus producing a shock wave output signal 160. A 50 ohm 
CPS waveguide would use the dimensions of FIG. 2 with a semi-insulating 
GaAs substrate 154. 
The above concept could be used with many different and standard waveguide 
geometries, not just microstrips and coplanar strips as discussed in the 
previous examples. 
In the arrangements of FIGS. 16 and 17 where a voltage is applied across 
the waveguide conductors, I have referred to the optically produced 
electrical signals as "photoconduction". Included within the meaning of 
"photoconduction" is conduction that occurs in the semiconductor due to 
optically-induced avalanche. Above some threshold of laser energy and 
voltage bias, optically-induced avalanche produces a localized 
semiconductor conductivity much higher than would be possible from similar 
optical energies in a strictly linear photoconductive semiconductor 
device. 
In another embodiment, the semiconductor waveguide is configured into a 
cavity or resonator arrangement with a continuous wave (CW) sinusoidal 
input 76 as shown in FIG. 18. The input sinusoid is coupled into the 
microstrip in the usual way. The conductors interfaced to the 
semiconductor substrate 78 are a semi-transparent top conductor 77, 
metallized top conductors 75, 80, and the usual ground plane 79. The gap 
83 capacitively couples the input signal into the central cavity section 
77 of length l. When l=(2n+1).lambda./4, the line behaves like a series 
resistance-capacitance-inductance (RCL) circuit, and when l=n.lambda./2, 
the line behaves like a parallel RCL circuit, where n is a positive 
integer, and .lambda.=2.pi./.beta.. .beta. is the phase constant of the 
microstrip line and is related to the free-space wavelength .lambda..sub.o 
of the input sinusoid by .beta.=2.pi.(e.sub.eff).sup.0.5 /.lambda..sub.o, 
where e.sub.eff is the effective dielectric constant for the microstrip 
geometry using a certain semiconductor substrate. Laser illumination 81 is 
swept over the central cavity section, preferably electro-optically, at 
such a velocity as to give a harmonic Doppler upshift of one of the 
travelling waves inside the cavity section. The backward and forward 
travelling waves inside the cavity have the frequency of the input 
sinusoid before interaction with the swept laser illumination. Following 
the up-shifting plasma-wave interaction, the gap 82 is laser illuminated 
to dump the stored cavity energy at the up-shifted frequency into the 
output line 80 and out from the microstrip through the usual ssma 
connector. 
The proper laser sweep velocity is chosen using equation 1 for a wave 
impinging on a moving reflecting boundary, where the propagating 
electrical wave velocity c in the waveguide is given by equation 5. Again, 
the effective dielectric constant e.sub.eff for a microstrip waveguide is 
found in the reference given in the discussion following introduction of 
equation 5. 
For the preferred cavity embodiment, a 50 ohm microstrip is realized using 
a 0.6 mm thick low temperature grown semi-insulating GaAs substrate 78 
with top conductive strips 0.43 mm in width. The input gap 83 has a 40 
.mu.m spacing and the output gap where stationary illumination sometimes 
occurs has a spacing of 0.4 mm. The applied input sinusoid is 5 GHz. The 
effective dielectric constant for this geometry is 8.36, which translates 
into a waveguide wavelength for 5 GHz of 2,075 cm. Setting the cavity 
length l=n.lambda./2, l=1.037 cm for n=1. A preferred harmonic of the 
input sinusoid is 25 GHz. For a Doppler frequency increase of 5, v/c=0.665 
from FIG. 1. From equation 5 the velocity of the electrical signal in the 
waveguide is c=1.038E8 m/s. Thus, the velocity of the swept laser 
illumination to satisfy v/c=0,665 and give an up-shift to 25 GHz is 6.9E7 
m/s. Laser illumination 82 at the output gap dumps the stored and 
up-shifted cavity energy into the line 80, which is connected to an ssma 
connector to accommodate a coaxial output cable that routes the signal 
into some desired load. The GaAs is a low temperature grown variety which 
has a fast recombination time so that the photogenerated plasma dies out 
rapidly when the laser illumination is extinguished from a given location 
on the waveguide. 
Though two conductor microwave waveguides such as the coplanar strip, 
microstrip, slotline, and others commonly use semiconductors such as GaAs 
for the dielectric separating the conductors, rectangular and circular 
"single" conductor waveguides can also be made with a semiconductor 
dielectric inside. Normally, these hollow-pipe guides have air or some 
other dielectric inside which are completely enclosed with a conductor. 
Using a semi-transparent conductor such as the types discussed previously 
on the outside of a semiconductor core, such a guide can accommodate a 
swept laser illumination, producing a moving reflecting plasma as before 
to interact with an impinging electromagnetic signal. It is not 
necessarily required that a true electrical "short" be produced in order 
to reflect an impinging electrical or electromagnetic signal, since a 
plasma boundary of sufficient density and gradient can act as a reflector 
to such radiation. Coupling the electromagnetic energy into the guide is 
well known and described in most microwave texts. 
In a manner quite similar to the description regarding directing the laser 
pulse through the length of the semiconductor down the same axis as the 
electromagnetic signal in a microstrip waveguide, the laser illumination 
can also be directed down the same axis as the electromagnetic signal in a 
bulk semiconductor that is not in a waveguide. Using the same 
considerations as in the discussion relating to FIG. 6 with regard to 
laser wavelength and power, the propagating laser pulse creates a moving 
plasma boundary which reflects an impinging electromagnetic signal. This 
impinging electromagnetic signal can be microwaves, light, or other 
frequencies. An optical semiconductor waveguide can be used to confine the 
propagation of impinging light energy to be up-shifted. The laser used to 
create a moving plasma front can be propagated down the same axis of this 
waveguide or directed in from the side as previously disclosed using 
electro-optic beam deflection or a passive prism coupled to the optical 
waveguide. In this embodiment, the swept laser illumination penetrates to 
the depth of the optical guiding layer in the waveguide from the side and 
is of sufficient intensity to produce a reflective plasma front to light 
frequencies, again in an entirely similar fashion to the up-shifting of 
microwaves. 
Finally, in each of the above embodiments in order to keep laser 
illumination out of areas where it is not desired on the waveguides, an 
opaque mask is placed on those areas of the waveguides. In the CPS 
geometry of FIG. 19, the opaque painted regions 170 prevent photocarrier 
generation in the semiconductor substrate 176 due to illumination which 
over-fills the gap region 178. 
While preferred embodiments of the invention have been described in detail 
along with various alternative configurations, it must be kept in mind 
that other modifications may also be made according to the teachings of 
this invention. For example, there are numerous waveguide geometries 
beyond the more common types mentioned, all of which may have potential 
application in this invention. Also, the interaction of a moving plasma in 
a semiconductor with impinging electromagnetic radiation will affect 
signals whose frequencies extend beyond what is commonly considered the 
microwave region. Therefore, the disclosure of the preferred embodiments 
of the present invention is intended to be illustrative, but not limiting, 
of the scope of the invention which is set forth in the following claims.