Abstract:
The present invention includes a semiconductor epitaxial structure optimized for photoconductive free space terahertz generation and detection; and amplifier circuits for photoconductively sampled terahertz detection which may employ the optimized epitaxial structures.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority to U.S. application No. 60/334,549, filed Nov. 29, 2001. 

   FIELD OF THE INVENTION 
   The present invention relates generally to the terahertz photoconductive sampling of optical signals, and more particularly to an optimized epitaxial photoconductive structure and associated voltage amplifier for such sampling. 
   BACKGROUND AND SUMMARY OF THE INVENTION 
   Epitaxial structures of low temperature grown gallium arsenide (GaAs) photoconductive devices can be tailored to the specific and differing needs of both free-space terahertz generation and free-space terahertz sampling. Low temperature GaAs can also be grown as a heterostructure with a tailored growth profile. When grown at approximately 600° C., GaAs has a recombination rate on the order of 1 nanosecond. Under these conditions, the strucutre is its natural or stoichiometric state, and has equal quantities of Ga and As forming the lattice. 
   It is also known that lowering the growth temperature to approximately 200° C. causes the As concentration to increase relative to Ga, forming a nonstoichiometric structure. A subsequent annealing step at the usual growth temperature of 600° C. for 10 minutes creates a form of single-crystal GaAs that possesses high resistively, high mobility, and subpicosecond carrier lifetime—all of which make such a structure well-suited to terahertz generation and sampling. Nevertheless, the performance of a free space terahertz generation by a low temperature GaAs photoconductive device can be enhanced by growing a lattice-matched heterostructure formed of low temperature GaAs and aluminum gallium arsenide (AlGaAs) on top of a normal GaAs substrate. 
   The benefits of the AlGaAs are numerous. For example, the barrier layer created by the AlGaAs confines the photogenerated carriers to the low temperature GaAs region. Generally, the incident light is absorbed in the low temperature GaAs region, but carrier diffusion can force both electrons and holes out of this region and into the substrate, where they recombine at a rate of approximately one nanosecond. The AlGaAs layer will prevent any carriers from thermalizing or tunneling to the GaAs substrate. Low temperature AlGaAs has a subpicosecond carrier lifetime similar to LT-GaAs. Additionally, the AlGaAs barrier layer reduces the dark and illuminated current of a biased device by removing the conduction path through the GaAs substrate layer. The increased resistivity allows a greater bias to be applied with reduced chance of damage by current or heat dissipation in the biased region. 
   Sampling of a free space terahertz waveform occurs when the illuminated photoconductive gate conducts for a time shorter than the entire terahertz wave cycle. During the conduction period, charge flows from one side to another of a dipole antenna structure due to the potential difference induced by the terahertz wave. The amount of current flow per sampling optical pulse is proportional to the terahertz voltage potential and the off-state resistance of the interaction area. The antenna structure has an inherent capacitance, and unless the illumination is near saturation to bring the resistance very low, the resistance-capacitance RC time constant is long enough that the sub-picosecond conduction period will not fully equilibrate the capacitance to bring the instantaneous antenna potential difference to zero. 
   Any amplifier circuit connected to the terahertz antenna must have very high impedance at terahertz frequencies. Non-ideal amplifiers with low impedance at terahertz frequencies will serve to equilibrate charge in the antenna in response to the terahertz field without the action of the photoconductive gate. 
   The combined off-state resistance and impedance of the amplifier with the capacitance of the antenna, leads, and amplifier yield an RC time constant which limits the rate at which the optical sampling gate can be swept through repeated identical terahertz waveforms. If this RC time constant is too large, the recorded terahertz waveform will be distorted in frequency response, phase and amplitude. Differing combined circuits can be chosen for the best signal to noise and RC time constant to meet the scanning rate. Existing designs have time constants limiting the terahertz waveform scanning to less than 10 Hz corresponding to electrical bandwidths of 100 Hz. For many applications it is desirable to scan at 10,000 to 1000 Hz corresponding to electrical bandwidths up to 1 MHz. 
   The photoconductive-gated current can either be amplified by measuring the current or the off state voltage of the THz antenna which has been (repetitively) charged by the photoconductive current. The voltage across the antenna is proportional to the charge divided by the capacitance of the antenna. 
   Existing terahertz receivers are generally current amplifiers. Current amplifiers typically have low impedance and therefore a minimal RC time constant. However, they are typically used at very slow scanning speeds to achieve adequate signal to noise. When the illumination of the photoconductive region is well below saturation, or when a terahertz field is small, the amplifier noise current and any off-state noise current may be large in comparison to the actual current induced by the terahertz field. The mismatch in amplifier impedance with the off-state photoconductive device impedance may yield low signal to noise ratios. Current amplification can be used when the active area is strongly illuminated and slow scanning speeds are used. These amplifiers are also ideal for systems where stray capacitance is present and slow scan speeds are acceptable. 
   The current state of the art employs a photoconductive gate/antenna assembly simply as a current source for an external amplifier. Integration and signal averaging occur after the external amplifier. At low signal levels, the noise floor of a current amplifier is sufficiently high limits the sensitivity of the device. Moreover, because current amplifiers are not integral to the photoconductive gate/antenna assembly, the connecting wires increased the inherent capacitance in the system and adversely affected the response time of the assembly. As such, there is a need in the art for an integrated photoconductive gate and amplifier assembly in which the response time is optimized and the signal to noise ratio is maximized. 
   Accordingly, the present invention includes an amplified photoconductive gate having an integrated antenna and voltage amplifier. Voltage amplifiers can yield superior signal to noise because their input impedance can be large and match the large impedance of the photoconductive sampling gate. When the illumination of the gate is well below saturation, the large amplifier impedance allows the antenna amplifier circuit capacitance to reach equilibrium as driven by the repeated terahertz potential on the antenna during the sample gate. This multi-sample integration allows the antenna voltage to rise well above that of a single sample. Furthermore, by reducing the stray capacitance inherent in cabling, equilibrium can be reached in a minimal number of illuminations, thus allowing rapid scanning of the signal. 
   Voltage amplifiers can be integrated into a single housing with the photoconductive antenna. Integrating a low-input capacitance amplifier with the terahertz receiver increases sampling speeds, lowers the cost, and improves product robustness over the use of a typical current preamplifier. The signal-to-noise ratio achieved with voltage amplifiers is equal to or better than that achieved by systems employing current amplifiers, while still gaining all of the noted benefits. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic illustration of a terahertz photoconductive semiconductor epitaxial structure having a barrier layer. 
       FIG. 2  is a diagrammatic illustration of a terahertz photoconductive semiconductor epitaxial structure. 
       FIG. 3  is a schematic diagram of an equivalent terahertz photoconductive sampled voltage amplifier receiver circuit. 
       FIG. 4  is a schematic diagram of a single stage voltage amplified photoconductive sampled terahertz antenna receiver circuit. 
       FIG. 5  is a schematic diagram of a high-speed, two-stage voltage-amplified photoconductive sampled terahertz antenna receiver circuit. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention relates generally to the terahertz photoconductive sampling of optical signals utilizing a photoconductor and an associated amplifier. More particularly, the present invention relates to an optimized epitaxial photoconductive structure and associated voltage amplifier for such sampling, as discussed further herein. 
     FIG. 1  is a diagrammatic illustration of a terahertz photoconductive semiconductor epitaxial structure having a barrier layer, termed a photoconductive gate  10 . The photoconductive gate  10  includes a substrate  12  composed of GaAs. A barrier layer  14  is disposed between the substrate  12  and the photoconductive layer  16 . A bipolar terahertz antenna comprised of a first pole  18  and a second pole  20  is disposed on the photoconductive layer  16 . Although the first pole  18  is designated positive and the second pole  20  is designated negative, it is understood that the respective charges of the poles of the terahertz antenna may be reversed. 
   As noted, the substrate  12  is preferably composed of intrinsic GaAs. Alternatively, the substrate  12  may be composed of Indium Arsenide (InAs), Indium Gallium Arsenide (InGaAs), Indium Phosphate (InP), Silicon or Sapphire. The substrate  12  is preferably between 100 and 2000 microns in thickness. 
   The barrier layer  14  is one of AlGaAs or low temperature AlGaAs. The barrier layer  14  has a preferred thickness between 0.4 and 0.6 microns, and is preferably composed of 70% aluminum. The photoconductive layer  16  is preferably low temperature GaAs having a thickness between 0.5 and 2 microns. Alternatively, the photoconductive layer is low temperature InGaAs or Silicon-on-Sapphire, both of which are suitable for generating sub-picosecond carrier lifetimes. In an alternative embodiment, a second barrier layer  14  composed of AlGaAs or low temperature AlGaAs is grown on the photoconductive layer  16  in order to optimize the performance of the photoconductive gate  10 . 
     FIG. 2  shows a diagrammatic illustration of a terahertz photoconductive semiconductor epitaxial structure in which there is no barrier layer. In this example, a photoconductive gate  10  is comprised of a substrate  24  supporting a photoconductive layer  26 . The substrate  24  is preferably composed of intrinsic GaAs. Alternatively, the substrate  24  may be composed of InAs, InGaAs, InP, Silicon or Sapphire. The substrate  12  is preferably between 100 and 2000 microns in thickness. The photoconductive layer  26  is preferably low temperature GaAs having a thickness between 0.5 and 2 microns. Alternatively, the photoconductive layer is low temperature InGaAs or Silicon-on-Sapphire, both of which are suitable for generating sub-picosecond carrier lifetimes. 
   As before, a bipolar terahertz antenna comprised of a first pole  28  and a second pole  30  is disposed on the photoconductive layer  26 . Although the first pole  28  is designated positive and the second pole  30  is designated negative, it is understood that the respective charges of the poles of the terahertz antenna may be reversed. 
   The photoconductive gate  10  and the respective terahertz antenna shown in  FIGS. 1 and 2  may be coupled to one of three voltage amplifiers, depicted in  FIGS. 3 ,  4  and  5 . 
     FIG. 3  is a schematic diagram of an equivalent terahertz photoconductive sampled voltage amplifier receiver circuit. The photoconductive gate  10  is coupled to a bipolar terahertz antenna having a first pole  18  and a second pole  20 . A resistor  34  and capacitor  36  are shown as illustrative of the inherent properties of the photoconductive gate and the terahertz antenna. The resistor  34  (R PC ) is indicative of the limiting resistance of the photoconductive gate with respect to the overall time constant, RC RECEIVER . The capacitor  36  (C ANT ) represents the terahertz antenna capacitance, which is typically 0.1 to 0.5 pF 
   The leads from the first pole  18  and the second pole  20  are coupled to an external resistor  38  (R EXT ), the value of which is predetermined to control the overall time constant RC RECEIVER . The maximum value of the external resistor  38  is the resistance between the leads from the first pole  18  and the second pole  20  to the instrumentation amplifier inputs. Similarly, an external capacitor  40  (C EXT ) is coupled to the first pole  18  and the second pole  20  for controlling the overall time constant, RC RECEIVER . The external capacitor  40  typically has a value between 0.3 and 5 pF. 
   The leads from the first pole  18  and the second pole  20  are coupled to an instrumentation amplifier  46 , which shows an instrument resistor  42  (R AMP ) indicative of the input impedance of the instrumentation amplifier, typically between 10 9  and 10 10  ohms. An instrument capacitor  44  (C AMP ) represents the input capacitance of the instrumentation amplifier, typically in the range of 0.5 to 20 pF. 
   If the gain of the instrumentation amplifier  46  is chosen such that the frequency response of instrumentation amplifier  46  alone is sufficiently high, the rate of response depends on the overall time constant RC RECEIVER  of the receiver circuit. 
   The overall time constant, RC RECEIVER , is determined by all of the input resistance and capacitance parameters of the equivalent circuit shown in FIG.  1 .
 
 RC   RECEIVER   =R   TOTAL   *C   TOTAL   (1) 
 
 R   TOTAL =1/(1 /R   PC +1 /R   EXT   +R   AMP )  (2) 
 
 C   TOTAL   =C   ANT   +C   EXT   +C   AMP   (3) 
 
   At high illumination and/or high optical repetition rate, the value R PC  approaches a minimum value where the maximum number of charge carriers have been created between the photoconductive gate electrodes. At low illumination and/or low optical repetition rate, the value of R PC  approaches a maximum value as shown.
 
 R   PC =1/(1 /R   photo (Power,Rate)+1 /R   off )  (4) 
 
where R photo (Power,Rate) is the reduced resistance of the photoconductive gate  10  induced by the optical illumination with pulses at a given power and rate. The value R off  is proportional to the intrinsic resistance of the semiconductor epitaxial structure between the first pole  18  and the second pole  20  of the antenna.
 
   The receiver circuit is optimized for high speed waveform sampling. High speed applications require minimizing RC RECEIVER . This is done by either reducing R TOTAL  or C TOTAL . The capacitance of the antenna represented by capacitor  36  is fixed by a particular antenna design, and typically C ANT &lt;C EXT  and C ANT &lt;C AMP . The value of the external capacitor  40  can be minimized by reducing the connection lead length as much as possible. The minimum length would be achieved when the instrumentation amplifier  46  is fabricated on the photoconductive gate  10 . Alternately, the amplifier can be housed in close proximity inside the same miniature electronic module. 
   In a typical system, R AMP  is between 10 9  and 10 10  ohms and R PC  is approximately 5×10 7  ohms. If R EXT &gt;&gt;R AMP , R EXT &gt;&gt;R PC , or both, and noting that R AMP &gt;&gt;R PC  in a preferred system, equation 1 can be approximated as:
 
 RC   RECEIVER   =R   PC *( C   ANT   +C   EXT   +C   AMP ).  (5) 
 
   Usually, increasing the optical power or repetition rate is not practical. Equation 5 shows that RC RECEIVER  can be minimized by tailoring the photoconductive gate  10  to achieve the desired time constant. 
   If it is not practical to alter the photoconductive gate  10 , then the external resistor  38  is added where R EXT &lt;R PC  and R EXT &lt;R AMP , and the overall constant can be approximated as follows:
 
 RC   RECEIVER   =R   EXT *( C   ANT   +C   EXT   +C   AMP ).  (6) 
 
   The external resistor  38  could be exterior to the photoconductive gate  10 , or lithographically patterned on the die itself. 
     FIG. 4  is a schematic diagram of a single stage voltage amplified photoconductive sampled terahertz antenna receiver circuit. The photoconductive gate  10  is coupled to a bipolar terahertz antenna having a first pole  18  and a second pole  20 . The first pole  18  and the second pole  20  are coupled to a bleedoff resistor pair  50  having two resistors tied to the ground state. Preferably the resistors in the bleedoff resistor pair  50  are of equal value, and that value is much greater than the inherent resistance of the photoconductive gate  10  in an off state (R PC off ). Given that the photoconductive gate is floating, the absolute voltage of each of the first pole  18  and the second pole  20  can drift outside the input range of the instrumentation amplifier  46  due to uneven charge buildup. The bleedoff resistor pair  50  thus drains the long term charge buildup. The leads from the first pole  18  and the second pole  20  are also coupled to an external resistor  48  to reduce the overall time constant. 
   The instrumentation amplifier  46  is coupled to a feedback resistor to set the instrumentation amplifier gain at a value between ten and one thousand. The instrumentation amplifier  46  is further coupled to an offset voltage divider  54  preferably comprising three resistors. The offset voltage divider  54  is used to trim the amplified output offset from ground to zero when no external signal is sampled by the photoconductive gate  10 . 
   The instrumentation amplifier  46  preferably has large input impedance and it can be referenced to an external voltage. The input impedance should be chosen high enough that little current is drawn by the instrumentation amplifier  46 . High CMMR of 100 dB allows the first pole  18  and the second pole  20  to float. As one side of the antenna is not directly connected to ground, noise current from the ground wiring cannot be introduced into photoconductive gate  10  and antenna poles  18 ,  20 . 
   A single stage of amplification will achieve an optimized signal to noise ratio. Typical instrumentation amplifiers have maximum bandwidth at unity gain. The amplification necessary for terahertz signals may be greater than one thousand, which may reduce the bandwidth of the amplifier by several orders of magnitude. However, for slower speed applications, the reduced bandwidth may be adequate, and the superior signal to noise properties of a single stage amplifier can be used. 
   As such, the external resistor  48  is typically not necessary for the slower applications which can use a single stage amplifier circuit. As before, the external resistor  48  could be exterior to the photoconductive gate  10 , or lithographically patterned on the die itself. The entirety of  FIG. 4  is usable as a set of discrete elements in one or more housings, combined inside a single housing, or fabricated on a single microelectronic circuit. 
     FIG. 5  is a schematic diagram of a high-speed, two-stage voltage-amplified photoconductive sampled terahertz antenna receiver circuit. The photoconductive gate  10  is coupled to a bipolar terahertz antenna having a first pole  18  and a second pole  20 . The first pole  18  and the second pole  20  are coupled to a bleedoff resistor pair  50  having two resistors tied to the ground state. Preferably the resistors in the bleedoff resistor pair  50  are of equal value, and that value is much greater than the inherent resistance of the photoconductive gate  10  in an off state (R PC off ). As the photoconductive gate is floating, the absolute voltage of each of the first pole  18  and the second pole  20  can drift outside the input range of the instrumentation amplifier  46  due to uneven charge buildup. The bleedoff resistor pair  50  thus drains the long term charge buildup. The leads from the first pole  18  and the second pole  20  are also coupled to an external resistor  48  to reduce the overall time constant. 
   The instrumentation amplifier  46  is coupled to a feedback resistor to set the instrumentation amplifier gain at a value between ten and one thousand. The instrumentation amplifier  46  is further coupled to an offset voltage divider  54  preferably comprising three resistors. The offset voltage divider  54  is used to trim the amplified output offset from ground to zero when no external signal is sampled by the photoconductive gate  10 . 
   The instrumentation amplifier  46  is further coupled to a second stage operation amplifier  58  by a load resistor  56 . The load resistor  56  is used to couple the output of the instrumentation amplifier  46  to the input of the second stage operation amplifier  58 . 
   The second stage operation amplifier  58  is coupled to a feedback loop  60  preferably including resistors  62 ,  64  such that the gain is between ten and one hundred. Alternatively, the resistors  62 ,  64  are fixed resistors such that the gain is fixed. 
   As described above, the instrumentation amplifier  46  is used because its buffered inputs have large input impedance and it can be referenced to an external voltage. However, as noted the amplification necessary for terahertz signals may be greater than one thousand, which may reduce the bandwidth of the amplifier by several orders of magnitude. This may not be adequate for high speed terahertz waveform sampling 
   In order to achieve sufficient combined amplifier bandwidth, the two stage amplifier of  FIG. 5  can be used to maximize the frequency response. The gain of each of the instrumentation amplifier  46  and the second stage operation amplifier  58  is set such that the bandwidth of each amplifier alone can meet the desired minimum response time. In order to achieve the maximal signal to noise, the gain of the first stage through the instrumentation amplifier  46  should be maximized without sacrificing the desired total bandwidth. As before, the entirety of  FIG. 5  is usable as a set of discrete elements in one or more housings, combined inside a single housing, or fabricated on a single microelectronic circuit. 
   As described, the present invention includes an optimized epitaxial photoconductive structure and associated voltage amplifier for the sampling of free space terahertz optical signals. Nevertheless, it should be apparent to those skilled in the art that the above-described embodiments are merely illustrative of but a few of the many possible specific embodiments of the present invention. Numerous and various other arrangements can be readily devised by those skilled in the art without departing from the spirit and scope of the invention as defined in the following claims.