In this discussion, we report two recent advances in femtosecond optical technology. Firstly, the generation and measurement of optical pulses as short as 30 fs is described. The pulses are produced using self-phase modulation of the 90 fs laser pulse in a short 15 cm optical fiber followed by a grating compressor. Finally, we report the operation of an optically synchronized streak camera which operates at 10 Hz repetition rate and has a system jitter of less than one picosecond. Synch-ronization is achieved with a Fe:InP photoconductive switch which allows for room temperature dc biasing.

Using the pump and probe technique the relaxation processes of photogenerated carriers in amorphous tetrahedral semiconductors and chalcogenide glasses in the time domain from 0.5 Ps to 1.4 ns have been studied. The results obtained on the following phenomena are reviewed: hot carrier thermalization in amorphous silicon; trapping of carriers in undoped a-Si:H; trapping of carriers in deep traps produced by doping; geminate recombination in As₂S_3-xSe_x glasses.

Time-resolved measurements of the decay of photoexcited carriers in InGaAsP revea1 the presence of Auger recombination at carrier concentrations near and above 3x10¹⁸ cm^-3. Our results suggest that while Auger recombination does contribute to the temperature dependence of the InGaAsP-1.3μm laser, it may not alone fully account for poorer room temperature dependence of InGaAsP-InP compared to GaAs-GaA1As lasers.

Carrier transport in the sub-picosecond regime is considered. Special attention is paid to the development of the proper equations required at this timescale, along with the difficulties imposed by many-body effects for high carrier density.

Time resolved kinetics of Ga_.5 In_.5 P was measured by a streak camera system. The emission recombination time was measured to be 40 ps at room temperature and 70 ps at T=78K. As excitation power density increased the band to band photoluminescence spectra at room temperature broadens, shifts to higher energy and at highest excitation power the emission spectra exhibits an overlapping of two peaks. At T=78K when excitation power density increased there appears to be another peak at the high energy side of the spectrum. The low energy band does not shift with excitation power while the high energy peak grows in intensity. We attributed these two bands to the recombination of electrons with holes in heavy and split off valence band.

3He+ bombardment is shown to be an effective means of achieving very short free carrier lifetimes in Fe-doped InP. Photoconductivity studies indicate free carrier mobilities (~600 cm²/Vs with carrier relaxation times of 6 psec. Preliminary correlation measurements demonstrate that carrier lifetimes less than 5 psec have been achieved in InP using ³He⁺ bombardment without compromising the dark resistance of the photoconductors. The upper limit to the lifetime is a consequence of using coaxial cables in the correlation circuit.

Examples of the use of picosecond optical pulses both as diagnostic probes and as triggering signals for transferred electron devices (TEDs) are given. The internal electric field distribution in a planar InP TED was measured with the spatial resolution of the focused beam of pulses. Domain formation in response to a triggering bias pulse is shown to require a minimum pulse duration; the bias dependence of this duration was measured for a gated GaAs TED.

Recent progress in picosecond pulse generation with semiconductor diode lasers is reviewed.

The pulse widths obtained from AlGaAs lasers have been reduced to the subpicosecond regime by passive and active mode locking of AlGaAs lasers containing a region of saturable absorption in an external cavity. Bursts of 4 pulses each of 0.65 psec long and separated by the 11 psec round trip time through the laser have been obtained by passive mode locking using a linear external cavity. By inserting a bandwidth limiting element in the cavity, the pulse width increased to ~ 20 psec. The widths obtained are in reasonable agreement with a theoretical model and correspond to dispersion and bandwidth limited propagation, respectively. Pulses as short as 0.56 psec have been obtained by the colliding pulse technique using a ring geometry external cavity.

Ultrashort cavity semiconductor lasers have been pumped at room temperature by AℓGaAs injection lasers. Peak powers of up to 10 mW were obtained in InP in the form of single longitudinal mode subnanosecond pulses. Conversion efficiency from the injection laser pump power to lasing power was up to 8 percent in InP. Tuning over 150 A° was seen.

Recent developments in two areas of high speed semiconductor lasers will be addressed: (1) passive mode-locking of a segmented-contact semiconductor laser with a reliable, controllable saturable absorber which produces stable picosecond optical pulses, and (2) realization of very high frequency (X-band) direct analog modulation of a semiconductor laser diode.

We describe tunable cw modelocked laser action from synchronously pumped CdS, CdSSe, CdSe, InGaAsP and HgCdTe lasers. The 2-50 pm thick semiconductor platelets are longitudinally pumped and have produced 40 mW at 495 nm unmodelocked and a peak power of 50 Watts modelocked at 1.2μm. Pulsewidths of 4-6 ps with time-bandwidth products of 0.6-1.7 are obtained using intracavity etalons and Lyot filters. Intracavity tuning achieves a single crystal range of 7 nm in CdS at 495 nm, 29 nm in InGaAsP at 1.2 μm and a range of 60 nm in HgCdTe at 1.8 μm. As many as two dozen crystals have been mounted together on a micrometer stage within the dewar to all an immediate and wide tuning range. Also tuning from 1.82 to 2.0 μm via the Burstein-Moss shift in a single HgCdTe epilayer has been accomplished by varying the loss in the external cavity. A modelocked threshold as law as 0.3 mW in CdS, a 20% conversion efficiency into the TEMoo mode, and lasing in InGaAsP with optical pumping a full 1.5 eV above a 1 eV bandaap shows that an efficient and convenient picosecond source tunable through the visible and near infrared is feasible.

Picosecond pulses from optically pumped, ultrashort cavity semiconductor film lasers have a large positive chirp that is not linear in time. Average chirp values are about 1 nm/psec for InGaAsP film lasers. The chirp is caused by transient change in the refractive index of the semiconductor film during the pulse due to variation of the free carrier concentration. The transient carrier concentrations of 10²⁰ cm^-3 caused by the intense picosecond pump pulses is sufficient to account for the observed chirp. For film lasers, the dominant mechanism by which the carriers affect the index of refraction is free carrier plasma refraction. The positively chirped pulses have been compressed by the negative linear group velocity dispersion of 8m of single-mode optical fiber. Pulse compression factors as large as 3.4 have been obtained. Pulses as short as 2.5 psec at 1.23 μm and 3.4 ps at 1.42 μm have been generated by this compression scheme.

Picosecond optoelectronic switches have been applied for high speed pulse modulation of double heterostructure (DHS) GaAs/(GaAl)As diode lasers. The picosecond switches made of crystalline GaAs doped with chronium (GaAs:Cr) are activated by the pulse train of a synchronously mode-locked dye laser. The generated electrical pulses with a typical duration of 60 ps are superimposed to a variable dc bias for direct modulation of different gain and index guided GaAs/(GaAl)As DHS laser diodes. The shortest pulses obtained from the diode laser are of 55 ps width. The emission spectra of the unbiased index guided as well as gain guided lasers are always multimode at this fast modulation and the spectral width of the individual laser modes is in the order of 10 GHz. Single mode operation can be obtained with index guided lasers if the short electrical pulses are superimposed to a dc current well above threshold, however, temporal broadening due to diffusion damping occurs under these operation conditions.

The characterization of waveform measurement systems and the development of reference waveforms is now underway at the National Bureau of Standards (NBS). Efforts to upgrade the state of the art of fast waveform measurements at NBS has resulted in the development of the first photoconductive switch using GaAs in addition to a patent on the sampling of electrical signals with optical signals and vice versa. These photoconductive switches are now being applied to the development of reference waveform generators in the form of a Maxwell-Wagner two-layer capacitor in silicon stripline to complement the liquid-filled coaxial tine fitters developed previously. The silicon filters are more compact, rugged, and less expensive to fabricate. In addition, they lend themselves well to integration with photoconductive switches resulting in a reduction in connector problems and pulse excitation errors.

Two experiments are reported in this paper. In the first experiment we have succeeded in using natural diamond as a photoconductor excited by picosecond laser pulses to switch high voltage with 80% efficiency. The conductivity was found to be sub-linear in input light intensity, and the carrier life-time deduced to be about 1 nanosecond. In the second experiment a Blumlein pulse generator with an ultrafast opto-electronic switch is reported. The Blumlein pulse generator possesses the potentiality of yielding a pulse amplitude which can be equal to the charging voltage in an ideal case. The opto-electronic switch was fabricated from high resistivity silicon with annular aluminum electrodes. With a pulsed bias voltage of 1100 volts, the switched-out amplitude is about 900 volts, greater than the maximum voltage attainable from a standard charge line pulser generator.

We have used 20 psec, 1.06 and 0.35 μm pulses to time resolve the photoconductive response of semiconducting and insulating natural diamonds, respectively. Pairs of diamonds were mounted in tandem in a biased coaxial transmission line and the time response of each was obtained by using a sample and gate technique that yields the cross-correlation of the induced photocurrents. Typical response times of 100-500 psec were determined for the semiconducting diamonds at room temperature; these times are explained in terms of capture of photoexcited holes by excited states of the boron acceptors in these p-type materials. The response time of the insulating diamonds was determined by using a semiconducting and insulating diamond in tandem, with the former illuminated by by 1.06 μm light and the latter by 0.35 μm light. In each case photoconductivity is induced via impurity to band transitions. The response time of the insulating diamonds is ≤500 psec. Because of this fast time response, as well as high dielectric strength and thermal conductivity, insulating diamond shows promise as a fast high voltage switch with voltage hold-off capability of (~10 kV.

A picosecond optoelectronic switch based on a new material, LPE grown semi-insulating InP:Co, is described. Pulse response of < 75 ps has been measured. It is observed that these pulse widths depend on both the position of the light beam and the applied voltage. From these dependencies, it is found that the metallic contacts play an important role in the performance of the devices.

We have been developing a silicon photoconductive switch for use as a Pockels cell driver in the pulse generation systems of the fusion lasers Nova and Novette. The objective has been to make 10 kV switches repeatably and which are reliable on an operating system. We found that nonlinear phenomena in nearly intrinsic silicon caused excessive conduction at high voltage resulting in breakdown. Our experiments with doped material show that this problem can be eliminated, resulting in useful devices.

This paper outlines the advantages and the potential of photoconductive switches applied to high-power systems. The photoconductive effect can be used to switch large amounts of energy by changing the conductivity of a solid-state circuit element many orders of magnitude with a high-power laser. The simplicity of these devices offers many advantages in high-power applications when combined with high-power pulsed lasers. The surge capability, the switched energy gain, and the maximum average power for photoconductive power switches are discussed. In addition, the results of a 100-kV, 100-MW photoconductive switch experiment transferring 20 J in 200 ns are presented.

We discuss the dynamic behavior of the propagation parameters of millimeter-waves in semiconductor waveguides when they are controlled by a laser induced electron-hole plasma. Depending upon the laser source and the semiconducting material utilized, high speed millimeter-wave phase shifters, modulators, switches, and gating devices may be constructed. Gated millimeter-wave pulses of 1 ns duration have been observed in a Si waveguide. Modulated millimeter-wave signals with pulse durations of ~ 400 ps and a modulation bandwidth approaching 1 GHz are readily available using a Cr:GaAs waveguide controlled by a mode-locked Nd:YAG laser. Control of the propagation properties of a Si waveguide has also been demonstrated using a GaAs diode laser. Simultaneous measurements of phase shift and attenuation in the waveguide as a function of time have been performed and a theoretical model has been developed which is in excellent agreement with the measured data.

This paper describes the test apparatus and circuitry used to make measurements of pulsed light conductance on samples of high resistivity (10⁷ Ω-cm) gallium arsenide switching devices, having nominal 25 PM and 700 Pm gap spacings. Differences in conductance are observed on variously grown samples. Maximum peak light conductance obtained was ≈2 millisiemens for an effective irradiated optical power on the (25 μm) gap of approximately 130 milliwatts (λ = 850 nm). Comparisons are made between the observed pulse measurements and the pulse waveforms generated by computer simulation using a model based on a theoretical analysis of the relationships between photoconductance and irradiated optical power.

We report on the recent advances of an electrooptic sampling technique for the characterization of electrical transients that has now achieved a temporal resolution of .5 ps. Voltage sensitivity is on the order of 50 μV. The electrooptic material is used in a traveling wave geometry as well as in an electrodeless manner, where the electrooptic material probe the fringing field associated with the signal as it propagates along the transmission line.

We report the development of an electro-optic sampling system and its application to the characterization of high speed GaAs Schottky photodiodes. This system achieves a temporal resolution of 2 ps and shot noise limited sensitivity of 11 μV√Hz.

A Ti-diffused lithium niobate, traveling wave modulator has been fabricated and tested. A Mach-Zehnder interferometer optical configuration and a coplanar waveguide electrical transmission line are used. For a 4mm interaction length, the modulator has a 3dB bandwidth of 13 GHz and requires only 2 volts to switch at λ = 840 nm. The frequency response is measured directly using an ultra-high speed photodiode, and the test set-up therefore constitutes the highest bandwidth electrooptical transmission system ever reported.

Calculations are made showing the dispersion of electrical pulses of a few picoseconds duration when propagating along microstrip transmission lines. The dispersion is primarily due to the geometrical factors affecting the quasi-TEM mode although material properties also contribute. Especially interesting are effects of pulse sharpening by dispersion for initially asymmetric pulses.

Progress in the development oil high-speed InP optoelectronic switches is reported. Two approaches are investigated for decreasing recombination time: proton bombardment and raising the Fe concentration. The former is found to produce greater increases in speed than the latter but with greater decrease in sensitivity to light. Application of the device is reported in two areas: an RF mixer and a sample-and-hold switch. For mixer applications, the off-state capacitance and on-state linearity are measured. The 20 to 75 fF of capacitance is found to limit high-freuency performance and a third-order intercept of +20 dBm for 14 mW of laser power is comparable to conventional mixers. Sample-and-hold preliminary measurements indicate a need for an oft-resistaLce > 1 MΩ and an on-resistance of < 100 Ω.

This presentation addresses a plan to incorporate the use of photoconductor switches in digitizing high frequency antenna array data. High frequency (HF) typically refers to the frequency band from 2 to 30 megahertz where, under current technology, almost all signal processing is done by analog techniques. If one is to bring all the tools and techniques of modern signal processing to this frequency band, a goal which is consistent with the Department of Defense programs in the development of very high speed integrated circuits (VHSIC), then digital-signal-processing silicon devices which can easily operate at 100-MHz sample rates will need to be utilized.

The direct addressing of ultrafast integrated circuits by picosecond optical pulses has numerous applications in electronics and data processing. These include new and unambiguous ways of characterizing the speeds of Gigahertz logic circuits, contact-free diagnosis of problems or failures in complex circuits, and the presentation of novel possibilities for data input and interconnections in future high-speed data processors and super-computers.

Ultrahigh speed photodiodes with -3dB frequency roll-off in excess of 100 GHz or equivalently in the time domain an impulse response with a full width half maximum (FWHM) of 5.4 psec have been developed and characterzed. These 100 GHz photodiodes have a planar configuration fabricated on n on n⁺ GaAs expitaxial layers on semi-insulating GaAs substrates which together with proton bombardment, have kept parasitic capacitances to ≤15 femto farads. The planar structure photodiode also lends itself easily to monolighic integration with a MESFET. The device operates at a reverse bias of -4 volts.

Optical techniques for time resolved studies of semiconductors are attractive because they are inherently limited only by the excitation laser pulsewidth. In recent years, various techniques have been exploited in measurements of photoexcited carrier dynamics and fast relaxation processes in GaAs and other semiconductors. Each optical technique has its own set of disadvantages. An absorption experiment requires a transparent sample or device. Because of the small signal, reflectivity experiments in the visible require good noise rejection. Time resolved luminescence measurements utilizing upconversion techniques have been difficult due to low upconversion efficiencies. However, the existence of a non-linearity in the luminescence of GaAs''' has made luminescent measurements easy, even at room temperature.

Single and double junction surface structures have been investigated for ultra-fast integrated optical detection. The M-I-S-I-M lateral structure, M-B-M tunnelling and photo-emissive structures are discussed.

We have measured the impulse response of InP:Fe photoconductors to mode-locked dye laser. excitation. Bias, excitation, and Fe concentration dependence of the impulse response are reported. Linearity of response over two decades in both bias and excitation and independence of transient-response pulse width to both bias and excitation are demonstrated. Correlation is observed between Fe concentration and transient response pulse width. Finally, optical electronic autocorrelation of impulse response consistent with the sampling oscilloscope measurements is reported.

Improvements in the quantum efficiency of both metal-oxide-metal (MOM) tunnel junction detectors and metal-insulator-metal (MIM) photoconductive detectors have been achieved by periodic microstructuring of their electrodes. E hancement in sensitivity are due to plasmon coupling, reduced reflection and coupling into a guided wave resonance.

Planar high-speed interdigital photoconductive detectors have been fabricated on MOCVD-grown GaInAs/GaAs heterostructures. The small intentional lattice mismatch at the GaInAs/GaAs interface allows a controlled surface recombination velocity which decreases the effective lifetime of minority carriers without significant active layer mobility degradation. The InGaAs detectors have the following characteristics: bandgap of ~ 1.25 eV; rise times of < 25 psec; fall times ~ 50-100 psec; FWHM < 45 psec; high speed responsivity ~0.3 A/W; 2-300 nA leakage current at 5-10 V operating voltages and flat analog response to ~ 15 GHz.

Interdigital silicon Schottky barrier diodes have been evaluated for picosecond pulse measurements. Structures with clearly defined receiving apertures and submicrometer contact spacings were created with electron beam lithography. The detectors exhibit saturation currents corresponding to the absorbed optical power. Impulse response widths were less than 50 ps, and response maps yielded uniform patterns. A peak quantum efficiency of over 30 percent was obtained, and the usable spectral responsivity extends beyond 2 μm.