S.D. Calawa

Generation and detection of coherent terahertz waves using two photomixers

A general technique has been demonstrated at microwave and submillimeter-wave frequencies for photoconductive sampling in the frequency domain using photomixers and continuous-wave laser diodes. A microwave version in which two photomixers were coupled by a transmission line was developed to quantitatively test the concept from 0.05 to 26.5 GHz. A quasioptical version using antenna-coupled photomixers was demonstrated from 25 GHz to 2 THz. Such a system can outperform systems based on time-domain photoconductive sampling in frequency resolution, spectral brightness, system size, and cost.

Lattice thermal conductivity of nanostructured thermoelectric materials based on PbTe

We report the through-thickness lattice thermal conductivity Λl of (PbTe)1−x/(PbSe)x nanodot superlattices (NDSLs) over a wide range of periods 5 nm≤h≤50 nm, compositions 0.15≤x≤0.25, growth temperatures 550 K≤Tg≤620 K, and growth rates 1 μm h−1≤R≤4 μm h−1. All of our measurements approach Λl of bulk homogenous PbTe1−xSex alloys with the same average composition. For 5 nm≤h≤50 nm, Λl is independent of h; a result we attribute to short mean-free paths of phonons in PbTe and small acoustic impedance mismatch between PbTe/PbSe. We alloyed the PbTe layers of four NDSLs with SnTe up to a mole fraction y=18%; Λl is reduced by <25%.

InGaAsP/InP avalanche photodiodes for photon counting at 1.06 μm

Geiger-mode (photon-counting) operation at 1.06 μm has been demonstrated with InGaAsP/InP avalanche photodiodes operated at room temperature. A photon detection efficiency of 33% was measured on uncoated detectors, representing an internal avalanche probability of 60%. Under identical bias conditions a dark count rate as low as 1.7 MHz was measured at 290 K, consistent with a primary dark current of <0.3 pA. Dark count rates drop by approximately 50–200× by cooling the detectors to 210 K (−63 °C).

Phase-controlled, heterodyne laser-induced transient grating measurements of thermal transport properties in opaque material

The methodology for a heterodyned laser-induced transient thermal grating technique for non-contact, non-destructive measurements of thermal transport in opaque material is presented. Phase-controlled heterodyne detection allows us to isolate pure phase or amplitude transient grating signal contributions by varying the relative phase between reference and probe beams. The phase grating signal includes components associated with both transient reflectivity and surface displacement whereas the amplitude grating contribution is governed by transient reflectivity alone. By analyzing the latter with the two-dimensional thermal diffusion model, we extract the in-plane thermal diffusivity of the sample. Measurements on a 5 μm thick single crystal PbTe film yielded excellent agreement with the model over a range of grating periods from 1.6 to 2.8 μm. The measured thermal diffusivity of 1.3 × 10−6 m2/s was found to be slightly lower than the bulk value.

Arrays of III-V semiconductor Geiger-mode avalanche photodiodes

In this paper, InGaAsP/InP APDs is designed for detection of near infrared (1-1.5 /spl mu/m wavelength) light and GaN APDs designed for detection of ultraviolet (<365 nm wavelength) light. This paper will also describe ladar measurements which use arrays of G-M APDs matched with timing circuits to produce 3D images with near-infrared photons.

Low-noise optical injection locking of a resonant tunneling diode to a stable optical frequency comb

Optical injection locking of a resonant tunneling diode (RTD) oscillator has been demonstrated using ultrashort pulses from a mode-locked Ti:sapphire laser operating at a 1GHz pulse rate. The source of the optical signal is a mode-locked femtosecond laser whose optical frequency comb is phase locked to a H-maser stabilized frequency synthesizer. An exceptionally large capture range of more than 5MHz is observed. The system produces stable microwave signals with low phase noise, which at 1GHz is less than −74dBc∕Hz for a 10Hz offset. The noise of the microwave injection-locked RTD signal matches that of the input optical pulses.

Resonant-tunneling-diode relaxation oscillator☆

Abstract Monolithic resonant-tunneling-diode (RTD) relaxation oscillators are fabricated. The highest repetition rate of this pulse generator is 6.7 GHz with a pulse width of approximately 60 ps. Oscillators with an RTD connected to an off-chip transmission line have been operated at a rate as low as 34 MHz while maintaining a similar pulse width. Characterization aided with simulations provides a better understanding of the RTD relaxation oscillator and the effects of the RTD characteristics on the performance of the oscillator.

Development of Geiger-mode APD arrays for 1.06 /spl mu/m

InGaAsP/InP Geiger-mode avalanche photodiodes (APDs) have been demonstrated with low dark count rates and high photon sensitivity at 1.06 /spl mu/m. APD arrays can be operated with a common bias voltage with excellent uniformity of response and dark count rates. By cooling, the dark count rates can be reduced from /spl sim/1 MHz at room temperature to /spl sim/35 kHz at 210 K. This operating temperature range is achievable with TE-coolers and should enable new active-imaging applications.

Molecular-beam epitaxial regrowth on oxygen-implanted GaAs substrates for device integration

Device-quality layers were regrown on GaAs wafers by molecular-beam epitaxy over conductive pregrown areas and on selectively patterned high-resistivity areas formed by oxygen implantation. The regrowth over both areas resulted in comparable device-quality GaAs. The high resistivity of the oxygen-implanted area was maintained after the regrowth and no oxygen incorporation was observed in the regrown layer. The cutoff frequency of a 1.5-μm-gate metal-semiconductor field-effect transistor fabricated on the regrown layer over the high-resistivity areas is 7 GHz. This demonstration shows that planar technology can be used in epitaxial regrowth, simplifying the integration of vastly different devices into monolithic circuits.