Accurate circuit models derived from electromagnetic simulations have been used to fabricate photomixer sources with optimized high-impedance antennas. Output powers on the order of 1 W were measured for various designs spanning 0.6-2.7 THz. The improvement in output power ranged from 3 to 10 dB over more conventionally designed photomixers using broad-band log-spiral antennas. Measured data on single dipoles, twin dipoles, and twin slots are in good agreement with the characteristics predicted by the design simulations.

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.

Low-temperature-grown (LTG) GaAs is used as an optical-heterodyne converter or photomixer, to generate coherent continuous-wave (CW) output radiation at frequencies up to 5 THz. Photomixers consist of an epitaxial LTG-GaAs layer that is patterned with interdigitated metal electrodes, on which two laser beams are focused with their frequencies offset by the desired difference frequency. The difference-frequency power is coupled out of the photomixer using coplanar waveguide at low frequencies and using log-spiral, dipole, and slot antennas at higher frequencies. Difference-frequency power is limited by the maximum optical-pump power that the photomixer can withstand. Fiber-coupled photomixers were operated at 77 K-a configuration in which they exhibited improved heatsinking and, therefore, withstood higher pump power. Progress has been made in the development of photomixer local oscillators (LO's) for space-based receivers that use superconducting tunnel junctions and hot-electron bolometers as heterodyne detectors.

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High-order harmonic generation at 4MHz as a light source for time-of-flight photoemission spectroscopy

The response time of photoconductive submillimeter-wave emitters based on low-temperature-grown (LTG) GaAs is known to increase at high applied bias, which limits the output power of these devices at frequencies near 1 THz. We performed measurements of an LTG GaAs photoconductor embedded in a coplanar waveguide with both static and dynamic illumination to investigate the increase in response time and an increase in direct-current photoconductance that occurs at the same bias voltages. We attribute both phenomena to a reduction of the electron capture cross section of donor states due to electron heating and Coulomb-barrier lowering. We discuss why the phenomena cannot be explained by space-charge-limited current or other injection-limited currents, or by impact ionization.

We have made direct measurements of the thermal boundary resistance, Rbd, between high quality epitaxial films of YBa2Cu3O7−δ and a variety of substrates with and without buffer layers. The boundary resistance was deduced from measurements of the electrical resistance changes in three parallel strips of YBa2Cu3O7−δ when one was electrically heated. Our measurements indicate that Rbd is weakly dependent on temperature and, for all the measured samples, has a value of 0.8–1.4×10−3 K cm2/W between 90 and 200 K, which is a factor of ≊80 larger than the prediction of the acoustic mismatch model at 100 K. The thermal response time of ≊1 ns which results from the heat capacity of the film and Rbd is observed in many experiments with pulsed laser sources.

We consider the phenomenon of afterpulsing in avalanche photodiodes (APDs) operating in gated and free-running Geiger mode. An operational model of afterpulsing and other noise characteristics of APDs predicts the noise behavior observed in the free-running mode. We also use gated-mode data to investigate possible sources of afterpulsing in these devices. For 30-μm-diam, 1.06-μm-wavelength InGaAsP∕InP APDs operated at 290K and 4V overbias, we obtained a dominant trap lifetime of τd=0.32μs, a trap energy of 0.11eV, and a baseline dark count rate 245kHz.