K. A. McIntosh

Photomixing up to 3.8 THz in low‐temperature‐grown GaAs

Low‐temperature‐grown (LTG) GaAs is used as an optical‐heterodyne converter or photomixer, to generate coherent continuous‐wave output radiation from microwave frequencies up to 3.8 THz. The photomixer consists of an epitaxial layer of LTG GaAs with interdigitated electrodes fabricated on the top surface. Terahertz photocurrents are generated in the gaps between the electrodes, and power is radiated into free space through a three‐turn self‐complementary spiral antenna. In a photomixer having a 0.27‐ps electron‐hole lifetime and small electrode capacitance, the output power is practically flat up to about 300 GHz and then rolls off at a rate of approximately 12 dB/oct.

Terahertz photomixing with diode lasers in low-temperature-grown GaAs

Recent optical heterodyne measurements with distributed‐Bragg‐reflector diode‐laser pumps demonstrate that low‐temperature‐grown (LTG) GaAs photomixers will be useful in a compact all‐solid‐state terahertz source. Electrical 3 dB bandwidths as large as 650 GHz are measured in mixers with low electrode capacitance. These bandwidths appear to be independent of pump‐laser wavelength over the range 780–850 nm. Shorter wavelength pumping results in a significant reduction of the bandwidth. The best LTG‐GaAs photomixers are used to generate coherent continuous‐wave output radiation at frequencies up to 5 THz.

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.

Optical and terahertz power limits in the low-temperature-grown GaAs photomixers

Optical heterodyne conversion, or photomixing, occurs in an epitaxial low-temperature-grown GaAs layer with voltage-biased metal electrodes on which two laser beams are focused with their frequencies offset by a desired difference frequency. Difference-frequency power couples out of the photomixer through a log-spiral antenna at THz frequencies. Pumping such a device with the maximum optical power of ∼90 mW at 77 K led to a measured output power of 0.2 μW at 2.5 THz, approximately twice the maximum output power of a photomixer operated near 300 K. Photomixers that were operated above the maximum optical power were destroyed, often because of a thermally induced fracture in the GaAs substrate. The fracture seemed to occur at high pump power when the temperature of the photomixer active area was elevated by roughly 110 K, independent of the bath temperature.

Increase in response time of low-temperature-grown GaAs photoconductive switches at high voltage bias

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.

Milliwatt output levels and superquadratic bias dependence in a low‐temperature‐grown GaAs photomixer

A cw output power up to 0.8 mW is obtained from a low‐temperature‐grown (LTG) GaAs, 0.3 μm gap, interdigitated‐electrode photomixer operating at room temperature and pumped by two modes of a Ti:Al2O3 laser separated in frequency by 0.2 GHz. The output power and associated optical‐to‐electrical conversion efficiency of 1% represent more than a sixfold increase over previous LTG‐GaAs photomixer results obtained at room temperature. A separate LTG‐GaAs photomixer having 0.6 μm gaps generated up to 0.1 mW at room temperature and up to 4 mW at 77 K. Low‐temperature operation is beneficial because it reduces the possibility of thermal burnout and it accentuates a nearly quartic dependence of output power on bias voltage at high bias. The quartic dependence is explained by space‐charge effects which result from the application of a very high electric field in the presence of recombination‐limited transport. These conditions yield a photocurrent‐voltage characteristic that is very similar in form to the well‐known Mott–Gurney square‐law current in trap‐free solids.

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).

Investigation of ultrashort photocarrier relaxation times in low-temperature-grown GaAs

Photocarrier relaxation times τr in low-temperature-grown (LTG) GaAs have been determined with time-resolved reflectance measurements. Measured τr values are extremely sensitive to the substrate temperature during LTG GaAs growth and postgrowth anneal. Photogenerated-electron relaxation times as short as 90 fs are found for LTG GaAs grown at temperatures near 200 °C and annealed at temperatures below 580 °C. We report the results of a systematic investigation of the dependence of τr on growth temperatures between 180 and 260 °C and anneal temperatures between 480 and 620 °C.

Terahertz measurements of resonant planar antennas coupled to low‐temperature‐grown GaAs photomixers

Resonant slot and dipole antennas coupled to low‐temperature‐grown GaAs photomixers have been fabricated and tested at terahertz operating frequencies. Enhanced output power is seen from the resonant structures compared to mixers coupled to broadband self‐complementary spiral antennas. Driving point impedances as high as 300 Ω are attained at the resonant frequencies. These devices will be useful as fixed frequency local oscillators for submillimeter heterodyne receivers.

GaN avalanche photodiodes grown by hydride vapor-phase epitaxy

Avalanche photodiodes have been demonstrated utilizing GaN grown by hydride vapor-phase epitaxy. Spatially uniform gain regions were achieved in devices fabricated on low-defect-density GaN layers that exhibit no microplasma behavior. A uniform multiplication gain up to 10 has been measured in the 320–360 nm wavelength range. The external quantum efficiency at unity gain is measured to be 35%. The electric field in the avalanche region has been determined from high-voltage C–V measurements to be ∼1.6 MV/cm at the onset of the multiplication gain. Electric fields as high as 4 MV/cm have been measured in these devices. Response times are found to be less than 5 μs, limited by the measurement system.