A large-aperture design for terahertz traveling-wave photomixers, continuously pumped free space by two detuned diode lasers, is proposed and experimentally verified for devices based on low-temperature-grown GaAs ͑LT-GaAs͒. It combines the advantages of conventional interdigitated small-area structures and traveling-wave devices. An output power of 1 µW at the mixing frequency of 1 THz was measured in initial testing, which meets local oscillator power requirements for superconducting heterodyne mixer devices. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1884262͔ Continuous-wave photonic mixer terahertz ͑THz͒ radiation sources are basically fast photoconductive switches modulated with the beat frequency of two detuned nearinfrared ͑IR͒ diode lasers. 1 One of the fastest materials to follow the beat frequency in the THz range is lowtemperature-grown GaAs ͑LT-GaAs͒ with photocarrier trap times down to 100 fs. 2 Besides the corresponding unmatched tuning range, photomixers are also particularly attractive for their all-solid state, noncryogenic, low power consumption, and relative low-cost approach. Therefore, they are interesting as local oscillators ͑LOs͒ for heterodyne submillimeter or terahertz receivers 3,4 based on superconductor-insulatorsuperconductor junctions ͑SIS͒ or hot-electron bolometers ͑HEB͒. Providing the power necessary for these mixers above 1 THz is a challenge, but is within reach of the current development of LT-GaAs photomixers. The latest report for smallest-area SIS junctions is p pump Ϸ 0.1 W at 1 THz at the mixer 5 ͑with a theoretical frequency dependency of p pump ϳ f 2 ͒, and for HEBs p pump Ϸ 0.2 W at 1.8 THz at the mixer Si lens 6 ͑with an expected frequency dependency of p pump ϳ f͒.With small-area photonic mixers, consisting of interdigitated metal-semiconductor-metal electrode structures, hereafter MSM ͓Fig 1͑c͔͒, it proved to be difficult to routinely reach these power levels above 1 THz. If small-area mixers are used with broadband antennas of load resistance, R a , the uncompensated device capacitance, C, introduces a rolloff, 2,7 ϳ1/͓1+͑2 · R a C · ͒ 2 ͔, which is usually around 1 THz. However, if it is located at the footpoint of resonant antennas, capacitance up to a certain value may be tuned out by the antenna inductance, 8 and the terahertz power at the resonance frequency, res , follows just the unavoidable rolloff, ϳ1/͓1+͑2 · e · res ͒ 2 ͔, given by the effective response time, e , for the electronic current seen locally at the electrodes. This in turn is not identical to the photoelectron trap time, 1,9-11 but is elongated by the intrinsic transit time of a space-charge dominated current pulse initiated by the shortliving photoelectrons between the electrodes. 9,12 The restriction on the capacitance first imposes upper limits on the device area and number of fingers. Because of a destruction intensity of Ͻ1.5 mW/ m 2 for LT-GaAs this limits IR pump power to Ͻ100 mW, so that improved passive thermal sinking is essential for small-area mixers, 3,13 unless coolin...

The authors report on fabrication and measurement of traveling-wave photomixers based on high energy and low dose nitrogen-ion-implanted GaAs. They used 3MeV energy to implant N+ ions into GaAs substrates with an ion concentration dose of 3×1012cm−2. The N+-implanted GaAs photomixers exhibit improvements in the output power in comparison with their counterparts, photomixers fabricated on low-temperature-grown GaAs. The maximal output power was 2.64μW at 850GHz. No saturation of the output power with increased bias voltage and optical input power was observed. These characteristics make N+-implanted GaAs the material of choice for efficient high power sources of terahertz radiation.

We have fabricated and characterized novel traveling-wave photomixers with recessed interdigitated metal-semiconductor-metal (MSM) contacts based on low-temperature-grown GaAs. The new recessed MSM geometry led to an improved electric-field distribution inside the photomixer structure and resulted in an up-to-100% increase in the output power of continuously operated devices, compared to conventional MSM devices with standard surface electrodes fabricated on an identical material. The recessed electrode structure also resulted in lower saturation of output power at higher input powers, enabling it to take advantage of higher input powers.

A pump experiment of two astronomical heterodyne receivers, a superconductorinsulator-superconductor ͑SIS͒ receiver at 450 GHz and a hot-electron-bolometer ͑HEB͒ receiver at 750 GHz, is reported. A low-temperature-grown GaAs metal-semiconductor-metal photonic local oscillator ͑LO͒ was illuminated by two near infrared semiconductor lasers, generating a beat frequency in the submillimeter range. I-V junction characteristics for different LO pump power levels demonstrate that the power delivered by the photomixer is sufficient to pump a SIS and a HEB mixer. SIS receiver noise temperatures were compared using a conventional solid-state LO and a photonic LO. In both cases, the best receiver noise temperature was identical ͑T sys = 170 K͒.