Large-area traveling-wave photonic mixers for increased continuous terahertz power

Abstract

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...

Ultra-narrow-linewidth continuous-wave THz sources based on multiplier chains

“…THz spectroscopy has also been performed using sources based on mixing two lasers [1,5] in a nonlinear medium (photomixers [6,9] and nonlinear-optical crystals [7,8]), producing a wave at the difference frequency. Accurate frequency stabilization of the two lasers is necessary in order to obtain a frequency-stable THz wave.…”

Terahertz generation by means of optical lasers

“…The THz radiation appears as a result of resonance photo-mixing [46][47][48][49]. The absorption edge of GaAs (∼ 0.9 µm) enables one to use powerful femtosecond radiation of Ti:Sapphire laser systems for pumping, but this material cannot be activated by solid-state or fiber lasers emitting at longer wavelengths.…”

Tunable, continuous-wave Terahertz photomixer sources and applications

“…Instead of the introduction of a path delay, the relative phase between neighboring devices is introduced by a tilt angle between the two near-IR laser beams (see Fig. 31, similar to a traveling wave approach 144 ). The phase difference between two adjacent sources becomes…”