Photoconductive Materials for THz Generation at 1550 nm: ErAs:GaAs vs InGaAs Based Materials

Since the 1990s the photoconductive material of choice for THz generation has been low-temperature grown GaAs (LTG-GaAs) thanks to its useful mobility (~200 cm2/V.s), high dark resistivity (~107 Ω.cm), and fast carrier lifetime (~0.2 ps). However, due to the band gap energy of GaAs, the usual optical drive wavelength is around 800 nm where suitable lasers, both cw and pulsed, tend to be marginal in output power and rather expensive. Consequently, research has pushed towards the development of InGaAs-on-InP-based ultra-fast photoconductive materials operating at 1550-nm where cw and pulsed lasers are superior in all respects. Recently, we discovered ultrafast extrinsic photoconductivity in erbium-doped GaAs at 1550 nm, allowing the generation of both cw and pulsed THz waves. GaAs-based materials have major electrical advantages over InGaAs-based because of their superior dark resistivity and breakdown field. In this paper, we show that 1550-nm-driven GaAs:Er is now a promising alternative to InGaAs-based photoconductive material. We focus on the ultrafast photoconductive behavior, and compare the performance of GaAs:Er photoconductive switches to those from state-of-the-art ultrafast InGaAs devices.