Monolithic echo-less photoconductive switches as a high-resolution detector for terahertz time-domain spectroscopy

Interdigitated photoconductive (iPC) switches are powerful and convenient devices for time-resolved spectroscopy, with the ability to operate both as sources and detectors of terahertz (THz) frequency pulses. However, reflection of the emitted or detected radiation within the device substrate itself can lead to echoes that inherently limit the spectroscopic resolution achievable for their use in time-domain spectroscopy (TDS) systems. In this work, we demonstrate a design of low-temperature-grown-GaAs (LT-GaAs) iPC switches for THz pulse detection that suppresses such unwanted echoes. This is realized through the growth of a buried multilayer LT-GaAs structure that retains its ultrafast properties, which, after wafer bonding to a metal-coated host substrate, results in an iPC switch with a metal plane buried at a subwavelength depth below the LT-GaAs surface. Using this device as a detector, and coupling it to an echo-less iPC source, enables echo-free THz-TDS and high-resolution spectroscopy, with a resolution limited only by the temporal length of the measurement governed by the mechanical delay line used. As a proof-of-principle, the 212-221 and the 101-212 rotational lines of water vapor have been spectrally resolved, demonstrating a spectral resolution below 10 GHz.