Tunable Terahertz Photomixer Emitters

In common CW terahertz photomixer emitters, two wavelength-tunable single-mode lasers with frequencies ${\nu }_1$ and ${\nu }_2$ are superimposed, resulting in a fast carrier frequency and a slow beat or difference frequency ( ${\nu }_{\text{diff}}=|{\nu }_1-{\nu }_2|$ ). The combined laser signal is focused onto a suitable semiconductor structure inducing an alternating photocurrent at the difference frequency of the lasers. Integrated planar antenna structures are used to radiate the generated electric field into free space. Tuning of the difference-frequency ${\nu }_{\text{diff}}$ of the lasers translates directly into the tunability of the terahertz radiation of these sources. The wavelengths of the laser excitation determines appropriate material systems with respect to band-gap properties of the semiconductor. For instance, photomixers based on Indium Gallium Arsenide (InGaAs) are commonly used with 1550 nm laser systems, while Gallium Arsenide (GaAs) based transmitters usually work with 850 nm radiation. Especially the use of 1550 nm optical lasers allows to benefit from the maturity of technological components for this wavelength. Components, which were originally developed for fiber-optical communication, can find their use in 1550 nm terahertz CW systems, which results in overall compactness, reliability, and cost efficiency^{18 (link),20 (link)}. The so called PiN photodiode (PiN-PD) is one of the commonly used types of CW emitters for the 1550 nm excitation wavelength. The structure of a PiN-PD is composed of an intrinsic absorption layer sandwiched between n-type and a p-type semiconducting layers. Photocarriers are exited in the intrinsic layer by laser illumination and accelerated by the intrinsic electrical field and an additional external bias field forming an alternating photocurrent, which can be irradiated by an appropriate antenna structure. For our experiments, we use an InGaAs waveguide-integrated PiN-PD (WiN-PD) as photomixer source for the generation of terahertz radiation, similar to the one in^{29 (link),30 (link)}. The transmitter is driven by two tunable 1550 nm semiconductor lasers.