Development of terahertz photomixer technology at telecommunications wavelength

Terahertz (THz) region is one of the least developed regions of the electromagnetic spectrum. Lack of compact and high power sources and detectors in this wavelength range has limited its use for various key applications. In this thesis, three different approaches adopted for the generation of THz radiation are discussed, quantum cascade lasers (QCLs), photoconductive emitters and photomixers and emphasis is given to photomixing.
Photomixers generate continuous wave THz radiation by beating two independent laser beams on a semiconductor material. Beat frequency between the laser beams determines the emission frequency. In this work, two different materials, iron (Fe)– doped indium gallium arsenide (Fe:InGaAs) and Fe–doped indium gallium arsenide phosphide (Fe:InGaAsP) is used for THz photomixing at telecommunications wavelength. Characterizing the materials gave an idea about its intrinsic properties. With a standard antenna design, exemplar performance in terms of bandwidth (>2.4 THz) and output power was obtained from these materials.
In order to improve the THz power from photomixers, two different electrode designs with nanometre dimensions were attempted on Fe:InGaAsP wafer. The spectral bandwidth and power from the emitters were studied at different bias orientations and polarizations. Mapping the emitters gave an insight into the geometrical dependence of the emission mechanism. The design was tested in a THz time domain system to confirm the results.
Using photomixers, a 2.0 THz QCL was injection locked to a heterodyne source. The emission frequency of the QCL was locked over ~20 MHz. QCL voltage modulation was monitored for different emitter modulation frequencies. Locking experiment was performed at different injected signal strengths and QCL biases. QCL emission frequency was monitored at the injection locked frequency and Fabry-Perot modes.