MEMS-Tunable VCSELs-Driven Terahertz Emitters Based on Conventional and Nanoelectrode Photomixers

Abstract

Continuous-wave (CW) terahertz (THz) photomixing setups require compact, widely tunable mode-hop-free driving lasers. This thesis reports, for the first time, on the performance of surface micromachined single-mode microelectromechanical system (MEMS)-tunable vertical-cavity surface-emitting lasers (VCSELs) around 1550 nm for application in a tunable THz CW photomixing system. An electrothermally tunable MEMS-VCSEL featuring a tuning range of 64 nm (7.92 THz) with a side-mode-suppression-ratio of > 50 dB over the entire tuning range is optically heterodyned with a distributed feedback (DFB) laser diode to drive the photomixing setup. The achieved THz bandwidth of 4.74 THz is yet only limited by the employed photomixers. This thesis addresses a systematic characterization of MEMS-VCSELs with a particular focus on the parameters relevant for THz photomixing, such as tunability, tuning speed, linewidth, polarization and wavelength stability. MEMSVCSELs have excellent polarization-suppression-ratio of > 20 dB and a linewidth of < 50 MHz, respectively over the entire tuning range. Due to the suspended structure of the MEMS-tunable mirror, MEMS-VCSELs show wavelength drifts and fluctuations. A wavelength control circuit compensates the slow drifts while the fast fluctuations show a long-time-scale bandwidth of < 350 MHz which is still sufficient for THz photomixing. The fluctuations are functions of the mechanical properties of the tunable mirror and analyzed in this thesis. The small active volume of VCSELs results in a low output power of few milliwatts. This thesis investigates the development of nanoelectrode-based CW photomixers with high optical to THz conversion efficiency. These devices have an order of magnitude lower capacitance to reduce the losses and improve the output power as compared to conventional interdigitated electrode structures. The photomixers have potential to produce sufficient photocurrent when VCSELs intensely illuminate the 1 µm–2 µm electrode gaps through lens systems. We first utilize dielectrophoresis technique to align silver nanowires as nanoelectrodes to fabricate the photomixers at 850 nm and 1550 nm using corresponding substrate materials. While DC characteristics look promising, so far no THz signal is generated due to the failure of the unprotected nanowires. The thesis concludes with a detailed analysis of the failure mechanisms with prospective solutions.