Fabrication and Characterization of Terahertz Plasmonic Photoconductive antennas

Abstract

A terahertz (THz) wave is an electromagnetic wave with a frequency range of 0.1 to 10 THz located between infrared and millimeter waves. This THz wave has attracted the attention of many researchers in various fields of science and industry because of the merits of its nonionizing radiation properties, unique spectral signatures of molecules, and ability to penetrate non-polar materials. In contrast, the THz frequency band has not been fully explored and thus has considerable potential for use in many applications due to the exotic properties of THz electromagnetic waves. With the development of femtosecond lasers, THz time-domain spectroscopy (THz-TDS) was realized in 1989 by Grischkowsky et al. THz-TDS has become an important technology for non-destructive materials testing, and medical diagnostics. Photoconductive sources and detectors are most commonly utilized in THz-TDS and provide a sufficient dynamic range over 1 THz. Photoconductive antennas (PCAs) consist of a couple of metallic antennas and a photo-absorbing semiconductor. A femtosecond laser optical pulse illuminates the area between two antenna electrodes, and the photo-generated carriers are separated by the electric field established between the antenna gap via external bias voltage. An ultrafast photocurrent is fed to the antenna, and electromagnetic THz pulses radiate into free space. In this doctoral thesis, we improved the performance of the PCAs by applying nano-plasmonic photonics. We took their commercial feasibility one step further by applying a micro cylindrical lens array (CLA) to increase their value as a terahertz source. Firstly, nanoscale plasmonic grating structures were realized between the electrodes of the large area PCA. The intensity of the terahertz radiation from the plasmonic PCA was enhanced two-fold compared with that from the PCA without the plasmonic grating structure. By using an electrically isolated nano-grating structure, the performance enhancement was found to be due to the plasmonic effect other than bias field enhancement. The photo-absorption enhancement near the GaAs surface and the plasmonic grating interface was investigated to elucidate the terahertz radiation enhancement mechanism in the plasmonic PCA. The highly localized electric field near the nanoscale grating shows optical absorption at the GaAs surface was enhanced. The enhanced optical absorption increased the photo-generated carrier density at the device surface, leading to enhanced intensity and bandwidth of the terahertz (THz) radiation from the plasmonic PCA. Secondly, the cylindrical lens array (CLA) structure is employed to maximize the efficiency of an interdigitated PCA. In conventional interdigitated PCAs, the shadow mask was an essential structure to enable the unidirectional carrier acceleration. An even-numbered electrode gap in the interdigitated structure is masked and only the odd-numbered electrode gaps are utilized to absorb the incident optical pump beam. Consequently, the destructive interference in the far field can be suppressed. However, due to the 75% metallic surface and high reflectivity of the GaAs surface, only under 25% of the optical pump beam was used for the THz generation. The CLA structure can substitute for the shadow mask and use over 80% of the incident optical pump beam for the THz generation. At an optical pump power of 80 mW and a bias voltage of 10 V, THz radiation enhancement was measured to be 558% by the effect of the CLA and plasmonic nano-grating.