Investigations on Insb Plasmonic Devices for Sensor Applications at Terahertz Frequencies

Abstract

Rapid advances in the field of Plasmonics over the last two decades have led to several practical nano-photonic devices and were largely confined to devices utilizing visible light. Recently, there has been growing interest in plasmonic devices operating at terahertz (THz) frequencies. The interest arose due to its potential applications in detecting minute quantities of certain materials by employing the techniques of THz spectroscopy. Many materials show significant absorption of energy in the THz frequency range, thus enabling their detection by transmission spectroscopy. Further, the fabrication of efficient sources and detectors at THz frequencies has led to development of the technique of THz Time – Domain Spectroscopy (THz-TDS), which has been employed to characterize devices such as THz waveguides, antennas, resonators and filters. Developments in THz devices have attracted considerable attention on THz sensing in the field of biochemistry and medicine. In the effort to develop a highly promising and sensitive THz chemical sensor, this work focuses on experimentally and theoretically investigating the transmission characteristics and hence, the sensing capabilities, of a Semiconductor–Insulator–Semiconductor (SIS) THz waveguide device with stubs. The stubs function as resonant cavities and the device itself functions as a narrow band filter. The devices were made using intrinsic Indium Antimonide (InSb), which is a promising material for THz plasmonics. The resonant transmission characteristics of the device at THz frequencies are exploited for sensing applications. The transmission characteristics of the waveguide device were simulated using finite element method techniques for various sizes of waveguide width and stub length. The waveguide devices were fabricated by laser micromachining and their transmission characteristics were measured by THzTDS. The experimental results are consistent with the simulation results. The stubs of the device were loaded with Bovine Serum Albumin (BSA) protein molecules and polystyrene dissolved in toluene. Significant change in the THz transmission and absorption coefficient was observed for different concentrations of BSA and polystyrene. Consequent change in the refractive index in the stubs alters the transmitted signal intensity. Results show that, a change in concentration of material loaded in the stub even by 1mol/L, leads to measurable change in the transmission coefficient of the device. Thus, InSb plasmonic waveguide device operating at THz frequencies shows promising potential as a good material sensor.