Thickness-Controlled Ti Optical Filters Designed for Tunable Terahertz Technology

Abstract

Herein, titanium (Ti) thin films deposited by a vacuum ion coating technique under a high vacuum pressure of 10 5 mbar are designed to perform as optical filters operatives in the terahertz frequency domain. The performance of these amorphous optical filters is controlled by varying the filter thickness in the range of 100–500 nm. Increasing the thickness of the filters decreases the optical transmittance, the absorption coefficient, the optical conductivity (σ), the terahertz cutoff frequency ( fT), the drift mobility, and the free carrier concentration. The thickness-based bulk limit of these filters is ≈300 nm. On the other hand, the increase in the optical filter thickness results in an increase the dielectric constant. Drude–Lorentz analyses on σ of the filters have shown the domination of four oscillators with oscillator frequencies of 1016, 859, 508, and 317 THz. These oscillators guarantee the good response of the Ti optical filters to optical excitations in the infrared, visible, and UV spectral domains. Based on the filter thickness, an extremely wide range of fT extending from ≈3.8 to 1248 THz can be achieved. The wide tunability of fT and σ of the Ti optical filters makes them promising for terahertz applications.