Noise Reduction in THz-TDS Dielectric Characterization of Thin Films with THz Interferometry

Abstract

The purpose of this study is to fully explore the nature of noise contributions in terahertz time domain spectroscopy systems when used to analyze dielectric properties of materials. Furthermore, the noise-reduction mechanisms are explored using THz interferometric TDS using a Michelson type interferometer. Limits of detection for this interferometer are measured first with a parallel-faced dielectric slab which has a simplistic analytical model. Other characterization measurements were done by photo-induced refractive index changes according to the Drude model. Measurements were performed using sequential scan TDS, differential TDS, and interferometric TDS. Finally a novel interferometric design is introduced based on constructive interference which eliminates beam asymmetry issues associated with destructive interference designs. The most significant noise contribution in THz TDS measurement systems arises from the THz transmitter due to the limited stability of the excitation laser source. In sequential scan TDS the noise component of the sample and reference scans are uncorrelated and add to the total noise floor of the system. THz interferometry has been found to reduce the overall measurement noise in a THz-TDS dielectric characterization measurement by for thin films. The primary reason is the correlation of the noise between the reference and sample signals, canceling out background fluctuation pulse-by-pulse. Using the tilted slab experiment an optical path difference of 1/700 pulse coherence length between propagation through sample and reference has been measured. Results from photo-induced index change of a semi-conducting wafer show an optical path length difference of 1/1250 with good extraction of index difference between dark and illuminated semi-conductor. The findings compare well with that of differential TDS and also confirm improvements over standard THz TDS.