A Reliable Method for Removing Fabry–Perot Effect in Material Characterization With Terahertz Time-Domain Spectroscopy

Terahertz Time-Domain Spectroscopy (THz-TDS) has been a powerful tool for material characterization. The Fabry-Perot (FP) effect caused by the multiple reflections within the sample exists inherently, which makes the echoes contaminated. In the case of thin samples, different reflection signals may merge together and even the echoes corresponding to the main reflection disappears, making the characterization improper or impossible. To deal with this problem, a novel time-domain optimization method is proposed for removing the FP effect. By introducing a new term which describes the variation of the waveform in time-domain, not only the complex refractive index but also the thickness of material are obtained for a very thin silicon wafer (~100 μm) whose thickness is less than 2/3 lambda at the central frequency of the THz-TDS system. The accuracy of the algorithm is quite comparable to an iterative method, which is considered to be the most accurate one at the moment, but it is more than 100 times faster, with a better convergence performance. Hence, it is promising for rapid material scenarios such as the in-line quality control and imaging-related applications.

I. Introduction

Terahertz Time-Domain Spectroscopy (THz-TDS) has been proofed to be a powerful tool to extract sample parameters. With many molecules having vibration and rotation energy levels in the terahertz (THz) region, THz-TDS is widely applied in composite analysis [1], [2]. Certain material parameters are needed in many fields, such as the design of electromagnetic devices, measurement of biological material, etc. The material parameters can be extracted by a free-space measurement in THz-TDS, which keep the sample noncontact [3], [4].