Metrology of complex refractive index for solids in the terahertz regime using frequency domain spectroscopy

Abstract

Frequency domain spectroscopy allows an experimenter to establish optical properties of solids in a wide frequency band including the technically challenging 10 THz region, and in other bands enabling metrological comparison between competing techniques. We advance a method for extracting the optical properties of high-index solids using only transmission-mode frequency domain spectroscopy of plane-parallel Fabry-Perot optical flats. We show that different data processing techniques yield different kinds of systematic error, and that some commonly used techniques have inherent systematic errors which are underappreciated. We use model datasets to cross-compare algorithms in isolation from experimental errors, and propose a new algorithm which has qualitatively different systematic errors to its competitors. We show that our proposal is more robust to experimental nonidealities such as noise or apodization, and extract the complex refractive index spectrum of crystalline silicon as a practical example. Finally, we advance the idea that algorithms are complementary rather than competitive, and should be used together as part of a toolbox for better metrology.

Metrology of complex refractive index for solids in the terahertz regime using frequency domain spectroscopy

“…This kind of methodology can allow future intercomparisons between algorithms using a standardised test which allows for comparison to exact analytic results. To ensure that fair comparisons can be drawn and our conclusions replicated, our analysis tools, data, and code for each technique discussed in the paper have been published under the GNU general license [29].…”

“…1. Our model data can be replicated trivially using our code published under the GNU general license [29]. The dataset shown in Figure 1 will be used for cross-comparison of techniques throughout this Theory section.…”

“…Although extracting ( ) is beyond the scope of this paper, we anticipate applications of this feature of the algorithm to study imperfections in samples over large surfaces or volumes. All of our data and analysis techniques are published for free under the GNU general license [29].…”

“…The central data for our paper and our code analysis tools are available to the public from a permanent data repository [29].…”