Terahertz time-domain spectroscopy (THz-TDS) is a powerful technique for material’s characterization and process control. It has been used for contact-free conductivity measurements of metals, semiconductors, 2D materials, and superconductors. Furthermore, THz-TDS has been used to identify chemical components such as amino acids, peptides, pharmaceuticals, and explosives, which makes it particularly valuable for fundamental science, security, and medical applications. This tutorial is intended for a reader completely new to the field of THz-TDS and presents a basic understanding of THz-TDS. Hundreds of articles and many books can be consulted after reading this tutorial. We explore the basic concepts of TDS and discuss the relationship between temporal and frequency domain information. We illustrate how THz radiation can be generated and detected, and we discuss common noise sources and limitations for THz-TDS. This tutorial concludes by discussing some common experimental scenarios and explains how THz-TDS measurements can be used to identify materials, determine complex refractive indices (phase delay and absorption), and extract conductivity.

Tutorial: An introduction to terahertz time domain spectroscopy (THz-TDS)

Abstract Surface plasmon polaritons and their localized counterparts, surface plasmons, are widely used at visible and near-infrared (near-IR) frequencies to confine, enhance, and manipulate light on the subwavelength scale. At these frequencies, surface plasmons serve as enabling mechanisms for future on-chip communications architectures, high-performance sensors, and high-resolution imaging and lithography systems. Successful implementation of plasmonics-inspired solutions at longer wavelengths, in the mid-infrared (mid-IR) frequency range, would benefit a number of highly important technologies in health- and defense-related fields that include trace-gas detection, heat-signature sensing, mimicking, and cloaking, and source and detector development. However, the body of knowledge of visible/near-IR frequency plasmonics cannot be easily transferred to the mid-IR due to the fundamentally different material response of metals in these two frequency ranges. Therefore, mid-IR plasmonic architectures for subwavelength light manipulation require both new materials and new geometries. In this work we attempt to provide a comprehensive review of recent approaches to realize nano-scale plasmonic devices and structures operating at mid-IR wavelengths. We first discuss the motivation for the development of the field of mid-IR plasmonics and the fundamental differences between plasmonics in the mid-IR and at shorter wavelengths. We then discuss early plasmonics work in the mid-IR using traditional plasmonic metals, illuminating both the impressive results of this work, as well as the challenges arising from the very different behavior of metals in the mid-IR, when compared to shorter wavelengths. Finally, we discuss the potential of new classes of mid-IR plasmonic materials, capable of mimicking the behavior of traditional metals at shorter wavelengths, and allowing for true subwavelength, and ultimately, nano-scale confinement at long wavelengths.

https://www.degruyter.com/downloadpdf/j/nanoph.2013.2.issue-2/nanoph-2012-0027/nanoph-2012-0027.xml

Towards nano-scale photonics with micro-scale photons: the opportunities and challenges of mid-infrared plasmonics

We demonstrate the extraordinary transmission of terahertz THz radiation through gratings of subwavelength apertures structured in indium antimonide InSb. This transmission can be attributed to the tunneling of surface plasmons polaritons which are excited in semiconductors at THz frequencies. By thermally controlling the permittivity of the grating the transmittance increases by more than one order of magnitude. This increase might be associated to the larger the skin depth in InSb at low temperatures, nwhich gives rise to a larger effective size of the apertures.

/pdf/transmission-of-thz-radiation-through-insb-gratings-of-3h2snuy6pb.pdf

Transmission of THz radiation through InSb gratings of subwavelength apertures

We present reflection THz-time domain spectroscopy measurements of the complex conductivity of n-type, 0.038 Ω cm GaAs and n-type, 0.22 Ω cm Si wafers. These measurements clearly demonstrate the efficacy of the reflection technique on highly conductive, optically dense samples and approach the precision of THz–TDS transmission measurements. Because the THz-bandwidth, reflection measurements extend beyond the carrier collision frequency, we obtain direct measures of the mobility and the carrier number density.

Characterization of optically dense, doped semiconductors by reflection THz time domain spectroscopy

We propose and demonstrate a terahertz (THz) time-domain spectroscopy combined with ellipsometry. The complex optical constants of a Si wafer with low resistivity are deduced from the measurements of the wave forms of reflected s- and p-polarized THz pulses without reference measurement. The obtained dispersion of refractive index above ∼0.2 THz shows good agreement with that predicted by the Drude theory. The complex optical constants deduced by the THz ellipsometry in the low-frequency region are strongly affected by the slight error of the ellipsometric angle originating mainly from the misalignment of the rotation angles of the polarizer and analyzer.

Measurement of complex optical constants of a highly doped Si wafer using terahertz ellipsometry

Using transient terahertz reflection spectroscopy, the far‐infrared (0.1–1.1 THz) reflection amplitude and associated phase change upon reflection from undoped InSb was measured between 80 and 260 K. Using the Drude model, the transient terahertz data were fit by adjusting the values for the electron mobility and carrier concentration. The close agreement between the data and model validates using the Drude model to describe the electronic and optical properties of InSb below 1 THz. These results suggest a possible application of transient terahertz reflection spectroscopy for measuring semiconductor properties of thick samples without surface contact.

L. A. Schlie

Papers

Transient terahertz reflection spectroscopy of undoped InSb from 0.1 to 1.1 THz