Terahertz measurements on subwavelength-size samples down to the tunneling limit

TL;DR

This study investigates how sample size affects terahertz spectroscopy accuracy, especially phase data, using waveguide models to analyze subwavelength samples down to the tunneling limit, with implications for precise optical property measurements.

Contribution

It introduces a waveguide-based approach to analyze terahertz measurements on very small samples, extending the usable frequency range and improving data interpretation near the tunneling limit.

Findings

Phase data can be reliably analyzed down to 10 cm$^{-1}$ for 0.2 mm apertures.

Cylindrical apertures exhibit a cut-off frequency around 29 cm$^{-1}$, affecting wave propagation.

Mounting samples on conical bores mitigates cut-off effects, enhancing measurement accuracy.

Abstract

For terahertz spectroscopy on single crystals, the wavelength $\lambda$ often is comparable to the size of the studied samples, emphasizing diffraction effects. Using a continuous-wave terahertz spectrometer in transmission geometry, we address the effect of the sample size on the achievable accuracy of the optical properties, focusing in particular on the phase data. We employ $\alpha$-lactose monohydrate as a paradigmatic example and compare data that were measured using apertures with diameters D in the range from 10 mm to 0.2 mm. For small D, strong diffraction typically invalidates a quantitative analysis of the transmitted amplitude at low frequencies. The phase data, however, can be evaluated to lower frequency and show a more systematic dependence on D. For a quantitative analysis, we employ a waveguide picture for the description of small apertures with a cylindrical bore. For D as small as 0.2 mm, corresponding to 1/D = 50 cm$^{-1}$, a circular waveguide does not support propagating waves below its cut-off frequency $1/\lambda_c = \omega_c/2\pi c \approx 29$ cm$^{-1}$. Experimentally, we confirm this cut-off for cylindrical apertures with a thickness $d_{ap} = 1$ mm. Close to $\omega_c$, the measured phase velocity is an order of magnitude larger than $c$, the speed of light in vacuum. The cut-off is washed out if a sample is mounted on a thin aperture with a conical bore. In this case, the phase data of $\alpha$-lactose monohydrate for D = 0.2 mm can quantitatively be described down to about 10 cm$^{-1}$ if the waveguide-like properties of the aperture are taken into account in the analysis.