Dielectric constants give significant information on how materials behave and relax under the influence of electromagnetic fields. N. Matsumoto and T. Hosokura from the Murata Manu-facturing Co. Ltd. and T. Nagashima and M. Hangyo from Osaka University have developed reflection-type time-domain terahertz spectrosopic ellipsometry (THz-TDSE). Reflection-type spectroscopy is a contactless technique used for high-absorption materials and thin films deposited on opaque substrates, and time-domain spectroscopy provides transmittance/reflectance as well as phase shift information. The accuracy of time-domain spectroscopy is limited by phase shift phenomena. The researchers implemented THz-TDSE, eliminating previous reference and phase shift problems, thus allowing for accurate, contactless, and nondestructive in situ measurements of complex dielectric constants in the THz range.

In the January 15th issue of Optics Letters (DOI: 10.1364/OL.36.000265; p. 265), the researchers report development of a setup that allows accurate measurements of the dielectric constants in the THz range of different materials, namely GaAs thin films and SrTiO₃ on Pt substrate. In the setup, THz waves are generated by irradiation of a photoconductive antenna (on low-T grown GaAs) by a pulsed 800 nm laser, and focused on the sample on a ~20 mm² area.

The polarizations of incident and emerging THz waves are controlled precisely in order to detect p- and s-waves independently. The detection of the reflected wave is done by another photoconductive antenna irradiated by the delayed probe laser beam. The photocurrent of the antenna is measured through a lock-in amplifier. The time domain waveforms of p- and s-polarized reflected waves are measured independently, and Fourier-transformed into complex reflection coefficients.

Since the optical path lengths of the s- and p-polarized waves are strictly equal, the phase shift information between the p- and s-polarization can be obtained precisely. Reflection coefficients are used to compute real and imaginary parts of the dielectric constant. Interferences arising from the different interfaces were taken into account and this technique permitted measurement of the dielectric constant for GaAs thin film, as well as soft-mode phonon dispersion spectra in SrTiO₃ thin films.