Development of a Terahertz Time-Domain Spectrometer Optimized at 5-8 THz and the Study of Surface Polaritons in NiO-SrTiO3 Nano-Composite Ceramics.

Abstract

In this work we develop an optimized spectroscopic instrument that generates and temporally detects terahertz waves at high frequencies and with high signal-to-noise ratio. This capability in combination with scanning electron microscopy and fourier transform infrared spectroscopy allows us to carry out an experimental investigation of a composite ceramic material made up of antiferromagnetic Nickel Oxide (NiO) with resonant nano-to-micron sized ferroelectric Strontium Titanate (STO) inclusions. This mixture holds promise for attaining bulk negative refractive index in the far-infrared wavelength range.

In particular, the experimental method we developed uses GaP to generate terahertz pulses via optical rectification in a collinear phase-matched configuration relying on the dispersion of the refractive index. The GaP-based time-domain system operates up to 8 THz and is especially well suited at high frequencies, where it has high signal-to-noise ratio and power conversion efficiency 30 times greater than those of commercial photoconductive emitters. These characteristics are demonstrated in measurements of a well-characterized material in the reflection geometry. We also discuss the power output and describe theoretically the observed THz field generation by nonlinear mixing, the field’s free space propagation, and its detection.

We fabricate the NiO-STO mixture, take reflection and transmission measurements in the range of 10-600 wavenumbers and develop a model based on the Clausius-Mosotti framework for describing the nano-particle behavior of STO inclusions in the ceramic composite. This model contrasts the proposed interpretation of volume averaged mixing based on bulk properties. The spectral features are dominated by the vibrational surface modes of the inclusion in a broad frequency region. Specifically, we see two dominant features: the Frohlich mode due to the spherically shaped inclusions as well as a broad band in the region of negative permittivity of bulk STO. Based on the analysis of the inclusion fraction, it is possible to identify the effect of clustering and information about cluster size from the spectral measurement. The Frohlich mode represents a well defined single-particle geometric resonance which opens the way for spectral engineering of metamaterial ceramics. We measure the magnetic behavior of sintered NiO which is found to agree well with bulk properties.