Governing limits of spatial resolution and spectral bandwidth for implementations of terahertz spectroscopy on the subwavelength scale

Abstract

In this work, we consider the governing limits of spatial resolution and spectral bandwidth in pursuing implementations of terahertz (THz) spectroscopy on the subwavelength scale. We discuss the need for effective focusing elements in driving sufficient levels of power from the (macroscopic) incident THz beam down to the (microscopic) focal spot. Such elements, when effective, enable large signal strengths and wide bandwidths, but this has proven to be challenging in contemporary implementations of near-field THz imaging and spectroscopy. To this end, we show theoretical and experimental results for focusing via parabolic mirrors, high-resolution lenses, and engineered dielectric spheres, with the latter yielding THz microjets with especially intense and small focal spots. We then discuss the need for near-field spatial constriction, to drive the spatial resolution down to an even smaller scale, and show that this constriction can lead to dispersive (i.e., frequencydependent) characteristics. In this work, we demonstrate spatial constriction via simple circular apertures, which function as high-pass filters. Ultimately, our theoretical and experimental results reveal that implementations of THz spectroscopy on the subwavelength scale are governed by a spatial-spectral product-whereby reductions in the aperture's diameter (to improve the spatial resolution) raise the aperture's cutoff frequency (at the expense of spectral bandwidth).