Terahertz microspectroscopy: far-field spectral fidelity degradation and recovery

Terahertz near-field microspectroscopy is an emerging technique essential for characterization of novel materials and biomolecules. A popular technique utilizes a subwavelength aperture and a detector placed in the far-field. Growing interest in this technique is due to the advent of high-power frequency tunable THz sources and high sensitivity room temperature THz detectors. Severe limitations are found with resulting spectral artifacts arising from diffraction, sample geometry, aperture size, and nearby resonances. We model transmission of a focused 400 μm diameter Gaussian beam through c-cut single crystal sucrose, with a well-defined resonance at 1.985 THz, mounted on 150 μm and 200 μm diameter apertures using HFSS. Modeling is validated with Beer’s Law with respect to sample thickness. Spectral fidelity is found to deteriorate beyond ~750 μm from the aperture, illustrating the need for near-field detection. We find that spectral fidelity may extended to the far-field with a low-loss THz waveguide that may facilitate a thermally-sensitive THz detector. This work is anticipated to be of great interest to a broad community in which far-field detection is commonly used and value to the growing interest in the design and development of compact THz microspectroscopy instruments.