Metrics for spectral and spatial performance in terahertz spectroscopic imaging

In this work, we investigate the spectral and spatial performance of coupled terahertz microjets and apertures, with knowledge that the broadband material characterizations desired in spectroscopy are often at odds with the fine spatial resolutions sought in imaging. To explore this, we characterize lactose masked by apertures of varied widths across a targeted spectrum of 0.3-0.9 THz. We observe the spectra in terms of Kramers-Kronig error, with our spectral cutoff frequency as a metric of spectral performance, and modulation depth, with spatial resolution as a metric of imaging performance. Overall, we see the apertures act as high-pass frequency filters, with the spectral cutoff frequency as a dividing line between inaccurate spectra at low frequencies (due to prevailing random and systematic error) and accurate spectra at high frequencies (due to high transmission). For spectral performance, our aperture widths of 1000, 500, and 250 µm give spectral cutoff frequencies that are less than 0.3 THz, at 0.41 THz, and at 0.84 THz, respectively. For imaging performance, we see frequencies of 0.3, 0.6, and 0.9 THz give spatial resolutions of 1000, 250, and 250 µm, respectively. Ultimately, we conclude that spectroscopic and imaging performance must be considered together in realizing an effective system for terahertz spectroscopic imaging.