Josh Paul Robert Nixon

We demonstrate a near-field terahertz (THz) imaging modality based on asynchronous optical sampling of on-chip THz sensors, and apply it to the sub-wavelength imaging of silicone soft-tissue phantoms. The sensor consists of a planar Goubau-line waveguide that incorporates a quarter-wavelength stub band-stop resonator, with photoconductive switches integrated into the waveguide for broadband THz signal generation and detection. By raster-scanning soft-tissue phantoms through the evanescent electric field above the resonator using a robotic manipulator, we reconstruct two-dimensional THz images with spatial resolution down to 50 μm - more than one order of magnitude below the free-space wavelength at the operating frequency (355 GHz) - and with pixel acquisition times down to 100 ms. Image contrast is obtained using both frequency shifts of the resonator and changes in power transmission through the device, enabled by the broadband nature of the on-chip system. These results simultaneously establish the potential of on-chip near-field THz sensors as a platform for high-resolution imaging of soft tissue analogues, while also highlighting their potential for future applications in biomedical imaging and real-time sensing.

This paper discusses the application of on-chip terahertz (THz) filters attached to waveguides that can act as sensor elements, including for scanned imaging applications. Our work presents a comparative numerical study of several different geometries (comprising five split-ring resonator geometries and a quarter-wavelength stub resonator, the latter being well established as a sensor at THz frequencies and therefore able to act as a benchmark). We designed each structure to have a resonant frequency of 500 GHz, allowing the impact of resonator geometry on sensing performance to be isolated; the performance was quantified by assessing each design using four figures of merit: resonance quality factor, sensitivity (relative frequency shift under dielectric loading), responsivity (sensitivity weighted by resonance sharpness), and the electric field confinement area. Simulations were conducted using Ansys HFSS using the properties of a commercially available photoresist (Shipley 1813) as a dielectric load to assess performance under conditions comparable to previous experimental studies. The analysis showed that while sensitivity remained broadly similar across geometries, responsivity and quality factor differed substantially between resonators. Furthermore, the spatial distribution of the electric field and current density, particularly in rotated configurations, was found to significantly impact coupling efficiency between the resonator and transmission line. Our findings provide guidance for the general design of systems employing THz sensors while establishing a framework with which to benchmark future sensor geometries.