Periodically-poled lithium niobate (PPLN) sources consisting of custom-built stacks of large-area wafers provide a unique opportunity to systematically study the multi-cycle terahertz (THz) generation mechanism as they are assembled layer-by-layer. Here we investigate and optimize the THz emission from PPLN wafer stacks as a function of wafer number, pump fluence, pulse duration and chirp, wafer separation, and pump focusing. Using 135 µm-thick, 2"-diameter wafers we generate high-energy, narrowband THz pulses with central frequencies up to 0.39 THz, directly suitable for THz-driven particle acceleration applications. We explore the multi-cycle pulse build-up with increasing wafer numbers using electro-optic sampling measurements, achieving THz conversion efficiencies up to 0.17%, while demonstrating unique control over the pulse length and bandwidth these sources offer. Guided by simulations, observed frequency-dependence on both stack-mounting and pump focusing conditions have been attributed to inter-wafer etalon and Gouy phase-shifts respectively, revealing subtle features that are critical to the understanding and performance of PPLN wafer-stack sources for optimal narrowband THz generation.

Full control of the ellipticity of broadband pulses of THz radiation, from linear to left- or right-handed circular polarization, was demonstrated via a four-pixel photoconductive emitter with an integrated achromatic waveplate. Excellent polarization purity and accuracy were achieved, with Stokes parameters exceeding 97% for linear and circular polarization, via a robust scheme that corrected electrically for polarization changes caused by imperfect optical elements. Furthermore, to assess the speed and precision of measurements of the THz polarization, we introduced a figure of merit, the standard error after one second of measurement, found to be 0.047° for the polarization angle.

We report on the generation of high-power narrow-bandwidth terahertz (THz) pulses by cryogenic cooling of hand-made periodically poled lithium niobate (PPLN) wafer stacks. As a proof-of-concept, we cool stacks with up to 48 wafers down to 97 K and achieve few-percent bandwidths at a center frequency of 0.39 THz, with pulse energy up to 0.42 mJ and average power of 21 mW. Supported by modeling, we observe effective cooling of PPLN wafer stacks that not only reduces terahertz absorption but critically maintains the micrometer-scale inter-wafer gaps for optimal terahertz transmission. Our results unlock the potential for scaling these large-area sources to greater numbers of wafers to push both the energy and bandwidth beyond current capability, opening up possibilities in areas such as terahertz-driven particle acceleration, terahertz imaging, and control over material properties.

We demonstrate that asynchronous optical sampling (ASOPS) can be used to measure the propagation of terahertz (THz) bandwidth pulses in a coplanar waveguide device with integrated photoconductive switches used for signal excitation and detection. We assess the performance of the ASOPS technique as a function of measurement duration, showing the ability to acquire full THz time-domain traces at rates up to 100 Hz. We observe a peak dynamic range of 40 dB for the shortest measurement duration of 10 ms, increasing to 88 dB with a measurement time of 500 s. Our work opens a route to real-time video-rate imaging via modalities using scanned THz waveguides, as well as real-time THz sensing of small volume analytes; we benchmark our on-chip ASOPS measurements against previously published simulations of scanning THz sensor devices, demonstrating sufficient dynamic range to underpin future video-rate THz spectroscopy measurements with these devices.

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.

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.