Ultrafast & Terahertz Photonics Group

In this work, we demonstrate significant modifications to our robotically controlled terahertz (THz) sensing system, the “PicoBot,” enabling it to perform in vivo imaging of skin rather than limiting it to single-point measurements. By integrating a robotic arm equipped with force-sensitive feedback control, we maintain consistent contact pressure between the probe and the skin surface throughout imaging. In conjunction with this hardware advancement, we introduce an accompanying image analysis pipeline that reduces noise and enhances repeatability across scans. These improvements allow for reliable intra- and inter-subject comparisons, a critical step toward the clinical utility of THz imaging. Our ultimate aim is to use THz imaging to detect skin cancer margins: this paper highlights progress towards this goal and skin evaluation in general.

Electro-optic sampling allows the electric field of THz, mid-infrared and visible light pulses to be measured directly as a function of time, with data analysis often performed in the frequency domain after fast Fourier transform. Here, we review aspects of Fourier theory relevant to the frequency-domain analysis of light pulses recorded in the time domain. We describe a “best practise” approach to using the discrete Fourier transform that ensures consistency with analytical results from the continuous Fourier transform. We summarise a phenomenological time-domain model of THz pulses, based on carrier and envelope waves, and show that it can reproduce a wide variety of experimental single- to multi-cycle THz pulses, with exemplary data from lab-based sources (photoconductive antennae, optical rectification, spintronic emitters) and a THz free-electron laser. A quantitative comparison of the spectral energy density of these distinct sources is enabled by the amplitude-accurate discrete Fourier transform. We describe a method that ensures the accurate calculation of the absolute spectral phase (valid for arbitrary sampling windows in the time domain) and summarise how the carrier-envelope phase, pulse arrival time, and chirp can be obtained from the phase. Our aim with this overview of THz pulse analysis is to highlight algorithms and concepts that are useful to newcomers to time-domain spectroscopy and experts, alike.

The dynamics of the drying process of polymer latexes after casting as a wet film onto a substrate are important to track as they influence the physical and mechanical properties and performance of the dried polymer films. Current methods used to follow this drying process include gravimetric analysis, coupled with advanced techniques like GARField-NMR or optical coherence tomography. The latter two methods provide height and spatial information in the z-direction, normal to the substrate, and occasionally in the xz- or yz-planes. Terahertz time-domain spectroscopy (THz-TDS) is a welcome addition as it provides both the structural and spectroscopic information in the parallel xy-plane, filling the geometric gap. Herein, we utilize THz-TDS to study the drying and film formation process of various polymer latexes with a broad range of glass transition temperatures. We showcase the applicability of this technique in obtaining 2D parallel hydration maps of the drying dispersions, in the form of droplets, using latex-dependent calibration lines. Our findings display known phenomena arising from the drying of the colloidal dispersions.

Improved knowledge of the influence of temperature upon layered perovskites is essential to enable perovskite-based devices to operate over a broad temperature range and to elucidate the impact of structural changes upon the optoelectronic properties. We examined the Ruddlesden–Popper layered perovskite 2-thiophenemethylammonium lead iodide (ThMA2PbI4) and observed a structural phase transition between a high- and a low-temperature phase at 220 K using temperature-dependent X-ray diffraction, UV–visible absorption, and photoluminescence (PL) spectroscopy. The structural phase transition altered the tilt pattern of the inorganic octahedra layer, modifying the absorption and PL spectra. Further, we found a narrow and intense additional PL peak in the low-temperature phase, which we assigned to radiative emission from a defect-bound exciton state. In both phases we determined the thermal expansion coefficient and found values similar to those of cubic 3D perovskites, i.e., larger than those of typical substrates such as glass. These results demonstrate that the organic spacer plays a critical role in controlling the temperature-dependent structural and optoelectronic properties of layered perovskites and suggests more widely that strain management strategies may be needed to fully utilize layered perovskites in device applications.