Off-axis parabolic mirrors (OAPMs) are widely used in the THz and mm-wave communities for spectroscopy and imaging applications, as a result of their broadband, low-loss operation and high numerical apertures. However, the aspherical shape of an OAPM creates significant geometric aberrations: these make achieving diffraction-limited performance a challenge, and lower the peak electric field strength in the focal plane. Here, we quantify the impact of geometric aberrations on the performance of the most widely used spectrometer designs, by using ray tracing and physical optics calculations to investigate whether diffraction-limited performance can be achieved in both the sample and the detector plane. We identify simple rules, based on marginal ray propagation, that allow spectrometers to be designed that are more robust to misalignment errors, and which have minimal aberrations for THz beams. For a given source, this allows the design of optical paths that give the smallest THz beam focal spot, with the highest THz electric field strength possible. This is desirable for improved THz imaging, for better signal-to-noise ratios in linear THz spectroscopy and optical-pump THz-probe spectroscopy, and to achieve higher electric field strengths in non-linear THz spectroscopy.

We report on the emission of high-intensity pulsed terahertz radiation from the metal-free halide perovskite single crystal methyl-DABCO ammonium iodide (MDNI) under femtosecond illumination. The power and angular dependence of the THz output implicate optical rectification of the 800 nm pump as the mechanism of THz generation. Further characterization finds that, for certain crystal orientations, the angular dependence of THz emission is modulated by phonon resonances attributable to the motion of the methyl-DABCO moiety. At maximum, the THz emission spectrum of MDNI is free from significant phonon resonances, resulting in THz pulses with a temporal width of <900 fs and a peak-to-peak electric field strength of approximately 0.8 kV cm–1─2 orders of magnitude higher than any other reported halide perovskite emitters. Our results point toward metal-free perovskites as a promising new class of THz emitters that brings to bear many of the advantages enjoyed by other halide perovskite materials. In particular, the broad tunability of optoelectronic properties and ease of fabrication of perovskite materials opens up the possibility of further optimizing the THz emission properties within this material class.

Light-emitting diodes (LEDs) are ubiquitous in modern society, with applications spanning from lighting and displays to medical diagnostics and data communications. Metal-halide perovskites are promising materials for LEDs because of their excellent optoelectronic properties and solution processability. Although research has progressed substantially in optimizing their external quantum efficiency, the modulation characteristics of perovskite LEDs remain unclear. Here we report a holistic approach for realizing fast perovskite photonic sources on silicon based on tailoring alkylammonium cations in perovskite systems. We reveal the recombination behaviour of charged species at various carrier density regimes relevant for their modulation performance. By integrating a Fabry–Pérot microcavity on silicon, we demonstrate perovskite devices with efficient light outcoupling. We achieve device modulation bandwidths of up to 42.6 MHz and data rates above 50 Mbps, with further analysis suggesting that the bandwidth may exceed gigahertz levels. The principles developed here will support the development of perovskite light sources for next-generation data-communication architectures. The demonstration of solution-processed perovskite emitters on silicon substrates also opens up the possibility of integration with micro-electronics platforms.

Transdermal drug delivery (TDD) has been widely used in medical treatments due to various advantages, including delivering drugs at a consistent rate. However, variations in skin hydration can have a significant effect on the permeability of chemicals. Therefore, it is essential to study the changes in skin hydration induced by TDD patches for better control of the delivery rate. In this work, in vivo terahertz (THz) spectroscopy is conducted to quantitatively monitor human skin after the application of patches with different backing materials and propylene glycol concentrations. Changes in skin hydration and skin response to occlusion induced by other patches are investigated and compared. Our work demonstrates the potential application of in vivo THz measurements in label-free, non-invasive evaluation of transdermal patches on human skin and further reveals the mechanism behind the effect.

Optical pump terahertz probe spectroscopy (OPTP) is a versatile non-contact technique that measures transient photoconductance decays with femtosecond temporal resolution. However, its maximum temporal range is limited to only a few nanoseconds by the mechanical delay lines used. We extended the temporal range of OPTP to milliseconds and longer while retaining sub-nanosecond resolution. A separate pump laser was electrically synchronized to the probe pulses, allowing the pump–probe delay to be controlled with an electronic delay generator. We demonstrated the capabilities of this technique by examining the photoconductance decays of semiconductors with lifetimes ranging over six orders of magnitude: III-Vs, metal halide perovskites, germanium, and silicon. A direct comparison of results on silicon from OPTP and inductively coupled photoconductance decay highlighted the higher spatial and temporal resolution of OPTP, which allowed in-plane and out-of-plane carrier diffusion to be studied.

The perturbed free induction decay (PFID) observed in ultrafast infrared spectroscopy was used to unveil the rates at which different vibrational modes of the same atomic-scale defect can interact with their environment. The N₃VH⁰ defect in diamond provided a model system, allowing a comparison of stretch and bend vibrational modes within different crystal lattice environments. The observed bend mode (first overtone) exhibited dephasing times T₂=2.8(1)  ps, while the fundamental stretch mode had surprisingly faster dynamics T₂<1.7  ps driven by its more direct perturbation of the crystal lattice, with increased phonon coupling. Further, at high defect concentrations the stretch mode’s dephasing rate was enhanced. The ability to reliably measure T₂ via PFID provides vital insights into how vibrational systems interact with their local environment.