A dual-beam THz spectrometer is reported that substantially reduces the influence of systematic errors in THz time-domain spectroscopy such as those caused by variations in femtosecond laser power or the environmental temperature and humidity. Dual THz beams with single-cycle waveforms were generated simultaneously using a dual-pixel interdigitated photoconductive antenna, allowing the simultaneous acquisition of sample and reference data in the spectrometer using the same optical components. A low-aberration optical geometry ensured diffraction-limited spatial profiles for both beams despite their off-axis propagation and was validated experimentally by measuring frequency-dependent beam profiles and theoretically via physical optics calculations. Although the experimental amplitudes and absolute phase spectra of both beams were very similar, we further provided a correction procedure to eliminate these small differences. The robustness of the dual-beam spectrometer design was evaluated by measuring the complex transmission of a thin plastic sheet after intentionally introducing a change in the relative humidity of the THz beam path. The dual-beam THz spectrometer was effective at removing systematic errors in the amplitude and phase by simultaneously measuring the two THz beams under the same conditions.

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.

Transdermal drug delivery patches are a good alternative to hypodermic drug injection. The drug delivery efficiency depends strongly on the hydration of the skin under treatment, and therefore, it is essential to study the effects on the skin induced by the application of these medical-grade patches. Terahertz (THz) spectroscopy shows great promise for non-invasive skin evaluation due to its high sensitivity to subtle changes in water content, low power and non-ionizing properties. In this work, we study the effects of transdermal drug delivery patches (three fully occlusive and three partially occlusive) applied on the upper arms of ten volunteers for a maximum period of 28 h. Three different levels of propylene glycol (0 %, 3 % and 6 %) are added to the patches as excipient. By performing multilayer analysis, we successfully retrieve the water content of the stratum corneum (SC) which is the outermost layer of skin, as well as its thickness at different times before and after applying the patches. This study demonstrates the potential of using THz sensing for non invasive skin monitoring and has wide applications for skin evaluation as well as the development of skin products.

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.

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.