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Strong exciton–photon coupling results in the formation of hybrid light–matter states whose relaxation dynamics often experience a bottleneck due to a large manifold of dark states. This bottleneck results in dynamics like those of the uncoupled material, hampering the observation and utilization of polaritonic properties. A possible solution to this problem is to use cavities with fewer dark states, suggesting a need for an increased understanding of the influence of the cavity platform on the polariton photophysics. Here, we use pump-probe spectroscopy to compare Fabry–Perot cavities and plasmonic nanorods coupled under identical conditions and analyze how the cavity influences relaxation kinetics. Fabry–Perot cavities host a large number of dark states, leading to dynamics fully dominated by the bottleneck. However, the relaxation from polaritonic states in nanorod systems is distinctly different compared to the uncoupled materials, with a sub-100 fs decay that dominates the kinetics. This fast relaxation emerges only when selectively exciting within the polaritonic region: off-resonance excitation yields plasmonic hot-carrier dynamics. These findings show that the cavity critically defines polariton relaxation. Moreover, this work suggests that plasmonic cavities can operate as either polaritons or plasmons, offering tunable functionality for applications in, for instance, polaritonic chemistry.

Phonon engineering has improved charge transport in semiconducting organic molecules by introducing high mass side chains, which ameliorate harmful transient localization. The influence of these side chains on thermal energy transfer and dynamic disorder has not been fully explored. In this work, we first use low temperature X-ray diffraction to probe thermally induced structural changes in functionalized acene molecular semiconductors, combined with low temperature IR spectroscopy to track changes to their vibrational energy landscape. Furthermore, time-resolved IR pump–IR probe spectroscopy is employed to measure IR absorption kinetics associated with the high mass side chains of molecules, revealing intramolecular vibrational energy redistribution pathways. Alkyne groups in the side chains are shown to act as vibrational energy traps, remaining hot for time scales of >2 ns. The results reveal nonequilibrium vibrational pathways associated with side chains that may influence the phonon manifold relevant to dynamic disorder.

Generalized ellipsometry can uncover the optical properties of anisotropic materials, in which the light–matter interaction alters the polarization state. In the terahertz frequency range, generalized ellipsometry has been infrequently realized due to the challenge of rapidly controlling and measuring THz polarization. Here, we report the development and calibration of a high-efficiency terahertz time-domain generalized ellipsometer based on two dual-channel photoconductive antennas. The fiber-coupled multi-pixel devices act as source and detector and achieve a high data throughput with four independent terahertz pulses in a single optical delay scan, without the need for any bulky or slow polarization elements such as polarizers. Following a one-off system calibration, an accurate optical characterization of a uniaxially birefringent Al₂O₃ crystal serves to validate the method. Our technique benefits from efficient data throughput, full polarization-resolved capability, and reduced system complexity, paving the way to in situ and real-time monitoring applications.

Terahertz (THz) sensing has gained significant attention as a non-ionizing modality capable of probing the superficial layers of biological tissue with high sensitivity to water content, structural changes, and biochemical composition. Its strong interaction with water makes it uniquely suited for investigating dermatological applications such as hydration assessment, wound and burn monitoring, and the detection and characterization of skin cancers. However, the shallow penetration depth of THz light, combined with the sensitivity of measurements to probe-skin coupling, contact pressure, and motion artifacts, poses persistent challenges for in vivo use. Overcoming these limitations requires carefully engineered hardware, robust measurement protocols, and advanced computational techniques tailored to biological variability. This article reviews the evolution of THz instrumentation for in vivo skin sensing, spanning early laboratory systems to emerging compact, handheld, robotic, and computationally enhanced platforms. Key developments in probe miniaturization, ergonomic design, and automated control of probe orientation and pressure are described, as these advances directly influence measurement reproducibility and clinical usability. We highlight recent progress in single-pixel imaging, which offers video-rate capabilities, the emergence of waveguide-integrated metamaterial sensors that push spatial resolution far below the diffraction limit, and THz ellipsometry that gives complementary contrast. In addition, we discuss the growing role of data-driven processing, compressed-sensing algorithms, and biophysical modeling in extracting clinically relevant parameters from THz measurements. Although promising, many of the emerging systems remain untested in in vivo dermatological scenarios, and challenges including motion artifacts, calibration stability, and inter-subject variability must be addressed. Continued interdisciplinary work between engineers, physicists, and clinicians is essential for transitioning THz systems toward practical, patient-compliant diagnostic tools.