Girish Lakhwani

Embedding molecules in optical microcavities is an emerging method of controlling excited-state processes of optoelectronic materials without synthetic modification. This has potential benefit in organic photovoltaics, where the strong light-matter coupling achieved in optical cavities could be used to assist energy transfer and charge formation, and mitigate loss pathways. Transient absorption (TA) spectroscopy is often used to gain insight into the photophysics of organic photovoltaic systems and would be an ideal technique for determining the effect of strong coupling on exciton dynamics. However, conducting TA spectroscopy on optical microcavities results in several challenges, such as excitonic signals being masked and large artifacts arising from transient changes in the cavity structure. Here, we study blend films of PTB7-Th:PC71BM cavities, which have previously been found to exhibit altered performance between noncavity and cavity photovoltaic devices. We use transfer matrix simulations to reproduce the cavity TA spectra, and show that, when artifacts are accounted for, there is no significant difference between cavity and noncavity photophysics for this photovoltaic blend.

Most applications in optoelectronics that utilize two-dimensional transition metal dichalcogenides (TMDs) are currently limited to micrometer sizes, making the scalability of these devices practically significant. This study investigates the interaction between plasmonic arrays featuring Archimedes spiral patterns and large-area MoS2 films measuring 1 cm2. Despite the presence of defect states in these large-area MoS2 monolayers, we recorded a maximum 40% contrast in photoluminescence when comparing left and right circularly polarized excitation, with a degree of polarization of 0.3 ± 0.1, considering positional variation using a wide-field excitation. We found that bringing the spirals closer together enhances the contrast of the degree of polarization in the absence of the diffractive lattice mode. Our study demonstrates the potential of large-area TMD-based devices and underscores the importance of improving or engineering material quality to achieve device performances comparable to those based on micrometer-sized TMD flakes.

A major challenge in chiral sensing technologies is the reliance on enantiopure materials to differentiate between enantiomers, as their interactions with a chiral system produce distinct detection signals. However, synthesizing and isolating enantiopure materials is often costly and time-consuming due to the difficulty of separating enantiomers. Here, we demonstrate that racemic materials can also function effectively as chiral sensors when the system exhibits non-inversion enantioselectivity. Fluorescent Cd(II) metal-organic frameworks (MOFs) were synthesised using enantiopure and racemic H8-BINOL-based ligands and their ability to enantioselectively sense aspartic acid dimethyl ester (Asp-diME) evaluated. Remarkably, the MOF synthesized from the racemic ligand retained measurable enantioselectivity, comparable to its enantiopure counterparts, highlighting its potential as a cost-effective and accessible alternative for chiral sensing. We further investigated the crystallization mechanism and found that secondary nucleation templated by the initial crystal plays a significant role in chiral induction during MOF formation, posing a challenge in obtaining a truly racemic MOF. These findings provide a promising strategy for designing chiral sensors based on racemic materials and underscore the importance of chiral induction in chiral MOF synthesis.

Non-inversion enantioselectivity, as exhibited by H ₈ -BINOL containing MOFs allows the use of non-enantiopure systems as fluorescent chiral sensors enabling a cost-effective alternative to enantiopure systems.

Semiconductor nanowires are sensitive to the polarization of light due to their one-dimensional structure and high dielectric contrast to the surrounding medium. This phenomenon enables configurations of polarization-sensitive nanoscale devices that can be potentially integrated onto a chip. Here, we demonstrate a hybrid plasmonic perovskite nanolaser that exhibits unconventional polarization dependence. Typical plasmonic nanolaser designs utilize a metallic substrate and a low-index buffer layer material. In this study, we use a birefringent CsPbBr₃ perovskite nanowire on a metal substrate separated by a thin Ta₂O₅ buffer layer, exhibiting a refractive index lower than that along the ordinary axes of the nanowire, but higher than that along the extraordinary axes. In these conditions, we experimentally show a lower lasing threshold when the incident field is orthogonally polarized, i.e., along the b-axis. This is due to stronger electric field confinement at the nanowire-buffer interface as shown in simulation when pumped by orthogonal polarized light. This polarization sensitivity is unique to the hybrid plasmonic configuration and is not observed in the photonic counterpart, such as a nanowire on a quartz substrate. Furthermore, we found that short plasmonic nanowires exhibit lower lasing thresholds in addition to a larger polarization dependence, contrary to longer plasmonic nanowires. Moreover, orthogonally polarized pumping induces a larger-emission blueshift than longitudinally polarized pumping, attributed to strong exciton-polariton interactions. This blueshift is pronounced in plasmonic nanowires with lower lasing thresholds. This polarization-sensitive plasmonic nanolaser with reduced lasing threshold has potential applications in nanophotonic integrated circuits and room-temperature perovskite polaritonics.

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters are beneficial for color pure organic light-emitting diodes (OLEDs) due to their narrow emission bandwidths. Understanding the photophysics of these materials is crucial for developing devices with better performance. In this letter, we demonstrate the excited state photophysical properties of the MR-TADF emitter OQAO(mes)₃ embedded in different host matrices with different polarities, including the OLED-relevant hosts. Transient absorption and time-resolved fluorescence experiments reveal a second lower lying triplet state in high polarity matrices, consistent with solution studies. The two triplet populations may interconvert; thus, the low-lying triplets can be partly harvested to bright singlets via reverse intersystem crossing (rISC) generating delayed fluorescence. Interestingly, we find no evidence of exciplex-like interactions in host-guest systems affecting the TADF process, contrary to previous reports.