Dr Alan Bowman

Abstract:

Thermo-optical nonlinearities (TONL) in metasurfaces enable dynamic control of optical properties—such as transmitted power, phase, and polarization—through external stimuli like laser irradiation or temperature. Due to the inherently slow thermal dynamics of extended systems, research has primarily focused on steady-state effects, as rapid modulation is typically considered challenging. In this study, photo-driven TONL is investigated in amorphous silicon (a-Si) metasurfaces under both steady-state and, more importantly, dynamic conditions using a modulated 488 nm continuous-wave pump laser. First, a non-monotonic change is observed in transmission as a function of irradiation intensity at a wavelength red-shifted by 15 nm from the electric-dipole resonance. Specifically, transmission initially decreases by 30% before increasing by 30% as the laser intensity reaches 5 mW/ (Formula presented.). Next, it is demonstrated that TONL decouple thermal and optical response times, with the optical response being up to seven times faster than the thermal response under tested conditions ((Formula presented.) (Formula presented.) vs. (Formula presented.) (Formula presented.)). Most remarkably, it is experimentally shown that the interplay of these effects enables optical modulation at twice (100 kHz) the excitation laser's modulation frequency (50 kHz). Finally, it is shown that exploiting these unique conditions allow thermo-optical metasurfaces to intrinsically encode multiple optical states within a single modulation cycle, realizing a self-modulating photonic platform. TONL thus open new avenues for engineering active metasurfaces that combine fast, high-amplitude modulation with self-modulating optical dynamics, making them promising for next-generation optical switching, dynamic holography, optical information processing, and neuromorphic computing.

Abstract:

Second-harmonic generation (SHG) is a powerful surface-specific probe for centrosymmetric materials, with broad relevance to energy and biological interfaces. Plasmonic nanomaterials have been extensively utilized to amplify this nonlinear response. Yet, material instability has constrained most studies to gold, despite the significance of plasmonic metals such as copper for catalysis. Here, we demonstrate stable and anisotropic SHG from monocrystalline copper, overcoming long-standing challenges associated with surface degradation. By leveraging an on-substrate synthesis approach that yields atomically flat and oxidation-resistant Cu microflakes, we enable reliable SHG measurements and reveal a strong cross-polarized response with C 3v surface symmetry. The SHG signal remains stable over 3 h of continuous femtosecond excitation, highlighting the remarkable optical robustness of the Cu microflakes. These results reinforce the viability of monocrystalline Cu as a robust platform for nonlinear nanophotonics and surface-sensitive spectroscopy, expanding the range of copper-based optical applications.

Abstract:

Copper is one of the most extensively studied materials for energy conversion and catalytic systems, with a wide range of other applications, from nanophotonics to biotechnology. However, existing synthesis methods are limited with many undesirable byproducts and poorly defined morphologies. Here, we report an on-substrate wet synthesis approach that yields purely metallic and monocrystalline Cu microflakes with an exposed (111) crystalline surface. By systematically studying the growth mechanism, we achieve unprecedented sizes of more than 130 μm, which is 2 orders of magnitude larger than reported in most previous studies, along with high aspect ratios of over 400. Furthermore, we show significantly higher stability against oxidation provided by the halide adlayer, which also eliminates the need for any organic surfactants in the synthesis. Overall, our facile synthesis approach delivers an exciting avenue for the emerging fields of catalysis and nanophotonics.

Abstract:

Optical filters that respond to the wavelength and direction of incident light can be used to increase the efficiency of tracking solar cells. However, as tracking solar cells are more expensive to install and maintain, it is likely that nontracking solar cells will remain the main product of the (terrestrial) solar cell industry. Here we demonstrate that directional spectral filters can also be used to increase the efficiency limit of nontracking solar cells at the equator beyond what is currently understood by up to ∼0.5% (relative ∼1.8%). We also reveal that such filters can be used to regulate the energy output of solar cells throughout a day or year, and can reduce the thickness of the absorber layer by up to 40%. We anticipate that similar gains would be seen at other latitudes. As this filter has complex wavelength-direction functionality, we present a proof-of-concept design based on Luneburg lenses, demonstrating these filters can be realized. Our results will enable solar cells with higher efficiency and more stable output while using less material.

Abstract:

Exploring nonequilibrium hot carriers from plasmonic metal nanostructures is a dynamic field in optoelectronics, with applications including photochemical reactions for solar fuel generation. The hot carrier injection mechanism and the reaction rate are highly impacted by the metal/molecule interaction. However, determining the primary type of reaction and thus the injection mechanism of hot carriers has remained elusive. In this work, we reveal an electron injection mechanism deviating from a purely outer-sphere process for the reduction of ferricyanide redox molecule in a gold/p-type gallium nitride (Au/p-GaN) photocathode system. Combining our experimental approach with ab initio simulations, we discovered that an efficient inner-sphere transfer of low-energy electrons leads to an enhancement in the photocathode device performance in the interband regime. These findings provide important mechanistic insights, showing our methodology as a powerful tool for analyzing and engineering hot-carrier-driven processes in plasmonic photocatalytic systems and optoelectronic devices.