arXiv subjects

Elif Nur Dayi

Publications and source records attributed to Elif Nur Dayi.

6 recordsLinked to original sources

arXiv2026

Metaphotonic Catalysis: Amorphous silicon metasurfaces encode photochemical activity

Solar-to-fuel conversion can benefit from photoelectrodes with engineered light-matter interactions, yet most nanostructured designs provide limited control over the spatial and spectral distribution of photochemical activity. Here, we present an all-dielectric amorphous-silicon metasurface photoelectrode that confines resonant light-matter interactions within a 220-nm-thick active layer. Tunable Mie-type and guided-mode resonances spectrally encode chemical reactivity and produce absorptance above 80% near the silicon band edge, compared with less than 30% for an unpatterned film of the same thickness. The metasurface simultaneously functions as the light absorber, carrier-transport layer, and catalytic interface without an added co-catalyst or engineered passivation layer. Operando photo-scanning electrochemical microscopy reveals wavelength- and structure-dependent redox activity and a tenfold enhancement in internal quantum efficiency near the silicon band edge relative to planar films. Power-dependent measurements support a photon-driven rather than nonlinear photothermal origin of the enhancement, while surface-sensitive ultrafast transient-reflectivity measurements probe the underlying carrier dynamics. Light-coupled scanning electrochemical cell microscopy further shows hydrogen-evolution enhancements of up to 21-fold under photocatalytic conditions and 15-fold under photoelectrochemical bias, corresponding to 11.2-fold and 7.7-fold enhancements after accounting for the estimated surface-area increase. The metasurfaces remain stable during more than 10 hours of immersion and prolonged laser illumination. These results establish amorphous silicon as a stable and versatile platform for resonantly programmed photocatalysis and solar-fuel generation.

physics.optics↗

arXiv2026

Revealing Wavelength- and Size-Dependent CO2 Reduction Selectivity via Operando Scanning Photo-Electrochemical Microscopy

Controlling product selectivity in plasmonic catalysis, particularly in CO2 reduction (CO2R), remains a central unsolved challenge with direct implications for light-driven fuel and chemical synthesis. Here, we deploy quantitative operando scanning photoelectrochemical microscopy (photo-SECM) to provide a direct demonstration that tuning photon energy switches CO2R selectivity through an electronically driven pathway. On plasmonic Au/p-GaN photocathodes, interband excitation (460-560 nm) drives selective CO production while intraband excitation (640-800 nm) favors H2 evolution. By maintaining constant absorbed power across wavelengths and confirming linear power dependence, we isolate the role of hot-carrier energy from photonic and photothermal contributions. Density functional theory calculations reveal that higher-energy interband excitation progressively increases the overlap between hot-electron-accessible states and the CO-producing intermediate, selectively promoting CO over formate, in excellent agreement with experiment. We further show that selectivity is geometrically gated by hot-carrier transport: sub-100 nm nanostructures sustain CO2R activity, while ~300 nm nanodisks suffer transport losses that suppress it, consistent with ab initio hot-carrier transport calculations. Together, these results establish photon energy, carrier transport, and nanostructure geometry as coupled design parameters for plasmonic CO2R selectivity, resolve a longstanding debate on the origin of plasmon-driven selectivity effects, and position photo-SECM as a broadly applicable operando platform for photo(electro)catalysis.

physics.chem-ph↗

arXiv2025

Photo-Thermally Tunable Photon-Pair Generation in Dielectric Metasurfaces

Photon-pair sources based on spontaneous four-wave mixing (SFWM) in integrated photonics are often spectrally static. We demonstrate and model a fundamental thermo-optical mechanism that modulates photon-pair generation in amorphous silicon (a-Si) thin films and metasurfaces via SFWM. Femtosecond-pulsed excitation yields g2(0) higher than 400 in unpatterned a-Si, confirming high-purity nonclassical emission. Resonant a-Si metasurfaces produce photon pairs at rates exceeding 3.8 kHz under 0.6 mW pump power through Mie-type modes. Pump absorption induces localized heating that redshifts resonances, altering modal overlap and SFWM efficiency, leading to deviations from the quadratic power scaling expected in the undepleted regime. Coupled electromagnetic and heat-transfer simulations quantitatively reproduce these trends. Polarization-resolved measurements show nearly isotropic nonlinear responses, with 3 times higher third-order susceptibility of a-Si compared to poly-Si. This work positions a-Si as a bright, CMOS-compatible quantum photonics platform and identifies thermo-optical detuning as a key mechanism that should be considered-and potentially harnessed-in integrated photon-pair sources.

physics.optics↗

arXiv2025

Cross-polarized and Stable Second Harmonic Generation from Monocrystalline Copper

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 C3v surface symmetry. The SHG signal remains stable over several minutes of continuous femtosecond excitation, highlighting the 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.

physics.optics↗

arXiv2025

Large area monocrystalline and surfactant-free copper microflake synthesis

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 by-products and poorly defined morphologies. Here, we report a surfactant-free on-substrate wet synthesis approach that yields monocrystalline metallic Cu microflakes with (111) crystalline exposed surface. By systematically studying the growth mechanism, we achieve unprecedented sizes of more than 130 μm, which is two orders of magnitude larger than reported in most previous studies, along with higher aspect ratios of over 400. Furthermore, we show distinctly higher stability against oxidation provided by the halide adlayer. Overall, our facile synthesis approach delivers an exciting venue for the emerging fields of catalysis and nanophotonics.

physics.chem-ph↗

arXiv2024

Decoupling Optical and Thermal Responses: Thermo-optical Nonlinearities Unlock MHz Transmission Modulation in Dielectric Metasurfaces

Thermo-optical nonlinearities (TONL) in metasurfaces enable dynamic control of optical properties like transmission, reflection, and absorption through external stimuli such as laser irradiation or temperature. As slow thermal dynamics of extended systems are expected to limit modulation speeds ultimately, research has primarily focused on steady-state effects. In this study, we investigate photo-driven TONL in amorphous silicon (a-Si) metasurfaces both under steady-state and, most importantly, dynamic conditions (50 kHz modulation) using a 488 nm continuous-wave pump laser. First, we show that a non-monotonic change in the steady-state transmission occurs at wavelengths longer than the electric-dipole resonance (800 nm). In particular, at 815 nm transmission first decreases by 30% and then increases by 30% as the laser intensity is raised to 5 mW/μm2. Next, we demonstrate that TONL decouple the thermal and optical characteristic times, the latter being up to 7 times shorter in the tested conditions (i.e τopt =0.5 μs vs τth =3.5 μs). Most remarkably, we experimentally demonstrate that combining these two effects enables optical modulation at twice the speed (100 kHz) of the excitation laser modulation. We finally show how to achieve all-optical transmission modulation at MHz speeds with large amplitudes (85%). Overall, these results show that photo-driven TONL produce large and fully reversible transmission modulation in dielectric metasurfaces with fast and adjustable speeds. Therefore, they open completely new opportunities toward exploiting TONL in dynamically reconfigurable systems, from optical switching to wavefront manipulation.

physics.optics↗