The majority of visible light-active plasmonic catalysts are often limited to Au, Ag, Cu, Al, etc., which have considerations in terms of costs, accessibility, and instability. Here, we show hydroxy-terminated nickel nitride (Ni3N) nanosheets as an alternative to these metals. The Ni3N nanosheets catalyze CO2 hydrogenation with a high CO production rate (1212 mmol g−1 h−1) and selectivity (99%) using visible light. Reaction rate shows super-linear power law dependence on the light intensity, while quantum efficiencies increase with an increase in light intensity and reaction temperature. The transient absorption experiments reveal that the hydroxyl groups increase the number of hot electrons available for photocatalysis. The in situ diffuse reflectance infrared Fourier transform spectroscopy shows that the CO2 hydrogenation proceeds via the direct dissociation pathway. The excellent photocatalytic performance of these Ni3N nanosheets (without co-catalysts or sacrificial agents) is suggestive of the use of metal nitrides instead of conventional plasmonic metal nanoparticles.
Tunable slow light systems have gained much interests recently due to their efficient control of strong light-matter interactions as well as their huge potential for realizing tunable device applications. Here, a dynamically tunable polarization independent slow light system is experimentally demonstrated via electromagnetically induced transparency (EIT) in a terahertz (THz) metasurface constituted by plus and dimer-shaped resonators. Optical pump-power dependent THz transmissions through the metasurface samples are studied using the optical pump terahertz probe technique. Under various photoexcitations, the EIT spectra undergo significant modulations in terms of its resonance line shapes (amplitude and intensity contrast) leading to dynamic tailoring of the slow light characteristics. Group delay and delay bandwidth product (DBP) values are modulated from 0.915 ps to 0.42 ps and 0.059 to 0.025 as the pump fluence increases from 0 to 62.5 〖nJ cm〗^(-2). This results in tunable slow THz light with group velocities ranging from 2.18×〖10〗^5 〖m s〗^(-1) to 4.76×〖10〗^5 〖m s〗^(-1), almost 54% change in group velocity. The observed tuning is attributed to the photo-induced modifications of the optoelectronic properties of the substrate layer. The demonstrated slow light scheme can provide opportunities for realizing dynamically tunable slow light devices, delay lines, and other ultrafast devices for THz domain.
The development of metamaterial-based photonic components has acquired a significant interest in technological developments at terahertz frequencies. The manipulation of the state of polarization is an important parameter in optical devices. In this study, we have investigated, both numerically and experimentally, a toroidal excitation-based metamaterial that is capable of converting terahertz from its linearly polarized state to an orthogonally polarized state over a broad spectrum. The meta-molecule unit of the proposed geometry is comprised of a pair of resonators connected to each other having a split gap in each arm. We have studied both the horizontal and vertical components of transmission for numerous in-plane rotations of the proposed geometry. A multipolar analysis confirms a significant contribution of the toroidal component. Polarization conversion of nearly 40% is observed over a broad spectrum of 1.19–2.5 THz. Such a broadband cross-polarization converter could have remarkable implications for the development of terahertz toroidal metamaterial devices.
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