ACS Photonics最新文献

In the rapidly evolving field of structured light, self-torque has been recently defined as an intrinsic property of light beams carrying time-dependent orbital angular momentum. In particular, extreme-ultraviolet (EUV) beams with self-torque, exhibiting a topological charge that continuously varies on the subfemtosecond time scale, are naturally produced in high-order harmonic generation (HHG) when driven by two time-delayed intense infrared vortex beams with different topological charges. Until now, the polarization state of such EUV beams carrying self-torque has been restricted to linear states due to the drastic reduction in the harmonic up-conversion efficiency with increasing the ellipticity of the driving field. In this work, we theoretically demonstrate how to control the polarization state of EUV beams carrying self-torque, from linear to circular. The extremely high sensitivity of HHG to the properties of the driving beam allows us to propose two different driving schemes to circumvent the current limitations to manipulate the polarization state of EUV beams with self-torque. Our advanced numerical simulations are complemented with the derivation of selection rules of angular momentum conservation, which enable precise tunability over the angular momentum properties of the harmonics with self-torque. The resulting high-order harmonic emission, carrying time-dependent orbital angular momentum with a custom polarization state, can expand the applications of ultrafast light–matter interactions, particularly in areas where dichroic or chiral properties are crucial, such as magnetic materials or chiral molecules.

Hyperbolic metamaterials (HMMs) are engineered materials with a hyperbolic isofrequency surface, enabling a range of interesting phenomena and applications including negative refraction, enhanced sensing, and subdiffraction imaging, focusing, and waveguiding. Existing HMMs primarily work in the visible and infrared spectral range due to the inherent properties of their constituent materials. Here, we demonstrate a THz-range Dirac HMM using topological insulators as the building blocks. We find that the structure houses up to three high-wavevector volume plasmon polariton (VPP) modes, consistent with transfer matrix modeling and effective medium theory calculations. The VPPs have mode indices greater than 100, significantly larger than observed for VPP modes in HMMs made from metals or doped semiconductors while maintaining comparable quality factors. We attribute these properties to the two-dimensional Dirac nature of the electrons occupying the topological insulator surface states. Because these are van der Waals materials, these structures can be grown at a wafer-scale on a variety of substrates, allowing them to be integrated with existing THz structures and enabling next-generation THz optical devices.

Dielectric media are very promising for near-field radiative heat transfer (NFRHT) applications as these materials can thermally emit surface phonon polaritons (SPhPs) resulting in large and quasi-monochromatic heat fluxes. Near-field radiative heat flux between dissimilar dielectric media is much smaller than that between similar dielectric media and is also not quasi-monochromatic. This is due to the mismatch of the SPhP frequencies of the two heat-exchanging dielectric media. Here, we experimentally demonstrate that NFRHT between dissimilar dielectric media increases substantially when a graphene sheet is deposited on the medium with a smaller SPhP frequency. An enhancement of ∼2.7 to 3.2 folds is measured for the heat flux between SiC and LiF separated by a vacuum gap of size ∼100–140 nm when LiF is covered by a graphene sheet. This enhancement is due to the coupling of SPhPs and surface plasmon polaritons (SPPs). The SPPs of graphene are coupled to the SPhPs of LiF resulting in coupled SPhP-SPPs with a dispersion branch monotonically increasing with the wavevector. This monotonically increasing branch of dispersion relation intersects the dispersion branch of the SPhPs of SiC causing the coupling of the surface modes across the vacuum gap, which resonantly increases the heat flux at the SPhP frequency of SiC. This surface phonon-plasmon coupling also makes NFRHT quasi-monochromatic, which is highly desired for applications such as near-field thermophotovoltaics and thermophotonics. This study experimentally demonstrates that graphene is a very promising material for tuning the magnitude and spectrum of NFRHT between dissimilar dielectric media.

Small organic molecules can possess extremely high hyperpolarizabilities. Their potential use in nonlinear photonics has, however, been limited by the fact that their bulk second-order nonlinearities often vanish in thin film form due to the centrosymmetric arrangement that results from most fabrication processes. The typical approach to overcome this problem has been to use electric field poling, which comes at the cost of considerably increased complexity. In polar films, however, molecules can spontaneously adopt an asymmetric out-of-plane orientation distribution that breaks the centrosymmetry. This phenomenon is at the origin of the spontaneous orientation polarization observed in organic thin films, a phenomenon that has recently attracted considerable attention from the organic optoelectronics community. In this work we show that spontaneous orientation can be leveraged to obtain evaporated thin films with bulk second-order nonlinear coefficients of χ₃₃⁽²⁾ ≃ 20 pm/V, on par with the inorganic nonlinear materials commonly used in integrated photonics. Additionally, we show that the evaporation rate and substrate treatments can be used to tune the nonlinear properties of these films. Finally, we show that the codeposition of a molecule possessing a large hyperpolarizability with a host molecule known for its strong spontaneous orientation can favor the spontaneous orientation of the nonlinear molecule and lead to large nonlinearities. This technique can lead to films with stronger nonlinearities than in neat films, even at low concentrations of nonlinear compounds (as low as 23%). This work paves the way for the direct integration of evaporated organic semiconductor thin films for second-order nonlinear optical processes on optical chips and metasurfaces, without the need for electrical poling.

Diamond photoconductive switch devices are expected to be candidates for microwave generation systems based on their attractive characteristics. Herein, a nitrogen-doped diamond single-crystal layer is grown on a boron-doped diamond substrate by microwave plasma chemical vapor deposition, which forms the NPN structure, extraordinarily reducing the on-resistance of the photoconductive switch. Compared with the traditional diamond photoconductive switch with a nitrogen-doped diamond substrate as well as a nitrogen-doped epilayer, the on-resistance of the NPN structure photoconductive switch is reduced by an order of magnitude. Especially, the rise time is only 62 ps when low laser energy is used to activate the NPN structure diamond photoconductive switch. At a 3.5 kV applied voltage and irradiation with a 4 mJ saturated energy laser, the output voltage waveform is observed with a voltage conversion efficiency of roughly 72.6% and a rise time of less than 150 ps as well as the minimum on-state resistance of approximately 18.9 Ω.