<i>Advanced Photonics</i>, co-published by SPIE and Chinese Laser Press, is a highly selective, Gold Open Access, international journal publishing innovative research in all areas of optics and photonics, including fundamental and applied research.

Stimulated Raman scattering (SRS) microscopy has shown superior chemical resolution due to the much narrower vibrational spectral bandwidth than its fluorescence counterpart. However, breaking the diffraction-limited spatial resolution of SRS imaging is much more challenging because of the intrinsically weak scattering cross section and inert/stable nature of molecular bond vibrations. We report superresolution SRS (SR-SRS) nanoscopy based on reversible-switchable vibrational photochromic probes integrated with point spread function engineering strategy. By introducing a Gaussian-shaped ultraviolet excitation beam and a donut-shaped visible depletion beam in addition to the pump and Stokes beams, SR-SRS could reach sub-100 nm resolution on photoswitchable nanoparticles (NPs). Furthermore, NP-treated live cell imaging was demonstrated with resolution improvement by a factor of ∼4. Our proof-of-principle work provides the potential for SR vibrational imaging to assist research on complex biological systems.

Diverse spatial mode bases can be exploited in mode-division multiplexing (MDM) to sustain the capacity growth in fiber-optic communications, such as linearly polarized (LP) modes, vector modes, LP orbital angular momentum (LP-OAM) modes, and circularly polarized OAM (CP-OAM) modes. Nevertheless, which kind of mode bases is more appropriate to be utilized in fiber still remains unclear. Here, we aim to find the superior mode basis in MDM fiber-optic communications via a system-level comparison in air-core fiber (ACF). We first investigate the walk-off effect of four spatial mode bases over 1-km ACF, where LP and LP-OAM modes show intrinsic mode walk-off, while it is negligible for vector and CP-OAM modes. We then study the mode coupling effect of degenerate vector and CP-OAM modes over 1-km ACF under fiber perturbations, where degenerate even and odd vector modes suffer severe mode cross talk, while negligible for high-order degenerate CP-OAM modes based on the laws of angular momentum conservation. Moreover, we comprehensively evaluate the system-level performance for data-carrying single-channel and two-channel MDM transmission with different spatial mode bases under various kinds of fiber perturbations (bending, twisting, pressing, winding, and out-of-plane moving). The obtained results indicate that the CP-OAM mode basis shows superiority compared to other mode bases in MDM fiber-optic communications without using multiple-input multiple-output digital signal processing. Our findings may pave the way for robust short-reach MDM optical interconnects for data centers and high-performance computing.

Lidar, a technology at the heart of autonomous driving and robotic mobility, performs 3D imaging of a complex scene by measuring the time of flight of returning light pulses. Many technological challenges, including enhancement of the observation field of view (FoV), acceleration of the imaging frame rate, improvement of the ambiguity range, reduction of fabrication cost, and component size, must be simultaneously addressed so that lidar technology reaches the performance needed to strongly impact the global market. We propose an innovative solution to address the problem of wide FoV and extended unambiguous range using an acousto-optic modulator that rapidly scans a large-area metasurface deflector. We further exploit a multiplexing illumination technique traditionally deployed in the context of telecommunication theory to extend the ambiguity range and to drastically improve the signal-to-noise ratio of the measured signal. Compacting our metasurface-scanning lidar system to chip-scale dimension would open new and exciting perspectives, eventually relevant to the autonomous vehicles and robotic industries.

<i>Advanced Photonics</i>, co-published by SPIE and Chinese Laser Press, is a highly selective, Gold Open Access, international journal publishing innovative research in all areas of optics and photonics, including fundamental and applied research.

We propose a framework that connects the spatial symmetries of a metasurface to its material parameter tensors and its scattering matrix. This provides a simple and universal way to effortlessly determine the properties of a metasurface scattering response, such as chirality or asymmetric transmission, and which of its effective material parameters should be taken into account in the prospect of a homogenization procedure. In contrast to existing techniques, this approach does not require any a priori knowledge of group theory or complicated numerical simulation schemes, hence making it fast, easy to use and accessible. Its working principle consists in recursively solving symmetry-invariance conditions that apply to dipolar and quadrupolar material parameters, which include nonlocal interactions, as well as the metasurface scattering matrix. The overall process thus only requires listing the spatial symmetries of the metasurface. Using the proposed framework, we demonstrate the existence of multipolar extrinsic chirality, which is a form of chiral response that is achieved in geometrically achiral structures sensitive to field gradients, even at normal incidence.

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On-demand modification of the electronic band structures of high-mobility two-dimensional (2D) materials is of great interest for various applications that require rapid tuning of electrical and optical responses of solid-state devices. Although electrically tunable superlattice (SL) potentials have been proposed for band structure engineering of the Dirac electrons in graphene, the ultimate goal of engineering emergent quasiparticle excitations that can hybridize with light has not been achieved. We show that an extreme modulation of one-dimensional (1D) SL potentials in monolayer graphene produces ladder-like electronic energy levels near the Fermi surface, resulting in optical conductivity dominated by intersubband transitions (ISBTs). A specific and experimentally realizable platform comprising hBN-encapsulated graphene on top of a 1D periodic metagate and a second unpatterned gate is shown to produce strongly modulated electrostatic potentials. We find that Dirac electrons with large momenta perpendicular to the modulation direction are waveguided via total internal reflections off the electrostatic potential, resulting in flat subbands with nearly equispaced energy levels. The predicted ultrastrong coupling of surface plasmons to electrically controlled ISBTs is responsible for emergent polaritonic quasiparticles that can be optically probed. Our study opens an avenue for exploring emergent polaritons in 2D materials with gate-tunable electronic band structures.

In the quest to realize a scalable quantum network, semiconductor quantum dots (QDs) offer distinct advantages, including high single-photon efficiency and indistinguishability, high repetition rate (tens of gigahertz with Purcell enhancement), interconnectivity with spin qubits, and a scalable on-chip platform. However, in the past two decades, the visibility of quantum interference between independent QDs rarely went beyond the classical limit of 50%, and the distances were limited from a few meters to kilometers. Here, we report quantum interference between two single photons from independent QDs separated by a 302 km optical fiber. The single photons are generated from resonantly driven single QDs deterministically coupled to microcavities. Quantum frequency conversions are used to eliminate the QD inhomogeneity and shift the emission wavelength to the telecommunication band. The observed interference visibility is 0.67 ± 0.02 (0.93 ± 0.04) without (with) temporal filtering. Feasible improvements can further extend the distance to ∼600 km. Our work represents a key step to long-distance solid-state quantum networks.

Naomi Halas, professor of electrical and computer engineering at Rice University and winner of the prestigious 2022 Eni Energy Transition Award, discusses her vision for the future of plasmonics and nanophotonics, in terms of fundamentals and applications, in conversation with Jia Zhu, professor in the College of Engineering and Applied Science at Nanjing University and associate editor for Advanced Photonics.