The concept of skyrmions, which refers to topologically stable vector textures and was originally introduced in particle physics, has recently been transferred to various types of fields, including electromagnetic ones. Its potential in optics and photonics stimulates the development of sophisticated strategies for skyrmionic beam shaping, but these are limited to bulky systems, thus hindering the emergence of practical applications. Here we propose and experimentally report on the realization of compact flat-optics skyrmion generators based on the spin-orbit interaction of light. Our approach highlights how the use of unitary wave plates beyond the uniform-retardance condition opens new avenues for the preparation of topological optical states.

This work explores the excitation of a two-mode exciton-polariton condensate within a ring-shaped trap, with each mode manifesting as a distinct eigenstate characterized by oppositely directed azimuthal polariton currents. The coexistence of these condensate states is facilitated by a positive gain-loss balance for both states, sustained by external noncoherent laser pumping. We utilize interferometric methods to investigate these states. Spherical wave interferometry visualizes the azimuthal polariton currents and elucidates the spatial phase properties of the condensates. Plane-wave interferometry with a self-delayed replica of the condensate’s photoluminescence provides a highly sensitive approach for estimating the energy splitting between the states. The experimental observations are validated by numerical simulations. The study emphasizes the potential of these condensates to form the basis for flux polariton qubits, leveraging the superposition of current states within an annular optical trap.

Electromagnetic wave polarization engineering with help from a pair of identical anisotropic metasurfaces of twisted optical axes, is introduced. Flawless linear polarization rotation and conversion to circular are demonstrated for lossless designs while symmetries between the responses of plasmonic and dielectric metasurfaces at different separating distances, are identified. The proposed concept expands the approach of twistronics in classical electrodynamics and reports optimal setups that can be directly utilized for polarization-controlled optical manipulation, holographic imaging, or angular-momentum multiplexing.

Silicon nitride (${\mathrm{Si}\mathrm{N}}_x$) is one of the most promising integrated photonics platforms, offering ultralow loss, significant Kerr nonlinearity, and extremely high integration capability. However, ${\mathrm{Si}\mathrm{N}}_x$ in the C band exhibits normal material group-velocity dispersion (GVD), while anomalous GVD, essential for most nonlinearity-based applications, can only be achieved in thick waveguides through geometric dispersion. The common challenge in the fabrication of thick waveguides without using the Damascene technique is the uniform filling of the gaps between closely located waveguides. To address this challenge, we elaborated a combined technological approach that includes plasma-enhanced chemical vapor deposition, reactive ion etching, and plasma-enhanced atomic-layer deposition. This approach reliably provides full and uniform filling of the complex etched profile gaps between 1-$\text{μ}\mathrm{m}$-thick silicon nitride waveguides, spaced at distances of the order of hundreds of nanometers. Additionally, using microring resonators, we experimentally demonstrated that voids within the upper cladding layer significantly affect the performance of the photonic integrated components.

The growing maturity of photonic integrated circuit (PIC) fabrication technology enables the high integration of an increasing number of optical components onto a single chip. With the incremental circuit complexity, the calibration of active phase shifters in a large-scale PIC becomes a crucial issue. The traditional one-by-one calibration techniques encounter significant hurdles with the propagation of calibration errors, and achieving the decoupling of all phase shifters for independent calibration is not straightforward. To address this issue, we propose a global calibration approach for large-scale PIC. Our method utilizes a custom network to simultaneously learn the nonlinear phase-current relations for all thermo-optic phase shifters on the PIC by minimizing the negative likelihood of the measurement datasets. Moreover, the reflectivities of all static beam splitter components can also be synchronizedly extracted using this calibration method. As an example, a quantum walk PIC with a circuit depth of 12 is calibrated, and a programmable discrete-time quantum walk is experimentally demonstrated. These results will greatly benefit the applications of large-scale PICs in photonic quantum information processing.

The introduction of low-loss optical fibers probably represents the single most important advance in the growth of telecommunication systems. To meet our needs for secure communications, it is likely that our classical network will soon be operating alongside what is known as a quantum network. The latter is very sensitive to loss and thus poses constraints to the performance of current fiber components. In particular, recent quantum network prototypes underline the absence of low-loss nonreciprocal fiber-based devices. Here, we present a solution to this issue by realizing low-loss (0.81 dB), broadband (at least 50-GHz bandwidth), and high-extinction (up to 27 dB) circulators, based on Mach-Zehnder interferometers including so-called fiber null-couplers. The latter are directional couplers, whose splitting ratio can be controlled by launching acoustic waves along the coupling region. Fabricated from standard single-mode fibers and actuated electrically, these circulators can be made to fit any existing optical fiber networks and could turn out to be key for the transmission and processing of optically encoded quantum information.

Optical resonances can increase the sensitivity of measurements to material perturbations and also accelerate photochemical reactions. Here, we show that these two effects can be combined multiplicatively, to enhance the detection via weak or low-concentration photochemical reactions far beyond what could previously be attained. For an optical resonance with quality factor $Q$, the sensitivity of our detection scheme is enhanced by $\sim Q^2$ (where $\sim$ denotes approximate proportionality), as demonstrated by both theoretical arguments and numerical simulations of a simple optical-grating resonance coupled with reaction-diffusion equations. Such an approach opens a door to further improvements by careful design of the resonance: even a three-parameter optimization of the grating resonance yields an additional $\approx 7$ times improvement.

Plasmonic nanostructures provide an excellent platform for tailoring the light-matter interaction, and have found applications ranging from perfect absorption to phase modulation. However, realizing narrowband perfect plasmonic absorbers and dynamical control of light response remains challenging, primarily due to the high intrinsic losses and the limited tunability of metallic components. Here, a gain-coupled-bound state in the continuum (BIC) strategy is proposed to achieve a narrowband perfect plasmonic absorber with tailored functionality. The bandwidth can be dramatically reduced, with quadratic scalability dependence on the perturbation parameters of the quasi-BIC, essentially arising from the powerful physics of BIC in radiation engineering. Moreover, this strategy can make the reflection phase of the plasmonic absorber undergo across a full phase diagram. In particular, it displays functional phase transitions upon crossing the critical coupling or lasing during tailoring loss by gain. This phase transition is further utilized to realize a highly sensitive phase interrogation refractometric sensor. This work provides a profound physical insight into the tailoring functionalities of absorption and phase response, and may open up possibilities for trace detection in biochemical and chemical analysis.

Ultrafast laser processing is a cutting-edge technology used in three-dimensional nanofabrication owing to its unique multiphoton absorption scheme, making it essential for applications such as microfluidics, photonic devices, and integrated optics. Nevertheless, the precise fabrication of three-dimensional structures within birefringent crystals faces challenges arising from a significantly distorted focused field caused by a refractive index mismatch and birefringence under tightly focused conditions. In this study, we extend the vectorial Debye diffraction theory to incorporate birefringence and deduce an analytical solution to accurately model the distorted focused patterns inside z-cut uniaxial crystals. Practical methods are also proposed to achieve a perfectly focused spot by simultaneously compensating for spherical aberration and birefringence. These findings provide valuable insights for the accurate fabrication of large-scale three-dimensional structures in crystals.

How to prepare and engineer an orbital angular momentum (OAM) array is crucial for optical micromanipulation, large-alphabet optical communications, and optical image processing. Here, by leveraging nonlinear frequency conversion, we propose a scheme for tailoring an OAM array in the manner of controlling light with light. Specifically, by guiding two fundamental waves into a nonlinear optical crystal, one of which carries a Dammann vortex grating (DVG) and the other carries a Gaussian or vortex mode, we equivalently construct a nonlinear DVG in a nonlinear crystal. Thus, we can prepare a one-dimensional and a two-dimensional vortex array particularly with equal energy distribution. Our present scheme can be directly extended to engineering other spatial modes and enrich the toolkit for light field engineering. Also, this nonlinear manner could inspire more attempts in the fields of structured light nonlinear optics.

It is a common requirement to lock the frequency of a laser to an optical cavity resonance in experimental physics research. While the Pound-Drever-Hall technique is among the most popular methods, it requires the use of auxiliary phase modulators, which increases the complexity and cost. Moreover, the laser-locking noise caused by unwanted residual amplitude modulation within the modulator often limits the achievable locking stability. In this work, we demonstrate an alternative approach that negates the need for any external modulators, and inherently mitigates the residual amplitude noise. We directly apply fast electro-optic modulation to a monolithic lithium niobate microresonator to efficiently generate laser sidebands at frequencies much higher than the optical resonance bandwidth. Such a modulation allows us to generate low-noise laser-locking signals after appropriate photodetection and demodulation. Our noise characterization and frequency instability measurement show that the direct modulation technique renders a locking instability that is one order of magnitude better than that of the conventional Pound-Drever-Hall technique. The residual locking instability is also numerically analyzed and experimentally verified.

Laser-intensity fluctuations cause undesired phase noise even in balanced heterodyne interferometers. However, in space missions in particular, such as LISA Pathfinder or LISA, direct measurements of these fluctuations at the relevant frequencies are often not available. Hence, it can be challenging to estimate their impact on the interference phase. To address this, we propose a new method for characterizing laser relative intensity noise (RIN) using differential wavefront sensing (DWS), with the latter being a well-established technique typically used for angular sensing and control. Unlike other methods, this approach does not require an additional reference interferometer and instead takes advantage of the inherent phase subtraction of DWS. This allows us to estimate the RIN value at the heterodyne frequency and its harmonic, relative to the sensor noise floor of the total measurement system. Moreover, it provides a strategy to identify the ideal set point for minimizing RIN couplings in DWS.

The classical scenario where a single-plane-wave field impinges a Dallenbach absorber is well studied both theoretically and experimentally. However, occasionally a spectrum of plane waves impinges the absorber. Such a scenario occurs, for example, if an antenna is located adjacent to the absorbing layer. In this paper, for this scenario we obtain the absorbing performance bound and design an optimized layered absorber that approaches the bound. In a numerical demonstration, we explore a realistic case where a dipole antenna is placed in the vicinity of a finite, electrically thin, Dallenbach absorber backed by a perfect-electric-conductor plane in the 6G frequency range. In the absence of the absorbing layer covering the perfect-electric-conductor plane, severe scattering from the plane distorts the radiated fields. These distortions are robustly mitigated by the specifically tailored optimal absorber to yield a more-desired radiation pattern. Additionally, we propose a metamaterial realization that emulates the required properties of the absorbing layer for all field polarizations.

Photonic topological insulators have unique advantages regarding ensuring lossless transmission and realizing stable energy locality of electromagnetic waves. More complex requirements to manipulate discrete degrees of freedom (DOF) of photons, such as orbital DOF, show great foresight. In this paper, we design and demonstrate two kinds of topological metamaterials, A and B, with arbitrary auxiliary orbital DOF. By applying continuous generalized Kekulé modulations $\pm m_0{e}^{i\phi }$, the topological Chern numbers of the A and B configurations are inverse and the orbital DOF is controlled independently. This provides a completely new idea for the controllability and applicability of the photonic topology phase. We construct three kinds of topological platforms based on the more generic type of topological photonic insulators with arbitrary orbital DOF, including a perfect electrical conductor (PEC) boundary of a finite metamaterial structure, a domain wall interface consisting of two topological phases, and line-defect channels. The findings of our system provide a new pathway for exploring multifunctional topological electromagnetic metamaterials. It is expected that the present results can become an alternative means to manipulate topological phase transition and provide an underlying platform for the exploration of additional DOF.

The long-lived and optically addressable high-spin state of the negatively charged silicon vacancy ($V_{\mathrm{Si}}$) in silicon carbide makes it a promising system for applications in quantum technologies. Most studies of its spin dynamics have been performed under external magnetic fields applied along the symmetry axis. Here, we find that the application of weak magnetic fields perpendicular to the symmetry axis leads to nontrivial behavior caused by the dynamical reorientation of the $V_{\mathrm{Si}}$ spin multipole under optical excitation. Particularly, we observe the inversion of the quadrupole-spin polarization in the excited state and the appearance of a dipole-spin polarization in the ground state. The latter is much higher than the thermal polarization and cannot be induced solely by optical excitation. Our theoretical calculations reproduce well all sharp features in the spin-resonance spectra and shine light on the complex dynamics of spin multipoles in these kinds of solid-state systems.

The measurement of optical vortices constitutes a critical aspect within the photonics domain, with the fast identification of orbital-angular-momentum (OAM) states in vector beams presenting considerable challenges, especially when compared with their counterparts in scalar beams. In this work, we introduce a swift and efficient identification scheme designed specifically for OAM states in vector beams. This innovative scheme enables the simultaneous detection of vortex modes and polarization characteristics through a single-shot measurement. Employing a vector mode decomposition method technique, our approach utilizes advanced polarization conversion and polarization preservation strategies. This methodology has been rigorously developed within a theoretical framework and has been demonstrated in a straightforward, easily implementable experimental setup. The proposed method holds substantial promise for advancing fundamental research in optical topology and quantum state engineering, while also facilitating the applications of OAMs in optical communication.

We present a semianalytical synthesis scheme for designing realistic printed-circuit-board- (PCB) based metagratings (MGs), capable of simultaneous beam steering and polarization conversion. These low-profile structures, comprised sparse periodic arrangements of subwavelength polarizable particles (meta-atoms), oriented by insightful analytical models leading to fabrication-ready designs, have gained growing interest in recent years, demonstrating exceptional diffraction engineering capabilities with very high efficiencies. Herein, to facilitate cross-polarization coupling, we introduce a tilted dogbone meta-atom, going beyond the popular vertical (loaded wire) or recently proposed horizontal (dipole) configurations used in common PCB-MG manifestations at microwave frequencies, strictly susceptible to either transverse-electric or transverse-magnetic fields, respectively. As shown, the anisotropy introduced by the in-plane rotation of the dogbones, integrated into the MG analytical model in the form of a rotated dipole line, serves as an additional degree of freedom that can be leveraged to nonlocally manipulate the polarization and trajectory of the incident fields. Verified experimentally, the resultant semianalytical synthesis framework enables valuable physical insights, opening the door to integration of efficient anisotropic MGs in applications requiring both polarization and wavefront control, such as satellite, radar, and advanced antenna systems.

Exploiting the strengths of different quantum hardware components may increase the capabilities of emerging quantum processors. Here we propose and analyze a quantum architecture that leverages the nonlocal connectivity of optics, along with the exquisite quantum control offered by superconducting microwave circuits, to produce entangled optical resource states. In contrast to previous proposals on optically distributing entanglement between superconducting microwave processors, we use squeezing between microwaves and optics to produce microwave-optical Bell pairs in a dual-rail encoding from a single microwave quantum processor. Moreover, the microwave quantum processor allows us to deterministically entangle microwave-optical Bell pairs into larger cluster states, from which entangled optical photons can be extracted through microwave measurements. Our scheme paves the way for small microwave quantum processors to create heralded entangled optical resource states for optical quantum computation, communication, and sensing using imperfect microwave-optics transducers. We expect that improved isolation of the superconducting processor from stray optical fields will allow the scheme to be demonstrated with currently available hardware.

Photonics is crucial for communication and computing, as it involves controlling how light scatters. Recent breakthroughs like bound states in the continuum (BICs) and parity-time-symmetric systems are linked to the scattering matrix, governed by passivity and causality. However, using complex-frequency excitations overcomes these limitations, allowing us to study systems with losses or gains and discover phenomena like virtual critical coupling and virtual gain. Applying complex-frequency-excitation concepts to more complicated systems is challenging. In this work, we extend the lumped-element model from circuit theory to analyze light-scattering anomalies in the complex-frequency domain. This approach helps us design and understand effects such as virtual perfect absorption, BICs, real and virtual critical coupling, exceptional points, and anisotropic transmission resonances. These insights broaden our knowledge of electromagnetic wave phenomena and open possibilities in photonics, offering innovative ways to create and optimize optical devices and systems for various applications.