Topological acoustics enables backscattering-immune wave transport along domain interfaces, whose directionality can be deterministically controlled through spin-momentum locking of the excitation source. In this work, we computationally demonstrate a monolithic two-dimensional Ge2Sb2Te5 (GST) phononic crystal plate in which hexagonally patterned crystalline inclusions are embedded within an amorphous GST host, where the impedance contrast between the two phases opens a topological bandgap. Here we show that the overlap integral between an external excitation source and the Bloch eigenstates governs directional selectivity. The source position and phase determine which topological pseudospin channel is excited, enabling deterministic routing through spin-momentum locking. Our system exploits [Formula: see text] symmetry, which supports degenerate [Formula: see text]-type and [Formula: see text]-type orbital modes at the [Formula: see text] point serving as pseudospin degrees of freedom. When a single harmonic force is applied, it projects onto both pseudospin channels, yielding bidirectional propagation. By contrast, a quadrature phased force pair on neighboring inclusions generates a rotating displacement field whose coupling to one pseudospin state identically vanishes, locking propagation to a single direction. Swapping the force positions reverses the routing direction, and this reversal is spatially invariant across the interface, providing evidence of spin-momentum locking. By varying only the source configuration, the same interface operates as a bidirectional waveguide, unidirectional isolator, or selective router. These results demonstrate how spin-momentum locking can serve as an efficient mechanism for directional selectivity of topological interface states in monolithic structures, with relevance to on-chip acoustic signal routing and frequency-selective wave filtering.

Electrically small antennas (ESA) are subject to fundamental bandwidth and radiation efficiency limitations due to their small size compared to the wavelength of operation. While the use of active loads has been long explored to overcome these limits, practical implementations have been hindered by unwanted instabilities and nonlinearities. Recently, various parametric phenomena have been investigated as a powerful tool to enhance the radiation properties of ESAs. However, increasing the parametric gain alone reduces the antenna bandwidth, highlighting the inherent gain-bandwidth trade-off. In this work, we overcome this challenge by designing a dual-resonant ESA, which is parametrically driven by two independent parametric pump tones oscillating at the sum and difference frequencies of tailored resonant modes of the ESA. By carefully engineering the pump power levels and frequencies, we surpass conventional limits associated with the gain-bandwidth product and demonstrate enhancement of both peak transmission and 3-dB bandwidth of ESAs, yielding tangible enhancements in data rate. Both analytical and experimental validations confirm the efficacy of this approach, underscoring its potential to transcend conventional gain-bandwidth product limitations for ESAs.

We investigate the effect of nonrelativistic motion on the emission dynamics of a dipole emitter moving next to a reflecting interface. Within the formalism of macroscopic QED, we obtain a general equation of motion for the dipole amplitude in terms of the dyadic Green's function, yielding a dynamical extension of the Drexhage effect. At short dipole-surface distances, the dipole can be described as a parametric oscillator featuring time-dependent dampings and Lamb shifts, both arising from the self-induced modulation of the surrounding electromagnetic environment. Importantly, these time-dependent parameters do not always average out, leading to amplification of the dipole amplitude and the radiated intensity when considering certain sinusoidal trajectories with specific modulation amplitudes and frequencies. We derive threshold modulation amplitudes as function of the relative permittivities at the interface. Qualitatively, in the vicinity of certain epsilon-near-zero materials, amplification is possible purely by modulation of the damping. Our findings open up avenues for the dynamic control of light-matter interaction in nanophotonic environments.

We present the design and modeling of a triplelayer metalens system for beam steering in the near-infrared (NIR) wavelength. Taking inspiration from the Risley prism concept, the system employs Pancharatnam-Berry (PB) phase modulation through nanopillar arrays to achieve precise and tunable wavefront control from a spherical incident wave. The first metalens collimates an incoming spherical wavefront into a planar one, while two mechanically rotatable metalenses introduce adjustable phase gradients, enabling independent steering in both azimuth and zenith directions. The proposed device, featuring an aperture of $16 \mu ~\mathrm{m}$ (approximately $21.3 \lambda$ at the operating wavelength of $0.75 \mu ~\mathrm{m}$), demonstrates efficient and continuous beam steering. Full-wave simulations performed using a commercial FDTD solver confirm the system's effectiveness, underscoring its potential for compact, lightweight, and low-cost applications in LiDAR, optical communications, and infrared imaging.

The Bode-Fano bound sets a fundamental trade-off between bandwidth and reflection in passive, linear, time-invariant matching networks. We show that an aperiodically time-modulated reactive element can emulate the non-Foster response required to match a prescribed pulse, achieving reflectionless, nearly distortionless energy transfer beyond the Bode-Fano limit. The approach introduces a new constraint: a minimum dc bias that scales with pulse bandwidth, derived from the requirement that the modulated capacitance remain positive at all times. This modulation-bias bound replaces the classical bandwidth-reflection trade-off with a bandwidth-energy trade-off. A realistic circuit simulation confirms broadband matching with preserved waveform fidelity, demonstrating that the scheme is physically realizable and not merely a mathematical circumvention.

Achieving broadband on-chip optical amplification spanning the visible and near-infrared (NIR) can enable diverse quantum sensing, metrology, and classical communication applications within a single unified device. However, conventional semiconductor and ion-doped amplifiers suffer from limited gain bandwidths set by fixed energy levels, while optical parametric amplifiers (OPAs) operating continuously from the visible to the NIR have remained elusive due to dispersion-limited bandwidth and the high pump powers required in the visible or ultraviolet (UV). Here, we overcome these limitations by introducing an electrically reconfigurable OPA architecture on lithium niobate integrated photonics. By synergistically combining ultra-high effective χ^(2) nonlinearity (∼7,000%/W-cm^2), high-order dispersion engineering, and local electro-thermal tuning of quasi-phase matching, our device achieves record gain spectral spanning more than an optical octave, from 770 to 1650 nm. This range covers key transitions of many photonic quantum systems and all telecommunication bands. Moreover, our approach eliminates the need for high-power, wavelength-tunable visible or UV pumps, delivering a peak on-chip gain of 23.67 dB with a single 1060 nm pump at 90 mW average on-chip power. This work opens new avenues for multi-functional, reconfigurable photonics unifying the visible and infrared regimes, with broad implications for quantum sensing and communications.

Actively tunable photonic devices are vital for next-generation optoelectronics, requiring rapid switching and high bandwidth. Although organic optoelectronic devices have found wide applications, their use as optical modulators has been limited by low absorption in the critical near-infrared (NIR) region, a slow response time, and weak nonlinearities. To address these limitations, we developed a scheme based on intermediate exciton-photon coupling in an NIR-absorbing squaraine dye-based photonic structure. Using energy-momentum-resolved pump-probe spectroscopy, we show that the sign and magnitude of the optical response of our system depend strongly on the energy detuning between the excitonic and photonic modes. These data are analyzed using temporal coupled-mode theory to show that near resonance, a distinct energy exchange process emerges in the crossover regime between strong and weak light-matter coupling. This effect enables dynamic control over the photoinduced response, providing a pathway for broadband optical signal modulation extending into the NIR spectral region.

Broadening of spectral and spatial responses due to intrinsic loss in real materials often hides sharp features. One recently recognized route to recover those features is to probe the system with complex-frequency (CF) signals that decay exponentially in time: a suitably tailored temporal decay can compensate for loss and reveal an intrinsic, narrow response. However, generating rapidly decaying optical waveforms in real time is often challenging (the required decay times may be in the range of tens of femtoseconds). A recently proposed alternative synthesizes the CF response numerically after detection of conventional, real-frequency signals using Fourier post-processing. Here we explore advantages and challenges of these approaches: we show that a physical CF excitation robustly sharpens spectral features in the presence of noise, while a post-detection synthesized CF response shows only limited improvement once realistic detection and readout noise is considered. At the same time, in low-noise conditions a much simpler post-detection filtering procedure attains equal or better recovery than the synthesized CF reconstruction, making the synthesis unnecessary in practice.

The classical analytical formulation of 2-D Mie scattering for coaxial cylindrical multilayers requires solving a 2Nx2N system of equations for each polarization, where N is the number of layers, which quickly becomes cumbersome as N increases. In this letter, we address this challenge by developing a compact recursive approach, based on Richmond's 1965 method for dielectric shells, reformulated as a sequence of simple 2x2 systems. We extend Richmond's formulation to the case of multilayer impedance boundaries (e.g., metasurfaces), making it particularly effective for modern scattering engineering problems, such as metasurface cloaking design. The effectiveness of the approach is demonstrated through the minimization of the scattering signature of a metallic cylinder surrounded by such multilayer impedance surfaces. The results are validated against independent full-wave electromagnetic simulations.

Metasurfaces are planar arrays of subwavelength elements that can efficiently control electromagnetic reflection, refraction, and scattering, being able to replace bulky components with ultrathin, engineered interfaces. This article traces their evolution in the past years with an emphasis on their applications in microwave engineering settings. We feature three classes of microwave metasurfaces: local surfaces, characterized by a tailored surface impedance that enables full control over the phase, amplitude, and polarization of the scattered waves; nonlocal architectures, which have been expanding this paradigm by introducing engineered coupling among the constituent elements of the metasurface and engineered dispersion, enabling angular and spectral selectivity, analog signal processing, and spatial compression; and time-varying and space-time-modulated metasurfaces, which further extend the design space, enabling broadband absorption and magnet-free nonreciprocal propagation, among other features that linear, time-invariant designs cannot support. Finally, digital space-time-coding metasurfaces (STCMs) unify these advances within programmable hardware, offering real-time reconfigurability, multifunctional beam control, and dynamically reconfigurable analog processing. Metasurfaces are rapidly expanding to become an essential tool for microwave engineering, thanks to their agility and ubiquitous relevance in many emerging application spaces.

Mie scattering describes the linear interaction of electromagnetic waves with spheres of arbitrary composition and size. Here, we introduce and experimentally validate an analog circuit emulator of Mie scattering by temporally dispersive spheres. The emulator reconstructs the full scattering response using a modular lumped-element network, in which excitation conditions are set by generators and filters, while distinct resistor-inductor-capacitor networks encode material dispersion and radiation properties of the supported resonant modes. We demonstrate a one-to-one correspondence between the time-averaged powers scattered, absorbed, and extinguished by a dispersive sphere, and the power absorbed or supplied within distinct circuit stages, yielding a circuit-level realization of optical power conservation. Our results establish an experimentally accessible platform for exploring and designing dispersion-engineered scatterers.

Surface polaritons in natural materials offer unique opportunities for tailoring light–matter interactions at the nanoscale. While various classes of surface polaritons have recently been unveiled via near-field imaging techniques, their connection to the underlying topological properties of bulk polaritonic bands has remained unexplored. Here, we unveil polaritonic topological nodal rings in unstructured natural materials, arising from the anisotropic coupling between photons and material excitations and supporting topologically nontrivial polaritonic bands. Through topological band theory, we identify drumhead-like surface polariton modes associated with such nodal rings, directly visualize them using scattering-type scanning near-field optical microscopy, and engineer their near-field features by tuning the orientation of the material optical axis relative to the interface. Furthermore, we demonstrate transverse spin-momentum locking, associated with these topological features, enabling control of the propagation direction through the handedness of circularly polarized excitations. Our findings establish an unexplored link between topological semimetals and polaritonic photonics, opening a new pathway for the exploration and application of topological polaritons in natural materials without requiring sophisticated nanofabrication. Recent experiments have demonstrated control of surface phonon polaritons (SPPs) in various schemes, yet their topological nature in relation to their polaritonic bands has not been explored. Here, the authors characterise drumhead-like SPPs via scattering type scanning near-field optical microscopy in uniaxial calcite, resolving their connection to topological nodal lines and demonstrating near field control.

Losses are a major roadblock in the technological implementation of surface plasmons at optical frequencies. The recent emergence of MoOCl2, a correlated van-der-Waals material with strongly anisotropic optical properties, offers new avenues to circumvent this limit. We report the far-field observation of high-Q surface plasmon polaritons in this material, arising from the anisotropic hybridisation of surface plasmons and dielectric modes. We then explore nonlinear pumping of intraband electrons to the conduction band in these structures, leading, contrary to intuition, to an abrupt increase in the lifetime of the polariton resonance, despite the injection of hot electrons. This counterintuitive phenomenon stems from the competition between photon and plasmon excitations in a lenticular polariton resonance, yielding a largely tuneable lifetime at ultrafast speeds.

We demonstrate a 160nm thick 3R-MoS 2 nonlinear metasurface supporting nonlocal quasi-BIC resonances, maximizing overlap and satisfying generalized phase matching for second harmonic generation (SHG). The platform delivers over 140× SHG enhancement and ∼ 10 − 4 conversion efficiency, enabling compact, integrable nonlinear photonic devices.

Kirchhoff's law fundamentally relates thermal emission to absorption. For linear, static, reciprocal media, it equates the emissivity and absorptivity for each direction and frequency, while in nonreciprocal systems emission and absorption are equal when the bias is time-reversed. In time-varying media, however, temporal modulation breaks time-translation invariance, converts frequencies, and enables energy exchange with the modulation drive. As a result, a same-frequency relation between absorptivity and emissivity can no longer be expected. Here, we derive a generalized Kirchhoff's law for linear time-varying Floquet media. We show that the emissivity at a given frequency equals a weighted sum of harmonic-resolved absorptivities of the adjoint system, with weights accounting for thermal occupation and photon-flux conversion. This relation has both practical and fundamental consequences. In practical terms, it allows emissivity to be calculated from absorption, simplifying the design of time-varying thermal emitters. More fundamentally, it reveals thermal radiation regimes inaccessible in static media. In particular, we identify time-varying structures that exhibit strong emission with negligible absorption at the same frequency for all directions, yielding a near-maximal violation of the conventional form of Kirchhoff's law.

The discovery of graphene plasmons (GPs) and hyperbolic phonon polaritons (HPhPs) in two-dimensional (2D) van der Waals (vdW) materials has enabled extreme light confinement and enhanced light-matter interactions, holding the promise for miniaturized mid-infrared (mid-IR) photonic devices. However, GPs and HPhPs suffer from limited propagation lengths, hindering their impact in various applications. Here, we demonstrate long-range surface polaritons (LRSPs) in 2D vdW materials, featuring much longer propagation lengths and faster group velocities, which may offer complementary opportunities for high-speed on-chip photonic applications in the mid-IR regime. The demonstrated LRSPs are supported by deep-subwavelength heterostructures consisting of a hexagonal boron nitride (h-BN) flake, a high-index germanium (Ge) layer, and a gold (Au) film. Within such geometry, we experimentally demonstrate propagation distances exceeding 80 micrometers crossing an entire h-BN flake without substantial decay. A theoretical prediction of ∼925-micrometer propagation length is obtained. These polaritons may be ideally suited for realizing polaritonic interconnects that bridge different components of compact and planar photonic systems, enabling mid-IR information transport and seamless integration with other vdW material-based mid-IR photonic devices.

Coherent interfaces between microwave-frequency quantum systems and low-loss optical links are essential for quantum networks. However, existing microwave-optical transducers often trade conversion efficiency against added noise, bandwidth, and device integrability. Here, we demonstrate coherent microwave-to-optical transduction based on magnon-exciton coupling in the layered antiferromagnet CrSBr. Driving the antiferromagnetic resonance with microwave signals imprints coherent modulation on a reflected optical probe, generating optical sidebands that are resonantly enhanced near excitonic transitions. While prior magnon-based approaches to microwave-to-optical transduction have typically relied on intrinsically weak off-resonant magneto-optical effects (e.g., Faraday rotation), our scheme exploits strong light-matter interactions at exciton resonances. Even in a bulk crystal without cavity enhancement, we observe coherent conversion over an intrinsically broadband window of 300 MHz. We further show that multiple exciton-polariton resonances inherit the magnon-coupled response, suggesting a route to broaden the usable optical detuning range and to mitigate optical dissipation. Our results establish magnon-coupled excitons in layered magnets as a scalable platform for broadband microwave-optical interfaces, with pathways to higher cooperativity via reduced magnetic volume and cavity integration.

Time reflection emerges as the dual phenomenon of conventional reflection at a spatial interface, and it is induced by an abrupt and spatially uniform change in time of the optical properties of a material, forming a temporal interface. Strong time reflections require ultrafast processes and large modulation amplitudes-conditions that are challenging to achieve. In this work, we demonstrate a new regime for efficient pulse reversal of a pulse that can be accessed through moderate, adiabatic modulation of a plasmonic waveguide. Considering a zinc oxide waveguide with optically tunable carrier concentrations, we show that a slow modulation of the plasma frequency can support efficient temporal reflections based on the reversal of the group velocity rather than of the phase velocity. While interference phenomena do not time-reverse in this setting, the pulse temporal evolution and dispersion do. Our theory unveils the role of broken spatial symmetries in this process and the associated momentum evolution, with opportunities to realize exotic temporal scattering processes within realistic optical setups.

We investigate the scattering of counterpropagating photon Fock states at a time interface, induced by an abrupt variation of the dielectric permittivity of the host material. We unveil the role of vacuum-generated photons in determining the final photon number distribution and coherence properties. Such vacuum-generated photons interfere with the photons in the initial Fock states, producing distinctive interference patterns, which can be broadly controlled, together with the temperature of the final photon gas, by the time-interface parameters and the number of photons in the initial states.

Nanostructures can be designed to absorb light efficiently at resonance despite their subwavelength footprint, but causality and passivity fundamentally limit the bandwidth over which strong absorption can be maintained. Here we derive fundamental absorption-bandwidth limits for passive, causal, linear, and temporally dispersive subwavelength objects by rigorously casting electromagnetic scattering as an equivalent impedance-matching problem. This mapping yields ultimate Bode-Fano-type constraints for optical absorption and provides rational synthesis guidelines for the material dispersion of passive nanoparticles that can approach the bounds. Our results clarify the ultimate limits for broadband light harvesting and dissipation, with implications for solar-energy conversion, photothermal hyperthermia, thermal management, and related nanophotonic technologies.