Metamaterials exhibiting hyperbolic dispersion achieve a large degree of control over light-matter interactions, from subdiffraction imaging to enhanced spontaneous emission. However, conventional plasmonic hyperbolic metamaterials suffer from limited tunability and lack intrinsic emission capabilities, constraining their utility for active photonic devices. Here, we demonstrate a room-temperature, electrically tunable, excitonic hyperbolic metamaterial using aligned films of chirality-pure semiconducting carbon nanotubes. Unlike plasmonic systems, these excitonic metamaterials of aligned nanotubes combine strong optical anisotropy with dynamic electrostatic tunability. Spectroscopic ellipsometry reveals that the hyperbolic dispersion window can be electrically shifted by 53 meV, enabling real-time switching between hyperbolic and elliptical regimes. Theory predicts that this tunability translates to the propagation angle being modulated by 34°, driven by a momentum enhancement 3.11 times that of free space, limited primarily by material losses that can be mitigated through improved alignment. In addition, simulations of the system exhibit a high Purcell factor of 1550 and a modulation of 37% without an optical cavity for a dipole placed 5 nm above the aligned nanotubes. These findings establish excitonic carbon nanotubes as a versatile platform for dynamically reconfigurable photonic metamaterials such as adaptive optical devices, electrically controlled spontaneous emission, and tunable hyper-lenses operating at room temperature.

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.

Materials with unusual optical properties are central to advanced control of light. Yet, in nature, such materials may be exceedingly rare and often difficult to obtain. To overcome this limitation, here we introduce the concept of temporal illusion: A temporally dynamic framework in which carefully programmed temporal variations in effective parameters generate responses akin to those of, in principle, any arbitrary time-invariant structure. We theoretically demonstrate that proper modulation of the permittivity of a conventional dielectric in space and time replicates the optical behavior associated with exotic materials. Besides, we reveal that, beyond steady-state effects, temporal illusion also enables control over transient responses, for instance, by effectively lowering the time constant of high-quality-factor resonators, therefore, allowing faster energy accumulation. Moreover, by incorporating detuning between modulation and excitation, we show that the framework unlocks additional functionalities. The temporal illusion paradigm thus broadens the capabilities of space-time varying systems, offering a powerful route to synthesize material responses on demand and paving the way for new theoretical and experimental directions in optics and wave physics.

Hyperbolic media enable unique optical phenomena including hyperlensing, negative refraction, enhanced photonic density of states (PDOS), and highly confined polaritons. While most hyperbolic media are artificially engineered metamaterials, certain natural materials with extreme anisotropy can exhibit hyperbolic dispersion. Here, we report the first observation of natural hyperbolic dispersion in hexagonal boron nitride (hBN) in the deep-ultraviolet (DUV) regime, induced by strong, anisotropic exciton resonances. Using imaging spectroscopic ellipsometry (ISE), we characterize the complex dielectric function along in-plane and out-of-plane directions down to 190 nm (6.53 eV), revealing a type-II hyperbolic window in the DUV regime. This hyperbolicity supports hyperbolic exciton polaritons (HEP) with high directionality and slow group velocity. Our findings establish hBN as a promising platform for nanophotonic applications in the technologically significant DUV spectral range.

Temporal metamaterials, created by modulating the refractive index in time, offer powerful means of controlling wave propagation but still lack a systematic design methodology. Here, we develop an analytic inverse-design framework rooted in space-time duality and the established theory of one-dimensional spatial inverse scattering. By prescribing reflection (backward-wave) and transmission (forward-wave) responses in rational-function form, we obtain closed-form refractive-index modulations that are guaranteed to be physically admissible. This approach avoids iterative optimization and provides direct analytic control of the modulation. We illustrate the method with syntheses of mathematical operators, such as derivatives and integrals, as well as Chebyshev- and Butterworth-type filters, and validate the results through finite-difference time-domain simulations. Our findings establish a general route to temporal media with tailored functional and spectral responses, enabling applications in wave-based information processing, programmable filtering, and amplification schemes inspired by photonic time crystals.

Optical manipulation of micro- and nanoparticles near surfaces is fundamental for applications in sensing and microfluidics, yet controlling particle-surface interactions remains challenging. Here we experimentally investigate light-induced forces on dielectric particles near epsilon-near-zero (ENZ) metamaterial surfaces using photonic force microscopy. By illuminating trapped particles with tunable visible light, we observe a wavelength-dependent repulsive force unique to ENZ surfaces, contrasting with the attractive forces near dielectric or metallic substrates. This repulsion peaks near the ENZ frequency and may be attributed to combined optical ENZ effects and thermophoretic forces. Our findings demonstrate that ENZ metamaterials can induce stable levitation of particles via light-driven forces, offering a novel mechanism for contactless manipulation in microfluidic environments. This work advances understanding of light-matter interactions at ENZ interfaces and suggests potential for ENZ-based optical control of micro- and nanoscale objects, with potential applications in micro- and nanofluidic environments.

In recent years, there has been growing interest in non-Hermitian phenomena in low-symmetry conductors, particularly optical gain driven by electro-optic effects. Conventional semiclassical treatments typically attribute these effects to nonlinear interactions associated with the anomalous velocity of Bloch electrons. Here, we present a phenomenological microscopic model that not only recovers these anomalous-velocity contributions, but also incorporates interband effects that become significant at higher frequencies. Our model captures a wide range of nonlinear interactions while remaining consistent with passivity and microscopic reversibility. Using this broader framework, we study the nonlinear interactions between free and bound electrons as an alternative mechanism for optical gain. We show that, under non-equilibrium conditions in low-symmetry conductors, the linearized electromagnetic response can exhibit both nonreciprocity and gain, even without anomalous velocity contributions. Finally, we analyze the stability of electrically biased systems and highlight potential applications such as optical isolators and traveling-wave amplifiers.

A time interface (a rapid change of the constitutive parameters of a material in time), applied within an unbounded medium where a wave travels, can enable frequency conversion, and is considered the temporal analogue of a spatial interface between two materials. Here, we study light-matter interactions in four dimensions, 4D (space, x,y,z, and time, t), by exploring the implications of applying time interfaces not to the entire space where a wave travels, but to certain regions of space in order to create spatial interfaces in time. Different configurations such as induced perpendicular, parallel, and oblique spatial interfaces via a temporal interface are discussed. It is shown how such four-dimensional combinations of temporal and spatial interfaces can enable interesting features such as the 4D generalized Snell law and the temporal chirp, temporal lensing, and temporal routing of electromagnetic waves. Such exotic possibilities may provide new ways to manipulate light-matter interactions via a combination of temporal and spatial interfaces.

We introduce a mechanism that can both hold and amplify electromagnetic waves by rapidly changing the permittivity of the medium during the wave travel from a positive to a dispersionless (i.e. non-Foster) negative value and then back again. The underlying physics behind this phenomenon is theoretically explored by considering plane wave and Gaussian pulse propagation in an unbounded medium. Interestingly, we show that a rapid positive-to-negative temporal change of ε(t) causes the propagation of the wave to stop (observed by a frozen phase in time) while the amplitude of the frozen field exponentially grows. Stepping the permittivity back to the original (or a new) positive value will cause the wave to thaw and resume propagation with the original (or the new) frequency, respectively. We numerically study the case of dipole radiation in such time-varying non-Foster structures. As a possible implementation, we propose a parallel plate waveguide platform loaded with time-dependent media emulating parallel lumped non-Foster negative capacitors. Such non-Foster time-varying structures may open new venues in controlling and manipulating wave-matter interaction. Here the authors proposes a mechanism to freeze and amplify electromagnetic waves via time interfaces where the permittivity of the medium is changed from positive to a Non-Foster negative value and then back to positive.

With his formal analysis in 1951, the physicist Pyotr Kapitza demonstrated that an inverted pendulum with an externally vibrating base can be stable in its upper position, thus overcoming the force of gravity. Kapitza's work is an example that an originally unstable system can become stable after a minor perturbation of its properties or initial conditions is applied. Inspired by his ideas, we show how nonFoster circuits can be stabilized with the application of external electrical vibration, i.e., time modulations. Non-Foster circuits are highly appreciated in the engineering community, since their bandwidth characteristics are not limited by passive-circuit bounds. Unfortunately, non-Foster circuits are usually unstable and they must be stabilized prior to operation. Here, we focus on the study of non-Foster L(t)C circuits with time-varying inductors and time-invariant negative capacitors. We find an intrinsic connection between Kapitza's inverted pendulum and non-Foster L(t)C resonators. Moreover, we show how positive timevarying modulations of L(t) > 0 can overcome and stabilize non-Foster negative capacitances C < 0. These findings open up an alternative manner of stabilizing electric circuits with the use of time modulations, and lay the groundwork for application of what we coin vibrational electromagnetics in more complex media.

In this paper, we show how time modulations can be conveniently used to stabilize non-Foster circuits, i.e., circuits whose components violate Foster's reactance theorem. As an example, we study the stabilization of an $L(t)C$ resonator constituted by a time-varying inductance $L(t)$ and a time-invariant negative capacitance $C < 0$.

Exotic forms of wave control have been emerging by engineering matter in space and time. In this framework, temporal photonic interfaces, i.e., abrupt changes in the electromagnetic properties of a material, have been shown to induce temporal scattering phenomena dual to spatial reflection and refraction, at the basis of photonic time crystals and space-time metamaterials. Despite decades-old theoretical studies on these topics, and recent experimental demonstrations, the careful modeling of these phenomena has been lagging behind. Here, we develop from first principles a rigorous model of the electrodynamics of temporal photonic interfaces, highlighting the crucial role of the mechanisms driving time variations. We demonstrate that the boundary conditions and conservation laws associated with temporal scattering may substantially deviate from those commonly employed in the literature, based on their microscopic implementation. Our results open new vistas for both fundamental investigations over light-matter interactions in time-varying structures and for the prospect of their future implementations and applications in optics and photonics.

The corner problem is a century-old canonical scattering problem describing wave diffraction at a quarter-plane spatial discontinuity. Here, we study its space-time analog: wave scattering at a corner in space-time, arising at a time-switched spatial interface. We highlight and resolve an inconsistency between spatial and temporal boundary conditions arising in this problem, and analytically demonstrate the emergence of shock waves launched by the scattering process. After numerically verifying our theory, we realize and experimentally probe the scattering at a space-time corner arising at the edge of a time-switched waveguide. Our results unveil and efficiently model the unusual phenomena arising at the spatial interface between time-modulated and static media, of great relevance for the growing field of spatiotemporal metamaterials.

Spatial inhomogeneity, temporal modulation, and engineered anisotropy of parameters of electromagnetic media offer numerous opportunities for manipulating light–matter interaction over the past decades. Here, we investigate a scenario in which we deal with the temporal interface, hyperbolic anisotropy in the form of layered structures, and frequency dispersion. We theoretically investigate how a monochromatic uniform plane wave – propagating in an unbounded, homogeneous, isotropic dielectric medium – undergoes changes due to the rapid temporal variation of such medium into a hyperbolic dispersive medium formed by the stack of thin metal–dielectric bilayers, in which the metal follows the lossless Drude dispersion and the dielectric is assumed to be dispersionless. We corroborate our analytical results by numerical simulations. We observe several interesting phenomena, such as conversion of the original frequency into three pairs of frequencies, resulting in three sets of forward (FW) and backward (BW) waves. We present the amplitudes and the time-averaged Poynting vectors for such FW and BW waves and discuss some of the salient features of such temporal interface.

Inspired by Kapitza’s inverted pendulum problem, we have been exploring how such phenomena can be brought into the field of electromagnetic and optical metamaterials. I will present several case studies we have explored so far. Vibrational electromagnetics in metamaterials: The problem of an inverted pendulum with a vibrating base has been an exciting topic in mechanics since long ago [1]. It is known that a stationary inverted pendulum is at an unstable equilibrium. However, the structure becomes stable when the base is vibrated with a small amplitude but high frequency. In 1951, Pyotr Kapitza developed a comprehensive theory describing quantitatively the physics behind this mechanical problem and providing the necessary stability conditions [2, 3]. His work on this theory launched the fields of vibrational mechanics and vibrational resonance with applications in various fields of science and technology. Inspired by Kapitza’s work, in my group we have been exploring how such phenomena can be brought into the fields of electromagnetics and optics. In other words, can an “unstable” electromagnetic problem be made “stable” by adding a small-amplitude, high-frequency modulation of a parameter? We have been investigating several scenarios for this purpose. One of these scenarios is the case of LC circuit with negative (non-Foster) capacitance [4]. Such a circuit exhibits instability due to the presence of negative C. However, we have shown theoretically that a properly time-modulated inductor L(t) can indeed make the circuit stable even in the presence of negative C [4]. This problem has led us to the next scenario of “Temporal Illusion”, in which a judiciously selected time-varying permittivity can imitate light-matter interaction in an object with unusual permittivity values [5]. Another case we are exploring is how monochromatic evanescent waves in Drude-negative-permittivity material can be significantly altered into monochromatic propagating waves by having periodic high-spatial-frequency distributions of positive-negative inhomogeneity. More cases are in progress now. In this talk, I will present an overview of our most recent results and discuss some of the salient features of these phenomena inspired by the Kapitza inverted pendulum problem.

Multistable Elastic Pixels (MEPs) are liquid crystal-based unit cells designed to enable nonvolatile reconfiguration of scattering inclusions. We experimentally demonstrate MEPs and build metasurfaces composed of MEP arrays to achieve tunable diffraction of visible wavelengths.

The ability to perform mathematical computations using metastructures is an emergent paradigm that carries the potential of wave-based analog computing to the realm of near-speed-of-light, low-loss, compact devices. We theoretically introduce and experimentally verify the concept of a reconfigurable metastructure that performs analog complex mathematical computations using electromagnetic waves. Reconfigurable, RF-based components endow our device with the ability to perform stationary and non-stationary iterative algorithms. After demonstrating matrix inversion (stationary problem), we use the machine to tackle two major non-stationary problems: root finding with Newton's method and inverse design (constrained optimization) via the Lagrange multiplier method. The platform enables possible avenues for wave-based, analog computations for general linear algebraic problems and beyond in compact, ultrafast, and parallelized ways.

We report on the design and experimental validation of a programmable silicon photonic architecture solving recursive mathematical problems directly in the optical domain. The reconfigurable kernel of the circuit is realized through a mesh of thermally tuneable Mach-Zehnder Interferometers, which is embedded into optical feedback loops and is used to implement arbitrary unitary matrices. The proposed architecture is employed to experimentally demonstrate matrix inversion without any optical-to-electrical conversion.