The field of low-loss dielectric metasurfaces has created a new paradigm of miniaturization for free-space optical elements. We study the effects of uniformly changing the refractive index of high-index metalenses. We experimentally demonstrate a modal refractive-index change $(\mathrm{\Delta }{n}_{\mathrm{MD}}=0.15)$ in the fundamental Mie resonances in an $\mathrm{In}\mathrm{Sb}$ resonator based on traditional thermo-optic effects. We develop a high aspect ratio metasurface design with simulated 75–90% transmission efficiency and 2π phase shift as a function of cylinder radius. A metalens is shown to have high (>60%) focusing efficiencies for large (up to 0.8) numerical aperture designs. The uniform thermal tuning of the metalens system is studied based on the variations in the spatial phase profile of the individual resonators. We demonstrate that both the operating wavelength $(\mathrm{\Delta }{\lambda }_f=500\mathrm{nm})$ and the focal length (Δf = 45 µm) can be dynamically modified based on the chromatic dispersion of the engineered metalens. The results show that static metasurfaces made of high-index semiconductors can be thermally actuated to tune the operating focal wavelength and focal length.

One-way edge states at the surface of photonic topological insulators are of significant interest for communications and nonlinear and quantum optics. Moreover, when reciprocity is broken in a photonic topological insulator, these states provide protection against disorder, which is of particular importance for slow-light applications. Achieving such a one-way edge state, however, requires the construction of a two-dimensional structure. Here, we show how unidiriectional Floquet bands can arise in purely one-dimensional, adiabatically modulated dynamic systems, in contrast with the higher dimensionality needed in topological insulators. We also show that, using realistic experimental parameters, the concept can be implemented using both a coupled-resonator optical waveguide and a photonic-crystal waveguide. Furthermore, we illustrate the associated protection against disorder and find it to be of a different nature when compared to Floquet topological insulators.

We demonstrate the generation of multimode entangled states of propagating microwaves. The entangled states are generated by our parametrically pumping a multimode superconducting cavity. By combining different pump frequencies, applied simultaneously to the device, we can produce different entanglement structures in a programable fashion. The Gaussian output states are fully characterized by our measuring the full covariance matrices of the modes. The covariance matrices are absolutely calibrated by our using an in situ microwave calibration source, a shot-noise tunnel junction. Applying a variety of entanglement measures, we demonstrate both full inseparability and genuine tripartite entanglement of the states. Our method is easily extensible to more modes.

Submonolayer quantum dots (SML QDs) combine the large gain cross section of quantum wells (QWs) with the potential benefits of a stronger confinement. We demonstrate here an optically pumped vertical external-cavity surface emitting laser (VECSEL) based on active SML QDs. The ultrafast SML QD VECSEL is optimized for passive modelocking with an intracavity semiconductor saturable absorber mirror (SESAM) around 1030 nm. We have achieved the highest cw output power of 11.2 W from a QD-based VECSEL to date. With a birefringent filter inside the VECSEL cavity, we obtain a tuning range of 47 nm centered at 1028 nm. Any attempt to passively modelock the VECSEL with a QW SESAM reveals fundamental limitations. The intrinsically higher linewidth enhancement factor of SML QDs compared to QWs or self-assembled Stranski-Krastanov QDs is further increased at higher pump powers. Our experiments confirm prior theoretical predictions that a strong amplitude-phase coupling can destabilize cw modelocking and introduce chaotic multiple pulse fluctuations.

We report a single-stage bidirectional interface capable of linking ${\mathrm{Sr}}^{+}$ trapped-ion qubits in a long-distance quantum network. Our interface converts photons between the ${\mathrm{Sr}}^{+}$ emission wavelength at 422 nm and the telecom $C$ band to enable low-loss transmission over optical fiber. We have achieved both up- and down-conversion at the single-photon level with efficiencies of 9.4% and 1.1%, respectively. Furthermore, we demonstrate noise levels that are low enough to allow for genuine quantum operation in the future.

Self-assembled quantum dots (QDs) provide an ideal platform for realization of on-demand maximally entangled photon pairs, which is essential for quantum-information technologies. When the two intermediate exciton states are indistinguishable in energy, the biexciton cascade process naturally yields the maximally entangled photon pairs. However, because of the low symmetry of QDs, the two bright states should belong to different irreducible representations in realistic experiments, and their energy difference, called the “fine-structure splitting” (FSS), is much larger than their homogeneous broadening, giving rise to only classically correlated photon pairs by erasing the “which-way” information. In this work, we show that since these two states belong to different representations, their lifetimes should also be slightly different, which is termed “exciton lifetime asymmetry.” In contrast to the extensively studied FSS, investigation of this lifetime asymmetry is missing in the literature. Here we conduct an investigation of the exciton lifetime asymmetry in self-assembled QDs and present a theory to deduce this asymmetry indirectly from measurable qualities of QDs. We further reveal that the intrinsic lifetimes and their asymmetries are more fundamental quantities of QDs, which determine the upper bound of the extrinsic lifetime asymmetries. Exact relations between lifetime asymmetries, FSS, and the polarization angle are also derived. These relations may be measured in experiments by studying the degree of polarization, as well its evolution under external stress. Our findings provide a complete description of the symmetry of the optical properties of QDs, which can give an important basis to deepen our understanding of QDs.

A stable lattice of topological defects occurring at the electric reorientation of a nematic fluid is studied by quantitative polarimetry and laser diffraction. Generation of optical vortices by topological defects is demonstrated in the case of two distinct mechanisms. First, individual defects convert circularly polarized light partially into a vortex beam with opposite handedness, while beams diffracted on the defect lattice do not carry vorticity. Second, dislocation of the lattice structure is a topological defect on a larger length scale; then beams diffracted on a single dislocation possess optical vortex character. The vortex-generation efficiency is tunable by the applied voltage.

We present a method to measure quadratic terahertz optical nonlinearities in terahertz time-domain spectroscopy. We use a rotating linear polarizer (a polarizing chopper) to modulate the amplitude of the incident terahertz pulse train. We use phase-sensitive lock-in detection at the fundamental and the second harmonic of the modulation frequency to separate the materials’ responses that are linear and quadratic in a terahertz electric field. We demonstrate this method by measuring the quadratic terahertz Kerr effect in the presence of the much stronger linear electro-optic effect in the (110) $\mathrm{GaP}$ crystal. We propose that the method can be used to detect terahertz second-harmonic generation in noncentrosymmetric media in time-domain spectroscopy, with broad potential applications in nonlinear terahertz photonics and related technology.

The exploration of the binary valley degree of freedom in topological photonic systems has inspired many intriguing optical phenomena such as the photonic Hall effect, robust delay lines, and perfect out-coupling refraction. In this work, we experimentally demonstrate the tunability of electromagnetic flow in a valley photonic crystal waveguide. By continuously controlling the phase differences of a microwave monopolar antenna array, the flow of electromagnetic waves can split into different directions according to the chirality of the phase vortex, and the splitting ratio varies from 0.9 to 0.1. Topological valley transport of edge states is also observed at the photonic domain wall. Tunable edge state dispersion, i.e., from gapless valley-dependent modes to gapped flat bands, is found at the photonic boundary between a valley photonic crystal waveguide and a perfect electric conductor, leading to the tunable frequency bandwidth of high transmission. Our work paves the way to the controllable and dynamic modulation of electromagnetic flow in topological photonic systems.

For a conventional higher-order Poincaré (HOP) sphere beam that generalizes optical vortices and vector beams, its donutlike intensity profile varies significantly with topological charges, which is undesirable for many applications in optical manipulations and communications. Recently, the perfect optical vortex and perfect vector beam have been reported to provide a solution. Until now, this strategy has not been extended to the whole HOP sphere. In this work, the concept of a perfect HOP sphere is proposed for the first time and realized by digitalizing the spiral geometric phase with specific structures of circular Dammann gratings (CDGs), which are demonstrated in liquid crystals through dynamic photo-patterning. Via selecting the incident spin, any point on the perfect HOP sphere can be obtained. Properly programming CDGs makes both single- and multi-ringed perfect HOP sphere beams achievable with merits of high efficiency and uniform energy distribution. All experimental results are qualitatively consistent with the theoretical calculations. This integrated liquid crystal device paves the way for perfect optical angular momentum manipulation and facilitates numerous cutting-edge applications.

Nonlocal (spatial-dispersion) effects in multilayered metamaterials composed of periodic stacks of alternating, deeply subwavelength dielectric layers are known to be negligibly weak. Counterintuitively, under certain critical conditions, weak nonlocality may build up strong boundary effects that are not captured by conventional (local) effective-medium models based on simple mixing formulas. Here we show that this phenomenon can be fruitfully studied and understood in terms of error propagation in the iterated maps of the trace and antitrace of the optical transfer matrix of the multilayer. Our approach effectively parameterizes these peculiar effects via remarkably simple and insightful closed-form expressions, which enable direct identification of the critical parameters and regimes. We also show how these boundary effects can be captured by suitable nonlocal corrections.

The processes of hole localization in ${\mathrm{Y}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ and ${\mathrm{Lu}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ single crystals are investigated by electron paramagnetic resonance (EPR) and thermally stimulated luminescence (TSL). It is found that holes created by x-ray irradiation at 77 K are predominantly self-trapped at regular oxygen ions forming an ${\mathrm{O}}^{-}$ hole center. This self-trapped hole (STH) center is thermally stable to about 100 K in both yttrium aluminum garnet ${\mathrm{Y}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ (YAG) and lutetium ${\mathrm{Lu}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$ garnet ($\mathrm{Lu}\mathrm{AG}$) crystals. At higher temperatures, thermally liberated holes are retrapped at oxygen ions in the vicinity of an acceptor ion such as ${\mathrm{Mg}}^{2+}$ and ${\mathrm{Al}}_{\mathrm{Y}}$ or ${\mathrm{Al}}_{\mathrm{Lu}}$ antisite ions which leads to an increase in the thermal stability of the trapped hole approximately equal to 150 K. TSL measurements show two composite glow peaks in the temperature range of 77–280 K, the temperature positions of which correlate well with the thermal stability of the ${\mathrm{O}}^{-}$ centers. The hole thermal ionization energy is determined from a numerical fit of the TSL peaks within the model of second-order kinetics. It is in the range of 0.25–0.26 eV for an ${\mathrm{O}}^{-}$ STH center, and increases to 0.41–0.45 eV for an ${\mathrm{O}}^{-}$ center stabilized by the acceptor. Revealed ${\mathrm{O}}^{-}$ centers can be attributed to ${\mathrm{O}}^{-}$ small polarons formed mainly due to hole stabilization by short-range interactions with the surrounding lattice. Knowledge of the nature of color centers and related trapping sites is critically important for further optimization of all garnet scintillation materials.

Magnetic resonance coupling theory for connected metamaterial resonators is studied and experimentally demonstrated. The subwavelength resonators have dominantly conductive coupling through the current exchange. Two distinct interactions are observed in two different connection configurations. Based on the strong coupling, one-dimensional localized surface magnetic modes propagating along a chain of subwavelength resonators are proposed. By changing the connection configuration, the surface wave can be switched between the forward and backward modes. The numerical simulations and experiments are in good agreement. The proposed mechanism of energy transport in the subwavelength scale has potential applications in integrated devices and circuits.

The duration and fidelity of qubit readout are critical factors for applications in quantum-information processing as they limit the fidelity of algorithms which reuse qubits after measurement or apply feedback based on the measurement result. Here we present fast multiplexed readout of five qubits using a single 1.2-GHz-wide readout channel. Using a readout pulse length of 80 ns and populating readout resonators for less than 250 ns, we find an average probability of correct assignment for the five measured qubits to be $97\mathrm{\%}$. The differences between the individual readout errors and those found when measuring the qubits simultaneously are within $1\mathrm{\%}$. We employ individual Purcell filters for each readout resonator to suppress off-resonant driving, which we characterize by measuring the dephasing imposed on unintentionally measured qubits. We expect the readout scheme presented here to become particularly useful for the selective readout of individual qubits in multiqubit quantum processors.

With the rapid development of optical metasurfaces, anticounterfeiting methods continue to evolve. Recently, the unprecedented capability of optical metasurfaces in the local manipulation of the light’s polarization has been used to generate arbitrary polarization profiles, providing an unusual method for anticounterfeiting. We experimentally demonstrate a metasurface device that can hide a quick response (QR) code in the polarization profile of a laser beam. The desired space-variant polarization profile originates from the superposition of two circularly polarized light beams with opposite handedness based on a single metasurface. A linear polarizer is used to reveal the hidden QR code. The anticounterfeiting property of such a device has potential applications in product identification, item tracking, and document management.

The ability to control the polarization of light at the extreme nanoscale has long been a major scientific and technological goal for photonics. Here we predict the phenomenon of polarization splitting through van der Waals heterostructures of nanoscale thickness, such as graphene-hexagonal boron nitride heterostructures, at infrared frequencies (near 25.35 THz). The underlying mechanism is that the designed heterostructures possess an effective relative permittivity with its in-plane (out-of-plane) component being unity (zero); such heterostructures are transparent to transverse-electric (TE) waves while opaque to transverse-magnetic (TM) waves, without resorting to the interference effect. Moreover, the predicted phenomenon is insensitive to incident angles. Our work thus indicates that van der Waals heterostructures are a promising nanoscale platform for the manipulation of light, such as the design of polarization beam nanosplitters and ε-near-zero materials, and the exploration of superscattering for TM waves and zero scattering for TE waves from deep-subwavelength nanostructures.

Conventional transparent materials at optical frequencies have refractive-index values (n) greater than unity—most commonly between about 1 and 4. This paper explores optical phenomena made possible by using materials with refractive indices less than unity. We focus primarily on fused silica ($\mathrm{Si}{\mathrm{O}}_2$), a well-studied dielectric with strong optical-phonon resonances in the mid infrared that result in a spectral region in which n < 1 with modest optical loss. Using this ubiquitous easy-to-deposit material, we demonstrate infrared-frequency external reflection, frustrated external reflection, and direct coupling to surface-plasmon polaritons from free space. Our work suggests that materials with refractive indices below unity can bring significant new functionalities to optical devices.

Time-frequency duality, which enables the control of optical waveforms by manipulating the amplitudes and phases of electromagnetic fields, plays a pivotal role in a wide range of modern optics. The conventional one-dimensional (1D) time-frequency duality has been successfully applied to characterize the behavior of classical light, such as ultrafast optical pulses from a laser. However, the 1D treatment is not enough to characterize quantum-mechanical correlations in the time-frequency behavior of multiple photons, such as the biphotons from parametric down-conversion. Two-dimensional (2D) treatment is essentially required, but has not been fully demonstrated yet due to technical problems. Here, we study the 2D time-frequency duality of biphotons, by measuring two-photon distributions in both the frequency and the time domain. It is discovered that the generated biphotons satisfy the Fourier-transform-limited conditions in a two-photon spectral distribution, but do not satisfy those conditions in the single-photon marginal distribution. Our study may pave the way for the quantum-optical synthesizer, the next-generation technology beyond the traditional optical synthesis in the time-frequency domain.

Presently, silicon photonics requires photodetectors that are sensitive in a broad infrared range, can operate at room temperature, and are suitable for integration with the existing $\mathrm{Si}$-technology process. Here, we demonstrate strong room-temperature sub-band-gap photoresponse of photodiodes based on $\mathrm{Si}$ hyperdoped with tellurium. The epitaxially recrystallized Te-hyperdoped $\mathrm{Si}$ layers are developed by ion implantation combined with pulsed-laser melting and incorporate $\mathrm{Te}$-dopant concentrations several orders of magnitude above the solid solubility limit. With increasing $\mathrm{Te}$ concentration, the $\mathrm{Te}$-hyperdoped layer changes from insulating to quasi-metallic behavior with a finite conductivity as the temperature tends to zero. The optical absorptance is found to increase monotonically with increasing $\mathrm{Te}$ concentration and extends well into the mid-infrared range. Temperature-dependent optoelectronic photoresponse unambiguously demonstrates that the extended infrared photoresponsivity from $\mathrm{Te}$-hyperdoped $\mathrm{Si}$ p-n photodiodes is mediated by a $\mathrm{Te}$ intermediate band within the upper half of the $\mathrm{Si}$ band gap. This work contributes to pave the way toward establishing a $\mathrm{Si}$-based broadband infrared photonic system operating at room temperature.

We investigate the lasing dynamics of optically-pumped submicrowatt-threshold Raman silicon lasers that employ a high-quality (high-Q) nanocavity design. The measurements reveal that free carriers generated by two-photon absorption induce dynamic effects during the initial lasing process, even at the very low threshold power of 0.12 µW. At higher excitation powers, the Raman laser signal exhibits a significant reduction within a few hundred nanoseconds after the initial rise, followed by clear oscillations. We find that the temporal behavior of the laser signal strongly depends on the excitation wavelength. However, the Raman laser signal converges to a stable continuous operation within a few microseconds after the initial rise for any excitation power employed in this work. Numerical simulations indicate that the oscillations reflect the dynamic shift of the resonant wavelength due to the thermo-optic effect and the carrier-plasma effect, which are induced by free carriers generated via two-photon absorption. These results are useful for understanding the correct design of future devices that employ Raman silicon nanocavity lasers.