The design of the scattering properties of electromagnetic structures is of fundamental interest in optical science and engineering. While there has been great practical success in applying local optimization methods to electromagnetic device design, it is unclear whether the performance of the resulting designs is close to that of the best possible design. This question remains open for absorptionless electromagnetic devices, since the absence of material loss makes it difficult to provide provable bounds on their scattering properties. We resolve this problem by providing nontrivial lower bounds on performance metrics that are convex functions of the scattered fields. Our bounding procedure relies on accounting for a constraint on the electric fields inside the device, which can be provably constructed for devices with small footprints or low dielectric contrast. We illustrate our bounding procedure by upper bounding the scattering cross sections of lossless dielectric and metallic particles.

We report a comprehensive study of low-power octave-bandwidth single-soliton microresonator frequency combs in both the 1550-nm and 1064-nm bands. Our experiments utilize fully integrated silicon-nitride Kerr microresonators and we demonstrate direct soliton generation with widely available distributed-Bragg-reflector lasers that provide less than 40 mW of chip-coupled laser power. We report measurements of soliton thermal dynamics and demonstrate how rapid laser-frequency control, consistent with the thermal time scale of a microresonator, facilitates stabilization of octave-bandwidth soliton combs. Moreover, since soliton combs are completely described by the fundamental linear and nonlinear dynamics of the intraresonator field, we demonstrate the close connection between modeling and generation of octave-bandwidth combs. Our experiments advance the development of self-referenced frequency combs with integrated-photonics technology and comb-laser sources with a tens-of-terahertz pulse bandwidth across the near infrared.

Embedded eigenstates are nonradiative modes of an open structure with momentum compatible with radiation, yet characterized by unboundedly large Q factors. Traditionally, these states originate from total destructive interference of radiation from two or more nonorthogonal modes in periodic structures. In this work, we demonstrate a class of embedded eigenstates based on Berreman modes in epsilon-near-zero layered materials and propose realistic silicon carbide structures that support high-Q ($-{10}^3$) resonances based on these principles. The proposed structures demonstrate strong absorption in a narrow spectral and angular range, giving rise to quasicoherent and highly directive thermal emission.

Controlling the polarization state of light at a fast speed is important in both classical and quantum regimes. Here, we present an easily integrated compact tunable linear polarization-state generator of single photons in the telecom wavelength band on a periodically poled lithium niobate on insulator (PPLNOI) chip. The linear polarization of single photons can be continuously rotated by applying a transverse electric field utilizing the electro-optic (EO) effect. With strong light confinement and the large EO coefficient in the PPLNOI ridge waveguide, dramatically reduced driving voltage and fast-speed modulation is realized. Moreover, quantum entanglement is well maintained during the switching of polarization states, as proved in the time-energy entanglement measurement. The scheme holds promise for realizing integrated quantum optics.

The aim of this work is to clarify the scintillation-yield enhancement in $\mathrm{Lu}\mathrm{YAG}:\mathrm{Pr}$ scintillators obtained by $\mathrm{Li}$ codoping via integrated study of the valence state of activators, the preferential site occupancy of $\mathrm{Li}$ codopants, and defect structures from experimental and theoretical insights. With $\mathrm{Li}$ codoping, the light yield and energy resolution of $10\times 10\times 10{\mathrm{mm}}^3$ $\mathrm{Lu}\mathrm{YAG}:\mathrm{Pr}$ samples are improved from 15 600 to 24 800 photons/MeV, and 5.3 to 4.3% at 662 keV, respectively. The optical absorption spectra indicate that $\mathrm{Li}$ codoping does not induce conversion of stable ${\mathrm{Pr}}^{3+}$ to ${\mathrm{Pr}}^{4+}$ in $\mathrm{Lu}\mathrm{YAG}:\mathrm{Pr}$ single crystals. Based on the formation energies of substitutional and interstitial $\mathrm{Li}$ sites using density-functional-theory (DFT) calculations and the ${}^7\mathrm{Li}$ nuclear magnetic resonance results, it is shown that the $\mathrm{Li}$ ions prefer to dominantly occupy the fourfold coordinated interstitial sites and fourfold coordinated $\mathrm{Al}$ sites. The systematic analysis of thermoluminescence glow curves, positron annihilation lifetime spectroscopies, and defect formation energies derived from DFT calculations reveals that the concentration of isolated $\mathrm{Lu}$ and $\mathrm{Al}$ vacancies as dominant acceptor defects is reduced by $\mathrm{Li}$ codoping, whilst the shallow ${\mathrm{Li}}_i$ interstitial defects and the deep $V_O$ oxygen vacancies are introduced simultaneously. We propose that the lowering of hole trapping at defects resulting from $\mathrm{Li}$ codoping contributes to the scintillation-yield enhancement.

The ability to control the directional and spectral properties of thermal emission is of fundamental importance for many applications. Here we propose a method to design directional and broadband thermal emitters (DBTEs) and directional and narrowband thermal emitters (DNTEs) based on classic thermal emitters and angular selectors. DBTEs exhibit strong thermal emission at a specific angle and within a broadband wavelength range. Besides, DBTEs achieve better radiative cooling performance than classic thermal emitters without special angular control when there are surrounding buildings. On the other hand, DNTEs can confine thermal emission within a narrow bandwidth and a narrow angular width. DNTEs show higher energy conversion efficiency than conventional narrowband thermal emitters, and have potential application in high-efficiency infrared light sources.

A laser diode subjected to optoelectronic feedback in which some light is converted to photocurrent that is fed back into the laser injection terminals can display periodic oscillations in its optical intensity. We demonstrate experimental and numerical evidence that, as the feedback level is varied, a stepwise change in the oscillation frequency manifests itself in the output optical intensity. These transitions in the dynamics can either correspond to an abrupt jump between two limit cycles, associated with a subcritical torus bifurcation of a limit cycle, or to a progressive switching mediated by an intermediate quasiperiodic state. Finally, when ramping the feedback level down, hysteresis is observed in the sequence of switching events between the limit cycles. Such devices are of interest for photonic microwave sources for wavelength-division multiplexed radio-over-fiber systems.

On-chip channelless diffraction-free beam guiding enables dense integration of optical circuits in a reconfigurable manner, where total internal reflection, which is considered the cornerstone of guided-wave optics, is utilized to confine light in the out-of-plane direction. Here, we theoretically propose a physical mechanism to achieve on-chip channelless diffraction-free beam guiding beyond the light cone, utilizing the physics of bound states in the continuum. A bound state in the continuum with a tailored spatial dispersion plays an important role in cancelling both the in-plane diffraction and the out-of-plane scattering when the condition for total internal reflection is not satisfied. As a proof-of-concept verification, we experimentally demonstrate such an effect based on an all-dielectric platform using microwaves. The on-chip channelless diffraction-free beam guiding beyond the light cone also allows direct free-space coupling to such self-collimation modes. Our results may open up an avenue for exploring the physics and applications of guided-wave optics.

This paper presents a low-loss and broadband impedance-interface waveguide and a broadband mode-matching technique to convert transverse electromagnetic (TEM) mode into line-wave (LW) mode, which is formed at the interface between two complementary impedance surfaces. Furthermore, an ultra-wideband Y splitter is designed based on the one-dimensional (1D) impedance-interface waveguides. Broad bandwidth, low loss, tunable mode confinement, and ability for integration are the benefits of 1D impedance-interface waveguides. However, their drawbacks may stem from the lack of and problematic excitation of a pure LW mode at the interface between two planes. Another limitation is the coupling efficiency between 1D impedance-interface waveguides and conventional microwave waveguides. To ratify such limitations, a broadband tapered-slot Vivaldi transition is designed to excite the proposed 1D impedance-interface waveguide. Subsequently, a Y splitter consisting of complementary impedance surfaces is designed, fabricated, and measured. The Y splitter is implemented in the frequency band of 4–16 GHz, having a bandwidth of $120\mathrm{\%}$ and an insertion loss of approximately 1 dB. The aforementioned characteristics of 1D impedance-interface waveguides are fully verified. Consequently, the proposed waveguide, mode-matching technique, and Y splitter could be used for the design of other microwave devices.

Single molecules in solids have been considered as an attractive class of solid-state single quantum systems because they can be chemically synthesized to have stable transitions at desired wavelengths. Here, we report and demonstrate single dibenzoterrylene molecules in crystalline anthracene nanosheets as a solid-state platform for delicate photophysics studies and as building blocks of single-photon devices. The nanosheet sample enables studies of single-molecule photophysics at room temperature, including in situ observation of single-molecule insertion-site jump, quantitative measurements of the associated changes of dipole-moment orientation and magnitude, and determination of the excitation-power-dependent intersystem crossing rate and the triplet lifetime. Moreover, we demonstrate the flexible assembly of the nanosheets into a planar Yagi-Uda antenna device to achieve a Gaussian-like angular emission pattern from single dibenzoterrylene molecules. The low thickness (approximately 100 nm), good photostability, and mechanical stability make the dibenzoterrylene-in-anthracene-nanosheet system an excellent candidate for static quantum nodes in integrated photonic circuits.

The nitrogen-vacancy center in diamond has been explored extensively as a light-matter interface for quantum information applications, however, it is limited by low coherent photon emission and spectral instability. Here, we present a promising interface based on an alternative defect with superior optical properties (the germanium-vacancy) coupled to a finesse-11 000 fiber cavity, resulting in a ${31}_{-15}^{+11}$-fold increase in the spectral density of zero-phonon-line emission. This work sets the stage for cryogenic experiments, where we predict a measurable increase in the spontaneous emission rate.

The synthesis of ultrathin photonic structures in order to effectively redirect and mold optical wave fronts into arbitrary shapes is of crucial importance in modern beam steering, imaging, and sensing technologies. To this end, planar subwavelength systems such as optical metasurfaces have been intensely investigated in recent years. Such arrangements rely on abrupt, yet controllable, phase shifts imparted on the incident beam, by means of judiciously designed anisotropic nanoantennas. Here, we propose and demonstrate an altogether different methodology in order to manipulate the flow of light, by adopting a diatomic parity-time ($\mathcal{P}\mathcal{T}$)-symmetric Bravais-lattice topology, the unit cell of which involves only a transparent and a lossy optical component. In this respect, a honeycomblike configuration is employed, the principal symmetries of which are progressively broken through specific geometric transformations. The complex near-field coupling interactions between neighboring diffractive elements give rise to a discerning enhancement or attenuation along specific directions in the far field, over a broad range of wavelengths in the visible domain. In this work, we report the realization of an all-passive $\mathcal{P}\mathcal{T}$-symmetric optical metasurface on a flexible polyimide substrate, capable of demonstrating selective directional scattering. Our study draws a clear connection between surface topology and radiation directivity, which can be systematically utilized toward observing unconventional transport effects in flat- and curved-space $\mathcal{P}\mathcal{T}$ lattices.

$\mathcal{P}\mathcal{T}$-symmetric systems can be used as ultrasensitive sensors by exploiting the non-Hermitian degeneracy at the exceptional point. However, operating and controlling these systems at the exceptional point are usually difficult. Here, we study the effect of rotation on threshold gain of circular Bragg lasers with radial $\mathcal{P}\mathcal{T}$ symmetry. We find that, even deviating away from the exceptional point, these lasers can provide a two-orders-of-magnitude enhancement in the rotation sensitivity compared with the conventional circular Bragg lasers of similar geometry and threshold gain. Additionally, these lasers are more robust against perturbation in operating condition than sensors that must operate at the exceptional point.

The existing techniques for measuring high-dimensional pure states of light in the orbital angular momentum (OAM) basis either involve a large number of single-pixel data acquisitions and substantial postselection errors that increase with dimensionality, involve substantial loss, or require interference with a reference beam of known phase. Here, we propose an interferometric technique that can measure an unknown pure state using only nine multipixel image acquisitions without involving postselection, loss, or a separate reference beam. The technique essentially measures two complex correlation functions of the input field and then employs a recursive postprocessing algorithm to infer the state. We experimentally demonstrate the technique for pure states up to dimensionality of 25, reporting a mean fidelity greater than $90\mathrm{\%}$ up to 11 dimensions. Our technique can significantly improve the performance of OAM-based information processing applications.

The growing need for classical as well as quantum optical sensing places increasingly stringent requirements upon the desired characteristics of the engendered fields. Specifically, achieving superior field enhancement plays a critical role in applications ranging from chem-bio sensing, Raman and infrared spectroscopies to ion trapping and qubit control in emerging quantum-information science. Due to their low optical losses and ability to exhibit resonant field enhancements, all dielectric multilayers are emerging as an optical material system not only useful to classical photonics and sensing but also of potential to be integrated with quantum materials and quantum sensing. The recently introduced concept of zero-admittance layers [1] within dielectric multilayer materials, enables the creation and control of resonant fields orders of magnitude larger than the exciting field. Here, invoking the zero-admittance concept, we design, fabricate, and characterize an all-dielectric nonabsorbing stack and demonstrate the engendered huge field enhancement. Describing the fields in terms of Bloch surface waves, we connect the surface field to the semiperiodicity in the dielectric domains of the stack. As a specific application of the resonant field, we propose and demonstrate refractive-index sensing for the detection of trace amounts of an analyte. The results include a quantification of the sensitivity of the device with respect to the profile of the exciting field. The experimental results are shown to be in good agreement with theoretical calculations.

Nitrogen-vacancy centers in diamond allow measurement of environment properties such as temperature, magnetic and electric fields at the nanoscale level, of utmost relevance for several research fields, ranging from nanotechnologies to biosensing. The working principle is based on the measurement of the resonance frequency shift of a single nitrogen-vacancy center (or an ensemble of them), usually detected by monitoring the center photoluminescence emission intensity. Albeit several schemes have already been proposed, the search for the simplest and most effective one is of key relevance for real applications. Here we present a continuous-wave lock-in-based technique able to reach high sensitivity in temperature measurement at microscale or nanoscale volumes (4.8 ${\mathrm{mK}/\mathrm{Hz}}^{1/2}$ in $\mu {\mathrm{m}}^3$). Furthermore, the present method has the advantage of being insensitive to environmental magnetic noise that in general introduces a bias in the temperature measurement.

We analyze the dynamics of passively mode-locked integrated external-cavity surface-emitting lasers (MIXSELs) using a first-principles dynamical model based on delay algebraic equations. We show that the third-order dispersion stemming from the lasing microcavity induces a train of decaying satellites on the leading edge of the pulse. Due to the nonlinear interaction with carriers, these satellites may get amplified, thereby destabilizing the mode-locked states. In the long-cavity regime, the localized structures that exist below the lasing threshold are found to be deeply affected by this instability. As it originates from a global bifurcation of the saddle-node infinite-period type, we explain why the pulses exhibit forms of behavior characteristic of excitable systems. Using the multiple-time-scale and the functional-mapping methods, we derive rigorously a master equation for MIXSELs in which third-order dispersion is an essential ingredient. We compare the bifurcation diagrams of the two models and assess their good agreement.

We theoretically study plasmonic antennas featuring areas of extremely concentrated electric or magnetic field, known as hot spots. We combine two types of electric-magnetic complementarity to increase the degree of freedom for the design of the antennas: bowtie and diabolo duality and Babinet’s principle. We evaluate the figures of merit for different plasmon-enhanced optical spectroscopy methods and optical trapping: field enhancement, decay rate enhancement, quality factor of the plasmon resonances, and trapping potential depth. The role of Babinet’s principle in interchanging electric and magnetic field hot spots and its consequences for practical antenna design are discussed. In particular, diabolo antennas exhibit slightly better performance than bowties in terms of larger field enhancement and larger $Q$ factor. For specific resonance frequency, diabolo antennas are considerably smaller than bowties, which makes them favorable for the integration into more complex devices but also makes their fabrication more demanding in terms of spatial resolution. Finally, we propose a Babinet-type dimer antenna featuring electromagnetic hot spot with both the electric and magnetic field components treated on an equal footing.

A single-photon source is an essential tool for the emerging field of quantum technologies. Ideally, it should be spectrally compatible with other photonic devices while providing a high flux of narrow-band photons. The single organic dye molecule dibenzanthanthrene under cryogenic conditions possesses the given characteristics and therefore constitutes a prominent single-photon source. Nevertheless, the implementation of such a single-photon source requires a complex experimental setup involving a cryostat with a confocal microscope for the effective collection of the molecular emission. In the approach presented here we use a single emitter coupled directly to the end facet of an optical fiber. This has the potential to transfer a single-photon source based on a quantum emitter from a proof-of-principle type of setup to a scalable “plug-and-play” device. We present successful coupling of a single organic molecule to an optical fiber and record the excitation spectrum, measure the saturation curve, and analyze the contributions of Raman background fluorescence. The single-photon nature is proven by an antibunched autocorrelation recording, which reveals coherent Rabi oscillations.

We present a method for efficient spin-wave guiding within the magnonic nanostructures. Our technique is based on the anomalous refraction in the metamaterial flat slab. The gradual change of the material parameters (saturation magnetization or magnetic anisotropy) across the slab allows tilting the wavefronts of the transmitted spin waves and controlling the refraction. Numerical studies of the spin-wave refraction are preceded by the analytical calculations of the phase shift acquired by the spin wave due to the change of material parameters in a confined area. We demonstrate that our findings can be used to guide the spin waves smoothly in curved waveguides, even through sharp bends, without reflection and scattering between different waveguide’s modes, preserving the phase, the quantity essential for wave computing.