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Nonreciprocal gyrotropic materials have attracted significant interest recently in material physics, nanophotonics, and topological physics. Most of the well-known nonreciprocal materials, however, only show nonreciprocity under a strong external magnetic field and within a small segment of the electromagnetic spectrum. Here, through first-principles density functional theory calculations, we show that due to strong spin-orbit coupling manganese-bismuth (MnBi) exhibits nonreciprocity without any external magnetic field and a large gyrotropy in a broadband long-wavelength infrared regime (LWIR). Further, we design a multi-layer structure based on MnBi to obtain a maximum degree of spin-polarized thermal emission at 7 $\mu$m. The connection established here between large gyrotropy and the spin-polarized thermal emission points to a potential use of MnBi to develop spin-controlled thermal photonics platforms.

Dimensionality plays a crucial role in long-range dipole-dipole interactions (DDIs). We demonstrate that a resonant nanophotonic structure modifies the apparent dimensionality in an interacting ensemble of emitters, as revealed by population decay dynamics. Our measurements on a dense ensemble of interacting quantum emitters in a resonant nanophotonic structure with long-range DDIs reveal an effective dimensionality reduction to $\bar{d} = 2.20 (12)$, despite the emitters being distributed in 3D. This contrasts the homogeneous environment, where the apparent dimension is $\bar{d} = 3.00$. Our work presents a promising avenue to manipulate dimensionality in an ensemble of interacting emitters.

Spectro-polarimetric imaging in the long-wave infrared (LWIR) region is a powerful tool for capturing temperature, material composition, and surface morphology information. However, current spectro-polarimetric LWIR imagers are often bulky and severely limited in spectral resolution and field of view (FOV). In this work, we present a new paradigm for spectro-polarimetric demultiplexing by combining large-area meta-optical devices and advanced computational imaging algorithms. We use the intrinsic dispersion and polarization modulation of anisotropic spinning metasurfaces to achieve simultaneous spectral and polarimetric resolution without the need for bulky filter wheels or interferometers. Our spinning-metasurface-based spectro-polarimetric module is robust, compact (< 10 x 10 x 10 cm) and has a wide field of view (20.5 degrees). Our approach represents a significant advancement in the field of thermal imaging, allowing for high-quality, information-rich thermal image data for a wide range of applications such as astronomical exploration, medical diagnosis, and agricultural monitoring.

The engineering of the spatial and temporal properties of both the electric permittivity and the refractive index of materials is at the core of photonics. When vanishing to zero, those two variables provide new knobs to control light-matter interactions. This perspective aims at providing an overview of the state of the art and the challenges in emerging research areas where the use of near-zero refractive index and hyperbolic metamaterials is pivotal, in particular light and thermal emission, nonlinear optics, sensing applications and time-varying photonics.

All optical switches offer advanced control over the properties of light at ultrafast timescales using optical pulses as both the signal and the control. Limited only by material response times, these switches can operate at terahertz speeds, essential for technology-driven applications such as all-optical signal processing and ultrafast imaging, as well as for fundamental studies such as frequency translation and novel optical media concepts such as photonic time crystals. In conventional systems, the switching time is determined by the relaxation response of a single active material, which is challenging to adjust dynamically. This work demonstrates that the zero-to-zero response time of an all-optical switch can instead be varied through the combination of so-called fast and slow materials in a single device. When probed in the epsilon-near-zero operational regime of a material with a slow response time, namely, plasmonic titanium nitride, the switch exhibits a relatively slow, nanosecond response time. The response time then decreases reaching the picosecond time scale in the ENZ regime of the faster material, namely, aluminum-doped zinc oxide. Overall, the response time of the switch is shown to vary by two orders of magnitude in a single device and can be selectively controlled through the interaction of the probe signal with the constituent materials. The ability to adjust the switching speed by controlling the light-matter interaction in a multi-material structure provides an additional degree of freedom in all-optical switch design. Moreover, the proposed approach utilizes slower materials that are very robust and allow to enhance the field intensities while faster materials ensure an ultrafast dynamic response. The proposed control of the switching time could lead to new functionalities within key applications in multiband transmission, optical computing, and nonlinear optics.

The dynamics of three interacting objects has been investigated extensively in Newtonian gravitational physics (often termed the three-body problem), and is important for many quantum systems, including nuclei, Efimov states, and frustrated spin systems. However, the dynamics of three macroscopic objects interacting through quantum vacuum fluctuations (virtual photons) is still an unexplored frontier. Here, we report the first observation of Casimir interactions between three isolated macroscopic objects. We propose and demonstrate a three terminal switchable architecture exploiting opto-mechanical Casimir interactions that can lay the foundations of a Casimir transistor. Beyond the paradigm of Casimir forces between two objects in different geometries, our Casimir transistor represents an important development for control of three-body virtual photon interactions and will have potential applications in sensing and information processing with the Casimir effect., Comment: 7 pages, 4 figures

Photonic spin density (PSD) in the near-field gives rise to exotic phenomena such as photonic skyrmions, optical spin-momentum locking and unidirectional topological edge waves. Experimental investigation of these phenomena requires a nanoscale probe that directly interacts with PSD. Here, we propose and demonstrate that the nitrogen-vacancy (NV) center in diamond can be used as a quantum sensor for detecting the spinning nature of photons. This room temperature magnetometer can measure the local polarization of light in ultra-subwavelength volumes through photon-spin-induced virtual transitions. The direct detection of light's spin density at the nanoscale using NV centers in diamond opens a new frontier for studying exotic phases of photons as well as future on-chip applications in spin quantum electrodynamics (sQED)., Comment: 21 pages, 9 figures

The strong electric and magnetic resonances in dielectric subwavelength structures have enabled unique opportunities for efficient manipulation of light-matter interactions. Besides, the dramatic enhancement of nonlinear light-matter interactions near so-called bound states in the continuum (BICs) has recently attracted enormous attention due to potential advancements in all-optical and quantum computing. However, the experimental realizations and the applications of high- Q factor resonances in dielectric resonances in the visible regime have thus far been considerably limited. In this work, we explore the interplay of electric and magnetic dipoles in arrays of dielectric nanoresonators to enhance light-matter interaction. We report on the experimental realization of high-Q factor resonances in the visible through the collective diffractive coupling of electric and magnetic dipoles. Providing direct physical insights, we also show that coupling the Rayleigh anomaly of the array with the electric and magnetic dipoles of the individual nanoresonators can result in the formation of different types of BICs. We utilize the resonances in the visible regime to achieve lasing action at room temperature with high spatial directionality and low threshold. Finally, we experimentally demonstrate multi-mode, directional lasing and study the BIC-assisted lasing mode engineering in arrays of dielectric nanoresonators. We believe that our results enable a new range of applications in flat photonics through realizing on-chip controllable single and multi-wavelength micro-lasers.

We introduce the concept of a photonic Dirac monopole, appropriate for photonic crystals, metamaterials and 2D materials, by utilizing the Dirac-Maxwell correspondence. We start by exploring vacuum where the reciprocal momentum space of both Maxwell's equations and the massless Dirac equation (Weyl equation) possess a magnetic monopole. The critical distinction is the nature of magnetic monopole charges, which are integer valued for photons but half-integer for electrons. This inherent difference is directly tied to the spin and ultimately connects to the bosonic or fermionic behavior. We also show the presence of photonic Dirac strings, which are line singularities in the underlying Berry gauge potential. While the results in vacuum are intuitively expected, our central result is the application of this topological Dirac-Maxwell correspondence to 2D photonic (bosonic) materials, as opposed to conventional electronic (fermionic) materials. Intriguingly, within dispersive matter, the presence of photonic Dirac monopoles is captured by nonlocal quantum Hall conductivity - i.e. a spatiotemporally dispersive gyroelectric constant. For both 2D photonic and electronic media, the nontrivial topological phases emerge in the context of massive particles with broken time-reversal symmetry. However, the bulk dynamics of these bosonic and fermionic Chern insulators are characterized by spin-1 and spin-1/2 skyrmions in momentum space, which have fundamentally different interpretations. This is exemplified by their contrasting spin-1 and spin-1/2 helically quantized edge states. Our work sheds light on the recently proposed quantum gyroelectric phase of matter and the essential role of photon spin quantization in topological bosonic phases., Comment: 25 pages, 4 figures. This is the fourth paper in a series of papers on spin and topological bosonic phases of light. The others can be found here: https://doi.org/10.1103/PhysRevA.98.023842, arXiv:1708.08192, and https://doi.org/10.1364/OPTICA.3.000118

Fundamental differences between fermions and bosons are revealed in their spin and distribution statistics as well as the discrete symmetries they obey (charge, parity and time). While significant progress has been made on fermionic topological phases of matter with time-reversal symmetry, the bosonic counterpart still remains elusive. We present here a spin-1 bosonic topological insulator for light by utilizing a Dirac-Maxwell correspondence. Departing from structural photonic approaches which mimic the pseudo-spin-\textonehalf{} behavior of electrons, we exploit the integer spin and discrete symmetries of the photon to formulate a distinct bosonic topological phase of matter. We introduce a bosonic Kramers theorem and the photonic equivalent of topological quantization, which arises solely from photon spin. Our continuum field theory predicts that photons acquire a mass in the presence of a spatio-temporally dispersive degenerate chirality, a unique form of magneto-electric coupling inside matter fundamentally different from well-known chirality, magneto-electricity, gyrotropy or bi-anisotropy. We predict that this unique dispersive (non-local) degenerate chiral medium has anomalous parity and time-reversal symmetries and if found in nature will exhibit a gapped Quantum spin-1 Hall bosonic phase. Photons do not possess a conductivity transport parameter which can be quantized (unlike electronic systems), but we predict that photon spin quantization of symmetry-protected edge states is amenable to experimental isolation leading to a new bosonic phase of matter., Comment: 13 pages, 4 figures

The discovery of photonic hyperbolic dispersion surfaces in certain van der Waals bonded solids, such as hexagonal boron nitride and bismuth selenide (a topological insulator), offers intriguing possibilities for creating strongly modified light-matter interactions. However, open problems exist in quantifying electromagnetic field fluctuations in these media, complicating typical approaches for modeling photonic characteristics. Here, we address this issue by linking the identifying traits of hyperbolic response to a coupling between longitudinal and transverse fields that can not occur in isotropic media. This description allows us to calculate the influence of hyperbolic response on electromagnetic fluctuations without explicitly imposing a characteristic size (model of nonlocality), leading to formally bounded expressions so long as material absorption is included. We then apply this framework to two exemplary areas: the optical sum rule for modified spontaneous emission enhancement in a general uniaxial medium, and thermal electromagnetic field fluctuations in hexagonal boron nitride and bismuth selenide. We find that while the sum rule is satisfied, it does not constrain the enhancement of light-matter interactions in either case. We also show that both hexagonal boron nitride and bismuth selenide possess broad spectral regions where the magnitude of electromagnetic field fluctuations are over 120 times larger, and over 800 times larger along specific angular directions, than they are in vacuum., Comment: 14 pages, 7 figures

Single atoms form a model system for understanding the limits of single photon detection. Here, we develop a non-Markov theory of single-photon absorption by a two-level atom to place limits on the absorption (transduction) time. We show the existence of a finite rise time in the probability of excitation of the atom during the absorption event which is infinitely fast in previous Markov theories. This rise time is governed by the bandwidth of the atom-field interaction spectrum and leads to a fundamental jitter in time-stamping the absorption event. Our theoretical framework captures both the weak and strong atom-field coupling regimes and sheds light on the spectral matching between the interaction bandwidth and single photon Fock state pulse spectrum. Our work opens questions whether such jitter in the absorption event can be observed in a multi-mode realistic single photon detector. Finally, we also shed light on the fundamental differences between linear and nonlinear detector outputs for single photon Fock state vs. coherent state pulses., Comment: 10 pages, 6 figures

We observe unique absorption resonances in silver/silica multilayer-based epsilon-near-zero (ENZ) metamaterials that are related to radiative bulk plasmon-polariton states of thin-films originally studied by Ferrell (1958) and Berreman (1963). In the local effective medium, metamaterial descrip- tion, the unique effect of the excitation of these microscopic modes is counterintuitive and captured within the complex propagation constant, not the effective dielectric permittivities. Theoretical anal- ysis of the band structure for our metamaterials shows the existence of multiple Ferrell-Berreman branches with slow light characteristics. The demonstration that the propagation constant reveals subtle microscopic resonances can lead to the design of devices where Ferrell-Berreman modes can be exploited for practical applications ranging from plasmonic sensing to imaging and absorption enhancement.