Publications

Spin and orbital angular momentum of light plays a central role in quantum nanophotonics as well as topological electrodynamics. Here, we show that the thermal radiation from finite-size bodies comprising nonreciprocal magneto-optical materials can exert a spin torque even in global thermal equilibrium. Moving beyond the paradigm of near-field heat transfer, we calculate near-field radiative angular momentum transfer between finite-size nonreciprocal objects by combining Rytov's fluctuational electrodynamics with the theory of optical angular momentum. We prove that a single magneto-optical cubic particle in nonequilibrium with its surroundings experiences a torque in the presence of an applied magnetic field (T-symmetry breaking). Furthermore, even in global thermal equilibrium, two particles with misaligned gyrotropy axes experience equal-magnitude torques with opposite signs which tend to align their gyrotropy axes parallel to each other. Our results are universally applicable to semiconductors like InSb (magnetoplasmas) as well as Weyl semimetals which exhibit the anomalous Hall effect (gyrotropic) at infrared frequencies. Our work paves the way towards near-field angular momentum transfer mediated by thermal fluctuations for nanoscale devices.

A chiral absorber of light can emit spin-polarized (circularly polarized) thermal radiation based on Kirchhoff’s law which equates spin-resolved emissivity with spin-resolved absorptivity for reciprocal media at thermal equilibrium. No such law is known for nonreciprocal media. In this work, we discover three spin-resolved Kirchhoff’s laws of thermal radiation applicable for both reciprocal and nonreciprocal planar media. In particular, these laws are applicable to multi-layered or composite slabs of generic bianisotropic material classes which include (uniaxial or biaxial) birefringent crystals, (gyrotropic) Weyl semimetals, magnetized semiconductors, plasmas, ferromagnets and ferrites, (magnetoelectric) topological insulators, metamaterials and multiferroic media. We also propose an experiment to verify these laws using a single system of doped indium antimonide (InSb) thin film in an external magnetic field. Furthermore, we reveal a surprising result that the planar slabs of all these material classes can emit partially circularly polarized thermal light without requiring any surface patterning, and identify planar configurations which can experience nontrivial thermal optomechanical forces and torques upon thermal emission into the external environment at lower temperature (nonequilibrium). Our work also provides a new fundamental insight of detailed balance of angular momentum (in addition to energy) of equilibrium thermal radiation, and paves the way for practical functionalities based on thermal radiation using nonreciprocal bianisotropic materials.

Chern-Simons theories have been very successful in explaining integer and fractional quantum Hall phases of matter, topological insulators, and Weyl semimetals. However, it remains an open question as to whether Chern-Simons theories can be adapted to topological photonics. We develop a viscous Maxwell-Chern-Simons theory to capture the fundamental physics of a topological electromagnetic phase of matter. We show the existence of a unique spin-1 skyrmion in the viscous Hall fluid arising from a photonic Zeeman interaction in momentum space. Our work bridges the gap between electromagnetic and condensed matter topological physics while also demonstrating the central role of photon spin-1 quantization in identifying new phases of matter.

When a neutral sphere is rotating near a surface in vacuum, it will experience a frictional torque due to quantum and thermal electromagnetic fluctuations. Such vacuum friction has attracted many interests but has been too weak to be observed. Here we investigate the vacuum frictional torque on a barium strontium titanate (BST) nanosphere near a BST surface. BST is a perovskite ferroelectric ceramic that can have large dielectric responses at GHz frequencies. At resonant rotating frequencies, the mechanical energy of motion can be converted to electromagnetic energy through resonant photon tunneling, leading to a large enhancement of the vacuum friction. The calculated vacuum frictional torques at resonances at subGHz and GHz frequencies are several orders larger than the minimum torque measured by an optically levitated nanorotor recently, and are thus promising to be observed experimentally. Moreover, we calculate the vacuum friction on a rotating sphere near a layered surface for the first time. By optimizing the thickness of the thin-film coating, the frictional torque can be further enhanced by several times.

The temporal dynamics of large quantum systems perturbed weakly by a single excitation can give rise to unique phenomena at the quantum phase boundaries. Here, we develop a time-dependent model to study the temporal dynamics of a single photon interacting with a defect within a large system of interacting spin qubits (N > 100). Our model predicts a quantum resource, giant susceptibility, when the system of qubits is engineered to simulate a first-order quantum phase transition (QPT). We show that the absorption of a single-photon pulse by an engineered defect in the large qubit system can nucleate a single shot quantum measurement through spin noise read-out. This concept of a single-shot detection event (“click”) is different from parameter estimation, which requires repeated measurements. The crucial step of amplifying the weak quantum signal occurs by coupling the defect to a system of interacting qubits biased close to a QPT point. The macroscopic change in long-range order during the QPT generates amplified magnetic noise, which can be read out by a classical device. Our work paves the way for studying the temporal dynamics of large quantum systems interacting with a single-photon pulse.

Over the past decade, topology has emerged as a major branch in broad areas of physics, from atomic lattices to condensed matter. In particular, topology has received significant attention in photonics because light waves can serve as a platform to investigate nontrivial bulk and edge physics with the aid of carefully engineered photonic crystals and metamaterials. Simultaneously, photonics provides enriched physics that arises from spin-1 vectorial electromagnetic fields. Here, we review recent progress in the growing field of topological photonics in three parts. The first part is dedicated to the basics of topological band theory and introduces various two-dimensional topological phases. The second part reviews three-dimensional topological phases and numerous approaches to achieve them in photonics. Last, we present recently emerging fields in topological photonics that have not yet been reviewed. This part includes topological degeneracies in nonzero dimensions, unidirectional Maxwellian spin waves, higher-order photonic topological phases, and stacking of photonic crystals to attain layer pseudospin. In addition to the various approaches for realizing photonic topological phases, we also discuss the interaction between light and topological matter and the efforts towards practical applications of topological photonics.

Superconducting nanowire single-photon detectors have emerged as a promising technology for quantum metrology from the mid-infrared to ultraviolet frequencies. Despite recent experimental successes, a predictive model to describe the detection event in these detectors is needed to optimize the detection metrics. Here, we propose a probabilistic criterion for single-photon detection based on single-vortex (flux quanta) crossing the width of the nanowire. Our model makes a connection between the dark counts and photon counts near the detection threshold. The finite-difference calculations demonstrate that a change in the bias current distribution as a result of the photon absorption significantly increases the probability of single-vortex crossing even if the vortex potential barrier has not vanished completely. We estimate the instrument response function and show that the timing uncertainty of this vortex tunneling process corresponds to a fundamental limit in timing jitter of the click event. We demonstrate a trade-space between this intrinsic (quantum) timing jitter, quantum efficiency, and dark count rate in TaN, WSi, and NbN superconducting nanowires at different experimental conditions. Our detection model can also explain the experimental observation of exponential decrease in the quantum efficiency of SNSPDs at lower energies. This leads to a pulse-width dependency in the quantum efficiency, and it can be further used as an experimental test to compare across different detection models.

We show that a single photon pulse incident on two interacting two-level atoms induces a transient entanglement force between them. After absorption of a multi-mode Fock state pulse, the time-dependent atomic interaction mediated by the vacuum fluctuations changes from the van der Waals interaction to the resonant dipole–dipole interaction (RDDI). We explicitly show that the RDDI force induced by the single photon pulse fundamentally arises from the two-body transient entanglement between the atoms. This single photon pulse induced entanglement force can be continuously tuned from being repulsive to attractive by varying the polarization of the pulse. We further demonstrate that the entanglement force can be enhanced by more than three orders of magnitude if the atomic interactions are mediated by graphene plasmons. These results demonstrate the potential of shaped single photon pulses as a powerful tool to manipulate this entanglement force and also provides a new approach to witness transient atom–atom entanglement.

Nearly all thermal radiation phenomena involving materials with linear response can be accurately described via semi-classical theories of light. Here, we go beyond these traditional paradigms to study a nonlinear system that, as we show, requires quantum theory of damping. Specifically, we analyze thermal radiation from a resonant system containing a χ (2) nonlinear medium and supporting resonances at frequencies ω1 and ω2 ≈ 2ω1, where both resonators are driven only by intrinsic thermal fluctuations. Within our quantum formalism, we reveal new possibilities for shaping the thermal radiation. We show that the resonantly enhanced nonlinear interaction allows frequency-selective enhancement of thermal emission through upconversion, surpassing the well-known blackbody limits associated with linear media. Surprisingly, we also find that the emitted thermal light exhibits non-trivial statistics (g (2) (0) , ∼2) and biphoton intensity correlations (at two distinct frequencies). We highlight that these features can be observed in the near future by heating a properly designed nonlinear system, without the need for any external signal. Our work motivates new interdisciplinary inquiries combining the fields of nonlinear photonics, quantum optics and thermal science.

We study the interplay of electron and photon spin in nonreciprocal materials. Traditionally, the primary mechanism to design nonreciprocal photonic devices has been magnetic fields in conjunction with magnetic oxides, such as iron garnets. In this work, we present an alternative paradigm that allows tunability and reconfigurability of the nonreciprocity through spintronic approaches. The proposed design uses the high spinorbit coupling (SOC) of a narrow-band-gap semiconductor (InSb) with ferromagnetic dopants. A combination of the intrinsic SOC and a gate-applied electric field gives rise to a strong external Rashba spin-orbit coupling (RSOC) in a magnetically doped InSb film. The RSOC which is gate alterable is shown to adjust the magnetic permeability tensor via the electron g factor of the medium. We use electronic band structure calculations (k · p theory) to show that the gate-adjustable RSOC manifest itself in the nonreciprocal coefficient of photon fields via shifts in the Kerr and Faraday rotations. In addition, we show that photon spin properties of dipolar emitters placed in the vicinity of a nonreciprocal electromagnetic environment are distinct from reciprocal counterparts. The Purcell factor (Fp) of a spin-polarized emitter (right-handed circular dipole) is significantly enhanced due to a larger g factor while a left-handed dipole remains essentially unaffected. Our search for novel nonreciprocal material platforms can lead to electron-spin-controlled reconfigurable photonic devices.