Engineering symmetries in nanostructures and metasurfaces provides a new paradigm to control incoherent heat radiation for applications in energy conversion, thermal sources, infrared imaging, and radiative cooling.

# Publications

## 2022

Picophotonics: Anomalous Atomistic Waves in Silicon

”. Physical Review Applied, 18, 4.The concept of photonic frequency-momentum (ω-q) dispersion has been extensively studied in artificial dielectric structures such as photonic crystals and metamaterials. However, the ω-q dispersion of electrodynamic waves hosted in natural materials at the atomistic level is far less explored. Here, we develop a Maxwell Hamiltonian theory of matter combined with the quantum theory of atomistic polarization to obtain the electrodynamic dispersion of natural materials interacting with the photon field. We apply this theory to silicon and discover the existence of anomalous atomistic waves. These waves occur in the spectral region where propagating waves are conventionally forbidden in a macroscopic theory. Our findings demonstrate that natural media can host a variety of yet to be discovered waves with subnanometer effective wavelengths in the picophotonics regime.

Quantum analog of the maximum power transfer theorem

”. Optics Express, 30, 20, Pp. 35840-35853.We discover the quantum analog of the well-known classical maximum power transfer theorem. Our theoretical framework considers the continuous steady-state problem of coherent energy transfer through an *N*-node bosonic network coupled to an external dissipative load. We present an exact solution for optimal power transfer in the form of the *maximum power transfer theorem* known in the design of electrical circuits. Furthermore, we introduce the concept of quantum impedance matching with Thevenin equivalent networks, which are shown to be exact analogs to their classical counterparts. Our results are applicable to both ordered and disordered quantum networks with graph-like structures ranging from nearest-neighbor to all-to-all connectivities. This work points towards universal design principles adapting ideas from the classical regime to the quantum domain for various quantum optical applications in energy harvesting, wireless power transfer, and energy transduction.

Fundamental efficiency bound for quantum coherent energy transfer in nanophotonics

”. Optics Express, 30, 19, Pp. 34725-34739.We derive a unified quantum theory of coherent and incoherent energy transfer between two atoms (donor and acceptor) valid in arbitrary Markovian nanophotonic environments. Our theory predicts a fundamental bound 𝜂𝑚𝑎𝑥=𝛾𝑎𝛾𝑑+𝛾𝑎ηmax=γaγd+γa for energy transfer efficiency arising from the spontaneous emission rates *γ*_{d} and *γ*_{a} of the donor and acceptor. We propose the control of the acceptor spontaneous emission rate as a new design principle for enhancing energy transfer efficiency. We predict an experiment using mirrors to enhance the efficiency bound by exploiting the dipole orientations of the donor and acceptor. Of fundamental interest, we show that while quantum coherence implies the ultimate efficiency bound has been reached, reaching the ultimate efficiency does not require quantum coherence. Our work paves the way towards nanophotonic analogues of efficiency-enhancing environments known in quantum biological systems.

Thermal Emission of Spinning Photons from Temperature Gradients

”. Physical Review Applied, 18, 1.The fluctuational electrodynamic investigation of thermal radiation from nonequilibrium or nonisothermal bodies remains largely unexplored because it necessarily requires volume integration over the fluctuating currents inside the emitter, which quickly becomes computationally intractable. Here, we put forth a formalism combining fast calculations based on modal expansion and fluctuational electrodynamics to accelerate research at this frontier. We employ our formalism on a sample problem: a long silica wire held under temperature gradient within its cross section. We discover that the far-field thermal emission carries a nonzero spin, which is constant in direction and sign, and interestingly, is transverse to the direction of the power flow. We clearly establish the origin of this transverse spin as arising from the nonequilibrium intermixing of the cylindrical modes of the wire, and not from any previously studied or intuitively expected origins such as chiral or nonisotropic materials and geometries, magnetic materials or fields, and mechanical rotations. This finding of nonequilibrium spin texture of emitted heat radiation can prove useful for advancing the noninvasive thermal metrology or infrared-imaging techniques.

A powerful result of topological band theory is that nontrivial phases manifest obstructions to constructing

localized Wannier functions. In Chern insulators, it is impossible to construct Wannier functions that respect

translational symmetry in both directions. Similarly, Wannier functions that respect time-reversal symmetry

cannot be formed in quantum spin Hall insulators. This molecular orbital interpretation of topology has been

enlightening and was recently extended to topological crystalline insulators which include obstructions tied to

space-group symmetries. In this paper, we introduce a class of two-dimensional topological materials known

as optical N-insulators that possess obstructions to construct localized molecular polarizabilities. The optical

N-invariant N ∈ Z is the winding number of the atomistic susceptibility tensor χ and counts the number of

singularities in the electromagnetic linear response theory. We decipher these singularities by analyzing the

optical band structure of the material—the eigenvectors of the susceptibility tensor—which constitutes the

collection of optical Bloch functions. The localized basis of these eigenvectors is optical Wannier functions

which characterize the molecular polarizabilities at different lattice sites. We prove that in a nontrivial optical

phase N = 0, such a localized polarization basis is impossible to construct. Utilizing the mathematical machinery

of K theory, these optical N-phases are refined further to account for the underlying crystalline symmetries of

the material, generating a complete classification of the topological electromagnetic phase of matter.

## 2021

One of the fundamental predictions of quantum mechanics is the occurrence of random fluctuations in a vacuum caused by zero-point energy. Remarkably, quantum electromagnetic fluctuations can induce a measurable force between neutral objects, known as the Casimir effect^{1}, and it has been studied both theoretically^{2,3} and experimentally^{4-9}. The Casimir effect can dominate the interaction between microstructures at small separations and is essential for micro-and nanotechnologies^{10,11}. It has been utilized to realize nonlinear oscillation^{12}, quantum trapping^{13}, phonon transfer^{14,15,} and dissipation dilution^{16}. However, a non-reciprocal device based on quantum vacuum fluctuations remains an unexplored frontier. Here we report quantum-vacuum-mediated non-reciprocal energy transfer between two micromechanical oscillators. We parametrically modulate the Casimir interaction to realize a strong coupling between the two oscillators with different resonant frequencies. We engineer the system's spectrum such that it possesses an exceptional point^{17-20} in the parameter space and explore the asymmetric topological structure in its vicinity. By dynamically changing the parameters near the exceptional point and utilizing the non-adiabaticity of the process, we achieve non-reciprocal energy transfer between the two oscillators with high contrast. Our work demonstrates a scheme that employs quantum vacuum fluctuations to regulate energy transfer at the nanoscale and may enable functional Casimir devices in the future.

**See also:**Vacuum Fluctuations in Matter

We numerically demonstrate that a planar slab made of magnetic Weyl semimetal (a class of topological materials) can emit high-purity circularly polarized (CP) thermal radiation over a broad mid- and long-wave infrared wavelength range for a significant portion of its emission solid angle. This effect fundamentally arises from the strong infrared gyrotropy or nonreciprocity of these materials, which primarily depends on the momentum separation between Weyl nodes in the band structure. We clarify the dependence of this effect on the underlying physical parameters and highlight that the spectral bandwidth of CP thermal emission increases with increasing momentum separation between the Weyl nodes. We also demonstrate, using the recently developed thermal discrete dipole approximation (TDDA) computational method, that finite-size bodies of magnetic Weyl semimetals can emit spectrally broadband CP thermal light, albeit over smaller portion of the emission solid angle compared to the planar slabs. Our work identifies unique fundamental and technological prospects of magnetic Weyl semimetals for engineering thermal radiation and designing efficient CP light sources.

**See also:**Vacuum Fluctuations in Matter

Nitrogen-vacancy (NV) centers in diamond have emerged as promising room-temperature quantum sensors for probing condensed matter phenomena ranging from spin liquids, two-dimensional (2D) magnetic materials, and magnons to hydrodynamic flow of current. Here we propose and demonstrate that the nitrogen-vacancy center in diamond can be used as a quantum sensor for detecting the photonic spin density, the spatial distribution of light’s spin angular momentum. We exploit a single spin qubit on an atomic force microscope tip to probe the spinning field of an incident Gaussian light beam. The spinning field of light induces an effective static magnetic field in the single spin qubit probe. We perform room-temperature sensing using Bloch sphere operations driven by a microwave field (XY8 protocol). This nanoscale quantum magnetometer can measure the local polarization of light in ultra-sub-wavelength volumes. We also put forth a rigorous theory of the experimentally measured phase change using the NV center Hamiltonian and perturbation theory involving only virtual photon transitions. The direct detection of the photonic spin density at the nanoscale using NV centers in diamond opens interesting quantum metrological avenues for studying exotic phases of photons, nanoscale properties of structured light as well as future on-chip applications in spin quantum electrodynamics.

**See also:**Spin Electrodynamics

Classical structured light with controlled polarization and orbital angular momentum (OAM)of electromagnetic waves has varied applications in optical trapping, bio-sensing, optical communications, and quantum simulations. However, quantum noise and photon statistics of three-dimensional photonic angular momentum are relatively less explored. Here, we develop a quantum framework and put forth the concept of quantum structured light for space-time wavepackets at the single-photon level. Our work deals with three-dimensional angular momentum observables for twisted quantum pulses beyond scalar-field theory as well as the paraxial approximation. We show that the spin density generates modulated helical texture and exhibits distinct photon statistics for Fock-state vs. coherent-state twisted pulses. We introduce the quantum correlator of photon spin density to characterize nonlocal spin noise providing a rigorous parallel with electronic spin noise. Our work can lead to quantum spin-OAM physics in twisted single-photon pulses and opens explorations for phases of light with long-range spin order.

**See also:**Spin Electrodynamics