Publications

2022

Sathwik Bharadwaj, Todd Van Mechelen, and Zubin Jacob. 10/27/2022.

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.

Cristian L. Cortes, Wenbo Sun, and Zubin Jacob. 9/15/2022.

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.

Cristian L. Cortes, Wenbo Sun, and Zubin Jacob. 9/8/2022.

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 γd and γa 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.

P.Y. Chen, C. Khandekar, R. Ayash, and Zubin Jacob. 7/21/2022.

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.

Farid Kalhor, Noah F. Opondo, Shoaib Mahmud, Leif Bauer, Li-Ping Yang, and Zubin Jacob. 6/29/2022.

Optically induced static magnetic field in the ensemble of nitrogen-vacancy centers in diamond

. OSA Publishing, 47, 13, Pp. 3347-3350.

Generation of a local magnetic field at the nanoscale is desirable for many applications such as spin-qubit-based quantum memories. However, this is a challenge due to the slow decay of static magnetic fields. Here, we demonstrate a photonic spin density (PSD)-induced effective static magnetic field for an ensemble of nitrogen-vacancy (NV) centers in bulk diamond. This locally induced magnetic field is a result of coherent interaction between the optical excitation and the NV centers. We demonstrate an optically induced spin rotation on the Bloch sphere exceeding 10 degrees which has potential applications in all-optical coherent control of spin qubits.

Li-Ping Yang, Farhad Khosravi, and Zubin Jacob. 5/31/2022. Quantum field theory for the spin operator of the photon. Physical Review Research, 4, 2, Pp. 023165.

All elementary particles in nature can be classified as fermions with half-integer spin and bosons with integer
spin. Within quantum electrodynamics (QED), even though the spin of the Dirac particle is well defined, there
exist open questions on the quantized description of the spin of the gauge field particle—the photon. Using quantum field theory, we discover the quantum operators for the spin angular momentum (SAM) SM = (1/c) d3xπ × A and orbital angular momentum (OAM) LM = −(1/c) d3xπμx × ∇Aμ of the photon, where πμ is the conjugate canonical momentum of the gauge field Aμ. We also reveal a perfect symmetry between the angular momentum commutation relations for Dirac fields and Maxwell fields. We derive the well-known OAM and SAM of classical electromagnetic fields from the above-defined quantum operators. Our work shows that the spin and OAM operators commute, which is important for simultaneously observing and separating the SAM and OAM. The correct commutation relations of orbital and spin angular momentum of the photon have applications in quantum optics, topological photonics as well as nanophotonics and can be extended in the future for the spin structure of nucleons.

Xueji Wang, Tyler Sentz, Sathwik Bharadwaj, Yifan Wang, Limei Qi, and Zubin Jacob. 5/12/2022. Observation of Non-vanishing Optical Helicity in Thermal Radiation from Symmetry-Broken Metasurfaces. arXiv.

Thermal radiation is traditionally an incoherent radiative signal, where the radiated heat is highly unpolarized, spectrally broad, and omnidirectional. The recent extensive interests in thermal photonics focus on tailoring the temporal coherence (spectrum) and spatial coherence (directivity) of thermal radiation. Here, we investigate the photon spin characteristics of the radiation excited by thermal fluctuations using a symmetrybroken metasurface. Utilizing spin-polarized angle-resolved thermal emission spectroscopy (SPARTES), we explicitly show when both mirror- and inversion- symmetries are broken, the summation of spin-angular momentum projected on wavevectors, namely the optical helicity, can be non-vanishing even without applying a magnetic field. We find the photon spin and the energy-momentum dispersion of thermal radiation can be effectively tailored through symmetry engineering. Our results firmly suggest the symmetry-based strategy provides a general pathway for comprehensively controlling the temporal, spatial, and especially spin coherence of thermal radiation.

Farid Kalhor, Noah F. Opondo, Shoaib Mahmud, Leif Bauer, Li-Ping Yang, and Zubin Jacob. 5/6/2022. Optically induced static magnetic field in the ensemble of nitrogen-vacancy centers in diamond. arXiv.

Generation of local magnetic field at the nanoscale is desired for many applications such as spinqubit-based quantum memories. However, this is a challenge due to the slow decay of static magnetic fields. Here, we demonstrate photonic spin density (PSD) induced effective static magnetic field for an ensemble of nitrogen-vacancy (NV) centers in bulk diamond. This locally induced magnetic field is a result of coherent interaction between the optical excitation and the NV centers. We demonstrate an optically induced spin rotation on the Bloch sphere exceeding 10 degrees which has potential applications in all optical coherent control of spin qubits.

Todd Van Mechelen, Sathwik Bharadwaj, and Zubin Jacob. 4/5/2022.

Optical N-insulators: Topological obstructions to optical Wannier functions in the atomistic susceptibility tensor

. Physical Review Research, 4, 2.

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.

Zohreh Poursoti, Wenbo Sun, Sathwik Bharadwaj, and Zubin Jacob. 3/31/2022. Deep ultra-violet plasmonics: exploiting momentum-resolved electron energy loss spectroscopy to probe germanium. Optics Express, 30, 8, Pp. 12630-12638.

Germanium is typically used for solid-state electronics, fiber-optics, and infrared applications, due to its semiconducting behavior at optical and infrared wavelengths. In contrast, here we show that the germanium displays metallic nature and supports propagating surface plasmons in the deep ultraviolet (DUV) wavelengths, that is typically not possible to achieve with conventional plasmonic metals such as gold, silver, and aluminum. We measure the photonic band spectrum and distinguish the plasmonic excitation modes: bulk plasmons, surface plasmons, and Cherenkov radiation using a momentum-resolved electron energy loss spectroscopy. The observed spectrum is validated through the macroscopic electrodynamic electron energy loss theory and first-principles density functional theory calculations. In the DUV regime, intraband transitions of valence electrons dominate over the interband transitions, resulting in the observed highly dispersive surface plasmons. We further employ these surface plasmons in germanium to design a DUV radiation source based on the Smith-Purcell effect. Our work opens a new frontier of DUV plasmonics to enable the development of DUV devices such as metasurfaces, detectors, and light sources based on plasmonic germanium thin films.