Polariton Spectrum Research Articles

(Invited) Electrical, Optical, and Mechanical Properties of Two-Dimensional Materials

Atomically thin two-dimensional materials are direct bandgap semiconductors with a rich interplay of the valley and spin degrees of freedom, which offer the potential for electronics and optoelectronics. A strong Coulomb interaction leads to tightly bound electron-hole pairs or excitons and two-electron one-hole quasiparticles or trions. We solve the two-particle and three-particle problems for the wavefunctions for excitons and trions in the basis set of the model- Hamiltonian for single particles. The calculated linear absorptions, photoluminescence spectra, and polariton spectra as a function of doping and temperature explain the experimental data in 2D monolayers and predict novel spectroscopic features due to the many-body Coulomb interactions. Exciton lifetime plays a crucial role in optoelectronic applications. I will also discuss the phonon-assisted Auger non-radiative decay mechanism of excitons in doped 2D materials. Finally, I will discuss the strain engineering in monolayer 2D materials transferred on 1D and 2D patterned substrates. This material is based upon work supported by the Air Force Office of Scientific Research under award number FA9550-22-1-0312.

Optical Emission Spectra of Molecular Excitonic Polariton Computed at the First‐Principles Level QED‐TDDFT

In microcavity, strong coupling between light and molecules leads to the formation of hybrid excitations, i.e., the polaritons, or exciton‐polaritons. Such coupling may alter the energy landscape of the system and the optical properties of the material, making it an effective approach for controlling the light emission from molecular materials. However, due to the complexity of vibrational modes, spectroscopic calculations for organic exciton‐polaritons remain to be challenging. In this work, based on the linear‐response quantum‐electrodynamical time‐dependent density functional theory (QED‐TDDFT), we employ the thermal vibrational correlation function (TVCF) formalism to calculate the molecular optical spectrum of the lower polaritons (LP) at first‐principles level for three molecules, i.e., anthracene, distyrylbenzenes (DSB), and rubrene. The polaron decoupling effect is confirmed from our first‐principles computations. The theoretical emission spectra of LP provide fruitful insights for molecular and device design in microcavities that are otherwise hindered due to the lack of vibrational information.

A Quantum Chemistry Approach to Linear Vibro-Polaritonic Infrared Spectra with Perturbative Electron-Photon Correlation.

In the vibrational strong coupling (VSC) regime, molecular vibrations and resonant low-frequency cavity modes form light-matter hybrid states, vibrational polaritons, with characteristic infrared (IR) spectroscopic signatures. Here, we introduce a molecular quantum chemistry-based computational scheme for linear IR spectra of vibrational polaritons in polyatomic molecules, which perturbatively accounts for nonresonant electron-photon interactions under VSC. Specifically, we formulate a cavity Born-Oppenheimer perturbation theory (CBO-PT) linear response approach, which provides an approximate but systematic description of such electron-photon correlation effects in VSC scenarios while relying on molecular ab initio quantum chemistry methods. We identify relevant electron-photon correlation effects at the second order of CBO-PT, which manifest as static polarizability-dependent Hessian corrections and an emerging polarizability-dependent cavity intensity component providing access to transmission spectra commonly measured in vibro-polaritonic chemistry. Illustratively, we address electron-photon correlation effects perturbatively in IR spectra of CO2 and Fe(CO)5 vibro-polaritonic models in sound agreement with nonperturbative CBO linear response theory.

The impact of hBN layers on guided exciton–polariton modes in WS2 multilayers

Abstract Guided exciton–polariton modes naturally exist in bare transition metal dichalcogenide (TMDC) layers due to self-hybridization between excitons and photons. However, these guided polariton modes exhibit a limited propagation distance owing to the substantial exciton absorption within the material. Here, we investigated the impact of hexagonal boron nitride (hBN) layers on guided exciton–polariton modes in WS2 multilayers. By integrating hBN layers, we demonstrate a notable enhancement in the quality of guided exciton–polariton modes. The hBN layers can reduce substrate surface roughness and provide surface protection for the WS2 layer, mitigating inhomogeneous broadening of the exciton resonance. Consequently, we experimentally observed that the propagation distance of polariton modes substantially increased with hBN layers. Additionally, the polariton spectrum broadened due to efficient exciton relaxation to the polariton states at lower energies. Comparison with simulation data emphasizes that the observed improvements are primarily attributed to enhanced exciton quality. The promising outcomes with hBN encapsulation suggest its potential to overcome strong excitonic losses of the guided exciton polariton in implementing nanophotonic devices. Furthermore, this approach provides a new avenue for exploring the novel physics of guided exciton–polariton modes and their potential applications in polariton-based all-optical integrated circuits.

Giant nonlocal Kerr nonlinearity and polaritonic solitons in a Rydberg-dressed Bose-Einstein condensate.

We present a scheme to generate nonlocal optical Kerr nonlinearity and polaritonic solitons via matter-wave superradiance in a Rydberg-dressed Bose-Einstein condensate (BEC). We show that the polariton spectrum of the scattered field generated by the superradiance is changed significantly due to the existence of the long-range Rydberg-Rydberg interaction between atoms, i.e. it has a roton-maxon form; moreover, the BEC structure factor displays a strong dependence on the Rydberg-dressing, which can be tuned in a controllable way. We also show that such a Rydberg-dressed BEC system can support a giant nonlocal optical Kerr nonlinearity, and hence allow the formation and stable propagation of polaritonic solitons, which have ultraslow propagation velocity and ultralow generation power. The results reported here are useful to understand the unique properties of Rydberg-dressing in BECs and have potential applications in optical information processing and transmission.

Vibrational strong coupling in liquid water from cavity molecular dynamics.

We assess the cavity molecular dynamics method for the calculation of vibrational polariton spectra using liquid water as a specific example. We begin by disputing a recent suggestion that nuclear quantum effects may lead to a broadening of polariton bands, finding instead that they merely result in anharmonic red shifts in the polariton frequencies. We go on to show that our simulated cavity spectra can be reproduced to graphical accuracy with a harmonic model that uses just the cavity-free spectrum and the geometry of the cavity as input. We end by showing that this harmonic model can be combined with the experimental cavity-free spectrum to give results in good agreement with optical cavity measurements. Since the input to our harmonic model is equivalent to the input to the transfer matrix method of applied optics, we conclude that cavity molecular dynamics cannot provide any more insight into the effect of vibrational strong coupling on the absorption spectrum than this transfer matrix method, which is already widely used by experimentalists to corroborate their cavity results.

Electron exchange effect on surface magnetoplasmon polaritons dynamics in a graphene-plasmonic structure

By employing a two-dimensional linearized magnetoquantum hydrodynamic model and Maxwell’s equations, the electron exchange effect on the dispersion spectrum of surface magneto-plasmon polaritons (SMPPs) is studied in a perpendicular configurated graphene-plasmonic structure where a graphene sheet is directly covered by two semi-infinite dielectrics. Besides, other influences (including the graphene electron density, the dielectric constant of the dielectric medium, and the external magnetic field) on dispersion characteristics in both classical and quantum regimes of graphene surface magneto plasmon polaritons (GSMPPs) have been investigated in the presence of an electron exchange effect. Our results show that these influences greatly affect the dynamics of GSMPPs. Also, it is found that in the presence of the electron exchange effect, the propagation speed and the dispersion spectrum shift of GSMPPs in the classical regime are largely increased more than those in the case of the quantum regime. Our findings demonstrate that the electron exchange effect has a vital function in the modulation of the dynamical behavior of SMPPs in graphene-nano optical and plasmonic devices.

Comment on "Isolating Polaritonic 2D-IR Transmission Spectra".

This Viewpoint responds to the analysis of 2D IR spectra of vibration cavity polaritons in the study reported in The Journal of Physical Chemistry Letters (Duan et al. 2021, 12, 11406). That report analyzed 2D IR spectra of strongly coupled molecules, such as W(CO)6 and nitroprusside anion, based on subtracting a background signal generated by polariton filtered free space signals. They assigned the resulting response as being due to excited polaritons. We point out in this Viewpoint that virtually all of the response can be properly reproduced using the physics of transmission through an etalon containing a material modeled with a complex dielectric function describing the ground- and excited-state absorber populations. Furthermore, such a coupled system cannot be described as a scaled sum of the bare molecular and cavity responses.

Controlling topological phases of matter with quantum light

Controlling the topological properties of quantum matter is a major goal of condensed matter physics. A major effort in this direction has been devoted to using classical light in the form of Floquet drives to manipulate and induce states with non-trivial topology. A different route can be achieved with cavity photons. Here we consider a prototypical model for topological phase transition, the one-dimensional Su-Schrieffer-Heeger model, coupled to a single mode cavity. We show that quantum light can affect the topological properties of the system, including the finite-length energy spectrum hosting edge modes and the topological phase diagram. In particular we show that depending on the lattice geometry and the strength of light-matter coupling one can either turn a trivial phase into a topological one or viceversa using quantum cavity fields. Furthermore, we compute the polariton spectrum of the coupled electron-photon system, and we note that the lower polariton branch disappears at the topological transition point. This phenomenon can be used to probe the phase transition in the Su-Schrieffer-Heeger model.

Novel Materials Based on Nonideal Supercrystal Formed by a Tunnel Connected Array of Microcavities Containing Ensembles of Quantum Dots

Numerical model for a defect-containing lattice of microcavities with embedded ultracold atomic clusters (quantum dots) is developed. It is assumed that certain fractions of quantum dots are absent, which leads to transformation of polariton spectrum of the overall structure. The dispersion relations for polaritonic modes are derived as functions of structure defects concentrations and elastic strain. It is shown that, as a result of elastic strain of the system and presence of structural defects under study, it is possible to achieve necessary changes in its energy structure (and, therefore, optical properties) determined by the rearrangement of the polariton spectrum.

Near-isotropic polariton heat transport along a polar anisotropic nanofilm

SummaryThe heat transport of surface phonon-polaritons propagating along a polar uniaxial anisotropic nanofilm is studied for different orientations of its optical axis, film thicknesses, and temperatures. For an hBN nanofilm, it is shown that i) the propagation of polaritons can be described in terms of even and odd modes that generalize the transverse magnetic and transverse electrical ones that typically appear in isotropic films. ii) The frequency spectrum of polaritons can efficiently be tuned with the angle between the film optical axis and their propagation direction. iii) The polariton thermal conductivity takes higher values for a thinner or hotter nanofilm. iv) The even and odd modes have a remarkable contribution to the total polariton thermal conductivity, which takes a value higher than 5.6 Wm−1K−1 for a 25-nm-thick nanofilm at 500 K. The obtained results thus uncover some key features of the propagation and heat transport of polaritons in uniaxial nanofilms.

Plasmon polaritons in 3D graphene periodic structure

ABSTRACT In this paper, we proposed a cubic graphene structure to generate both transverse electric and transverse magnetic plasmon polaritons. The component graphene squares are finite in size and the structure is thus realizable. The temperature effects were studied comprehensively and the results showed that our plasmon polariton dispersion relations behave stably with respective to the temperature. Polaritons can be generated and modulated effectively by altering the size and medium (permittivity), and we acquired a broadband spectrum of plasmon polaritons in the structure. In addition, our proposed structure requires no external stimuli, such as magnets and power. The study supplies guidelines to optoelectronic device design and selection.

Phonon signatures in spectra of exciton polaritons in transition metal dichalcogenides

Embedding a monolayer of a transition metal dichalcogenide in a high-$Q$ optical cavity results in the formation of distinct exciton polariton modes. The polaritons are affected by the strong exciton-phonon interaction in the monolayer. We use a time convolutionless master equation to calculate the phonon influence on the spectra of the polaritons. We discuss the nontrivial dependence of the line shapes of both branches on temperature and detuning. The peculiar polariton dispersion relation results in a linewidth of the lower polariton being largely independent of the coupling to acoustic phonons. For the upper polariton, acoustic phonons lead to a low-energy shoulder of the resonance in the linear response. Furthermore, we analyze the influence of inhomogeneous broadening being the dominant contribution to the lower polariton linewidth at low temperatures. Our results point towards interesting phonon features in polariton spectra in transition metal dichalcogenides.

Isolating Polaritonic 2D-IR Transmission Spectra.

Strong coupling between vibrational transitions in molecules within a resonant optical microcavity leads to the formation of collective, delocalized vibrational polaritons. There are many potential applications of "polaritonic chemistry", ranging from modified chemical reactivity to quantum information processing. One challenge in obtaining the polaritonic response is removing a background contribution due to the uncoupled molecules that generate an ordinary 2D-IR spectrum whose amplitude is filtered by the polariton transmission spectrum. We show that most features in 2D-IR spectra of vibrational polaritons can be explained by a linear superposition of this background signal and the true polariton response. Through a straightforward correction procedure, in which the filtered bare-molecule 2D-IR spectrum is subtracted from the measured cavity response, we recover the polaritonic spectrum.

Direct measurement of a non-Hermitian topological invariant in a hybrid light-matter system.

Topology is central to understanding and engineering materials that display robust physical phenomena immune to imperfections. Different topological phases of matter are characterized by topological invariants. In energy-conserving (Hermitian) systems, these invariants are determined by the winding of eigenstates in momentum space. In non-Hermitian systems, a topological invariant is predicted to emerge from the winding of the complex eigenenergies. Here, we directly measure the non-Hermitian topological invariant arising from exceptional points in the momentum-resolved spectrum of exciton polaritons. These are hybrid light-matter quasiparticles formed by photons strongly coupled to electron-hole pairs (excitons) in a halide perovskite semiconductor at room temperature. We experimentally map out both the real (energy) and imaginary (linewidth) parts of the spectrum near the exceptional points and extract the novel topological invariant—fractional spectral winding. Our work represents an essential step toward realization of non-Hermitian topological phases in a condensed matter system.

Exciton–polaritons in GaAs-based slab waveguide photonic crystals

We report the observation of bandgaps for low loss exciton–polaritons propagating outside the light cone in GaAs-based planar waveguides patterned into two-dimensional photonic crystals. By etching square lattice arrays of shallow holes into the uppermost layer of our structure, we open gaps on the order of 10 meV in the photonic mode dispersion, whose size and light–matter composition can be tuned by proximity to the strongly coupled exciton resonance. We demonstrate gaps ranging from almost fully photonic to highly excitonic. Opening a gap in the exciton-dominated part of the polariton spectrum is a promising first step toward the realization of quantum-Hall-like states arising from topologically nontrivial hybridization of excitons and photons.

Coherent Surface Plasmon Hole Burning via Spontaneously Generated Coherence.

Surface plasmon (SP)—induced spectral hole burning (SHB) at the silver-dielectric interface is investigated theoretically. We notice a typical lamb dip at a selective frequency, which abruptly reduces the absorption spectrum of the surface plasmons polaritons (SPP). Introducing the spontaneous generated coherence (SGC) in the atomic medium, the slope of dispersion becomes normal. Additionally, slow SPP propagation is also noticed at the interface. The spectral hole burning dip is enhanced with the SGC effect and can be modified and controlled with the frequency and intensity of the driving fields. The SPP propagation length at the hole-burning region is greatly enhanced under the effect of SGC. A propagation length of the order of 600 µm is achieved for the modes, which is a remarkable result. The enhancement of plasmon hole burning under SGC will find significant applications in sensing technology, optical communication, optical tweezers and nano-photonics.

Valley-selective optical Stark effect of exciton-polaritons in a monolayer semiconductor

Selective breaking of degenerate energy levels is a well-known tool for coherent manipulation of spin states. Though most simply achieved with magnetic fields, polarization-sensitive optical methods provide high-speed alternatives. Exploiting the optical selection rules of transition metal dichalcogenide monolayers, the optical Stark effect allows for ultrafast manipulation of valley-coherent excitons. Compared to excitons in these materials, microcavity exciton-polaritons offer a promising alternative for valley manipulation, with longer lifetimes, enhanced valley coherence, and operation across wider temperature ranges. Here, we show valley-selective control of polariton energies in WS2 using the optical Stark effect, extending coherent valley manipulation to the hybrid light-matter regime. Ultrafast pump-probe measurements reveal polariton spectra with strong polarization contrast originating from valley-selective energy shifts. This demonstration of valley degeneracy breaking at picosecond timescales establishes a method for coherent control of valley phenomena in exciton-polaritons.

Understanding radiative transitions and relaxation pathways in plexcitons

Summary Molecular aggregates on plasmonic nanoparticles have emerged as attractive systems for the studies of polaritonic light-matter states, called plexcitons. Such systems are tunable, scalable, easy to synthesize, and offer sub-wavelength confinement, all while giving access to the ultrastrong light-matter coupling regime, promising a plethora of applications. However, the complexity of these materials prevented the understanding of their excitation and relaxation phenomena. Here, we follow the relaxation pathways in plexcitons and conclude that while the metal destroys the optical coherence, the molecular aggregate coupled to surface processes significantly contributes to the energy dissipation. We use two-dimensional electronic spectroscopy with theoretical modeling to assign the different relaxation processes to either molecules or metal nanoparticle. We show that the dynamics beyond a few femtoseconds has to be considered in the language of hot electron distributions instead of the accepted lower and upper polariton branches and establish the framework for further understanding.

Ground state properties and infrared spectra of anharmonic vibrational polaritons of small molecules in cavities.

Recent experiments and theory suggest that ground state properties and reactivity of molecules can be modified when placed inside a nanoscale cavity, giving rise to strong coupling between vibrational modes and the quantized cavity field. This is commonly thought to be caused either by a cavity-distorted Born-Oppenheimer ground state potential or by the formation of light-matter hybrid states, vibrational polaritons. Here, we systematically study the effect of a cavity on ground state properties and infrared spectra of single molecules, considering vibration-cavity coupling strengths from zero up to the vibrational ultrastrong coupling regime. Using single-mode models for Li-H and O-H stretch modes and for the NH3 inversion mode, respectively, a single cavity mode in resonance with vibrational transitions is coupled to position-dependent molecular dipole functions. We address the influence of the cavity mode on polariton ground state energies, equilibrium bond lengths, dissociation energies, activation energies for isomerization, and on vibro-polaritonic infrared spectra. In agreement with earlier work, we observe all mentioned properties being strongly affected by the cavity, but only if the dipole self-energy contribution in the interaction Hamiltonian is neglected. When this term is included, these properties do not depend significantly on the coupling anymore. Vibro-polaritonic infrared spectra, in contrast, are always affected by the cavity mode due to the formation of excited vibrational polaritons. It is argued that the quantized nature of vibrational polaritons is key to not only interpreting molecular spectra in cavities but also understanding the experimentally observed modification of molecular reactivity in cavities.