Polaritonic Phenomena Research Articles

Twist-Induced Hyperbolic Shear Metasurfaces

Following the discovery of moiré-driven superconductivity and density waves in twisted-graphene multilayers, twistronics has spurred a surge of interest in tailored broken symmetries through angular rotations enabling new properties, from electronics to photonics and phononics. Analogously, in monoclinic polar crystals a nontrivial angle between nondegenerate dipolar phonon resonances can naturally emerge due to asymmetries in their crystal lattice, and its variations are associated with intriguing polaritonic phenomena, including axial dispersion, i.e., the rotation of the optical axis with frequency, and microscopic shear effects that result in an asymmetric distribution of material loss. So far, these phenomena have been restricted to specific midinfrared frequencies difficult to access with conventional laser sources and fundamentally limited by the degree of asymmetry and by the strength of light-matter interactions available in natural crystals. Here, we leverage the twistronics concept to demonstrate maximal axial dispersion and loss redistribution of hyperbolic waves in elastic metasurfaces, achieved by tailoring the angle between coupled metasurface pairs featuring tailored anisotropy. We show extreme control over elastic wave dispersion and absorption via the twist angle and leverage the resulting phenomena to demonstrate enhanced propagation distance, in-plane reflection-free negative refraction and diffraction-free defect detection. Our work welds the concepts of twistronics, non-Hermiticity, and extreme anisotropy, demonstrating the powerful opportunities enabled by metasurfaces for tunable, highly directional surface-acoustic-wave propagation of great interest for a wide range of applications spanning from seismic mitigation to on-chip phononics and wireless communication systems, hence paving the way toward their translation into emerging photonic and polaritonic metasurface technologies. Published by the American Physical Society 2024

Embrace the darkness: An experimental perspective on organic exciton–polaritons

Organic polaritonics has emerged as a captivating interdisciplinary field that marries the complexities of organic photophysics with the fundamental principles of quantum optics. By harnessing strong light–matter coupling in organic materials, exciton–polaritons offer unique opportunities for advanced device performance, including enhanced energy transport and low-threshold lasing, as well as new functionalities like polariton chemistry. In this review, we delve into the foundational principles of exciton–polaritons from an experimental perspective, highlighting the key states, processes, and timescales that govern polariton phenomena. Our review centers on the spectroscopy of exciton–polaritons. We overview the primary spectroscopic approaches that reveal polariton phenomena, and we discuss the challenges in disentangling polaritonic signatures from spectral artifacts. We discuss how organic materials, due to their complex photophysics and disordered nature, not only present challenges to the conventional polariton models but also provide opportunities for new physics, like manipulating dark electronic states. As the research field continues to grow, with increasingly complex materials and devices, this review serves as a valuable introductory guide for researchers navigating the intricate landscape of organic polaritonics.

Polariton Phenomena in Superradiant Lasing with a Low-Q Asymmetric Fabry–Perot Cavity

Polariton Phenomena in Superradiant Lasing with a Low-Q Asymmetric Fabry–Perot Cavity

Real-space nanoimaging of hyperbolic shear polaritons in a monoclinic crystal.

Various optical crystals possess permittivity components of opposite signs along different principal directions in the mid-infrared regime, exhibiting exotic anisotropic phonon resonances. Such materials with hyperbolic polaritons-hybrid light-matter quasiparticles with open isofrequency contours-feature large-momenta optical modes and wave confinement that make them promising for nanophotonic on-chip technologies. So far, hyperbolic polaritons have been observed and characterized in crystals with high symmetry including hexagonal (boron nitride), trigonal (calcite) and orthorhombic (α-MoO3 or α-V2O5) crystals, where they obey certain propagation patterns. However, lower-symmetry materials such as monoclinic crystals were recently demonstrated to offer richer opportunities for polaritonic phenomena. Here, using scanning near-field optical microscopy, we report the direct real-space nanoscale imaging of symmetry-broken hyperbolic phonon polaritons in monoclinic CdWO4 crystals, and showcase inherently asymmetric polariton excitation and propagation associated with the nanoscale shear phenomena. We also introduce a quantitative theoretical model to describe these polaritons that leads to schemes to enhance crystal asymmetry via the damping loss of phonon modes. Ultimately, our findings show that polaritonic nanophotonics is attainable using natural materials with low symmetry, favouring a versatile and general way to manipulate light at the nanoscale.

Valley-polarized hyperbolic exciton polaritons in few-layer two-dimensional semiconductors at visible frequencies

Polaritons are quasiparticles describing the coupling between light and matter. In two-dimensional materials, polaritonic phenomena are abundant and unique, owing to their low dimensionality and the extraordinary properties of the supported quasiparticles. In this work, we predict the existence of hyperbolic exciton polaritons (HEPs) in few-layer transition-metal dichalcogenides (TMDs) at visible frequencies. We show that hyperbolicity can be induced in the TMD under certain conditions owing to the resonant behavior of the supported excitons, leading to the existence of HEPs. We derive the HEPs' dispersion relation under these conditions and analyze their confinement and loss properties, incorporating nonlocal corrections stemming from the high momentum of the modes. In addition, we show that owing to the valley properties of TMDs, the HEPs are coupled to the valley degree of freedom, leading to a hyperbolic spin-valley Hall effect. Such highly confined and valley-polarized HEPs provide opportunities to control strong light-matter interaction at the atomic scale.

Surface exciton polariton resonances (SEPR)–based sensors

A new type of resonance in the development of sensors using long-range surface exciton polariton (LRSEP) phenomena has been coined: surface exciton polariton resonance (SEPR). The resonance was obtained in the reflected spectrum of a Kretschmann-Raether setup with a two-coupled-interface structure composed of 412 nm magnesium fluoride and 50 nm chromium thin films. The roles of different parameters such as thicknesses of the films and the incidence angles have been simulated. Some preliminary experimental results show a promising performance with a shift of the resonance central wavelength with changes in the incidence angle of -136.52 nm/° and a sensitivity of 23,221 nm/refractive index unit.

A Theoretical Perspective on Molecular Polaritonics.

In the past decade, much theoretical research has focused on studying the strong coupling between organic molecules (or quantum emitters, in general) and light modes. The description and prediction of polaritonic phenomena emerging in this light–matter interaction regime have proven to be difficult tasks. The challenge originates from the enormous number of degrees of freedom that need to be taken into account, both in the organic molecules and in their photonic environment. On one hand, the accurate treatment of the vibrational spectrum of the former is key, and simplified quantum models are not valid in many cases. On the other hand, most photonic setups have complex geometric and material characteristics, with the result that photon fields corresponding to more than just a single electromagnetic mode contribute to the light–matter interaction in these platforms. Moreover, loss and dissipation, in the form of absorption or radiation, must also be included in the theoretical description of polaritons. Here, we review and offer our own perspective on some of the work recently done in the modeling of interacting molecular and optical states with increasing complexity.

Analysis of the classical trajectory treatment of photon dynamics for polaritonic phenomena.

Simulating photon dynamics in strong light-matter coupling situations via classical trajectories is proving to be powerful and practical. Here, we analyze the performance of the approach through the lens of the exact factorization approach. Since the exact factorization enables a rigorous definition of the potentials driving the photonic motion, it allows us to identify that the underestimation of photon number and intensities observed in earlier work is primarily due to an inadequate accounting of light-matter correlation in the classical Ehrenfest force rather than errors from treating the photons quasiclassically per se. The latter becomes problematic when the number of photons per mode begins to exceed a half.

Cavity-Mediated Hybridization of Bright and Dark Excitons in an Ultrastrongly Coupled Carbon Nanotube Microcavity

Semiconducting single-walled carbon nanotubes (SWCNTs) are ideal materials for studying strong light–matter coupling and related polaritonic phenomena at room temperature. SWCNTs possess strong, re...

Plasmonics: A Necessity in the Field of Sensing-A Review (Invited)

ABSTRACT Surface plasmon resonance (SPR) and surface plasmon polariton (SPP) is substantially developing optical sensing approaches that have been working in a wide range of applications, such as biological and chemical analyte detection, bioimaging, food safety, and environmental monitoring to security. Recently, optical fiber-based Surface plasmon resonance (SPR) sensors has gained much attention due to the invention of Photonic crystal fiber (PCF) and microstructured optical fiber (MOF) which offer compact size and light regulatory competencies in unmatched ways. Moreover, the field of sensing has witnessed another type of plasmonic sensors which are based on metal-insulator-metal (MIM) waveguide structure utilizing surface plasmon polariton (SPP) phenomena. This review paper provides an insight into the technological growths in plasmonic sensors based on optical fibers and the MIM platform. We believe that this paper is one of its kind which provides a diverse range of novel plasmonic sensor structures to its readers.

Polariton panorama

AbstractIn this brief review, we summarize and elaborate on some of the nomenclature of polaritonic phenomena and systems as they appear in the literature on quantum materials and quantum optics. Our summary includes at least 70 different types of polaritonic light–matter dressing effects. This summary also unravels a broad panorama of the physics and applications of polaritons. A constantly updated version of this review is available athttps://infrared.cni.columbia.edu.

ASNOM mapping of SiC epilayer doping profile and of surface phonon polariton waveguiding

Apertureless scanning near-field optical microscopy mapping of a slightly doped 4H-SiC epitaxial layer grown on a heavily doped 4H-SiC substrate was performed in a cleaved edge geometry. Surface phonon polariton waves excited by an external coherent light were observed on a sample surface that contains such an epilayer-defined strip near its edge. The light frequency was tuned close to the lattice resonance. Due to a low doping level in an epilayer, its electromagnetic response is determined mainly by the SiC lattice resonance. The rest of the sample surface corresponds to a substrate whose electromagnetic response is determined mainly by the free carriers so that phonon polariton phenomena get suppressed. Such an epilayer-defined strip (vanishing at 895cm−1 frequency) becomes more pronounced at 920cm−1 and, finally, the excited state gets completely confined within such a strip (938cm−1) due to the differences in the electromagnetic properties of doped and undoped SiC.

Nanoimaging and Nanospectroscopy of Polaritons with Time Resolved s‐SNOM

AbstractThe development of electronics and photonics is entering a new era of ultrahigh speed sensing, data processing, and telecommunication. The carrier frequencies of the next‐generation electronic devices inevitably extend beyond radio frequencies, marching toward the nominally photonics‐dominated territories, e.g., terahertz and beyond. As a result, electronic and photonic techniques naturally merge and seek common ground. At the forefront of this technical trend is the field of polaritonics, where polaritons are half‐light, half‐matter quasiparticles that carry the properties of both “bare” photons and “bare” dipole‐carrying excitations. The Janus‐faced nature of polaritons renders the unique capability of operando control using photoexcitation or applied electric field. Here, state‐of‐the‐art ultrafast polaritonic phenomena probed by scattering‐type scanning near‐field optical microscope (s‐SNOM) techniques is reviewed. The ultrafast dynamical control and loss‐reduction of the polariton propagation are discussed with special emphasis on the creation and probing of the tip or edge induced plasmon– and phonon–polaritons in low‐dimensional systems. The detailed technical aspects of s‐SNOM and its possible future development are also presented.

Cavity-Modified Exciton Dynamics in Photosynthetic Units.

Recently, exciton–photon strong coupling has been proposed as a means to control and enhance energy transfer in ensembles of organic molecules. Here, we demonstrate that the exciton dynamics in an archetypal purple bacterial photosynthetic unit, composed of six LH2 antennas surrounding a single LH1 complex, is greatly modified by its interaction with an optical cavity. We develop a Bloch–Redfield master equation approach that accounts for the interplay between the B800 and B850 bacteriochlorophyll molecules within each LH2 antenna, as well as their interactions with the central LH1 complex. Using a realistic parametrization of both the photosynthetic unit and optical cavity, we investigate the formation of polaritons in the system, revealing that these can be tuned to accelerate its exciton dynamics by 3 orders of magnitude. This yields a significant occupation of the LH1 complex, the stage immediately prior to the reaction center, with only a few-femtosecond delay after the initial excitation of the LH2 B800 pigments. Our theoretical findings unveil polaritonic phenomena as a promising route for the characterization, tailoring, and optimization of light-harvesting mechanisms in natural and artificial photosynthetic processes.

Photonic-crystal exciton-polaritons in monolayer semiconductors

Semiconductor microcavity polaritons, formed via strong exciton-photon coupling, provide a quantum many-body system on a chip, featuring rich physics phenomena for better photonic technology. However, conventional polariton cavities are bulky, difficult to integrate, and inflexible for mode control, especially for room-temperature materials. Here we demonstrate sub-wavelength-thick, one-dimensional photonic crystals as a designable, compact, and practical platform for strong coupling with atomically thin van der Waals crystals. Polariton dispersions and mode anti-crossings are measured up to room temperature. Non-radiative decay to dark excitons is suppressed due to polariton enhancement of the radiative decay. Unusual features, including highly anisotropic dispersions and adjustable Fano resonances in reflectance, may facilitate high temperature polariton condensation in variable dimensions. Combining slab photonic crystals and van der Waals crystals in the strong coupling regime allows unprecedented engineering flexibility for exploring novel polariton phenomena and device concepts.

Mid-infrared nanospectroscopy of Berreman mode and epsilon-near-zero local field confinement in thin films

Thin films with vanishing real part of the dielectric function (Re[e] = 0) in the mid-infrared (MIR) region are promising photonic materials for manipulating and enhancing IR light–matter interactions at the nanoscale. We present a nanospectroscopic characterization of two fundamental polaritonic phenomena near Re[e] = 0 by atomic force microscope infrared spectroscopy (AFM-IR): the Berreman mode (BE) in 100 nm SiO2 and Si3N4 films on Si, and epsilon-near-zero (ENZ) local field confinement in a 2 nm native SiO2 layer on Si. AFM-IR is an emerging photothermal technique that provides direct information on nanoscale IR absorption, allowing unambiguous identification of BE and ENZ effects supported by simulations. We demonstrate far-field applicability of polaritonic AFM-IR studies by characterizing a nanoscale plasmonic ENZ grating on Si with 2 nm native SiO2 using polarization-dependent IR microscopy.

Room-temperature exciton-polaritons with two-dimensional WS2.

Two-dimensional transition metal dichalcogenides exhibit strong optical transitions with significant potential for optoelectronic devices. In particular they are suited for cavity quantum electrodynamics in which strong coupling leads to polariton formation as a root to realisation of inversionless lasing, polariton condensation and superfluidity. Demonstrations of such strongly correlated phenomena to date have often relied on cryogenic temperatures, high excitation densities and were frequently impaired by strong material disorder. At room-temperature, experiments approaching the strong coupling regime with transition metal dichalcogenides have been reported, but well resolved exciton-polaritons have yet to be achieved. Here we report a study of monolayer WS2 coupled to an open Fabry-Perot cavity at room-temperature, in which polariton eigenstates are unambiguously displayed. In-situ tunability of the cavity length results in a maximal Rabi splitting of ħΩRabi = 70 meV, exceeding the exciton linewidth. Our data are well described by a transfer matrix model appropriate for the large linewidth regime. This work provides a platform towards observing strongly correlated polariton phenomena in compact photonic devices for ambient temperature applications.

Soliton physics with semiconductor exciton–polaritons in confined systems

In the past decade, there has been a significant progress in the study of non-linear polariton phenomena in semiconductor microcavities. One of the key features of non-linear systems is the emergence of solitons. The complexity and the inherently strong nonlinearity of the polariton system made it a perfect sandpit for observing solitonic effects in half-light half-matter environment. This review focuses on the theory and the latest experimental elucidating physics as well as potential applications of conservative and dissipative solitons in exciton–polariton systems.

Nonlinear spoof surface plasmon polariton phenomena based on conductor metamaterials

We present nonlinear phenomena produced from spoof surface plasmon polariton (SSPP) modes. Below the THz spectrum, artificially textured conducting metastructures on a subwavelength scale generate surface-bound modes and are called SSPP modes, similar to surface plasmon polariton (SPP) modes in the visible spectrum. Even though nonlinear effects in the THz domain are negligible, subwavelength metallic gap structures are ideal candidates to realize nonlinear behavior in the THz domain because of slow light propagation, strong electromagnetic confinement, and a high quality factor Q. In particular, when SSPP structures are combined with Kerr nonlinear materials, nonlinear-bistable curves can be observed below the THz spectrum.

Inhomogeneous origin of the interface roughness broadening of intersubband transitions

The impact of the interface roughness on intersubband transitions (ISTs) in quantum wells is analyzed as an inhomogeneous broadening due to localization rather than a traditional scattering process. The results offer simple explanation for the temperature dependent spectra of gain and absorption in quantum cascade lasers and also for the strong IST polariton phenomena.