Properties Of Polaritons Research Articles

Lossless and stable propagation of surface plasmon polaritons in quasi-P T potential.

We propose a scheme to study the propagation properties of surface plasmon polaritons (SPPs) in a dielectric/metal/dielectric waveguide with a refraction index of the dielectric modulated as quasi-P T-symmetric potentials. By treating the propagation loss of SPPs as the absorption background, we prove that a total gain-loss balance system which allows SPPs losslessly and stably propagating can be achieved. In addition, the propagation robustness of single peak and dipole peak transverse structured SPPs in quasi-P T-symmetric potentials under noise perturbations is discussed. The results may have certain significance for expanding applications of non-Hermitian optics in micro-/nano-optical information processing.

Direct extraction of optical constants of organic semiconductors in ultrastrongly coupled microcavities and their application in antireflection design for polariton devices

The strongly bound Frenkel excitons in organic semiconductors enable strong or even ultrastrong exciton-photon coupling in room-temperature cavities, with the resulting polariton states typically resolved through reflectance measurements. This paper demonstrates that the distinct features of exciton and polariton modes in the reflectance spectra of strongly/ultrastrongly coupled organic microcavities can be effectively utilized to extract the optical constants and physical thickness of the embedded organic semiconductor. We investigate metal-clad microcavities based on two prototype conjugated polymers, poly[2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and poly(3-hexylthiophene) (P3HT), both exhibiting ultrastrong coupling characteristics. The (n,k) spectra and thickness of these polymer films are determined by fitting the normal incidence reflectance spectra of organic microcavities, using Kramers-Kronig transformation and transfer-matrix calculations with varying optical and thickness parameters. We also examine the individual effects of the main fitting parameters on the spectrum, establishing a close correlation with the underlying polariton properties. Moreover, we analyze the optical admittance at exciton and polariton modes to understand reflectance variations with different parameters, which facilitates precise control of optical properties at specific modes through cavity design. Finally, using the extracted optical constants of MDMO-PPV and P3HT, we propose optimized microcavity designs that exhibit antireflection at the lower polariton mode for potential luminescence and photodetection device applications.

Electrical spectroscopy of polaritonic nanoresonators

One of the most captivating properties of polaritons is their capacity to confine light at the nanoscale. This confinement is even more extreme in two-dimensional (2D) materials. 2D polaritons have been investigated by optical measurements using an external photodetector. However, their effective spectrally resolved electrical detection via far-field excitation remains unexplored. This hinders their exploitation in crucial applications such as sensing, hyperspectral imaging, and optical spectrometry, banking on their potential for integration with silicon technologies. Herein, we present the electrical spectroscopy of polaritonic nanoresonators based on a high-quality 2D-material heterostructure, which serves at the same time as the photodetector and the polaritonic platform. Subsequently, we electrically detect these mid-infrared resonators by near-field coupling to a graphene pn-junction. The nanoresonators simultaneously exhibit extreme lateral confinement and high-quality factors. This work opens a venue for investigating this tunable and complex hybrid system and its use in compact sensing and imaging platforms.

A novel and effective oxidation-resistant approach in plasmonic MIM biosensors for real-time detection of urea and glucose in urine for monitoring diabetic and kidney disease severity

In this research article, we introduce a novel oxidation-resistant approach in a Metal–Insulator–Metal (MIM) plasmonic biosensor, specifically designed for the detection and diagnosis of diabetes and kidney disease severity using urea and glucose biomarkers in urine. The proposed biosensor features a four-hand-shaped fan resonator with integrated nanodots and stubs within its waveguide. We optimized its performance through Finite Element Method (FEM) simulations, achieving a maximum sensitivity of 2470.48 nm/RIU and a Figure of Merit (FOM) of 50.15. To ensure chemical stability, we utilized an oxidation-resistant plasmonic material (Au) in our sensor, a critical factor for reliable biosensing applications. Additionally, we focus on Diabetic Kidney Disease (DKD), a significant global health challenge, emphasizing the need for early detection and point-of-care treatment. By exploiting minute changes in resonant wavelength resulting from light–matter interactions through the properties of Surface Plasmon Polaritons (SPPs) and MIM structures, our biosensor demonstrates promising capabilities for real-time and quantitative analysis of biomarkers critical for monitoring the stages of DKD.

Tunable Phonon Polariton Hybridization in a Van der Waals Hetero-Bicrystal.

Phonon polaritons, the hybrid quasiparticles resulting from the coupling of photons and lattice vibrations, have gained significant attention in the field of layered van der Waals heterostructures. Particular interest has been paid to hetero-bicrystals composed of molybdenum oxide (MoO3) and hexagonal boron nitride (hBN), which feature polariton dispersion tailorable via avoided polariton mode crossings. In this work, the polariton eigenmodes in MoO3-hBN hetero-bicrystals self-assembled on ultrasmooth gold are systematically studied using synchrotron infrared nanospectroscopy. It is experimentally demonstrated that the spectral gap in bicrystal dispersion and corresponding regimes of negative refraction can be tuned by material layer thickness, and these results are quantitatively matched with a simple analytic model. Polaritonic cavity modes and polariton propagation along "forbidden" directions are also investigated in microscale bicrystals, which arise from the finite in-plane dimension of the synthesized MoO3 micro-ribbons. The findings shed light on the unique dispersion properties of polaritons in van der Waals heterostructures and pave the way for applications leveraging deeply sub-wavelength mid-infrared light-matter interactions.

Deciphering between enhanced light emission and absorption in multi-mode porphyrin cavity polariton samples

Abstract It remains unclear how the collective strong coupling of cavity-confined photons to the electronic transitions of molecular chromophore leverages the distinct properties of the polaritonic constituents for future technologies. In this study, we design, fabricate, and characterize multiple types of Fabry-Pérot (FP) mirco-resonators containing copper(II) tetraphenyl porphyrin (CuTPP) to show how cavity polariton formation affects radiative relaxation processes in the presence of substantial non-Condon vibronic coupling between two of this molecule’s excited electronic states. Unlike the prototypical enhancement of Q state radiative relaxation of CuTPP in a FP resonator incapable of forming polaritons, we find the light emission processes in multimode cavity polariton samples become enhanced for cavity-exciton energy differences near those of vibrations known to mediate non-Condon vibronic coupling. We propose the value of this detuning is consistent with radiative relaxation of Herzberg-Teller polaritons into collective molecular states coupled to the cavity photon coherently. We contrast the feature stemming from light emission from the HT polariton state with those that occur due to polariton-enhanced light absorption. Our results demonstrate the landscape of molecular and photonic interactions enabled by cavity polariton formation using complex chromophores and how researchers can design resonators to leverage these interactions to characterize and control polaritonic properties.

Beyond single-molecule chemistry for electrified interfaces using molecule polaritons

Abstract This review introduces the preparation, observation, and characterization of systems of molecule polaritons. We summarize recent progress by our group on the control of molecular properties in the spaces of various optical cavities, such as plasmonic metal nanostructures and Fabry–Perot mirrors. After stating our motivations, we introduce topics on single-molecule observations based on surface-enhanced Raman scattering (SERS) spectroscopy. Our original findings are not limited to the ultrasensitive detection of molecules/materials, but include unique resonant enhancements of SERS through exotic electronic excitation processes at electrified interfaces. Comprehensive analyses of SERS spectra provide novel routes to electronic excitation using plasmonic metal nanostructures. This behavior leads to the distinct photoenergy conversion based on excited electrons and holes with anomalous electrochemical potentials. Such unique resonant electronic excitation emerges as a large optical force that can be used to manipulate small single molecules on surfaces in solution, even at room temperature. We also introduce systems for examining unique interactions between molecules and the optical modes of cavities, from single molecules to molecular ensembles. Plasmonic surface lattices and Fabry–Perot mirrors allow the formation of electronic and vibrational strong coupling states, respectively, showing unique properties of molecule polaritons, even under dark conditions, i.e. without photoillumination.

Spin Chaos of Exciton Polaritons in a Magnetic Field

The spin properties of exciton polaritons in a micropillar cavity placed in a static magnetic field and excited by a resonant light wave are studied theoretically. Owing to the Zeeman effect, a nonlinear polariton system has two branches of optical response that are characterized by opposite circular polarizations. An indirect mechanism of polarization reversal is predicted, according to which the current state of the system undergoes a transition to dynamical chaos, and then the alternative spin state is established spontaneously. Such spin switches, mediated by a chaotic stage, proceed in both directions near the same critical excitation amplitude, so that the sign of the circular polarization of the cavity radiation is directly determined by the intensity of the optical pump.

Thermal and electrostatic tuning of surface phonon-polaritons in LaAlO3/SrTiO3 heterostructures

Phonon polaritons are promising for infrared applications due to a strong light-matter coupling and subwavelength energy confinement they offer. Yet, the spectral narrowness of the phonon bands and difficulty to tune the phonon polariton properties hinder further progress in this field. SrTiO3 – a prototype perovskite oxide - has recently attracted attention due to two prominent far-infrared phonon polaritons bands, albeit without any tuning reported so far. Here we show, using cryogenic infrared near-field microscopy, that long-propagating surface phonon polaritons are present both in bare SrTiO3 and in LaAlO3/SrTiO3 heterostructures hosting a two-dimensional electron gas. The presence of the two-dimensional electron gas increases dramatically the thermal variation of the upper limit of the surface phonon polariton band due to temperature dependent polaronic screening of the surface charge carriers. Furthermore, we demonstrate a tunability of the upper surface phonon polariton frequency in LaAlO3/SrTiO3 via electrostatic gating. Our results suggest that oxide interfaces are a new platform bridging unconventional electronics and long-wavelength nanophotonics.

Spin Chaos of Exciton Polaritons in a Magnetic Field

The spin properties of exciton polaritons in a micropillar cavity placed in a static magnetic field and excited by a resonant light wave are studied theoretically. Owing to the Zeeman effect, a nonlinear polariton system has two branches of optical response that are characterized by opposite circular polarizations. An indirect mechanism of polarization reversal is predicted, according to which the current state of the system undergoes a transition to dynamical chaos, and then the alternative spin state is established spontaneously. Such spin switches, mediated by a chaotic stage, proceed in both directions near the same critical excitation amplitude, so that the sign of the circular polarization of the cavity radiation is directly determined by the intensity of the optical pump.

A versatile platform for gas-phase molecular polaritonics.

Cavity coupling of gas-phase molecules will enable studies of benchmark chemical processes under strong light-matter interactions with a high level of experimental control and no solvent effects. We recently demonstrated the formation of gas-phase molecular polaritons by strongly coupling bright ν3, J = 3 → 4 rovibrational transitions of methane (CH4) to a Fabry-Pérot optical cavity mode inside a cryogenic buffer gas cell. Here, we further explore the flexible capabilities of this infrastructure. We show that we can greatly increase the collective coupling strength of the molecular ensemble to the cavity by increasing the intracavity CH4 number density. In doing so, we can tune from the single-mode coupling regime to a multimode coupling regime in which many nested polaritonic states arise as the Rabi splitting approaches the cavity mode spacing. We explore polariton formation for cavity geometries of varying length, finesse, and mirror radius of curvature. We also report a proof-of-principle demonstration of rovibrational gas-phase polariton formation at room temperature. This experimental flexibility affords a great degree of control over the properties of molecular polaritons and opens up a wider range of simple molecular processes to future interrogation under strong cavity-coupling. We anticipate that ongoing work in gas-phase polaritonics will facilitate convergence between experimental results and theoretical models of cavity-altered chemistry and physics.

Polaritonic linewidth asymmetry in the strong and ultrastrong coupling regime

Abstract The intriguing properties of polaritons resulting from strong and ultrastrong light–matter coupling have been extensively investigated. However, most research has focused on spectroscopic characteristics of polaritons, such as their eigenfrequencies and Rabi splitting. Here, we study the decay rates of a plasmon–microcavity system in the strong and ultrastrong coupling regimes experimentally and numerically. We use a classical scattering matrix approach, approximating our plasmonic system with an effective Lorentz model, to obtain the decay rates through the imaginary part of the complex quasinormal mode eigenfrequencies. Our classical model automatically includes all the interaction terms necessary to account for ultrastrong coupling without dealing with the rotating-wave approximation and the diamagnetic term. We find an asymmetry in polaritonic decay rates, which deviate from the expected average of the uncoupled system’s decay rates at zero detuning. Although this phenomenon has been previously observed in exciton–polaritons and attributed to their disorder, we observe it even in our homogeneous system. As the coupling strength of the plasmon–microcavity system increases, the asymmetry also increases and can become so significant that the lower (upper) polariton decay rate reduction (increase) goes beyond the uncoupled decay rates, γ − < γ 0,c < γ +. Furthermore, our findings demonstrate that polaritonic linewidth asymmetry is a generic phenomenon that persists even in the case of bulk polaritons.

Quasi-equilibrium polariton condensates in the non-linear regime and beyond

We investigate the many-body behavior of polaritons formed from electron-hole pairs strongly coupled to photons in a two-dimensional semiconductor microcavity. We use a microscopic mean-field BCS theory that describes polariton condensation in quasi-equilibrium across the full range of excitation densities. In the limit of vanishing density, we show that our theory recovers the exact single-particle properties of polaritons, while at low densities it captures non-linear polariton-polariton interactions within the Born approximation. For the case of highly screened contact interactions between charge carriers, we obtain analytic expressions for the equation of state of the many-body system. This allows us to show that there is a photon resonance at a chemical potential higher than the photon cavity energy, where the electron-hole pair correlations in the polariton condensate become universal and independent of the details of the carrier interactions. Comparing the effect of different ranged interactions between charge carriers, we find that the Rytova-Keldysh potential (relevant to transition metal dichalcogenides) offers the best prospect of reaching the BCS regime, where pairs strongly overlap and the minimum pairing gap occurs at finite momentum. Finally, going beyond thermal equilibrium, we argue that there are generically two polariton branches in the driven-dissipative system and we discuss the possibility of a density-driven exceptional point within our model.

Manipulating nonlinear exciton polaritons in an atomically-thin semiconductor with artificial potential landscapes

Exciton polaritons in atomically thin transition-metal dichalcogenide microcavities provide a versatile platform for advancing optoelectronic devices and studying the interacting Bosonic physics at ambient conditions. Rationally engineering the favorable properties of polaritons is critically required for the rapidly growing research. Here, we demonstrate the manipulation of nonlinear polaritons with the lithographically defined potential landscapes in monolayer WS2 microcavities. The discretization of photoluminescence dispersions and spatially confined patterns indicate the deterministic on-site localization of polaritons by the artificial mesa cavities. Varying the trapping sizes, the polariton-reservoir interaction strength is enhanced by about six times through managing the polariton–exciton spatial overlap. Meanwhile, the coherence of trapped polaritons is significantly improved due to the spectral narrowing and tailored in a picosecond range. Therefore, our work not only offers a convenient approach to manipulating the nonlinearity and coherence of polaritons but also opens up possibilities for exploring many-body phenomena and developing novel polaritonic devices based on 2D materials.

Harnessing exciton-polaritons for digital computing, neuromorphic computing, and optimization [Invited

Polaritons are quasiparticles resulting from the strong quantum coupling of light and matter. Peculiar properties of polaritons are a mixture of physics usually restricted to one of these realms, making them interesting for study not only from the fundamental point of view but also for applications. In recent years, many studies have been devoted to the potential use of exciton-polaritons for computing. Very recently, it has been shown experimentally that they can be harnessed not only for digital computing but also for optical neural networks and for optimization related to hard computational problems. Here, we provide a brief review of recent studies and the most important results in this area. We focus our attention, in particular, on the emerging concepts of non-von-Neumann computing schemes and their realizations in exciton-polariton systems.

Fine-tuning biexcitons-plasmon coherent states in a single nanocavity

Abstract A tunable plexcitonic material that sustains multimode hybridization is highly desirable, which is vital for advanced quantum devices. However, the research about regulations of biexcitons-plasmon coherent states has rarely been reported. Here we apply single-nanoparticle scattering spectroscopy correlative with SEM imaging to identify biexcitons-plasmon interaction in a metal-semiconductor hybrid structure composed of a single Au@Ag nanoparticle, J-aggregates molecules and tungsten disulfide (WS2) monolayer. The mode competition within the localized plasmonic hotspots (∼240 nm3) is revealed by continuously regulating the J-aggregates spacer. Two distinct anticrossings are observed at both excitons resonances, and large double Rabi splittings (137 meV and 124 meV) are obtained successfully. We establish experimentally that J-aggregates and WS2 monolayer are responsible for the middle polariton states, while plasmon rarely contributes. Further calculations show that plasmonic nanocavity enables coherent energy exchange with different excitons by providing a highly enhanced localized E-field. In addition, we find that the multimode coupling strengths can be efficiently tuned by changing the cavity morphology and environment temperature, where the tuning spectral accuracy can reach up to 1 nm. Our findings uncover the distinctive properties of biexcitons-plasmon polaritons, suggest an easily obtainable multiqubit states platform, and open up a new way to construct nanoscale photonic devices.

Dispersive Propagation of Terahertz Pulses in a Plasmonic Fiber

The dispersion properties of surface plasmon polaritons (SPPs) during propagation on metal wires with a dielectric coating in the terahertz frequency range were investigated theoretically. An analytical expression was obtained for a pulsed electric field using the solution of Maxwell equations taking into account high-order dispersion terms. The influence of the dielectric coating on the distortion of the pulse shape was investigated. Unlike uncoated wire, the propagation of surface plasmon pulses along a coated wire is highly dispersive. It was shown that the coating leads to the appearance of a long-chirped signal with a propagation of only a few millimeters, i.e., when a terahertz pulse propagates along a coated wire, it acquires a long oscillatory tail, the frequency of which depends on time.

Near‐Field Nano‐Optical Imaging of van der Waals Materials

Abstract2D van der Waals (vdW) materials are emerging as the next generation platform for optical and electronic devices with their wide coverage of the energy bandgaps. The strong light–matter interactions in 2D vdW layers allow for exploring novel optical and electronic phenomena such as 2D polaritons exhibiting ultrahigh field confinement, defects‐induced new quantum states, and strain‐modulated quantum confinement of 2D excitons. Far‐field optical imaging techniques are extensively used to characterize the 2D vdW materials so far, however, subdiffraction spatial resolution is required for comprehensive investigations of 2D vdW materials of which physical properties are greatly influenced by local defects and strain. This article aims to cover historical advances, fundamental principles, and distinct features of emerging near‐field optical imaging techniques: scattering‐type scanning near‐field optical microscopy, tip‐enhanced Raman spectroscopy, tip‐enhanced photoluminescence techniques, and photo‐induced force microscopy. The recent developments toward spectroscopic analysis of near‐field imaging and applications for unveiling unique properties of 2D polaritons, nanoscale defects, and mechanical strains in 2D vdW materials, are also discussed. This review article provides an understanding of emerging near‐field imaging techniques and suggests prospective applications for exploring 2D vdW materials.

Polaritons in Van der Waals Heterostructures.

2D monolayers supporting a wide variety of highly confined plasmons, phonon polaritons, and exciton polaritons can be vertically stacked in van der Waals heterostructures (vdWHs) with controlled constituent layers, stacking sequence, and even twist angles. vdWHs combine advantages of 2D material polaritons, rich optical structure design, and atomic scale integration, which have greatly extended the performance and functions of polaritons, such as wide frequency range, long lifetime, ultrafast all-optical modulation, and photonic crystals for nanoscale light. Here, the state of the art of 2D material polaritons in vdWHs from the perspective of design principles and potential applications is reviewed. Some fundamental properties of polaritons in vdWHs are initially discussed, followed by recent discoveries of plasmons, phonon polaritons, exciton polaritons, and their hybrid modes in vdWHs. The review concludes with a perspective discussion on potential applications of these polaritons such as nanophotonic integrated circuits, which will benefit from the intersection between nanophotonics and materials science.

Analysis, Reduction, and Utilization of Loss in Reconfigurable Spoof Surface Plasmon Polaritons

Reconfigurability of dispersion properties of spoof surface plasmon polaritons (SSPPs) have been realized by loading tunable elements such as varactors and p-i-n diodes. However, the parasitic resistance of tunable elements will lead to transmission loss of SSPPs, which still lacks investigation. In this article, we propose to comprehensively answer three critical questions in this area: 1) how to analyze the loss of reconfigurable SSPPs; 2) how to reduce the loss of reconfigurable SSPPs; and 3) how to utilize the loss of reconfigurable SSPPs. For the first question, the complex characteristic impedance of reconfigurable SSPPs is theoretically obtained, which indicates the mechanism of the enhanced loss in reconfigurable SSPPs. For the second question, a low-loss reconfigurable SSPP structure loading parallel varactors is proposed to reduce and reconfigure the loss of reconfigurable SSPPs. For the third question, a multifunctional 2-unit antenna array is designed and tested using a feeding network composed of the loss-reconfigurable SSPPs. Experimental results demonstrate that two functions of sum-difference beam forming and amplitude modulation are achieved by the single sample. Thus the technique of loss reconfiguration in SSPPs may find wide applications in efficiency-sensitive circuits and smart microwave systems in the future.