Exciton-plasmon Polaritons Research Articles

Full Quantitative Near-Field Characterization of Strongly Coupled Exciton–Plasmon Polaritons in Thin-Layered WSe2 on a Monocrystalline Gold Platelet

Full Quantitative Near-Field Characterization of Strongly Coupled Exciton–Plasmon Polaritons in Thin-Layered WSe₂ on a Monocrystalline Gold Platelet

Incorporation strategy for organic dyes into gold nanoparticle supercrystals.

Ordered arrays of plasmonic nanoparticles, supercrystals can lead to the formation of plasmon-polaritons. Coupling light emitters with plasmon polaritons might allow the formation of exciton-plasmon polaritons with properties tuneable by the supercrystal design. To construct such optically active materials, the inclusion of emitters is imperative. The addition of organic dyes without affecting the periodic order of the nanocrystals is difficult, as post-formation protocols might dissolve the supercrystals, and pre-formation addition might affect the self-assembly process. Here, we present an exemplary strategy to functionalize gold nanoparticles prior to self-assembly with a cyanine isothiocyanate dye that was obtained by a straightforward reaction of the amine functionalized dye with carbon disulfide. In the second step, the nanoparticles are functionalized with a thiol-terminated polystyrene, which stabilizes the nanoparticles and governs the self-assembly process. The dye can be integrated in a quantitative fashion, and the nanoparticles can be self-assembled into supercrystals. The strategy should be applicable in general for amine functionalized dyes, which is a common modification.

Degenerate parametric down-conversion facilitated by exciton-plasmon polariton states in a nonlinear plasmonic cavity

We study the effect of degenerate parametric down-conversion (DPDC) in an ensemble of two-level quantum emitters (QEs) coupled via near-field interactions to a single surface plasmon (SP) mode of a nonlinear plasmonic cavity. For this purpose, we develop a quantum driven-dissipative model capturing non-equilibrium dynamics of the system in which incoherently pumped QEs have transition frequency tuned near the second-harmonic response of the SPs. Considering the strong coupling regime, i.e. the SP-QE interaction rate exceeds system dissipation rates, we find a critical SP-QE coupling attributed to the phase transition between normal and lasing steady states. Examining fluctuations above the system’s steady states, we predict new elementary excitations, namely, the exciton-plasmon polaritons formed by the two-SP quanta and single-exciton states of QEs. The contribution of two-SP quanta results in the linear scaling of the SP-QE interaction rate with the number of QEs, , as opposed to the -scaling known for the Dicke and Tavis–Cummings models. We further examine how SP-QE interaction scaling affects the polariton dispersions and power spectra in the vicinity of the critical coupling. For this purpose, we compare the calculation results assuming a finite ensemble of QEs and the model thermodynamic limit. The calculated power spectra predict an interplay of coherent photon emission by QEs near the second-harmonic frequency and correlated photon-pair emission at the fundamental frequency by the SPs (i.e. the photonic DPDC effect).

Ultra-confined Propagating Exciton–Plasmon Polaritons Enabled by Cavity-Free Strong Coupling: Beating Plasmonic Trade-Offs

Hybrid coupling systems consisting of transition metal dichalcogenides (TMD) and plasmonic nanostructures have emerged as a promising platform to explore exciton–plasmon polaritons. However, the requisite cavity/resonator for strong coupling introduces extra complexities and challenges for waveguiding applications. Alternatively, plasmonic nano-waveguides can also be utilized to provide a non-resonant approach for strong coupling, while their utility is limited by the plasmonic confinement-loss and confinement-momentum trade-offs. Here, based on a cavity-free approach, we overcome these constraints by theoretically strong coupling of a monolayer TMD to a single metal nanowire, generating ultra-confined propagating exciton–plasmon polaritons (PEPPs) that beat the plasmonic trade-offs. By leveraging strong-coupling-induced reformations in energy distribution and combining favorable properties of surface plasmon polaritons (SPPs) and excitons, the generated PEPPs feature ultra-deep subwavelength confinement (down to 1-nm level with mode areas ~ 10–4 of λ2), long propagation length (up to ~ 60 µm), tunable dispersion with versatile mode characters (SPP- and exciton-like mode characters), and small momentum mismatch to free-space photons. With the capability to overcome the trade-offs of SPPs and the compatibility for waveguiding applications, our theoretical results suggest an attractive guided-wave platform to manipulate exciton–plasmon interactions at the ultra-deep subwavelength scale, opening new horizons for waveguiding nano-polaritonic components and devices.

The Role of Molecular Arrangement on the Strongly Coupled Exciton–Plasmon Polariton Dispersion in Metal–Organic Hybrid Structures

Metal–organic hybrid structures have been demonstrated to be a versatile platform to study primary aspects of light–matter interaction by means of emerging states comprising excitonic and plasmonic properties. Here we are studying the wave-vector-dependent photoexcitations in gold layers covered by molecular films of zinc phthalocyanine and its fluorinated derivatives (FnZnPc, with n = 0, 4, 8, 16). These layered metal–organic samples show up to four anticrossings in their dispersions correlating in energy with the respective degree of ZnPc fluorination. By means of complementary structural and theoretical data, we attribute the observed anticrossings to three main scenarios of surface plasmon coupling: (i) to aggregated α-phase regions within the FnZnPc layers at 1.75 and 1.85 eV, (ii) to a coexisting F16ZnPc β-polymorph at 1.51 eV, and (iii) to monomers, preferentially located at the metal interface, at 2.15 eV. Whereas energy and splitting of the monomer anticrossings depend on strength and average tilting of the molecular dipole moments, the aggregate-related anticrossings show a distinct variation with degree of fluorination. These observations can be consistently explained by a change in FnZnPc dipole density induced by an increased lattice spacing due to the larger molecular van der Waals radii upon fluorination. The reported results prove Au/FnZnPc bilayers a model system to demonstrate the high sensitivity of exciton–plasmon coupling on the molecular alignment at microscopic length scales.

Nano-optical imaging of exciton-plasmon polaritons in WSe2/Au heterostructures.

We report a nano-optical imaging study of exciton-plasmon polaritons (EPPs) in WSe2/Au heterostructures with scattering-type scanning near-field optical microscopy (s-SNOM). By mapping the interference fringes of EPPs at various excitation energies, we constructed the dispersion diagram of the EPPs, which shows strong exciton-plasmon coupling with a sizable Rabi splitting energy (∼0.19 eV). Furthermore, we found a sensitive dependence of the polariton wavelength (λp) on WSe2 thickness (d). When d is below 40 nm, λp decreases rapidly with increasing d. As d reaches 50 nm and above, λp drops to 210 nm, which is over 4 times smaller than that of the free-space photons. Our simulations indicate that the high spatial confinement of EPPs is due to the strong localization of the polariton field inside WSe2. Our work uncovers the transport properties of EPPs and paves the way for future applications of these highly confined polaritons in nanophotonics and optoelectronics.

Rabi splitting obtained in exciton-plasmon polaritons coupling between monolayer black phosphorus with metal

The strong coupling between the exciton polariton and the plasmon polariton produced by the interaction of light and matter forms a new hybrid polariton. We investigated the interaction between black phosphorus and metal generated polaritons. The spectrum with angular resolution as a variable shows anti-crossing characteristics in the strong coupling region. The proposed structure can achieve 65–150 meV Rabi splitting. The dispersion relation and coupled harmonic oscillator model also prove that increasing dielectric permittivity can effectively enlarge Rabi splitting. The research provides a backdrop for the application of polarization devices and strong coupling platforms.

Exciton–Plasmon Coupling in 2D Semiconductors Accessed by Surface Acoustic Waves

We theoretically demonstrate the coupling between excitons in 2D semiconductors and surface plasmons in a thin metal film by means of a surface acoustic wave (SAW), proving that the generated exciton-plasmon polaritons (or plexcitons) are in the strong coupling regime. The strain field of the SAW creates a dynamic diffraction grating providing the momentum match for the surface plasmons, whereas the piezoelectric field, that could dissociate the excitons, is cancelled out by the metal. This is exemplified for monolayer MoS$\mathrm{_{2}}$ and mono- and few-layer black phosphorus on top of a thin silver layer on a LiNbO$\mathrm{_{3}}$ piezoelectric substrate, providing Rabi splittings of 100-150 meV. Thus, we demonstrate that SAWs are powerful tools to modulate the optical properties of supported 2D semiconductors by means of the high-frequency localized deformations tailored by the acoustic transducers, that can serve as electrically switchable launchers of propagating plexcitons suitable for active high-speed nanophotonic applications.

Patterning of transition metal dichalcogenides catalyzed by surface plasmons with atomic precision

Summary Plasmon-induced charge transfer has drawn considerable interests and spurred rapid developments of photovoltaics and photocatalysis. Various plasmonic metal/transition metal dichalcogenide (TMD) heterostructures have been developed to promote charge separation. For plasmon-induced catalysis, the transformation of TMDs is rare, and the erosion of TMDs has not yet been reported. Here, we report the etching of two-dimensional TMDs by planar surface plasmon polaritons (SPPs). We found that SPPs can etch various TMDs into desired layers and lateral size through controlling the light power and incident direction. In aqueous media, the strong plasmonic coupling at Au/MoS2 interface generates holes in the valence band of MoS2 that can weaken its interlayer interaction. As a synergistic effect, the plasmonic hot electrons produce oxidizing H2O2 to dissolve MoS2 from the top layers. Our results provide a new perspective for understanding the plasmonic coupling at metal-TMD interfaces and a reliable route toward fabricating well-defined TMDs.

Ultrafast Investigation of Individual Bright Exciton–Plasmon Polaritons in Size‐Tunable Metal–WS2 Hybrid Nanostructures

AbstractStrong light–matter interactions between resonantly coupled metal plasmons and spin–orbit‐coupled bright excitons from 2D transition metal dichalcogenides (TMDs) can produce discrete bright exciton–plasmon polaritons (plexcitons). A few efforts have been made to perceive the spin‐induced exciton‐polaritons in nanocavities at cryogenic conditions, however, successful realization of bright plexcitons in time domain is still lacking. Here, both the bright plexcitons are identified discretely at room temperature and their ultrafast temporal dynamics in size‐tunable Au–WS2 hybrid nanostructures are investigated using a helicity‐controlled femtosecond pump‐probe spectroscopy technique. The zero detuning between both the bright excitons and plasmons is achieved at ≈7.0 ps, matching with the stable plexciton formation time. The Rabi‐splitting energy is as high as ≈250 meV for both the plexcitons, validating the strong‐coupling conditions of polariton formation. Realization of these novel bright exciton–plasmon interactions in the metal–TMDs platform is, therefore, interesting for both fundamental understanding and their possible futuristic applications in quantum photonics operating at room temperature.

Exciton-plasmon polariton coupling and hot carrier generation in two-dimensional SiB semiconductors: a first-principles study

Abstract Exciton (strong electron–hole interactions) and hot carriers (HCs) assisted by surface plasmon polaritons show promise to enhance the photoresponse of nanoelectronic and optoelectronic devices. In the current research, we develop a computational quantum framework to study the effect of coupled exciton and HCs on the photovoltaic energy distribution, scattering process, polarizability, and light emission of two-dimensional (2D) semiconductors. Using a stable 2D semiconductor (semihydrogenated SiB) as our example, we theoretically show that external strain and thermal effect on the SiB can lead to valley polarized plasmon quasiparticles and HC generation. Our results reveal that the electron–phonon and electron–electron (e–e) interactions characterize the correlation between the decay rate, scattering of excitons, and generation of HCs in 2D semiconductors. Moreover, phonon assisted luminescence spectra of SiB suggest that light emission can be enhanced by increasing strain and temperature. The polarized plasmon with strong coupling of electronic and photonics states in SiB makes it as a promising candidate for light harvesting, plasmonic photocurrent devices, and quantum information.

Electrical Control of Hybrid Monolayer Tungsten Disulfide-Plasmonic Nanoantenna Light-Matter States at Cryogenic and Room Temperatures.

Hybrid light-matter states-polaritons-have attracted considerable scientific interest recently, motivated by their potential for development of nonlinear and quantum optical schemes. To realize such states, monolayer transition metal dichalcogenides (TMDCs) have been widely employed as excitonic materials. In addition to neutral excitons, TMDCs host charged excitons, which enables active tuning of hybrid light-matter states by electrical means. Although several reports demonstrated charged exciton-polaritons in various systems, the full-range interaction control attainable at room temperature has not been realized. Here, we demonstrate electrically tunable charged exciton-plasmon polaritons in a hybrid tungsten disulfide (WS2) monolayer-plasmonic nanoantenna system. We show that electrical gating of monolayer WS2 allows tuning the oscillator strengths of neutral and charged excitons not only at cryogenic but also at room temperature, both at vacuum and atmospheric pressure. Such electrical control enables a full-range tunable switching from strong neutral exciton-plasmon coupling to strong charged exciton-plasmon coupling. Our experimental findings allow discussing beneficial and limiting factors of charged exciton-plasmon polaritons, as well as offer routes toward realization of charged polaritonic devices at ambient conditions.

Gate-Tunable Emission of Exciton–Plasmon Polaritons in Hybrid MoS2-Gap-Mode Metasurfaces

The advance in designing arrays of ultrathin two-dimensional optical nano-resonators, known as metasurfaces, is currently enabling a large variety of novel flat optical components. The remarkable control over the electromagnetic fields offered by this technology can be further extended to the active regime in order to manipulate the light characteristics in real-time. In this contribution, we couple the excitonic resonance of atomic thin MoS2 monolayers with gap-surface-plasmon (GSP) metasurfaces, and demonstrate selective enhancement of the exciton-plasmon polariton emissions. We further demonstrate tunable emissions by controlling the charge density at interface through electrically gating in MOS structure. Straddling two very active fields of research, this demonstration of electrically tunable light-emitting metasurfaces enables real-time manipulation of light-matter interactions at the extreme subwavelength dimensions.

Room-Temperature Strong Coupling of CdSe Nanoplatelets and Plasmonic Hole Arrays.

Exciton polaritons are hybrid light–matter quasiparticles that can serve as coherent light sources. Motivated by applications, room-temperature realization of polaritons requires narrow, excitonic transitions with large transition dipoles. Such transitions must then be strongly coupled to an electromagnetic mode confined in a small volume. While much work has explored polaritons in organic materials, semiconductor nanocrystals present an alternative excitonic system with enhanced photostability and spectral tunability. In particular, quasi-two-dimensional nanocrystals known as nanoplatelets (NPLs) exhibit intense, spectrally narrow excitonic transitions useful for polariton formation. Here, we place CdSe NPLs on silver hole arrays to demonstrate exciton–plasmon polaritons at room temperature. Angle-resolved reflection spectra reveal Rabi splittings up to 149 meV for the polariton states. We observe bright, polarized emission arising from the lowest polariton state. Furthermore, we assess the dependence of the Rabi splitting on the hole-array pitch and the number N of NPLs. While the pitch determines the in-plane momentum for which strong coupling is observed, it does not affect the size of the splitting. The Rabi splitting first increases with NPL film thickness before eventually saturating. Instead of the commonly used dependence, we develop an analytical expression that includes the transverse confinement of the plasmon modes to describe the measured Rabi splitting as a function of NPL film thickness.

Pliable polaritons: Wannier exciton-plasmon coupling in metal-semiconductor structures

Abstract Plasmonic structures are known to support the modes with sub-wavelength volumes in which the field/matter interactions are greatly enhanced. Coupling between the molecular excitations and plasmons leading to the formation of “plexcitons” has been investigated for a number of organic molecules. However, plasmon-exciton coupling in metal/semiconductor structures has not experienced the same degree of attention. In this work, we show that the “very strong coupling” regime in which the Rabi energy exceeds the exciton binding energy is attainable in semiconductor-cladded plasmonic nanoparticles and leads to the formation of Wannier exciton-plasmon polariton (WEPP), which is bound to the metal nanoparticle and characterized by dramatically smaller (by a factor of a few) excitonic radius and correspondingly higher ionization energy. This higher ionization energy, which exceeding approaches 100 meV for the CdS/Ag structures, may make room-temperature Bose-Einstein condensation and polariton lasing in plasmonic/semiconductor structures possible.

Temporal Dynamics of Localized Exciton-Polaritons in Composite Organic-Plasmonic Metasurfaces.

We use femtosecond transient absorption spectroscopy to study the temporal dynamics of strongly coupled exciton-plasmon polaritons in metasurfaces of aluminum nanoantennas coated with J-aggregate molecules. Compared with the thermal nonlinearities of aluminum nanoantennas, the exciton-plasmon hybridization introduces strong ultrafast nonlinearities in the composite metasurfaces. Within femtoseconds after the pump excitation, the plasmonic resonance is broadened and shifted, showcasing its high sensitivity to excited-state modification of the molecular surroundings. In addition, we observe temporal oscillations due to the deep subangstrom acoustic breathing modes of the nanoantennas in both bare and hybrid metasurfaces. Finally, unlike the dynamics of hybrid states in optical microcavities, here, ground-state bleaching is observed with a significantly longer relaxation time at the upper polariton band.

(Invited) Novel Nanophotonic Devices By Engineering Light-Matter Interactions in Low-Dimensional Systems

Strongly confined electrical, optical and thermal excitations drastically modify material’s properties and break local symmetries that can enable precisely tunable novel responses and new functionalities. With an emphasis on low-dimensional materials such as nanowires and monolayer MoS2, we will discuss how extreme confinement of fields interacting with materials produces new and unexpected materials response. For example, we will discuss how the strong plasmonic fields can lead to a new paradigm of nanoscale Si photonics such as optical emission in the visible region and nonlinear optical devices. Furthermore, by utilizing the fundamental symmetry breaking properties of fields, new quantum phenomena such as chirality-dependent optical and electronic properties will be discussed in non-chiral materials and utilized to enable new functionalities that are only possible in strong spin-orbit coupled materials. The role of geometry in low-dimensional systems to produce new properties in the presence of symmetry breaking fields will be discussed. Finally, effect of plasmons on light matter interactions in 2D excitonic crystals will be discussed, which can be engineered to produce novel responses such as enhanced and tunable emission, Fano resonances and strong exciton-plasmon polaritons, which can be precisely controlled by geometry and applied fields to produce novel device concepts.

Control of electromagnetically induced transparency via a hybrid semiconductor quantum dot–vanadium dioxide nanoparticle system

We numerically investigate the electromagnetically induced transparency (EIT) of a hybrid system consisting of a three-level quantum dot (QD) in the vicinity of vanadium dioxide nanoparticle (VO2NP). VO2NP has semiconductor and metallic phases where the transition between the two phases occurs around a critical temperature. When the QD-VO2NP hybrid system interacts with continuous wave laser fields in an infrared regime, it supports a coherent coupling of exciton–polariton and exciton–plasmon polariton in semiconductor and metal phases of VO2NP, respectively. In our calculations a filling fraction factor controls the VO2NP phase transition. A probe and control laser field configuration is studied for the hybrid system to measure the absorption of QD through the filling fraction factor manipulations. We show that for the VO2NP semiconductor phase and proper geometrical configuration, the absorption spectrum profile of the QD represents an EIT with two peaks and a clear minimum. These two peaks merge to one through the VO2NP phase transition to metal. We also show that the absorption spectrum profile is modified by different orientations of the laser fields with the axis of the QD-VO2NP hybrid system. The innovation in comparison to other research in the field is that robust variation in the absorption profile through EIT is due to the phase transition in VO2NP without any structural change in the QD-VO2NP hybrid system. Our results can be employed to design nanothermal sensors, optical nanoswitches, and energy transfer devices.

Electrical Tuning of Exciton-Plasmon Polariton Coupling in Monolayer MoS2 Integrated with Plasmonic Nanoantenna Lattice.

Active control of light-matter interactions in semiconductors is critical for realizing next generation optoelectronic devices with real-time control of the system's optical properties and hence functionalities via external fields. The ability to dynamically manipulate optical interactions by applied fields in active materials coupled to cavities with fixed geometrical parameters opens up possibilities of controlling the lifetimes, oscillator strengths, effective mass, and relaxation properties of a coupled exciton-photon (or plasmon) system. Here, we demonstrate electrical control of exciton-plasmon coupling strengths between strong and weak coupling limits in a two-dimensional semiconductor integrated with plasmonic nanoresonators assembled in a field-effect transistor device by electrostatic doping. As a result, the energy-momentum dispersions of such an exciton-plasmon coupled system can be altered dynamically with applied electric field by modulating the excitonic properties of monolayer MoS2 arising from many-body effects. In addition, evidence of enhanced coupling between charged excitons (trions) and plasmons was also observed upon increased carrier injection, which can be utilized for fabricating Fermionic polaritonic and magnetoplasmonic devices. The ability to dynamically control the optical properties of a coupled exciton-plasmonic system with electric fields demonstrates the versatility of the coupled system and offers a new platform for the design of optoelectronic devices with precisely tailored responses.

Coherent control of optical absorption and the energy transfer pathway of an infrared quantum dot hybridized with a VO2 nanoparticle

We systematically investigate the optical response of a semiconductor quantum dot (QD) hybridized with a vanadium dioxide nanoparticle (VO2NP) in the infrared (IR) region. The VO2NP features a semiconductor to metal phase change characteristic below and above a critical temperature that leads to an abrupt change in the particle’s optical properties. This feature means that the QD-VO2NP hybrid system can support the coherent coupling of exciton-polaritons and exciton-plasmon polaritons in the semiconductor and metal phases of the VO2NP, respectively. In our calculations, the VO2NP phase transition is modelled with a filling fraction (f), representing the fraction of the VO2NP in the metallic phase. The phase transition is driven by the hybrid system’s interaction with a continuous wave (CW) IR laser field. In this paper, we show how control over the filling fraction results in the enhancement or suppression of the QD’s linear absorption. These variations in the QD absorption is due to dramatic changes in the effective local field experienced by the QD and the non-radiative energy transfer from the QD to the VO2NP. The presented results have the potential to be applied to the design of thermal sensors at the nanoscale.