Strong Light-matter Interaction Research Articles

Storing light near an exceptional point

Photons with zero rest mass are impossible to be stopped. However, a pulse of light can be slowed down and even halted through strong light-matter interaction in a dispersive medium in atomic systems. Exceptional point (EP), a non-Hermitian singularity point, can introduce an abrupt transition in dispersion. Here we experimentally observe room-temperature storing light near an exceptional point induced by nonlinear Brillouin scattering in a chip-scale 90-μm-radius optical microcavity, the smallest platform up to date to store light. Through nonlinear coupling, a Parity-Time (PT) symmetry can be constructed in optical-acoustical hybrid modes, where Brillouin scattering-induced absorption (BSIA) can lead to both slow light and fast light of incoming pulses. A subtle transition of slow-to-fast light reveals a critical point for storing a light pulse up to half a millisecond. This compact and room-temperature scheme of storing light paves the way for practical applications in all-optical communications and quantum information processing.

Polarization-independent quasi-BIC supported by non-rotationally symmetric dimer metasurfaces.

Asymmetric metasurfaces supporting quasi-bound states in the continuum (-BICs) have recently attracted significant interest in the field of nanophotonics due to their high quality factor and strong light-matter interaction properties. However, asymmetric metasurface structures are susceptible to the polarization state of the incident light, which constrains their potential applications. In this Letter, we present a new, to our knowledge, scheme of polarization-independent quasi-BIC resonance supported by a non-rotationally symmetric nanorod dimer metasurface. By tuning the asymmetry parameter, the designed metasurface exhibits a consistent quasi-BIC response for incident plane waves of arbitrary polarization. The physical mechanism of the quasi-BIC resonance is elucidated by the study of the far-field multipole decomposition and the near-field electromagnetic distribution. We then point out that the realization of the polarization-independent quasi-BIC resonance depends on the transition between magnetic and electric quadrupoles. Furthermore, the designed metasurface is demonstrated to have excellent refractive index sensing performance. This work provides a new idea for the design of polarization-independent and high-performance resonators.

Dual Plasmons with Bioinspired 3D Network Structure Enabling Ultrahigh Efficient Solar Steam Generation.

Plasmonic nanomaterials such as Au, Ag, and Cu are widely recognized for their strong light-matter interactions, making them promising photothermal materials for solar steam generation. However, their practical use in water evaporation is significantly limited by the trade-off between high costs and poor stability. In this regard, we introduce a novel, nonmetallic dual plasmonic TiN/MoO3-x composite. This composite features a three-dimensional, urchin-like biomimetic structure, with plasmonic TiN nanoparticles embedded within a network of plasmonic MoO3-x nanorods. As a solar absorber, the TiN/MoO3-x composite achieves a high evaporation rate of ∼2.05 kg m-2 h-1 with an energy efficiency up to 106.7% under 1 sun illumination, outperforming the state-of-the-art plasmonic systems. The high photothermal stability and unique dual plasmonic nanostructure of the TiN/MoO3-x composite are demonstrated by advanced in situ laser-heating transmission electron microscopy and photon-induced near-field electron microscopy/electron energy-loss spectroscopy, respectively. This work provides new inspiration for the design of plasmonic materials.

Self-Driven Graphene Photodetector Arrays Enabled by Plasmon-Induced Asymmetric Electric Field.

Gap surface plasmon (GSP) modes enhance graphene photodetectors (GPDs)' performance by confining the incident light within nanogaps, giving rise to strong light absorption. Here, we propose an asymmetric plasmonic nanostructure array on planar graphene comprising stripe- and triangle-shaped sharp tip arrays. Upon light excitation, the noncentrosymmetric metallic nanostructures show strong light-matter interactions with localized field close to the surface of tips, causing an asymmetric electric field. These features can accelerate the hot electron generation in graphene, forming a directional diffusion current. Accordingly, the artificial GPDs exhibit a wavelength-dependence behavior covering the wavelength range from 0.8 to 1.6 μm, with three photoresponse maxima corresponding to the nanostructures' resonances. Additionally, the polarization-dependent GPDs can realize a responsivity of ∼25 mA/W and a noise equivalent power of ∼0.44 nW/Hz1/2 at zero bias when excited at the resonance of 1.4 μm. Overall, our study offers a new strategy for preparing compact and multifrequency infrared GPDs.

Van Hove Singularity-Enhanced Raman Scattering and Photocurrent Generation in Twisted Monolayer-Bilayer Graphene.

Twisted monolayer-bilayer graphene (TMBG) has recently emerged as an exciting platform for exploring correlated physics and topological states with rich tunability. Strong light-matter interaction was realized in twisted bilayer graphene, boosting the development of broadband graphene photodetectors from the visible to infrared spectrum with high responsivity. Extending this approach to the case of TMBG will help design advanced quantum nano-optoelectronic devices because of the reduced symmetry of the system. Here, we observe the formation of van Hove singularities (VHSs) in TMBG by monitoring the significant enhancement of the Raman intensity of the G peak and the intensity ratio of G and 2D peaks. The strong interlayer coupling also leads to the appearance of twist-angle-dependent Raman R and R' peaks in TMBG. Furthermore, the constructed graphene photodetectors from 13.5°-TMBG show significantly enhanced photoresponsivity (∼31 folds of monolayer graphene and ∼15 folds of trilayer graphene) when the energy of incident photons matches the interval energy between the two VHSs in the conduction and valence bands. Our findings establish TMBG as a tunable platform for investigating the light-matter interaction and designing high-performance graphene photodetectors with combined high responsivity and high selectivity.

Strongly coupled plasmon-exciton polaritons for photobleaching suppression

Abstract Strong light–matter interactions have received a lot of attention, for example in the pursuit of plasmonic-excitonic structures as coherent light sources with low-power threshold. In this study, we investigate the influence of room temperature strong coupling between surface plasmon polaritons (SPP) and excitons on fluorescence lifetimes and photobleaching effects. Our plasmonic-photonic structure, comprising of thin silver (Ag) and gold (Au) layers with a Rhodamine 6G (R6G) dye layer, shows a clear shift in the plasmon resonance and R6G absorption lines with varying incident angles, indicative of strong coupling, with a measured Rabi splitting of approximately 90 meV. Fluorescence lifetime imaging microscopy (FLIM) was then employed to assess photobleaching, revealing a significant reduction in photobleaching effect for in strongly coupled plasmonic-excitonic structures compared to single Rhodamine R6G layers. Our findings indicate the pivotal role of strong light–matter interactions in reducing photobleaching effects and stabilizing fluorescence intensities, offering promising avenues for developing quantum multiparticle nanophotonic devices with enhanced stability and performance.

Dynamically Tunable Circularly Polarized Selectivity in Plasmon-Enhanced Halide Perovskite Nanocrystal Glasses.

Ultrafast spin manipulation for optical spin-logic applications requires material systems with strong spin-selective light-matter interactions. The optical Stark effect can realize spin-selective light-matter interactions by breaking the degeneracy of spin-selective transitions with an external electric field. Halide perovskites have large exciton binding energies, which enable a room-temperature optical Stark effect. However, halide perovskites are prone to degradation when interacting with light and polar solvents, limiting further integration with nanophotonic structures. We demonstrate a hybrid material system consisting of CsPbBr3 nanocrystal glass weakly coupled to resonant plasmonic silver nanoparticles, showing ultrafast tunable spin-based polarization selectivity at room temperature. We performed circularly polarized pump-probe characterizations to investigate the optical Stark effect in this material system, which resulted in a maximum energy shift of ∼3.67 meV (detuning energy of 0.11 eV and pump intensity of 0.62 GW/cm2). We show that halide perovskite nanocrystal glasses have excellent resistance to heat and moisture, which may be favorable for integration with nanophotonic structures for further engineering polarization states, energy tuning, and coherence time.

Cavity-Mediated Enhancement of the Energy Transfer in the Reduced Fenna-Matthews-Olson Complex.

Strong light-matter interaction leads to the formation of hybrid polariton states and can alter the light-harvesting properties of natural photosynthetic systems without modifying their chemical structure. In the present study, we computationally investigate the effect of the resonant cavity on the efficiency and the rate of the population transfer in a quantum system coupled to the cavity and the dissipative environment. The parameters of the model system were chosen to represent the Fenna-Matthews-Olson natural light-harvesting complex reduced to the three essential sites. The dynamics of the total system was propagated using the hierarchical equations of motion. Our results show that the strong light-matter interaction can accelerate the population transfer process compared to the cavity-free case but at the cost of lowering the transfer efficiency. The transition to the strong coupling regime was found to coincide with the degeneracy of polariton eigenvalues. Our findings indicate the potential and the limit of tuning the energy transfer in already efficient natural light-harvesting systems.

Strong coupling between excitons and chiral quasibound states in the continuum of the bulk WS2 metasurface

In the fields of optics and photonics, the topic of light-matter interactions, particularly strong coupling effects, is a developing area of research. Exciton polariton, a hybridized state brought about by strong coupling, is a hot topic, especially from the standpoint of chiral optics. Under the incidence of right circularly polarized light and left circularly polarized (RCP and LCP) light, we investigate the strong coupling between excitons and quasibound states in the continuum (Q-BICs) resonance in a bulk WS2 metasurface. It is discovered that the Q-BICs are affected obviously on center frequencies and linewidth by structure parameters; while the exciton peaks are impacted insignificantly. Furtherly Q-BIC shows chiral enhancing features. When we take into account both Q-BIC and excitons of the metasurface simultaneously, there is a strong coupling as evidenced by the Rabi splitting up to 182 meV and the clear anti-crossing behavior in the transmittance and reflectance spectra of left and right circularly polarized light, respectively. Notably, a quasi-induced transparency window forms in the circular dichroism (CD) spectrum around the exciton band due to the non-chirality of exciton peaks. This enables the removal of excitons that are not involved in the strong coupling. The bulk WS2 chiral metasurface as a strong coupling system not only provides a way to understand the strong Light-matter interaction, but also generate a potential possibility for realizing the application of chiral optics. Additionally, it can realize the strong chiral coupling with a single self-hybridized element in the structure. Our results have potential implications in the chiral optical field and provide a chiral perspective on the study of strong photon-exciton coupling.

Layer and orientation dependent optical anisotropic response of CVD grown MoS2 films under resonance/non-resonance excitation

The MoS2hasgained worldwide research interest owing to their strong light-matter interaction and their rich physics that offers unprecedented potential for next-generation nanotechnology applications. Theirelectrical and thermal properties are mainly governed by interaction mechanism of electrons with phonons and thus, studying the polarization-dependent anisotropic phonon response ofMoS2are of great interest. Here, we discuss the evolution of phonon modes with increasing layer number and their respective phonon DOS. Further, the optical anisotropic responseis experimentally investigated using ARPRSstudy for CVD-grown triangular MoS2with different layer numbers and different excitations, and has been discussedbased ontheory that includes deformation potential and Fröhlich interaction.Further, the ARPRS study of horizontally and vertically oriented MoS2films are studied. Thedifferent anisotropic response of vertically oriented film is observed due to symmetry breaking of the vertical nanosheets, thus providing wide-field applications in MoS2-based photonic and optoelectronic devices.

Engineering the Interfacing of Molecules with 2D Transition Metal Dichalcogenides: Enhanced Multifunctional Electronics.

ConspectusEngineering all interfaces between different components in electronic devices is the key to control and optimize fundamental physical processes such as charge injection at metal-semiconductor interfaces, gate modulation at the dielectric-semiconductor interface, and carrier modulation at semiconductor-environment interfaces. The use of two-dimensional (2D) crystals as semiconductors, by virtue of their atomically flat dangling bond-free structures, can facilitate the tailoring of such interfaces effectively. In this context, 2D transition metal dichalcogenides (TMDs) have garnered tremendous attention over the past two decades owing to their exclusive and outstanding physical and chemical characteristics such as their strong light-matter interactions and high charge mobility. These properties position them as promising building blocks for next-generation semiconductor materials. The combination of their large specific surface area, unique electronic structure, and properties highly sensitive to environmental changes makes 2D TMDs appealing platforms for applications in optoelectronics and sensing. While a broad arsenal of TMDs has been made available that exhibit a variety of electronic properties, the latter are unfortunately hardly tunable. To overcome this problem, the controlled functionalization of TMDs with molecules and assemblies thereof represents a most powerful strategy to finely tune their surface characteristics for electronics. Such functionalization can be used not only to encapsulate the electronic material, therefore enhancing its stability in air, but also to impart dynamic, stimuli-responsive characteristics to TMDs and to selectively recognize the presence of a given analyte in the environment, demonstrating unprecedented application potential.In this Account, we highlight the most enlightening recent progress made on the interface engineering in 2D TMD-based electronic devices via covalent and noncovalent functionalization with suitably designed molecules, underlining the remarkable synergies achieved. While electrode functionalization allows modulating charge injection and extraction, the functionalization of the dielectric substrate enables tuning of the carrier concentration in the device channel, and the functionalization of the upper surface of 2D TMDs allows screening the interaction with the environment and imparts molecular functionality to the devices, making them versatile for various applications. The tailored interfaces enable enhanced device performance and open up avenues for practical applications. This Account specifically focuses on our recent endeavor in the unusual properties conferred to 2D TMDs through the functionalization of their interfaces with stimuli-responsive molecules or molecular assemblies. This includes electrode-functionalized devices with modulable performance and charge carriers, molecular-bridged TMD network devices with overall enhanced electrical properties, sensor devices that are highly responsive to changes in the external environment, in particular, electrochemically switchable transistors that react to external electrochemical signals, optically switchable transistors that are sensitive to external light inputs, and multiresponsive transistors that simultaneously respond to multiple external stimuli including optical, electrical, redox, thermal, and magnetic inputs and their application in the development of unprecedented memories, artificial synapses, and logic inverters. By presenting the current challenges, opportunities, and prospects in this blooming research field, we will discuss the powerful integration of such strategies for next-generation electronic digital devices and logic circuitries, outlining future directions and potential breakthroughs in interface engineering.

Cathodoluminescence from interlayer excitons in a 2D semiconductor heterobilayer.

Photoluminescence has widely been used to study excitons in semiconducting transition metal dichalcogenide (MX$_2$) monolayers, demonstrating strong light matter interactions and locked spin and valley degrees of freedom. In heterobilayers composed of overlapping monolayers of two different MX$_2$, an interlayer exciton can form, with the hole localised in one layer and the electron in the other. These interlayer excitons are long-lived, field-tunable, and can be trapped by moir'e patterns formed at small twist angles between the layers. Here we demonstrate that emission from radiative recombination of interlayer excitons can be observed by cathodoluminescence from a WSe$_2$/MoSe$_2$ heterobilayer encapsulated in hexagonal boron nitride. The higher spatial resolution of cathodoluminescence, compared to photoluminescence, allows detailed analysis of sample heterogeneity at the 100s of nm lengthscales over which twist angles tend to vary in dry-transfer fabricated heterostructures.

(Invited) Electronic Doping in Two-Dimensional Semiconductors

Quantum-confined semiconductors provide highly tunable optical and electrical properties for a wide variety of emerging applications. Two-dimensional semiconductors possess tunable bandgaps, high charge carrier mobilities, and strong light-matter interactions that can enable applications in photovoltaics, quantum information processing, and spintronics. Electronic doping, the intentional introduction of controlled charge carrier density and type (electrons or holes) is a fundamental enabler for many emerging applications of two-dimensional semiconductors. While electrostatic doping in transistor geometries is useful for certain applications, it is important to develop additional methods based on e.g. molecular redox doping and substitutional atomic doping that can provide tunable Fermi level control in the absence of an applied bias. In this presentation, I will discuss our efforts to develop such doping strategies for a variety of two-dimensional semiconductors. I will discuss how electronic doping modulates opto-electronic properties, including ground-state absorbance, charge transport, and excited-state dynamics in these 2D semiconductors. These studies provide fundamental insights into the degree to which molecular and substitutional doping strategies can modulate Fermi level, charge transport, and excitonic optical transitions in 2D semiconductors.

"Nonperturbative Nonlinearities": Perhaps Less than Meets the Eye.

We address challenges in characterizing changes in permittivity and refractive index beyond standard perturbative methods with special attention given to transparent conductive oxides (TCOs). We unveil a realistic limit to permittivity changes under high optical power densities. Our study covers both slow and ultrafast nonlinearities, demonstrating that all nonlinearities induce refractive index changes accurately described by a simple curve with saturation electric field (or irradiance) and maximum change of permittivity at saturation. Our model, grounded in material properties, like oscillator strength and characteristic times, offers a robust framework for understanding and predicting nonlinear optical phenomena in TCOs and other materials. We differentiate between the significance of higher-order nonlinear susceptibilities in ultrafast and slow nonlinear scenarios. We aim to provide valuable insights for researchers exploring strong light-matter interaction.

Cavity Controlled Upconversion in CdSe Nanoplatelet Polaritons.

Exciton-polaritons provide a versatile platform for investigating quantum electrodynamics effects in chemical systems, such as polariton-altered chemical reactivity. However, using polaritons in chemical contexts will require a better understanding of their photophysical properties under ambient conditions, where chemistry is typically performed. Here, we used cavity quality factor to control strong light-matter interactions and in particular the excited state dynamics of colloidal CdSe nanoplatelets (NPLs) coupled to a Fabry-Pérot optical cavity. With increasing cavity quality factor, we observe significant population of the upper polariton (UP) state, exemplified by the rare observation of substantial UP photoluminescence (PL). Excitation of the lower polariton (LP) states results in upconverted PL emission from the UP branch due to efficient exchange of population between the LP, UP and the reservoir of dark states present in collectively coupled polaritonic systems. In addition, we measure time scales for polariton dynamics ∼100 ps, implying great potential for NPL based polariton systems to affect photochemical reaction rates. State-of-the-art quantum dynamical simulations show outstanding quantitative agreement with experiments, and thus provide important insight into polariton photophysical dynamics of collectively coupled nanocrystal-based systems. These findings represent a significant step toward the development of practical polariton photochemistry platforms.

Wide-Dynamic-Range Control of Quantum-Electrodynamic Electron Transfer Reactions in the Weak Coupling Regime.

Catalyzing reactions effectively by vacuum fluctuations of electromagnetic fields is a significant challenge within the realm of chemistry. As opposed to most studies based on vibrational strong coupling, we introduce an innovative catalytic mechanism driven by weakly coupled polaritonic fields. Through the amalgamation of macroscopic quantum electrodynamics (QED) principles with Marcus electron transfer (ET) theory, we predict that ET reaction rates can be precisely modulated across a wide dynamic range by controlling the size and structure of nanocavities. Compared to QED-driven radiative ET rates in free space, plasmonic cavities induce substantial rate enhancements spanning the range from 103- to 10-fold. By contrast, Fabry-Perot cavities engender rate suppression spanning the range from 10-2- to 10-1-fold. This work overcomes the necessity of using strong light-matter interactions in QED chemistry, opening up a new era of manipulating QED-based chemical reactions in a wide dynamic range.

Influence of Edges and Interlayer Electron-phonon Coupling in WS2/h-BN Heterostructure.

The semiconducting layered transition metal dichalcogenides (e.g., WS2) are excellent candidates for the realization of optoelectronic and nanophotonic applications on account of their band gap tunability, high binding energy and oscillator strength of the excitons, strong light-matter interaction, appreciable charge carrier mobility, and valleytronic properties. However, the photoluminescence (PL) emissions are reported to show a nonuniform spatial distribution, with the edges emitting features like defect-bound excitons and biexcitons at low temperatures in addition to the typical excitons and trions. The appearance of these additional PL features has been shown in the literature to have a strong dependence on the presence of S-vacancies and excess charge carriers. We demonstrate an enhancement of the defect-bound excitons and biexcitons by creating a heterostructure of WS2 with h-BN where the coupling between the charge carriers in WS2 with the polar phonons in h-BN governs the enhancement. Furthermore, we have performed a comprehensive resonant Raman study with varying polarization and magnetic field which not only confirms the presence of electron-phonon coupling in WS2/h-BN heterostructure, it further demonstrates a thermally induced differential resonance behavior with the excitonic level and the defect-induced midgap states (due to S-vacancies at the edge of WS2) exhibited by a dome-shaped behavior of the Raman intensities with temperature for the normal and defect-induced phonon modes. The defect-bound Raman modes exhibit maximum resonance at ∼240 K while normal Raman modes show at ∼280 K owing to a thermal variation of the electronic states.

Reversible modulation of superconductivity in thin-film NbSe2 via plasmon coupling

In recent years, lightwave has stood out as an ultrafast, non-contact control knob for developing compact superconducting circuitry. However, the modulation efficiency is limited by the low photoresponse of superconductors. Plasmons, with the advantages of strong light-matter interaction, present a promising route to overcome the limitations. Here we achieve effective modulation of superconductivity in thin-film NbSe2 via near-field coupling to plasmons in gold nanoparticles. Upon resonant plasmon excitation, the superconductivity of NbSe2 is substantially suppressed. The modulation factor exceeds 40% at a photon flux of 9.36 × 1013 s−1mm−2, and the effect is significantly diminished for thicker NbSe2 samples. Our observations can be theoretically interpreted by invoking the non-equilibrium electron distribution in NbSe2 driven by the plasmon-associated evanescent field. Finally, a reversible plasmon-driven superconducting switch is realized in this system. These findings highlight plasmonic tailoring of quantum states as an innovative strategy for superconducting electronics.

Femtosecond laser direct writing wedge metallic microcavities for terahertz sensing

Metallic resonators allow the manipulation of electromagnetic fields in subwavelength volumes for strong light-matter interaction, underpinning emerging flat-optics devices. Metamaterials designed with optimized effective mode volume (Veff) for stimulating and maximizing the utilization of high-density photons are highly desirable for sensing applications. Here, we demonstrate a bowtie-type terahertz (THz) metallic aperture metamaterial structure with low Veff for enhanced biosensing. The device leverages the inherent characteristic of the femtosecond laser fabrication process enabling wedge cavities down to 3 µm by tailoring the pulse energy, whose sensing performance is superior to trapezoidal ones. Such a substrate-free, microcavity metallic metamaterial sensor with extremely strong interactions achieves an experimentally normalized sensitivity of 0.654 RIU−1, and demonstrates the detection of L-proline down to 0.87 nmol. These findings provide new insights into the design and optimization of efficient THz metamaterials with tightly confined fields for ultrasensitive biomolecular detection.

Enhancing photoluminescence of WSe2 in vapor grown WSe2/VOCl bilayer heterojunctions via surface passivation.

Monolayer tungsten selenide (WSe2) has attracted attention due to its direct bandgap-generated strong light emission and light-matter interaction. Herein, vertical WSe2/VOCl bilayer heterojunctions with enhanced PL of WSe2 were synthesized by the vapor growth method. The morphology, crystal structure, and chemical composition of the WSe2/VOCl heterojunctions were systematically investigated, which confirmed the successful formation of the heterojunctions. The PL emission intensity of WSe2 obtained from the WSe2/VOCl heterojunction was about 2.4 times higher than that of the WSe2 monolayer, demonstrating the high optical quality of the WSe2/VOCl heterojunction, which was further confirmed by time-resolved PL measurements. The insulator top VOCl, which was deposited on the surface of the semiconductor bottom WSe2 as a surface passivation material, reducing the impurities and resulting in an atomically clean surface, successfully enhanced the PL emission of the bottom WSe2. This vertical WSe2/VOCl bilayer heterojunction with PL enhancement could provide a promising platform for optical devices.