Strong Light-matter Interaction Research Articles

Molecular Energy Transfer under the Strong Light-Matter Interaction Regime.

Research into strong light-matter interactions continues to fascinate, being spurred on by unforeseen and often spectacular experimental observations. Properties that were considered to depend exclusively on material composition have been found to be drastically altered when a material is placed inside a resonant optical cavity. This is nowhere more the case than in the field of intermolecular energy transfer, where polaritonic states formed as a result of strong light-matter interactions have been shown to promote energy transfer over distances vastly exceeding conventional limits. In this review, we provide the reader with a succinct account of the fundamental concepts of intermolecular energy transfer, and how they are modified by strong light-matter interactions. We also summarize recent experimental advances in the area, including in optoelectronic device contexts, and highlight both the potential and challenges that remain in this exciting field of research going forward.

High-performance 2D WS2 photodetector enhanced by charge-transfer doping through NH3 annealing

Transition metal dichalcogenides (TMDs) are potential candidates towards the next-generation photodetector, owing to their tunable energy bandgap and strong light–matter interaction. However, the low charge-carrier mobility and insufficient quantum efficiency of TMDs-based devices restrain their optoelectronic performances. Therefore, an effective and facile doping method needs to be developed for overcoming their poor optoelectronics properties. Here, we demonstrate a controllable nondegenerate n-doping technique for 2D TMDs materials by annealing under NH3 flow. The on/off current ratio and field-effect mobility of the device after annealing treatment increases about 2 orders of magnitude and 23 times, respectively. We attribute this remarkable doping effect to the physical adsorption and chemical adsorption of NH3 as well as the associated formation of isolated sulfur vacancies, which is dynamically activated during the annealing treatment, and further verified by the atomic-scale characterization and density functional theory calculations. The annealing temperature and time present a controllable modulation on the electron doping levels of the WS2 channel. The fabricated WS2 photodetector can operate in a broad range of visible light at room temperature. At 532 nm wavelength, the responsivity of 2.97 A/W, external quantum efficiency of 699% and specific detectivity of over 1010 Jones indicate its great potential in sensitive and power-efficient photodetectors. Our results provide an effective method to realize controllable n-doping of 2D materials, aiming for fabricating high-performance 2D photodetectors.

Sustainable chemistry with plasmonic photocatalysts

Abstract There is a pressing global need to increase the use of renewable energy sources and limit greenhouse gas emissions. Towards this goal, highly efficient and molecularly selective chemical processes that operate under mild conditions are critical. Plasmonic photocatalysis uses optically-resonant metallic nanoparticles and their resulting plasmonic, electronic, and phononic light-matter interactions to drive chemical reactions. The promise of simultaneous high-efficiency and product-selective reactions with plasmon photocatalysis provides a compelling opportunity to rethink how chemistry is achieved. Plasmonic nanoparticles serve as nanoscale ‘antennas’ that enable strong light–matter interactions, surpassing the light-harvesting capabilities one would expect purely from their size. Complex composite structures, combining engineered light harvesters with more chemically active components, are a focal point of current research endeavors. In this review, we provide an overview of recent advances in plasmonic catalysis. We start with a discussion of the relevant mechanisms in photochemical transformations and explain hot-carrier generation and distributions from several ubiquitous plasmonic antennae. Then we highlight three important types of catalytic processes for sustainable chemistry: ammonia synthesis, hydrogen production and CO2 reduction. To help elucidate the reaction mechanism, both state-of-art electromagnetic calculations and quantum mechanistic calculations are discussed. This review provides insights to better understand the mechanism of plasmonic photocatalysis with a variety of metallic and composite nanostructures toward designing and controlling improved platforms for green chemistry in the future.

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.

An Optoelectronic Synapse Based on Two-Dimensional Violet Phosphorus Heterostructure.

Neuromorphic computing can efficiently handle data-intensive tasks and address the redundant interaction required by von Neumann architectures. Synaptic devices are essential components for neuromorphic computation. 2D phosphorene, such as violet phosphorene, show great potential in optoelectronics due to their strong light-matter interactions, while current research is mainly focused on synthesis and characterization, its application in photoelectric devices is vacant. Here, the authors combined violet phosphorene and molybdenum disulfide to demonstrate an optoelectronic synapse with a light-to-dark ratio of 106 , benefiting from a significant threshold shift due to charge transfer and trapping in the heterostructure. Remarkable synaptic properties are demonstrated, including a dynamic range (DR) of > 60dB, 128 (7-bit) distinguishable conductance states, electro-optical dependent plasticity, short-term paired-pulse facilitation, and long-term potentiation/depression. Thanks to the excellent DR and multi-states, high-precision image classification with accuracies of 95.23% and 79.65% is achieved for the MNIST and complex Fashion-MNIST datasets, which is close to the ideal device (95.47%, 79.95%). This work opens the way for the use of emerging phosphorene in optoelectronics and provides a new strategy for building synaptic devices for high-precision neuromorphic computing.

Multifunctional interface between integrated photonics and free space

The combination of photonic integrated circuits and free-space metaoptics has the ability to untie technological knots that require advanced light manipulation due to their conjoined ability to achieve strong light–matter interaction via wave-guiding light over a long distance and shape them via large space-bandwidth product. Rapid prototyping of such a compound system requires component interchangeability. This represents a functional challenge in terms of fabrication and alignment of high-performance optical systems. Here, we report a flexible and interchangeable interface between a photonic integrated circuit and the free space using an array of low-loss metaoptics and demonstrate multifunctional beam shaping at a wavelength of 780 nm. We show that robust and high-fidelity operation of the designed optical functions can be achieved without prior precise characterization of the free-space input nor stringent alignment between the photonic integrated chip and the metaoptics chip. A diffraction limited spot of ∼3 μm for a hyperboloid metalens of numerical aperture 0.15 is achieved despite an input Gaussian elliptical deformation of up to 35% and misalignments of the components of up to 20 μm. A holographic image with a peak signal-to-noise ratio of >10 dB is also reported.

Strongly enhanced light-matter coupling of monolayer WS2 from a bound state in the continuum.

Exciton-polaritons derived from the strong light-matter interaction of an optical bound state in the continuum with an excitonic resonance can inherit an ultralong radiative lifetime and significant nonlinearities, but their realization in two-dimensional semiconductors remains challenging at room temperature. Here we show strong light-matter interaction enhancement and large exciton-polariton nonlinearities at room temperature by coupling monolayer tungsten disulfide excitons to a topologically protected bound state in the continuum moulded by a one-dimensional photonic crystal, and optimizing for the electric-field strength at the monolayer position through Bloch surface wave confinement. By a structured optimization approach, the coupling with the active material is maximized here in a fully open architecture, allowing to achieve a 100 meV photonic bandgap with the bound state in the continuum in a local energy minimum and a Rabi splitting of 70 meV, which results in very high cooperativity. Our architecture paves the way to a class of polariton devices based on topologically protected and highly interacting bound states in the continuum.

All-optical switching of liquid crystals at terahertz frequencies enabled by metamaterials.

Nematic liquid crystals integrated with metallic resonators (metamaterials) are intriguing hybrid systems, which not only offer added optical functionalities, but also promote strong light-matter interactions. In this report, we show with an analytical model that the electric field generated by a conventional oscillator-based terahertz time domain spectrometer is strong enough to induce partial, all-optical switching of nematic liquid crystals in such hybrid systems. Our analysis provides a robust theoretical footing for the mechanism of all-optical nonlinearity of liquid crystals, which was recently hypothesised to explain an anomalous resonance frequency shift in liquid crystal-loaded terahertz metamaterials. The integration of metallic resonators with nematic liquid crystals offers a robust approach to explore optical nonlinearity within such hybrid material systems in the terahertz range; paves the way towards increased efficiency of existing devices; and broadens the range of applications of liquid crystals in the terahertz frequency range.

Ultrafast nonequilibrium dynamics and high-harmonic generation in two-dimensional quantum spin Hall materials

We develop the theoretical framework of nonequilibrium ultrafast photonics in monolayer quantum spin Hall insulators supporting a multitude of topological states. In these materials, ubiquitous strong light-matter interactions in the femtosecond scale lead to nonadiabatic quantum dynamics, resulting in topology-dependent nonlinear optoelectronic transport phenomena. We investigate the mechanism driving topological Dirac fermions interacting with strong ultrashort light pulses and uncover various experimentally accessible physical quantities that encode fingerprints of the quantum material's topological electronic state from the high-harmonic generated spectrum. Our work sets the theoretical cornerstones to realize the full potential of time-resolved harmonic spectroscopy for understanding nonequilibrium processes in quantum topological systems and identifying topological invariants in two-dimensional quantum spin Hall solid state systems.

Computational study on the catalytic control of endo/exo Diels-Alder reactions by cavity quantum vacuum fluctuations

Achieving control over chemical reaction’s rate and stereoselectivity realizes one of the Holy Grails in chemistry that can revolutionize chemical and pharmaceutical industries. Strong light-matter interaction in optical or nanoplasmonic cavities might provide the knob to reach such control. In this work, we demonstrate the catalytic and selectivity control of an optical cavity for two selected Diels-Alder cycloaddition reactions using the quantum electrodynamics coupled cluster (QED-CC) method. Herein, we find that by changing the molecular orientation with respect to the polarization of the cavity mode the reactions can be significantly inhibited or selectively enhanced to produce major endo or exo products on demand. This work highlights the potential of utilizing quantum vacuum fluctuations of an optical cavity to modulate the rate of Diels-Alder cycloaddition reactions and to achieve stereoselectivity in a practical and non-intrusive way. We expect that the present findings will be applicable to a larger set of relevant reactions, including the click chemical reactions.

Ultrasensitive in-plane excitons-dominated pseudo-Brewster angle of transition metal dichalcogenides monolayers

Strong light-matter interactions rooted in excitons enable the transitional metal dichalcogenides (TMDCs) monolayers to exhibit unique optical properties that are promising for breaking the limitation of dimensions to create new nanophotonic techniques and applications. Herein, we theoretically and experimentally investigated the fundamental pseudo-Brewster angle (pBA) of TMDCs monolayers (MoS2, MoSe2, WS2, and WSe2) coated on dielectric substrates. We found the pBA undergoes significant angular shifts up to 3.1–4.7° in the visible region by inserting the TMDCs monolayers on a dielectric sapphire substrate, which exhibit close association with their excitonic transitions. Moreover, we demonstrated the pBA has surprisingly deep sub-monolayer resolution of by investigating the MoS2 sub-monolayers with varied surface coverage ratios (CRs). Experiment and calculation results show the sub-monolayer sensitivity of ∼ 0.047°/1% CRs, which is super advanced in resolving subtle differences of disconnected monolayers. Our results provide a basic reference for the studies on nano-optics, flat optics, and a wide range of low-dimensional materials towards creating functional and outstanding nano-photonic devices and sensors.

Direct nano-imaging of light-matter interactions in nanoscale excitonic emitters

Strong light-matter interactions in localized nano-emitters placed near metallic mirrors have been widely reported via spectroscopic studies in the optical far-field. Here, we report a near-field nano-spectroscopic study of localized nanoscale emitters on a flat Au substrate. Using quasi 2-dimensional CdSe/CdxZn1-xS nanoplatelets, we observe directional propagation on the Au substrate of surface plasmon polaritons launched from the excitons of the nanoplatelets as wave-like fringe patterns in the near-field photoluminescence maps. These fringe patterns were confirmed via extensive electromagnetic wave simulations to be standing-waves formed between the tip and the edge-up assembled nano-emitters on the substrate plane. We further report that both light confinement and in-plane emission can be engineered by tuning the surrounding dielectric environment of the nanoplatelets. Our results lead to renewed understanding of in-plane, near-field electromagnetic signal transduction from the localized nano-emitters with profound implications in nano and quantum photonics as well as resonant optoelectronics.

Near-unity broadband infrared absorption in a graphene-black phosphorus bimodal triple-layer structure

It is well known that both graphene and monolayer black phosphorus (BP) can excite and generate surface plasmonic resonances in the infrared spectrum, providing suitable conditions for strong light-matter interactions. To take full advantage of this benefit, a three-layered infrared absorbing structure with broadband absorption effect based on a hybrid graphene-BP metamaterial was developed. Using numerical research methods, the absorption effect of the structure could be well modulated by adjusting the geometrical parameters of the structure and the doping levels of graphene and BP. In addition, the sensitivity of the structure to the angle of incidence is also investigated. By optimizing the previous parameters, the structure achieves an absorption of more than 87.5% in the wavelength range from 10.1 to 24.5 um and a maximum absorption of 99.97%. The proposed broadband infrared structure could have many potential applications in infrared sensing, bio-imaging and environmental monitoring.

Directional Emission from Electrically Injected Exciton–Polaritons in Perovskite Metasurfaces

We present a new approach to achieving strong coupling between electrically injected excitons and photonic bound states in the continuum of a dielectric metasurface. Here a high-finesse metasurface cavity is monolithically patterned in the channel of a perovskite light-emitting transistor to induce a large Rabi splitting of ∼200 meV and more than 50-fold enhancement of the polaritonic emission compared to the intrinsic excitonic emission of the perovskite film. Moreover, the directionality of polaritonic electroluminescence can be dynamically tuned by varying the source-drain bias, which induces an asymmetric distribution of exciton population within the transistor channel. We argue that this approach provides a new platform to study strong light-matter interactions in dispersion engineered photonic cavities under electrical injection and paves the way to solution-processed electrically pumped polariton lasers.

Polarization‐Driven Ultrafast Optical Switching in TiS3 Nanoribbons via Anisotropic Hot Carrier Dynamics

AbstractLayered quasi‐1D nanomaterials exhibit strong linear dichroism and effective light‐matter interactions, promising for novel information devices driven by light‐polarization. In particular, their optical anisotropy and ultrafast photoresponse allow for polarization‐controlled nanophotonic switches and modulators. However, the technology still requires substantial further studies due to an insufficient understanding of modulation mechanisms and limited investigations of a few materials. Here, transient absorption (TA) microscopy is employed to investigate the ultrafast polarization‐based optical switching in TiS3 nanoribbons, a rising quasi‐1D material of the transition metal trichalcogenide family. Highly anisotropic sub‐picosecond near‐infrared modulation is observed, which is most pronounced when the polarization of the probe pulse is aligned with the nanoribbon axis but disappears as the polarization is rotated to the perpendicular orientation. The authors attribute this significant dependence on the probe polarization to the hot carrier‐induced broadening of an anisotropic interband resonance, as supported by analyses of transient TA line shapes and first principles calculated optical transitions. Furthermore, a strong pump polarization‐dependent modulation is obtained by tuning the pumping energy, enabling highly anisotropic all‐optical switching controlled solely by polarization. These results offer comprehensive information on both probe‐ and pump‐polarization‐based modulations and a deeper physical understanding of polarization‐dependent photoresponse through ultrafast hot carrier dynamics.

Structural, elastic, electronic, optical and vibrational properties of single-layered, bilayered and bulk molybdenite MoS2-2H.

In recent years, transition metal dichalcogenides have received great attention since they can be prepared as two-dimensional semiconductors, presenting heterodesmic structures incorporating strong in-plane covalent bonds and weak out-of-plane interactions, with an easy cleavage/exfoliation in single or multiple layers. In this context, molybdenite, the mineralogical name of molybdenum disulfide, MoS2, has drawn much attention because of its very promising physical properties for optoelectronic applications, in particular a band gap that can be tailored with the material's thickness, optical absorption in the visible region and strong light-matter interactions due to the planar exciton confinement effect. Despite this wide interest and the numerous experimental and theoretical articles in the literature, these report on just one or two specific features of bulk and layered MoS2 and sometimes provide conflicting results. For these reasons, presented here is a thorough theoretical analysis of the different aspects of bulk, monolayer and bilayer MoS2 within the density functional theory (DFT) framework and with the DFT-D3 correction to account for long-range interactions. The crystal chemistry, stiffness, and electronic, dielectric/optical and phonon properties of single-layered, bilayered and bulk molybdenite have been investigated, to obtain a consistent and detailed set of data and to assess the variations and cross correlation from the bulk to single- and double-layer units. The simulations show the indirect-direct transition of the band gap (K-K' in the first Brillouin zone) from the bulk to the single-layer structure, which however reverts to an indirect transition when a bilayer is considered. In general, the optical properties are in good agreement with previous experimental measurements using spectroscopic ellipsometry and reflectivity, and with preliminary theoretical simulations.

Water-Assisted Growth of Twisted 3R-Stacked MoSe2 Spirals and Its Dramatically Enhanced Second Harmonic Generations.

Enhanced second-harmonic generation (SHG) responses are reported in monolayer transition metal dichalcogenides (e.g., MX2 , M: Mo, W; X: S, Se) due to the broken symmetries. The 3R-like stacked MX2 spiral structures possessing the similar broken inversion symmetry should present dramatically enhanced SHG responses, thus providing great flexibility in designing miniaturized on-chip nonlinear optical devices. To achieve this, the first direct synthesis of twisted 3R-stacked chiral molybdenum diselenide (MoSe2 ) spiral structures with specific screw dislocations (SD) arms is reported, via designing a water-assisted chemical vapor transport (CVT) approach. The study also clarifies the formation mechanism of the MoSe2 spiral structures, by precisely regulating the precursor supply accompanying with multiscale characterizations. Significantly, an up to three orders of magnitude enhancement of the SHG responses in twisted 3R stacked MoSe2 spirals is demonstrated, which is proposed to arise from the synergistic effects of broken inversion symmetry, strong light-matter interaction, and band nesting effects. Briefly, the work provides an efficient synthetic route for achieving the 3R-stacked TMDCs spirals, which can serve as perfect platforms for promoting their applications in on-chip nonlinear optical devices.

Fano Resonance in Near-Field Thermal Radiation of Two-Dimensional Van der Waals Heterostructures.

Two-dimensional (2D) materials and their vertically stacked heterostructures have attracted much attention due to their novel optical properties and strong light-matter interactions in the infrared. Here, we present a theoretical study of the near-field thermal radiation of 2D vdW heterostructures vertically stacked of graphene and monolayer polar material (2D hBN as an example). An asymmetric Fano line shape is observed in its near-field thermal radiation spectrum, which is attributed to the interference between the narrowband discrete state (the phonon polaritons in 2D hBN) and a broadband continuum state (the plasmons in graphene), as verified by the coupled oscillator model. In addition, we show that 2D van der Waals heterostructures can achieve nearly the same high radiative heat flux as graphene but with markedly different spectral distributions, especially at high chemical potentials. By tuning the chemical potential of graphene, we can actively control the radiative heat flux of 2D van der Waals heterostructures and manipulate the radiative spectrum, such as the transition from Fano resonance to electromagnetic-induced transparency (EIT). Our results reveal the rich physics and demonstrate the potential of 2D vdW heterostructures for applications in nanoscale thermal management and energy conversion.

Topologically protected multiple resonant modes in a one-dimensional photonic crystal heterostructure

This paper proposed and theoretically examined a double-interfaced one-dimensional photonic crystal heterostructure for strong localization of topologically protected resonant modes. It is challenging to achieve these modes for a range of wavelengths using a single photonic crystal. The design proposed in this study is a heterostructure of photonic crystals 1 (PC1) and 2 (PC2), and it provides distributed localized modes from the infrared to ultraviolet wavelength ranges. In addition, multiple resonant modes occur at certain photonic bandgaps due to the addition of the third photonic crystal (PC3), which is analytically modeled with the heterostructure of PC1 and PC2. The enhancement in the number of resonant modes depends on the PC2 number of unit-cells and the reflection phase of the proposed heterostructure. The reflection phase is abruptly changing from 0 to π for several wavelengths inside the bandgap. These resonance modes are also dependent on the topological behavior of each connected photonic crystal and are immune to small disorder and back-scattering within the crystal. The high-quality factor (Q-factor ∼107) shows strong light–matter interaction of these multiple resonant modes. The characterization was done in terms of the Zak phase, sign of the reflection phase, and bandgap overlapping. Consequently, this heterostructure may pave the way for new topological photonics and new applications in optoelectronics, frequency up-conversion, photonic devices, rainbow trapping, multiwavelength optical filters, and so on.

Versatile Tunning of Compact Microring Waveguide Resonator Based on Lithium Niobate Thin Films

With the advancement of modulation technology and the requirement for device miniaturization and integration, lithium niobate on insulator (LNOI) can be a versatile platform for this pursuit, as it can confine the transmitted light at the nanoscale, leading to a strong light–matter interaction, which can sensitively capture external variations, such as electric fields and temperature. This paper presents a compact microring modulator with versatile tuning based on X-cut LNOI. The LNOI modulator equipped with electrodes with a coverage angle of 120∘ achieved a maximum electro-optic (EO) tuning efficiency of 13 pm/V and a maximum extinction ratio of 11 dB. The asymmetry in the static or quasi-static electro-optic tuning of the microring modulator was also analyzed. Furthermore, we measured the thermal-optic effect of the device with a sensitivity of 26.33 pm/∘C, which can potentially monitor the environment temperature or compensate for devices’ functional behavior. The demonstrated efficient and versatile compact microring modulator will be an important platform for on-chip active or passive photonic components, microring-based sensor arrays and integrated optics.