Enhancement Of Light-matter Interaction Research Articles

Unconventional plasmonic sensitization of graphene in mid-infrared

Light–matter interaction in graphene can be engineered and substantially enhanced through plasmonic sensitization, which has led to numerous applications in photodetection, sensing, photocatalysis and spectroscopy. The majority of these designs have relied on conventional plasmonic materials such as gold, silver and aluminum. This limits the implementation of such devices to the ultraviolet and visible regimes of the electromagnetic spectrum. However, for many practical applications, including those relevant to security and defense, the development of new strategies and materials for sensing and detection of infra red (IR) light is crucial. Here we use surface enhanced Raman spectroscopy (SERS), for direct visualization and estimation of enhanced light–matter interaction in graphene in the mid-IR regime, through sensitization by an unconventional plasmonic material. Specifically, we fabricate a hybrid device consisting of a single layer graphene and a two-dimensional array of nanodiscs of aluminum doped zinc oxide (AZO), which is a highly doped semiconductor, exhibiting plasmonic resonance in the mid-IR. We find that the enhancement in the SERS signal of graphene is of similar magnitude to what has been achieved previously in the visible using conventional plasmonic materials. Our results establish the potential of such hybrid systems for graphene-based optical and optoelectronic applications in the mid-IR.

Enhancing Light-Matter Interactions in MoS2 by Copper Intercalation.

The intercalation of layered compounds opens up a vast space of new host-guest hybrids, providing new routes for tuning the properties of materials. Here, it is shown that uniform and continuous layers of copper can be intercalated within the van der Waals gap of bulk MoS2 resulting in a unique Cu-MoS2 hybrid. The new Cu-MoS2 hybrid, which remains semiconducting, possesses a unique plasmon resonance at an energy of ≈1eV, giving rise to enhanced optoelectronic activity. Compared with high-performance MoS2 photodetectors, copper-enhanced devices are superior in their spectral response, which extends into the infrared, and also in their total responsivity, which exceeds 104 A W-1 . The Cu-MoS2 hybrids hold promise for supplanting current night-vision technology with compact, advanced multicolor night vision.

Tuning higher order electric field resonances in plasmonic hexagonal arrays by oxygen-plasma treatment

Experimental and simulation studies show that a hexagonal array of polystyrene particles on a gold substrate, capped with gold semi-shells, results in increased optical confinement of the resonance modes in comparison with those for arrays without metallic substrate or capping. Well-ordered arrays of polystyrene beads were fabricated by convective deposition, a solution-based self-assembly process. The polystyrene particles exhibited strong optical concentration, with photonic and plasmonic modes as detected in optical spectra and modelling. The resonance wavelengths of the plasmonic hexagonal array were modified by treating the hexagonal arrays with oxygen plasma at fixed power of 150 W for time periods of 0–10 min. The plasma treatment reduced the vertical height of the polystyrene particles, and the resonances were tuned and sharpened after plasma treatment. This increase is attributed to the improved uniformity of the gold semi-shell coating distributed over the polystyrene particles. Additionally, the plasma treatment increased the refractive index of the polystyrene particles due to cross-linking. Devices with these plasmonic hexagonal arrays enable enhanced light–matter interactions, with the flexibility to post-tune the resonance wavelengths.

Wavefront-selective Fano resonant metasurfaces

Fano resonances are conventionally understood as sharp spectral features with selectivity in the momentum-frequency domain, implying that they can be excited only by plane waves with specific frequencies and incident angles. We demonstrate that Fano resonances can be made generally selective in the space-frequency domain. They can be tailored to resonate only when excited by a frequency, polarization, and wavefront of choice. This generalization reveals that Fano systems are characterized by eigenwaves that scatter to their time-reversed image upon reflection. Although in conventional Fano systems this trivially occurs for normally incident plane waves, we show that, in general, the selected wavefront is locally retroreflected everywhere across the device. These results show that conventional Fano resonances are a subset of a broader dichroic phenomenon with spin, spatial, and spectral selectivity. We demonstrate these concepts with nonlocal metasurfaces whose governing principles are deeply rooted in the symmetry features of quasi-bound states in the continuum. Enhanced light–matter interactions and symmetry-protection make these phenomena uniquely suited for enriching applications in quantum optics, non-linear optics, augmented reality, and secure optical communications, laying the groundwork for a range of novel compact optical sources and devices.

High-performance bimetallic surface plasmon resonance biochemical sensor using a black phosphorus–MXene hybrid structure

A highly sensitive surface plasmon resonance biochemical sensor based on black phosphorus–MXene (Ti3C2Tx) hybrid structure and copper–nickel (Cu–Ni) bimetal layers is investigated analytically under angular interrogation technique. The use of Ni over Cu shows much sensitivity enhancement for the proposed sensor than on using single metal. Black phosphorus serves as an effective medium for enhanced light–matter interaction due to its layer-dependent direct and tunable bandgap. High metallic conductivity, hydrophilicity, larger surface area, and the large number of adsorption sites on the functionalized surface of Ti3C2Tx provide better sensing of analytes. We have achieved the highest sensitivity (304.47°/RIU) for the Cu–Ni bimetal configuration among all other MXene-based SPR biosensors proposed to date. It can be fabricated as a sensor chip for efficient sensing of analytes or biochemical molecules considering careful control of surface termination of Ti3C2Tx. The transverse magnetic electric field |Ez| is also plotted for the biochemical sensor to compute the sensing medium’s penetration depth.

Room Temperature Light‐Mediated Long‐Range Coupling of Excitons in Perovskites

AbstractPerovskites have been the focus of attention due to their multitude of outstanding optoelectronic properties and structural versatility. 2D halide perovskites such as (C6H5C2H4NH3)2PbI4, or simply PEPI, form natural multiple quantum wells with enhanced light–matter interactions, making them attractive systems for further investigation. This work reports tunable splitting of exciton modes in PEPI resulting from strong light–matter interactions, manifested as multiple dips (modes) in the reflection spectra. While the origin of the redder mode is well understood, that for the bluer dip at room temperature is still lacking. Here, it is revealed that the presence of the multiple modes originates from an indirect coupling between excitons in different quantum wells. The long‐range characteristic of the mediated coupling between excitons in distant quantum wells is also demonstrated in a structure design along with its tunability. Moreover, a device architecture involving an end silver layer enhances the two excitonic modes and provides further tunability. Importantly, this work will motivate the possibility of coupling of the excitonic modes with a confined light mode in a microcavity to produce multiple exciton‐polariton modes.

Highly Enhanced Light-Matter Interaction in MXene Quantum Dots-Monolayer WS2 Heterostructure.

Since the Ti3 C2 wasdiscovered in 2011, the family of MXenes has attracted much attention. MXenes offer great potential in the tuning of many fundamental properties by the synthesis of new structures. The synthesis methods of MXene mainly require steps including immersing a MAX phase in hydrofluoric acid (HF) and processing at high temperatures. However, the HF may be hard to acquire in many countries and processing at high temperatures may cause risk issues. In this article, a simple and cost-effective synthesis of Ti3 C2 Tx quantum dots (QDs) via chemical solution method that follows the long-time magnetic stirring process-initiated etching of Al atoms from commercial Ti3 AlC2 powder at room temperature is introduced. With WS2 monolayer sitting over the MXenes QD arrays, a higher level of photoluminescence (PL) enhancement is found in the heterostructure with increasing laser power at room temperature and a few novel quasi-particles species in the heterostructure at -190°C. The observations show that the possible plasmonic behavior initiated by QD arrays and the suspension state of WS2 may coplay the roles to trigger multiple quasi-particles species. This study can be an important benchmark for the extensive understanding of quasi-particles species, and their dynamics.

2D Cs2AgBiBr6 with Boosted Light–Matter Interaction for High‐Performance Photodetectors

AbstractLead‐free double perovskite Cs2AgBiBr6 has attracted significant research interests for optoelectronic applications because of its nontoxicity, inherent stability, and high detection sensitivity. In this work, the 2D Cs2AgBiBr6 with a thickness of ≈5 nm and lateral length larger than 50 µm is successfully fabricated by a space‐confined method. The fabricated ultra‐thin 2D Cs2AgBiBr6 exhibits significant advantages on photodetection, due to its enhanced light–matter interaction. Remarkably, compared with bulk Cs2AgBiBr6, 2D Cs2AgBiBr6‐based photodetectors exhibit dramatically improved optoelectronic properties including ultra‐high detectivity (D*) of 7.4 × 1014 Jones (more than ten times), photoresponsivity (R) of 54.6 A W−1 (exceeding 4.7 times), an on/off ratio of 7.4 × 104 (more than ten times), and a fast response time of ≈1.7 ms (exceeding 30 times). In addition, due to the strong photon recycling effect of Cs2AgBiBr6, optical properties in both light absorption and emission can be effectively engineered by the material thickness, which enables a tunable wavelength‐dependent photodetection. The results provide further insights on the light–matter interaction of environmentally friendly 2D perovskites related materials and shine light on their high‐performance optoelectrical applications.

Colloidal Mie Resonators for All‐Dielectric Metaoptics

High refractive index dielectric nanostructures exhibiting electric and magnetic Mie resonances offer a powerful platform for realizing unprecedented control of electromagnetic waves at the nanoscale and enhanced light–matter interactions. Dielectric metasurfaces with a variety of functionalities are realized due to the recent developments of nanofabrication technologies. In addition, colloidal dispersions of high refractive index dielectric nanoparticles have emerged as a special form of metamaterials that leads to a wider range of applications in photonics. This review focuses on the recent developments of colloidal Mie resonators made of moderate (2.5–3.5) to high (>3.5) refractive index dielectrics operating in the visible to near‐infrared range. Starting from the brief summary of fundamental properties of spherical dielectric nanoparticles, fabrication methods of colloidal Mie resonators composed of different materials and optical functionalities such as the enhanced light–matter interactions, the chirality, and the structural color generation are overviewed.

From Fano to Quasi-BIC Resonances in Individual Dielectric Nanoantennas.

Sharp optical resonances in high-index dielectric nanostructures have recently attracted significant attention for their promising applications in nanophotonics. Fano resonances, as well as resonances associated with bound states in the continuum (BIC), have independently shown a great potential for applications in nanoscale lasers, sensors, and nonlinear optical devices. Here, we demonstrate experimentally a close connection between Fano and quasi-BIC resonances excited in individual dielectric nanoantennas. We analyze systematically the resonant response of AlGaAs nanoantennas pumped with a structured light in the near-infrared range. We trace a variation of the scattering spectrum that fully agrees with an analytical expression governed by a Fano parameter and observe directly a transition to a quasi-BIC resonance. Our results suggest a unified approach toward the analysis of sharp resonances in subwavelength nanostructures originating from strong coupling of optical modes that can provide high energy localization for enhanced light-matter interactions.

Experimental Realization of Optimal Energy Storage in Resonators Embedded in Scattering Media

AbstractThe ability to enhance light–matter interactions by increasing the energy stored in optical resonators is inherently dependent on the resonators' coupling to the incident wavefront. In practice, weak coupling may result from resonators' irregular shapes and/or the scrambling of waves in the surrounding scattering environment. Here, a blind and non‐invasive wavefront shaping technique providing optimal coupling to resonators is presented. The coherent control of the incident wavefront relies on the lengthening of delay times of waves efficiently exciting the resonator. Using a modal approach, the optimality of the proposed technique is proven and its limitations are quantified. The proposed concept is demonstrated in microwave experiments by injecting in situ optimal wavefronts that maximize the energy stored in high‐permittivity dielectric scatterers and extended leaky cavities embedded in a complex environment. The introduced framework is expected to find important applications in the enhancement of light–matter interactions in photonic materials as well as to enhance energy harvesting.

Recent Advances in Opal/Inverted Opal Photonic Crystal Photocatalysts

Solar light is one of the most renewable energy resources. However, the limited light absorption range and low quantum efficiency of current semiconductors has immensely hindered the development of solar energy utilization technology and has been a matter of research priority. Opal/inverted opal photonic crystals (PCs) display a unique periodic structure, which can spontaneously enhance light–matter interactions and extend light propagation path, thus showing great potential in harvesting solar energy. The construction of opal/inverted opal photonic crystal photocatalyst (PCP) featured with attractive optical performance, which is originated from slow photons effect and multiple scatter effect, has been considered as an effective strategy to boost photocatalytic property. This review starts with the introduction of optical characteristics of PC, followed by discussion of fabrication strategies. Then, an overview of progress toward the application of PCP in photocatalytic hydrogen production, pollutant degradation, and CO2 reduction is provided as well. Finally, the challenging points and perspective are presented. It is expected that this review can provide valuable guidance for future design and synthesis of PCPs.

Symmetry analysis of trimer-based all-dielectric metasurfaces with toroidal dipole modes

Herein, we discuss the conditions for excitation of symmetry-protected toroidal dipole modes in all-dielectric metasurfaces composed of trimer or twin-trimer clusters of dielectric disks. Such metasurfaces permit enhanced light–matter interaction due to spatially confined light in resonant systems with a high-quality factor. To describe characteristics of toroidal modes existing in the clusters, we use the magnetic dipole moments approximation, group-theoretical methods, group representation theory, symmetry-adapted linear combination method, and circuit theory. To validate the obtained theoretical results, we fulfill both full-wave numerical simulations and microwave experiments. In particular, we have shown that the toroidal dipole mode appears as a quasi-dark state of the trimer. It can be excited in the metasurface by the field of a linearly polarized wave, providing the symmetry of the trimer is properly reduced. In the metasurface, the properties of the toroidal dipole mode are determined primarily by the parameters of a single trimer and are not a consequence of the periodicity of the array. The coupling of the toroidal dipole modes in the twin-trimers can appear in both bonding and anti-bonding fashion resulting in different orders of the net toroidal dipole moment. Due to the unique field configuration of these modes, the proposed metasurfaces can be considered as a platform for efficient light–matter interaction for enhanced absorption, non-linear switching, and sensing.

Angular dependence and absorption properties of the anapole mode of Si nano-disks

The polarization- and angle-resolved optical responses of the anapole mode in silicon nano-disks array have been experimentally and theoretically investigated. The good agreement between measured data and simulations yields to a consistent description of the anapole mode behavior that exhibits different features for TE or TM polarization excitation. Scattering matrix calculation allows us to disentangle scattered and diffused light contributions and to provide a quantitative estimation of the absorbance enhancement associated with 2D excitation of the anapole mode. We performed the multipolar decomposition of the far-field scattered radiation for both TE and TM polarizations and unambiguously identified the anapole resonant condition in excellent agreement with the experimental results over a large range of incident angles. Our findings demonstrate the controlled excitation of electromagnetic anapole modes in engineered arrays of silicon nano-disks for the development of optical nanostructures with enhanced light–matter interaction.

Cavity-enhanced light–matter interaction in Vogel-spiral devices as a platform for quantum photonics

Enhancing light–matter interactions on a chip is of paramount importance for classical and quantum photonics, sensing, and energy harvesting applications. Several photonic geometries have been developed, allowing high extraction efficiencies, enhanced light–matter interactions, and control over the spontaneous emission dynamics of solid-state quantum light sources. To this end, a device geometry resilient to nanofabrication imperfections, providing high-quality light confinement and control over the emitted light properties, would be desirable. We demonstrate that aperiodic arrangements, whose geometry is inspired by natural systems where scattering elements are arranged following Fibonacci series, represent a platform for enhancing the light–matter interaction in on-chip nanophotonic devices, allowing us to achieve efficient visible light confinement. We use optically active defect centers in silicon nitride as internal light sources to image and characterize, by means of microphotoluminescence spectroscopy, the individual optical modes confined by photonic membranes with Vogel-spiral geometry. By studying the statistics of the measured optical resonances, in combination with rigorous multiple scattering theory, we observe lognormal distributions and report quality factors with values as high as 2201 ± 443. Our findings improve the understanding of the fundamental physical properties of light-emitting Vogel-spiral systems and show their application to active nanophotonic devices. These results set the basis for further development of quantum devices that leverage the unique properties of aperiodic Vogel spiral order on a chip, including angular momentum states, thus producing mode structures for information processing and communications.

Fast and Slow Nonlinearities in Epsilon‐Near‐Zero Materials

AbstractNovel materials, with enhanced light–matter interaction capabilities, play an essential role in achieving the lofty goals of nonlinear optics. Recently, epsilon‐near‐zero (ENZ) media have emerged as a promising candidate to enable the enhancement of several nonlinear processes including refractive index modulation and harmonic generation. Here, the optical nonlinearity of ENZ media is analyzed to clarify the commonalities with other nonlinear media and its unique properties. Transparent conducting oxides as the family of ENZ media with near‐zero permittivity in the near‐infrared (telecom) band are focused on. The instantaneous and delayed nonlinearities are investigated. By identifying their common origin from the band nonparabolicity, it is shown that their relative strength is entirely determined by a ratio of the energy and momentum relaxation (or dephasing) times. Using this framework, ENZ materials are compared against the many promising nonlinear media that are investigated in literature and show that while ENZ materials do not radically outpace the strength of traditional materials in either the fast or slow nonlinearity, they pack key advantages such as an ideal response time, intrinsic slow light enhancement, and broadband nature in a compact platform making them a valuable tool for ultrafast photonics applications for decades to come.

Maximal single-frequency electromagnetic response

Modern nanophotonic and meta-optical devices utilize a tremendous number of structural degrees of freedom to enhance light–matter interactions. A fundamental question is how large such enhancements can be. We develop an analytical framework to derive upper bounds to single-frequency electromagnetic response, across near- and far-field regimes, for any materials, naturally incorporating the tandem effects of material- and radiation-induced losses. Our framework relies on a power-conservation law for the polarization fields induced in any scatterer. It unifies previous theories on optical scattering bounds and reveals new insight for optimal nanophotonic design, with applications including far-field scattering, near-field local-density-of-states engineering, optimal wavefront shaping, and the design of perfect absorbers. Our bounds predict strikingly large minimal thicknesses for arbitrarily patterned perfect absorbers, ranging from 50–100 nm for typical materials at visible wavelengths to micrometer-scale thicknesses for polar dielectrics at infrared wavelengths. We use inverse design to discover metasurface structures approaching the minimum-thickness perfect-absorber bounds.

Surface plasmon polariton–enhanced photoluminescence of monolayer MoS2 on suspended periodic metallic structures

Abstract Plasmonic nanostructures have garnered tremendous interest in enhanced light–matter interaction because of their unique capability of extreme field confinement in nanoscale, especially beneficial for boosting the photoluminescence (PL) signals of weak light–matter interaction materials such as transition metal dichalcogenides atomic crystals. Here we report the surface plasmon polariton (SPP)-assisted PL enhancement of MoS2 monolayer via a suspended periodic metallic (SPM) structure. Without involving metallic nanoparticle–based plasmonic geometries, the SPM structure can enable more than two orders of magnitude PL enhancement. Systematic analysis unravels the underlying physics of the pronounced enhancement to two primary plasmonic effects: concentrated local field of SPP enabled excitation rate increment (45.2) as well as the quantum yield amplification (5.4 times) by the SPM nanostructure, overwhelming most of the nanoparticle-based geometries reported thus far. Our results provide a powerful way to boost two-dimensional exciton emission by plasmonic effects which may shed light on the on-chip photonic integration of 2D materials.

Real-space observation of vibrational strong coupling between propagating phonon polaritons and organic molecules

Phonon polaritons in van der Waals materials can strongly enhance light–matter interactions at mid-infrared frequencies, owing to their extreme field confinement and long lifetimes1–7. Phonon polaritons thus bear potential for vibrational strong coupling with molecules. Although the onset of vibrational strong coupling was observed spectroscopically with phonon-polariton nanoresonators8, no experiments have resolved vibrational strong coupling in real space and with propagating modes. Here we demonstrate by nanoimaging that vibrational strong coupling can be achieved between propagating phonon polaritons in thin van der Waals crystals (hexagonal boron nitride) and molecular vibrations in adjacent thin molecular layers. We performed near-field polariton interferometry, showing that vibrational strong coupling leads to the formation of a propagating hybrid mode with a pronounced anti-crossing region in its dispersion, in which propagation with negative group velocity is found. Numerical calculations predict vibrational strong coupling for nanometre-thin molecular layers and phonon polaritons in few-layer van der Waals materials, which could make propagating phonon polaritons a promising platform for ultrasensitive on-chip spectroscopy and strong-coupling experiments. Real-space mid-infrared nanoimaging reveals vibrational strong coupling between molecules and propagating phonon polaritons in unstructured, thin hexagonal boron nitride layers, which could provide a platform for testing strong coupling and local control of chemical properties.

Terahertz radiation processes in critically coupled graphene plasmonic nanostructures

Plasmonic excitations in graphene nanostructures provide a particularly effective means to enhance light–matter interactions at THz frequencies. Here, we investigate the use of graphene nanoribbons for narrowband THz light emission based on the excitation of plasmonic oscillations under current injection and their resonant decay into free-space radiation. A detailed theoretical model of the underlying plasmon-enhanced thermal emission mechanism is presented, whose predictions are in good agreement with the recent experimental demonstration of this phenomenon. This model highlights the key role played by the nanostructure absorption efficiency to maximize the output radiation at the plasmonic resonance frequency. Based on this idea, we explore the integration of graphene nanoribbons with nearby metallic antennas in an open cavity configuration in order to promote critical coupling to free-space radiation and correspondingly enhance the absorption (and, therefore, radiation) efficiency by up to two orders of magnitude. The simulation results indicate that this approach is promising for the development of novel THz sources with technologically relevant emission characteristics.