Enhancement Of Light-matter Interaction Research Articles

Multifunctional terahertz device based on plasmon-induced transparency

Abstract Enhancing light-matter interaction is crucial in optics for boosting nanophotonic device performance, which can be achieved via plasmon-induced transparency (PIT). In this study, a polarization-insensitive PIT effect at terahertz frequencies is achieved using a novel metasurface composed of a cross-shaped graphene structure surrounded by four graphene strips. The high symmetry of this metasurface ensures its insensitivity to changes in the polarization angle of incident light. The PIT effect, stemming from the coupling of graphene bright modes, was explored through finite difference time domain (FDTD) simulations and coupled mode theory (CMT) analysis. By tuning the Fermi level in graphene, we effectively modulated the PIT transparent window, achieving high-performance optical switching with a modulation depth (88.9% < MD < 98.0%) and insertion losses (0.17 dB < IL < 0.51 dB) at a carrier mobility of 2 m2/(V·s). Furthermore, the impact of graphene carrier mobility on the slow-light effect was examined, revealing that increasing the carrier mobility from 0.5 m2/(V·s) to 3 m2/(V·s) boosts the group index from 126 to 781. These findings highlight the potential for developing versatile terahertz devices, such as optical switches and slow-light apparatus.

Giant goos–hanchen shift with heigh reflection base on guided mode resonance in trapezoidal hBN/dielectric metasurface

hBN is a natural hyperbolic van der Waals material that can enhance light–matter interactions. In this work, the maximum Goos-Hanchen(GH) shift has a magnitude of −4680 μm with high reflection using the central beam method in the trapezoidal dielectric/hBN grating (TDG) metasurface (MS) where the beam waist w0≥4λ. Analysis of the electromagnetic field distribution indicated that the amplified GH shift was a result of guided mode resonances excited in the waveguide dielectric layer. The frequency and magnitude of GH shifts can be effectively controlled by adjusting the height of the trapezoidal hBN, as well as by manipulating the widths of the top and bottom dielectric layers, the period of the TDG structure, and the height of the multi-layered structure. Moreover, an evaluation of the structure-sensing properties based on the GH shift was conducted. The enhanced and controllable GH shift exhibited by the TDG MS presents promising prospects for applications in optical sensors, optical switches, and optoelectronic detectors.

Ultra-high Q-factor quasi-BIC BaTiO3 metasurface for electro-optic modulation

Metasurfaces play a crucial role in trapping electromagnetic waves with specific wavelengths, serving as a significant platform for enhancing light–matter interactions. In all kinds of dynamic modulation metasurfaces, electro-optic modulation metasurfaces have attracted much attention due to its advantages of fast, stable and high efficiency. In order to respond to the extremely weak refractive index change of the electro-optical effect of the materials, the metasurfaces are required to support optical signals with high Q values. The quasi-bound state in the continuum (Q-BIC) is often used to enhance the light-field modulation capability of metasurfaces and to improve the modulation sensitivity of electro-optic modulators due to its ability to generate high Q-factor resonances. However, the design of an electro-optic modulation metasurface that facilitates the application of voltage and achieves modulation efficiency of nearly 100% is still in urgent need of development. In this study, single-crystal BTO metasurfaces are modeled using finite-difference time-domain method, and the structural symmetry is broken to introduce a Q-BIC resonance to generate a high Q-factor optical signal of 2.45 × 104 for high-depth electro-optic modulation. By simulating an applied electric field of 143 V/mm on the metasurface, a slight refractive index change of BTO of 8 × 10−4 was produced, leading to an electro-optical intensity modulation depth of 100%. Furthermore, the nanostructure of the metasurface was carefully designed to facilitate nano-fabrication and voltage application, and it is ideal for the development of low-power, CMOS-compatible, and miniaturized electro-optic modulation devices. Although the results of this study are based on simulations, they provide a crucial theoretical basis and guidance for the realization of efficient and realistic design of dynamic metasurfaces.

Enhanced Light-Matter Interaction in Metallic Nanoparticles: A Generic Strategy of Smart Void Filling.

The intrinsic properties of materials play a substantial role in light-matter interactions, impacting both bulk metals and nanostructures. While plasmonic nanostructures exhibit strong interactions with photons via plasmon resonances, achieving efficient light absorption/scattering in other transition metals remains a challenge, impeding various applications related to optoelectronics, chemistry, and energy harvesting. Here, we propose a universal strategy to enhance light-matter interaction, through introducing voids onto the surface of metallic nanoparticles. This strategy spans nine metals including those traditionally considered optically inactive. The absorption cross section of void-filled nanoparticles surpasses the value of plasmonic (Ag/Au) counterparts with tunable resonance peaks across a broad spectral range. Notably, this enhancement is achieved under arbitrary polarizations and varied particle sizes and in the presence of geometric disorder, highlighting the universal adaptability. Our strategy holds promise for inspiring emerging devices in photocatalysis, bioimaging, optical sensing, and beyond, particularly when metals other than gold or silver are preferred.

Single-Mode Control and Individual Nanoparticle Detection in the Ultraviolet Region Based on Boron Nitride Microdisk with Whispering Gallery Mode.

Optical microcavities are known for their strongly enhanced light-matter interactions. Whispering gallery mode (WGM) microresonators have important applications in nonlinear optics, single-mode output, and biosensing. However, there are few studies on resonance modes in the ultraviolet spectrum because most materials with high absorption properties are in the ultraviolet band. In this study, the performance of a microdisk cavity based on boron nitride (BN) was simulated by using the Finite-difference time-domain (FDTD) method. The WGM characteristics of a single BN microdisk with different sizes were obtained, wherein the resonance modes could be regulated from 270 nm to 350 nm; additionally, a single-mode at 301.5 nm is achieved by cascading multiple BN microdisk cavities. Moreover, we found that a BN microdisk with a diameter of 2 μm has a position-independent precise sensitivity for the nanoparticle of 140 nm. This study provides new ideas for optical microcavities to achieve single-mode management and novel coronavirus size screening, such as SARS-CoV-2, in the ultraviolet region.

High-efficiency light manipulation using a single layer of folded graphene microribbons

Since its one-atom thickness, it remains an open question to enhance light–matter interactions in graphene, which is usually implemented through external resonant structures such as Fabry–Perot cavity. Here, we propose an alternative scheme to enhance light–matter interactions in a single layer of folded graphene microribbons (FGMRs), and remarkably, for normal incidences rather than oblique incidences in most studies. By optimizing structural parameters (e.g., the location of folding axis and folding angle), three light manipulations such as perfect absorption, perfect reflection, and perfect transmission can be achieved independently. More interestingly, any one of the three functionalities can be actively switched to the other via changing material parameters (Fermi level and carrier mobility ), which is actually the most attractive feature of graphene plasmonics. Finally, we show FGMRs can also support triple functionalities, i.e., via changing material parameters, one of the three functionalities can be switched to the second one and then the third one. Our results will be of great interest to fundamental physics and pave the way for graphene plasmonic device applications.

High-Sensitivity Optical Sensors Empowered by Quasi-Bound States in the Continuum in a Hybrid Metal-Dielectric Metasurface.

Enhancing light-matter interaction is a key requisite in the realm of optical sensors. Bound states in the continuum (BICs), possessing high quality factors (Q factors), have shown great advantages in sensing applications. Recent theories elucidate the ability of BICs with hybrid metal-dielectric architectures to achieve high Q factors and high sensitivities. However, the experimental validation of the sensing performance in such hybrid systems remains equivocal. In this study, we propose two symmetry-protected quasi-BIC modes in a metal-dielectric metasurface. Our results demonstrate that, under the normal incidence of light, the quasi-BIC mode dominated by dielectric can achieve a high Q factor of 412 and a sensing performance with a high bulk sensitivity of 492.7 nm/RIU (refractive index unit) and a figure of merit (FOM) of 266.3 RIU-1, while the quasi-BIC mode dominated by metal exhibits a stronger surface affinity in the biotin-streptavidin bioassay. These findings offer a promising approach for implementing metasurface-based sensors, representing a paradigm for high-sensitivity biosensing platforms.

Terahertz bi-functional polarization converter based on interference mechanism supported by diatomic metasurfaces

Polarization manipulation based on the Jones matrix facilitates the enhancement of light-matter interactions. Recently, arbitrarily tailorable polarization states generated with the assistance of a diatomic metasurface effectively reduce the complexity of the system. Nevertheless, a single polarization switching behavior hinders the application of meta-platforms in cryptographic imaging. Here, we theoretically propose and design a single-layer diatomic all-dielectric metasurface working in the terahertz band, which can efficiently realize bi-functional polarization switching according to the Jones matrix. Such a meta-platform is assembled from two anisotropic silicon pillars with carefully optimized lateral dimensions and in-plane twist angles. Benefiting from the flexible assembly of half-wave plate and quarter-wave plate, the polarization states generated by the constructed metasurfaces in the transmission mode can be arbitrarily tailored. The feasibility of this diatomic metasurface is further validated by a broadband near-field imaging device, paving the way for broader system applications in cryptographic imaging, data storage, and chiral sensing.

Enhancement of Spontaneous Photon Emission in Inverse Photoemission Transitions in Semiconductor Quantum Dots.

Inverse photoemission (IPE) is a radiative electron capture process where an electron is transiently captured in the conduction band (CB) followed by intraband de-excitation and spontaneous photon emission. IPE in quantum dots (QDs) bypasses optical selection rules for populating the CB and provides insights into the capacity for electron capture in the CB, the propensity for spontaneous photon emission, intraband transition energies where both initial and final states are in the CB, and the generation of photons with frequencies lower than the bandgap. Here, we demonstrate using time-dependent perturbation theory that judicious application of electric fields can significantly enhance the IPE transition in QDs. For a series of CdSe, CdS, PbSe, and PbS QDs, we present evidence of field-induced enhancement of IPE intensities (188% for Cd54Se54), field-dependent control of emitted photon frequencies (Δω = 0.73 eV for Cd54Se54), and enhancement of light-matter interaction using directed Stark fields (103% for Cd54Se54).

Optical Microcavities Empowered Biochemical Sensing: Status and Prospects

Optical microcavities are compact structures that confine resonant photons in microscale dimensions for long periods of time, greatly enhancing light–matter interactions. Plentiful and profound physical mechanisms within these microcavities or functional microcavities have been extensively explored, including mode shift/splitting/broadening, lasing and gain enhancements, surface plasmon resonance, fluorescence resonance energy transferring, optical frequency comb spectroscopy, optomechanical interaction, and exceptional point. The versatility in design and the diverse range of materials, particularly composites involving metals and 2-dimensional materials, have paved a way for innovative approaches and improved performance in biochemical sensing applications. Leveraging the advantages ranging from miniaturization, high sensitivity, rapid response, and inherent stability, optical microcavity-based biochemical sensors have emerged to address the growing and increasingly complex demands of biochemical detection. This review commences with an exploration of fundamental mechanisms and structures and then delves into typical applications in recent advancements, covering the detection of biomacromolecules, cells, solid particles, liquid ions, and gas molecules. This review also culminates with a forward-looking perspective, highlighting future development trends and crucial research directions.

Second harmonic generation in an anisotropic lithium niobate metasurface governed by quasi-BICs

Resonant metasurfaces can greatly trap the light fields, so that they are widely used to enhance light–matter interactions at the nanoscale, such as promoting nonlinear effects of materials. Lithium niobate (LN) is an excellent nonlinear optical material and is often employed to generate harmonic signals. In this Letter, we numerically study the second harmonic generation (SHG) characteristics of the LN metasurface based on the quasi-bound states in the continuum (QBIC). The designed BIC and excited QBIC metasurfaces always hold C4v symmetry, and the BIC is demonstrated to degenerate into two BICs due to the anisotropic characteristics of LN. Moreover, the excited two high Q-factor QBICs can effectively enhance the SHG in LN, although the device maintains C4v symmetry, the SHG signal still shows polarization dependence. In addition, with the increase of Q-factor of quasi-BIC, the power and conversion efficiency (η) of SHG increase significantly. The calculated η can reach 6.04 × 10−3 and can be further improved when the resonance mode is closer to BIC. These results have important implications for high-quality nonlinear light sources based on LN materials.

Amplified spontaneous emission at the band edges of Ag-coated Al nanocone array

Surface lattice resonances (SLRs) with ultra-narrow linewidth (high quality factor) can enhance light–matter interactions at the nanoscale and modulate the propagating light from the emission wavelength direction to efficiency by photonic band engineering. Therefore, SLRs can serve as an excited candidate to enhance and, more importantly, modulate amplified spontaneous emission (ASE) with more optical parameters. Here, this work presents a system of two-dimensional Ag-coated Al nanocone array (Ag-NCA) packaged with Nile red, and a normal ASE with 15-fold enhancement is observed under external driving light. This enhancement fades away, obviously, in the case of the off-normal condition, as the optical feedback evolves from the band edge steady state to the propagating state. The ASE of this hybrid plasmonic system expands the possibilities of interaction between light and matter and has great promise for applications in nanolasing, super-resolution imaging, and photonic integration circuits.

Dielectric resonances of the cylindrical micro/nano cavity within epsilon-near-zero materials.

The dielectric resonances of spherically symmetric micro/nano cavity in zero-index materials have been systematically studied. However, the resonance properties of other shaped dielectric cavities in zero-index materials remain unclear. Here, we theoretically investigate the electromagnetic resonances of the dielectric cavity with cylindrical symmetry in the epsilon-near-zero materials. This kind of cavity supports a set of resonances with strong light confinement, including dipole, quadrupole and higher-order modes with multiple nodes. Furthermore, there is a redshift of the resonance wavelength with an increment of its size, obeying a law as the function of diameter and height. Also, we find that the redshift will be slower for higher-order modes. Through the infinite refractive index contrast and extra degree of freedom, they should have potential application in the enhancement of light-matter interaction and multiple-functional light manipulation in the integrated optical systems.

Protein aggregation monitoring in microdisk optofluidic sensor through microcavity enhanced Raman scattering

Protein aggregation plays a crucial role in amyloid lesions but it is a huge challenge to monitor the slight changing process dynamically. Here, we propose an optofluidic surface-enhanced Raman scattering (SERS) sensing chip. It consists of several microdisks deposited with gold nanoparticles (Au NPs), where the microdisk acts as a whispering gallery microcavity (WGM) for light confinement and the Au NPs generate the localized surface plasma for electromagnetic enhancement. The effects of different stimulus factors on the secondary structure of lysozyme protein are revealed by observing the change of Raman spectroscopy of amide band with the enhancement of light-matter interaction. The optofluidic SERS design illuminates the potential for dynamic sensing of biological fluid and cellular microenvironment on chip.

Inverse design of all-dielectric metasurfaces with accidental bound states in the continuum

Abstract Metasurfaces with bound states in the continuum (BICs) have proven to be a powerful platform for drastically enhancing light–matter interactions, improving biosensing, and precisely manipulating near- and far-fields. However, engineering metasurfaces to provide an on-demand spectral and angular position for a BIC remains a prime challenge. A conventional solution involves a fine adjustment of geometrical parameters, requiring multiple time-consuming calculations. In this work, to circumvent such tedious processes, we develop a physics-inspired, inverse design method on all-dielectric metasurfaces for an on-demand spectral and angular position of a BIC. Our suggested method predicts the core–shell particles that constitute the unit cell of the metasurface, while considering practical limitations on geometry and available materials. Our method is based on a smart combination of a semi-analytical solution, for predicting the required dipolar Mie coefficients of the meta-atom, and a machine learning algorithm, for finding a practical design of the meta-atom that provides these Mie coefficients. Although our approach is exemplified in designing a metasurface sustaining a BIC, it can, also, be applied to many more objective functions. With that, we pave the way toward a general framework for the inverse design of metasurfaces in specific and nanophotonic structures in general.

Tunable flatband plasmonic quasi-bound states in the continuum based on graphene-assisted metasurfaces

Bound states in the continuum (BICs) of plasmonic systems offer a powerful method for enhancing light–matter interaction at the nanoscale. The recent emergence of flatband quasi-BICs has alleviated the limitation of the incident angle of the excitation light on generating high-quality-factor (high-Q-factor) resonances, which makes it feasible to produce substantial near-field enhancement by focused light. However, the current works are limited to passive systems with fixed amplitude and Q-factor, hindering the dynamic tunability of light field enhancement. Here, we design a plasmonic metasurface integrated with monolayer graphene to achieve tunable flatband quasi-BICs. Under the illumination of a tightly focused transverse-magnetic wave, our simulations show that adjusting the chemical potential of graphene can increase Q-factor from 52.5 to 75.9 and improve absorption amplitude from 81% to 95%. These results pave the way for dynamically adjustable near-field enhancement with tightly focused light.

Dual-Dewetting Process for Self-Assembled Nanoparticle Clusters in Wafer Scale.

Plasmonic molecules, which are geometrically well-defined plasmonic metal nanoparticle clusters, have attracted significant attention due to their enhancement of light-matter interactions owing to a stronger electric field enhancement than that by single particles. High-resolution lithography techniques provide precise positioning of plasmonic nanoparticles, but their fabrication costs are excessively high. In this study, we propose a lithography-free, self-assembly fabrication method, termed the dual-dewetting process, which allows the control of the size and density of gold nanoparticles. This process involves depositing a gold thin film on a substrate and inducing dewetting through thermal annealing, followed by a second deposition and annealing. The method achieves a uniform distribution of particle size and density, along with increased particle density, across a 6-inch wafer. The superiority of the method is confirmed by a 30-fold increase in the signal intensity of surface-enhanced Raman scattering following the additional dewetting with an 8 nm film, compared to single dewetting alone. Our findings indicate that the dual-dewetting method provides a simple and efficient approach to enable a variety of plasmonic applications through efficient plasmonic molecule large-area fabrication.

High performance, single crystal gold bowtie nanoantennas fabricated via epitaxial electroless deposition

Material quality plays a critical role in the performance of nanometer-scale plasmonic structures and represents a significant hurdle to large-scale device integration. Progress has been hindered by the challenges of realizing scalable, high quality, ultrasmooth metal deposition strategies, and by the poor pattern transfer and device fabrication yields characteristic of most metal deposition approaches which yield polycrystalline metal structure. Here we highlight a novel and scalable electrochemical method to deposit ultrasmooth, single-crystal (100) gold and to fabricate a series of bowtie nanoantennas through subtractive nanopatterning. We investigate some of the less well-explored design and performance characteristics of these single-crystal nanoantennas in relation to their polycrystalline counterparts, including pattern transfer and device yield, polarization response, gap-field magnitude, and the ability to model accurately the antenna local field response. Our results underscore the performance advantages of single-crystal nanoscale plasmonic materials and provide insight into their use for large-scale manufacturing of plasmon-based devices. We anticipate that this approach will be broadly useful in applications where local near-fields can enhance light–matter interactions, including for the fabrication of optical sensors, photocatalytic structures, hot carrier-based devices, and nanostructured noble metal architectures targeting nano-attophysics.

Tuning of graphene plasmons by ferroelectric superdomain for mid-infrared photodetector with high responsivity

A new strategy is demonstrated for confining graphene plasmons to resonantly enhance light–matter interactions for tunable mid-IR detection. Our devices consist of integrating monolayer graphene without patterning onto a nanoribbon-connected ring-shaped ferroelectric superdomain with alternately up- and down-polarization. The simulations show that our devices have a tunable spectral response from 11.7 to 19.5 μm by both reconfiguring the ferroelectric superdomain and varying the ferroelectric-gated graphene Fermi level. A highest photoresponsivity of 796–947 A W−1 has been achieved in 10–20 μm. The proof-of-concept photodetector offers the possibility to simplify the fabrication of plasmonic devices and helps the development of applications of tunable mid-IR detection.

Intrinsic strong light-matter coupling with self-hybridized bound states in the continuum in van der Waals metasurfaces.

Photonic bound states in the continuum (BICs) provide a standout platform for strong light-matter coupling with transition metal dichalcogenides (TMDCs) but have so far mostly been implemented as traditional all-dielectric metasurfaces with adjacent TMDC layers, incurring limitations related to strain, mode overlap and material integration. Here, we demonstrate intrinsic strong coupling in BIC-driven metasurfaces composed of nanostructured bulk tungsten disulfide (WS2) and exhibiting resonances with sharp, tailored linewidths and selective enhancement of light-matter interactions. Tuning of the BIC resonances across the exciton resonance in bulk WS2 is achieved by varying the metasurface unit cells, enabling strong coupling with an anticrossing pattern and a Rabi splitting of 116 meV. Crucially, the coupling strength itself can be controlled and is shown to be independent of material-intrinsic losses. Our self-hybridized metasurface platform can readily incorporate other TMDCs or excitonic materials to deliver fundamental insights and practical device concepts for polaritonic applications.