Wideband mid-infrared thermal emitter based on stacked nanocavity metasurfaces

Efficient thermal radiation in the mid-infrared(M-IR)region is of supreme importance for many applications including thermal imaging and sensing,thermal infrared light sources,infrared spectroscopy,emissivity coatings,and camouflage.The ability to control light makes metasurfaces an attractive platform for infrared applications.Recently,different metamaterials have been proposed to achieve high thermal radiation.To date,broadening the radiation bandwidth of a metasurface emitter(meta-emitter)has become a key goal to enable extensive applications.We experimentally demonstrate a broadband M-IR thermal emitter using stacked nanocavity metasurface consisting of two pairs of circular-shaped dielectric(Si;N;)–metal(Au)stacks.A high thermal radiation can be obtained by engineering the geometry of nanocavity metasurfaces.Such a meta-emitter provides wideband and broad angular absorptance of both p-and s-polarized light,offering a wideband thermal radiation with an average emissivity of more than 80%in the M-IR atmospheric window of 8–14μm.The experimental illustration together with the theoretical framework establishes a basis for designing broadband thermal emitters,which,as anticipated,will initiate a promising avenue to M-IR sources.

Dynamic modulation in graphene-integrated silicon photonic crystal nanocavity

Silicon-based electro-optic modulators are the key devices in integrated optoelectronics. Integration of the graphene layer and the photonic crystal(PC) cavity is a promising way of achieving compact modulators with high efficiency. In this paper, a high-quality(Q) acceptor-type PC nanocavity is employed to integrate with a single-layer graphene for realizing strong modulation. Through tuning the chemical potential of graphene, a large wavelength shift of 2.62 nm and a Q factor modulation of larger than 5 are achieved. A modulation depth(12.8 dB) of the reflection spectrum is also obtained.Moreover, the optimized PC nanocavity has a large free spectral range of 131.59 nm, which can effectively enhance the flexibility of the modulator. It shows that the proposed graphene-based PC nanocavity is a potential candidate for compact,high-contrast, and low-power absorptive modulators in integrated silicon chips.

Terahertz phononic crystal in plasmonic nanocavity

Interaction between photons and phonons in cavity optomechanical systems provides a new toolbox for quantum information technologies.A GaAs/AlAs pillar multi-optical mode microcavity optomechanical structure can obtain phonons with ultra-high frequency(~THz).However,the optical field cannot be effectively restricted when the diameter of the GaAs/AlAs pillar microcavity decreases below the diffraction limit of light.Here,we design a system that combines Ag nanocav-ity with GaAs/AlAs phononic superlattices,where phonons with the frequency of 4.2 THz can be confined in a pillar with~4 nm diameter.The Q_(c)/V reaches 0.22 nm^(-3),which is~80 times that of the photonic crystal(PhC)nanobeam and~100 times that of the hybrid point-defect PhC bowtie plasmonic nanocavity,where Q_(c) is optical quality factor and V is mode volume.The optome-chanical single-photon coupling strength can reach 12 MHz,which is an order of magnitude larger than that of the PhC nanobeam.In addition,the mechanical zero-point fluctuation amplitude is 85 fm and the efficient mass is 0.27 zg,which is much smaller than the PhC nanobeam.The phononic superlattice-Ag nanocavity optomechanical devices hold great potential for applications in the field of integrated quantum optomechanics,quantum information,and terahertz-light transducer.

A compact frequency selective stop-band splitter by using Fabry Perot nanocavity in a T-shaped waveguide

By utilizing a Fabry–Perot （FP） nanocavity adjacent to T-shaped gap waveguide ports, spectrally selective filtering is realized. When the wavelength of incident light corresponds to the resonance wavelength of the FP nanocavity, the surface plasmons are captured inside the nanocavity, and light is highly reflected from this port. The resonance wavelength is determined by using Fabry–Perot resonance condition for the nanocavity. For any desired filtering frequency the dimension of the nanocavity can be tailored. The numerical results are based on the two-dimensional finite difference time domain simulation under a perfectly matched layer absorbing boundary condition. The analytical and simulation results indicate that the proposed structure can be utilized for filtering and splitting applications.

Quantum mechanical solution to spectral lineshape in strongly-coupled atom-nanocavity system

The strongly coupled system composed of atoms,molecules,molecule aggregates,and semiconductor quantum dots embedded within an optical microcavity/nanocavity with high quality factor and/or low modal volume has become an excellent platform to study cavity quantum electrodynamics(CQED),where a prominent quantum effect called Rabi splitting can occur due to strong interaction of cavity-mode single-photon with the two-level atomic states.In this paper,we build a new quantum model that can describe the optical response of the strongly-coupled system under the action of an external probing light and the spectral lineshape.We take the Hamiltonian for the strongly-coupled photon-atom system as the unperturbed Hamiltonian H_(0)and the interaction Hamiltonian of the probe light upon the coupled-system quantum states as the perturbed Hamiltonian V.The theory yields a double Lorentzian lineshape for the permittivity function,which agrees well with experimental observation of Rabi splitting in terms of spectral splitting.This quantum theory will pave the way to construct a complete understanding for the microscopic strongly-coupled system that will become an important element for quantum information processing,nano-optical integrated circuits,and polariton chemistry.

A high-quality factor hybrid plasmonic nanocavity based on distributed Bragg reflectors

Herein,we propose a high-quality（Q） factor hybrid plasmonic nanocavity based on distributed Bragg reflectors（DBRs） with low propagation loss and extremely strong mode confinement.This hybrid plasmonic nanocavity is composed of a high-index cylindrical nanowire separated from a metal surface possessing shallow DBRs gratings by a sufficiently thin low-index dielectric layer.The hybrid plasmonic nanocavity possesses advantages such as a high Purcell factor（Fp） of up to nearly 20000 and a gain threshold approaching 266 cm^（-1）at 1550 nm,promising a greater potential in deep sub-wavelength lasing applications.

Static and Dynamic Analysis of Lasing Action from Single and Coupled Photonic Crystal Nanocavity Lasers

The single and coupled photonic crystal nanocavity lasers are fabricated in the InGaAsP material system and their static and dynamic features are compared. The coupled-cavity lasers show a larger lasing e^ciency and generate an output power higher than the single-cavity lasers, results that are consistent with the theoretical results obtained by rate equations. In dynamic regime, the single-cavity lasers produce pulses as short as 113 ps, while the coupled-cavity lasers show a significantly longer lasing duration. These results indicate that the photonic crystal laser is a promising candidate for the light source in high-speed photonic integrated circuit.

Whispering-gallery mode hexagonal micro-/nanocavity lasers [Invited]

Whispering-gallery-mode(WGM) hexagonal optical micro-/nanocavities can be utilized as high-quality(Q) resonators for realizing compact-size low-threshold lasers. In this paper, the progress in WGM hexagonal micro-/nanocavity lasers is reviewed comprehensively. High-Q WGMs in hexagonal cavities are divided into two kinds of resonances propagating along hexagonal and triangular periodic orbits, with distinct mode characteristics according to theoretical analyses and numerical simulations;however, WGMs in a wavelength-scale nanocavity cannot be well described by the ray model. Hexagonal micro-/nanocavity lasers can be constructed by both bottom-up and top-down processes, leading to a diversity of these lasers. The ZnO-or nitride-based semiconductor material generally has a wurtzite crystal structure and typically presents a natural hexagonal cross section. Bottom-up growth guarantees smooth surface faceting and hence reduces the scattering loss effectively.Laser emissions have been successfully demonstrated in hexagonal micro-/nanocavities synthesized with various materials and structures. Furthermore, slight deformation can be easily introduced and precisely controlled in top-down fabrication, which allows lasing-mode manipulation. WGM lasing with excellent singletransverse-mode property was realized in waveguide-coupled ideal and deformed hexagonal microcavity lasers.

Auxiliary-cavity-assisted vacuum Rabi splitting of a semiconductor quantum dot in a photonic crystal nanocavity

The coherent light-matter interaction has drawn an enormous amount of attention for its fundamental importance in the cavity quantum-electrodynamics (C-QED) field and great potential in quantum information applications. Here, we design a hybrid C-QED system consisting of a quantum dot (QD) driven by two-tone fields implanted in a photonic crystal (PhC) cavity coupled to an auxiliary cavity with a single-mode waveguide and investigate the hybrid system operating in the weak, intermediate, and strong coupling regimes of the light-matter interaction via comparing the QD-photon interaction with the dipole decay rate and the cavity field decay rate.The results indicate that the auxiliary cavity plays a key role in the hybrid system, which affords a quantum channel to influence the absorption of the probe field. By controlling the coupling strength between the auxiliary cavity and the PhC cavity, the phenomenon of the Mollow triplet can appear in the intermediate coupling regime,and even in the weak coupling regime. We further study the strong coupling interaction manifested by vacuum Rabi splitting in the absorption with manipulating the cavity-cavity coupling under different parameter regimes.This study provides a promising platform for understanding the dynamics of QD-C-QED systems and paving the way toward on-chip QD-based nanophotonic devices.

Near-field and photocatalytic properties of mono-and bimetallic nanostructures monitored by nanocavity surface-enhanced Raman scattering

Localized surface plasmon resonances(LSPR)generated in a particle-film nanocavity enhance electric fields within a nanoscale volume.LSPR can also decay into hot carriers,highly energetic species that catalyze photocatalytic reactions in molecular analytes located in close proximity to metal surfaces.In this study,we examined the intensity of the electric field(near-field)and photocatalytic properties of plasmonic nanocavities formed by single nanoparticles(SNP)on single nanoplates(SNL).Using 4-nitrobenzenethiol(4-NBT)as a molecular reporter,we determined the near-field responses,as well as measured rates of 4-NBT dimerization into 4,4-dimercaptoazobenzene(DMAB)in the gold(Au)SNP on AuSNL nanocavity(Au-Au),as well as in AuSNP on AgSNL(Au-Ag),AgSNP on AuSNL(Ag-Au),and AgSNP on AgSNL(Ag-Ag)nanocavities using 532,660,and 785 nm excitations.We observed the strongest near-field signals of 4-NBT at 660 nm in all examined plasmonic systems that is found to be substantially red-shifted relative to the LSPR of the corresponding nanoparticles.We also found that rates of DMAB formation were significantly greater in heterometal nanocavities(Au-Ag and Ag-Au)compared to their monometallic counterparts(Au-Au and Ag-Ag).These results point to drastic differences in plasmonic and photocatalytic properties of mono and bimetallic nanostructures.

Switching dynamics in InP photonic-crystal nanocavity

In this paper, we presented switching dynamic investigations on an InP photonic-crystal （PhC） nanocavity structure using homodyne pump-probe measurements. The measurements were compared with simulations based on temporal nonlinear coupled mode theory and carrier rate equations for the dynamics of the carrier density governing the cavity properties. The results provide insight into the nonlinear optical processes that govern the dynamics of nanocavities.

Twisted lattice nanocavity with theoretical quality factor exceeding 200 billion

Simultaneous localization of light to extreme spatial and spectral scales is of high importance for testing fundamental physics and various applications.However,there is a longstanding trade-off between localizing a light field in space and in frequency.Here we discover a new class of twisted lattice nanocavities based on mode locking in momentum space.The twisted lattice nanocavity hosts a strongly localized light field in a 0.048𝜆3 mode volume with a quality factor exceeding 2.9×1011(∼250𝜇s photon lifetime),which presents a record high figure of merit of light localization among all reported optical cavities.Based on the discovery,we have demonstrated silicon-based twisted lattice nanocavities with quality factor over 1 million.Our result provides a powerful platform to study light-matter interaction in extreme conditions for tests of fundamental physics and applications in nanolasing,ultrasensing,nonlinear optics,optomechanics and quantum-optical devices.

A universal method to calculate the surface energy density of spherical surfaces in crystals

Surface energy plays an important role in the mechanical performance of nanomaterials; however, determining the surface energy density of curved surfaces remains a challenge. In this paper, we conduct atomic simulations to calculate the surface energy density of spherical surfaces in various crystalline metals. It is found that the average surface energy density of spherical surfaces remains almost constant once its radius exceeds 5 nm. Then, using a geometrical analysis and the scaling law, we develop an analytical approach to estimate the surface energy density of spherical surfaces through that of planar surfaces. The theoretical prediction agrees well with the direct atomic simulations, and thus provides a simple and general method to calculate the surface energy density in crystals.

Numerical analysis of surface plasmon nanocavities formed in thickness-modulated metal-insulator-metal waveguides

The enhancement characteristics of the local field in the surface plasmon nanocavities are investigated numerically. The cavity is constructed by placing a defect structure in the thickness-modulated metal-insulator-metal waveguide Bragg gratings. The characteristic impedance based transfer matrix method is used to calculate the transmission spectra and the resonant wavelength of the cavities with various geometric parameters. The finite-difference time- domain method is used to obtain the field pattern of the resonant mode and validate the results of the transfer matrix method. The calculation and simulation results reveal the existence of resonant wavelength shift and intensity variation with structural parameters, such as the modulation period of the gratings, the length and the width of the defect structure. Both numerical analysis and theoretical interpretation on these phenomena are given in details.

Resonant magneto-optical Kerr effect induced by hybrid plasma modes in ferromagnetic nanovoids

With nanovoids buried in Co films, resonant structures were observed in spectra of polar magneto-optical Kerr effect（MOKE）, where both a narrow bandwidth and high intensity were acquired. Through changing the thickness of Co films and the lattice of voids, different optical modes were introduced. For a very shallow array of voids, the resonant MOKE was induced by Ag plasma edge resonance, for deeper ones, hybrid plasma modes, such as void plasmons in the voids, surface lattice plasmons between the voids, and the co-action of them, etc. resulted in resonant MOKE. We found that resonant MOKE resulted from the void plasmons resonance which possesses the narrowest bandwidth for the lowest absorption of voids. The simulated electromagnetic field（EF） distribution consolidated different effects of these three optical modes on resonant MOKE modulation. Such resonant polar MOKE possesses high sensitivity, which might pave the way to on-chip MO devices.

Mechanically induced Cu active sites for selective C–C coupling in CO_(2)electroreduction

Developing a convenient method to endow bulk Cu-based electrode with high activity of electrocatalytic CO_(2)reduction reaction(CO_(2)RR)to multicarbon(C_(2+))products is desirable but challenging.Herein,for the first time,we report that mechanical polishing induces highly reactive Cu sites for selective C-C coupling in CO_(2)RR.We find that mechanical polishing could endow Cu foil with abundant nanocavity surface structure,which efficiently confines the carbonaceous intermediates to enhance the probability of C-C coupling reaction.By confining the carbonaceous intermediates with Cu nanocavity,the as-prepared electrode delivers a Faradaic efficiency toward C_(2+)products of 65.7%at-1.3 V vs.RHE,which is enhanced up to 1.7 folds compared with that of commercial Cu foil.This work provides a new method to enable Cu foil with high activity of CO_(2)RR to C_(2+)products.

Horizontally slotted photonic crystal nanobeam cavity with embedded active nanopillars for ultrafast direct modulation

A horizontally slotted photonic crystal nanobeam cavity with an embedded active nanopillar structure is proposed for ultrafast direct modulation. By designing the thicknesses of both the nanobeam and the horizontal slot layer, the quality factor （Q factor） and the mode volume （Vn） of the proposed cavity can be engineered independently. As a result, the spontaneous emission （SpE） rate is enhanced with a small Vn of 2.4 while the SpE rate and the cavity photon lifetime have an optimal Q factor of ~ 1000. In our simulation, the modulation bandwidth could be enhanced up to 170 GHz with different emission linewidths nf the~ netiv~ nnnnnillnr

Tunneling Electrons Triggered Energy Transfer between Coherently Coupled Donor-Acceptor Molecules

Energy transfer is ubiquitous in natural and artificial lightharvesting systems,and coherent energy transfer,a highly efficient energy transfer process,has been accepted to play a vital role in such systems.However,the energy oscillation of coherent energy transfer is exceedingly difficult to capture because of its evanescence due to the interaction with a thermal environment.Here a microscopic quantum model is used to study the time evolution of electrons triggered energy transfer between coherently coupled donoracceptor molecules in scanning tunneling microscope(STM).A series of topics in the plasmonic nanocavity(PNC)coupled donor-acceptor molecules system are discussed,including resonant and nonresonant coherent energy transfer,dephasing assisted energy transfer,PNC coupling strength dependent energy transfer,Fano resonance of coherently coupled donor-acceptor molecules,and polariton-mediated energy transfer.

λ/20-Thick cavity for mimicking electromagnetically induced transparency at telecommunication wavelengths

Optical cavities play crucial roles in enhanced light-matter interaction,light control,and optical communications,but their dimensions are limited by the material property and operating wavelength.Ultrathin planar cavities are urgently in demand for large-area and integrated optical devices.However,extremely reducing the planar cavity dimension is a critical challenge,especially at telecommunication wavelengths.Herein,we demonstrate a type of ultrathin cavities based on large-area grown Bi_(2)Te_(3)topological insulator(TI)nanofilms,which present distinct optical resonance in the near-infrared region.The result shows that the Bi_(2)Te_(3)TI material presents ultrahigh refractive indices of>6 at telecommunication wavelengths.The cavity thickness can approach 1/20 of the resonance wavelength,superior to those of planar cavities based on conventional Si and Ge high refractive index materials.Moreover,we observed an analog of the electromagnetically induced transparency(EIT)effect at telecommunication wavelengths by depositing the cavity on a photonic crystal.The EIT-like behavior is derived from the destructive interference coupling between the nanocavity resonance and Tamm plasmons.The spectral response depends on the nanocavity thickness,whose adjustment enables the generation of obvious Fano resonance.The experiments agree well with the simulations.This work will open a new door for ultrathin cavities and applications of TI materials in light control and devices.

Defect-modulated and heteroatom-functionalized Ti_(3−x)C_(2)T_(y)MXene 3D nanocavities induce growth of MoSe_(2)nanoflakes toward electrocatalytic hydrogen evolution in all pH electrolytes

Reducing kinetic energy barriers and developing accessible active sites are critical to deliver high hydrogen evolution reaction(HER)efficiency.In this paper,we synthesized defect-modulated and heteroatom(boron)-functionalized three-dimensional(3D)bowl-shaped Ti_(3−x)C_(2)T_(y)MXene(B-TCT)nanocavities coupled with the vertical growth of MoSe_(2)nanoflakes.The B-TCT@MoSe_(2)nanohybrids catalyst delivers the overpotentials of 49.9,52.7,and 67.8 mV to reach a HER current density of 10 mA·cm^(−2)under acidic,alkaline,and neutral conditions,respectively.Such outstanding HER activity is predominantly attributed to the heteroatom functionalization,self-adapting Ti vacancy(VTi)defect modulation,and spatial configuration design in the 3D B-TCT nanocavity,which synergistically regulate the electronic structure,activate the basal plane/edge unsaturated sites,and reduce the reaction energy barrier.Experimental and theoretical calculations demonstrate that strong heterogeneous interfacial bonding interactions between B-TCT and MoSe_(2)can dramatically reduce the free energy of hydrogen adsorption and facilitate efficient interfacial charge migration,thus essentially improving the HER kinetics.We used this 3D porous nanohybrid system assembled by defectrich lamellar structures to elucidate the advantageous synergistic effects of multiple mechanisms among defect structure,heteroatom functionalization,and interfacial coupling,which provided important insights for the development of efficient hybridtype catalysts.