Single Nanocavity Research Articles

How local electric field regulates C–C coupling at a single nanocavity in electrocatalytic CO2 reduction

C–C coupling is of utmost importance in the electrocatalytic reduction of CO2, as it governs the selectivity of diverse product formation. Nevertheless, the difficulties to directly observe C–C coupling pathways at a specific nanocavity hinder the advances in catalysts and electrolyzer design for efficient high-value hydrocarbon production. Here we develop a nano-confined Raman technology to elucidate the influence of the local electric field on the evolution of C–C coupling intermediates. Through precise adjustments to the Debye length in nanocavities of a copper catalyst, the overlapping of electrical double layers drives a transition in the C–C coupling pathway at a specific nanocavity from *CHO–*CO coupling to the direct dimerization of *CO species. Experimental evidence and simulations validate that a reduced potential drop across the compact layer promotes a higher yield of CO and promotes the direct dimerization of *CO species. Our findings provide insights for the development of highly selective catalyst materials tailored to promote specific products.

Few-emitter lasing in single ultra-small nanocavities.

Lasers are ubiquitous for information storage, processing, communications, sensing, biological research and medical applications. To decrease their energy and materials usage, a key quest is to miniaturise lasers down to nanocavities. Obtaining the smallest mode volumes demands plasmonic nanocavities, but for these, gain comes from only a single or few emitters. Until now, lasing in such devices was unobtainable due to low gain and high cavity losses. Here, we demonstrate a form of 'few emitter lasing' in a plasmonic nanocavity approaching the single-molecule emitter regime. The few-emitter lasing transition significantly broadens, and depends on the number of molecules and their individual locations. We show this non-standard few-emitter lasing can be understood by developing a theoretical approach extending previous weak-coupling theories. Our work paves the way for developing nanolaser applications as well as fundamental studies at the limit of few emitters.

Reconfigurable moiré nanolaser arrays with phase synchronization.

Miniaturized lasers play a central role in the infrastructure of modern information society. The breakthrough in laser miniaturization beyond the wavelength scale has opened up new opportunities for a wide range of applications1-4, as well as for investigating light-matter interactions in extreme-optical-field localization and lasing-mode engineering5-19. An ultimate objective of microscale laser research is to develop reconfigurable coherent nanolaser arrays that can simultaneously enhance information capacity and functionality. However, the absence of a suitable physical mechanism for reconfiguring nanolaser cavities hinders the demonstration of nanolasers in either a single cavity or a fixed array. Here we propose and demonstrate moiré nanolaser arrays based on optical flatbands in twisted photonic graphene lattices, in which coherent nanolasing is realized from a single nanocavity to reconfigurable arrays of nanocavities. We observe synchronized nanolaser arrays exhibiting high spatial and spectral coherence, across a range of distinct patterns, including P, K and U shapes and the Chinese characters '' and '' ('China' in Chinese). Moreover, we obtain nanolaser arrays that emit with spatially varying relative phases, allowing us to manipulate emission directions. Our work lays the foundation for the development of reconfigurable active devices that have potential applications in communication, LiDAR (light detection and ranging), optical computing and imaging.

Tailoring Fluorescence-Phosphorescence Emission with a Single Nanocavity.

Realizing the dual emission of fluorescence-phosphorescence in a single system is an extremely important topic in the fields of biological imaging, sensing, and information encryption. However, the phosphorescence process is usually in an inherently "dark state" at room temperature due to the involvement of spin-forbidden transition and the rapid non-radiative decay rate of the triplet state. In this work, we achieved luminescent harvesting of the dark phosphorescence processes by coupling singlet-triplet molecular emitters with a rationally designed plasmonic cavity. The achieved Purcell enhancement effect of over 1000-fold allows for overcoming the triplet forbidden transitions, enabling radiation enhancement with selectable emission wavelengths. Spectral results and theoretical simulations indicate that the fluorescence-phosphorescence peak position can be intelligently tailored in a broad range of wavelengths, from visible to near-infrared. Our study sheds new light on plasmonic tailoring of molecular emission behavior, which is crucial for advancing research on plasmon-tailored fluorescence-phosphorescence spectroscopy in optoelectronics and biomedicine.

λ/20 surface nanostructuring of ZnO by mask-less ultrafast laser processing

Abstract Fabrication of nanostructures with a feature size much smaller than the laser wavelength is challenging due to the optical diffraction limit. It’s well known that the irradiation of polarized ultrafast laser generates periodic nanostructures, so called laser-induced periodic surface structures (LIPSS). Owing to the modulated field, the surface is periodically ablated to form specific patterns, which can be used for some photonic applications including surface-enhanced Raman scattering (SERS). In this paper, we investigate the morphologies of LIPSS on ZnO substrates by mask-less ultrafast laser processing. By adjusting the laser processing parameters, including fluence, pulse number, polarization, and pulse duration, the homogenous nanostrip array and nanopillar array are created. Furthermore, by adjusting the laser fluence, a single nanogroove with a width of ∼20 nm and a single nanocavity with a diameter of ∼24 nm are created. The gold nanoparticles are then coated on the ZnO nanopillar array for SERS application. We found that the concentration of defects in ZnO substrate is increased by the laser irradiation, which is beneficial for SERS performances to achieve an enhancement factor of SERS as high as 2.28 × 107.

Bright single-nanocrystal upconversion at sub 0.5 W cm−2 irradiance via coupling to single nanocavity mode

Bright single-nanocrystal upconversion at sub 0.5 W cm−2 irradiance via coupling to single nanocavity mode

Detection of Strong Light-Matter Interaction in a Single Nanocavity with a Thermal Transducer.

The concept of strong light-matter coupling has been demonstrated in semiconductor structures, and it is poised to revolutionize the design and implementation of components, including solid state lasers and detectors. We demonstrate an original nanospectroscopy technique that permits the study of the light-matter interaction in single subwavelength-sized nanocavities where far-field spectroscopy is not possible using conventional techniques. We inserted a thin (∼150 nm) polymer layer with negligible absorption in the mid-infrared range (5 μm < λ < 12 μm) inside a metal-insulator-metal resonant cavity, where a photonic mode and the intersubband transition of a semiconductor quantum well are strongly coupled. The intersubband transition peaks at λ = 8.3 μm, and the nanocavity is overall 270 nm thick. Acting as a nonperturbative transducer, the polymer layer introduces only a limited alteration of the optical response while allowing to reveal the optical power absorbed inside the concealed cavity. Spectroscopy of the cavity losses is enabled by the polymer thermal expansion due to heat dissipation in the active part of the cavity, and performed using atomic force microscopy (AFM). This innovative approach allows the typical anticrossing characteristic of the polaritonic dispersion to be identified in the cavity loss spectra at the single nanoresonator level. Results also suggest that near-field coupling of the external drive field to the top metal patch mediated by a metal-coated AFM probe tip is possible, and it enables the near-field mapping of the cavity mode symmetry including in the presence of a strong light-matter interaction.

Strong coupling with directional scattering features of metal nanoshells with monolayer WS2 heterostructures

Realizing and manipulating strong light–matter coupling in 2D monolayer semiconductors are of the utmost importance in the development of photonic devices. Hollow nanostructures of noble metals are particularly interesting because of their stronger local electromagnetic field compared with solid nanoparticles, which facilitate the strong coupling of single metal nanostructures. Here, the tunable single nanocavity plasmon–exciton coupling was demonstrated at room temperature in hybrid systems consisting of Ag@Au hollow nanocubes (HNCs) and monolayer WS2 underneath, where a large vacuum Rabi splitting of 131.3 meV was observed. Mode splitting can be clearly observed from the dark-field scattering spectrum of the single hybrid nanocavity, which is ascribed to the strong coupling between the nanocavity mode and the excitonic mode. Then, we used the finite difference time domain method to simulate these hybrid systems. By changing the thickness of the shell of the Ag@Au HNC, we can tune the surface plasmon resonance peak position of HNCs to match the exciton energy of the monolayer WS2. The strong couplings were realized via the calculated scattering spectra. The calculated results were consistent with the experimental results. Furthermore, the mode volume of different nanostructures was discussed, and the mode volume of HNCs is smaller than other solid ones at the same plasmonic resonance wavelength, which also indicates that its ability to restrict an electromagnetic field is stronger. This study provides an ideal platform for the strong coupling of a single nanocavity at room temperature and has broad application prospects in the field of single-photon devices.

Broadband Nanoscale Surface‐Enhanced Raman Spectroscopy by Multiresonant Nanolaminate Plasmonic Nanocavities on Vertical Nanopillars

AbstractSurface‐enhanced Raman spectroscopy (SERS) has become a sensitive detection technique for biochemical analysis. Despite significant research efforts, most SERS substrates consisting of single‐resonant plasmonic nanostructures on the planar surface suffer from limitations of narrowband SERS operation and unoptimized nano‐bio interface with living cells. Here, it is reported that nanolaminate plasmonic nanocavities on 3D vertical nanopillar arrays can support a broadband SERS operation with large enhancement factors (&gt;106) under laser excitations at 532, 633, and 785 nm. The multi‐band Raman mapping measurements show that nanolaminate plasmonic nanocavities on vertical nanopillar arrays exhibit broadband uniform SERS performance with diffraction‐limited resolution at a single nanopillar footprint. By selective exposure of embedded plasmonic hotspots in individual metal–insulator–metal (MIM) nanogaps, nanoscale broadband SERS operation at the single MIM nanocavity level with visible and near‐infrared (vis–NIR) excitations is demonstrated. Numerical studies reveal that nanolaminate plasmonic nanocavities on vertical nanopillars can support multiple hybridized plasmonic modes to concentrate optical fields across a broadband wavelength range from 500 to 900 nm at the nanoscale.

Unified Scattering and Photoluminescence Spectra for Strong Plasmon-Exciton Coupling.

The strong coupling between excitons and single plasmonic nanocavities enables plexcitonic states in nanoscale systems at room temperature. Here we demonstrate the strong coupling of surface plasmon modes of metal nanowires and excitons in monolayer semiconductors, with Rabi splitting manifested in both scattering and photoluminescence (PL) spectra. By utilizing the propagation properties of surface plasmons on the nanowires, the PL emitted through the scattering of plasmon-exciton hybrid modes is extracted. The analytically calculated scattering and PL spectra well reproduce the experimental results. These findings unify the scattering and PL spectra in the plexcitonic system and eliminate the ambiguities of PL emission, shedding new light on understanding the rich spectral phenomena in the plasmon-exciton strong coupling regime.

Giant Enhancement of Nonlinear Harmonic Generation in a Silicon Topological Photonic Crystal Nanocavity Chain

AbstractStrongly enhanced third‐harmonic generation (THG) by the topological localization of an edge mode in a Su‐Schrieffer‐Heeger (SSH) chain of silicon photonic crystal nanocavities is demonstrated. The edge mode of the nanocavity chain not only naturally inherits resonant properties of the single nanocavity, but also exhibits the topological feature with mode robustness extending well beyond individual nanocavity. By engineering the SSH nanocavities with alternating strong and weak coupling strengths on a silicon slab, the edge mode formation that entails a strong THG signal similar to that obtained from a single nanocavity is observed, which both show three orders of magnitude enhancement compared with that in a trivial SSH structure. The results indicate that the photonic crystal nanocavity chain can provide a promising on‐chip platform for topology‐driven nonlinear photonics.

Energy-resolved plasmonic chemistry in individual nanoreactors.

Plasmonic resonances can concentrate light into exceptionally small volumes, which approach the molecular scale. The extreme light confinement provides an advantageous pathway to probe molecules at the surface of plasmonic nanostructures with highly sensitive spectroscopies, such as surface-enhanced Raman scattering. Unavoidable energy losses associated with metals, which are usually seen as a nuisance, carry invaluable information on energy transfer to the adsorbed molecules through the resonance linewidth. We measured a thousand single nanocavities with sharp gap plasmon resonances spanning the red to near-infrared spectral range and used changes in their linewidth, peak energy and surface-enhanced Raman scattering spectra to monitor energy transfer and plasmon-driven chemical reactions at their surface. Using methylene blue as a model system, we measured shifts in the absorption spectrum of molecules following surface adsorption and revealed a rich plasmon-driven reactivity landscape that consists of distinct reaction pathways that occur in separate resonance energy windows.

Pushing the limit of high-Q mode of a single dielectric nanocavity

High index dielectric nanostructure supports different types of resonant modes. However, it is very challenging to achieve high-Q factor in a single subwavelength dielectric nanoresonator due to non-hermtian property of the open system. Here, we present a universal approach of finding out a series of high-Q resonant modes in a single nonspherical dielectric nanocavity by exploring quasi-bound state in the continuum. Unlike conventional method relying on heavy computation (ie, frequency scanning by FDTD), our approach is built upon leaky mode engineering, through which many high-Q modes can be easily achieved by constructing avoid-crossing (or crossing) of the eigenvalue for pair leaky modes. The Q-factor can be up to 2.3*10^4 for square subwavelength nanowire (NW) (n=4), which is 64 times larger than the highest Q-factor (Q=360) reported so far in single subwavelength nanodisk. Such high-Q modes can be attributed to suppressed radiation in the corresponding eigenchannels and simultaneously quenched electric(magnetic) at momentum space. As a proof of concept, we experimentally demonstrate the emergence of the high-Q resonant modes (Q=380) in the scattering spectrum of a single silicon subwavelength nanowire.

Recent advances in nanocavities and their applications.

High quality factor and small mode volume in nanocavities enable the demonstration of efficient nanophotonic devices with low power consumption, strong nonlinearity, and high modulation speed, due to the strong light-matter interaction. In this review, we focus on recent state-of-the-art nanocavities and their applications. We introduce single nanocavities including semiconductor nanowires, plasmonic cavities, and nanostructures based on quasi-bound states in the continuum (quasi-BIC), for laser, photovoltaic, and nonlinear applications. In addition, nanocavity arrays with unique feedback mechanisms, including BIC cavities, parity-time symmetry coupled cavities, and photonic topological cavities, are introduced for laser applications. These various cavity designs and underlying physics in single and array nanocavities are useful for the practical implementation of promising nanophotonic devices.

Temperature-dependent dark-field scattering of single plasmonic nanocavity

Abstract Plasmonic materials have long been exploited for enhanced spectroscopy, integrated nanophotonic circuits, sensing, light harvesting, etc. Damping is the key factor that limits their performance and restricts the development of the field. Optical characterization of single nanoparticle at low temperature is ideal for investigating the damping of plasmons but is usually technically impractical due to the sample vibration from the cryostat and the surface adsorption during the cooling process. In this work, we use a vibration-free cryostat to investigate the temperature-dependent dark-field scattering spectroscopy of a single Au nanowire on top of a Au film. This allows us to extract the contribution of electron-phonon scattering to the damping of plasmons without performing statistics over different target nanoparticles. The results show that the full width at half-maximum of the plasmon resonance increases by an amount of 5.8%, over the temperature range of 5−150 K. Electromagnetic calculations reveal that the temperature-insensitive dissipation channels into photons or surface plasmon polaritons on the Au film contribute up to 64% of the total dissipations at the plasmon resonance. This explains why the reduction of plasmon linewidth seems small at the single-particle level. This study provides a more explicit measurement on the damping process of the single plasmonic nanostructure, which serves as basic knowledge in the applications of nanoplasmonic materials.

Array of Single Nanocavity Electrodes for Lab-on-a-Chip

Electrochemical methods have been frequently employed in lab-on-a-chip systems for biosensing and biomedical analysis of analytes. For this goal, fabrication of electrode is one of important steps because material and structure of electrode have significant effects on performance of electrochemical detection. In these days, various nano-sized electrodes were prepared by several methods because they have some advantages compared to macro/micro-sized electrodes such as enhanced mass transport, improved signal to noise ratio, and minimal iR drop. In this study, to get higher current and to keep the advantages of nano-sized electrode, the array of a nanocavity electrode with controlled diameter and structure was fabricated by photolithography and focused ion beam. Each electrode in the array is designed to work individually, which indicates that simultaneous and multiplex detection of several biomarkers is enabled for efficient lab-on-a-chip system. First of all, it is necessary to investigate the electrochemical behavior of single nanocavity electrodes. The redox behavior was intensively studied according to scan rates, kinds of redox species, concentrations of redox species, kinds of supporting electrolyte, and concentrations of supporting electrolytes.

Broad-band, reversible nonreciprocal light transmission based on a single nanocavity.

We present a general yet simple method for achieving broad-band, high-contrast-ratio, and reversible all-optical diodes. The mechanism is based on the controllable photonic transitions inside a nonlinear nanocavity, triggered by only one pulse. We demonstrate that the interaction between the signal, pump pulse, and the cavity mode plays a crucial role in dynamically controlling the nonreciprocal light transmissions. Using this mechanism, we show that the nonreciprocal light transmission can be controllably reversed without changing the signal light's wavelength or power, with a broad operation bandwidth over 7.5nm and a transmission contrast rate over 20dB. This approach provides a promising avenue in flexible manipulation of on-chip all-optical signal processing.

Coexistence of air and dielectric modes in single nanocavity.

A deterministic design method and experimental demonstration of single photonic crystal nanocavity supporting both air and dielectric modes in the mid-infrared wavelength region are reported here. The coexistence of both modes is realized by a proper design of photonic dispersion to confine air and dielectric bands simultaneously. By adding central mirrors to make the resonance modes be confined at the bandgap edges, high experimental Q-factors of 2.32 × 104 and 1.59 × 104 are achieved at the resonance wavelength of about 3.875μm and 3.728μm for fundamental dielectric and air modes, respectively. Moreover, multiple sets of air and dielectric modes can be realized by introducing central aperiodic mirrors with multiple bandgaps. The realization of coexistence of air and dielectric modes in single nanocavity will offer opportunities for multifunctional devices, paving the way to integrated multi-parameter sensors, filters, nonlinear devices, and compact light sources.

Thermal Redistribution of Exciton Population in Monolayer Transition Metal Dichalcogenides Probed with Plasmon–Exciton Coupling Spectroscopy

Inversion symmetry breaking and spin–orbit coupling result in spin-splitting of both valence and conduction bands in transition metal dichalcogenide (TMDC) monolayers. The optical transitions between band edges with opposite spins are termed dark excitons that are decoupled with in-plane polarized photons. Here, we find that the presence of dark excitons modifies the temperature-dependent plasmon–bright-exciton coupling strength of a TMDC monolayer interacting with a single plasmonic nanocavity. Quite interestingly, we observe that the modifications are in an opposite manner for WS2 and MoS2 monolayers. Coupled-oscillator analysis reveals that the WS2–nanocavity coupling strength increases with rising temperature, yet that for the MoS2–nanocavity diminishes, which both follow the temperature evolution of the respective exciton oscillator strength obtained by fitting the reflectance spectra of pristine TMDC monolayers with a multi-Lorentz oscillator model. Full-wave electromagnetic simulations with experim...

Deterministic aperiodic photonic crystal nanobeam supporting adjustable multiple mode-matched resonances.

We investigate nanocavities in deterministic aperiodic photonic crystal (PhC) nanobeams. We reveal that even a single nanocavity can support multiple mode-matched resonances, which show an almost perfect field overlap in the cavity region. The unique property is enabled by the existence of adjustable multiple bandgaps in deterministic aperiodic PhC nanobeams. Our investigation may inspire related studies on low threshold lasers, integrated nonlinear devices, optical filters, and on-chip sensors.