Local Density of States for Nanoplasmonics

Spectroscopy in Nanoscopic Cavities: Models and Recent Experiments

The ability of nanophotonic cavities to confine and store light to nanoscale dimensions has important implications for enhancing molecular, excitonic, phononic, and plasmonic optical responses. Spectroscopic signatures of processes that are ordinarily exceedingly weak such as pure absorption and Raman scattering have been brought to the single-particle limit of detection, while new emergent polaritonic states of optical matter have been realized through coupling material and photonic cavity degrees of freedom across a wide range of experimentally accessible interaction strengths. In this review, we discuss both optical and electron beam spectroscopies of cavity-coupled material systems in weak, strong, and ultrastrong coupling regimes, providing a theoretical basis for understanding the physics inherent to each while highlighting recent experimental advances and exciting future directions.

Relations between near-field enhancements and Purcell factors in hybrid nanostructures of plasmonic antennas and dielectric cavities

Strong near-field enhancements (NFEs) of nanophotonic structures are believed to be closely related to high Purcell factors (FP). Here, we theoretically show that the correlation is partially correct; the extinction cross section (σ) response is also critical in determining FP. The divergence between NFE and FP is especially pronounced in plasmonic-dielectric hybrid systems, where the plasmonic antenna supports dipolar plasmon modes and the dielectric cavity hosts Mie-like resonances. The cavity's enhanced-field environment can boost the antenna's NFEs, but the FP is not increased concurrently due to the larger effective σ that is intrinsic to the FP calculations. Interestingly, the peak FP for the coupled system can be predicted by using the NFE and σ responses. Furthermore, the limits for FP of coupled systems are considered; they are determined by the sum of the FP of a redshifted (or modified, if applicable) antenna and an individual cavity. This contrasts starkly with the behavior of NFE which is closely associated with the multiplicative effects of the NFEs provided by the antenna and the dielectric cavity. The differing behaviors of NFE and FP in hybrid cavities have varied impacts on relevant nanophotonic applications such as fluorescence, Raman scattering and enhanced light-matter interactions.

Constructing Multiphase-Induced Interfacial Polarization to Surpass Defect-Induced Polarization in Multielement Sulfide Absorbers

The extremely weak heterointerface construction of high-entropy materials (HEM) hinders them being the electromagnetic wave (EMW) absorbers with ideal properties. To address this issue, this study proposes multiphase interfacial engineering and results in a multiphase-induced interfacial polarization loss in multielement sulfides. Through the selection of atoms with diverse reaction activities, the multiphase interfacial components of CuS (1 0 5), Fe0.5 Ni0.5 S2 (2 1 0), and CuFe2 S3 (2 0 0) are constructed to enhance the interfacial polarization loss in multielement Cu-based sulfides. Compared with single-phase high-entropy Zn-based sulfides (ZnFeCoNiCr-S), the multiphase Cu-based sulfides (CuFeCoNiCr-S) possess optimized EMW absorption properties (effective absorption bandwidth (EAB) of 6.70 GHz at 2.00 mm) due to the existence of specific interface of CuS (1 0 5)/CuFe2 S3 (2 0 0) with proper EM parameters. Furthermore, single-phase ZnFeCoNiCr-S into FeNi2 S4 (3 1 1)/(Zn, Fe)S (1 1 1) heterointerface through 400 °C heat-treated is decomposed. The EMW absorption properties are enhanced by strong interfacial polarization (EAB of 4.83 GHz at 1.45 mm). This work reveals the reasons for the limited EMW absorption properties of high-entropy sulfides and proposes multiphase interface engineering to improve charge accumulation and polarization between specific interfaces, leading to the enhanced EMW absorption properties.

Plasmonic Nanodiamonds

While nitrogen-vacancy (NV) centers in diamonds have emerged as promising solid-state quantum emitters for sensing applications, the tantalizing possibility of coupling them with photonic or broadband plasmonic nanostructures to create ultrasensitive biolabels has not been fully realized. Indeed, it remains technologically challenging to create free-standing hybrid diamond-based imaging nanoprobes with enhanced brightness and high temporal resolution. Herein, we leverage the bottom-up DNA self-assembly to develop hybrid free-standing plasmonic nanodiamonds, which feature a closed plasmonic nanocavity completely encapsulating a single nanodiamond. Correlated single nanoparticle spectroscopical characterizations suggest that the plasmonic nanodiamond displays dramatically and simultaneously enhanced brightness and emission rate. We believe that they hold huge potential to serve as a stable solid-state single-photon source and could serve as a versatile platform to study nontrivial quantum effects in biological systems with enhanced spatial and temporal resolution.

Nonlinear Optical Signal Generation Mediated by a Plasmonic Azimuthally Chirped Grating

Plasmonic gratings are simple and effective platforms for nonlinear signal generation since they provide a well-defined momentum for photon-plasmon coupling and local hot spots for frequency conversion. Here, a plasmonic azimuthally chirped grating (ACG), which provides spatially resolved broadband momentum for photon-plasmon coupling, was exploited to investigate the plasmonic enhancement effect in two nonlinear optical processes, namely two-photon photoluminescence (TPPL) and second harmonic generation (SHG). The spatial distributions of the nonlinear signals were determined experimentally by hyperspectral mapping with ultrashort pulsed excitation. The experimental spatial distributions of nonlinear signals agree very well with the analytical prediction based on photon-plasmon coupling with the momentum of the ACG, revealing the "antenna" function of the grating in plasmonic nonlinear signal generation. This work highlights the importance of the antenna effect of the gratings for nonlinear signal generation and provides insight into the enhancement mechanism of plasmonic gratings in addition to local hot spot engineering.

Engineering Electromagnetic Field Distribution and Resonance Quality Factor Using Slotted Quasi-BIC Metasurfaces

Dielectric metasurfaces governed by bound states in the continuum (BIC) are actively investigated for achieving high-quality factors and strong electromagnetic field enhancements. Traditional approaches reported for tuning the performance of quasi-BIC metasurfaces include tuning the resonator size, period, and structure symmetry. Here we propose and experimentally demonstrate an alternative approach through engineering slots within a zigzag array of elliptical silicon resonators. Through analytical theory, three-dimensional electromagnetic modeling, and infrared spectroscopy, we systematically investigate the spectral responses and field distributions of the slotted metasurface in the mid-IR. Our results show that by introducing slots, the electric field intensity enhancement near the apex and the quality factor of the quasi-BIC resonance are increased by a factor of 2.1 and 3.3, respectively, in comparison to the metasurface without slots. Furthermore, the slotted metasurface also provides extra regions of electromagnetic enhancement and confinement, which holds enormous potential in particle trapping, sensing, and emission enhancement.

Plasmon-induced coherence, exciton-induced transparency, and Fano interference for hybrid plasmonic systems in strong coupling regime

We present an analytical model describing the transition to a strong coupling regime for an ensemble of emitters resonantly coupled to a localized surface plasmon in a metal-dielectric structure. The response of a hybrid system to an external field is determined by two distinct mechanisms involving collective states of emitters interacting with the plasmon mode. The first mechanism is the near-field coupling between the bright collective state and the plasmon mode, which underpins the energy exchange between the system components and gives rise to exciton-induced transparency minimum in scattering spectra in the weak coupling regime and to emergence of polaritonic bands as the system transitions to the strong coupling regime. The second mechanism is the Fano interference between the plasmon dipole moment and the plasmon-induced dipole moment of the bright collective state as the hybrid system interacts with the radiation field. The latter mechanism is greatly facilitated by plasmon-induced coherence in a system with the characteristic size below the diffraction limit as the individual emitters comprising the collective state are driven by the same alternating plasmon near field and, therefore, all oscillate in phase. This cooperative effect leads to scaling of the Fano asymmetry parameter and of the Fano function amplitude with the ensemble size, and therefore, it strongly affects the shape of scattering spectra for large ensembles. Specifically, with increasing emitter numbers, the Fano interference leads to a spectral weight shift toward the lower energy polaritonic band.

Light emission from plasmonic nanostructures

The mechanism of light emission from metallic nanoparticles has been a subject of debate in recent years. Photoluminescence and electronic Raman scattering mechanisms have both been proposed to explain the observed emission from plasmonic nanostructures. Recent results from Stokes and anti-Stokes emission spectroscopy of single gold nanorods using continuous wave laser excitation carried out in our laboratory are summarized here. We show that varying excitation wavelength and power change the energy distribution of hot carriers and impact the emission spectral lineshape. We then examine the role of interband and intraband transitions in the emission lineshape by varying the particle size. We establish a relationship between the single particle emission quantum yield and its corresponding plasmonic resonance quality factor, which we also tune through nanorod crystallinity. Finally, based on anti-Stokes emission, we extract electron temperatures that further suggest a hot carrier based mechanism. The central role of hot carriers in our systematic study on gold nanorods as a model system supports a Purcell effect enhanced hot carrier photoluminescence mechanism. We end with a discussion on the impact of understanding the light emission mechanism on fields utilizing hot carrier distributions, such as photocatalysis and nanothermometry.

Comparative electron and photon excitation of localized surface plasmon resonance in lithographic gold arrays for enhanced Raman scattering

The ability to tune the localized surface plasmon resonance (LSPR) of nanostructures is desirable for surface enhanced Raman spectroscopy (SERS), plasmon-assisted chemistry and other nanophotonic applications. Although historically the LSPR is mainly studied by optical techniques, with the recent advancement in electron monochromators and correctors, it has attracted considerable attention in transmission electron microscopy (TEM). Here, we use electron energy loss spectroscopy (EELS) in a scanning TEM to study individual gold nanodiscs and bowties in lithographic arrays with variable LSPRs by adjusting the size, interspacing, shape and dielectric environment during the nanofabrication process. We observe the strongest Raman signal enhancement when the LSPR frequency is close to the incident laser frequency in Raman spectroscopy. A simplified harmonic oscillator model is used to estimate SERS enhancement factor (EF) from EELS, bridging the connection between electron and photon excitation of plasmonic arrays. This work demonstrates that STEM-EELS shows promise for revealing the contributions of specific LSPR modes to SERS EF. Our results provide guidelines to fine-tune nanoparticle parameters to deliver the maximum signal enhancement in biosensing applications, such as early cancer detection.

Laser-Induced Periodic Ag Surface Structure with Au Nanorods Plasmonic Nanocavity Metasurface for Strong Enhancement of Adenosine Nucleotide Label-Free Photoluminescence Imaging

The label-free detection of biomolecules by means of fluorescence spectroscopy and imaging is topical. The developed surface-enhanced fluorescence technique has been applied to achieve progress in the label-free detection of biomolecules including deoxyribonucleic acid (DNA) bases. In this study, the effect of a strong enhancement of photoluminescence of 5'-deoxyadenosine-monophosphate (dAMP) by the plasmonic nanocavity metasurface composed of the silver femtosecond laser-induced periodic surface structure (LIPSS) and gold nanorods or nanospheres has been realized at room temperature. The highest value of 1220 for dAMP on the Ag-LIPSS/Au nanorod metasurface has been explained to be a result of the synergetic effect of the generation of hot spots near the sharp edges of LIPSS and Au nanorod tips together with the excitation of collective gap mode of the cavity due to strong near-field plasmonic coupling. A stronger plasmonic enhancement of the phosphorescence compared to the fluorescence is achieved due to a greater overlap of the phosphorescence spectrum with the surface plasmon spectral region. The photoluminescence imaging of dAMP on the metasurfaces shows a high intensity in the blue range. The comparison of Ag-LIPSS/Au nanorod and Ag-LIPSS/Au-nanosphere metasurfaces shows a considerably higher enhancement for the metasurface containing Au nanorods. Thus, the hybrid cavity metasurfaces containing metal LIPSS and nonspherical metal nanoparticles with sharp edges are promising for high-sensitive label-free detection and imaging of biomolecules at room temperature.

Dynamic control of spontaneous emission rate using tunable hyperbolic metamaterials

We numerically investigate the dynamic control over the spontaneous emission rate of quantum emitters using tunable hyperbolic metamaterials (HMMs). The dispersion of a metal-dielectric thin-film stack at a given frequency can undergo a topological transition from an elliptical to a hyperbolic dispersion by incorporating a tunable metal or dielectric film in the HMM. This transition modifies the local density of optical states of the emitter and, hence, its emission rate. In the visible range, we use an HMM consisting of TiN and ${{\rm Sb}_2}{{\rm S}_3}$Sb₂S₃ and show considerable tunability in the Purcell enhancement and quantum efficiency as ${{\rm Sb}_2}{{\rm S}_3}$Sb₂S₃ phase changes from amorphous to crystalline. Similarly, we show tunable Purcell enhancement in the telecommunication wavelength range using a ${\rm TiN}/{{\rm VO}_2}$TiN/VO₂- HMM. Finally, tunable spontaneous emission rate in the mid-IR range is obtained using a ${\rm graphene}/{\rm MgF}_2$graphene/MgF₂ HMM by modifying the graphene conductivity through changing its chemical potential. We show that using a metal nitride (for the visible and NIR HMMs) and a fluoride (for the mid-IR HMM) is important to get an appreciable change in the effective permittivity of the thin-film multilayer stack.

Ultrastrong coupling of CdZnS/ZnS quantum dots to bonding breathing plasmons of aluminum metal-insulator-metal nanocavities in near-ultraviolet spectrum

Strong coupling originating from excitons of quantum dots and plasmons in nanocavities can be realized at room temperature due to the large electromagnetic field enhancement of plasmons, offering building blocks for quantum information systems, ultralow-power switches and lasers. However, most of the current strong coupling effects were realized by the interaction between excitons and far-field light excited bright plasmon modes in the visible range. Beyond that, there is still a lack of direct imaging of polariton modes at the nanoscale. In this work, by using cathodoluminescence, ultrastrong coupling with Rabi splitting exceeding 1 eV between bonding breathing plasmons of aluminum (Al) metal-insulator-metal (MIM) cavities and excited states of CdZnS/ZnS quantum dots was observed in the near-ultraviolet (UV) spectrum. Further, the hybridization of the QDs excitons and bonding breathing plasmonic modes is verified by deep-subwavelength images of polaritonic modes in real-space. Analytic analysis based on the coupled oscillator model and full-wave electromagnetic simulations is consistent with our experimental results. Our work not only indicates the great potential of electron excited plasmon modes for strong coupling applications, but also extends the polaritonic frequency to the UV range with Al nanocavities.

Photonic crystal for graphene plasmons

Photonic crystals are commonly implemented in media with periodically varying optical properties. Photonic crystals enable exquisite control of light propagation in integrated optical circuits, and also emulate advanced physical concepts. However, common photonic crystals are unfit for in-operando on/off controls. We overcome this limitation and demonstrate a broadly tunable two-dimensional photonic crystal for surface plasmon polaritons. Our platform consists of a continuous graphene monolayer integrated in a back-gated platform with nano-structured gate insulators. Infrared nano-imaging reveals the formation of a photonic bandgap and strong modulation of the local plasmonic density of states that can be turned on/off or gradually tuned by the applied gate voltage. We also implement an artificial domain wall which supports highly confined one-dimensional plasmonic modes. Our electrostatically-tunable photonic crystals are derived from standard metal oxide semiconductor field effect transistor technology and pave a way for practical on-chip light manipulation.

Traditional photonic crystals consist of periodic media with a pre-defined optical response. Here, the authors combine nanostructured back-gate insulators with a continuous layer of graphene to demonstrate an electrically tunable two-dimensional photonic crystal suitable for controlling the propagation of surface plasmon polaritons.

Cooperative Energy Transfer Controls the Spontaneous Emission Rate Beyond Field Enhancement Limits

Quantum emitters located in proximity to a metal nanostructure individually transfer their energy via near-field excitation of surface plasmons. The energy transfer process increases the spontaneous emission (SE) rate due to plasmon-enhanced local field. Here, we demonstrate a significant acceleration of the quantum emitter SE rate in a plasmonic nanocavity due to cooperative energy transfer (CET) from plasmon-correlated emitters. Using an integrated plasmonic nanocavity, we realize up to sixfold enhancement in the emission rate of emitters coupled to the same nanocavity on top of the plasmonic enhancement of the local density of states. The radiated power spectrum retains the plasmon resonance central frequency and line shape, with the peak amplitude proportional to the number of excited emitters indicating that the observed cooperative SE is distinct from superradiance. Plasmon-assisted CET offers unprecedented control over the SE rate and allows us to dynamically control the spontaneous emission rate at room temperature which can enable SE rate based optical modulators.

Exciton-Plasmon Energy Exchange Drives the Transition to a Strong Coupling Regime

We present a model for exciton-plasmon coupling based on an energy exchange mechanism between quantum emitters (QE) and localized surface plasmons in metal-dielectric structures. Plasmonic correlations between QEs give rise to a collective state exchanging its energy cooperatively with a resonant plasmon mode. By defining carefully the plasmon mode volume for a QE ensemble, we obtain a relation between QE-plasmon coupling and a cooperative energy transfer rate that is expressed in terms of local fields. For a single QE near a sharp metal tip, we find analytically the enhancement factor for QE-plasmon coupling relative to QE coupling to a cavity mode. For QEs distributed in an extended region enclosing a plasmonic structure, we find that the ensemble QE-plasmon coupling saturates to a universal value independent of system size and shape, consistent with the experiment.

Three-level spaser for next-generation luminescent nanoprobe

The development of modern biological and medical science highly depends on advanced luminescent probes. Current probes typically have wide emission spectra of 30 to 100 nm, which limits the number of resolvable colors that are simultaneously labeled on samples. Spasers, the abbreviation for surface plasmon lasers, have ultranarrow lasing spectra by stimulated light amplification in the plasmon nanocavity. However, high threshold (>10² mJ cm^-2) and short lasing lifetime (approximately picoseconds to nanoseconds) still remain obstacles for current two-level spaser systems. We demonstrated a new type of a three-level spaser using triplet-state electrons. By prolonging the upper state lifetime and controlling the energy transfer, high gain compensation was generated. This probe, named delayed spasing dots (dsDs), about 50 to 60 nm in size, exhibited a spectral linewidth of ~3 nm, an ultralow threshold of ~1 mJ cm^-2, and a delayed lasing lifetime of ~10² μs. As the first experimental realization of the three-level spaser system, our results suggested a general strategy to tune the spasing threshold and dynamics by engineering the energy level of the gain medium and the energy transfer process. These dsDs have the potential to become new-generation luminescent probes for super-multiplex biological analysis without disturbance from short lifetime background emission.

Optics of exciton-plasmon nanomaterials

This review provides a brief introduction to the physics of coupled exciton-plasmon systems, the theoretical description and experimental manifestation of such phenomena, followed by an account of the state-of-the-art methodology for the numerical simulations of such phenomena and supplemented by a number of FORTRAN codes, by which the interested reader can introduce himself/herself to the practice of such simulations. Applications to CW light scattering as well as transient response and relaxation are described. Particular attention is given to so-called strong coupling limit, where the hybrid exciton-plasmon nature of the system response is strongly expressed. While traditional descriptions of such phenomena usually rely on analysis of the electromagnetic response of inhomogeneous dielectric environments that individually support plasmon and exciton excitations, here we explore also the consequences of a more detailed description of the molecular environment in terms of its quantum density matrix (applied in a mean field approximation level). Such a description makes it possible to account for characteristics that cannot be described by the dielectric response model: the effects of dephasing on the molecular response on one hand, and nonlinear response on the other. It also highlights the still missing important ingredients in the numerical approach, in particular its limitation to a classical description of the radiation field and its reliance on a mean field description of the many-body molecular system. We end our review with an outlook to the near future, where these limitations will be addressed and new novel applications of the numerical approach will be pursued.

Low-Loss Plasmonic Dielectric Nanoresonators

Material losses in metals are a central bottleneck in plasmonics for many applications. Here we propose and theoretically demonstrate that metal losses can be successfully mitigated with dielectric particles on metallic films, giving rise to hybrid dielectric-metal resonances. In the far field, they yield strong and efficient scattering, beyond even the theoretical limits of all-metal and all-dielectric structures. In the near field, they offer high Purcell factor (>5000), high quantum efficiency (>90%), and highly directional emission at visible and infrared wavelengths. Their quality factors can be readily tailored from plasmonic-like (∼10) to dielectric-like (∼10³), with wide control over the individual resonant coupling to photon, plasmon, and dissipative channels. Compared with conventional plasmonic nanostructures, such resonances show robustness against detrimental nonlocal effects and provide higher field enhancement at extreme nanoscopic sizes and spacings. These hybrid resonances equip plasmonics with high efficiency, which has been the predominant goal since the field's inception.