Confined three-dimensional plasmon modes inside a ring-shaped nanocavity on a silver film imaged by cathodoluminescence microscopy

Theoretical and computational methods for tip- and surface-enhanced Raman scattering

Raman spectroscopy is a versatile tool for acquiring molecular structure information. The incorporation of plasmonic fields has significantly enhanced the sensitivity and resolution of surface-enhanced Raman scattering (SERS) and tip-enhanced Raman spectroscopy (TERS). The strong spatial confinement effect of plasmonic fields has challenged the conventional Raman theory, in which a plane wave approximation for the light has been adopted. In this review, we comprehensively survey the progress of a generalized theory for SERS and TERS in the framework of effective field Hamiltonian (EFH). With this approach, all characteristics of localized plasmonic fields can be well taken into account. By employing EFH, quantitative simulations at the first-principles level for state-of-the-art experimental observations have been achieved, revealing the underlying intrinsic physics in the measurements. The predictive power of EFH is demonstrated by several new phenomena generated from the intrinsic spatial, momentum, time, and energy structures of the localized plasmonic field. The corresponding experimental verifications are also carried out briefly. A comprehensive computational package for modeling of SERS and TERS at the first-principles level is introduced. Finally, we provide an outlook on the future developments of theory and experiments for SERS and TERS.

The Reststrahlen Effect in the Optically Thin Limit: A Framework for Resonant Response in Thin Media

Sharp resonances can strongly modify the electromagnetic response of matter. A classic example is the Reststrahlen effect - high reflectivity in the mid-infrared in many polar crystals near their optical phonon resonances. Although this effect in bulk materials has been studied extensively, a systematic treatment for finite thickness remains challenging. Here we describe, experimentally and theoretically, the Reststrahlen response in hexagonal boron nitride across more than 5 orders of magnitude in thickness, down to a monolayer. We find that the high reflectivity plateau of the Reststrahlen band evolves into a single peak as the material enters the optically thin limit, within which two distinct regimes emerge: a strong-response regime dominated by coherent radiative decay and a weak-response regime dominated by damping. We show that this evolution can be explained by a simple two-dimensional sheet model that can be applied to a wide range of thin media.

Linewidth reduction effect of a cavity-coupled dual-passband plasmonic filter

We propose a novel cavity-coupled MIM nano-hole array structure that exhibits a tunable dual passband in the near-infrared regime. When compared with the traditional single metal film, the designed structure provides a coupling effect between Gspp and SPP to significantly reduce the linewidths of the two transmission peaks. We also reveal the physical origin of the positive and negative influence of the cavity effect on the transmission of high-frequency and low-frequency peaks. This work supplies a new modulation theory for plasmonic devices based on the EOT phenomenon and has a wide application prospect in the fields of infrared sensor, plasmonic filter, and hyperspectral imaging.

Effects of Plasmon Modes on Resonant Raman Images of a Single Molecule

Localized surface plasmons (LSPs) are excellent light sources at the nanoscale. How to precisely describe the interaction between LSPs and molecules has become an important issue. We present here a comprehensive study on the dependence of resonant Raman images on LSP modes generated by two typical nanostructures. Theoretical calculations demonstrate that the Raman images are sensitive to not only the spatial distribution but also the phase of the localized field, which should be attributed to the quantum nature of the interaction between LSP modes and molecules. We also find that the rotation of noncylindrical symmetry modes could affect the details of images, which offers an extra means to extract molecular information. Our findings extend the understanding of the LSP-matter interaction, which would be useful for the rational design of nanostructures and thus further applications of LSPs.

Cathodoluminescence nanoscopy of open single-crystal aluminum plasmonic nanocavities

Exact understanding of the plasmon response of aluminum (Al) nanostructures in deep subwavelengths is critical for the design of Al based plasmonic applications, such as the emission control of quantum dots and surface-enhanced resonance Raman scattering in the ultraviolet (UV) range. Here, the plasmonic properties of open triangle cavities patterned by a focused ion beam in single-crystal bulk Al were explored using cathodoluminescence. The resonant modes were determined by experimental spectra and deep subwavelength real-space mode patterns ranging from the visible to the UV, which agreed well with full-wave electromagnetic simulations. The dispersion relation of the cavity modes was consistent with that at the interface between Al and vacuum, showing strong electromagnetic field confinement in the cavities. Open Al triangle cavities provided room for the interaction between optical emitters and confined electromagnetic fields, paving the way for plasmonic devices for a variety of applications, such as plasmonic light-emitting devices or nanolasers in the UV range.

Free electrons excited SPASER

Surface plasmon amplification by stimulated emission of radiation (SPASER) is discovered and used for realizing lasers at nanometer scale. The conventional gain media that are applied in SPASER are solid materials, such as organic dye or semiconductor, which limits the frequency range of SPASER. The free electrons could be considered as a kind of gain medium for emitting radiation. Here, we investigate theoretically the SPASER, which is excited by free electrons. We also demonstrate the tunable, deep-ultraviolet, and ultracompact laser numerically by having free electrons interact with surface plasmon polariton mode supported on metal surface. The output power density could reach about 30 W/μm² and the wavelength in deep ultraviolet could be widely tuned by varying the electron energy. This work offers a way of realizing integrated free electron laser in the ultraviolet frequency region.

Investigation of plasmon resonance in metal/dielectric nanocavities for high-efficiency photocatalytic device

Photocatalytic nanostructures loaded with metallic nanoparticles are being considered as a potential candidate for designing efficient water splitting devices. Here, we aim to unveil the plasmonic behavior of a device made of Au-TiO₂ nanostructures through in-depth investigations combining electron energy loss spectroscopy (EELS) and cathodoluminescence (CL). The experiments confirm the existence of Au bulk plasmon excitation, intrinsic interband transitions, and plasmon losses over a wide range of energies (0.6-2.4 eV). Depending on the size and the shape of the obtained nanostructures, such as fishing hook (FH), asymmetric nanorod (AR), and a/symmetric nanoparticles, in our devices, the dephasing times and the quality factors of the modes vary. Finite difference time domain simulations were then carried out on FH and AR structures. These simulations indicate good agreement between the electric field enhancement and the obtained plasmon excitation as observed in EELS. Moreover, the plasmonic activity obtained by CL and EELS was correlated with the photocurrent measurements recorded with the device, which confirmed that the localized plasmons in Au generate hot electrons and enhance the photoresponse of the device. This study confirms the functionality of the metal dielectric photocatalyst device over a wide range of wavelengths ranging from UV to near IR.

Unveiling quasi-dark surface plasmon modes in Au nanoring cavities by cathodoluminescence

Spectral resolving and imaging surface plasmon modes in noble metal nanostructures are important for applications in nanophotonics. Here, we use cathodoluminescence (CL) spectroscopy to excite and probe quasi-dark plasmon modes of Au nanoring cavities. Numerical simulations of both the spectra and the electromagnetic field distribution are carried out by using boundary element method. Good agreement between the experimental and simulated results is obtained. Particularly, CL is shown as an efficient method to probe quadrupole modes, which is difficult for traditional optical means. Moreover, a high Purcell factor in excess of 100 is obtained for the dark quadrupole modes in gold ring cavities. Our work provides an efficient way to explore the initial nature of surface plasmon modes in metal nanostructures.

Large Absorption Enhancement in Ultrathin Solar Cells Patterned by Metallic Nanocavity Arrays

A new type of light trapping structure utilizing ring-shaped metallic nanocavity arrays is proposed for the absorption enhancement in ultrathin solar cells with few photonic waveguide modes. Dozens of times of broadband absorption enhancement in the spectral range of 700 to 1100 nm is demonstrated in an ultrathin Si₃N₄/c-Si/Ag prototype solar cell by means of finite-difference time-domain (FDTD) simulation, and this dramatic absorption enhancement can be attributed to the excitation of plasmonic cavity modes in these nanocavity arrays. The cavity modes optimally compensate for the lack of resonances in the longer wavelength range for ultrathin solar cells, and eventually a maximum J_sc enhancement factor of 2.15 is achieved under AM 1.5G solar illumination. This study opens a new perspective for light management in thin film solar cells and other optoelectronic devices.

Probing plasmons in three dimensions by combining complementary spectroscopies in a scanning transmission electron microscope

The nanoscale optical response of surface plasmons in three-dimensional metallic nanostructures plays an important role in many nanotechnology applications, where precise spatial and spectral characteristics of plasmonic elements control device performance. Electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) within a scanning transmission electron microscope have proven to be valuable tools for studying plasmonics at the nanoscale. Each technique has been used separately, producing three-dimensional reconstructions through tomography, often aided by simulations for complete characterization. Here we demonstrate that the complementary nature of the two techniques, namely that EELS probes beam-induced electronic excitations while CL probes radiative decay, allows us to directly obtain a spatially- and spectrally-resolved picture of the plasmonic characteristics of nanostructures in three dimensions. The approach enables nanoparticle-by-nanoparticle plasmonic analysis in three dimensions to aid in the design of diverse nanoplasmonic applications.

Fabrication of metallic nanopatterns with ultrasmooth surface on various substrates through lift-off and transfer process

The smooth surface of the metallic nanostructure is essential for the propagation of surface plasmon polaritons. In this paper, we present a novel method to fabricate the metallic nanopatterns with ultra-smooth surface on various substrates. By using a silica film as the sacrificial layer, we show that the prefabricated metallic nanopatterns produced by electron beam lithography and film deposition can be hydrolyzed and transferred onto a designated substrate. The ultra-smooth surface morphology of nanopatterns has been characterized and verified by scanning electron microscopy and atomic force microscopy. More importantly, we demonstrate that this method can successfully produce a variety of nanostructures with high product yield, even onto the uneven substrate. The results indicate that our proposed method is a promising and versatile means to fabricate multiplicate smooth metallic nanostructure on various substrates for the application of nanophotonic devices.

Low-loss, extreme subdiffraction photon confinement via silicon carbide localized surface phonon polariton resonators

Plasmonics provides great promise for nanophotonic applications. However, the high optical losses inherent in metal-based plasmonic systems have limited progress. Thus, it is critical to identify alternative low-loss materials. One alternative is polar dielectrics that support surface phonon polariton (SPhP) modes, where the confinement of infrared light is aided by optical phonons. Using fabricated 6H-silicon carbide nanopillar antenna arrays, we report on the observation of subdiffraction, localized SPhP resonances. They exhibit a dipolar resonance transverse to the nanopillar axis and a monopolar resonance associated with the longitudinal axis dependent upon the SiC substrate. Both exhibit exceptionally narrow linewidths (7-24 cm(-1)), with quality factors of 40-135, which exceed the theoretical limit of plasmonic systems, with extreme subwavelength confinement of (λ(res)3/V(eff))1/3 = 50-200. Under certain conditions, the modes are Raman-active, enabling their study in the visible spectral range. These observations promise to reinvigorate research in SPhP phenomena and their use for nanophotonic applications.

Radiation-suppressed plasmonic open resonators designed by nonmagnetic transformation optics

How to confine light energy associated with surface plasmon polaritons (SPPs) in a physical space with minimal radiation loss whereas creating maximum interacting section with surrounding environment is of particular interest in plasmonic optics. By virtue of transformation optics, we propose a design method of forming a polygonal surface-plasmonic resonator in fully open structures by applying the nonmagnetic affine transformation optics strategy. The radiation loss can be suppressed because SPPs that propagate in the designed open structures will be deceived as if they were propagating on a flat metal/dielectric interface without radiation. Because of the nonmagnetic nature of the transformation strategy, this design can be implemented with dielectric materials available in nature. An experimentally verifiable model is subsequently proposed for future experimental demonstration. Our design may find potential applications in omnidirectional sensing, light harvesting, energy storage and plasmonic lasing.

Plasmonic whispering gallery cavities as optical nanoantennas

We study the resonant modes of surface plasmon whispering gallery cavities based on a circular groove in a Au surface. We use spatially, angle-, and polarization-resolved cathodoluminescence spectroscopy to measure the resonant plasmonic local field distribution at deep-subwavelength resolution and determine the far-field radiation distribution for each plasmonic mode. We show mode-selective excitation of the plasmonic modes and resolve the modal angular radiation pattern. The results show that plasmonic whispering gallery resonators can be used as versatile antennas both in receiving and transmitting mode.

Three-dimensional subwavelength confinement of terahertz electromagnetic surface modes in a coupled slit structure

We report on the three-dimensional subwavelength confinement of the electromagnetic waves at a coupled metallic slit structure beyond diffraction limit in terahertz region. Lateral confinement behavior, leading to the three-dimensional confinement, is caused by a strong funneling effect of the light which occurs at the intersection of slits with a sharp metal geometry. Tunability of the resonant frequency and the position of the light confinement is achieved by controlling the slit length and the position of the intersection of slits, respectively.

Dispersion control in plasmonic open nanocavities

We experimentally demonstrate the dependence of plasmonic resonant properties on cavity height in open circular cylinder nanocavities as a result of a strong three-dimensional confinement of the electromagnetic field, which shows a new way to tailor the dispersion of surface plasmon polaritons in cavities. The azimuthal and the axial symmetric plasmonic mode patterns are directly observed at resonant wavelengths using cathodoluminescence spectroscopy. Plasmonic modes and optical vertical cavity modes can be simultaneously excited and can coexist in a nanocavity with sufficient height. The highest quality factor, which is up to 73, is obtained in a 500 nm high cavity. The smallest mode volume is only 0.031 λ(SPP)(3), and the corresponding Purcell factor is 71. Open nanocavities provide space for the interaction between an optical emitter and a confined electromagnetic field. Many applications can be expected, such as plasmonic light-emitting devices and nanolasers.

Vertical plasmonic resonant nanocavities

We demonstrate plasmonic modes in a vertical nanocavity with an air output window at the top surface and Ag reflectors. The resonances of surface plasmon polaritons are investigated using cathodoluminescence spectroscopy. The resonant modes are determined by comparing experiment and theoretical simulations. The plasmon dispersion relation in the vertical nanocavities shows a strong confinement to the electromagnetic field, and the smallest modal volume is only 0.0014 μm(3). Our work provides insights into the development of nanoscale plasmonic vertical cavity surface-emitting lasers.