Plasmon resonance tuning in metallic nanocavities

Probing plexciton emission from 2D materials on gold nanotrenches

Probing strongly coupled quasiparticle excitations at their intrinsic length scales offers unique insights into their properties and facilitates the design of devices with novel functionalities. In this work, we investigate the formation and emission characteristics of plexcitons, arising from the interaction between surface plasmons in narrow gold nanotrenches and excitons in monolayer WSe2. We study this strong plasmon-exciton coupling in both the far-field and the near-field. Specifically, we observe a Rabi splitting in the far-field reflection spectra of about 80 meV under ambient conditions, consistent with our theoretical modeling. Using a custom-designed near-field probe, we find that plexciton emission originates predominantly from the lower-frequency branch, which we can directly probe and map its local field distribution. We precisely determine the plexciton's spatial extension, similar to the trench width, with nanometric precision by collecting spectra at controlled probe locations. Our work opens exciting prospects for nanoscale mapping and engineering of plexcitons in complex nanostructures with potential applications in nanophotonic devices, optoelectronics, and quantum electrodynamics in nanoscale cavities.

Integrating collapsible plasmonic gaps on near-field probes for polarization-resolved mapping of plasmon-enhanced emission in 2D material

Scanning near-field optical microscopy (SNOM) is an important technique used to study the optical properties of material systems at the nanoscale. In previous work, we reported on the use of nanoimprinting to improve the reproducibility and throughput of near-field probes including complicated optical antenna structures such as the 'campanile' probe. However, precise control over the plasmonic gap size, which determines the near-field enhancement and spatial resolution, remains a challenge. Here, we present a novel approach to fabricating a sub-20 nm plasmonic gap in a near-field plasmonic probe through the controlled collapse of imprinted nanostructures using atomic layer deposition (ALD) coatings to define the gap width. The resulting ultranarrow gap at the apex of the probe provides a strong polarization-sensitive near-field optical response, which results in an enhancement of the optical transmission in a broad wavelength range from 620 to 820 nm, enabling tip-enhanced photoluminescence (TEPL) mapping of 2-dimensional (2D) materials. We demonstrate the potential of this near-field probe by mapping a 2D exciton coupled to a linearly polarized plasmonic resonance with below 30 nm spatial resolution. This work proposes a novel approach for integrating a plasmonic antenna at the apex of the near-field probe, paving the way for the fundamental study of light-matter interactions at the nanoscale.

On the giant enhancement of light in plasmonic or all-dielectric gratings containing nano-voids

We report the possibility of generating tremendous light-field enhancements within shallow nano-trenches made in a high-index dielectric material because of resonant behaviors that are reminiscent of what we get with sub-wavelength plasmonic cavities. High quality factors are explained through a modal analysis which gives explicit design rules. The thin dielectric void gratings could be a relevant alternative to plasmon-based devices for chemical sensing, or could inspire novel efficient wavelength-selective photo-absorbers by taking poorly absorbing materials.

Strong and weak couplings in molecular vibration-plasmon hybrid structures

Molecular vibration-plasmon couplings in a hybrid structure, which are composed of a silver grating filled with polymethyl methacrylate (PMMA) molecules (SG-PMMA), have been investigated theoretically. It is found that the interaction between the vibrational transitions and plasmons can transform from weak coupling into strong coupling by reducing the distance between the elements. When the space between grating elements is large, the localized surface plasmon resonance (LSP) of the silver elements greatly enhances the absorption of the PMMA molecules. As the gap between elements becomes small, the plasmonic nanocavity (NC) mode emerges and couples strongly with the molecular vibrational mode of PMMA. The strong coupling results in two new hybridized modes and the Rabi splitting energy is about 15 meV. Our work provides an effective way to alter the coupling strength of the molecular vibration-plasmon hybrid system and may be beneficial to the further biochemical and biophysical applications.

Ultranarrow-band metagrating absorbers for sensing and modulation

Nanostructured plasmonic metamaterials are an excellent platform for narrowband optical absorption, which has wide applications in sensing, filtering, modulation, and emission tailoring. However, achieving a subnanometer absorption bandwidth for optical sensing and dynamical control of light is still challenging. Here, we propose an asymmetric metagrating structure and make use of the propagating surface plasmonic mode that has a small dissipation rate, to achieve perfect optical absorption with a bandwidth of 0.28 nm near the wavelength of 1.55 μm. Our proposed structure can be used in solution environments as a chemical or biological sensor in the visible spectral range just by changing the structural parameters. The sensor possesses a sensitivity of 440 nm/RIU and figure of merit of 1333.33 RIU^-1. In addition, by combining an organic electro-optic material with this metagrating, our device can be reconfigurable with a dynamic range of 15.52 dB. Therefore, our proposed metagrating platform not only works as an ultranarrow-band absorber, but also can be employed for optical sensing and dynamic control of light.

Plasmonic Sensor Based on Dielectric Nanoprisms

A periodic array of extruded nanoprisms is proposed to generate surface plasmon resonances for sensing applications. Nanoprisms guide and funnel light towards the metal-dielectric interface where the dielectric acts as the medium under test. The system works under normal incidence conditions and is spectrally interrogated. The performance is better than the classical Kretschmann configurations, and the values of sensitivity and figure of merit are competitive with other plasmonic sensor technologies. The geometry and the choice of materials have been made taking into account applicable fabrication constraints.

UV-visible broadband wide-angle polarization-insensitive absorber based on metal groove structures with multiple depths

A new periodic Al groove structure coated by SiO₂ thin film is designed and numerically investigated for wide-angle and polarization-insensitive broadband absorption. A metal groove array presents optical absorption enhancement due to the cavity mode resonance, with the absorption peak capable of being shifted by controlling the depth of the metal groove. Broadband absorption can be realized by a periodic array of metal grooves with different depths combined in one single period. A two-dimensional Al structure with four different grooves in each period is designed to realize polarization-insensitive broadband absorption from 220 nm to 800 nm with average absorption efficiency over 80% within the incident angle of 40°. These wide-angle and broadband absorption structures can be applied in UV/visible-related biochemical sensors, solar cells, or photocatalysts.

Infrared Perfect Ultra-narrow Band Absorber as Plasmonic Sensor

We propose and numerically investigate a novel perfect ultra-narrow band absorber based on a metal-dielectric-metal-dielectric-metal periodic structure working at near-infrared region, which consists of a dielectric layer sandwiched by a metallic nanobar array and a thin gold film over a dielectric layer supported by a metallic film. The absorption efficiency and ultra-narrow band of the absorber are about 98 % and 0.5 nm, respectively. The high absorption is contributed to localized surface plasmon resonance, which can be influenced by the structure parameters and the refractive index of dielectric layer. Importantly, the ultra-narrow band absorber shows an excellent sensing performance with a high sensitivity of 2400 nm/RIU and an ultra-high figure of merit of 4800. The FOM of refractive index sensor is significantly improved, compared with any previously reported plasmonic sensor. The influences of structure parameters on the sensing performance are also investigated, which will have a great guiding role to design high-performance refractive index sensors. The designed structure has huge potential in sensing application.

Optical resonance problem in metamaterial arrays: a lattice dynamics approach

A systematic dynamic theory is established to deal with the optical collective resonance in metamaterial arrays. As a reference model, we consider an infinite split ring resonator (SRR) array illuminated by a linearly polarized wave and introduce an N-degree-of-freedom forced oscillator equation to simplify the coupled-mode vibration problem. We derive a strict formula of resonance frequency (RF) and its adjustable range from the steady-state response. Unlike a single SRR possesses invariant RF, it successfully explains the mechanism of RF shift effect in the SRR array when the incident angle changes. Instead of full wave analysis, only one or two adjacent resonance modes can give an accurate response line shape. Our approach is applicable for metallic arrays with any N-particle cell at all incident angles and well matched with numerical results. It provides a versatile way to study the vibration dynamics in optical periodic many-body systems.

Fluid-controlled tunable infrared filtering in hollow plasmonic nanofin cavities

Subwavelength structures sustaining surface plasmons have been employed in numerous fields due to their small size and ability to manipulate light beyond the diffraction limit. Light filtering using small-size plasmonic devices is a promising means of portable spectroscopy for purposes such as on-site chemical analyses. However, most plasmonic filters can only tune the resonance band by modifying the geometry of the structure or changing the incident light angle. Here, we present a plasmonic nanofin-cavity structure having a narrow band with its resonance wavelength controlled by varying the fluid in the hollow cavities of the filter. Control of the narrow-band resonance is realized over a wide range because of the coupling between the stationary surface plasmons generated from the nanofin-cavity mode and the propagating surface plasmons. The hollow cavity design enables fluid to be easily injected and removed, so that the filtered band can be controlled without the need for a complex and bulky structure or application of an external voltage.

Nanoscale TiO2 dielectric resonator absorbers

We demonstrate a narrow-band plasmonic absorber based on a uniform array of nanoscale cylindrical dielectric resonators (DRs) on a metallic substrate at visible frequencies. Under a normally incident plane-wave excitation, the DRs resonate in their horizontal magnetic dipolar mode, which can be seen as localized plasmonic hot spots. Such a localized resonance also couples incident waves into surface plasmon polaritons (SPPs) bidirectionally, and perfect absorption is achieved by creating SPP standing waves. The simulation shows perfect absorption at 633 nm and 1.8% relative bandwidth with >90% absorption, while the measurement demonstrates maximum absorption of 90% at 636 nm. Both simulation and measurement results are analyzed with coupled mode theory. An additional numerical study elaborates on the dependence of absorption on the resonator size, period, and incidence angle.

Monochromatic X-Ray Induced Novel Synthesis of Plasmonic Nanostructure for Photovoltaic Application

It has been universally delineated that the plasmonic metal nanoparticles can enhance the efficiency of photovoltaic cell by increasing the probability of energetic solar photons capturing phenomena using localized surface plasmonic resonance response. In this paper, we developed a novel in-situ simple approach to synthesize noble plasmonic silver nanoparticles (AgNP) from aqueous poly-vinyl-pyrrolidone solution of metal salt using radiolysis of water via synchrotron monochromatic X-ray irradiation without any chemical reducing agent. X-ray irradiation of water produces hydrated electrons (e(-)aq), superoxide (O(-)2) and atom radicals H*, which triggers the reaction and reduces metal salt. X-ray radiolysis based synthesis provides the control over the reaction and prevent the formation of secondary products as occurs in case of chemical reduction route. In the previous studies, synchrotron "white" X-rays had been examined for the synthesis of metal nanoparticles, but that technique limits only upto the material synthesis while in this work we explored the role of "monochromatic" X-rays for the production of bulk amount of nanoparticles which would also provide the feasibility of in-situ characterization. Transmission electron micrographs show that the synthesized AgNP appears spherical with diameter of 2-6 nm and is in agreement with the size estimation from uv-vis spectra by "Mie theory".

Polarization-independent and omnidirectional nearly perfect absorber with ultra-thin 2D subwavelength metal grating in the visible region

A polarization-independent and omnidirectional nearly perfect absorber in the visible region has been proposed. The absorber is two-layer structure consisting of a subwavelength metal grating layer embedded in the high refractive index and lossless dielectric layer on the metal substrate. Extraordinary optical absorption with absorption peaks of over 99% can be achieved over the whole visible region for both TM and TE polarization. This absorption is attributed to cavity mode (CM) resonance caused by the coupled surface plasmon polaritons (SPP). Through adjusting the grating thickness, the absorption peak can be tuned linearly, which is highly advantageous to design various absorbers. Furthermore, the absorbance retains ultra-high over a wide angular range of incidence for both TM and TE polarization. This nearly perfect absorber offers great potential in the refractive index (RI) sensors, integrated photodetectors, solar cells and so on.

Distinguishing plasmonic absorption modes by virtue of inversed architectures with tunable atomic-layer-deposited spacer layer

We demonstrated the distinguishing between plasmonic absorption modes by exploiting an inversed architecture with tunable atomic-layer-deposited dielectric spacer layer. The dielectric spacer layer was manipulated between the bottom metal-nanoparticle monolayer and the upper metal film to inspect the contributions of metal nanoparticles and dielectric film in a step-by-step manner. The experimental and simulated differences between the two peak absorption positions (Δf) and between the corresponding half width at half maxima (Δw) confirmed the evolutions of gap plasmon and interference-enhanced local surface plasmon resonance absorption modes in the plasmonic metamaterial absorbers (PMAs), which were useful for understanding the underlying mechanism of amorphous PMAs.

Plasmon resonance tuning in metal nanostars for surface enhanced Raman scattering

We report the fabrication of Au nanostar arrays by means of electron beam lithography, in which the plasmon resonance energy can be tuned via the nanostar size from the visible into the near-infrared region. The spectral response of the nanostar arrays was investigated by optical extinction (transmittance) experiments, and their surface enhanced Raman scattering performance has been tested at two different excitation wavelengths, 633 nm and 830 nm, using chemisorbed Cresyl violet molecules as analyte. The experimental results are supported by numerical simulations of the spatial and spectral electric field enhancement.

Direct and accurate patterning of plasmonic nanostructures with ultrasmall gaps

We report an improved method to directly and accurately fabricate plasmonic nanostructures with ultrasmall gaps. The fabrication is based on high-resolution focused ion beam milling with closely packed nanoring patterns. With fine and precise adjustment of the ion beam, elegant plasmonic nanostructures with ultrasmall dimensions down to 10 nm are achieved. We also show that the gap dimensions have a strong effect on the optical reflectance and transmittance of the plasmonic nanostructures. Measured results show reasonable agreement with finite-difference time-domain calculations. Our approach could find promising applications in plasmon-assisted sensing and surface-enhanced spectroscopy.