This PDF file contains the front matter associated with SPIE Proceedings Volume 11557, including the Title Page, Copyright information, and Table of Contents.

In this talk, we present our recent work on tuning the polarization states of light wave with 2D or 3D metastructures. First, we illustrate an approach to tune the phase difference of light via time retardation in a microstructured surface. Second, we demonstrate a general mechanism to construct the dispersion-free metastructure. Third, we present a freely tunable polarization rotator for broadband terahertz waves using a metastructure, and also an example on dynamically-switching the polarization state based on the phase transition of vanadium dioxide. The investigations provide some guidelines to control the polarization state of light at subwavelength scale.

The phase of an optical system being a circularly variable becomes undefined when the intensity vanishes, which is generally referred to as optical singularity. At the intensity vanishing point, singular phase or topological phase appears. In fact, optical singularity is pervasive in many physical phenomena such as vortex beams, reflection at Brewster’s angle, and perfect absorption and so on. Associated with the singularity point, the optical systems exhibit many nontrivial behaviors which could underpin tremendous nanophotonic applications. In this talk, I will present the utilization of such singular optics for metasurfaces as well as for far-field superresolution imaging. In the first part, we show that the losses of atomic thin layered materials can be used to create points of darkness (zero reflection). The singular phase behavior of the optical systems crossing the darkness point can lead to an abrupt phase jump of pi. Harnessing the Heaviside phase jump, we demonstrate atomic thin metasurface for light field manipulations. In the second part, we show that high-index dielectric nanostructures can support radiationless anapole state allowing vanishing far-field scattering accompanying with strong near fields. The unique feature can be utilized for giant photothermal nonlinear scattering modulations as well as application in far-field superresolution localization microscopy with an accuracy up to 40 nm.

Although many schemes have been proposed to manipulate propagating waves (PWs) and surface waves (SWs), usually each operation needs a different meta-device, being highly unfavorable for optics integrations. Here, we propose a scheme to design a single meta-device that can efficiently generate SWs and/or PWs with pre-designed wavefronts, illuminated by circularly polarized wave with different chirality. As a proof of concept, we design and fabricate a microwave meta-device and experimentally demonstrate that it can efficiently convert a CP wave with left or right handedness into a SW beam focused to a point or a SW beam travelling to a pre-designed direction. We further develop our scheme to design a meta-device and numerically demonstrate that it can either excite a SW beam or generate a far-field PW with pre-designed wavefront. Our work may stimulate simultaneous controls on far- and near-field EM environments based on a single ultra-compact platform.

This Conference Presentation, “Electromagnetic field manipulations in two-dimensional polaritonic crystals,” was recorded for the Photonics Asia 2020 Digital Forum.

Metasurfaces are well known for their functionality to control the reflection and refraction of waves via the introduction of subwavelength units along the surface. The efficiency is an important issue for enhancing metasurface functionalities. High efficiency metasurfaces in the transmission geometry have been recently designed by using various approaches including Huygen’s surface, etc. However, there are still severe limits in the present approaches. In this work, we demonstrate new mechanisms to realize transparent metasurfaces, leading to unprecedented new functionalities beyond current techniques. These advances in fundamental physics may also have important implications in practical applications.

This Conference Presentation, “Single molecule-doped crystalline nanosheet for delicate photophysics study and Gaussian mode single-photon emission,” was recorded for the Photonics Asia 2020 Digital Forum.

Weak intensity, low emission rate and poor emission directionality are three main challenges of quantum emitters. Here, we report on realizing enhanced photon emission from quantum dots mediated by plasmonic hybrid nanoantennas. Firstly, a plasmonic hybrid structure is proposed, where a silver nanocube is positioned at the center of a metallic concentric-ring structure, to simultaneously enhance the emission directionality and the decay rate of quantum dots embedded in the vertical nanogap while maintaining a high quantum efficiency. Secondly, a crystalline spherical silicon nanoparticle on metal nanoantenna is realized that can largely enhance the spontaneous emission intensity and the emission decay rate. A high quantum efficiency of over 80% is obtained. These hybrid nanoantennas can be combined with various nanoscale optical emitters and easily extended to form large area light-emitting devices for applications such as advanced display and wireless optical communication.

Epsilon-near-zero materials such as transparent conductive oxides (TCOs) have been attracting extensive research interests due to its extremely strong light field confinement. Combined with plasmonic/metasurface structures, remarkable intensity and phase modulations have been observed. However, the operation wavelength bandwidth is limited by the narrowband property of both the resonators and the ENZ effect. In this talk, two schemes to expand the bandwidth will be discussed. One is a dual-resonance coupling method, where the ENZ mode of TCOs is coupled to both magnetic resonance and FP resonance respectively by tuning the bias. The other scheme is a dual-ENZ coupling method, where the ENZ modes of both silicon and TCOs are coupled to the resonant modes of nanotrenches. Both schemes are found to significantly increase the modulation depth-bandwidth product. The results are useful to understand the physics in ENZ-resonance coupling and its operating mechanism in light modulations.

This paper performs detailed study of Raman scattering for platelets mixed with gold nanoparticles in presence with R6G molecules on quartz surface. Spectral properties of main spectral bands corresponding to proteins and dye in the complexes with gold nanoparticles have been performed. The perspectives SERS for applications in modern physics and biophotonics have been shown. Results of the study can be applied for SERS detection and investigation of blood components such as cells and platelets. Paper describes characteristic maxima of different cell components and its identification in platelets.

Maxwell's fish-eye lens is a special lens that can focus any point source perfectly. Such a lens is also called an Absolute Instrument, and has been widely used to design invisibility cloaks. In this talk, I will share our recent works along this line, both theoretically and experimentally. It includes self-focusing and Talbot effect, waveguide crossing, conformal caustics and singularities, and geodesic lenses.

This Conference Presentation, “Mechanics-enabled extreme nanofabrication for ultrasmall plasmonic nanogaps,” was recorded for the Photonics Asia 2020 Digital Forum.

Surface plasmon-mediated photocatalysis has attracted great interest. One model system is that p-aminothiophenol (PATP) could be dimerized into a new molecule of p,p′-dimercaptoazobenzene on noble metal surfaces under laser illumination. The mechanism of this catalytic reaction has caused wide discussion, but it is not fully understood yet. We performed a series of experiments for PATP and p-nitrothiophenol by tip-enhanced Raman spectroscopy and surface-enhanced Raman spectroscopy in a well-controlled gas environment (high vacuum, air, N2, and O2), and on different substrates (Au, Ag, Cu, Al, and corresponding oxides). The experimental results indicate that the electron acceptor or donor plays a decisive role in whether the reactions can occur or not.

Light-matter interactions can be hugely enhanced in plasmonic nanostructures by strong confinement of electromagnetic fields, which has incited many efforts to develop ultra-compact nonlinear devices among other types of nanoplasmonic devices. Previous work has focused upon wave mixing and modulation at quite low efficiencies. In this study, we reveal bistable switching in plasmonic antennas with a monolayer graphene or MoS2 sheet in their sub-nanometer hotspot junctions. The bistability derives from the interplay between a highly efficient wave mixing interaction and a strong positive feedback by the Purcell effect. We build a theoretical model that well reproduces the experiment results. The discovery of bistable plasmonic nano-switches is key to understanding and making efficient nonlinear plasmonic devices.

The utilization of surface plasmons (SPs) in the form of hot electrons has a great potential for applications in photodetection. Unfortunately, the metallic nanostructures usually support only narrowband plasmon resonances and the hot-electron thermalization loss results in an inefficient internal quantum process. Here, we demonstrate a broadband super absorber based on the metallic nanorod arrays (NRs). The average absorption across the entire visible band is up to 0.8 and the conversion efficiency is over 30-fold enhanced relative to the reference. Furthermore, considering the metallic nanostructures are usually complex with a high fabrication challenge, we present a purely planar hot-electron photodetector based on Tamm plasmons (TPs). More than 87% of the light incidence can be absorbed by the top metal layer. This enables a strong and unidirectional photocurrent and a photoresponsivity that can even be higher than that of the conventional nanostructured system.

We first investigate the nondipole effect of asymmetric quantum dots (QDs) in plasmonic antennas of shell, rod, triangle, and disk. The rod and triangle are found to have greater nondipole effect compared to shell and disk due to the larger electric field gradients resulting from high surface curvature. For a 3nm-radius QD adjacent to the rod end or the corner of triangle, the coupling strength of plasmon-QD interaction has a 10% enhancement for dipolar plasmonic mode. The multipole expansion shows that the enhancement is mainly contributed by the dipole-quadrupole transition interference, which will be suppressed in a symmetrical dimer structure. We further explore the nondipole effect in the plasmonic nanocavity formed by a gold nanotip and substrate. The nondipole effect is found to increase the quantum nonlinearities and lead to better single-photon purity. Our work demonstrates the impact of nondipole effect on plasmon-QD strong coupling and potential applications in quantum optics.

Conventional optical-resolution photoacoustic microscopy is limited by poor axial imaging resolution because of insufficient ultrasound detection bandwidth. Here we propose a polarization-differential surface plasmon sensor for photoacoustic detection, and realized an enhanced noise-equivalent-pressure sensitivity of ~120 Pa and a much broader photoacoustic bandwidth over 200 MHz, which provides an axial resolution of ~6.5 µm. We demonstrated that the capability in such micrometer-scale resolution enables in vivo volumetric label-free imaging of the microvasculature in not only the thin ear but also the thick forelimb of living mice. With advantages of reflection-mode signal capture, improved photoacoustic detection sensitivity and bandwidth, our system offers more opportunities for biomedical investigation.

The plasmon effect is of great significance for photoemission in metallic nanostructure. We introduced the photoemission electron microscope (PEEM) in detail, and used it to study the effects of polarization on the far-field and near-field of the plasmon. We further investigate the photoelectron energy spectrum obtained by PEEM and demonstrated the spatial distribution of photoelectrons with different energies. These experimental results help us to further understand the mechanism of photoemission and laid the foundation for the future development of plasmon device and technology.

Accurately grasping and controlling the plasmon dynamics and dephasing time is a prerequisite for the application of plasmons. Here, we report on the investigation of dynamics and dephasing time of different plasmonic hot spots in a single bowtie structure under varied light polarization using time-resolved photoemission electron microscopy (PEEM). In contrast to those previous global-parameter descriptions, we here report the experimental observation of apparently spatially diverse plasmon dynamic characteristics and spatially different dephasing time within a plasmonic bowtie. We experimentally obtain different plasmon dynamics in the tips of the bowtie nanostructure with different light polarization and actively control dephasing time by changing the light polarization which transforms the plasmon mode. Experimental results got the minimum dephasing time of 8.5fs and the maximum dephasing time of 17fs, which has a large adjustment range. In addition, we found that structural defects can prolong the dephasing time, and we analyzed its role in the influence of plasmon dynamics and dephasing time.

The theory of compound zoom system and its influence and control on aberration are the basis for the design of such systems and have practical value in engineering. The principle, modeling and error analysis are analyed. Based on the system configuration of the compound zoom system, moreover, this paper advances the algorithm analysis.

The plasma induced when laser interacts with matter (solid, gas, etc.) can radiate a wide range of electromagnetic waves. Its electromagnetic radiation bands range from extreme ultraviolet, ultraviolet, visible, infrared, terahertz to radio frequency microwave. Radiation in these wavelength bands has a wide range of applications, so it is of great significance to study laser plasma radiation. We studied the characteristics of nanosecond laser (1064nm, 8ns) induced plasma optical radiation. The influence of the laser parameters on the plasma radiation intensity and the time evolution of the radiation were obtained. Furthermore, we the obtained effect of the characteristics of the target on the radiation characteristics of the plasma. Finally, we calculated the electron temperature of the air plasma. The experimental results show that: the linear spectrum in the visible spectrum of laser-induced air plasma is mainly the ion spectrum of nitrogen and oxygen; as the laser energy increases the intensity of visible light radiation of air and Al plasma is gradually increasing; when the delay is 15ns, the spectral line intensity reaches the maximum; as the laser energy increases, the plasma electron temperature increases.

In this study, by using numerical simulations and scanning near-field optical microscopy, we investigate the excitation of HPhPs in MoO₃ with plasmonic nanoantennas. We find that the excitation efficiency is strongly dependent on the antenna length and orientation. By changing the size, shape and direction of the metal antenna, the excitation of the phonon Polariton mode can be controlled.

Hybrid system composed of metal nanoparticles and two-dimensional transition metal dichalcogenides (TMDs) is important for studying the light-matter interaction at the nanoscale. In particular, the 2D excitons are very sensitive to external stimulus, allowing their active control via temperature scanning, gating, and optical excitation. In comparison with other active tuning strategies, chemical treatment is another effective method. In this study, we realize tuning of the resonance coupling, through a chemical approach, in heterostructure composed of an individual gold nanorod integrated with monolayer WS₂ flake. Treating the heterostructure with an organic superacid: bis(trifluoromethane)sulfonimide (TFSI), the photoluminescence intensity can be enhanced by 7 times. The coupled system has enhanced energy splitting about 50 meV. The chemical treatment based on the TFSI solution provides a new approach for tuning the resonance coupling effect.

A metamaterial perfect absorber working in the ultraviolet-to-near infrared region is proposed. It consists of a periodic nanoarray of quadrangular frustum pyramids with a homogeneous titanium nitride (TiN) film as the ground. Each unit cell of the nanoarray is composed of alternately stacked TiN and dielectric patches with their width tapered linearly from the bottom to the top. The absorption mechanism was qualitatively explained by analyzing the electric and magnetic field distributions at the resonance wavelengths, and the absorption performance was simulated as a function of its geometric parameters and the angle and polarization of incident beam as well. Optimal geometric parameters are presented. With the optimal parameters, the absorption efficiency at normal incidence is above 99.3% over the entire spectral region from 300 to 2500 nm, and a high average absorption of 99.9% is achieved. The ultrabroadband perfect absorption behaviors are attributed to the localized surface plasmonic resonance effect of the pyramid, the intrinsic loss of the TiN material and the coupling of resonance modes between two adjacent pyramids. In addition, the absorber also shows polarization insensitivity and good angular acceptance capability of oblique incidence up to 70°. The unique and compact structure provides a new way for achieving an ultrabroadband solar absorber, showing great application prospects in the fields of solar energy harvesting and the corresponding (thermo-)photovoltaics.

Laser induced breakdown spectroscopy(LIBS)was an analysis technique based on laser plasma atomic emission spectroscopy. Because of its fast, non-destructive and high sensitivity, it was widely used in various fields. In order to study the laser induced breakdown spectra of heavy metal element and non-metal element in the soil, A method based on traditional laser induced breakdown spectroscopy (LIBS) technology with cavity constraints was proposed to improve the plasma emission spectrum. In this paper, the cylindrical aluminum cavity with the diameter of 2mm, 3mm, 4mm, 5mm, and 6mm at the same thickness were placed on the surface of the soil sample, the laser energy was set to 30mJ, and the pulse repetition frequency was set to 5Hz. The plasma lines of heavy metal element (Ba II 455.4nm) and non-metal element (Si I 288.158nm) under different cavity constraints were obtained, and the enhancement factor and signal-to-noise ration(SNR)of these two characteristic lines under different diameter cavity constraints were studied. The experimental results show that the plasma line has a certain enhancement under the condition of cavity restriction than without cavity restriction. As the diameter of the circular cavity increases, the enhancement factor tends to increase first and then decrease, and the spectral enhancement effect is the best when the diameter is 5mm. The change trend of the signal-to-noise ratio is consistent with the enhancement factor, which reaches the maximum when the cavity diameter is 5 mm. The study of this technology can provide guidance for the qualitative analysis of Ba and Si elements in soil.

In this paper, the efficiency of plasmon-driven catalytic reaction on double nanowires was investigated experimentally. The dimerization of 4-nitrobenzenethiol (4NBT) to p,p’-dimercaptoazobenzene (DMAB) can be driven by hot electrons generated by the non-radiative decay of surface plasmons, which can be monitored by Raman spectroscopy. It was demonstrated under the same excitation conditions, the Raman signal of the reactant molecules on the double nanowires was significantly stronger than that on the single nanowire. So does the catalytic reaction efficiency. The electromagnetic field intensity distribution of a single nanowire and double nanowires under the same excitation condition was analyzed through the finite difference time domain (FDTD) simulation . It was shown that the stronger “hot spot ” in the double nanowire part is induced. The theoretical and experimental results were consistent with each other. This research may provide a new way for catalytic reactions driven by double nanowire plasmonic structure.