Volume 9, Issue 4

The emergence of metamaterials (MMs) has led to groundbreaking photo-physical phenomena, which arise from their novel structure-dependent properties. Consisting of “meta-atom” building blocks, MMs can be organized into subwavelength metal/dielectric structures using bottom-up or top-down nanofabrication techniques. Optical metal metasurfaces are a class of MMs with macroscopic lateral dimensions but composed of one to few subwavelength layers of precisely oriented metal-based elements over a large surface area. In this review, we focus on gold metasurfaces, highlighting their fabrication methods, morphological characterization, as well as linear and nonlinear optical properties. We then review our recent work on fabricating and characterizing self-assembled gold metasurface. An interesting characteristics of the gold metasurfaces is their wide range of linear refractive indices, varying from n 0  ~ 0.5 in the visible to n 0  ~ 4 in the short wavelength infrared spectral region. Third-order nonlinearities are characterized by the Z-scan technique at wavelengths on- and off-plasmonic resonance of the gold metasurface. Experimental results on the relationship between the third-order nonlinearity of the self-assembled gold metasurface as a function of the linear response are presented for the first time. We conclude by discussing the potential applications and future outlook of self-assembled plasmonic metasurfaces.

Absorption, scattering, and fluorescence are processes that increase with electric field intensity. The most prominent way to enhance electric field intensity is to use localized or propagating surface plasmon polaritons (SPPs) based on metallic particles and nanostructures. In addition, several other, much less well-known, photonic structures that increase electric field intensity exist. Interference enhancement provided by thin dielectric coatings on reflective substrates is able to provide electric field intensity enhancement over the whole substrate and not only at certain hotspots, thereby being in particular suitable for the spectroscopy of thin surface layers. The same coatings on high refractive index substrates may be used for interference-enhanced total internal reflection-based spectroscopy in much the same way as Kretschmann or Otto configuration for exciting propagating SPPs. The latter configurations can also be used to launch Bloch surface waves on 1D photonic crystal structures for the enhancement of electric field intensity and thereby absorption, scattering, and fluorescence-based spectroscopies. High refractive index substrates alone can also, when nanostructured, enhance infrared absorption or Raman scattering via Mie-type resonances. As a further method, this review will cover recent developments to employ phonon polaritons in the reststrahlen region.

Carbon nanotubes (CNTs) are carbon based nanomaterials with long hollow structure and diameter at the nanometer scale. The chirality in combination with the radius determines the unique properties of CNTs. These CNTs with different properties have wide applications in the nanotechnology, electronics, photonics, and other fields in materials science and technology. In this review, we highlight the recent investigations on the nonlinear optical properties and applications in the lasers. The future research aspects and potential applications are discussed at the end of the review.

We present localization with stimulated emission depletion (LocSTED) microscopy, a combination of STED and single-molecule localization microscopy (SMLM). We use the simplest form of a STED microscope that is cost effective and synchronization free, comprising continuous wave (CW) lasers for both excitation and depletion. By utilizing the reversible blinking of fluorophores, single molecules of Alexa 555 are localized down to ~5 nm. Imaging fluorescently labeled proteins attached to nanoanchors structured by STED lithography shows that LocSTED microscopy can resolve molecules with a resolution of at least 15 nm, substantially improving the classical resolution of a CW STED microscope of about 60 nm. LocSTED microscopy also allows estimating the total number of proteins attached on a single nanoanchor.

The electromagnetic properties of ordered hole nanostructures in very thin metal films are characterized using CdSe/CdS nanocrystals (NCs) as nanoprobes. The characterization of the local density of optical states (LDOS) on the nanostructure is possible by the measurement of their photoluminescence decay rate. Statistical measurements are performed in the far field to show the average increase of optical modes. A determinist approach using an active single NC nanoprobe in the near field gives access to a more precise characterization of the LDOS. The optical properties of the structure come from the coupling between localized surface plasmons created by the holes and surface plasmon polaritons. A strong concentration of optical modes is observed around the holes thanks to the active near-field nanoprobe. With different NC orientations, the strong influence of the component perpendicular to the surface in the very near field of the LDOS is observed. Finite differential time domain simulations of the different components of the electric field in the very near field of the structure confirm that the localization of the electric field around the holes is only due to the normal component as observed with the nanoprobe.

Light-matter interactions at the nanoscale constitute a fundamental ingredient for engineering applications in nanophotonics and quantum optics. In this regard, Mie resonances supported by high-refractive index dielectric nanoparticles have recently attracted interest, due to their lower losses and better control over the scattering patterns compared to their plasmonic counterparts. The emergence of several resonances in high-refractive index dielectric nanoparticles results in an overall high complexity, where the electric and magnetic dipoles can show a significant spectral overlap, especially at optical frequencies, thus hindering possible light-matter coupling mechanisms arising in the optical spectrum. This behavior can be properly adjusted by using non-spherical geometries, an approach that has already been successfully exploited to tune directional scattering from dielectric nanoresonators. Here, by using cylindrical nanoparticles, we show, experimentally and theoretically, the emergence of peak splitting for both magnetic and electric dipole resonances of individual silicon nanodisks coupled to a J-aggregated organic semiconductor. In the two cases, we find that the different character of the involved resonances leads to different light-matter coupling regimes. Crucially, our results show that the observed energy splittings are of the same order of magnitude as the ones reported using similar plasmonic systems, thereby confirming dielectric nanoparticles as promising alternatives for localized strong coupling studies. The coupling of both the electric and magnetic dipole resonances can offer interesting possibilities for the control of directional light scattering in the strong coupling regime and the dynamic tuning of nanoscale light-matter hybrid states by external fields.

Highly sensitive characterization of surface plasmon resonance (SPR) modes lays the solid foundation for wide SPR-related applications. Herein, we discover that these SPR modes based on all-metal nanostructures without any probed molecule can be characterized with ultrahigh sensitivities at both excitation and emission wavelengths by utilizing plasmon-enhanced sum frequency generation (PESFG) spectroscopy. The theory of PESFG for sensitively characterizing SPR modes is first validated experimentally. Moreover, we have elaborately demonstrated that PESFG strongly depends on both the resonant wavelengths of SPR modes and spatial mode distributions when azimuthal angles of excitations are varied. Our study not only enhances the understanding of the mechanism that governs PESFG, but also offers a potentially new method for exploring new-style SPR modes (e.g. plasmon-induced magnetic resonance and bound states in the continuum) by PESFG.

Metalenses made of artificial sub-wavelength nanostructures have shown the capability of light focusing and imaging with a miniaturized size. Here, we report the demonstration of mass-producible amorphous silicon metalenses on a 12-inch glass wafer via the complementary metal-oxide-semiconductor compatible process. The measured numerical aperture of the fabricated metalens is 0.496 with a focusing spot size of 1.26 μm at the wavelength of 940 nm. The metalens is applied in an imaging system to test the imaging resolution. The minimum bar of the resolution chart with a width of 2.19 μm is clearly observed. Furthermore, the same system demonstrates the imaging of a fingerprint, and proofs the concept of using metalens array to reduce the system size for future compact consumer electronics.

Stimulated emission depletion (STED) microscopy breaks the optical diffraction barrier and has become a powerful tool for biological study. However, its application for in vivo study is limited because of its high demand for depletion power. Here, we propose digitally enhanced STED (DE-STED) as a method for reducing the depletion power that is required for STED superresolution imaging. A donut image is the key in this approach, which is composed of the depleted photons by STED laser and represents the intensity difference between confocal and STED images from the same imaging position. The depletion efficiency is digitally enhanced by multiplying the intensity of the donut image with a factor greater than 1, and then the photons from the periphery of the diffraction-limited spot are fully depleted by subtracting the enhanced donut image from the original confocal image. Finally, DE-STED achieves a resolution of ~λ/8 in biological samples with a depletion power that is an order of magnitude lower than that in traditional STED imaging. Furthermore, the proposed method helps to relax the restrictions on the fluorophore because of its low phototoxicity and photobleaching.

Axial diffraction-limited multiple foci are a kind of investigated focal field for trapping multiple nano-particles. We first experimentally generated diffraction-limited axial double foci by optimization-free binary planar lens and theoretically demonstrated it, which can be applied in multi-particle trapping. The proposed binary planar lens was analytically designed. The BPL has a numerical aperture of 0.9 and a focal length of 150 μm. The focal field of the binary planar lens, which is composed of diffraction-limited axial double foci, was first experimentally validated. The measured maximum lateral full widths at half maximum of the two generated focal spots were diffraction-limited and consistent with the theoretical. The axial double foci formed two stable optical traps that can trap two Rayleigh dielectric particles simultaneously. The radial, azimuthal and axial optical forces of the double optical traps are in good uniformity, which are 0.98, 0.99 and 0.96, respectively.

Thermal camouflage, which is used to conceal objects in the infrared vision for confrontation with infrared detection in civilian or military applications, has garnered increasing attraction and interest recently. Compared with conductive thermal camouflage, that is to tune heat conduction to achieve equivalent temperature fields, radiative thermal camouflage, based on emissivity engineering, is more promising and shows much superiority in the pursuit of dynamic camouflage technology when resorting to stimuli-responsive materials. In this paper, we demonstrate the emissivity-engineered radiative metasurface to realize dynamic thermal camouflage functionality via a flying laser heat source on the metal-liquid-crystal-metal (MLCM) platform. We employ a rigorous coupled-wave algorithm to calculate the surface emissivity of Au/LC/Au microstructures, where the LC-orientation angle distribution is quantified by minimizing the emitted thermal energy standard deviation throughout the whole plate. Emissivity engineering on the MCLM platform is attributed to multiple magnetic polariton resonance, and agrees well with the equivalent electric circuit analysis. Through this electrical modulation strategy, the moving hot spot in the original temperature field is erased and a uniform temperature field is observed in the infrared camera instead, demonstrating the very good dynamic thermal camouflage functionality. The present MLCM-based radiative metasurface may open avenues for high-resolution emissivity engineering to realize novel thermal functionality and develop new applications for thermal metamaterials and meta-devices.

We propose a nanospaser made of an achiral plasmonic–metal nanodisk and a two-dimensional chiral gain medium – a monolayer nanoflake of a transition-metal dichalcogenide (TMDC). When one valley of the TMDC is selectively pumped (e.g. by a circular-polarized radiation), the spaser (surface plasmon amplification by stimulated emission of radiation) generates a mode carrying a topological chiral charge that matches that of the gain valley. There is another, chirally mismatched, time-reversed mode with exactly the same frequency but the opposite topological charge; it is actively suppressed by the gain saturation and never generates, leading to a strong topological protection for the generating matched mode. This topological spaser is promising for use in nano-optics and nanospectroscopy in the near field especially in applications to biomolecules that are typically chiral. Another potential application is a chiral nanolabel for biomedical applications emitting in the far field an intense circularly polarized coherent radiation.

As a prominent alternative to toxic dyes/pigments, nanostructural color pixels have garnered tremendous attention in applications related to display/imaging devices and color printings. However, current color pixels mostly offer static color responses. In relation to this, dynamic color tuning properties must be investigated in order to expand their functionalities and promote their use in the fields of encryption and anti-counterfeiting. In this study, a simple array of hydrogenated amorphous silicon nanogratings is proposed to realize polarization-encrypted full-color images via the coupling of incident light into different leaky mode resonances within the nanogratings. The proposed pixels can readily switch from vivid full colors to indistinguishable orange color by altering the incident polarization state. Hence, unlike the reported polarization-tuned color generation schemes that merely allow for the color variation of the image or require complicated designs to hide the color information, the proposed approach can encrypt arbitrary full-color images via a simple tuning of the incident polarization state. Owing to the localized leaky mode resonances supported by the nanogratings, the pixel can still implement the polarization-encrypted functionality even when it contains only four gratings, thus enabling a remarkably high resolution. The proposed simple scheme may provide a credible new pathway for accelerating the practical applications of high-resolution encryption and anti-counterfeiting.

The active and analog tuning of the phase of light by metamaterials is needed to boost the switching performance of photonic devices. However, demonstrations of this type of tuning in the pivotal visible spectral region are still scarce. Herein, we report the active analog tuning of the phase of visible light reflected by a bismuth (Bi)-based metamaterial, enabled by a reversible solid-liquid transition. This metamaterial, fabricated by following a lithography-free approach, consists of two-dimensional assemblies of polydisperse plasmonic Bi nanostructures embedded in a refractory and transparent aluminum oxide matrix. The analog tuning of the phase is achieved by the controlled heating of the metamaterial to melt a fraction of the nanostructures. A maximum tuning of 320° (1.8 π) is observed upon the complete melting of the nanostructures at 230°C. This tuning is reversible by cooling to 25°C. In addition, it presents a wide hysteretic character due to liquid Bi undercooling. This enables the phase achieved by this analog approach to remain stable over a broad temperature range upon cooling and until re-solidification occurs around 100°C. Therefore, Bi-based metamaterials are endowed with analog optical memory capabilities, which are appealing for a wide range of applications, including optical data storage with enhanced information density or bistable photonic switching with a tunable “on” state.

We demonstrate experimentally that Tamm plasmons in the near infrared can be supported by a dielectric mirror interfaced with a metasurface, a discontinuous thin metal film periodically patterned on the sub-wavelength scale. More crucially, not only do Tamm plasmons survive the nanopatterning of the metal film but they also become sensitive to external perturbations as a result. In particular, by depositing a nematic liquid crystal on the outer side of the metasurface, we were able to red shift the spectral position of Tamm plasmon by 35 nm, while electrical switching of the liquid crystal enabled us to tune the wavelength of this notoriously inert excitation within a 10-nm range.

Metasurfaces are planar devices containing delicately designed nanoantenna or resonator arrays that allow for beam shaping, super resolution imaging, and holography. Hybrid metasurface – by integrating with tunable materials such as two-dimensional materials and phase change materials (PCMs) – provides a potential platform for active modulation of wavefronts. Specifically, PCMs can flexibly switch between crystalline and amorphous states with nonvolatile property under external stimuli and provide a large refractive permittivity contrast. Using metasurfaces based on PCM to manipulate wavefronts may provide new opportunities for switchable functionalities. Here, we propose two types of metasurface devices based on whole PCM films to realize switchable holography and simultaneous phase and interference encryption. This feature can be used to encrypt information in a switched state and store camouflage information in the other state by simply applying external thermal stimuli to the entire metasurface. This method can be applied in areas such as beam shaping, optical encryption, and anti-counterfeiting.

Semiconductor planar microcavities significantly enhance the interaction between light and matter and are thus crucial as a fundamental research platform for investigations of quantum information processing, quantum dynamics, and exciton-polariton observations. Microcavities also serve as a very agile basis for modern resonant-cavity light-emitting and detecting devices now in large-scale production for applications in sensing and communication. The fabrication of microcavity devices composed of both common materials now used in photonics and uncommon or arbitrary materials that are new to photonics offers great freedom in the exploration of the functionalities of novel microcavity device concepts. Here we propose and carefully investigate two unique microcavity designs. The first design uses a monolithic high-index-contrast grating (MHCG) and a distributed Bragg reflector (DBR) as the microcavity mirrors. The second design uses two MHCGs as the microcavity mirrors. We demonstrate by numerical analysis that MHCG-DBR and MHCG-MHCG microcavities, whose lateral radial dimension is 16 μm, reach very large quality factors at the level of 10 4 and nearly 10 6 , as well as purposely designed wavelength tuning ranges of 8 and 60 nm in both configurations, respectively. Our MHCG-MHCG microcavities with a very small size of 600 nm in the vertical dimension show extremely large quality factors, which can be explained by treating the optical modes as quasi-bound states in a continuum (BICs). Moreover, we verify our theoretical analysis and calibrate our simulation parameters by comparing to the experimental characteristics of an electrically injected MHCG-DBR microcavity vertical-cavity surface-emitting laser (VCSEL) emitting at a peak wavelength of about 980 nm. We use the calibrated parameters to simulate the emission characteristics of electrically injected VCSELs in various MHCG-DBR and MHCG-MHCG microcavity configurations to illustrate the influence of microcavity designs and their quality factors on the predicted lasing properties of the devices.

Opto-thermoelectric tweezers present a new paradigm for optical trapping and manipulation of particles using low-power and simple optics. New real-life applications of opto-thermoelectric tweezers in areas such as biophysics, microfluidics, and nanomanufacturing will require them to have large-scale and high-throughput manipulation capabilities in complex environments. Here, we present opto-thermoelectric speckle tweezers, which use speckle field consisting of many randomly distributed thermal hotspots that arise from an optical speckle pattern to trap multiple particles over large areas. By further integrating the speckle tweezers with a microfluidic system, we experimentally demonstrate their application for size-based nanoparticle filtration. With their low-power operation, simplicity, and versatility, opto-thermoelectric speckle tweezers will broaden the applications of optical manipulation techniques.

Hybrid lead halide perovskites have made great strides in next-generation photovoltaic and optoelectronic devices. Random lasers based on perovskite materials have been intensively investigated, but the miniaturization of perovskite random lasers has not been achieved up to now. Here, we report the fabrication of perovskite random lasers based on perovskite films deposited on the optical fiber facets using a dip-coating method. Under optical pumping conditions, random lasing was observed with minimum threshold energy of 32.3 μJ/cm 2 at ~550 nm; also, the lasing threshold decreased with increasing diameter of the optical fiber. The results show that the random lasing emission originates from the scattering between the perovskite crystal grains, and the decreased threshold is attributed to the increased active area of the perovskite film. The directionality of perovskite random lasers indicates that the divergence angle of the laser beam is less than 60°. We also demonstrate that the perovskite random laser on the fiber facet can prevent speckle formation and improve image quality. These results may promote the applications of random lasers in compact sources and integrated optoelectronic devices.

High-index dielectric nanostructures have emerged as an appealing complement to plasmonic nanostructures, offering similar light management capabilities at the nanoscale but free from the inherent optical losses. Despite the great interest in these all-dielectric architectures, their fabrication still requires cumbersome fabrication techniques that limit their implementation in many applications. Hence, the great interest in alternative scalable procedures. Among those, the fabrication of silicon spheres is at the forefront, with several routes available in the literature. However, the exploitation of the Mie modes sustained by these silicon resonators is limited over large areas by polydispersity or a lack of long-range order. Here, we present an all-dielectric metamaterial fabricated with a low cost and highly scalable technique: a combination of soft imprinting nanolithography and chemical vapor deposition. The resulting all-dielectric metasurface is composed of an array of silicon hemispheres on top of a high refractive index dielectric substrate. This architecture allows the exploitation of high-quality Mie resonances at a large scale due to the high monodispersity of the hemispheres organized in a single crystal two-dimensional lattice. The optical response of the metasurface can be engineered by the design parameters of the nanoimprinted structure. We further demonstrate the potential of this platform to enhance light emission by coupling dye molecules to the sustained Mie resonances and measuring both an eight-fold amplified signal and a triple lifetime reduction.

Striving for nanometer-sized solid-state single-photon sources, we investigate atom-like quantum emitters based on single germanium-vacancy (GeV) centers isolated in crystalline nanodiamonds (NDs). Cryogenic characterization indicated symmetry-protected and bright (>10 6 counts/s with off-resonance excitation) zero-phonon optical transitions with up to 6-fold enhancement in energy splitting of their ground states as compared to that found for GeV centers in bulk diamonds (i.e. up to 870 GHz in highly strained NDs vs. 150 GHz in bulk). Utilizing lithographic alignment techniques, we demonstrate an integrated nanophotonic platform for deterministic interfacing plasmonic waveguides with isolated GeV centers in NDs, which enables 10-fold enhancement of single-photon decay rates along with the emission direction control by judiciously designing and positioning a Bragg reflector. This approach allows one to realize the unidirectional emission from single-photon dipolar sources, thereby opening new perspectives for the realization of quantum optical integrated circuits.

Optical metasurfaces have gained attention due to their considerable prospects, which can benefit consumers and industries in terms of virtual reality devices and digital displays in various multimedia and entertainment-related applications. To date, most of the metasurface optimisation strategies involving isotropic or anisotropic nanoantennas encounter a trade-off between polarisation insensitivity and operating bandwidth under unpolarised white light, which is a fundamental requirement for many applications in digital displays. We presented a novel technique to break the trade-off between polarisation insensitivity and bandwidth by engineering the displacement dependent phase delay of the adjacent parallel and perpendicular anisotropic nanoantennas in a dielectric metasurface. As a result, we numerically demonstrate a highly efficient metasurface operating at an almost entire visible domain (wavelength of 450 nm–650 nm) while maintaining a constant phase modulation under all polarisation states, thus enabling the polarisation-insensitive metadevices for unpolarised white light.

Using an all-fiber mode selective coupler (MSC) at the visible band, here we experimentally demonstrate a generating and wavelength multiplexing scheme for the cylindrical vector (CV) and vortex beams (VBs). The proposed MSCs act as efficient mode converters to produce spectrally insensitive high-order modes (HOMs) at the wavelength ranging from 450 to 980 nm, which have broad operation bandwidth (more than 7 nm), high mode conversion efficiency (94%), and purity (98%), and low insert loss (below 0.5 dB). By adjusting the polarization state and the phase shift of linear polarization (LP) 11 mode respectively, the donut-shaped CVs and circular-polarization VBs are achieved. The focused intensity distribution of the donut beam on the cross- and axial-sections is monitored by using a confocal system. The all-fiber solution of producing and multiplexing HOMs opens a new route for stimulated emission depletion microscopy applications.