Volume 9, Issue 12 - Special Issue: Interdisciplinary Nanophotonics; Guest Editors: Hatice Altug, Laura Na Liu

The unusual electromagnetic responses of nanostructured metasurfaces endow them with an ability to manipulate the four fundamental properties (amplitude, phase, polarization, and frequency) of lightwave at the subwavelength scale. Based on this, in the past several years, a lot of innovative optical elements and devices, such as metagratings, metalens, metaholograms, printings, vortex beam generators, or even their combinations, have been proposed, which have greatly empowered the advanced research and applications of metasurfaces in many fields. Behind these achievements are scientists’ continuous exploration of new physics and degrees of freedom in nanostructured metasurface design. This review will focus on the progress on the design of different nanostructured metasurfaces for lightwave manipulation, including by varying/fixing the dimensions and/or orientations of isotropic/anisotropic nanostructures, which can therefore provide various functionalities for different applications. Exploiting the design degrees of freedom of optical metasurfaces provides great flexibility in the design of multifunctional and multiplexing devices, which can be applied in anticounterfeiting, information encoding and hiding, high-density optical storage, multichannel imaging and displays, sensing, optical communications, and many other related fields.

On-chip nanophotonic devices are a class of devices capable of controlling light on a chip to realize performance advantages over ordinary building blocks of integrated photonics. These ultra-fast and low-power nanoscale optoelectronic devices are aimed at high-performance computing, chemical, and biological sensing technologies, energy-efficient lighting, environmental monitoring and more. They are increasingly becoming an attractive building block in a variety of systems, which is attributed to their unique features of large evanescent field, compactness, and most importantly their ability to be configured according to the required application. This review summarizes recent advances of integrated nanophotonic devices and their demonstrated applications, including but not limited to, mid-infrared and overtone spectroscopy, all-optical processing on a chip, logic gates on a chip, and cryptography on a chip. The reviewed devices open up a new chapter in on-chip nanophotonics and enable the application of optical waveguides in a variety of optical systems, thus are aimed at accelerating the transition of nanophotonics from academia to the industry.

Metasurfaces, composed of specifically designed subwavelength units in a two-dimensional plane, offer a new paradigm to design ultracompact optical elements that show great potentials for miniaturizing optical systems. In the past few decades, metasurfaces have drawn broad interests in multidisciplinary communities owing to their capability of manipulating various parameters of the light wave with plentiful functionalities. Among them, pixelated polarization manipulation in the subwavelength scale is a distinguished ability of metasurfaces compared to traditional optical components. However, the inherent ohmic loss of plasmonic-type metasurfaces severely hinders their broad applications due to the low efficiency. Therefore, metasurfaces composed of high-refractive-index all-dielectric antennas have been proposed to achieve high-efficiency devices. Moreover, anisotropic dielectric nanostructures have been shown to support large refractive index contrast between orthogonal polarizations of light and thus provide an ideal platform for polarization manipulation. Herein, we present a review of recent progress on all-dielectric metasurfaces for polarization manipulation, including principles and emerging applications. We believe that high efficient all-dielectric metasurfaces with the unprecedented capability of the polarization control can be widely applied in areas of polarization detection and imaging, data encryption, display, optical communication and quantum optics to realize ultracompact and miniaturized optical systems.

Although the first lasers invented operated in the visible, the first on-chip devices were optimized for near-infrared (IR) performance driven by demand in telecommunications. However, as the applications of integrated photonics has broadened, the wavelength demand has as well, and we are now returning to the visible (Vis) and pushing into the ultraviolet (UV). This shift has required innovations in device design and in materials as well as leveraging nonlinear behavior to reach these wavelengths. This review discusses the key nonlinear phenomena that can be used as well as presents several emerging material systems and devices that have reached the UV–Vis wavelength range.

Concepts adapted from radio frequency devices have brought forth subwavelength scale optical nanoantenna, enabling light localization below the diffraction limit. Beyond enhanced light–matter interactions, plasmonic nanostructures conjugated with active materials offer strong and tunable coupling between localized electric/electrochemical/mechanical phenomena and far-field radiation. During the last two decades, great strides have been made in development of active plasmonic nanoantenna (PNA) systems with unconventional and versatile optical functionalities that can be engineered with remarkable flexibility. In this review, we discuss fundamental characteristics of active PNAs and summarize recent progress in this burgeoning and challenging subfield of nano-optics. We introduce the underlying physical mechanisms underpinning dynamic reconfigurability and outline several promising approaches in realization of active PNAs with novel characteristics. We envision that this review will provide unambiguous insights and guidelines in building high-performance active PNAs for a plethora of emerging applications, including ultrabroadband sensors and detectors, dynamic switches, and large-scale electrophysiological recordings for neuroscience applications.

Recently, nitrogen-doped graphene quantum dots (NGQDs), as a new type of quantum semiconductor and photoelectrochemical material, are promising candidates in photoelectric sensing, water splitting, and biological imaging and have various potential application prospects. In this work, NGQDs were prepared by a simple calcination method, and then a photoelectrochemical sensing platform based on the NGQDs electrode with superior photoelectrochemical activity was designed and fabricated for the detection of dopamine (DA). Benefitting from the quantum effect and size effect, NGQDs displayed an enhanced photocurrent effective within ultra-low detection limit (0.03 μ m ), wide detection range (0.03–450 and 450–9680 μ m ), and high sensitivity in detecting DA with the assistance of ultraviolet light irradiation. The NGQDs electrode also showed continuous and stable photocurrent densities after long-term experiment, indicating the excellent durability of NGQDs for DA detection.

Recent discoveries in the field of graphene-based heterostructures have led to the demonstration of high-performance photodetectors. However, the studies to date have been largely limited to the heterojunction with a Schottky barrier, restricted by an inevitable compromise between photoresponsivity and photodetectivity. Here, a new class of graphene-based tunneling photodetectors is introduced by inserting the Al 2 O 3 tunneling layer between silicon and graphene. The photocarriers can tunnel through the designed insulator layer which simultaneously blocks the dark current, thus maintaining high photodetectivity with desirable photoresponsivity. We further modulate the thickness of the Al 2 O 3 layer to explore the tunneling mechanism for the photocarriers, in which a photoresponsivity of 0.75 A/W, a high current ratio of 4.8 × 10 3 and a photodetectivity of 3.1 × 10 12 Jones are obtained at a 13.3-nm-thick Al 2 O 3 layer. In addition, the fabrication process is compatible with conventional semiconductor processing, providing further flexibility to large-scale integrated photodetectors with high performance.

Dielectric nanoresonators uniquely support both magnetic and electric resonances across a wide wavelength range. They are thus being exploited in a growing number of groundbreaking applications. In particular, they have been recently suggested as promising nanoheaters. However, while the thermo-optical properties of silicon and germanium resonators have been exploited to realize tunable metasurfaces based on external thermal inputs, the effect of self-induced optical heating onto their resonances has so far been neglected. In this study, we address the problem of self-heating of a thermo-optical resonator. In particular, employing a recursive procedure to account for the interdependence between the absorption cross section and the temperature of the resonator, we show that self-heating gives rise to a complex, nonlinear relationship between illumination intensity and temperature. Using both analytical and numerical models, we also observe that self-induced optical heating has nonnegligible effects on the spectral position of electric and magnetic resonances of spheres as well as anapole modes of nanodisks, even for moderate illumination intensities relevant for applications such as Raman scattering. Thus, our work demonstrates that self-induced optical heating must be properly accounted for when designing dielectric resonators for a wide range of devices.

Fluorescent metallic nanoparticles with high quantum yield are highly desirable for optical imaging in the fields of biology and materials science. We investigate the photoluminescence (PL) properties of Ag nanoplates, particularly triangular ones, systematically at the single-particle level under different excitation conditions. We find that their emission is controlled by surface plasmon resonances (SPRs). Because of their high geometric symmetry, the shape and intensity of the PL spectrum from Ag triangular nanoplates are independent of the excitation polarization. Under excitation wavelengths of 473, 532 and 633 nm, the PL quantum yield (QY) of Ag triangular nanoplates is found to be twice that of Au nanorods at similar SPR wavelengths. It can be attributed to the stronger and broader SPR response, which can make the PL more intense not only in the spatial domain but also over a larger range in the frequency domain. Our investigations will extend the application of Ag nanoplates in imaging, labeling, and emitting, especially at the single-molecule level, due to their ultra-bright, stable, and polarization-independent PL. This method to enhance the PL signal, in both the spatial and frequency domains, can be easily expanded to other systems.

We report on a Q-switched Yb-doped all-fiber laser based on a solution-processed Ag nanoplates saturable absorber. Optical deposition procedure is implemented to transfer the Ag nanoplates onto the fiber core area through the thermal effect. The saturable absorber is sandwiched between two fiber connectors, providing simplicity, flexibility, and easy integration into the laser oscillator. The modulation depth and saturation incident fluence are measured to be ~5.8% and ~106.36 μJ/cm 2 at 1-μm region, respectively. Self-started stable Q-switched operation is achieved for a threshold pump power of 180 mW. The repetition rates of the pulse trains range from 66.6 to 184.8 kHz when the pump power scales from 210 to 600 mW. The maximum average output power is 10.77 mW, corresponding to the single-pulse energy of 58.3 nJ and minimum pulse duration of ~1.01 μs. To the best of our knowledge, it is the first time that the Ag nanoplates saturable absorbers are utilized in the 1-μm Yb-doped Q-switched fiber laser.

We report on multiple excitonic resonances in bilayer tungsten diselenide (BL-WSe 2 ) stacked at different angles and demonstrate the use of the stacking angle to control the occurrence of these excitations. BL-WSe 2 with different stacking angles were fabricated by stacking chemical vapour deposited monolayers and analysed using photoluminescence measurements in the temperature range 300–100 K. At reduced temperatures, several excitonic features were observed and the occurrences of these exitonic resonances were found to be stacking angle dependent. Our results indicate that by controlling the stacking angle, it is possible to excite or quench higher order excitations to tune the excitonic flux in optoelectronic devices. We attribute the presence/absence of multiple higher order excitons to the strength of interlayer coupling and doping effect from SiO 2 /Si substrate. Understanding interlayer excitations will help in engineering excitonic devices and give an insight into the physics of many-body dynamics.

We propose a radio frequency (RF) and visibly transparent composite metasurface design comprising newly developed transparent multilayer conductive coatings. Detailed experimental and theoretical analysis of the RF/visible transparency of the proposed meta-glass is provided. The proposed nature-inspired symmetrical honeycomb-shaped meta-glass design, alters the electromagnetic properties of the glass substrate in the RF spectrum by utilizing visibly transparent Ag-based conductive coatings on each side. Furthermore, the competing effect of the Ag thickness on optical and RF transparency is discussed. We show that using multilayer dielectric-metal coatings, specifically 5-layered spectrally selective coatings, RF transparency of the meta-glass can be enhanced while preserving visible transparency. Herein we demonstrate high transparency meta-glass with 83% and 78% peak RF and optical transmission at 28 GHz and 550 nm, respectively. The meta-glass yields enhanced RF transmission by 80% and 10% when compared to low-emissivity glass and bare glass, respectively. The meta-glass design presented here is amenable to a variety of 5G applications including automobile radar systems. This work provides a superior alternative to the standard indium-tin-oxide (ITO) transparent material which is becoming scarce. Moreover, this study paves the way for the design of new visibly transparent metamaterials and artificial dielectrics.

We provide a critical analysis of some of the commonly used theoretical models to describe quantum plasmons in finite size media. We summarize the standard approach based on a Fano diagonalization and we show explicit discrepancies in the obtained results by taking the limit of vanishing coupling between the electromagnetic field and the material medium. We then discuss the derivation of spontaneous emission in a plasmonic environment, which usually relies on a Green tensor and is based on an incomplete identity. The effect of the missing terms is calculated in a one-dimensional model.

Mid-infrared (MIR) plasmon-phonon features of heterostructures composing of a plasmonic anisotropic two-dimensional material (A2DM) on a hexagonal boron nitride (hBN) film are analyzed. We derive the exact dispersion relations of plasmon-phonons supported by the heterostructures and demonstrate the possibility of topological transitions of these modes within the second Reststrahlen band of hBN. The topological transitions lead to enhanced local density of plasmon-phonon states, which intensifies the spontaneous emission rate, if the thickness of the hBN layer is appropriately chosen. We also investigate a lateral junction formed by A2DM/hBN and A2DM, demonstrating that one can realize asymmetric guiding, beaming, and unidirectionality of the hybrid guided modes. Our findings demonstrate potential capabilities of the A2DM/hBN heterostructures for active tunable light–matter interactions and asymmetric in-plane polariton nanophotonics in the MIR range.

The steadily increasing demand for accurate analysis of vitamin D level, via measurement of its best general marker, 25-hydroxyvitamin D (25(OH)D), pushes for the development of novel automated assays capable of working at very low concentrations. Here, we propose a plasmonic biosensor of 25(OH)D3 (calcifediol) based on surface-enhanced infrared absorption spectroscopy, which exploits the resonant coupling between plasmonic nanoantennas and vibrational excitation of small molecules. Specifically, our proposed platform features a large-area (several mm 2 ) metasurface made of gold nanoantennas fabricated on a silicon substrate, comprising different macroregions (“pixels”) of area 500 × 500 µm 2 . In each pixel, the nanoantenna geometrical parameters are tuned so as to support localized surface plasmon resonances (and hence large field enhancements at the nanoscale) within different regions of the infrared spectrum. As a result, a single chip is capable of performing analysis from the region of functional groups to that of fingerprint. Two different designs are fabricated via electron beam lithography, functionalized with a correlated antibody for the detection of 25(OH)D3, and characterized via Fourier-transform infrared spectroscopy. Our experiments demonstrate the capability to detect a concentration as low as 86 pmol/L, and an amount of immobilized small molecules of 25(OH)D3 monohydrate (molecular weight: 418.65 g/mol) as low as 4.31 amol over an area of 100 × 100 µm 2 .

In an optical Pancharatnam-Berry (PB) phase metasurface, each sub-wavelength dielectric structure of varied spatial orientation can be treated as a point source with the same amplitude yet varied relative phase. In this work, we introduce an optimized genetic algorithm (GA) method for the synthesis of one-dimensional (1D) PB phase-controlled dielectric metasurfaces by seeking for optimized phase profile solutions, which differs from previously reported amplitude-controlled GA method only applicable to generate transverse optical modes with plasmonic metasurfaces. The GA–optimized phase profiles can be readily used to construct dielectric metasurfaces with improved functionalities. The loop of phase-controlled GA consists of initialization, random mutation, screened evolution, and duplication. Here random mutation is realized by changing the phase of each unit cell, and this process should be efficient to obtain enough mutations to drive the whole GA process under supervision of appropriate mutation boundary. A well-chosen fitness function ensures the right direction of screened evolution, and the duplication process guarantees an equilibrated number of generated light patterns. Importantly, we optimize the GA loop by introducing a multi-step hierarchical mutation process to break local optimum limits. We demonstrate the validity of our optimized GA method by generating longitudinal optical modes (i. e., non-diffractive light sheets) with 1D PB phase dielectric metasurfaces having non-analytical counter-intuitive phase profiles. The produced large-area, long-distance light sheets could be used for realizing high-speed, low-noise light-sheet microscopy. Additionally, a simplified 3D light pattern generated by a 2D PB phase metasurface further reveals the potential of our optimized GA method for manipulating truly 3D light fields.

Semiconductor superlattices are strongly nonlinear media offering several technological challenges associated with the generation of high-frequency Gigahertz radiation and very effective frequency multiplication up to several Terahertzs. However, charge accumulation, traps and interface defects lead to pronounced asymmetries in the nonlinear current flow, from which high harmonic generation stems. This problem requires a full non-perturbative solution of asymmetric current flow under irradiation, which we deliver in this paper within the Boltzmann-Bloch approach. We investigate the nonlinear output on both frequency and time domains and demonstrate a significant enhancement of even harmonics by tuning the interface quality. Moreover, we find that increasing arbitrarily the input power is not a solution for high nonlinear output, in contrast with materials described by conventional susceptibilities. There is a complex combination of asymmetry and power values leading to maximum high harmonic generation.

We put forward the multipolar model which captures the physics behind linear and nonlinear response driven by high-quality (high- Q ) supercavity modes in subwavelength particles. We show that the formation of such trapped states associated with bound states in the continuum (quasi-BIC) can be understood through multipolar transformations of coupled leaky modes. The quasi-BIC state appears with increasing the order of the dominating multipole, where dipolar losses are completely suppressed. The efficient optical coupling to this state in the AlGaAs nanodisk is implemented via azimuthally polarized beam illumination matching its multipolar origin. We establish a one-to-one correspondence between the standard phenomenological non-Hermitian coupled-mode theory and multipolar models. The derived multipolar composition of the generated second-harmonic radiation from the AlGaAs nanodisk is then validated with full-wave numerical simulations. Back-action of the second-harmonic radiation onto the fundamental frequency is taken into account in the coupled nonlinear model with pump depletion. A hybrid metal-dielectric nanoantenna is proposed to augment the conversion efficiency up to tens of per cent due to increasing quality factors of the involved resonant states. Our findings delineate novel promising strategies in the design of functional elements for nonlinear nanophotonics applications.

Plasmonics offers a unique opportunity to break the diffraction limit of light and bring photonic devices to the nanoscale. As the most prominent example, an integrated nanolaser is a key to truly nanoscale photonic circuits required for optical communication, sensing applications and high-density data storage. Here, we develop a concept of an electrically driven subwavelength surface-plasmon-polariton nanolaser, which is based on a novel amplification scheme, with all linear dimensions smaller than the operational free-space wavelength λ and a mode volume of under λ 3 /30. The proposed pumping approach is based on a double-heterostructure tunneling Schottky barrier diode and gives the possibility to reduce the physical size of the device and ensure in-plane emission so that the nanolaser output can be naturally coupled to a plasmonic or nanophotonic waveguide circuitry. With the high energy efficiency (8% at 300 K and 37% at 150 K), the output power of up to 100 μW and the ability to operate at room temperature, the proposed surface plasmon polariton nanolaser opens up new avenues in diverse application areas, ranging from ultrawideband optical communication on a chip to low-power nonlinear photonics, coherent nanospectroscopy, and single-molecule biosensing.

Over the last five years, inorganic lead halide perovskite nanowires have emerged as prospective candidates to supersede standard semiconductor analogs in advanced photonic designs and optoelectronic devices. In particular, CsPbX 3 (X = Cl, Br, I) perovskite materials have great advantages over conventional semiconductors such as defect tolerance, highly efficient luminescence, and the ability to form regularly shaped nano- and microcavities from solution via fast crystallization. However, on the way of electrically pumped lasing, the perovskite nanowires grown on transparent conductive substrates usually suffer from strong undesirable light leakage increasing their threshold of lasing. Here, we report on the integration of CsPbBr 3 nanowires with nanostructured indium tin oxide substrates possessing near-unity effective refractive index and high conductivity by using a simple wet chemical approach. Surface passivation of the substrates is found out to govern the regularity of the perovskite resonators’ shape. The nanowires show room-temperature lasing with ultrahigh quality factors (up to 7860) which are up to four times higher than that of similar structures on a flat indium tin oxide layer, resulting in more than twofold reduction of the lasing threshold for the nanostructured substrate. Numerical modeling of eigenmodes of the nanowires confirms the key role of low-refractive-index substrate for improved light confinement in the Fabry–Pérot cavity which results in superior laser performance.

In this work we present a simple transfer-matrix based modeling tool for arbitrarily layered stacks of resonant plasmonic metasurfaces interspersed with dielectric and metallic multilayers. We present the application of this model by analyzing three seminal problems in nanophotonics. These are the scenario of perfect absorption in plasmonic Salisbury screens, strong coupling of microcavity resonances with the resonance of plasmon nano-antenna metasurfaces, and the hybridization of cavities, excitons and metasurface resonances.

Multi-resonant plasmonic simple geometries like nanocylinders and nanorods are highly interesting for two-photon photoluminescence and second harmonic generation applications, due to their easy fabrication and reproducibility in comparison with complex multi-resonant systems like dimers or nanoclusters. We demonstrate experimentally that by using a simple gold nanocylinder we can achieve a double resonantly enhanced two-photon photoluminescence of quantum dots, by matching the excitation wavelength of the quantum dots with a dipolar plasmon mode, while the emission is coupled with a radiative quadrupolar mode. We establish a method to separate experimentally the enhancement factor at the excitation and at the emission wavelengths for this double resonant system. The sensitivity of the spectral positions of the dipolar and quadrupolar plasmon resonances to the ellipticity of the nanocylinders and its impact on the two-photon photoluminescence enhancement are discussed.

Förster Resonance Energy Transfer (FRET) is a widely applied technique in biology to accurately measure intra- and inter-molecular interactions at the nanometre scale. FRET is based on near-field energy transfer from an excited donor to a ground state acceptor emitter. Photonic nanoantennas have been shown to modify the rate, efficiency and extent of FRET, a process that is highly dependent on the near-field gradient of the antenna field as felt by the emitters, and thus, on their relative distance. However, most of the experiments reported to date focus on fixed antennas where the emitters are either immobilized or diffusing in solution, so that the distance between the antenna and the emitters cannot be manipulated. Here, we use scanning photonic nanoantenna probes to directly modulate the FRET efficiency between individual FRET pairs with an unprecedented nanometric lateral precision of 2 nm on the antenna position. We find that the antenna acts as an independent acceptor element, competing with the FRET pair acceptor. We directly map the competition between FRET and donor-antenna transfer as a function of the relative position between the antenna and the FRET donor-acceptor pair. The experimental data are well-described by FDTD simulations, confirming that the modulation of FRET efficiency is due to the spatially dependent coupling of the single FRET pair to the photonic antenna.

Magneto-optical materials have become a key tool in functional nanophotonics, mainly due to their ability to offer active tuning between two different operational states in subwavelength structures. In the long-wavelength limit, such states may be considered as the directional forward- and back-scattering operations, due to the interplay between magnetic and electric dipolar modes, which act as equivalent Huygens sources. In this work, on the basis of full-wave electrodynamic calculations based on a rigorous volume integral equation (VIE) method, we demonstrate the feasibility of obtaining magnetically-tunable directionality inversion in spherical microresonators (THz antennas) coated by magneto-optical materials. In particular, our analysis reveals that when a high-index dielectric is coated with a magneto-optical material, we can switch the back-scattering of the whole particle to forward-scattering simply by turning off/on an external magnetic field bias. The validity of our calculations is confirmed by reproducing the above two-state operation, predicted by the VIE, with full-wave finite-element commercial software. Our results are of interest for the design of state-of-the-art active metasurfaces and metalenses, as well as for functional nanophotonic structures, and scattering and nanoantennas engineering.

Lattice light-sheet microscopy (LLSM) was developed for long-term live-cell imaging with ultra-fine three-dimensional (3D) spatial resolution, high temporal resolution, and low photo-toxicity by illuminating the sample with a thin lattice-like light-sheet. Currently available schemes for generating thin lattice light-sheets often require complex optical designs. Meanwhile, limited by the bulky objective lens and optical components, the light throughput of existing LLSM systems is rather low. To circumvent the above problems, we utilize a dielectric metasurface of a single footprint to replace the conventional illumination modules used in the conventional LLSM and generate a lattice light-sheet with a ~3-fold broader illumination area and a significantly leveraged illumination efficiency, which consequently leads to a larger field of view with a higher temporal resolution at no extra cost of the spatial resolution. We demonstrate that the metasurface can manipulate spatial frequencies of an input laser beam in orthogonal directions independently to break the trade-off between the field of view and illumination efficiency of the lattice light-sheet. Compared to the conventional LLSM, our metasurface module serving as an ultra-compact illumination component for LLSM at an ease will potentially enable a finer spatial resolution with a larger numerical-aperture detection objective lens.