This PDF file contains the front matter associated with SPIE Proceedings Volume 11695, including the Title Page, Copyright Information, and Table of Contents.

Introduction to SPIE Photonics West OPTO conference 11695: High Contrast Metastructures X.

High contrast grating has been demonstrated in VCSELs for over one decade with wavelengths in 850nm, 940nm, 980nm, 1050nm, 1300nm and 1550nm. Several device configurations were reported with HCG freely suspended as MEMS structure, monolithic with AlOx layer, flip-chip bonded onto a Si/SiO2 HCG and with a deposited thin film Si/SiO2. Wavelength swept VCSELs and multi-wavelength VCSELs were also published. In this talk, we will review all different results. We will particularly address polarization and far field pattern engineering. New applications in high speed modulation, 3D sensing and LIDAR will be discussed.

Nonlinear optical phenomena in nanostructured materials have attracted much attention due to their wide range of applications, ranging from sensing to novel light sources. Here, we demonstrate the enhanced harmonic generation in single-crystal transition-metal-dichalcogenide metasurfaces. Our nonlinear metasurface offers the highest refractive index to date and operation in a sub-diffractive regime for both second- and third-harmonic waves. Importantly, the interplay between the metasurface Mie resonances allows for control of the second-harmonic generation in forward or backward direction in a sub-diffractive regime. Our results open new opportunities for nonlinear light sources based on metasurfaces, including nonlinear mirrors and entangled-photon generation.

Diffractive photonic devices manipulate light via local and nonlocal optical modes. Local devices, such as metasurfaces, can shape a wavefront at multiple selected wavelengths, but inevitably modify light across the spectrum; nonlocal devices, such as grating filters, offer great frequency selectivity but limited spatial control. In this talk, I will introduce a rational design paradigm using quasi-bound states in the continuum to realize multifunctional nonlocal devices: metasurfaces that produce narrowband spatially tailored wavefronts at multiple selected wavelengths and yet are otherwise transparent.

Metamaterials are the sub-wavelength arrays of the composite structure made of metallic and/or dielectric materials. The metal-dielectric structure provides a significant enhancement in the field but has a high loss, low Q-Factor and poor spectral contrast in the visible and infrared frequencies. To overcome these problems, all-dielectric metamaterials structures are the better alternative which offers negligible losses due to their low absorption and hence they have narrow resonance peak and high spectral contrast. When all-dielectric metamaterial with specific geometry (asymmetric oscillators) interacts with the electromagnetic field in visible and infrared (IR) wavelength range, the interaction produces the Fano-Resonance. The Fano resonance depends on the shape and size of the metasurface structure and the refractive index of the surrounding. The Fano-resonance based on all-dielectric metamaterials can be used as a refractive index sensor for biomedical sensing and optical modulation in telecommunication. All-dielectric metamaterials based on Fano resonance can be utilized to have a high Figure-of-merit (FoM) refractive index sensor device. In this work, we are proposing a Fano resonance-based refractive index (RI) sensor which has a high FoM of the order of 2465.

All-dielectric focusing metasurfaces have attracted a lot of attention in recent years because of their lightweight, superior performance, and compact form factor. Traditionally, metasurfaces are fabricated from dielectric material shaped into pillar-like structures. Pillars are fragile and tend to fall, especially for high aspect ratio structures. We have developed and characterized a monolithic metasurface in the infrared spectral range. The monolithic design of realized metasurfaces allows for the creation of rugged structures that can withstand harsh cleaning and handling, while maintaining good focusing performance. This metasurface platform thus widens the range of practical applications for metalenses.

By enabling high reflectivity from a single layer of dielectric, high-contrast gratings (HCGs) provide an exciting platform for optomechanics. In the interest of enhancing the mechanical quality factor of such devices, we have combined photonic and phononic engineering to fabricate HCG mirrors within a structure containing phononic bandgaps. In this way, we realize optomechanical devices that simultaneously exhibit high reflectivity, ultralow mass, and ultrahigh mechanical quality factors. Our initial devices are designed to trade off optimal acoustic and optical performance. We have incorporated these devices into optical cavities, which we probe in order to determine their optical and mechanical properties.

We developed a method to easily transfer optical structures from a semiconductor substrate to a fiber-tip facet without the need for glues and preserving the pristine or functionalized condition of the structure surface. An opening is etched on the back of the fabrication wafer and the structure is suspended via breakable support. The transfer is achieved by mechanical contact with the fiber facet. Using a photonic crystal structure designed for high vertical coupling at the Gamma point a reflectance fiber-tip sensor with refractive index sensitivity of 120 nm/RIU has been assembled and could be further functionalized for application in biosensing.

High contrast gratings (HCGs) are diffraction gratings whose period is less than the wavelength of light, made of a material with a high refractive index. Monolithic HCGs (MHCGs) are made of the same material as the cladding. They can be made of almost any material used in optoelectronics. We show experimentally and via simulations that shaping the cross-section of the MHCG stripes enables very broad high reflection spectrum.

The field of metasurfaces holds the promise of fundamentally innovating different areas of electromagnetics, optics and photonics. In this talk, we present our recent efforts on some exciting topics at the frontiers of this field, with particular focus on ultra-broadband metalenses and nonlocal meta-optics. We present fundamental bandwidth limits on achromatic metalenses, independent of their implementation, and we discuss potential directions to approach and bypass these bounds. In the second part of the talk, we present our recent efforts on metasurfaces that are both nonlocal and laterally inhomogeneous, which open new opportunities for ultra-compact focusing systems and more advanced functionalities.

Recent work on beams with light structured along the propagation direction will be presented, where the polarization and the OAM of the beam changed along the propagation direction.

We experimentally demonstrate phase encoding in SHG with transparent all-dielectric metasurfaces. While a similar task was previously achieved with plasmonic metasurfaces for THG beam shaping, here we obtain three-order-of-magnitude higher generation efficiency without thermal dissipation.

We report luminescent solar concentrators designs using two-dimensional photonic crystal slabs as light trapping waveguides, resulting in high concentration factors. In all designs, we can achieve luminophore light trapping efficiency above 90%, leading to concentration factors as high as 100.

In this work, we investigate the scattering behavior of nanorods that are randomly packed at various densities and aspect ratios. We show that the maximum packing density, maximum scattering density, and the percolation threshold are all tightly connected to Onsager excluded-area principle.

I will discuss the status and outlook for electronically tunable and reconfigurable dielectric and plasmonic metasurfaces whose elements are arbitrarily reprogrammable, enabling a wide array of functions, including steering, focusing, and frequency multiplexing of scattered radiation.

This Conference Presentation, “Dielectric nanophotonics for reconfigurable planar optics and biosensing,” was recorded for the Photonics West 2021 Digital Forum.

This Conference Presentation, “Directed and focused light emission from high-contrast GaN quantum-well metasurfaces,” was recorded for the Photonics West 2021 Digital Forum.

For the first time, we present an active 2D metasurface array and its demonstrated versatile beam steering. The array is composed of individually-addressable, gate-controlled 10×10 pixels where each pixel modulates the phase of light in reflection. Each pixel is a gated plasmonic nanoresonator with an indium tin oxide (ITO) layer embedded in its middle. When proper gate biases are applied to the array, the refractive index of the ITO layer changes, generating a phase gradient necessary for dynamic beam steering. By generating a reconfigurable binary phase grating, we have successfully demonstrated full-area, 2D arbitrary beam steering.

A tuning metalens in transmission mode with GeSbTe (GST) phase-change material is proposed in this paper. Traditional single-layer metalenses with fixed geometry structure face the difficulty of focusing the wide-angle incident wave on the same focal point. Combining GST phase-change material with metasurface structure, the proposed single-layer metalens can converge wide-angel beam from −30° to 30° on a focal point with high efficiency without geometric structure variation. The unit cell of the proposed metalens consists of a GST nanopost and a silicon ring. GST phase-change material excited by a series of pulses switches among amorphous, crystalline and partially crystallized states. The phase shift controlled by each unit cell can well match the phase distribution of metalens required by focusing for different incident beams by changing the crystallization level of the GST. The simulation results show that the proposed metalens operating at 1550 nm wavelength can convert the incident plane wave to a converging spherical wave and focus it on a focal point with more than 27.68% efficiency when the incident angle changes from −30° to 30°. The focal length is in the range of 4535.4 to 4967.1 nm, in good agreement with the designed focal length of 4650 nm. Moreover, the full width at half maximum (FWHM) is from 0.581λ to 0.625λ approaching the diffraction limit of 0.6538λ. The proposed metalens is applicable for near-infrared light focusing with large view field.

We report an all-dielectric active metasurface based on indium phosphide (InP) multiple quantum wells (MQWs) operating at telecom wavelengths (λ=1.55μm). Our design exhibits high reflectance of >80% and is based on localized Mie supported by the metasurface. Our calculations show that the proposed metasurface can steer the beam up to polar angles of 35° while maintaining high efficiency of >80% and a side mode suppression ratio of 7 dB. The anticipated modulation frequencies are >1 MHz. Our metasurface can be used in future chip-scale light detection and ranging systems as well as for free-space optical communications.

Active metasurfaces represent a new class of flat optical elements, which can dynamically control the wavefront of the reflected or transmitted light at a subwavelength scale. Here, we theoretically investigate thermal performance of gate-tunable conducting oxide metasurfaces, which are illuminated with high-power laser beams (~kW/cm2). We develop strategies to mitigate and limit temperature increase of our active metasurfaces. To anchor our approach, we experimentally investigate the short pulse laser-induced damage of thin gold, indium tin oxide, and titanium nitride films. Our analysis reveals that our metasurfaces can support irradiances necessary for free space optical communication or light detection and ranging applications.

Metasurfaces infiltrated with liquid crystals have become a particularly promising means of tuning their optical properties, due to liquid crystals’ large and broadband optical anisotropy. In order to fully explore the parameter space of broadband all-dielectric liquid crystal tunable metasurfaces in the visible, we undertake a comprehensive study based on TiO2 nanoresonator superarrays, sweeping geometric parameters (i.e. disc radius and disc-to-disc gap). We demonstrate both electrical and thermal switching, and visualize the resonance change caused by either the orientation or phase change of liquid crystal, which provides a practical library for the rational design of liquid crystal tunable metasurfaces.

We report our development of Indium tin oxide (ITO) films with thicknesses greater than the typical optical telecommunication wavelength bands (~1550 nm) having epsilon-near-zero (ENZ) property at 1550 nm wavelength for the purpose of providing a new ENZ material platform for building high-contrast metastructure and metasurface devices. The films were grown using a high-power impulse magnetron sputtering (HiPIMS) tool, which allows for more control over film growth. A post-growth thermal annealing allowed the ITO film to reach the ENZ condition at the desirable wavelength. Our goal is to understand how deposition parameters and post deposition annealing conditions affect the film’s optical properties, therefore obtaining a controllable fabrication process for a desired optical property. Using spectroscopic ellipsometry to characterize the films, we show that the thick ITO films grown with HiPIMS exhibit ENZ behavior after post deposition annealing. The regime in which the material exhibits ENZ behavior is shown to be tunable within the wavelength range of 1500-1650 nm by varying the anneal temperature, anneal time, and oxygen exposure during anneal. In comparison with other thick ITO films grown with conventional pulsed DC magnetron sputtering, the optical constants of HiPIMS ITO films are shown to be much more constant with less variation throughout the bulk of the film. This result shows that these ITO films can be used to design a new family of opto-electronic devices that use ENZ ITO as the low-index base for high-contrast metasurface devices and as cladding for waveguides or optical cavities.

Metasurfaces are planar nanostructured optical elements for lensing, wave-front shaping, and polarization control. A general but time-consuming metasurface design tool is the finite difference time domain (FDTD) technique. The discrete dipole approximation (DDA) is a rigorous and fast alternative for computing the electromagnetic field scattered by particles, but has not been widely used in metasurface design because of the complicated numerical difficulties imposed by the nanostructure-substrate interaction. Here we present a substrate-compatible DDA formulation using a 1D Green’s function method under cylindrical coordinates, proving that DDA can be a promising alternative method to design and optimize nanophotonic devices.

Three-dimensional metastructures are capable of surpassing planar metasurfaces in performance and range of functionality. Design of 3D devices is less intuitive than 2D metasurfaces, but is tractable with the help of inverse-design techniques. This talk will discuss the difficulties with employing gradient-based inverse optimization on high index-contrast devices, and will present innovative solutions to address this. The design methodology is applied to submillimeter-wave antenna design fabricated entirely with etched Silicon. This talk may be interesting to those interested in electromagnetic design for any frequency band, and the targeted application may be interesting to the terahertz astrophysics community.

We show that deep residual generative neural networks, based on global topology optimization networks (GLOnets), can be configured to perform the multi-objective and categorical global optimization of photonic devices. We demonstrate the unity of our method in the optical thin-film stack design.

We report a framework comprising of a combination of sequence models (e.g. RNN, LSTM, Bi LSTM etc) and deep neural networks (DNNs) to tackle the forward problem of predicting the optical response for a given geometry of a broadband, terahertz metamaterial absorber based on Au split-ring resonators. We obtained a training and validation losses of 0.0062 and 0.0042 respectively. The test dataset for this model yielded a loss of 0.0026. Using our model, we were able to predict the spectral response of similar metamaterial absorber geometries in less than 0.5 seconds.

We propose a new design scheme to incorporate hard experimental constraints to the global optimization of inversely designed nanophotonic devices. We demonstrate the concept by design globally optimized freeform high performance deflection gratings with robustness and hard constraints imposed..

We discuss our recent advances on metasurfaces based on highly nonlocal features, stemming from long-range interactions and lattice phenomena. Different from conventional metasurface approaches, nonlocality offers tailored spectral control, both temporally and spatially, ideal for signal processing applications, and combined with enhanced light-matter interactions. We achieve these features by combining quasi-bound states in the continuum with geometric phase variations, tailoring at will the supported eigenwaves. The resulting high-contrast metasurfaces support ultrasharp responses selective to the impinging wave properties, effectively realizing ultrathin transparent films that highly reflect light only when illuminated by selected polarization, frequency and wavefront distribution. The demonstrated wavefront selectivity of nonlocal metasurfaces opens opportunities for augmented reality, secure communications, optical modulators and enhanced light-matter interactions.

Recently, we observe the emergence of a new field of all-dielectric resonant metaphotonics aiming at the manipulation of strong optically-induced electric and magnetic Mie-type resonances in dielectric nanostructures with high refractive index. Unique advantages of dielectric resonant nanostructures over their metallic counterparts are low dissipative losses and the enhancement of both electric and magnetic fields that provide competitive alternatives for some problems in plasmonics including optical nanoantennas, biosensors, active metasurfaces, and metadevices. This talk aims to highlight the recent advances in the physics and applications of all-dielectric Mie-resonant metaphotonics, including nonlinear effects, light-emitting metasurfaces, and applications of bound states in the continuum.

Supersymmetric (SUSY) design is an intuitive procedure for the inverse design of structures from known spectral features. We design one-dimensional corrugated waveguides using SUSY for the desired spectral response. Due to the finite length of the grating, inserted states have a finite lifetime. We obtain a bound state in the continuum by the interference of two states at the same frequency decaying in the same waveguide. The transmission coefficient has no imaginary part at the BIC point, leading to an infinite lifetime. For a finite structure, the phase changes abruptly by 2pi without changing the density of states.

We present numerical modeling and experimental characterization of the photonic bound states in high-contrast Si-based subwavelength grating waveguide structures. The resonant modes in the grating waveguides show some of the unique features of the photonic bound states in the continuum: continuous narrowing of the resonance linewidth and cancellation of radiative waves. The calculated field distributions show strong internal field buildup around resonances. To verify our simulation results, a Si-based subwavelength grating waveguide was fabricated and experimentally characterized. The measured reflection spectra show two resonance peaks around λ₀ = 1490 nm and λ₀ = 1505 nm. According to the simulated results, these two peaks are located near a BIC condition. The captured infrared microscope images in the reflection measurement reveal the dynamical interaction between the incident light and the subwavelength grating waveguide. The demonstrated Si grating waveguides has potential to be used as highly efficient frequency-selective couplers between free-space optical waves to in-plane guided optical waves in existing Si integrated photonic circuits.

Mie scatterer resonantly scatters when wavelength of incident light is similar to the size of the scatterer. The scattering of Mie resonator can be analyzed using multipole decomposition; silicon nanostructure has multipole scattering modes in visible regime. When the Mie scatterers are arrayed, the scattering response can be greatly amplified. To properly design array of Mie scatterer, i.e. metasurface, the hybridization of radiation mode of scatterer and lattice effect, i.e. guided-mode resonance (GMR), must be understood. Herein, we would like to provide the scattering mechanisms behind the hybridization between individual scattering mode and lattice effect, and use them to realize gradient structural coloration by silicon-based metasurface. We believe that a solid understanding of the coupling between individual Mie resonators and the lattice resonances can be a strong basis for designing planar spectral filters.

Image processing has become a critical technology in a variety of science and engineering disciplines. While most image processing is performed digitally, optical analog processing has the advantages of being low-power and high-speed. Here, we demonstrate optical analog imaging processing using flat optics including multi-layer architectures. The use of flat meta-optics opens new doors in optical image processing, such as edge imaging filters, as well as the freedom to spatially multiplex optical functions for off-loading processing tasks from the digital system.

Three-dimensional elements, with refractive index distribution structured at subwavelength scale, provide an expansive optical design space that can be harnessed for demonstrating multifunctional free-space optical devices. We present three dimensional dielectric elements, designed to be placed on top of the pixels of image sensors that provide different functionalities like sorting and focusing of light based on its color and polarization with efficiency significantly surpassing two dimensional absorptive and diffractive filters, and ultra-compact polarimetry. The devices are designed via iterative gradient-based optimization to account for multiple target functions while ensuring compatibility with existing nanofabrication processes, and they are experimentally validated using a scaled device that operates at microwave frequencies. This approach combines arbitrary functions into a single compact element, even where there is no known equivalent in bulk optics, enabling novel integrate

In this work I will show how meta-optics and computational backend can be used to perform sensing with lower power, and performance not achievable by just meta-optics or just computation.

Designing a metalens to achieve high resolution, large field-of-view imaging is challenging due to the geometrical aberration of metalens. Here, we report a new approach to tackle this challenge. We first optimize the phase profile of the metalens to balance the focusing ability for light incidences of different angles. While the image suffers some resolution loss due to a small degradation of on-axis focus performance, the field-of-view is significantly increased. We then use a deconvolution method to computationally recover the high-resolution image. Using this hybrid approach, we achieve high quality, high resolution, large field-of-view metalens imaging.

Dielectric meta-surface optical elements are of interest for a variety of interests, one of which is due to their high efficiency. Elements with binary structures that are easy to fabricate can have more than two phase levels, unlike diffractive elements. Previously reported work on meta-surfaces for complex light generation has focused mainly on beams with orbital angular momentum or Bessel beams. A few papers also talk about Airy beams. The latter have been found to have important applications in areas such as light sheet microscopy. These earlier papers describe the meta-surfaces in detail without dwelling on the various design and fabrication steps required to achieve a highly efficient element. Therefore, in this paper, we describe the design process in detail and highlight the key parameters that must be carefully optimized for successful fabrication. Finally, test results of a highly efficient, dielectric meta-surface cubic phase plate (CPP) are presented. The element has 8 phase levels and generates an Airy beam at the wavelength of 1064 nm. The focus of this work is to present a detailed step-by-step design flow, as well as the means by which to optimize the fabrication of those 8 levels. The phase variation is achieved by a change in the lateral dimensions. While the sizes are unique for each phase level, they are all sub-micron and this is where the fabrication challenge lies.

Quantitative phase imaging (QPI) solutions are developed to characterize metasurfaces through single-shot. SID4 wavefront analyzer based on the quadriwave lateral shearing interferometry, is used to provide the quantitative phase information. The work presents the ability of the wavefront sensor to characterize metasurfaces regardless its natures (effective refractive index or Pancharatnam-Berry phase, namely), or its phase functions. Advanced analyses are also performed on MetaLenses such as wavefront, PSF, MTF, Zernike polynomial analysis.

Metasurfaces may be structured with anisotropic constituent elements, leading to form birefringence. In recent years, these devices have enabled a variety of new polarization-dependent optical elements. The most general of these are holograms; a wide variety of polarization-dependent holograms have been demonstrated with metasurfaces, including those that can be switched by an arbitrarily specified basis of (in general) elliptical polarization states. In this presentation, we will thoroughly review work of this nature. In doing so, we will show that the design freedom afforded by form-birefringent metasurfaces to produce polarization-dependent holograms has not been fully exploited: metasurfaces may be used to produce what we dub “Jones matrix holograms”, in which the polarization response is not limited to an orthogonal basis of polarization states. Examples of these are shown.

Metasurfaces are arrays of artificially engineered subwavelength nanostructures. Owing to the strong form birefringence of these nanostructures, metasurfaces provide a fascinating platform to realize novel polarization optics. Recently, we proposed a more general design strategy for polarization-dependent metasurfaces using Fourier optics principles applied to the Jones calculus. We use this to design metasurface devices with arbitrarily chosen polarization responses embedded on diffraction orders, such as polarizers, waveplates, and cases that are mixtures of the two. We fabricate these gratings (for operation at visible wavelengths) and test them with Mueller matrix polarimetry, showing agreement with design.

In this study, by using glancing angle deposition technique, subsequent and repeated depositions of silicon(Si) and silver(Ag) lead to nanometer-dimension chiral subsegments, and thereby, we successfully fabricated spatially coherent, highly porous, super lattice type helical heterostructure thin films. We theoretically and experimentally investigate the chiro-optical properties of this new type plasmonic metamaterial via finite element modeling calculations and Mueller matrix spectroscopic ellipsometry method, respectively. The systematic changes in the morphology of helical structures by incorporating the plasmonic subsegments reveal an extra-ordinary chiro-optical response with fine spectral tunability over the entire visible spectral range into the ultra-violet.

The chiroptical effects are omnipresent throughout the universe and play a vital role in the sorting and detecting enantiomers in numerous applications like life sciences, pharmaceuticals, agrochemicals, food industry, etc. These chiroptical effects, along with polarization retention and full phase modulation, can have a significant potential for applications such as chiral imaging, anti-counterfeiting, and security. For strong chiroptical effects, all-dielectric metadevices offer a compact and efficient substitute to three-dimensional (3D) chiral metamaterials and flat plasmonic metadevices, which are prone to complex fabrication and ohmic losses, respectively. Here, we propose a unique metasurface based on the combination of achiral structures to achieve chiroptical effect with polarization retention and wavefront shaping. The proposed structure reflects the left hand circularly polarized (LHCP) light while preserving its handedness with complete absorption of the right hand circularly polarized (RHCP) and vice versa. Meanwhile, the structure provides full 2π phase modulation designed by hydrogenated amorphous silicon (a-Si:H), which is a low-loss, CMOS (complementary metal-oxide-semiconductor) compatible material with fabrication ease. The spin-selective reflection with circular dichroism and full phase modulation of designed structure find application in integrated optics, quantum optics, detection, and chiral imaging.

Owing to their high optical and mechanical quality suspended silicon nitride thin films are widely used for photonics and sensing applications. We discuss the fabrication and characterization of highly reflective one- dimensional subwavelength gratings patterned on commercial high-tensile stress Si₃N₄ membranes. Their non- invasive structural characterization using Atomic Force Microscopy provides detailed information on both the grating transverse profile and the deflection of the films after etching, which are compared with optical measurements and mechanical simulations, respectively. We then apply these ultrathin, low-loss optical components to optical spatial differentiation and demonstrate high quality first- and second-order spatial differentiation of the transverse profile of a Gaussian beam.

Monolithic high contrast grating (MHCG) is a particular type of a grating where both substrate and grating bars are made of the same material, in our case this is GaAs. Here we present the numerical simulations of GaAs-based planar focusing MHCG mirrors. In particular we compare the dependence of their reflectivity and the maximum intensity of the reflected light at the focal point with conventional parabolic reflectors of the same size and identical focal lengths. Our study is performed for both TE and TM polarizations. Moreover, we analyze the influence of geometrical imperfections (i.e. local disturbance of the height, period or fill factor of the grating) on the focusing properties of the grating mirrors. The project (POIR.04.04.00-00-4358/17) is carried out within the HOMING programme of the Foundation for Polish Science co-financed by the European Union under the European Regional Development Fund.

We discuss the design of partially etched 1D silicon nitride medium-index contrast sub-wavelength grating structures for generating strong longitudinally polarized resonant field and compare this with focal-field simulations obtained for radially and linearly polarized incident light. Normal incident plane-wave polarized perpendicular to the silicon nitride grating lines (TM polarization) generates both in-plane and out-of-plane field components relative to the grating plane, also here called as field depolarization, the ratio of which can be engineered by varying the grating dimensions. To have maximum depolarization above the structure, the etch depth of the gratings was fixed low at 30 nm, keeping the pitch and duty cycle fixed at 1 𝜇m and 50% respectively and the unetched thickness was varied. Maximum depolarization ratio, defined as the ratio of maximum longitudinal to transverse electric field intensity above the structure was observed to be 1.2 for an unetched thickness of 300 nm at resonant wavelength of 1489 nm. With thicker unetched thickness of 1000 nm, the depolarization within the structure can be maximized to 4.8 with resonance at 1826 nm. Often the generation of strong longitudinally polarized focal-fields relies on the use of tightly focused radially polarized incident light and imposes restrictions on the specimen due to the use of high-index immersion media. Such depolarization ratios are typically achieved with high numerical aperture (NA) focusing objective lens with NA greater than 1.25. Furthermore, we also report a simulation-based study of these structures for enhancing dark excitonic photoluminescence from Tungsten Diselenide(WSe2) monolayer integrated with these structures and observed the photoluminescence in presence of grating to be 20 times enhanced than that off grating.

Here we present how the monolithic high contrast grating (MHCG) mirror focuses light. The studied grating has a shape of a square with 300 micrometers side. The light focuses along one of the sides of the square. As a light source in our experiment we use a vertical-cavity surface-emitting laser that emits 980 nm. In our setup the light shines from above and the grating is on the bottom of the substrate. Based on numerous images taken by a camera attached to an optical microscope we generated a movie showing how the light intensity changes as a function of height above the grating. The FWHM at the focal point is around 5 micrometers and is observed around 200 micrometers above the top surface of the substrate. The measured focal length is in perfect agreement with the simulated data. Moreover, the light intensity at the focal point is more than 10 times larger as compared to the light intensity reflected by Au mirror reference. The project (POIR.04.04.00-00-4358/17) is financed by FNP

We propose a grating that design is inverted with respect to a design of conventional high-contrast grating (HCG) in which low refractive index grating is implemented on high refractive index cladding. We show that inverted HCG can achieve power reflectance of nearly 100% even if the refractive index of the grating is as low as 1.8. Inverted HCG facilitates implementation of highly reflecting mirrors composed of etched SiN, HfO or 3D micro-printed IP-Dip on semiconductors such as GaAs that processing is less technologically demanding with respect to processing of HCGs or monolithic HCGs.