Broadband high-efficiency dielectric metasurfaces for the visible spectrum.

Metasurfaces are planar optical elements that hold promise for overcoming the limitations of refractive and conventional diffractive optics. Original dielectric metasurfaces are limited to transparency windows at infrared wavelengths because of significant optical absorption and loss at visible wavelengths. Thus, it is critical that new materials and nanofabrication techniques be developed to extend dielectric metasurfaces across the visible spectrum and to enable applications such as high numerical aperture lenses, color holograms, and wearable optics. Here, we demonstrate high performance dielectric metasurfaces in the form of holograms for red, green, and blue wavelengths with record absolute efficiency (>78%). We use atomic layer deposition of amorphous titanium dioxide with surface roughness less than 1 nm and negligible optical loss. We use a process for fabricating dielectric metasurfaces that allows us to produce anisotropic, subwavelength-spaced dielectric nanostructures with shape birefringence. This process is capable of realizing any high-efficiency metasurface optical element, e.g., metalenses and axicons.

Single-Layer Metasurface with Controllable Multiwavelength Functions.

In this paper, we report dispersion-engineered metasurfaces with distinct functionalities controlled by wavelength. Unlike previous approaches based on spatial multiplexing or vertical stacking of metasurfaces, we utilize a single phase profile with wavelength dependence encoded in the phase shifters' dispersion. We designed and fabricated a multiwavelength achromatic metalens (MAM) with achromatic focusing for blue (B), green (G), yellow (Y), and red (R) light and two wavelength-controlled beam generators (WCBG): one focuses light with orbital angular momentum (OAM) states ( l = 0,1,2) corresponding to three primary colors; the other produces ordinary focal spots ( l = 0) for red and green light, while generating a vortex beam ( l = 1) in the blue. A full color (RGB) hologram is also demonstrated in simulation. Our approach opens a path to applications ranging from near-eye displays and holography to compact multiwavelength beam generation.

Immersion Meta-Lenses at Visible Wavelengths for Nanoscale Imaging.

Immersion objectives can focus light into a spot smaller than what is achievable in free space, thereby enhancing the spatial resolution for various applications such as microscopy, spectroscopy, and lithography. Despite the availability of advanced lens polishing techniques, hand-polishing is still required to manufacture the front lens of a high-end immersion objective, which poses major constraints for lens design. This limits the shape of the front lens to spherical. Therefore, several other lenses need to be cascaded to correct for spherical aberration, resulting in significant challenges for miniaturization and adding design complexity for different immersion liquids. Here, by using metasurfaces, we demonstrate liquid immersion meta-lenses free of spherical aberration at various design wavelengths in the visible spectrum. We report water and oil immersion meta-lenses of various numerical apertures (NA) up to 1.1 and show that their measured focal spot sizes are diffraction-limited with Strehl ratios of approximately 0.9 at 532 nm. By integrating the oil immersion meta-lens (NA = 1.1) into a commercial scanning confocal microscope, we achieve an imaging spatial resolution of approximately 200 nm. These meta-lenses can be easily adapted to focus light through multilayers of different refractive indices and mass-produced using modern industrial manufacturing or nanoimprint techniques, leading to cost-effective high-end optics.

Spin-to-orbital angular momentum conversion in dielectric metasurfaces: erratum.

We would like to clarify our paper [Opt. Express25, 377 (2017)] abstract sentence "These beams carry orbital angular momentum proportional to the number of intertwined helices constituting the wavefront."

Spin-to-orbital angular momentum conversion in dielectric metasurfaces.

Vortex beams are characterized by a helical wavefront and a phase singularity point on the propagation axis that results in a doughnut-like intensity profile. These beams carry orbital angular momentum proportional to the number of intertwined helices constituting the wavefront. Vortex beams have many applications in optics, such as optical trapping, quantum optics and microscopy. Although beams with such characteristics can be generated holographically, spin-to-orbital angular momentum conversion has attracted considerable interest as a tool to create vortex beams. In this process, the geometrical phase is exploited to create helical beams whose handedness is determined by the circular polarization (left/right) of the incident light, that is by its spin. Here we demonstrate high-efficiency Spin-to-Orbital angular momentum-Converters (SOCs) at visible wavelengths based on dielectric metasurfaces. With these SOCs we generate vortex beams with high and fractional topological charge and show for the first time the simultaneous generation of collinear helical beams with different and arbitrary orbital angular momentum. This versatile method of creating vortex beams, which circumvents the limitations of liquid crystal SOCs and adds new functionalities, should significantly expand the applications of these beams.

High-quality-factor planar optical cavities with laterally stopped, slowed, or reversed light.

In a planar optical cavity, the resonance frequencies increase as a function of in-plane wavevector according to a standard textbook formula. This has well-known consequences in many different areas of optics, from the shifts of etalon peaks at non-normal angles, to the properties of transverse modes in laser diodes, to the effective mass of microcavity photons, and so on. However, this standard formula is valid only when the reflection phase of each cavity mirror is approximately independent of angle. There is a certain type of mirror-a subwavelength dielectric grating near a guided mode resonance-with not only a strongly angle-dependent reflection phase, but also very high reflectance and low losses. Simulations show that by using such mirrors, high-quality-factor planar cavities can be designed that break all these textbook rules, leading to resonant modes that are slow, stopped or even backward-propagating in the in-plane direction. In particular, we demonstrate experimentally high-Q planar cavities whose resonance frequency is independent of in-plane wavevector-i.e., the resonant modes have zero in-plane group velocity, for one polarization but both in-plane directions. We discuss potential applications in various fields including lasers, quantum optics, and exciton-polariton condensation.

High efficiency near diffraction-limited mid-infrared flat lenses based on metasurface reflectarrays.

We report the first demonstration of a mid-IR reflection-based flat lens with high efficiency and near diffraction-limited focusing. Focusing efficiency as high as 80%, in good agreement with simulations (83%), has been achieved at 45° incidence angle at λ = 4.6 µm. The off-axis geometry considerably simplifies the optical arrangement compared to the common geometry of normal incidence in reflection mode which requires beam splitters. Simulations show that the effects of incidence angle are small compared to parabolic mirrors with the same NA. The use of single-step photolithography allows large scale fabrication. Such a device is important in the development of compact telescopes, microscopes, and spectroscopic designs.

Broadband Multifunctional Efficient Meta-Gratings Based on Dielectric Waveguide Phase Shifters.

Molding the wavefront of light is a basic principle of any optical design. In conventional optical components such as lenses and waveplates, the wavefront is controlled via propagation phases in a medium much thicker than the wavelength. Metasurfaces instead typically produce the required phase changes using subwavelength-sized resonators as phase shift elements patterned across a surface. This "flat optics" approach promises miniaturization and improved performance. Here we introduce metasurfaces which use dielectric ridge waveguides (DRWs) as phase shift elements in which the required phase accumulation is achieved via propagation over a subwavelength distance. By engineering the dispersive response of DRWs, we experimentally realize high resolving power meta-gratings with broadband (λ = 1.2-1.7 µm) and efficient routing (splitting and bending) into a single diffraction order, thus overcoming the limits of blazed gratings. In addition, we demonstrate polarization beam splitting capabilities with large suppression ratios.

Achromatic Metasurface Lens at Telecommunication Wavelengths.

Nanoscale optical resonators enable a new class of flat optical components called metasurfaces. This approach has been used to demonstrate functionalities such as focusing free of monochromatic aberrations (i.e., spherical and coma), anomalous reflection, and large circular dichroism. Recently, dielectric metasurfaces that compensate the phase dispersion responsible for chromatic aberrations have been demonstrated. Here, we utilize an aperiodic array of coupled dielectric nanoresonators to demonstrate a multiwavelength achromatic lens. The focal length remains unchanged for three wavelengths in the near-infrared region (1300, 1550, and 1800 nm). Experimental results are in agreement with full-wave simulations. Our findings are an essential step toward a realization of broadband flat optical elements.

Color generation and refractive index sensing using diffraction from 2D silicon nanowire arrays.

Tunable structural color generation from vertical silicon nanowires arranged in different square lattices is demonstrated. The generated colors are adjustable using well-defined Bragg diffraction theory, and only depend on the lattice spacing and angles of incidence. Vivid colors spanning from bright red to blue are easily achieved. In keeping with this, a single square lattice of silicon nanowires is also able to produce different colors spanning the entire visible range. It is also shown that the 2D gratings also have a third grating direction when rotated 45 degrees. These simple and elegant solutions to color generation from silicon are used to demonstrate a cost-effective refractive index sensor. The sensor works by measuring color changes resulting from changes in the refractive index of the medium surrounding the nanowires using a trichromatic RGB decomposition. Moreover, the sensor produces linear responses in the trichromatic decomposition values versus the surrounding medium index. An index resolution of 10(-4) is achieved by performing basic image processing on the collected images, without the need for a laser or a spectrometer. Spectral analysis enables an increase in the index resolution of the sensor to a value of 10(-6) , with a sensitivity of 400 nm/RIU.

Optimization of multiple-slot waveguides for biochemical sensing.

In this work, we analyze and optimize an optical biochemical sensor using silicon multiple-slot waveguides. The rigorous optimization procedure considers parameters such as ridge width, slot width, the number of slots, and the effect of residual silicon left at the bottom of the slot region. These parameters are then optimized using a figure of merit to achieve the highest possible sensitivity to bulk and surface changes in the upper cladding of the sensor. The multiple-slot structure is then studied in a bend configuration in order to construct ring-resonator-based sensors. A bulk sensitivity of 912 nm/refractive index unit is achieved for a change in bulk refractive index, which is three times better than single-slot waveguides.

Colorimetric sensors using nano-patch surface plasmon resonators.

A two-dimensional array of gold nano-patches on a highly reflective mirror is proposed for refractive index sensing based on changes in the reflected colors. The grating on the mirror creates localized surface plasmon resonances resulting in a minimum in the visible reflectance spectra. The wavelength of the resonance can be tuned by changing the width of the nano-patches and is also dependent on the refractive index of the surrounding medium. The color variation due to change in the refractive index is measured and used to realize a simple low-cost sensor with a refractive index resolution better than 10⁻5 just using image processing. The efficacy of the proposed sensor is also demonstrated for surface sensing by depositing thin layers of silicon dioxide. The color difference due to the addition of a 3 nm thick layer of silicon dioxide is detectable by the naked eye and deposition thickness of 2 Šcan be resolved using image processing.

Optical bio-chemical sensors on SNOW ring resonators.

In this paper, we propose and analyze novel ring resonator based bio-chemical sensors on silicon nanowire optical waveguide (SNOW) and show that the sensitivity of the sensors can be increased by an order of magnitude as compared to silicon-on-insulator based ring resonators while maintaining high index contrast and compact devices. The core of the waveguide is hollow and allows for introduction of biomaterial in the center of the mode, thereby increasing the sensitivity of detection. A sensitivity of 243 nm/refractive index unit (RIU) is achieved for a change in bulk refractive index. For surface attachment, the sensor is able to detect monolayer attachments as small as 1 Å on the surface of the silicon nanowires.

Silicon nanowire optical waveguide (SNOW).

In this paper, we propose a novel optical waveguide consisting of arrays of silicon nanowires in close proximity. We show that such a structure can guide an optical mode provided the electric field is polarized along the length of the nanowires. Furthermore, such guidance can happen even if the nanowires are arranged randomly albeit at a higher scattering loss. On the other hand, high radiation losses are observed if the electric field is polarized in the transverse direction to the nanowires. We calculate the optical radiation loss for different structures using Finite Difference Time Domain (FDTD) method. We also show that the arrayed nanowire region can be approximated using an effective index bulk waveguide. The approximation allows for design and optimization of optical structures using integrated optics methodology resulting in significant savings in time and resources. The advantage of the proposed waveguide structure is that it allows for increased optical confinement while using the enhanced optical interactions of nanowire structures compared to single nanowire photonic waveguide for diameters smaller than 100 nm. For a diameter of 50 nm for the silicon nanowire, an optical confinement factor of 33 % was achieved in the proposed waveguide as opposed to 0.1 % that is achieved for a single nanowire photonic waveguide. A radiation loss of 0.12 cm(-1) is achieved for nanowires of the same diameter spaced 75 nm apart. While our analysis is done on silicon nanowires at 1550 nm, the proposed structures can be extended to other materials and wavelength regimes also.

Gallium phosphide optical metasurfaces for visible light applications.

There are few materials that are broadly used for fabricating optical metasurfaces for visible light applications. Gallium phosphide (GaP) is a material that, due to its optical properties, has the potential to become a primary choice but due to the difficulties in fabrication, GaP thin films deposited on transparent substrates have never been exploited. In this article we report the design, fabrication, and characterization of three different amorphous GaP metasurfaces obtained through sputtering. Although the material properties can be further optimized, our results show the potential of this material for visible applications making it a viable alternative in the material selection for optical metasurfaces.

All-optical logic gates using nonlinear effects in silicon-on-insulator waveguides.

We propose multiple all-optical logic operations in complementary metal oxide semiconductor compatible silicon-on-insulator waveguides based on three nonlinear phenomena, stimulated Raman scattering, free carrier absorption, and cross phase modulation. The performance of three optical logic operations is simulated by use of the finite-difference time-domain method. We achieved an extinction ratio of approximately 13 dB between two logic levels.

Modified Fabry-Perot interferometric method for waveguide loss measurement.

An extension to the Fabry-Perot interferometric method is demonstrated to calculate the optical loss and the reflectivity for optical waveguides simultaneously. The method uses an excitation of the waveguide with a broadband amplified spontaneous emission source (a superluminescent diode in our case) and curve fitting to account for the change of input power, thereby simplifying the measurement procedure. The use of a broadband source as opposed to tunable lasers allows for simultaneous measurements over multiple wavelengths and decreased sensitivity to reflections in the cavity. Further, waveguides of different lengths are measured to calculate the optical loss and the reflectivity simultaneously. It is shown that, if the value for reflectivity is assumed, there could be a large error in the measurement of loss especially for short waveguides. Optical loss for ridge waveguides is measured and compared by using a tunable laser as the input source. The method can be used for a generic case where it is suspected that the input power changes during the measurement.

A broadband achromatic metalens for focusing and imaging in the visible.

A key goal of metalens research is to achieve wavefront shaping of light using optical elements with thicknesses on the order of the wavelength. Such miniaturization is expected to lead to compact, nanoscale optical devices with applications in cameras, lighting, displays and wearable optics. However, retaining functionality while reducing device size has proven particularly challenging. For example, so far there has been no demonstration of broadband achromatic metalenses covering the entire visible spectrum. Here, we show that by judicious design of nanofins on a surface, it is possible to simultaneously control the phase, group delay and group delay dispersion of light, thereby achieving a transmissive achromatic metalens with large bandwidth. We demonstrate diffraction-limited achromatic focusing and achromatic imaging from 470 to 670 nm. Our metalens comprises only a single layer of nanostructures whose thickness is on the order of the wavelength, and does not involve spatial multiplexing or cascading. While this initial design (numerical aperture of 0.2) has an efficiency of about 20% at 500 nm, we discuss ways in which our approach may be further optimized to meet the demand of future applications.

Nano-optic endoscope for high-resolution optical coherence tomography in vivo.

Acquisition of high-resolution images from within internal organs using endoscopic optical imaging has numerous clinical applications. However, difficulties associated with optical aberrations and the trade-off between transverse resolution and depth-of-focus significantly limit the scope of applications. Here, we integrate a metalens, with the ability to modify the phase of incident light at sub-wavelength level, into the design of an endoscopic optical coherence tomography catheter (termed nano-optic endoscope) to achieve near diffraction-limited imaging through negating non-chromatic aberrations. Remarkably, the tailored chromatic dispersion of the metalens in the context of spectral interferometry is utilized to maintain high-resolution imaging beyond the input field Rayleigh range, easing the trade-off between transverse resolution and depth-of-focus. We demonstrate endoscopic imaging both in resected human lung specimens and in sheep airways in vivo. The combination of the superior resolution and higher imaging depth-of-focus of the nano-optic endoscope will likely increase the clinical utility of endoscopic optical imaging.

Giant intrinsic chiro-optical activity in planar dielectric nanostructures.

The strong optical chirality arising from certain synthetic metamaterials has important and widespread applications in polarization optics, stereochemistry and spintronics. However, these intrinsically chiral metamaterials are restricted to a complicated three-dimensional (3D) geometry, which leads to significant fabrication challenges, particularly at visible wavelengths. Their planar two-dimensional (2D) counterparts are limited by symmetry considerations to operation at oblique angles (extrinsic chirality) and possess significantly weaker chiro-optical responses close to normal incidence. Here, we address the challenge of realizing strong intrinsic chirality from thin, planar dielectric nanostructures. Most notably, we experimentally achieve near-unity circular dichroism with ~90% of the light with the chosen helicity being transmitted at a wavelength of 540 nm. This is the highest value demonstrated to date for any geometry in the visible spectrum. We interpret this result within the charge-current multipole expansion framework and show that the excitation of higher-order multipoles is responsible for the giant circular dichroism. These experimental results enable the realization of high-performance miniaturized chiro-optical components in a scalable manner at optical frequencies.