Alexander Y. Zhu

Metalenses at visible wavelengths: Diffraction-limited focusing and subwavelength resolution imaging

Multifunction planar optics Specially designed two-dimensional (2D) arrays of nanometer-scale metallic antennas, or metasurfaces, may allow bulky optical components to be shrunk down to a planar device structure. Khorasaninejad et al. show that arrays of nanoscale fins of TiO can function as high-end optical lenses. At just a fraction of the size of optical objectives, such planar devices could turn your phone camera or your contact lens into a compound microscope. Maguid et al. interleaved sparse 2D arrays of metal antennas to get multifunctional behavior from the one planar device structure (see the Perspective by Litchinitser). The enhanced functionality of such designed metasurfaces could be used in sensing applications or to increase the communication capacity of nanophotonic networks. Science, this issue pp. 1190 and 1202; see also p. 1177 A planar metasurface can function as a high-end optical lens. Subwavelength resolution imaging requires high numerical aperture (NA) lenses, which are bulky and expensive. Metasurfaces allow the miniaturization of conventional refractive optics into planar structures. We show that high-aspect-ratio titanium dioxide metasurfaces can be fabricated and designed as metalenses with NA = 0.8. Diffraction-limited focusing is demonstrated at wavelengths of 405, 532, and 660 nm with corresponding efficiencies of 86, 73, and 66%. The metalenses can resolve nanoscale features separated by subwavelength distances and provide magnification as high as 170×, with image qualities comparable to a state-of-the-art commercial objective. Our results firmly establish that metalenses can have widespread applications in laser-based microscopy, imaging, and spectroscopy.

Graphene-enabled silver nanoantenna sensors.

Silver is the ideal material for plasmonics because of its low loss at optical frequencies but is often replaced by a more lossy metal, gold. This is because of silvers tendency to tarnish and roughen, forming Ag(2)S on its surface, dramatically diminishing optical properties and rendering it unreliable for applications. By passivating the surface of silver nanostructures with monolayer graphene, atmospheric sulfur containing compounds are unable to penetrate the graphene to degrade the surface of the silver. Preventing this sulfidation eliminates the increased material damping and scattering losses originating from the unintentional Ag(2)S layer. Because it is atomically thin, graphene does not interfere with the ability of localized surface plasmons to interact with the environment in sensing applications. Furthermore, after 30 days graphene-passivated silver (Ag-Gr) nanoantennas exhibit a 2600% higher sensitivity over that of bare Ag nanoantennas and 2 orders of magnitude improvement in peak width endurance. By employing graphene in this manner, the excellent optical properties and large spectral range of silver can be functionally utilized in a variety of nanoscale plasmonic devices and applications.

Multispectral Chiral Imaging with a Metalens

The vast majority of biologically active compounds, ranging from amino acids to essential nutrients such as glucose, possess intrinsic handedness. This in turn gives rise to chiral optical properties that provide a basis for detecting and quantifying enantio-specific concentrations of these molecules. However, traditional chiroptical spectroscopy and imaging techniques require cascading of multiple optical components in sophisticated setups. Here, we present a planar lens with an engineered dispersive response, which simultaneously forms two images with opposite helicity of an object within the same field-of-view. In this way, chiroptical properties can be probed across the visible spectrum using only the lens and a camera without the addition of polarizers or dispersive optical devices. We map the circular dichroism of the exoskeleton of a chiral beetle, Chrysina gloriosa, which is known to exhibit high reflectivity of left-circularly polarized light, with high spatial resolution limited by the numerical aperture of the planar lens. Our results demonstrate the potential of metasurfaces in realizing a compact and multifunctional device with unprecedented imaging capabilities.

Achromatic Metalens over 60 nm Bandwidth in the Visible and Metalens with Reverse Chromatic Dispersion

We demonstrate an achromatic metalens with a constant focal length over 60 nm bandwidth (λ= 490 nm to 550 nm). We also design metalenses with reverse chromatic dispersion, opposite of a Fresnel lens.

Voltage tuning of plasmonic absorbers by indium tin oxide

We experimentally demonstrate electrical tuning of plasmonic mid-infrared absorber resonances at 4 μm wavelength. The perfect infrared absorption is realized by an array of gold nanostrip antennas separated from a back reflector by a thin dielectric layer. An indium tin oxide active layer strongly coupled to the optical near field of the plasmonic absorber allows for spectral tunability.

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.Controlling the geometry of each dielectric element of a nanostructured surface enables frequency-dependent group delay and group delay dispersion engineering, and the fabrication of an achromatic metalens for imaging in the visible in transmission.

Traditional and emerging materials for optical metasurfaces

Abstract One of the most promising and vibrant research areas in nanotechnology has been the field of metasurfaces. These are two dimensional representations of metaatoms, or artificial interfaces designed to possess specialized electromagnetic properties which do not occur in nature, for specific applications. In this article, we present a brief review of metasurfaces from a materials perspective, and examine how the choice of different materials impact functionalities ranging from operating bandwidth to efficiencies. We place particular emphasis on emerging and non-traditional materials for metasurfaces such as high index dielectrics, topological insulators and digital metamaterials, and the potentially transformative role they could play in shaping further advances in the field.

Optoelectromechanical Multimodal Biosensor with Graphene Active Region

A general, overarching theme in nanotechnology is the integration of multiple disparate fields to realize novel or expanded functionalities. Here, we present a graphene enabled, integrated optoelectromechanical device and demonstrate its utility for biomolecular sensing. We experimentally achieve an ultrawide linear dynamic sensing range of 5 orders of magnitude of protein concentration, an improvement over state-of-the-art single mode nanosensors by approximately 2-3 orders of magnitude, while retaining a subpicomolar lowest detection limit. Moreover, the ability to monitor and characterize adsorption events in the full optoelectromechanical space allows for the extraction of key intrinsic parameters of adsorbates and has the potential to extend the capabilities of nanosensors beyond the traditional binary-valued test for a single type of molecule. This could have significant implications for molecular detection applications at variable concentrations, such as early disease detection in biomedical diagnostics.

Wavelength Tunable Microdisk Cavity Light Source with a Chemically Enhanced MoS2 Emitter

In this work, we report an integrated narrowband light source based on thin MoS2 emissive material coupled to the high quality factor whispering gallery modes of a microdisk cavity with a spatial notch that enables easy out-coupling of emission while it yields high spatial coherence and a Gaussian intensity distribution. The active light emitting material consists of chemically enhanced bilayer MoS2 flakes with a thin atomic layer deposited SiO2 protective coating that yields 20-times brighter chemically enhanced photoluminescence compared to as-exfoliated monolayers on the microdisk. Quality factors ≈ 1000 are observed as well as a high degree of spatial coherence. We also experimentally achieve effective index tuning of cavity coupled emission over a full free spectral range. The thermal response of this system is also studied. This work provides new insights for nanophotonic light sources with atomically thin active media.

Generation of wavelength-independent subwavelength Bessel beams using metasurfaces

Bessel beams are of great interest due to their unique non-diffractive properties. Using a conical prism or an objective paired with an annular aperture are two typical approaches for generating zeroth-order Bessel beams. However, the former approach has a limited numerical aperture (NA), and the latter suffers from low efficiency, as most of the incident light is blocked by the aperture. Furthermore, an additional phase-modulating element is needed to generate higher-order Bessel beams, which in turn adds complexity and bulkiness to the system. We overcome these problems using dielectric metasurfaces to realize meta-axicons with additional functionalities not achievable with conventional means. We demonstrate meta-axicons with high NA up to 0.9 capable of generating Bessel beams with full width at half maximum about as small as ~λ/3 (λ=405 nm). Importantly, these Bessel beams have transverse intensity profiles independent of wavelength across the visible spectrum. These meta-axicons can enable advanced research and applications related to Bessel beams, such as laser fabrication, imaging and optical manipulation.