Jaewon Oh

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.

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 optics1-3. Dielectric metasurfaces demonstrated thus far4-10 are limited to transparency windows at infrared wavelengths because of significant optical absorption and loss at visible wavelengths. It is critical that new materials and fabrication techniques be developed for dielectric metasurfaces at visible wavelengths to enable applications such as three-dimensional displays, wearable optics and planar optical systems11. Here, we demonstrate high performance titanium dioxide 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 that exhibits low surface roughness of 0.738 nm and ideal optical properties. To fabricate the metasurfaces we use a lift-off-like process that allows us to produce highly anisotropic nanofins with shape birefringence. This process is applicable to any optical element and can be implemented using a broad class of materials.Significance Metasurfaces are optical elements that can mimic and expand on the functionality of refractive optics with a comparatively thin and planar profile. Transmissive metasurfaces suffer from high optical loss at visible wavelengths because of currently used materials and fabrication techniques. This work introduces metasurfaces that operate across the visible spectrum with high efficiencies. As the basis of our metasurfaces, we used a common material, titanium dioxide, and a fabrication method based an atomic layer deposition that creates highly anisotropic nanostructures. The results presented here provide an important advance for realizing optical components at visible wavelengths—e.g., lenses, holograms, and phase shifters—with orders of magnitude reduction in thickness compared with traditional refractive optics. 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.

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.

Super-Dispersive Off-Axis Meta-Lenses for Compact High Resolution Spectroscopy.

Metasurfaces have opened a new frontier in the miniaturization of optical technology by allowing exceptional control over the wavefront. Here, we demonstrate off-axis meta-lenses that simultaneously focus and disperse light of different wavelengths with unprecedented spectral resolution. They are designed based on the geometric phase via rotated silicon nanofins and can focus light at angles as large as 80°. Due to the large angle focusing, these meta-lenses have superdispersive characteristics (0.27 nm/mrad) that make them capable of resolving wavelength differences as small as 200 pm in the telecom region. In addition, by stitching several meta-lenses together, we maintain a high spectral resolution for a wider wavelength range. The meta-lenses have measured efficiencies as high as 90% in the wavelength range of 1.1 to 1.6 μm. The planar and compact configuration together with high spectral resolution of these meta-lenses has significant potential for emerging portable/wearable optics technology.

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.

Ultra-compact visible chiral spectrometer with meta-lenses

Conventional compact spectrometers have a fixed spectral resolution and cannot resolve the polarization properties of light without additional optical elements, while their larger counterparts are bulky and costly. Here, we demonstrate multiple off-axis meta-lenses in the visible integrated on a single planar substrate. They possess both focusing and strongly dispersive properties and are designed to provide different spectral resolutions as well as working wavelength ranges on the same chip. We realize a compact spectrometer using only these meta-lenses and a CMOS camera and achieve detector-limited spectral resolutions as small as 0.3 nm and a total working wavelength range exceeding 170 nm for a beam propagation length of only a few cm. In addition, this spectrometer has the capability to resolve different helicities of light in a single measurement. This chip-camera setup represents the most compact configuration so far achieved for a spectrometer with similar performance and functionality, and its co...

Visible Wavelength Planar Metalenses Based on Titanium Dioxide

We present recent advances in metasurface-based photonics, which enables the realization of high performance planar lenses (metalenses) in the visible spectrum. They are enabled by a technique based on atomic layer deposition of titanium dioxide allowing for the fabrication of nanostructures with high fidelity. First, we demonstrate highly efficient metalenses with numerical aperture NA = 0.8 using the Pancharatnam-Berry phase approach. These metalenses can focus light into a diffraction-limited spot. They have efficiencies as high as 86% and provide high imaging resolution. Furthermore, by judicious design of the phase-shifting elements, we achieve a multispectral chiral metalens realized with a single metasurface layer. This chiral metalens can resolve both the chiral and spectral information of an object without the requirement of any additional optical components. Finally, we discuss the experimental realization of polarization-insensitive metalenses with NAs as high as 0.85. They are able to focus incident light to a spot as small as ~0.64λ with efficiencies up to 60%. Due to its straightforward and CMOS-compatible fabrication, this platform is promising for a wide range of applications ranging from camera modules, displays, laser-based imaging, microscopy, and spectroscopy to laser fabrication and lithography.

High‐Operating‐Temperature Direct Ink Writing of Mesoscale Eutectic Architectures

High-operating-temperature direct ink writing (HOT-DIW) of mesoscale architectures that are composed of eutectic silver chloride-potassium chloride. The molten ink undergoes directional solidification upon printing on a cold substrate. The lamellar spacing of the printed features can be varied between approximately 100 nm and 2 µm, enabling the manipulation of light in the visible and infrared range.

Macro to nanoscale imaging using planar lenses at visible wavelengths

The bulky nature of optical components has long hindered their convenient integration into our daily lives. Digital single-lens reflex (DSLR) cameras, for example, require multiple sets of bulky lenses that must be interchanged depending on the desired imaging parameters. Analogous to the miniaturization of silicon chips that has revolutionized electronic devices over the past few decades, lightweight and planar optical components are key factors for enabling the development of easy-to-carry or wearable devices. Traditionally, optical devices shape the wavefront of light via propagation through a bulky medium, like a lens. Furthermore, to improve the image quality or extract additional information (e.g., polarization), optical components must be cascaded, resulting in heavy and bulky systems. In this scenario, metasurfaces1–4—which enable wavefronts to be controlled without requiring volumetric propagation—open up a new frontier in optics. Multifunctional, high-performance metasurfaces have been reported, but the materials that are commonly used (i.e., noble metals5 or silicon6, 7) are lossy in the visible spectrum. As a result, practical demonstrations in this region are lacking. We have recently developed a fabrication approach based on the atomic-layer deposition (ALD) of titanium dioxide (TiO2/, yielding high-aspect-ratio nanostructures with negligible material and scattering losses.8 With this TiO2-based metasurface platform,8–11 we have fabricated metalenses that operate at visible wavelengths with unprecedented imaging quality.9 Our metalenses are also capable of resolving chirality, thereby opening Figure 1. Metalens design and fabrication.9 (A) Side view and (B) top view of our metalens building block, a titanium dioxide (TiO2) nanofin on a glass substrate. Phase is imparted via the rotation of each nanofin by the angle

Planar optics at visible wavelengths based on titanium dioxide (Conference Presentation)

We present a new platform that realizes high performance metasurfaces in the visible spectrum. This platform is based on atomic layer deposition of titanium dioxide and allows molding incident light wavefront to desired shapes including holographic images, optical vortices, and Bessel beams. The focus of this work will be on the design and demonstration of planar metalenses. We report on our recent experimental realization of high numerical aperture metalenses with efficiency as high as 86%. These metalenses can focus light into a diffraction-limited spot and can be employed for imaging purposes to provide sub-wavelength imaging resolution. In addition, by the judicious design of metalens building blocks, one can achieve a multispectral chiral metalens (MCML) within a single metasurface layer. The MCML can simultaneously resolve chiral and spectral information of an object without the requirement of additional optical components such as polarizers, wave-plates, or even gratings. Using this MCML, we map the chiroptical properties of a macroscopic chiral biological specimen across the visible range. Finally, since many applications require polarization insensitive planar lenses, we discuss the experimental realization of such metalenses with numerical apertures as high as NA=0.85. These metalenses can focus incident light to a spot as small as ~0.6lambda with efficiencies up to 70%. The straightforward and CMOS-compatible fabrication process of this platform is promising for a wide range of optics-based applications in multidisciplinary science and technology.