Shane Colburn

Low contrast dielectric metasurface optics

We demonstrate low contrast dielectric metasurface optical elements for operation at visible frequencies. Our devices show transmission efficiencies as high as 90% and focal spots on the order of the design wavelength.

Tunable metasurfaces via subwavelength phase shifters with uniform amplitude.

Metasurfaces with tunable spatial phase functions could benefit numerous applications. Currently, most approaches to tuning rely on mechanical stretching which cannot control phase locally, or by modulating the refractive index to exploit rapid phase changes with the drawback of also modulating amplitude. Here, we propose a method to realize phase modulation at subwavelength length scales while maintaining unity amplitude. Our device is inspired by an asymmetric Fabry-Perot resonator, with pixels comprising a scattering nanopost on top of a distributed Bragg reflector, capable of providing a nearly 2π nonlinear phase shift with less than 2% refractive index modulation. Using the designed pixels, we simulate a tunable metasurface composed of an array of moderately coupled nanopost resonators, realizing axicons, vortex beam generators, and aspherical lenses with both variable focal length and in-plane scanning capability, achieving nearly diffraction-limited performance. The experimental feasibility of the proposed method is also discussed.

Metasurface Freeform Nanophotonics

Freeform optics aims to expand the toolkit of optical elements by allowing for more complex phase geometries beyond rotational symmetry. Complex, asymmetric curvatures are employed to enhance the performance of optical components while minimizing their size. Unfortunately, these high curvatures and complex forms are often difficult to manufacture with current technologies, especially at the micron scale. Metasurfaces are planar sub-wavelength structures that can control the phase, amplitude, and polarization of incident light, and can thereby mimic complex geometric curvatures on a flat, wavelength-scale thick surface. We present a methodology for designing analogues of freeform optics using a silicon nitride based metasurface platform for operation at visible wavelengths. We demonstrate a cubic phase plate with a point spread function exhibiting enhanced depth of field over 300 micron along the optical axis with potential for performing metasurface-based white light imaging, and an Alvarez lens with a tunable focal length range of over 2.5 mm corresponding to a change in optical power of ~1600 diopters with 100 micron of total mechanical displacement. The adaptation of freeform optics to a sub-wavelength metasurface platform allows for further miniaturization of optical components and offers a scalable route toward implementing near-arbitrary geometric curvatures in nanophotonics.

Metasurface optics for full-color computational imaging

We design metalenses to capture colored images with white light by combining metasurfaces and computational imaging. Conventional imaging systems comprise large and expensive optical components that successively mitigate aberrations. Metasurface optics offers a route to miniaturize imaging systems by replacing bulky components with flat and compact implementations. The diffractive nature of these devices, however, induces severe chromatic aberrations, and current multiwavelength and narrowband achromatic metasurfaces cannot support full visible spectrum imaging (400 to 700 nm). We combine principles of both computational imaging and metasurface optics to build a system with a single metalens of numerical aperture ~0.45, which generates in-focus images under white light illumination. Our metalens exhibits a spectrally invariant point spread function that enables computational reconstruction of captured images with a single digital filter. This work connects computational imaging and metasurface optics and demonstrates the capabilities of combining these disciplines by simultaneously reducing aberrations and downsizing imaging systems using simpler optics.

Ultrathin van der Waals Metalenses

Ultrathin and flat optical lenses are essential for modern optical imaging, spectroscopy, and energy harvesting. Dielectric metasurfaces comprising nanoscale quasi-periodic resonator arrays are promising for such applications, as they can tailor the phase, amplitude, and polarization of light at subwavelength resolution, enabling multifunctional optical elements. To achieve 2π phase coverage, however, most dielectric metalenses need a thickness comparable to the wavelength, requiring the fabrication of high-aspect-ratio scattering elements. We report ultrathin dielectric metalenses made of van der Waals (vdW) materials, leveraging their high refractive indices and the incomplete phase design approach to achieve device thicknesses down to ∼λ/10, operating at infrared and visible wavelengths. These materials have generated strong interest in recent years due to their advantageous optoelectronic properties. Using vdW metalenses, we demonstrate near-diffraction-limited focusing and imaging and exploit their layered nature to transfer the fabricated metalenses onto flexible substrates to show strain-induced tunable focusing. Our work enables further downscaling of optical elements and opportunities for the integration of metasurface optics in ultraminiature optoelectronic systems.

Dielectric metasurface-based freeform optics

As light propagates through a material, it accumulates changes (i.e., to its phase, polarization, and amplitude) that transform the incident wavefront.1 With conventional refractive optics, the thickness profile of an element is varied to induce spatially variant changes to these properties. In this manner, the function of an optical element is heavily dependent on its geometry. Historically, manufacturability has limited component types to those with rotational symmetry and low-curvature profiles. However, profiles that are characterized by higher-order polynomials (i.e., more than two) with high curvatures and asymmetry can exhibit useful properties that enable the correction of aberrations,2 offaxis imaging,3 field-of-view expansion,4 and an increase to the depth of field.5 In the field of freeform optics, components of this nature are developed with an aim to minimizing their weight and size.6 Recent interest in freeform optics has been driven by potential applications in near-eye displays7, 8 and compact optical systems for medical, aerospace, and mobile devices (i.e., for which there are stringent size and weight constraints). Unfortunately, elements with high curvatures and complex forms are often difficult to manufacture with existing technologies, particularly at the micron scale. Metasurfaces—planar structures that are composed of quasiperiodic arrays of sub-wavelength scatterers (i.e., optical antennas)—can control the phase, amplitude, and polarization of incident light. Such surfaces can mimic complex geometric curvatures on a flat, wavelength-scale-thick surface,1, 9, 10 thus providing an excellent platform for nanoscale freeform optics. Furthermore, inexpensive and simple fabrication methods can be used in the development of such freeform elements. We have designed and fabricated metasurfaces that operate at visible frequencies using silicon nitride (Si3N4): see Figure 1(a).11, 12 Si3N4 is an ideal material for visible-frequency Figure 1. Dielectric metasurface freeform optics. (a) Scanning electron micrograph of the fabricated cubic phase plates, which together form a composite Alvarez lens. (b) By laterally displacing these two cubic phase plates, we can realize a lenslike (quadratic) phase profile with varying curvature and, hence, different focal lengths. The phase profiles shown are for lateral displacements of 10, 20, and 80 the metasurface periodicity. (c) Experimentally measured focal length as a function of the lateral displacement, showing a nonlinear change.

Metasurface-based freeform optics for biosensing and augmented reality systems

We demonstrate the capability of metasurfaces for freeform optics by performing FDTD simulations of a metasurface-based lens with adjustable focal length, and conducting ray optics analysis to enable a small form-factor diffractive augmented reality visor.