Alan Zhan

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.

Phase-matched nonlinear optics via patterning layered materials

The ease of integration and a large second-order nonlinear coefficient of atomically thin layered two-dimensional (2D) materials presents a unique opportunity to realize second-order nonlinearity in a silicon compatible integrated photonic system. However, the phase-matching requirement for second-order nonlinear optical processes makes the nanophotonic design difficult. We show that by nano-patterning the 2D material, quasi-phase-matching can be achieved. Such patterning-based quasi-phase-matching could potentially compensate for inevitable fabrication errors and significantly simplify the design process of the nonlinear nanophotonic devices.

Cavity nonlinear optics with layered materials

Abstract Unprecedented material compatibility and ease of integration, in addition to the unique and diverse optoelectronic properties of layered materials, have generated significant interest in their utilization in nanophotonic devices. While initial nanophotonic experiments with layered materials primarily focused on light sources, modulators, and detectors, recent efforts have included nonlinear optical devices. In this paper, we review the current state of cavity-enhanced nonlinear optics with layered materials. Along with conventional nonlinear optics related to harmonic generation, we report on emerging directions of nonlinear optics, where layered materials can potentially play a significant role.

Active metasurfaces based on phase-change memory material digital metamolecules

Tunable metasurfaces are a promising candidate for the next generation of spatial light modulators which will require higher refresh rates, smaller pixel sizes, and compact form factors. Phase-change memory materials present a unique platform for nonvolatile reconfigurable metasurfaces which could undergo phase transitions at MHz frequencies if actuated electrically, more than three orders of magnitude higher than refresh rates of existing commercial SLMs. While stable intermediate phases of GeSbTe (GST) exist which can be used for imparting differential phase shifts, the stochasticity of the material properties would limit the robustness of such a phase shifter, whereas the fully crystalline and amorphous states exhibit more consistent behavior. To overcome this, we design GST digital metamolecules comprising constituent meta-atoms which individually are in either the SET or RESET state, but which together form a tunable metamolecule with a set of robust phase shifts. We simulate active metasurface lenses based on these metamolecules, showing successful focusing, and demonstrate nano-patterning of a GST film with isolated nanoposts of material which could be electrically actuated, unlike counterparts which must be optically reconfigured.

Dielectric metasurface-based freeform optics

The miniaturization of current image sensors is primarily limited by the volume of optical elements. Using a subwavelength patterned quasi-periodic structure known as a metasurface, we can build planar optical elements based on the principle of diffraction. This platform allows us to mimic complex, asymmetric curvatures with ease and is ideal for the adaptation of freeform optics to the micron scale. The implementation of freeform optics on metasurfaces allows for extreme miniaturization of optical components. In our research we have demonstrated metasurface based optical elements such as lenses, vortex beam generators, and cubic phase plates near visible frequencies. Our fabricated lenses achieved beam spots of less than 1 μm with numerical apertures as high as ~ 0.75. We observed a transmission efficiency of 90% and focusing efficiency of 40% in the visible wavelengths. In addition, we have demonstrated a dynamic metasurface optical system called the 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 m of total mechanical displacement.

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.