Zhujun Shi

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.

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.

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.

Metasurfaces with wavelength-controlled functions

We demonstrate single-layer metasurfaces with controllable multi-wavelength functions. A multiwavelength achromatic metalens for red, yellow, green and blue light, and metasurfaces generating focused beams with different orbital angular momentum states are designed and fabricated.

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 subwavelength 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 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 is likely to increase the clinical utility of endoscopic optical imaging.A metalens is integrated into the design of an endoscopic optical coherence tomography catheter to achieve near-diffraction-limited imaging free of non-chromatic aberrations, offering high-resolution imaging well beyond the Rayleigh range of the input field.

Wide field-of-view waveguide displays enabled by polarization-dependent metagratings

We proposed a waveguide display design based on polarization dependent metagratings. By encoding the left and right half of field of view (FOV) in two orthogonal polarization channels, we achieved an overall horizontal FOV of 67° at 460 nm using a single waveguide, which is 70% larger than that achieved with conventional diffractive gratings. Metagratings that selectively diffract out TE or TM polarized light are designed and simulated using rigorous coupled wave analysis (RCWA). High polarization selectivity is achieved, with minimal crosstalk between the two channels. The transmission spectrum at normal incidence is calculated to assess the see-through effect. Remaining challenges such as fabrication and efficiency issues are discussed. The concept of multiplexing information in the polarization domain enables wide FOV waveguide displays for future AR devices.

Polarization-dependent metasurfaces for 2D/3D switchable displays

We proposed a 2D/3D switchable display design based on polarization-dependent metasurfaces. Metasurfaces are ultrathin planar optical devices patterned with subwavelength nanostructures. We design the metasurfaces such that they can simultaneously deflect right-hand circularly polarized (RCP) light to an angle and transmit left-hand circularly polarized (LCP) light to the normal direction. Combined with an active polarization rotator, the device can be switched between high resolution 2D display mode and multiview 3D display mode. Proof-of-principle metasurface designs are demonstrated. The far field radiation patterns in the 2D and 3D mode are simulated and analyzed. The effects of spectral bandwidth and beam directionality are also discussed. Compared with liquid crystal lenses, which is the key element in previous 2D/3D switchable displays, metasurfaces 1) deliver more precise phase profile control, thus less aberrations and higher image quality; 2) offer additional degrees of freedom in polarization manipulation; and 3) can be adapted to much smaller sizes.

Planar dielectric metasurfaces for immersion optics (Conference Presentation)

Using immersion lenses is a common approach to enhance the resolving power in various fields of optics such as microscopy and lithography. However, conventional immersion lenses are bulky, high-cost and are typically designed for only a few specific immersion liquids. The development of meta-surfaces provides a promising approach to manipulate light in a compact configuration, enabling many optical devices such as polarizers, waveplates and lenses. These are mainly focused in the near-infrared or the long-wavelength region of the visible spectrum due to fabrication challenges and intrinsic losses of materials used. Here, we demonstrate oil immersion planar lenses with a numerical aperture of 1.1 at visible wavelengths. The lenses provide diffraction-limited focal spots with Strehl ratios higher than 0.9 and 0.8 at their design wavelengths of 532 nm and 405 nm, respectively. Fabrication is based on an atomic-layer deposition (ALD) of TiO2. The loss of TiO2 in the visible is negligible and the surface roughness is well-controlled due to the precise monolayer growth of the TiO2 film. By applying the lens (designed at 532 nm) in a confocal scanning microscopy setup, we are able to achieve high-quality images with sub-wavelength resolution. It should be noted that this lens can be efficiently tailored for any liquid. We demonstrate another design for water-immersion lenses, which are highly applicable to super-resolution bio-imaging applications. The compactness and design flexibility of this platform is highly promising for widespread applications in imaging and spectroscopy.