Metasurfaces and flat optics

Introduction

Traditional optics - lenses, waveplates, filters, and the like - have existed for centuries. In many cases these elements are fashioned by techniques like machining and turning glass that, despite refinements, have also gone largely unchanged. More recently, metasurface optics have emerged where phase-shifting occurs at wavelength-scale, offering compactness and new functionality not accessible with bulk optical elements. These metasurface devices are fabricated using lithography, thin film deposition, and etching, techniques borrowed from semiconductor manufacturing. The production of these devices can be scaled up, with major consequences for optics.

Metalenses

Metasurfaces comprise sub-wavelength nanostructures. By the design and arrangement of the constituent nanostructures, metasurfaces are able to impart customized polarization, amplitude and phase to incident light. Additionally, metasurfaces can be lithographically mass-produced, enabling miniature and multifunctional metasurface optical elements. Metalenses, as an example of metasurface optical elements, has attracted widespread attention and was hailed as one of the top ten emerging technologies of 2019 by the World Economic Forum and Scientific American. The group is working on developing large diameter, high-efficiency and achromatic metalenses for applications in imaging, illumination, virtual and augmented reality etc. Here are three YouTube videos for those who are interested to learn more:

Science Magazine: Shrinking microscope lenses

TED talk given by Prof. Federico Capasso

Metalens demonstrations

Metasurface J-plates and metasurface-enhanced lasers
 

OAM manipulation with metasurfaces

Structured light refers to the tailoring or shaping of light in all its degrees of freedom, which can be used for micromanipulation and enhancing the capacity of optical communication channels. Chiral light is foremost among the family of structured light fields which carries spin angular momentum and orbital angular momentum (OAM). The metasurface J-plate is a metasurface converter for optical states that couples between arbitrary spin and optical angular momentum states of light in a compact planar. J-plates overcome a key limitation in alternative technologies such as Q-plates and spatial light modulators: conjugate symmetry of light’s angular momentum. And the relatively small pixel size of J-plates improves the beam quality, efficiency, and capability of generating higher OAM states.
 
Using a J-plate, a new metasurface laser that produces high-purity and non-symmetric super-chiral light never yet observed from lasers, creating of arbitrary spin-orbital chiral states of structured light at the source. Our laser conveniently outputs in the visible, offering a compact and power-scalable source that harnesses intra-cavity structured matter for the creation of arbitrary chiral states of structured light. The metasurface-enhanced laser is a new milestone in the history of structured light lasers as it breaches spin-orbit coupling symmetry as well as sets up a novel record for what high-purity and high-order OAM states can be created from a laser.
 

Nano-optic endoscopes with metalenses

Metasurfaces for biomedical imaging.

Improvement in the accuracy of endoscopic biopsy for small peripheral lesions is necessary if bronchoscopy will play a major role in lung cancer diagnosis. Endoscopic optical coherence tomography (OCT) with commercial catheters that rely on graded-index (GRIN) lenses or ball lenses, however, exhibit strong astigmatism and spherical aberration and thus deviate from diffraction-limited focusing. Shown is an artistic impression of the nano-optic endoscope that uses a metalens, with the ability to modify the phase of incident light at subwavelength level, to enable high-resolution endoscopic imaging at extended depth-of-focus by avoiding monochromatic aberrations. High-resolution three-dimensional images are captured by inserting the nano-optic endoscope into the lungs endo-bronchially visualize airway tissue microstructures. 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.
 

Polarization Imaging with Metasurfaces

Polarization camera

Conventional cameras and imaging systems are not sensitive to light’s polarization state. In order to measure this fundamental degree-of-freedom of light, imaging systems must often rely on complex beam paths, moving parts, or special sensors. Using a metasurface, however, all polarization components necessary for a full measurement of light’s polarization state can be combined into a single optical element, rendering a polarization-measuring camera not altogether more complicated than an ordinary one; this has broad potential applications in remote sensing and machine vision.
For more information, see Harvard SEAS news.

Large-area metalenses with Deep UV lithography

Large metalens

Metalenses are flat lenses that are ultrathin and lightweight, and are typically realized by placing millions to billions of nano-structures on a surface. The large number of structures makes scalability in both size and production scale a major concern. In our recent work, we demonstrate mass-producible, all-glass, centimeter-scale metalenses capable of focusing visible light, using deep-ultraviolet (DUV) projection lithography, a manufacturing technique widely used by computer chip foundries. These metalenses exhibit diffraction-limited performance and are suitable for potential applications in virtual reality (VR) devices and biological imaging techniques. Since the metalenses are also very light, we believe that they are ideal for camera applications where payload weight and footprint size are important, such as in drones and cubesats. 

Large metalens

Metasurface spectrometer

Spectrometer

One of the applications of flat lenses is in spectroscopy. Optical spectroscopy is an essential tool used in many areas such as food monitoring and medical diagnostics. However, spectrometers typically face a trade-off between their spectral resolution, working wavelength range and overall device size due to optical aberrations in their focusing elements. To preserve high spectral resolution over a broad bandwidth, additional corrective optical components must be used. These add significant bulk and prevent easy integration with portable devices. Here we present a highly compact, aberration-corrected spectrometer using meta-lenses, which are comprised of subwavelength scale nanostructures. It achieves nanometer resolution across 200 nm in the visible spectrum, with an overall footprint at the centimeter-scale.

Metasurface aberration correctors

Metacorrector

In lens design, the most widely used approach for chromatic aberration correction is based on adding refractive lenses made of different glasses. This approach not only increases volume and weight, but also relies on the difficult process of developing new glasses with suitable dispersion properties. Here, we demonstrated a metasurface aberration corrector (metacorrector) whose phase and dispersion (group delay and group delay dispersion) profiles are tailored to correct the monochromatic and chromatic aberrations of a fused silica singlet. The metacorrector was designed by engineering the effective refractive index of each constituent nanostructure over a large bandwidth. The images above show experimental imaging results under incoherent white-light illumination for cases with and without the metacorrector. The same design method is applicable for sophisticated microscope objectives and is promising for realizing super-achromatic and diffraction-limited lenses.