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Above: a near-infrared metalens employed in Coherent Raman scattering imaging (APL Photonics 6, 096107, 2021). Render by Kerolos M.A. Yousef.
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.
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
Metasurface J-plates and metasurface-enhanced lasers
Nano-optic endoscopes with metalenses
Polarization Imaging with Metasurfaces
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
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.
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
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.