Optical technology predates Archimedes by more than two thousand years. In ancient Egypt, early refractive lenses were used for artistic purposes. Though centuries have passed, comparable technology is still used today for the camera lenses in many products, including smartphones.
Traditionally, lenses are made with multiple bulky refractive lenses or lens elements stacked on top of each other. Though they produce high-quality images, their downside is their large size and weight. Luckily, the size and complexity of lens systems can be reduced through flat optics that are based on pioneering meta optical elements (MOEs).
Meta Optical Elements
Meta optical elements — also known as metasurfaces — are artificially engineered subwavelength-spaced arrays of nanophotonic phase-shifting elements that can be programmed to manipulate wavefront and polarization.
Depending on the application, NILT’s meta optical elements are comprised of arrays of pillars to sub-100 nm diameters with vertical sidewalls. Instead of the curved surface of a traditional lens element, MOEs are entirely flat but effectively create the same result. They bend and redirect the light, using a single MOE in place of several lenses stacked on top of each other.
The benefits of MOEs are many. For example, MOEs reduce the number of elements in the optics system, are easier to assemble, and provide increased functionality.
They allow you to rethink how you define your optical system and hold the promise of the perfect lens — perfect in the sense that they offer significantly more than the (refractive) lenses we have today. Previously, low efficiency has been the main barrier to manufactured meta optics. With the introduction of this new technology, the barrier has finally been removed. In late 2021, NILT designed, built, and characterized multiple meta optical element (MOE) lenses with 94% absolute efficiency.
High efficiency makes MOEs ideal for sensing and machine vision applications that target consumer electronics, industrial, IoT, medical, and automotive markets. These innovative markets will greatly benefit from MOE optical solutions, as market requirements call for precision, excellent performance, and lightweight, compact designs.
MOEs have enormous potential. They can advance numerous applications, such as simplified Time-of-Flight (ToF) systems, with high relative illumination and ultra-compact NIR cameras used for eye-tracking and driver monitoring.
Meta Optical Elements Compared to Diffractive Optical Elements
Although most lens systems today are based on refractive optics, MOEs are not the first generation of optical system miniaturization. For many years, flat optics have been developed using diffractive optics technology. Diffractive optical elements (DOEs) are created by structuring the surface of transparent materials to produce wavelength-scale height variations. The contour map of spatially varying heights results in spatially varying phase shifts that shape the wavefront as light propagates through the element. The phase-inducing mechanism remains similar to a refractive lens, which is based on the length of the ray path inside the lens material. In this way, the thickness of the optical element is reduced to ~1 μm (comparable to the wavelength) with the expense of a higher sensitivity to wavelength variation (in optical lingo, this is called “chromatic aberration”).
This contrasts with MOEs, where the phase shift is induced via the response of nanoantennas. Nanoantennas are binary subwavelength structures with varying geometry built on the surface of the substrate material. This further reduces the thickness of the element. The principle of an MOE is still diffractive, but the optical parameters are mainly controlled by geometric parameters rather than the material composition of the nanostructures.
This opens new degrees of design freedom to allow unprecedented control over the phase, amplitude, polarization, and wavelength. This results in single-surface optical elements that would otherwise require multiple optical components, or — especially in polarization optics — require components that do not have a counterpart in conventional optics.
Comparing lenses allows us to highlight the underlying differences in physical mechanisms for imparting phase delays. A DOE lens is typically made by multi-layer etching to form a digitized version of the sawtooth Fresnel lens. In the metalens, varying the nanopillars’ diameter is a common way of imparting the phase delay. Each nanopillar can be treated as a truncated waveguide; the smaller the diameter, the more the electric field leaks into the air, lowering its effective index.
Ready for Mass Production
Whereas most of the work in MOEs in the last 20 years has been concentrated in academia, NILT is now ready to commercialize MOE technology.
MOEs from NILT are ready to be delivered and mass-produced to customers’ specifications. A design can be generated and made into a prototype in only two weeks. MOEs can be designed with complex phase functions and multi-MOE stacked components can be designed to meet required specifications. The MOE itself is made in silicon on a glass substrate, making it strong, rigid, reliable, and thermally stable. It can be customized for wavelengths in NIR and SWIR bands.
NILT has more than 15 years of experience in high precision nanostructuring and is applying this expertise to realize fast prototyping and mass MOE production. The entire process of MOE production, from design, prototyping, manufacturing, and assembling is done internally.
Production of customized single surface MOEs is based on a combination of high expertise, and proven, validated, and reliable methodology. NILT is an established company with more than 1,500 purchase orders of experience. In addition, we have made customized prototypes for many years.
E-Beam Lithography versus Deep UV Lithography
Because of the sub-wavelength lateral size of nanostructures, most meta-surfaces demonstrated by academic institutions are fabricated with E-beam lithography (EBL). EBL is a high-resolution, high-fidelity technology that is ill-suited as a standalone technology for commercial purposes due to the time-consuming rasterization of the electron-beam.
However, combined with Nano-Imprint Lithography (NIL), EBL provides the possibility of fabricating high-quality, low-cost MOEs in batch sizes suitable for mass production. This is obtained by using mechanical deformation of imprint resist to create copies of the initial E-beam written master.
The fabrication of an NIL master using EBL secures high design freedom and low-cost prototyping. It provides a significantly quicker turnaround time than Deep UV Lithography (DUV), which requires the fabrication of a new mask when a design parameter is changed.
Extensive experience in EBL and NIL, combined with sophisticated optical design methods and advanced in-house optical characterization allowed NIL Technology to recently introduce a single surface metalens. This metalens is groundbreaking in its form, performance, compactness, and efficiency.
Single meta optics lenses, fabricated on silicon wafers, have the potential to replace the several refractive lenses currently used in today’s mobile devices, eliminating the so-called “smartphone camera bump.”
NIR MOE Camera Module
A demonstration of the capabilities of NILT MOEs is a 940 nm near-infrared (NIR) wavelength imaging lens with a single metasurface, used for 3D sensing and face recognition in smartphones and driver-monitoring in automobiles.
A complete NIR camera module is built using a 940 nm single meta optical element and an NIR sensor. Both demonstrator and customer-specific metalenses are currently being prototyped, shipped, and made ready for mass production.
This article was written by Theodor Nielsen, CEO and founder, and Michael Juhl, Optical Engineer, NIL Technology. For more information, visit here .