Microscopic robots with small-scale features that can control light at the microscale offer the potential to probe the microscopic world in more detail – with the scattering of light from such microbots able to induce diffractive optical effects.

To date, this combination of diffractive optics and tuneable mechanics has primarily exploited microelectromechanical systems (MEMS) devices, but creating actuatable microbots with features on the scale of the wavelength of light has been challenging.

To address this challenge, researchers at Cornell University turned to magnetically controlled microbots. While such robots have been developed at millimetre scales, the ability to perform magnetic actuation at the micron scale only became possible recently, due to the creation of protocols that encode magnetic information into microscale robotics and the use of atomic layer deposition (ALD) to create nanoscale hinges that make flexible micromachines capable of advanced navigation.

The team has now created magnetically controlled microbots that operate at the visible-light diffraction limit, so-called diffractive robots.

“A walking robot that’s small enough to interact with and shape light effectively takes a microscope’s lens and puts it directly into the microworld,” says team leader Paul McEuen in a press statement. “It can perform up-close imaging in ways that a regular microscope never could.”

New magnetic microbots

Using nanometre-scale mechanical membranes, rigid panels, programmable nanomagnets and diffractive optical elements, McEuen and colleagues created untethered microbots that are small enough to diffract visible light. They used the ALD hinges to connect the microbot’s rigid panels with magnetically actuatable joints, enabling them to reconfigure and move in millitesla-scale magnetic fields.

The core elements of the diffractive microbots comprise the light-diffracting panels with integrated nanomagnet arrays and the flexible hinges; the platform can also embed optical elements such as an optical diffraction grating. To enable the required mechanical, diffractive and magnetic performance, these integrated elements span several orders of magnitude in terms of their individual scales. The light diffracting grating panels were tens of microns in size, with each panel 1 µm wide, whereas the diffractive grating lines were on the scale of light wavelengths, the hinges had a thickness of 5 nm, and the magnetic domains were in the nanoscale realm.

The hinges played a crucial role, the researchers note, by providing a high degree of flexibility to an otherwise rigid robot. This flexibility allowed the microbots to rotate and reorientate themselves to dynamically change how light is diffracted, focused and redirected.

When manipulated with a magnetic field, the microrobots were able to simultaneously change shape, locomote along a surface and control diffracted light. This locomotion capability was due to the array of nanomagnets integrated into the light-diffracting grating panels.

By selectively controlling the aspect ratio of the nanomagnet domains and programming them using the strength of the external magnetic field, the researchers could control the movement of the microbots – including crawling forward on a solid surface and “swimming” through fluids while simultaneously steering and diffracting light.

“These robots are 5 microns to 2 microns,” says co-author Itai Cohen. “They’re tiny. And we can get them to do whatever we want by controlling the magnetic fields driving their motions.”

The researchers note that the tuneability of the optical elements could be further improved by adding more magnetic material to the microbots and/or increasing the size of the magnetic fields used to control them. And while this study centred around individual microbots, it should also be possible to use multiple microbots in magnetically actuated robot swarms to introduce collective optical effects.

Potential applications

As a generalized robotics platform, the microbots could easily be modified and produced with differing sizes, geometries and optical elements according to the intended application. Some key optical elements that could be integrated include meta-atoms, subwavelength apertures and plasmonic resonant probes.

The researchers have already demonstrated that the microbots have capabilities including force sensing with piconewton sensitivity, subdiffractive imaging using a type of structured illumination microscopy, and light beam steering and focusing using tunable diffractive optical elements. Other potential applications include endoscopic imaging and tissue ablation, high-resolution fluorescence microscopy of cells, and the high-resolution sensing of magnetic fields and current in integrated circuits.

The research is described in Science.

The post Magnetically controlled microbots are small enough to diffract visible light appeared first on Physics World.

A research team headed up at Seoul National University has pioneered an innovative metasurface-based folded lens system, paving the way for a new generation of slimline cameras for use in smartphones and augmented/virtual reality devices.

Traditional lens modules, built from vertically stacked refractive lenses, have fundamental thickness limitations, mainly due to the need for space between lenses and the intrinsic volume of each individual lens. In an effort to overcome these restrictions, the researchers – also at Stanford University and the Korea Institute of Science and Technology – have developed a lens system using metasurface folded optics. The approach enables unprecedented manipulation of light with exceptional control of intensity, phase and polarization – all while maintaining thicknesses of less than a millimetre.

Folding the light path

As part of the research – detailed in Science Advances – the team placed metasurface optics horizontally on a glass wafer. These metasurfaces direct light through multiple folded diagonal paths within the substrate, optimizing space usage and demonstrating the feasibility of a 0.7 mm-thick lens module for ultrathin cameras.

“Most prior research has focused on understanding and developing single metasurface elements. I saw the next step as integrating and co-designing multiple metasurfaces to create entirely new optical systems, leveraging each metasurface’s unique capabilities. This was the main motivation for our paper,” says co-author Youngjin Kim, a PhD candidate in the Optical Engineering and Quantum Electronics Laboratory at Seoul National University.

According to Kim, creation of a metasurface folded lens system requires a wide range of interdisciplinary expertise, including a fundamental understanding of conventional imaging systems such as ray-optic-based lens module design, knowledge of point spread function and modulation transfer function analysis and imaging simulations – both used in imaging and optics to describe the performance of imaging systems – plus a deep awareness of the physical principles behind designing metasurfaces and the nano-fabrication techniques for constructing metasurface systems.

“In this work, we adapted traditional imaging system design techniques, using the commercial tool Zemax, for metasurface systems,” Kim adds. “We then used nanoscale simulations to design the metasurface nanostructures and, finally, we employed lithography-based nanofabrication to create a prototype sample.”

Smoothing the “camera bump”

The researchers evaluated their proposed lens system by illuminating it with an 852 nm laser, observing that it could achieve near-diffraction-limited imaging quality. The folding of the optical path length reduced the lens module thickness to half of the effective focal length (1.4 mm), overcoming inherent limitations of conventional optical systems.

“Potential applications include fully integrated, miniaturized, lightweight camera systems for augmented reality glasses, as well as solutions to the ‘camera bump’ issue in smartphones and miniaturized microscopes for in vivo imaging of live animals,” Kim explains.

Kim also highlights some more general advantages of using novel folded lens systems in devices like compact cameras, smartphones and augmented/virtual reality devices – especially when compared with existing approaches – including include the ultraslim and lightweight form factor, and the potential for mass production using standard semiconductor fabrication processes.

When it comes to further research and practical applications in this area over the next few years, Kim points out that metasurface folded optics “offer a powerful platform for light modulation” within an ultrathin form factor, particularly since the system’s thickness remains constant regardless of the number of metasurfaces used.

“Recently, there has been growing interest in co-designing hardware-based optical elements with software-based AI-based image processing for end-to-end optimization, which maximizes device functionality for specific applications,” he says. “Future research may focus on combining metasurface folded optics with end-to-end optimization to harness the strengths of both advanced hardware and AI.”

The post Lens breakthrough paves the way for ultrathin cameras appeared first on Physics World.