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freeform and metasurfaces, understanding how both of them interact with light, and leveraging that to get a good image was a major challenge,’ said lead author Daniel Nikolov, an optical engineer in Rolland’s group.

Fabrication challenges Another obstacle was bridging from macroscale to nanoscale. The actual focusing device measures about 2.5mm across, but even that is 10,000 times larger than the smallest of the nanostructures imprinted on the freeform optic. ‘From a design standpoint, that meant changing the shape of the freeform lens and distributing the nanostructures on the lens in a way that the two of them work in synergy, so you get an optical device with a good optical performance,’ said Nikolov. This required finding a way to

circumvent the inability to directly specify metasurfaces in optical design software. In fact, different programs were used to achieve an integrated metaform device. Nikolov said fabrication was daunting, It required using electron-beam lithography, in which beams of electrons were used to cut away sections of the thin-film metasurface

where the silver nanostructures needed to be deposited. Writing with electron beams on curved freeform surfaces is atypical, and required developing fabrication processes. The researchers used a JEOL

electron-beam lithography machine at the University of Michigan’s Lurie Nanofabrication Facility. To write the metasurfaces on a curved freeform optic they first created a 3D map of the freeform surface using a laser- probe measuring system. The 3D map was then programmed into the JEOL to specify at what height each of the nanostructures needed to be fabricated. ‘We were pushing the capabilities of the machine,’ Nikolov said. Successful fabrication was

achieved after multiple iterations of the process. ‘This is a dream come true,’

said Rolland. ‘This required integrated teamwork where every contribution was critical to the success of this project.’

Reference 1

Science Advances Vol. 7, no. 18: Nikolov et al. ‘Metaform optics: Bridging nanophotonics and free- form optics’, DOI: 10.1126/sciadv. abe5112

LCOS spatial light modulators for colour- sequential holographic applications

Holoeye offers two fast spatial light modulator models which are optimised for colour sequential phase operation. The devices are capable of addressing 3 x 8 bit within a frame (180 Hz) using fast display versions for the visible range. The SLMs also feature an RGB light source sync connector for use with RGB colour- switchable laser sources. The Leto-3 SLM has a

resolution of 1920 x 1080 pixels with 6.4µm pixel pitch. The standard driver

unit has a size of only 97 x 80 x 19mm and the SLM can conveniently be integrated in optical setups.

The Luna SLM is based on an 0.39” Lcos microdisplay with a resolution of 1920 x 1080 pixels and 4.5µm pixel pitch. The driver Asic is embedded in the Lcos microdisplay itself. This saves board space which enables a very compact driver.

The display can even

accept video data input via a 4-lane MIPI DSI, which enables even more compact electronics for industrial implementations. light-modulators/

Malet Street in central London by sending out millions of lidar pulses from multiple positions on the street. The lidar data was then combined with point cloud data, building up a 3D model. ‘This way, we can stitch the scans together, building a whole scene, which doesn’t only capture trees, but cars, trucks, people, signs and everything else you would see on a typical city street,’ said co-author Phil Wilkes, a geographer who normally uses lidar to scan tropical forests. ‘Although the data we captured was from a stationary platform, | @electrooptics

“We can stitch scans together, building a whole scene... trees, cars, trucks, people, signs and everything else you would see on a typical city street”

it’s similar to the sensors that will be in the next generation of autonomous or semi-autonomous vehicles.’ When the 3D model of Malet

Street was completed, the researchers then transformed various objects on the street into

holographic projections. The point cloud data was processed by separation algorithms to identify and extract the target objects. Another algorithm was used to convert the target objects into computer-generated diffraction patterns. These data points were implemented into the optical setup to project 3D holographic objects into the driver’s field of view.

The optical setup is capable

of projecting multiple layers of holograms with the help of advanced algorithms. The holographic projection can

appear at different sizes and is aligned with the position of the represented real object on the street. For example, a hidden street sign would appear as a holographic projection relative to its actual position behind the obstruction, acting as an alert mechanism. The researchers are now

working to miniaturise the optical components used in their holographic setup so they can fit into a car. Once the setup is complete, vehicle tests on public roads in Cambridge will be carried out. EO

June 2021 Electro Optics 29

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