Adaptive planar optics are software-reconfigurable
The SmartLens they describe in a recent paper “Tunable and free-form planar optics” published in Nature Photonics consists of a 1mm thick sheet of thermally-responsive optical silicone (here low cost Polydimethylsiloxane or PDMS) embedded with ultra-tin gold-based 200μm-diameter spiral heaters. As the authors first demonstrated through electro-thermo-optical modelling, the resistive microwires can be powered at specific values to heat the polymer and modulate its refractive index locally, which affects the optical path without any mechanical deformation.
In order to design a SmartLens useful for optical wavefront shaping, the researchers had to precisely design the micro-heaters so they would heat the optical silicon and create the appropriate refractive index profiles, either to correct optical aberrations on top of a conventional lens or to refocus light locally.
To do so, they developed a genetic algorithm optimization routine to solve the inverse electro-thermo-optical modelling problem, starting from the targeted wavefront to reach the optimal spiral heater design. For a fixed process-based wire thickness of about 50nm, the genetic algorithm was let to play with the number, radius and width of resistive wire loops, delivering optimized designs within 30mn (or about 60 iterations) that approached the targeted wavefronts.
Interestingly, once the spiral heaters have been optimized and integrated into the transparent polymer, they remain transparent to the optical system they are affixed to, acting as a plane-parallel plate until they are powered to modulate the refractive index. And that modulation can be tuned continuously to different magnitudes, which means that a micro-array of SmartLenses could be distributed across a large conventional optical system and locally switched Off (transparent) or On to modulate the refractive index locally and refocus objects from different planes.
The authors validated their approach experimentally, designing a number of SmartLenses able to create a wide range of wavefront modifications, including ring refocusing of a collimated laser beam and simultaneous refocusing across multiple planes (to bring in focus several coloured objects located at various distances from an imaging system). But they report that such SmartLenses can also be optimized to generate complex functions based on Zernike polynomials for dynamic beam-forming applications.
Image credit: ICFO/M. Montagut.
Among the results reported in the paper, the authors note that the focal length decreases faster with the applied voltage V for smaller heaters, as the radius of curvature of the generated lens is shorter. One example, is a 10μm-diameter spiral delivering an f-number of 11 at 2.1V (from an infinite focal length at 0V). Here, the accuracy and precision of the focal length are only limited by the applied voltage accuracy and precision. The researchers tested spiral heaters as small as 10μm in diameter offering response times of around 0.5 milliseconds. In this research, the use of gold tracks as conductors slightly reduced the SmartLens’ transparency (down to 60%), but the authors anticipate this could be improved to over 90% using transparent conductive materials such as ITO. Anti-reflection coatings could further improve their design.
As well as being low-cost compared to today’s SLMs , the SmartLens is polarization-insensitive, and because it operates in a refractive rather than diffractive regime, it can also be used over a broad wavelength range without incurring chromatic aberrations or image distortion. Unlike deformable mirrors, SmartLens elements can also be used indifferently in transmission or reflection modes (with the appropriate optimized design).
When interviewed by eeNews Europe, professor Romain Quidant, leader of the Plasmon nano-optics group at the ICFO revealed his group had applied for several patents regarding this technology, which he aims to commercialize in some form or another.
“There are two aspects of the technology, on one hand, the fixed spiral design that needs to be optimized through iterative genetic algorithms, and on the other hand, the electrical reconfigurability of the SmartLens within the application”, Quidant explained.
“If you rely on one SmartLens as one pixel to shape an image, then the design is very important. But if you rely on a large number of pixels with many adjacent SmartLens spirals each affecting part of a pixelated field of vision, then there is inherent redundancy in the information and the design rules can be relaxed a bit” he hinted.
For smartphone cameras and even microscopes, a low cost SmartLens coating could easily be integrated into the optical path to enable multi-plane refocusing. And for robotic Simultaneous Localization and Mapping (SLAM) applications, large number of micro-sized SmartLens could probably be designed to create the equivalent of a flat multi-faceted compound eye, with arrays comprising a distribution of progressively optimized designs. One could even imagine a multi-layered design capable of multiplexed beam-shaping configurations, with layers optimized for different wavefront shaping functions.
In fact, the applications are so far reaching that Quidant didn’t want to disclose a roadmap yet, only hinting that the optimization algorithms would remain the lab’s secret recipe while different SmartLens configurations and driving software may become available as licensable IP for optical system manufacturers. SmartLens is at an incubation stage at the ICFO and a startup is in the making.
Institute of Photonic Sciences – www.icfo.eu
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