Adaptive planar optics are software-reconfigurable

September 10, 2019 //By Julien Happich
planar optics
Rather than modulating an optical path through mechanical deformation, like it is the case with commonly used spatial light modulators (SLMs), be it liquid-crystal or micromirror-based SLMs, researchers from the Institute of Photonic Sciences (ICFO, Barcelona) have devised a novel type of planar optics whose refractive index can be reconfigured locally and progressively.

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.

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