Flat patterned surfaces, traditionally realized through lithographic techniques, can manipulate the wavefront of light within minimal thicknesses comparable to the wavelength of light, offering significant potential for advances in science and technology. Amorphous polymers containing azobenzene molecules introduce a revolutionary approach to the direct fabrication of reprogrammable flat optical devices. In my work, I use azopolymers in combination with digital holographic illumination to enable maskless and reversible surface patterning. This innovative approach allows the design and the fabrication of reconfigurable optical elements, where light is used to fabricate the device, to tune its functionality or even completely reprogram it while the optical component is still maintained aligned in an operational setup.
The fabrication of Diffractive Optical Elements (DOEs) involves the structuration of material surfaces with geometries on the scale of light wavelength. Differently to standard photoresists, thin films of amorphous azobenzene-containing polymers (azopolymers) directly develop surface reliefs in the irradiated area, directly usable as phase-modulating masks, acting as DOEs. Surface structuration is the result of an intrinsically reversible light-induced material displacement, which makes azopolymer films usable as photo-transformable planar diffractive optical devices. Here we demonstrate reprogrammable and ready-to-use diffractive gratings, lenses, and holographic projectors, directly obtained in a single photo-structuration step through the projection of a grayscale holographic pattern over the azopolymer film free surface. The all-optical scheme, based on the principles of computer-generated holography, allows a simple and accurate engineering of the light pattern used for the azopolymer surface structuration. Additionally, direct pattern transferring opens to the real-time optimization of the device allowing to test and prototype its diffraction properties right during the developing of structured surfaces. The proposed approach offers a versatile, efficient, and full-optical reversible fabrication framework for DOEs, making it a promising option to overcome the demanding, burdensome, and irreversible manufacturing processes typically involved in the realization of planar diffractive optical devices.
Multiplexing is a method in which multiple information is stored or combined in a common medium. In photolithographic applications, spatial and temporal multiplexing can be used to design complex structured surfaces as superposition of simple profiles on the surface of a photosensitive materials able to respond to a time-averaged light pattern. Azopolymers are promising material systems in that sense, as they can be directly photo-structured in a reversible way over large scales with high quality, by simply controlling the spatiotemporal distribution of light irradiated on their surface. Not involving the typical chemical development of standard photoresists, the direct light-induced surface topography of azopolymer can be further modified after structuration, resulting a suitable platform to encode multiplexed information in a topographic surface relief pattern. Here, we show a method that allows the fabrication of multiplexed azopolymer surface relief gratings through a computer controlled holographic illumination system. The ability of our setup in accurately and digitally manipulating the evolution of the geometry of a simple sinusoidal intensity pattern is here exploited for the realization of quasicrystalline surface relief gratings, that can be tuned to have both positive and negative topography while preserving the multiplexed grating-vector information encoded in their far-field light diffraction pattern. Our results pave the way toward the realization structured surfaces able to convert multiple information in a single complex topographic profile for application in optics and cryptography.
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