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International audience; SummaryMaximizing phase modulation in photopolymers remains a challenge in order to use these materials to fabricate photonics devices. Different material compositions and irradiation conditions have been studied in order to achieve it. One of the main conclusions has been that with continuous laser exposure better results are achieved. However, our results show that higher phase modulation can be achieved using pulsed laser. The study has been done with crosslinked acrylamide based photopolymers and Holographic Polymer-Dispersed Liquid Crystals (H-PDLC) exposed with a pulsed laser (532 nm). The increment of the phase modulation between the pulsed laser and continuous laser exposition is 17%, achieving a maximum phase depth of 3π radians and a refractive index modulation of 0.0084 at the zero spatial frequency limit, where monomer diffusion does not take place. This opens the door to use this photopolymer in large scale manufacturing.In this sense, we have used the H-PDLC photopolymers material to fabricate tunable lenses by using the Laser Induced Forward Transfer (LIFT) technique. Using a laser of 1030 nm with a pulse duration of 250 fs and a power window very small (90mW), we have achieved to print for the first time micro-lenses of low absorbency photopolymer without using an absorption layer (DRL, or dynamic release layer, exclusively metallic). This DRL is used to absorb the laser energy and, when ejected, drives the polymer layer. It has been possible to print lenses on both on glass and ITO slides. On glass, the lens is 10 µm in diameter and 400 nm in height, compared to 15 µm and 350 nm achieved on ITO.AbstractThe fabrication of polymeric lens arrays for its use in wide-field visual systems is a challenge that is being addressed by using the Laser Induced Forward Transfer (LIFT) technique but in processes that require intermediate steps [ ], which slows down manufacturing time. Our goal is therefore to achieve one-step, adjustable laser printing process improving the overall printing quality. Another challenge when using tunable polymers is the customization of tunable focal lenses dynamically controlling their optical properties. In this work we address both aspects by using a tunable photopolymer thanks to liquid crystal molecules, such as H-PDLC [ ].In the first stage of the study, we check the behavior of photopolymers when exposed with a pulsed laser (532 nm). In particular, the effect of high-frequency irradiation has been proven in comparison with low-frequency in previous studies. The main conclusion is that with high frequencies, more than 20 kHz (period of 50 µs), higher phase depth values (until 3π) than the CW irradiations can be achieved with increments of up to a maximum of 17% and a refractive index modulation of until 0.0084. This is because the pulse off time is long enough to allow a high regeneration of the starting dye but not too long to avoid the reaction between growing polymer-chain and new free-radical, leading to higher polymerization rates. That means a good performance of these photopolymers when irradiated with pulsed laser.The use of the LIFT with polymers has a limitation since the polymer is not capable of absorbing the energy of the laser and, therefore, it cannot be ejected. That is why it is common to use an absorption layer (DRL, or dynamic release layer), which absorbs the laser energy and, when ejected, drives the polymer layer [ ].This DRL is often a problem as it can contaminate the top of the printed polymer with small ejected particles, affecting its optical performance.In this study, we show for the first time that it was possible to print a low absorbency photopolymer without DRL. For this we used a laser of 1030 nm, pulse duration between 250 fs, and a power window very small (90mW), as slightly higher power generates a splashing process that stains the surface. In addition, the distance between recipient and donor substrates should also be very small (20- 90 µm), so that the cavitation bubble touches the receiver, due to the high density and viscosity of the polymer it does not allow the formation of ink jets. It has been possible to print lenses on both on glass and ITO slides with lenses size of 10-15 µm in diameter and 350-400 nm in height.ACKNOWLEDGEMENTS: The work was supported by the “Ministerio de Ciencia e Innovación” of Spain (projects FIS2017-82919-R; PID2019-106601RB-I00), by the ‘‘Universidad de Alicante’’ (UATALENTO18-10; ACIE-20-10), and by Generalitat Valenciana (projects BEST/2021/021; IDIFEDER/2021/014, potential FEDER funding), and Marie Skłodowska Curie Postdoctoral Global Fellowship “FOCUSIS” grant agreement 844977.

International audience