Photoinitiator-free Laser Fabrication of Ultra-compact, Low-loss Waveguides in Polydimethylsiloxane

Owing to its excellent elasticity, wide spectral range of transparency, and outstanding chemical and thermal stability, polydimethylsiloxane (PDMS) is an elastomer of great technological importance, being widely used in the fabrication of microfluidic and optofluidic devices in particular. Compact, low loss optical waveguides are crucial in such devices for a dense integration of optical functionalities. Compared with photolithographic methods, multiphoton laser direct writing through photopolymerization has shown great promise in the fabrication of three-dimensional (3D) optical waveguides in PDMS without the limit of planar structures. Here we demonstrate a multiphoton laser direct writing process that produces low-loss, ultra-compact waveguides in PDMS. The fabrication employs a self-initiated multiphoton polymerization of phenylacetylene infused in PDMS without additional photoinitiator. The elimination of the photoinitiator results in an excellent refractive index homogeneity and thus a low propagation loss due to scattering, as well as a greater biocompatibility by involving the fewest possible chemicals (monomer only) in the polymerization reaction. Our characterizations show that the waveguides fabricated as such are on average 1.3 mu m wide with a refractive index contrast of 0.06 and a propagation loss of 0.03 dB/ cm in the spectral band of 650-700 nm. Our technique will enable a broad range of applications spanning from wearable photonics to chip-scale optical interconnects.

Photoinitiator-free multi-photon fabrication of compact optical waveguides in polydimethylsiloxane

Christophe Moser, Giulia Panusa, Demetri Psaltis, Ye Pu, Jieping Wang

Compact, low loss flexible optical waveguides are crucial in optofluidic and microfluidic devices for a dense integration of optical functionalities. We demonstrate the fabrication of compact optical waveguides in polydimethylsiloxane through multiphoton laser direct writing using phenylacetylene as the photosensitive monomer. Our fabrication technique employs photo-induced radical chain polymerization initiated by the monomer molecule itself without a photoinitiator. Because of the dense pi-electrons in phenylacetylene, we achieved a high refractive index contrast (Delta n >= 0.06) between the waveguide core and the PDMS cladding. This allowed for efficient waveguiding with a core size of 1.3-mu m with a measured loss of 0.03 dB/cm in the spectral band of 650-700 nm. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

OPTICAL SOC AMER2019

Three-dimensional fabrication of sub-micron optical waveguides in PDMS and other polymer materials

Giulia Panusa

Optical waveguides are one of the most important photonic components. They are indispensable tools in many of todayâs technologies because of their capacity of guiding light. In particular, compact, low loss, flexible polymer optical waveguides are crucial in optofluidic and microfluidic devices for a dense integration of optical functionalities. The sought after suitable materials and innovative fabrication techniques to achieve low loss long polymer optical waveguides and interconnects has proven to be challenging. In a fiber optic endoscope, thousands of single-mode (SM) optical fibers acting as single pixels are closely packed together, delivering the local information to a camera chip. Biocompatible polymer-based waveguides increasingly became valid alternatives to silica fibers to deliver and collect light for optical diagnosis, therapy and surgery. In particular, polydimethylsiloxane (PDMS) is well known to be a suitable polymer for biomedical implantation devices, thanks to its excellent physical and chemical properties. In this thesis, I demonstrate the fabrication of compact optical waveguides in PDMS through multiphoton laser direct writing (MP-LDW). The core of this research consists of the investigation of suitable combinations of monomer and PI capable of efficient photopolymerization in a cured PDMS matrix. We achieved, for the first time, the photoinitiator-free fabrication of optical waveguides employing phenylacetylene as the photosensitive monomer via multi-photon absorption. Because of the dense Ï-electrons in phenylacetylene, we achieved a high refractive index contrast (În â¥ 0.06) between the waveguide core and the PDMS cladding. This allowed for efficient waveguiding at a core size of 1.3-Âµm with a measured loss of 0.03 dB/cm in the spectral band of 650-700 nm.Motivated by the need of minimizing self-focusing, we investigated alternative chemical schemes and demonstrated the fabrication of submicron optical waveguides in PDMS using divinylbenzene (DVB) as the monomer through two-photon polymerization (2PP). We show that the commercial oxime ester photoinitiator Irgacure OXE02 is suitable for triggering the DVB polymerization, resulting in a stable and controllable fabrication process for the fabrication of defect-free, 5-cm long waveguides. Moreover, I present the methodologies we have developed for the fabrication of polymer rectangular step-index (STIN) optical waveguides using a commercial 3D printing system (Photonic Professional GT, Nanoscribe GmbH), using the proprietary IP-dip resist. We performed a full calibration and implemented a printing strategy for the fabrication of a 720 ÎŒm long SM-fiber bundle. We characterized it in terms of refractive index, transmission loss and imaging capabilities. We further demonstrate how a convolutional neural network (CNN) can reconstruct the original images from a scrambled output from the waveguide bundle due to crosstalk. To achieve this, we have constructed a CNN of the U-net type and trained the network with the input images and their corresponding output from the bundle. Overall, the presented work provides innovative materials and new insights into the fabrication of polymer optical microstructures, opening new scenarios for future technological development in the field of microfabrication using MP-LDW. We expect such waveguides will receive a wide range of applications in biosensors, microfluidic flow cytometry and wearable photonic devices.

EPFL2021

Tunable and Broadband Nanostructured Photonic Devices

Pierre-Yves Baroni

The main topic of this thesis is the fabrication and characterization of structures smaller than the wavelength of light, for operation in the visible or near infrared spectral range. In the first part of this thesis, the fabrication of periodic structures in one and two dimensions with an interference photolithography technique is described in detail. The structure periodicities that have been fabricated with this process ranges between 270 nm and a few microns on areas that can be as large as 100 cm2. Typical structure thicknesses are approximately equal to the period. In particular, a square lattice of pillars with a period of 270 nm has been created on the (non-flat) surface of a quartz microlens array and exhibits anti-reflective properties. Experimental results show a 15 % attenuation of the reflectivity and a 3 % enhancement of the transmissivity over the visible spectral range. In the second part of the thesis the structures are fabricated by e-beam lithography to ensure very precise devices shapes. The experimental results are obtained in the infrared range with two different structures, called photonic crystals. The first structure, a superprism, is a triangular lattice of pillars infiltrated by liquid crystals. A displacement of the output light spot is measured to be 20.5 µm for a wavelength variation of 27 nm. The structure length is 70 µm. The device, based on standard silicon technology, should allow integration of the device as a multiplexer/demultiplexer system into optical micro-circuits. The second structure, a tunable resonant cavity, is a wavelength filter working in the near infrared spectrum. The active area is composed of a photonic waveguide with a triangular lattice made of sub-micrometer holes. Additional nanostructuring in the light waveguide acts as a resonant cavity. The tunability of the device is obtained due to the liquid crystals which are infiltrated into the nanostructure. A 32 nm shift of the transmitted light peak inside the photonic band gap is measured by changing the temperature from room temperature to 45 °C.