A Laser Written 4D Optical Microcavity for Advanced Biochemical Sensing in Aqueous Environment

The topic of 4D printing, that is based on 3D printing of structures capable to response on external disturbances over the time, has evolved enormous interest in science and engineering since its first statement in 2013. Besides multi-material printing originally intended to provide self-assembly function, the interest in use of smart materials capable for self-sensing and/or self-actuating rises. Here the necessity in precise 3D fabrication techniques and solutions for structural response control are current challenges that 4D printing faces. In this paper we report on design, fabrication and characterization of a first example of 4D optical microcavity that shows advanced sensing performance boosted by self-sensing in the form of interaction of the polymer matrix with sensed molecules in aqueous environment. Distinctive features of 4D microcavity spectral response for bulk refractive index variations and sensitivity enhancement for more than one order of magnitude have been experimentally observed and discussed. Possibility for parallel interrogation with a single coupling and detection unit for a set of 4D microresonators fabricated with two-photon polymerization out of quencher-doped SZ2080 material with quality factor of $4\times 10^4$ at 685 nm without any post-processing steps has been experimentally demonstrated. The proposed 4D microcavity-based sensor opens a new avenue for designing of a multiplexed 4D printed optical sensing platform with advanced performance compared to conventional optical detection techniques for broad variety of detection tasks.