Polarization-dependent Bloch oscillations in optical waveguides.

Optical systems provide a new and practical platform for studying Bloch oscillations. This study investigates the fundamental-mode propagation of polarization-dependent Bloch oscillations. By using the three-dimensional properties of femtosecond laser direct writing, we fabricate a polymer-based gradient waveguide array and determine the Bloch oscillations under different polarization inputs by using the birefringence gradient and the equivalent refractive index, thus exhibiting a polarization-dependent Bloch period. Our results provide a new, to the best of our knowledge, paradigm for two-dimensional optical Bloch modes and highlight the influence of optical polarization in the same system, which provides a possibility to observe richer physics related to Bloch oscillations in one structure.

UV-NIR femtosecond laser hybrid lithography for efficient printing of complex on-chip waveguides.

We propose UV-IR femtosecond laser hybrid lithography for the efficient printing of complex on-chip waveguides, which offers good performance in terms of processing efficiency and accuracy. With this three-dimensional printing technology, waveguides with complex cross-section shapes, such as owls and kittens, can be easily fabricated with an efficiency increased by 1500% (for ${6}\;\unicode{x00B5} {\rm m}\; \times \;{6}\;\unicode{x00B5} {\rm m}$6µm×6µm). In addition, a circular cross-section waveguide with an extremely low birefringence and complex ${8} \times {8}$8×8 random walk networks were quickly customized, which implies that in the design and preparation of the large-scale optical chips, the proposed maskless method allows for the preparation of highly customized devices.

Smart bio-gel optofluidic Mach-Zehnder interferometers multiphoton-lithographically customized with chemo-mechanical-opto transduction and bio-triggered degradation.

Stimulus-responsive optical polymers, especially gels, are enabling new-concept energy-transducing "smart" optics. Full exploitation of their molecule-derived tuning and integration with traditional micro/nano-optics/optoelectronics rely on the implementation of devices by advanced "intelligent" micro/nano-manufacturing technologies, especially photolithographies with wide compatibility. In light of the increasing need for an organic combination of smart optical materials and digital micro/nano-manufacturing, novel "smart" optical micro-switches, namely, stimulus-actuated Mach-Zehnder interferometers as a proof-of-concept demonstration, were prototyped with protein-based hydrogels via aqueous multiphoton femtosecond laser direct writing (FsLDW). Protein-based Mach-Zehnder-interferometric smart optical devices here display a morphological quality sufficient for optical applications (average surface roughness ≤∼20 nm), nano-precision three-dimensional (3D) geometry of these millimeter-scale devices and purposely structured distribution of photo-crosslinking degree. Moreover, the device configuration was customized with unbalanced branches in which meticulous stimulus-responsive ability can be realized by simply tuning the surrounding chemical stimuli (i.e., Na2SO4 concentration here). The "heterogeneous" configuration with unbalanced branches (i.e., different optical and stimulus-responsive features) exhibits as-designed "smart" switching of propagated near-infrared light (∼808 nm). These capabilities, along with total biodegradation, indicate the application promise of this gel-based optic construction strategy towards novel "intelligent", bio/eco-friendly, self-tuning or sensing photonic integrated systems like optofluidics.

Bioinspired Zoom Compound Eyes Enable Variable-Focus Imaging.

Natural compound eyes provide the inspiration for developing artificial optical devices that feature a large field of view (FOV). However, the imaging ability of artificial compound eyes is generally based on the large number of ommatidia. The lack of a tunable imaging mechanism significantly limits the practical applications of artificial compound eyes, for instance, distinguishing targets at different distances. Herein, we reported zoom compound eyes that enable variable-focus imaging by integrating a deformable poly(dimethylsiloxane) (PDMS) microlens array (MLA) with a microfluidic chamber. The thin and soft PDMS MLA was fabricated by soft lithography using a hard template prepared by a combined technology of femtosecond laser processing and wet etching. As compared with other mechanical machining strategies, our combined technology features high flexibility, efficiency, and uniformity, as well as designable processing capability, since the size, distribution, and arrangement of the ommatidia can be well controlled during femtosecond laser processing. By tuning the volume of water injected into the chamber, the PDMS MLA can deform from a planar structure to a hemispherical shape, evolving into a tunable compound eye of variable FOV up to 180°. More importantly, the tunable chamber can functionalize as the main zoom lens for tunable imaging, which endows the compound eye with the additional capability of distinguishing targets at different distances. Its focal length can be turned from 3.03 mm to infinity with an angular resolution of 3.86 × 10-4 rad. This zoom compound eye combines the advantages of monocular eyes and compound eyes together, holding great promise for developing advanced micro-optical devices that enable large FOV and variable-focus imaging.

Protein-Based Three-Dimensional Whispering-Gallery-Mode Micro-Lasers with Stimulus-Responsiveness.

For the first time, proteins, a promising biocompatible and functionality-designable biomacromolecule material, acted as the host material to construct three-dimensional (3D) whispering-gallery-mode (WGM) microlasers by multiphoton femtosecond laser direct writing (FsLDW). Protein/Rhodamine B (RhB) composite biopolymer was used as optical gain medium innovatively. By adopting high-viscosity aqueous protein ink and optimized scanning mode, protein-based WGM microlasers were customized with exquisite true 3D geometry and smooth morphology. Comparable to previously reported artificial polymers, protein-based WGM microlasers here were endowed with valuable performances including steady operation in air and even in aqueous environments, and a higher quality value (Q) of several thousands (without annealing). Due to the "smart" feature of protein hydrogel, lasing spectrum was responsively adjusted by step of ~0.4 nm blueshift per 0.83-mmol/L Na2SO4 concentration change (0 ~ 5-mmol/L in total leading to ~2.59-nm blueshift). Importantly, other performances including Q, FWHM, FSR, peak intensities, exhibited good stability during adjustments. So, these protein-based 3D WGM microlasers might have potential in applications like optical biosensing and tunable "smart" biolasers, useful in novel photonic biosystems and bioengineering.