Zhen-Sheng Zhao

Micronanofabrication of assembled three-dimensional microstructures by designable multiple beams multiphoton processing

The authors proposed a customizable multiple beam multiphoton polymerization micronanofabrication method for parallel processing of assembled structures. The configuration and geometry of multiple beams were designed by changing the parameters of lenses set as well as aperture masks. Various assembled two-dimensional and three-dimensional microstructures including microgears and photonic crystals were demonstrated under nanometer scale resolution and high fabrication efficiency. The proposed micronanofabrication method could be expected to play an important role in the fabrication of micromachines and microdevices.

Improving spatial resolution and reducing aspect ratio in multiphoton polymerization nanofabrication

The authors investigate the phenomena affecting lateral spatial resolution (LSR) and aspect ratio (AR) in multiphoton polymerization (MPP) nanofabrication. A LSR of 50nm and an AR of 1.38 were achieved for photocured polymer lines on the surface of a substrate by continuing scanning mode. Theoretical analysis based on the distribution of light intensity in this setup indicates that the LSR could be improved to better than 20nm. The asymmetric shrinkage of voxel in the axial and lateral directions has a significant impact for obtaining features of low AR, which are a critical requirement for construction of micro/nanodevices by MPP nanofabrication.

Novel photoinitiator with a radical quenching moiety for confining radical diffusion in two-photon induced photopolymerization

We designed and synthesized a novel two-photon induced photoinitiator with radical quenching moiety, 3,6-bis[2-(4-nitrophenyl)-ethynyl]-9-(4-methoxybenzyl)-carbazole (BNMBC), and successfully demonstrated its optical properties, high initiating efficiency in two-photon induced polymerization (TPIP) and capability to fabricate 3D structures with high resolution. The high resolution in TPIP using BNMBC was achieved as compared to the reported photoinitiator 3,6-bis[2-(4-nitrophenyl)-ethynyl]-9-benzylcarbazole (BNBC). BNMBC was confirmed to have a large two-photon absorption cross-section of 2367 GM by Z-scan measurement. We investigated the photopolymerization properties of resins R1–R3 in which BNBC, BNBC/phenyl methyl ether (PhOMe) and BNMBC were used as photoinitiators, respectively. The TPIP fabrication experiments exhibited that BNMBC possessed high TPIP initiating efficiency and an effective radical quenching effect. The volumes of polymer fibers fabricated by the TPIP of the photoresist using BNMBC as the photoinitiator were decreased to 20%–30% of those synthesised using BNBC as a photoinitiator. The introduction of a radical quenching group to the photoinitiator resulted in the effective confining effect of radical diffusion compared to the addition of the same molar ratio of radical quencher. This kind of photoinitiator with radical quenching group should benefit the fabrication of precise structures.

Femtosecond direct laser writing of gold nanostructures by ionic liquid assisted multiphoton photoreduction

Amino-terminated ionic liquid assisted multiphoton photoreduction (IL-MPR) was developed for the direct writing of subwavelength gold nanostructures in AuCl4- ions aqueous solution by femtosecond laser. It was revealed that the carbon chain length was crucial for morphology and size control of gold nanostructures. A 228 nm width of gold nanostructure, which was beyond the optical diffraction limit, was fabricated by the matching between IL and the power and scanning speed of the laser beam. The measured conductivity is of the same order as that of bulk gold. Furthermore, we successfully fabricated a U-shaped terahertz planar metamaterial whose spectral response is consistent with the theoretical expectation. The IL-MPR nanofabrication protocol is expected to play an important role in the fabrication of fine metallic micro/nanostructures for applications in microelectromechanical systems, nanoelectronics and nanophotonics.

High efficiency solar cell based on ZnO nanowire array prepared by different growth methods

A high efficiency solar cell based on ZnO nanowire (NW) arrays of different morphology and crystalline quality has been assembled and investigated. According to the basic source of the hydrothermal growing solution, ZnO NW was prepared by using (a) ammonia solution (N method), (b) hexamethylene tetramine (H method) and (c) alternating of N and H (NH method), respectively. The morphology and crystalline quality of the ZnO NW array have been characterized by using scanning electron microscopy, Raman spectroscopy and transmission electron microscopy. The CdS and CdSe nanoparticles were deposited on ZnO NW array, which is applied as a ZnO/CdS/CdSe core/interlay/shell in solar cell with a polysulfide electrolyte and a CoS counter electrode. The results indicate that the photovoltaic behaviour strongly depends on the morphology and crystalline quality of the ZnO NW array. We observe an increasing short-circuit current density of N < NH < H by using ZnO NWs prepared with different methods. The best power conversion efficiency of 2.81% and a short-circuit current of 14.6 mA cm−2 were obtained. The results would open up new avenues towards the potential applications in designing high efficiency quantum dot sensitized solar cells.

Novel carbazole-based two-photon photosensitizer for efficient DNA photocleavage in anaerobic condition using near-infrared light

Two novel carbazole derivatives, BMEPC and BMEMC, were designed, synthesized and first reported as two-photon photosensitizers for DNA photodamage, which showed efficient DNA photocleavage ability under near-infrared light exposure via a two-photon process in anaerobic condition.

Complementary chiral metasurface with strong broadband optical activity and enhanced transmission

We present the design and realization of ultra-thin chiral metasurfaces with giant broadband optical activity in the infrared wavelength. The chiral metasurfaces consisting of periodic hole arrays of complementary asymmetric split ring resonators are fabricated by femtosecond laser two-photon polymerization. Enhanced transmission with strong polarization conversion up to 97% is observed owing to the chiral surface plasmons resulting from mirror symmetry broken. The dependence of optical activity on the degree of structural asymmetry is investigated. This simple planar metasurface is expected to be useful for designing ultra-thin active devices and tailoring the polarization behavior of complex metallic nanostructures.

Photonic bandgap of gradient quasidiamond lattice photonic crystal

A three-dimensional (3D) photonic crystal (PhC) structure consisting of gradient quasidiamond lattices was fabricated using multiphoton photopolymerization nanofabrication technique. The photonic bandgap (PBG) of this 3D PhC was experimentally confirmed by reflection and transmission measurements and simulated with finite-difference time domain calculations. The results indicate that a 3D PhC with gradient lattices could effectively expand the width of the PBG and may be beneficial for developing complete-bandgap PhCs with low refractive index materials for applications in polymer based optoelectronic devices and integrated systems.

Asymmetric fishnet metamaterials with strong optical activity.

We investigate the optical properties of mono- and double-layer asymmetric fishnet metamaterials with orientated elliptical holes, which exhibit exotic spectral and polarization rotating characteristics in the visible spectral range. Our results show that nontrivial orientations of the holes with respect to the reciprocal lattice vectors of the periodic lattice in both systems produce strong polarization rotation as well as additional enhanced optical transmission peaks. Analysis of the electromagnetic field distribution shows the unusual effect is produced by the spinning localized surface plasmon resonances due to the asymmetric geometry. High sensitivity of the hybridized mode on the dielectric spacing, the aspect ratio of the holes and the embedding media in double-layer structure is also observed. The dependence of spectral and polarization response on the orientation of the holes and the embedding media is useful for design of chiral metamaterials at optical frequencies and tailoring the polarization behavior of the metallic nano-structures.

Asymmetric microstructure of hydrogel: two-photon microfabrication and stimuli-responsive behavior

We propose an asymmetric three-dimensional (3D) multiphoton polymerization microfabrication method, and successfully demonstrate it through fabricating the size- and shape-controlled stimuli-responsive asymmetric hydrogel microcantilevers. The reversible ion-responsive hydrogel microcantilevers exhibit attractive and controllable bending behavior due to their asymmetric deformation to external ions. The reversible bending direction of microcantilevers completed within only 0.133 s when the surrounding environment was alternated from water to 1 M NaCl solution. Furthermore, the microcantilevers exhibited large total bending angles θT up to 32.9°. The bending behavior can be also strongly influenced by increasing the length of the microcantilevers. The 3D stimuli-responsive hydrogels were demonstrated to be promising for the application of microactuators and micromanipulators. This designable microfabrication technique and the stimuli-responsive behavior of asymmetric microstructure of hydrogel with versatility in the shape would be prospective for developing biomedical microdevices.