Xian-Zi Dong

Reduction in feature size of two-photon polymerization using SCR500

Fabricated by femtosecond laser pulses at wavelength λ of 780nm, the feature size of two-photon polymerization using SCR500 was reduced to λ∕50. Lines with sub-25-nm width were produced by controlling the incident laser power and the laser focus scan speed up to 700μm∕s. Based on repolymerization between two structures close to each other, the feature size was further reduced to ∼15nm, which demonstrated the potential for three-dimensional nanofabrication with high spatial resolution.

Improving spatial resolution of two-photon microfabrication by using photoinitiator with high initiating efficiency

The lateral spatial resolution (LSR) in two-photon induced polymerization was improved to 80nm by using an anthracene derivative (9,10-bis-pentyloxy-2,7-bis[2-(4-dimethylamino-phenyl)-vinyl]anthracene (BPDPA)) as a highly sensitive and efficient photoinitiator. Photocurable resin containing 0.18mol% BPDPA exhibited a low polymerization threshold of 0.64mW at 800nm. Theoretical calculations showed that the LSR can be increased by reducing the laser power, indicating that the LSR could be improved using more sensitive initiators in the future.

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.

Laser emission from dye-doped polymer film in opal photonic crystal cavity

The authors demonstrated the lasing emission of the cavity constructed by the allyl derivative of fluorescein doped dendrimer/polymer film and two opal photonic crystals. A dendrimer encapsulated the structure modified laser dye and increased its concentration up to 5.3wt% in the polymer film with high optical gain. Laser emission of the cavity was observed, which is located at 589nm with a full width at half maximum of 1.7nm above the lasing threshold of 15μJ∕pulse. The lasing output was observed at an unordinary angle owing to the superprism effect of opal photonic crystals.

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.

Design of high efficiency for two-photon polymerization initiator: combination of radical stabilization and large two-photon cross-section achieved by N-benzyl 3,6-bis(phenylethynyl)carbazole derivatives

Novel A–π–D–π–A V-shaped 3,6-bis(phenylethynyl)carbazole based chromophores were designed and synthesized as two-photon polymerization (TPP) initiators combining a large two-photon absorption cross-section with facilitated radical formation. 9-Benzyl-3,6-bis(4-nitrophenylethynyl)carbazole (4d) shows a strong two-photon absorption around 800 nm and exhibits very high two-photon polymerization initiating sensitivity with a threshold power of 0.8 mW at the concentration of 0.18 mol%, which is much lower than the threshold power of 6.37 mW found for benzil. The corresponding threshold of laser exposure intensity for TPP is 3.0 × 107 mJ cm−2. The lowest loading of 4d is up to 0.012 mol% with a threshold power of 3.2 mW.

Amplified spontaneous emission from dye-doped polymer film sandwiched by two opal photonic crystals

The authors observed amplified spontaneous emission (ASE) from dye-doped polymer film sandwiched by two opal photonic crystals (PhCs). The ASE effect occurred at 599nm with a full width at half maximum of about 5.1nm, which corresponds to the L-point gap edge of the opal PhCs. Photoluminescence lifetimes of both dye-doped polymer films with and without opal PhCs were measured and corroborated that the ASE of the dye-doped polymer film emission is due to the presence of the photonic stop band.

Carbazole-based 1D and 2D hemicyanines: synthesis, two-photon absorption properties and application for two-photon photopolymerization 3D lithography

One and two dimensional (1D and 2D) carbazole based hemicyanines, where methyl pyridinium, methyl indolium and methyl benzothiazolium were used as acceptor group, were synthesized by Knoevenagel condensation. One-photon absorption, fluorescence and two-photon fluorescence spectra were investigated. The experimental results indicated that the different ionic acceptors affect their one-photon and two-photon properties. Among them, 2D methyl pyridinium carbazole derivatives exhibited low quantum yields and large two-photon absorption cross sections more than 1600 GM. The synthesized compounds were used as photoinitiator of two-photon photopolymerization (TPP), and three-dimensional (3D) microstructure was successfully fabricated by TPP 3D lithography. They could be utilized as effective two-photon polymerization photoinitiators.