In this work, we demonstrate a developed 3D printing based on two-photon polymerization for achieving millimeter-scale, micron-accuracy 3D structures (MM-3DS), which combines the femtosecond laser of 800 nm and low magnification objective lens of 10×. The commercial photoresist SU-8 is used in 3D printing system for improving mechanical strength and chemical stability of MM-3DS. The 3D microstructures are preprogrammed and optimized by considering the scanning mode and experiment parameters. During 3D printing process, micron features are written within the interior of SU-8 film via localized polymerization driven by nonlinear two-photon absorption process. By the 3D movement in ~1 mm scale of the focused beam, a customized MM-3DS can be produced. We have fabricated a customized MM-3DS with a size of 1.6 mm and an accuracy of 10 μm. The influence of volume for the printing structures V<sub>s</sub> on the printing time T exhibits a linear behavior, indicating that the printing speed is 0.248 mm<sup>3</sup>/h under the current conditions. This technology offers a flexible and low-cost method of generating highly customizable, precisely controlled MM-3DS, which is promising for the manufacture of complex functional structures and devices for the microfluidics, microelectronics, photonics and so on.
Poly(ethylene) glycol diacrylate(PEGDA)-based hydrogel materials with the excellent biocompatibility are widely used not only for cell culturing and tissue engineering but also for damaged bone repairing. In order to guarantee the stability of PEGDA as cell scaffolds and the compatibility with the host tissue, a further knowledge of interaction between PEGDA micronanostructure and cells is indispensable. In this study, two kinds of three-dimensional PEGDA micronanostructures have been designed and fabricated by two-photon polymerization for fibroblasts culturing. The PEGDA (average molecular weight 700) used in our study can preferably reconcile with a quantitative crosslinker to enhance the mechanical strength of structures. The polymerized line width as a function of the experimental conditions such as laser power and scanning speed during the two-photon polymerization has been investigated. Through investigating the adhesion, proliferation, and spread of fibroblasts on PEGDA micronanostructures in vitro, a response between micro-structures and fibroblast has been investigated. This study would provide the potential application of PEGDA hydrogel in biophotonics and tissue engineering.
Multi-photon laser lithography (MPLL) is an economical maskless means for high resolution and intrinsic three-dimensional micro/nanostructures fabrication. Here, we report MPLL of AR-N 4340 photoresist, and a spatial resolution of 40 nm is obtained. The relationships between laser parameters and line morphologies are systematically investigated. In the MPLL process, standing wave interference generated by the reflected light from photoresist/air interface and the incident light could greatly influence the bonding capacity between the fabricated lines and glass substrate. Therefore, lines with width smaller than 150 nm can be easily taken away in the development process. In order to obtain line with higher resolution, two rectangular photoresist plates were fabricated for immobilization of the fabricated lines, and a nanoline with a feature size of 40 nm was achieved between them through carefully adjusting the incident laser power. This work is one of the evidences for high fabricating resolution characteristic of MPLL, and it exhibits the potential for fabricating high resolution semiconductor and electronic micro/nanostructures.
Here, we report a preparation method to fabricate 3D micro-cones with controllable morphology based on two-photon photolithography. Two-photon photolithography offers the unique ability to create arbitrarily complex 3D polymeric structures. The voxel shape of polymerization point is crucial for the topography of the micro-cone structure. Therefore, the relationship between focused voxel features of femtosecond laser and the shape of micro-cone were analyzed systematically, and a micro-cone structure with a cone height of 2 μm, cone tip of 50 nm, and a cone angle of 20° was successfully obtained in this study. In addition, 3D micro-cone structures with 10°, 20°, and 30° sharp corners have been fabricated by means of controlling the relative movement between laser focus and moving direction. Besides, the structures with a varied slope angle from 0° to 90° on the substrate surface can be obtained by controlling the post treatment process. Furthermore, a large array of 3D micro-cones has been achieved based on the proposed preparation method.
We focus on investigating the optical transmittance of silver grid transparent electrodes (SGTEs) in variable angle view theoretically and experimentally, rather than the optical transmittance under the normal incidence. The variable angle transmittance (VAT) values of SGTEs are measured on a home-made experimental setup. The experimental results about difference of the transmittance difference under different angles are small and negligible, although the measured angle is changed. Theoretically, the physical mechanism on nearly constant transmittance for different angle view can be well explained according to the theory of geometrical optics. This study provides an approach for investigating the VAT values of SGTEs in a controllable fashion and the influence of viewing angle of the touch screen.
Considering the axially symmetric polarization and intensity distribution, radially polarized (RP) laser beam has comparatively higher axial component of electric field and smaller size of focal spot compared to linearly polarized (LP) laser. In this study, the effect of radial polarization on multiphoton fabrication has been studied, and polymer spots and lines are chosen as the study objects of 2D micro/nano structures of multiphoton lithography. These structures were fabricated with IP-L, a commercial negative photoresist, by RP fs-pulse laser beam which was tightly focused by an objective lens with high numerical aperture. Multiple experimental conditions, such as fabrication power, exposure time and scanning velocity, were verified in order to observe the structural variation of these polymer structures. On the basis of measurement from images of the scanning electron microscope, the transverse and longitudinal sizes of polymer spots and lines could be analyzed, and the relationship between the aspect ratio (AR) and the above experimental conditions could be acquired. The statistical results agree with our predictions that the RP laser beam can significantly reduce the AR, and the AR in RP laser fabrication has little correlation with conditions besides fabrication power, such as exposure time and scanning velocity.
The photonic band gap effect which originates from the translational invariance of the periodic lattice of dielectrics has been widely applied in the technical applications of microwave, telecommunication and visible wavelengths. Among the various examples, polymers based three dimensional (3D) photonic crystals (PhCs) have attracted considerable interest because they can be easily fabricated by femo-second (fs) ultrafast laser direct writing (DLW) method. However, it is difficult to realize complete band gap in polymers PhCs due to the low index contrast between polymers and air. Here, we report the design and experimental realization of light's nonreciprocal propagation in woodpile PhCs fabricated with DLW method. Firstly, we fabricated several polymers woodpile PhCs on glass substrate with different crystal planes. The Fourier transform infrared spectroscopy (FTIR) measurements are in agreement with the theoretical predictions, which proves the validity and the accuracy of our DLW method. Further measurements of the transmission spectra with respect to the incident angle reveal that the surface crystal planes and incident wave vectors play important roles in the optical response. Furthermore, we designed and fabricated a 30° PhC wedge. And we find nonreciprocal transmission effect between the forward and backward waves, resulting from the nonsymmetrical refraction of the light in different planes. Our results may find potential applications in future 3D photonic integrated circuits and pave the way for the fabrication of other photonic and optical devices with DLW method.
In this study, a series of acryloyloxy-substituted azobenzene derivatives, 3-(tert-butyl)-4,4’-bisacryoloxy-azobenzene
(tBu-Azo-AO), 3-(tert-butyl)-4,4’-bis[3-(acryoloxy)propoxy]-azobenzene (tBu-Azo-AO3) and 3-(tert-butyl)-4,4’-bis[6-(acryoloxy)hexyloxy]-azobenzene (tBu-Azo-AO6) were synthesized and employed as monomers to prepare polymer films by copolymerizing dipentaerythritol hexaacrylate (DPE-6A) and methyl methacrylate (MMA), respectively. When exposed to a nanosecond laser beam at the wavelength of 355 nm, ultraviolet-visible (UV-Vis) absorption spectra of the resultant polymer films with different irradiation time were monitored. On the basis of the absorbance of the π-π* electronic transition, the kinetics of trans-to-cis photoisomerization of three kinds of azobenzene moieties were demonstrated and found to be influenced by both the pump energy and azobenzene concentration.
Femtosecond lasers have been found suitable for maskless photolithography with submicron resolution, which is very attractive for solving the problem of high photomask cost. Direct femtosecond laser writing of lithographic patterns is reported with submicron feature width on thin positive photoresist film. We use a scanning electron microscope to investigate the feature sizes of femtosecond laser lithography, which are determined by the incident laser power, the number of scan times and the substrate materials. Submicron T-shaped gates have been fabricated using a two-step process of femtosecond laser lithography where the gate foot and head can be separately defined on positive AZ4620 photoresist film. This work has led to the stable fabrication of sub-300 nm T-gates on the samples of GaN on sapphire substrate and AlGaN/GaN on Si substrate.