Additive manufacturing in-space is considered to have the potential for achieving logistical support in future space exploration. In order to meet the future metal additive manufacturing in-space, our research team is developing an advanced metal-wire laser additive manufacturing technology. The laser system is composed by 8 laser beams in an annular array. In the manufacturing process, 8 laser beams focus on the substrate to create molten pool and the metal-wire is vertically fed into the molten pool. This technology has been evaluated on ground environment and the metallurgical microstructure of the fabricated metallic parts is studied. In the immediate future, we are planning to evaluate the technology in microgravity environment using aircraft parabolic flights and carry out the comparison experiments. The experiments are used to study the effects of microgravity on the molten pool behavior, metal parts geometry, microstructure and mechanical properties.
Micro-pits array structure, micro-gratings structure covered with finer nanostructures were generated on titanium surfaces using femtosecond laser pulse. The wetting behavior of the two types of surface structures were studied. Micropits array structure with a size of dozens of microns exhibits hydrophilic properties, and the hydrophilicity increased with increasing the number of laser pulses. Micro-gratings structure covered with finer nanostructures exhibits hydrophobic properties, and the hydrophobicity increased with decreasing the laser energy density. The results demonstrated that wettability of metallic surface can be facilely controlled by adjusting the femtosecond laser processing parameters. The formation processes of the two types of surface microstructures as well as the underlying mechanisms for their special wettability are also discussed. This research might provide a facile controllable strategy to realize special wettability for metal or other solid surfaces, even expand the functions and applications of metals.
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.
KEYWORDS: 3D printing, Printing, Two photon polymerization, 3D modeling, Photoresist materials, Additive manufacturing, Manufacturing, Polymers, Scanning electron microscopy, Surface roughness
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 Vs on the printing time T exhibits a linear behavior, indicating that the printing speed is 0.248 mm3/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.
Unstable thermocapillary convection in metal liquid bridge is a typical phenomenon during the laser metal-wire additive manufacturing process in microgravity environment. The evolution and dynamic mechanism of the liquid bridge will influence the manufacturing process and quality for the forthcoming on-orbit space metal additive manufacturing. Therefore, it is very important to investigate the evolution and instability of thermocapillary convection in liquid bridges in microgravity. In present investigation, a numerical model is developed to reveal the characteristics of thermocapillary convection. The effects of aspect ratio and gravity on the critical Reynolds number for convection instability of thermocapillary convection in metal (Ti6Al4V) liquid bridge are investigated numerically. The results indicate that the critical Reynolds number for convection instability decreases with the increase of aspect ratio number at first, and then increases both in the gravity or microgravity environment. The numerical results also reveal that the critical Reynolds number for convection instability under gravity environment with natural convection in metal liquid bridge is larger than microgravity environment. The research shows that the influence of microgravity leads to a distinctly different behaviour of thermocapillary convection in metal liquid bridge compared to the gravity environment. A more comprehensive study will be conducted to cover the parameter space more systematically to identify the factors which significantly influence the stability of the thermocapillary convection in metal liquid bridge under microgravity environment, which is important for the on-orbit space metal additive manufacturing.
Additive manufacturing of metal parts in space is one of the potential means to realize on-orbit maintenance of aircraft. However, the basic phenomena such as the rapid melting and solidification behavior of metallic materials under the action of high-energy beams in space are unclear. It is necessary to observe those phenomena and reveal basic laws through space experiments. Therefore, an experimental platform for rapid melting and solidification of metal materials is developed. There are two parts included in this platform. A detailed design of the manufacturing system in space is described at first while the in lab experimental system on the ground is introduced also. In order to simulate the vacuum environment in space, a vacuum chamber is used to contain the core unit of the experimental system. Laser is used to melt a metal wire during the experiment while a positioning stage is adopted to shape the melted wire. The melting and solidification process is controlled automatically while it is monitored by a machine vision system at the same time.
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.
As complex ceramic multicomponent materials, the lunar regolith and minerals still challenge people on onsite utilization technology limited to the lunar surface environment. In this paper, we investigate the suitability of lunar regolith simulants and ilmenite powders for 3D printing (aka Additive Manufacturing) of hypothetical brick aimed at lunar habitat construction. The first generation of laser 3D printing equipment (Lunar 1.0) for our experiments has been designed and assembled, which is suitable for selective laser sintering (SLS) process out of many kinds of ceramic powders to manufacturing samples with different geometrical shapes. The lunar regolith simulants and ilmenite powders are demonstrated obvious spectral absorbance from ultraviolet to near-infrared spectra, which are successfully performed during the SLS process in Lunar 1.0. The 3D printing technologies are constantly improved by adjusting the parameters of laser process and mechanical movement. The morphological features of 3D printed samples, including surface and porosity are investigated by using SEM. The evaluation of size and micro-hardness tests are also conducted to reveal the printing qualities of samples. The EDS and XRD results characterize the elements and components of 3D printed samples. Obviously, the strong heating process by laser source in Lunar 1.0 has a great impact on materials, because the complex multicomponent materials and solid state reaction in high temperature by SLS process for regolith simulants and ilmenite. However, this influence of heat treatment by laser source is quite different from continuously thermal treatment for ceramics such as normal high temperature furnace. In the future, the research for 3D printing of lunar regolith simulants and ilmenite powders for hypothetical brick in vacuum and low gravity will carry out for approaching the extreme manufacturing environment on lunar surface.
Luneburg lens is a fascinating gradient refractive index (GRIN) lens that can focus parallel light rays on a perfect geometrical point without aberration in geometrical optics, compared with conventional lens made of homogeneous dielectric materials with specially designed surfaces. Constructing a three dimensional (3D) Luneburg lens at optical frequencies is a challenging task due to the difficulty of fabricating the desired GRIN materials in the lab. Reported experimental realizations of Luneburg lens at optical frequencies are mainly based on two dimensional metamaterials structures, or plasmonic structures. Here, we present the first practical implementation of the 3D version of Luneburg lens at optical frequencies based on the effective medium approximation. The 3D Luneburg lens is designed with GRIN 3D simple cubic metamaterials (SCMs) structures, and fabricated with no-resonant dielectric metamaterials by laser direct writing method in the commercial negative photoresist IP-L. We have experimentally demonstrated the feasibility of tailoring inhomogeneous metamaterials structures to realize 3D Luneburg lens with the effective GRIN profiles. The effective refractive index has been spatially and gradually modified by tailoring the volume filling fraction of SCMs structures. Simulated and experimental results simultaneously exhibit interesting 3D ideal focusing performance of the 3D Luneburg lens for the infrared light at wavelengths of 6.25m. This study would provide the protocol for developing the 3D Luneburg lens with wide field-of-view and ideal focusing theoretically and experimentally, which would further prompt the potential applications in integrated light-coupled devices and lab-on-chip integrated biological sensors based on infrared light.
Hydrogels with the precise three-dimensional configuration (3D hydrogels) have played important roles in the fields of tissue engineering and drug delivery, which will help to mimic the behavior of bio-inspired systems. Thus, the improvement of the spatial resolution on both the microscopic and the nanometric scale has been urgently demanded. There are some developed techniques to fabricate the 3D microstructures, however, these techniques require either high energy, strict operation environment or expensive mask.
As a promising maskless and flexible photolithography technique, two-photon polymerization (TPP) is a promising protocol to achieve 3D hydrogels. However, TPP of 3D hydrogels has been hindered due to the lack of high efficient water-soluble photoinitiator. In this presentation, the development of water soluble TPP photoinitiator of high initiating efficiency, as well as its fabrication capability and the resolution will be illustrated. The high resolution of 92 nm has been achieved benefiting from the high efficency of the developed photoinitiators and the optimized optical parameters. Furthermore, the microstructures with high accuracy such as those simulating the morphology of adenovirus and red blood cell have been fabricated, demonstrating the promising fabrication capability for achieving arbitrary architectures. The design of the photoinitiators, and the TPP fabrication technique mentioned in this study would provide high potential for the futher application in the research fields of cell imaging and tissue engineering.
F-theta lens is an important unit for selective laser melting (SLM) manufacture. The dual wavelength f-theta lens has not been used in SLM manufacture. Here, we present the design of the f-theta lens which satisfies SLM manufacture with coaxial 532 nm and 1030 nm~1080 nm laser beams. It is composed of three pieces of spherical lenses. The focal spots for 532 nm laser and 1030 nm~1080 nm laser are smaller than 35 μm and 70 μm, respectively. The results meet the demands of high precision SLM. The chromatic aberration could cause separation between two laser focal spots in the scanning plane, so chromatic aberration correction is very important to our design. The lateral color of the designed f-theta lens is less than 11 μm within the scan area of 150 mm x 150 mm, which meet the application requirements of dual wavelength selective laser melting.
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.
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.
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.
In our study, two carbazole-based cyanines, 3,6-bis[2-(1-methylpyridinium)vinyl]-9-methyl carbazole diiodide (A) and 6,6'-bis[2-(1-methylpyridinium)vinyl]-bis(9-methyl-carbazol-3yl)methane diiodide (B) were synthesized and employed as light-up probes for DNA and cell imaging. Both of the cyanine probes possess a symmetric structure and bis-cationic center. The obvious induced circular dichroism signals in circular dichroism spectra reveal that the molecules can specifically interact with DNA. Strong fluorescence enhancement is observed when these two cyanines are bound to DNA. These cyanine probes show high binding affinity to oligonucleotides but different binding preferences to various secondary structures. Confocal microscopy images of fixed cell stained by the probes exhibit strong brightness and high contrast in nucleus with a very low cytoplasmic background.
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.
We present a fabrication method of gold nanorod/ polymer composite microstructures by means of a femtosecond
near-infrared laser light. The mechanism of this method is based on a cooperation of two optical reactions;
two-photon polymerization (TPP) reaction only at the surface of gold nanorods, and optical accumulation of gold
nanorods in photo-polymerizable resin. Gold nanorods were mass-produced by seed mediated growth method, and
were mono-dispersed in photo-resin. The wavelength of the laser light was tuned resonant to two-photon
absorption of the photo-resin, and also close to a longitudinal local surface plasmon resonance (LSPR) mode of the
gold nanorods. The laser light excited LSPR onto gold nanorods, resulting in the formation of thin polymer layer
only at their surface through TPP. Concurrently occurring optical accumulation of gold nanorods by continuous
irradiation of laser light, gold nanorods got together into focus spot. The TPP layer at the surface of gold nanorods
worked as a glue to stick one another for forming their aggregated structure in micro/nano scale. By controlling
the intensity and the exposure time of laser light, an optimal condition was found to induce dominant polymerization
without any thermal damages. The scanning of the focus spot makes it possible to create arbitrary micro/nano
structures. This method has a potential to create plasmonic optical materials by controlling the alignment of gold
nanorods.
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.
In order to obtain high optical gain medium for low-threshold and compact polymeric lasers, a tert-butyl
substitutional rhodamine B (t-Bu-RhB) was synthesized and doped in PMMA film. The optical properties were
investigated and the results showed that the concentration of
t-Bu-RhB in PMMA could be increased to 6.9 wt % without
decrease of photoluminescence efficiency. Polystyrene (PS) PCs with bandgap of 595nm were fabricated by
vertical-deposition method. Resonator cavity was constructed by sandwiching t-Bu RhB doped films with two PS PCs.
When excited by a Nd:YAG laser beam(532nm,10Hz,8ns), single-mode laser emission was observed from the cavities
with dye concentration of 3, 5, 6.9, 10 wt %. Lasing emission is located at 592nm, with the full-width at half-maximum
(FWHM) of 1nm. Lasing threshold of the cavities dramatically decreased from 49.56 to 0.70 μJ/Pulse when the
concentration of t-Bu RhB increased from 3 to 6.9 wt%. Through enhancing the dye concentration to decrease lasing
threshold will save pump energy, which is not only important for economy of energy, but also in favor of extending the
life of such laser devices.
The two-photon polymerizations of metal ions doped acrylate monomers and oligomers have been investigated, which has been applied for three-dimensional (3-D) micro/nano-structure fabrication. Titanium (IV) ions doped urethane acrylate photopolymerisable resins were synthesized, and their properties of optical and polymerization were
investigated. The resolution of two-photon polymerization for micro/nanofabrication was evaluated. Titanium oxide (TiO2) nanoparticles were generated in the polymer matrix of micro-sized polymer structures. A 3-D diamond photonic crystal structure, which consisted of polymer composite materials of TiO2 nanoparticles, was successfully fabricated by direct laser writing technique, and its photonic bandgap was confirmed. This work would give us a new solution for producing 3-D micro/nanodevices of functional polymer composite materials.
The hyper Rayleigh scattering (HRS) method was applied to determine the absolute value of second-order hyperpolarizabilities ((beta) ) of stilbazolium p-toluenesulfonates and related ionic species in methanol. In general, the (beta) values of stilbazolium derivatives become large with increasing wavelength of absorption maximum and are 1.1 X 10-28 - 3.9 X 10-28 esu, i.e., 3 - 10 times larger than (beta) of p-nitroaniline. By way of exception, halogenated stilbazoliums showed rather larger (beta) compared with the unsubstituted stilbazolium having almost the same absorption cutoff and maximum. The calculated d value of hydroxystilbazolium p-toluenesulfonate obtained using the present (beta) and the known crystal structure agreed well with the experimental d value by the Maker fringe method.
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