Ultrashort pulse (USP) laser processing has great potential for precise microfabrication. Toward higher quality and productivity, we have developed the data-driven USP laser processing in which process parameters can be controlled based on data such as in-process monitoring and artificial intelligence (AI) optimization. In this work, stable formation of laser-induced periodic surface structure (LIPSS) in nanoscale on transparent glass materials has been demonstrated by the data-driven UPS laser processing.
As a platform for high resolution bioimaging, biochips with a refractive index very close to that of a culture medium (water) are highly demanded. CYTOP (AGC Corporation) is a promising material, since its refractive index (1.34) is almost similar to that of water (1.33). However, high transparency in also the ultraviolet region and high chemical resistance of CYTOP make it difficult to fabricate 3D microstructures for biochips. In this study we present fabrication method of 3D biochips based on CYTOP for cell observation by the molding process using 3D micro/nanostructures fabricated by two-photon polymerization with femtosecond laser.
We report on recent research on the development of data-driven ultrashort pulse laser processing to achieve higher productivity and quality. We are developing in-process monitoring, artificial intelligence (AI) optimization, and fast active control of the laser based on them. These key technologies are introduced for micro-drilling of metals and transparent materials and laser-induced periodic surface structure (LIPSS) formation on a silica glass. We demonstrate a fast pulse-to-pulse modulation of the fluences to control the ablation efficiency. A deep neural network was utilized to predict the 3-dimensional shapes of the ablation craters depending on the laser parameters (fluence and pulse duration). The scheme was extended to 10 sequential modulations of fluences. An in-process monitoring of the crack formation on glasses was implemented by optical transmission imaging with deep neural network. The optical reflection/transmission technique was also employed to probe the quality of the LIPSS formation on a silica glass.
The GHz burst mode femtosecond (fs) laser pulses have attracted considerable attention because they can perform better quality and higher efficiency ablation compared to the conventional irradiation scheme of fs pulses (single-pulse mode). Recently, we have demonstrated that the GHz burst mode fs laser pulses can create two-dimensional (2D) periodic surface structures (LIPSS) on Si surfaces. In this paper, we extend the GHz burst mode fs laser processing to form LIPSS on Ti plates. Our aim was to further investigate the more detailed mechanism and explore practical applications. Although the material characteristics of Ti are significantly different from Si, the GHz burst mode fs laser pulses can also create 2D-LIPSS. Then, mesenchymal stem cells cultured on the formed 2D-LIPSS were found to exhibit different behavior on 1D-LIPSS as compared with bare Ti surfaces.
KEYWORDS: Super resolution, Polymers, Microfluidics, 3D microstructuring, Two photon polymerization, Fabrication, Water, Femtosecond phenomena, Transparency, Refractive index
Development of biochips enabling distortion-free imaging in the microchannel filled with a culture medium (water) is required to investigate behavior of living cells in micro and nano environments. Fluoropolymer CYTOP is a promising material as a platform of biochips for the distortion-free imaging, because the refractive index of CYTOP (1.34) is almost same as 1.33 of water. In this study, we have developed a new 3D fabrication method for CYTOP by two-photon polymerized structures using a femtosecond laser as molds, which will be used for fabrication of micro and nano environment platforms for super-resolution bioimaging.
In the last several years, femtosecond laser processing using GHz burst mode, which consists of the ultrashort laser pulse (intra-pulse) trains with the pulse-to-pulse interval of several hundred ps, has attracted much attention, as it can achieve higher-processing quality with enhanced processing efficiency than the conventional femtosecond laser irradiation scheme (single-pulse mode). However, most of the research using the GHz burst mode was aimed at ablation. In this study, we extend the GHz burst mode femtosecond laser processing to the formation of laser-induced periodic surface structures (LIPSS) to explore the possibility of novel nanostructuring, over the single-pulse mode.
We have demonstrated surface plasmon polaritons excited on Si transiently metalized with an intense femtosecond laser pulse by measuring the reflectivity of a Si grating changed in the incident angle of the fs pulse. We observed that the incident angle where the sharp dip of the reflectivity appeared was changed by the thickness of SiO2 films. The result demonstrates that the plasmon wavelength is controlled through SiO2 film deposited on Si.
Laser-induced periodic surface structure (LIPSS) in nanometer scale formed by femtosecond (fs) laser pulses depends strongly on laser parameters such as fluence and superimposed pulse number, as well as the surface condition of target materials. Process control based on in-process monitoring is one of the solutions to form the nanostructure stably and homogeneously. However, it is difficult to monitor the nanostructure in the process, because it is much smaller than the light wavelength. This paper reports on a new technique for in-process monitoring of the periodic nanostructure using its anti-reflection property. As a target, we used a synthetic quartz plate. The linear-polarized 1030-nm, 250-fs laser pulses from a Yb fiber laser amplification system operated at a repetition rate of 20 kHz were focused with an objective lens and scanned on the target surface. Microscopic images of the target surface with coaxial epi-illumination and transillumination were acquired with two CMOS cameras. From these images, the surface reflectivity and transmittance were evaluated. After the ablation experiment, the surface morphology was observed with a scanning electron microscope. The surface of which transmittance increased as reflectivity decreased, had a line-like periodic nanostructure with a period of ~200 nm and a depth of ~1 μm. On the other hand, the surface of which both transmittance and reflectivity decreased did not have the nanostructure. These results demonstrate that an observation technique using anti-reflection property is much more effective in monitoring fs-laser-induced nanostructure on glass in the process.
We report that few-cycle laser pulses at low fluence can produce sub-100 nm periodic nanostructures on DLC surfaces through nanoablation induced by short-range surface plasmon polaritons (SR-SPPs). The 7-fs and 100-fs laser pulses from Ti:sapphire lasers were used. The period of the nanostructures with 7-fs pulses was much smaller than those with 100-fs pulses. The Raman spectrum showed that the modified layer in a DLC surface irradiated with 7-fs pulses was thinner than that with 100-fs. By using a model target, the period calculated for the excitation of SR-SPPs was in good agreement with that in the experiments.
Intense near-fields of surface plasmon polaritons (SPPs) excited with femtosecond (fs) laser pulses can sculpt nanometer-size structures on various kinds of solid materials through ablation. To control the formation, we need to understand the characteristic properties of the SPPs such as plasmon wavelength, damping, and spatial modes. Recently we succeeded to measure surface plasmon resonance curves of Si gratings with the intense p-polarized 100-fs laser pulses and observe the nanoablation on the surface. The experimental results and calculation for model targets have shown that SPPs with low internal damping can be excited with the fs pulse at higher fluence. This indicates that the propagation length of SPPs on Si can be controlled by the laser fluence of fs laser pulses.
Superimposed multiple shots of low-fluence femtosecond (fs) laser pulses form a periodic nanostructure on various kinds of solid surfaces through ablation. It has been proposed that the periodicity of the nanostructures is attributed to the excitation of surface plasmon polaritons (SPPs). However, the excitation of SPPs on non-metallic material surfaces was never been directly observed. We have observed anomalies appearing in the reflection of intense fs laser pulses at a Si grating surface with a grating structure. The results have exhibited an abrupt decrease to create a sharp dip at a specific incident angle, where the Si grating surface was deeply ablated along the edge of the grooves periodically. The experimental results and model calculation provide directly evidence that SPPs can resonantly be excited at the interface between air and the non-metallic material surface and that enhanced near fields can form periodic structures on the surface.
Superimposed multiple shots of low-fluence femtosecond (fs) laser pulses form a periodic nanostructure on solid surfaces through ablation. We have demonstrated that the self-organization process of nanostructuring can be regulated to fabricate a homogeneous nanograting on the target surface in air. A simple two-step ablation process and an ablation technique using interfering fs laser beams were developed to control plasmonic near-fields generated by fs laser pulses. The results have shown the nature of a single spatial standing wave mode of surface plasmon polaritons of which periodically enhanced near-fields ablate the target surface, to fabricate the nanograting on gallium nitride (GaN) and metals such as stainless steel (SUS) and titanium (Ti).
This paper describes two topics of our recent studies on ultrafast strong-field interactions with atoms, molecules and
solid surfaces. One is concerned with the high-order harmonic generation (HHG) from molecules nonadiabatically
aligned with intense femtosecond (fs) laser pulses in a pump and probe experiment. The HHG is very sensitive to the
molecular orbital and its spatial orientation with respect to the laser polarization. Experimental and theoretical studies
demonstrate the characteristic properties of HHG from coherently rotating molecules. The other topic is the periodic
nanostructure formation observed in fs laser ablation of dielectric materials. The major interest is in the ultrafast
interaction process of nanostructuring on solid surfaces, for the purposes of potential applications of fs lasers to nanoprocessing.
The experimental results have shown that enhanced near-field initiates the ablation of surface area much
smaller than the laser wavelength and the origin of nanoscale periodicity can be attributed to the excitation of surface
plasmon polaritons in the surface layer.
We have studied periodic nanostructure formation processes on hard thin film surfaces in femtosecond laser ablation. Using diamond-like carbon films patterned with submicrometer-size stripes, we found that the nanoscale ablation is preferentially initiated by the enhancement of a local field on the stripe surface having high curvature. Based on the experimental results for the initial stage of nanostructuring, it is concluded that the nanoscale ablation is initiated with the enhanced local field, and the periodicity is developed with the excitation of surface plasmon polaritons.
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