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.
We fabricated hierarchical glass biochips with interior characteristics that can be tailored by adjusting process parameters. By a new derived processing method, termed Femtosecond Laser Assisted chemical Etching Nanoscale Glass Deformation (FLAE-NGD), we have developed graded and hierarchical configurations with dimensions from several hundred micrometers to several hundred nanometers as relevant glass model platforms that mimic cancer cell intravasation-extravasation processes. We were able to control the dimensions of both the widths and lengths of the channels as well as shape and curvatures of interior glass pillars. Various curvatures were successfully prepared for the study of the migration and invasion processes of cancer. We have further evaluated the effect of x-ray exposure on melanoma cells grown in glass biochips and determined the increase of intracellular reactive oxygen species production and cellular DNA breaks with the applied irradiation dose.
A method is demonstrated for large-scale micro-through-hole (MTH) array fabrication in glass substrates for an advanced digital nucleic acid amplification technique (NAAT). The fabricated chip is advantageous to current partitioning devices in terms of speed, cost, and simplicity. To satisfy the requirement of MTH quantities for valid nucleic acid statistics, we improve the laser processing speed by focusing ultrafast Bessel pulses into a glass substrate under continuous translation. A single Bessel pulse can result a single MTH, and hundreds to thousands of MTHs can be produced per second. Preliminary digital NAAT experiment shows promising results of reagent partitioning of the fabricated chip. This work offers a highly efficient and low-cost scheme for glass-based reagent partitioning that will contribute to the wide accessibility of advanced digital NAATs.
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.
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.
Femtosecond laser pulses with GHz burst mode, which consists of a series of femtosecond laser pulse trains with a pulse interval of several hundred ps is expected to achieve high-efficiency and high-quality materials processing that cannot be performed by the conventional irradiation scheme. In this paper, we show the results of GHz burst mode ablation of silicon and copper. We further extend the GHz burst mode to surface nanostructuring for formation of novel two-dimenional laser-induced periodic surface structures (LIPSS), two-photon plolymerization (TPP) for improvement of fabrication resolution, and laser-induced plasma-assisted ablation (LIPAA) for high-quality microfabrication of transparent materials.
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.
Recently, GHz burst-mode fs laser pulses, which consists of pulse trains with extremely short intervals of several hundred ps, show superior processing characteristics in laser ablation to the conventional femtosecond laser processing (single mode) at the same average power. The GHz bursts of fs laser enables to induce ablation before the residual thermal energy deposited by preceding pulses diffuses away from the processed area, resulting in the reduction of the ablation threshold and enhancement of ablation efficiency. In this study, we apply the GHz burst mode fs laser pulses for MPP of photo-sensitive polymers to explore the effect of the burst on photochemical reaction.
Laser-induced near-field effect concentrates the laser energy to be enhanced on a localized area much smaller than the wavelength for nanoprocessing. Owing to the superhigh fabrication resolution, the laser near-field processing has been used for the surface nanostructuring to create photonic devices. The near-field processing is typically performed by using scanning optical microscope or scanning probe microscope combined with laser, while nano/microspheres provide the unique advantages of maskless, time-saving schemes.
In this paper, laser near-field reduction of metal ions assisted by silica spheres is presented for fabrication of plasmonic superlattices on silicon substrate, which can tune localized surface plasmonic resonance wavelengths from the visible to the near-infrared region by adjustment of the lattice periods. In the laser near-field reduction, the incident laser is tightly focused at the bottom side of the silica sphere to confine the reaction the in near-field.
Trace detection of bio-molecules with large molecular weight using surface-enhanced Raman scattering (SERS) method is still challenging, because the troublesome treatment of SERS substrate using coupling or cross-linking agents is required. In this paper, we apply liquid interface assisted SERS (LI-SERS) method, which provides unique features of collection and self-immobilization of analyte molecules on the SERS substrate, to realize the label-free trace detection of bio-molecules with detection limits of pM ~ fM. Specifically, DNA discrimination and quantitative detection of β-Amyloid (Aβ) in trace-concentration are demonstrated to illustrate the ultrahigh sensitivity and versatility of the LI-SERS method.
Ultrashort pulsed lasers are becoming used in multiple applications thanks to their extremely short pulse durations, which confine processing within the irradiated zone and ensure a precise material ablation. However, ultrashort pulsed lasers encounter some challenges at high-speed material removal. In this situation, the use of higher power lasers for increasing ablation rates leads to detrimental effects due to heat accumulation. Recently, GHz burst mode laser ablation has been proposed as a method to overcome this limitation by applying ablation cooling.
Following this approach, we study the influence of laser irradiation parameters in burst mode on the ablation efficiency and surface microfabrication quality with special interest in the use of different wavelengths, since most studies are often limited to use the fundamental wavelength in the near infrared. Bursts of pulses used in this study contain multiple pulses happening at an ultrafast ultrafast repetition rate of 5 GHz.
We have demonstrated UV laser photo-polymerization of elastic 2D/3D structures using UV-curable Polydimethylsiloxane (PDMS). In this technique, UV curable PDMS was locally polymerized to fabricate 1D and 2D single layer structures, as well as 3D multi-layer structures with resolution in micrometer-scale and structure dimension in millimeter-scale. In addition, a hybrid technique of aerosol jet printing followed by UV-LDW was developed to realize 2D patterning of thinner UV curable PDMS layer on free-form surface without solvents. In addition, 3D micro/nano processing for fabrication of 3D microfluidic surface-enhanced Raman spectroscopy (SERS) sensors will be presented as a subtractive process using photo-sensitive glass.
Cancer metastasis is the process in which cancer cells developed in the primary tumor start to spread in the body through bloodstream or lymphatic systems and home in on secondary sites, where they may generate new tumors. At the first stage, individual cancer cells migrate through narrow confined nanometric spaces or channels of micrometric interstitial spaces. It is thus challenging to create synthetic environments that mimic in vivo characteristics, fabricating relevant biosystems along with imaging techniques for sub-cellular visualization in order to understand mechanism of cancer cell migration, in particular in confining environments. Femtosecond laser assisted chemical etching (FLAE) is a technology performing subtractive processing of glass in order to create 3D microfluidic structures embedded in a microchip with the micrometric feature size. We evaluate herein relevant glass platforms capable to offer both observation of collective cancer cells migration over long periods and individual visualization at unicellular and subcellular levels on the target cell. Glass microfluidic biochips with micrometric characteristics are first fabricated by FLAE, hosting in vivo like microenvironments. Then, by applying two photon polymerization one may generate biomimetic polymeric architectures with confining channels inside microchannels. The fabricated 3D glass nanofluidics is applied to observe behavior of cancer cell deformation and migration in narrow spaces, providing new findings.
Recently Fourier-Scatterometry has become of increasing interest for quantitative wafer metrology. But also in other
fields the fast and precise optical characterization of periodical gratings of sub 100 nm size is of great interest. We
present the application of Fourier-Scatterometry, extended by the use of the coherent properties of white light for the
characterization of sub-wavelength periodic gratings of photosensitive material structured by two-photon polymerization.
First a simulation-based sensitivity comparison of Fourier-Scatterometry at one fixed wavelength, Fourier-Scatterometry
using a white light light source and also additionally using a reference-branch for white-light-interference has been
carried out. The investigated structures include gratings produced by two-photon polymerization of photosensitive
material and typical semiconductor test gratings. The simulations were performed using the rigorous-coupled-waveanalysis
included in our software package MicroSim. A sensitivity comparison between both methods is presented for
the mentioned structure types. We also show our experimental implementation of the measurement setup using a whitelight-
laser and a modified microscope with a high-NA (NA: 0.95) objective as well as a Linnik-type reference branch
for the phase sensitive measurements. First measurements for the investigation of the performance of this measurement
setup are presented for comparison with the simulation results.
A parallel processing of two-photon polymerization structuring is demonstrated with spatial light modulator. Spatial light
modulator generates multi-focus spots on the sample surface via phase modulation technique controlled by computer
generated hologram pattern. Each focus spot can be individually controlled in position and laser intensity with computer
generated hologram pattern displayed on spatial light modulator. The multi-focus spots two-photon polymerization
achieves the fabrication of asymmetric structure. Moreover, smooth sine curved polymerized line with amplitude of 5
μm and a period of 200 μm was obtained by fast switching of CGH pattern.
Laser-induced plasma-assisted ablation (LIPAA) process developed for glass materials has been applied for micromachining of a variety of transparent hard and soft materials. We have developed the proto-type LIPAA system using a second harmonic of diode pumped Q-switched Nd:YAG laser for the practical use. In this paper, micromachining and scribing of glass and sapphire is demonstrated using the developed system. Additionally, another application such as selective metallization of glass and polyimide with successive metal plating process is investigated. However, mechanism of this process is complex and still remains unknown. To have a better understanding of this process, double-pulse irradiation of a near-IR femtosecond (fs) laser with a delay time is also investigated. A possible mechanism is discussed based on the obtained results.
Ablation dynamics of fused silica by multiwavelength excitation process using F2 and KrF excimer laser has been investigated by energy analyzed mass spectrometry of ablated species. The number of Si ion generated by multiwavelength excitation process corresponds to that by single-F2 laser ablation and to approximately 2.1 times larger than that by single-KrF excimer laser ablation. In addition, kinetic energy distribution of Si+ ablated by multiwavelength excitation process shows almost same as that by single-F2 laser ablation. We regard that absorption of KrF excimer laser by excited state generated by F2 laser (excited-state absorption: ESA) causes similar electron excitation process to single-F2 laser irradiation, resulting in enhancement of Si ion with higher kinetic energy and then in high-quality ablation.
Three kinds of advanced technologies using lasers for glass microprocessing are reviewed. Simultaneous irradiation of vacuum ultraviolet (VUV) laser beam, which possesses extremely small laser fluence, with ultraviolet (UV) laser achieves enhanced high surface and edge quality ablation in fused silica and other hard materials with little debris deposition as well as high-speed and high-efficiency refractive index modification of fused silica (VUV-UV multiwavelength excitation processing). Metal plasma generated by the laser beam effectively assists high-quality ablation of transparent materials, resulting in surface microstructuring, high-speed holes drilling, crack-free marking, color marking, painting and metal interconnection for the various kinds of glass materials (laser-induced plasma-assisted ablation (LIPAA)). In the meanwhile, a nature of multiphoton absorption of femtosecond laser by transparent materials realizes fabrication of true three-dimensional microstructures embedded in photosensitive glass.
A multiwavelength excitation process using F2 and KrF excimer lasers for high-efficiency and high-speed refractive index modification of fused silica is demonstrated. We find that this process is essentially superior to conventional single-wavelength F2 laser processing. The multiwavelength excitation process achieves twice of diffraction efficiency compared with that of single-wavelength F2 laser irradiation sample at the same number of total photons supplied to the sample. This high-speed and high-efficiency modification is realized within ±50 ns of the delay time of each laser beam irradiation. In addition, the refractive index change of the multiwavelength sample was increased to 8.2×10-3, which is 1.78 times larger than that of single-wavelength F2 laser irradiation sample at same irradiation time. This superiority of the wavelength excitation process is attributed to resonance photoionization-like process based on excited state absorption in fused silica.
Hybrid laser processing for precision microfabrication of glass materials, in which the interaction of a conventional pulsed laser beam and another medium on the material surface leads to effective ablation and modification, is reviewed. The main role of the medium is to produce strong absorption of the nanosecond laser beam by the materials. Simultaneous irradiation of the vacuum ultraviolet (VUV)laser beam, which possesses extremely small laser fluence, with the ultraviolet (UV) laser greatly improves the ablation quality and modification efficiency for fused (VUV-UV multiwavelength excitation processing). Metal plasma generated by the laser beam effectively for assists high- quality ablation of transparent materials, resulting in microstructuring, cutting, color marking, printing and selective metallization of glass materials (laser-induced plasma-assisted ablation (LIPAA)). The detailed discussion described in this paper includes the ablation mechanism of hybrid laser processing.
Novel materials processing by a multiwavelength excitation process using F2 and KrF excimer lasers for highefficiency and high-speed refractive index modification of fused silica is demonstrated. We find that this process is essentially superior to single-wavelength F2 laser processing. The multiwavelength excitation process achieves twice of diffraction efficiency compared with that of single F2 laser irradiation sample at the same number oftotal photons supplied to the sample. This high-efficiency and high-speed modification is realized within ns ofthe delay time ofeach laser beam irradiation. In addition, the refractive index change of the multiwavelength sample was increased by 8.2x 1 Ø3, which is 1 .78 times larger than that of single wavelength F2 laser irradiation sample at same irradiation time. This superiority of the multiwavelength excitation process is attributed to resonance photoionization-like process based on excited state absorption in fused silica.
VUV-UV multiwavelength excitation process for precision microfabrication of hard materials, in which simultaneous irradiation of the vacuum ultraviolet (VUV) laser beam with extremely small laser fluence and the ultraviolet (UV) laser beam leads to effective ablation, is reviewed. A collinear irradiation system of F2 and KrF excimer lasers has been developed for this process. This system achieves well- defined micropatterning of fused silica and GaN with little thermal influence and little debris deposition. Ablation mechanism is explained as absorption of KrF excimer laser by excited-states formed by F2 laser (excited-state absorption: ESA). In addition, this technique is applied for much more efficient refractive index modification of fused silica compared with single F2 laser irradiation, which is attributed to resonance photoionization-like process based on ESA. These processes are characterized by experiments carried out at various conditions such as laser fluence, irradiation timing of each laser beam, and pulse number.
The novel technique for high-quality ablation and high- efficiency refractive index modification of fused silica by VUV-UV multiwavelength excitation process has been developed, in which both of the commercially available F2 and KrF excimer lasers are simultaneously irradiated to the sample. The high-quality ablation is ascribed to absorption of KrF excimer laser by excited states formed by F2 laser (excited-state absorption mechanism). Dependences of ablation rate on KrF excimer laser fluence at three deferent F2 laser fluences and irradiation timing of each laser beam are investigated. The multiwavelength excitation process is also applied for effective refractive index change. The multiwavelength excitation process achieves twice of diffraction efficiency compared with single wavelength irradiation of F2 laser at same number of incident total photon number. This is caused by the resonance photoionization-like process through the excited state.
GaN ablative etching using coaxial irradiation of KrF excimer laser and F2 laser has been explored. The etching process is consisted of pulsed laser ablation and successive acid treatment. Single pulse ablation using KrF excimer planarizes etched surface, however, multiple KrF irradiation roughens etched surface significantly. Small fraction of F2 laser simultaneously irradiated with KrF excimer laser improves surface roughness caused in the case of single-KrF irradiation. Irradiating delay time between both lasers is varied in order to investigate excitation mechanism as well as find optimum irradiation condition.
Collinear irradiation system of VUV-UV multiwavelength excitation process using F2 and KrF excimer lasers has been developed. This system achieves high-quality ablation of fused silica. In addition, dependence of ablation rate on various conditions such as laser fluence, delay time of each laser irradiation, and pulse number is investigated. Multiwavelength excitation effect is strongly affected by the delay time and extremely high etching rate over 30 nm/pulse is obtained during -10 ns to 10 ns of the delay time. KrF excimer laser ablation threshold decreases and its effective absorption coefficient increases with increasing simultaneously irradiated F2 laser fluence.
Etching of GaN by ablation using KrF excimer or F2 laser has been demonstrated, as well as simultaneous irradiation of F2 laser with KrF excimer laser to GaN has been explored. The GaN etching process is consisted of the following sequential procedures: laser ablation and an acid chemical treatment for residue removal. Single-pulse irradiation of KrF excimer laser as well as F2 laser planarizes the etched GaN surface. Multiple KrF irradiation roughens etched GaN surface significantly; however, low intensity F2 laser simultaneously irradiated with the KrF excimer laser improves the surface roughness. Complete removal of 700 nm-GaN is accomplished by 10 pulses with a laser intensity of approximately 40 x 106 W/cm2, besides, very sharp etching sidewall and extremely flat sapphire surface are obtained.
F2 laser ablation etching of GaN has been demonstrated. The etching geometry, etching rate and microroughness were investigated, and compared to the case of KrF excimer laser ablation etching. The etching process is consisted of the ablation and hydrochloric acid treatment. Very sharp edge was found along the etched area. The microroughness of etched surface is reduced as the laser intensity increases. The f2 laser ablation of GaN is thought to be initiated by direct photoionization by single-7.9 eV photon absorption.
Combination of PLD and nitrogen radical beam has grown high quality TiN films on Si substrate without silicidation at the interface between TiN thin film and Si substrate even at growth temperature more than 700 degrees C. Additionally, X- ray photoelectron spectroscopy revealed that this method achieved synthesis of almost stoichiometric TiN films. Diffusion barrier characteristics of the grown film were examined by deposition of Al thin films of about 400 nm thick on the TiN grown films, followed by post-thermal treatment at 500 degrees C for 30 minutes. Scanning electron microscopy (SEM) observation and Rutherford backscattering spectroscopy analysis revealed that sharp interfaces between Al and TiN were maintained after the thermal treatment, indicating excellent property of the TiN films as Si barrier metal.
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