Focus movement distance per pulse dependence of electrical conductivity and diameter of diamond internal modification induced by picosecond laser.

Internal and local modifications via ultrashort pulsed laser illumination to diamond are promising for manufacturing diamond electronic devices. The relationship between the diameter/electrical conductivity of modified regions and the laser fluence distribution was investigated. Picosecond laser illumination without scanning the laser focus fabricated short modified regions in diamond. As a result, the calculated laser fluence distribution matches the distribution of the modified regions. Wire-shaped modified regions were fabricated via laser illumination with scanning of the laser focus, and the corresponding diameter and electrical conductivity were investigated by controlling the laser focus movement distance per pulse (Vf). The modified regions fabricated with varying Vf were divided into three categories depending on the trend of the relationship between the diameter and electrical conductivity. The diameters of the modified regions were constant at the maximum values when Vf was sufficiently small, decreased with increasing Vf, and reached a minimum when Vf was sufficiently large. The modified regions became more electrically conductive with increasing Vf, even when the deposited energy per unit length decreased. Moreover, the electrical conductivity decreased significantly when the diameter became constant at the minimum value. Finally, the relationship between the diameter/electrical conductivity of the modified regions and the laser fluence distribution was elucidated.

Local control of optical absorption properties of glass using precipitation of gold nanoparticles via gold sphere movement driven by laser.

Glass embedded with metal nanoparticles is a promising material necessary for optical devices because of its absorption properties associated with the surface plasmon resonance (SPR) of metal nanoparticles. We demonstrated that continuous-wave laser illumination of the metal sphere in glass migrates the metal sphere and dopes the migration trajectory. In this study, we have attempted to locally control the absorption properties of borosilicate glass via gold nanoparticle precipitation using gold sphere migration. The gold sphere was moved in the glass via laser illumination, and the gold nanoparticles were precipitated in the movement trajectory. The trajectory was colored in red and purple gradations because of the SPR of gold nanoparticles. The particle size of the precipitated gold nanoparticles and the absorption peak wavelength increased with increasing velocity of the gold sphere. The absorption properties of the trajectory depend on the velocity of the gold sphere migration, which enabled local control of the absorption properties of the glass.

Clarification of fast metal sphere movement in glass.

In this study, metal spheres were implanted into glass by continuous-wave (CW) laser illumination, which manipulated the metal sphere inside the glass. The spheres moved at approximately 100 mm/s, which is 100 times faster compared to conventional movement. The movement mechanism was clarified by in situ, cross-sectional, and microscopic observations. With a high laser power density, the metal spheres moved fast with plasma emission, and their trajectory contained fine iron particles. The temperatures of the metal sphere with slow (<0.1 mm/s) and fast (>1 mm/s) speeds were 1,900 and 2,900 K, respectively.

Compositional redistribution in CaO-Al2O3-SiO2 glass induced by the migration of a steel microsphere due to continuous-wave-laser irradiation.

A high-power continuous-wave (CW) laser was used to move a steel microsphere through a CaO-Al2O3-SiO2 glass block at room temperature along a trajectory toward the laser source. A compositional analysis revealed that the CaO concentration in the glass decreased at the center of the microsphere's trajectory but increased in the area adjacent to it; the SiO2 concentration showed an opposite trend while the Al2O3 concentration did not change. Further, the compositional difference between the center and the area adjacent to the microsphere trajectory depends on the velocity of the microsphere, which is controllable by tuning the laser power.

Experimental and theoretical study on the driving force and glass flow by laser-induced metal sphere migration in glass.

Light is able to remotely move matter. Among various driving forces, laser-induced metal sphere migration in glass has been reported. The temperature on the laser-illuminated side of the sphere was higher than that on the non-illuminated side. This temperature gradient caused non-uniformity in the interfacial tension between the glass and the melted metal as the tension decreased with increasing temperature. In the present study, we investigated laser-induced metal sphere migration in different glasses using thermal flow calculations, considering the temperature dependence of the material parameters. In addition, the velocity of the glass flow generated by the metal sphere migration was measured and compared with thermal flow calculations. The migration velocity of the stainless steel sphere increased with increasing laser power density; the maximum velocity was 104 µm/s in borosilicate glass and 47 µm/s in silica glass. The sphere was heated to more than 2000 K. The temperature gradient of the interfacial tension between the stainless steel sphere and the glass was calculated to be -2.29 × 10-5 N/m/K for borosilicate glass and -2.06 × 10-5 N/m/K for silica glass. Glass flowed in the region 15-30 µm from the surface of the sphere, and the 80-µm sphere migrated in a narrow softened channel.

Picosecond optical vortex pulse illumination forms a monocrystalline silicon needle.

The formation of a monocrystalline silicon needle by picosecond optical vortex pulse illumination was demonstrated for the first time in this study. The dynamics of this silicon needle formation was further revealed by employing an ultrahigh-speed camera. The melted silicon was collected through picosecond pulse deposition to the dark core of the optical vortex, forming the silicon needle on a submicrosecond time scale. The needle was composed of monocrystalline silicon with the same lattice index (100) as that of the silicon substrate, and had a height of approximately 14 µm and a thickness of approximately 3 µm. Overlaid vortex pulses allowed the needle to be shaped with a height of approximately 40 µm without any changes to the crystalline properties. Such a monocrystalline silicon needle can be applied to devices in many fields, such as core-shell structures for silicon photonics and photovoltaic devices as well as nano- or microelectromechanical systems.

Moving force of metal particle migration induced by laser irradiation in borosilicate glass.

We optically manipulated a metal particle in borosilicate glass. The glass in the neighborhood of the laser-heated metal particle softened; hence, the metal particle was able to migrate in the glass. In this letter, the driving force of the metal particle toward the light source in the glass provided by laser illumination was investigated. The variation in the surface tension of the glass at the interface between the glass and the metal particle induced by the temperature gradient was calculated via a numerical temperature calculation. It was found that the temperature at the laser-illuminated surface of a stainless-steel particle with a radius of 40 µm was ~320 K higher than that on the nonilluminated side. The force applied to the metal particle from the surrounding glass was calculated to be ~100 µN, which was approximately equal to the viscous resistance force. In addition, the experimental and numerically calculated speeds of the moving particle, which was measured while varying the laser power, are discussed.

Metal particle manipulation by laser irradiation in borosilicate glass.

We propose a new technique of manipulating a metal particle in borosilicate glass. A metal particle that is heated by laser illumination heats the surrounding glass by radiation and conduction. A softened glass enabled metal particle migration. A 1-µm-thick platinum film was deposited on the back surface of a glass plate and irradiated with a green CW laser beam through the glass. As a result, the platinum film was melted and implanted into the glass as a particle. Platinum particles with diameters of 3 to 50 µm migrated at speeds up to 10 mm/s. In addition to platinum particles, nickel and austenitic stainless steel (SUS304) particles can be implanted.

The effect of micronscale anisotropic cross patterns on fibroblast migration.

Cell movement on adhesive surfaces is a complicated process based on myriad cell-surface interactions. Although both micron and nanoscale surface topography have been known to be important in understanding cell-materials interactions, typically only simple patterns (e.g., parallel lines or aligned posts) have been used in studying cell morphology, migration, and behavior. This restriction has limited the understanding of the multidirectional aspects of cell-surface response. The present study was performed to investigate cell morphology and motility on micronscale anisotropic cross patterns and parallel line patterns having different aspect ratios (1:2, 1:4, and 1:infinity), grid size (12-, 16-, and 24-mum distance neighboring longer side ridges), and height of ridges (3- and 10-mum). The movement characteristics were analyzed quantitatively with respect to cell migration speed, migration angle, persistence time (P) and motility coefficient (mu). A significant effect of the 1:4 grid aspect ratio cross patterns and parallel line patterns on cell alignment and directionality of migration was observed. Cell motility was also dependent on the patterned surface topography: the migration speed was significantly enhanced by the 1:2 and 1:4 cross patterns when the grid size was smaller than the size of individual cells (i.e., approximately 16 microm). In addition, the migration speed of cells on lower patterns was greater than on higher ridges. Overall, cell morphology and motility was influenced by the aspect ratio of the cross pattern, the grid size, and the height of ridges.

Fabrication of arbitrary polymer patterns for cell study by two-photon polymerization process.

Topographically patterned surfaces are known to be powerful tools for influencing cellular functions. Here we demonstrate a method for fabricating high aspect ratio ( approximately 10) patterns of varying height by using two-photon polymerization process to study contact guidance of cells. Ridge patterns of various heights and widths were fabricated through single laser scanning steps by low numerical aperture optics, hence at much higher processing throughput. Fibroblast cells were seeded on parallel line patterns of different height ( approximately 1.5-microm, approximately 0.8-microm, and approximately 0.5-microm) and orthogonal mesh patterns ( approximately 8-microm and approximately 4-microm height, approximately 5-microm and approximately 5.5-microm height, approximately 5-microm and approximately 6-microm height). Cells experienced different strength of contact guidance depending on the ridge height. Our results demonstrate that a height threshold of nearly 1 microm influences cell alignment on both parallel line and orthogonal mesh patterns. This fabrication technique may find wide application in the design of single cell traps for controlling cell behavior in microdevices and investigating signal transduction as influenced by surface topology.

Self-standing aligned fiber scaffold fabrication by two photon photopolymerization.

Development of materials and fabrication techniques lead the growth of three-dimensional cell culture matrices in biomedical engineering. In this work, we present a method for fabricating self-standing fiber scaffolds by two-photon polymerization induced by a femtosecond laser. The aligned fibers are 330 microm long with a diameter of 6-9 microm. Depending on the pitch of the aligned fibers, various cell morphologies are distinguished via three-dimensional images. Furthermore, the morphologies of fibroblast cells (NIH-3T3) and epithelial cells (MDCK) on the fiber scaffolds are studied to show the effect of high curvature (3-4.5 microm radii) on cell morphology. NIH-3T3 cells that contain straight pattern of actin microfilament bundles are extended and partly wrap single fibers or tend to reside between fibers. On the other hand, MDCK cells that contain circular pattern of actin microfilament bundles cover the fiber peripheral surface exhibiting high aspect ratio elongation. These results indicate that cell morphology on fiber scaffolds is influenced by the pattern of actin microfilament bundles.