Godai Miyaji

Mechanism of femtosecond-laser-induced periodic nanostructure formation on crystalline silicon surface immersed in water

Focused on silicon surface in water, superimposed multiple shots of linearly polarized 800-nm, 100-fs, 10-Hz laser pulses at lower fluence than the single-pulse ablation threshold are shown to produce two kinds of periodic nanostructures with almost constant periods of 150 nm and 400 nm. Surface plasmon polaritons excited in the surface layer illustrates well the formation of nanostructures and its dynamic properties observed. Pump and probe measurements of the ultrafast change in surface reflectivity during the interaction have demonstrated that the multiple low-fluence fs pulses are crucial to the nanostructuring through the accumulation of non-thermal bonding structure change and the subsequent nanoscale ablation.

Ultrafast dynamics of periodic nanostructure formation on diamondlike carbon films irradiated with femtosecond laser pulses

Using a pump-probe technique the authors have measured reflectivity of diamondlike carbon (DLC) film irradiated with femtosecond laser pulses to understand dynamic processes responsible for periodic nanostructure formation. The results have shown that characteristic reflectivity change observed as a function of superimposed laser shots is closely associated with the nanostructure formation and the bonding structure change to induce surface swelling, leading to a conclusion that the nanostructure formation on the DLC surface is certainly preceded by the bonding structure change. The nanoscale ablation to produce the nanostructure is discussed based on the local field generation on the surface.

Nanoscale ablation on patterned diamondlike carbon film with femtosecond laser pulses

The authors have studied the origin of nanostructure formation on diamondlike carbon film in femtosecond laser ablation at low fluence. Using the thin film target patterned with submicrometer-size stripes, they have observed that the nanostructure starts to be formed on the crest of stripes along the direction perpendicular to the laser polarization. The experimental results have shown that nanoscale ablation for the nanostructuring would preferentially be initiated by the enhancement of localized electric field on the stripe surface with high curvature.

Nanograting formation through surface plasmon fields induced by femtosecond laser pulses

Ablation of solid surfaces irradiated with superimposed multiple shots of low fluence femtosecond (fs) laser pulses often results in the formation of periodic nanostructures on the target surface. We demonstrate 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 was used to control the nanoscale energy deposition that should be developed through the excitation of surface plasmon polaritons (SPPs) during the fs laser-surface interaction. The results obtained for crystalline gallium nitride represent exactly the nature of a single spatial standing wave mode of SPPs of which periodically enhanced near-fields ablate the target surface to form the nanograting with a period of ∼200 nm. The calculated results for a model target reproduce well the observed nanograting period and explain the origin of its characteristic properties.

Role of multiple shots of femtosecond laser pulses in periodic surface nanoablation

Using a pump and probe technique, we observed time-dependent change in reflectivity of crystalline silicon surface to study the dynamic process of periodic surface nanostructure formation in femtosecond (fs) laser ablation. The results have shown that multiple shots of low-fluence fs laser pulses play the crucial role in the non-thermal process for nanostructuring through the increasing bonding structure change to amorphous silicon and resulting decrease in the ablation threshold.

Measurement of molecular rotational temperature in a supersonic gas jet with high-order harmonic generation.

We apply high-order harmonic generation to sensitive measurements of the molecular rotational temperature in a thin supersonic gas beam. The method uses nonresonant pump and probe femtosecond laser pulses to generate harmonic radiation from coherently rotating molecules. The rotational temperature of molecules can be derived accurately with high spatial and temporal resolutions from the Fourier spectrum of time-dependent signals. The validity of this method was tested for an expanding flow of an N(2) beam with a rapid temperature decrease. The results show the versatile applicability of this method.

Nanograting formation on metals in air with interfering femtosecond laser pulses

It is demonstrated that a homogeneous nanograting having the groove period much smaller than the laser wavelength (∼800 nm) can be fabricated on metals in air through ablation induced by interfering femtosecond laser pulses (100 fs at a repetition rate of 10 Hz). Morphological changes on stainless steel and Ti surfaces, observed with an increase in superimposed shots of the laser pulses at a low fluence, have shown that the nanograting is developed through bonding structure change at the interference fringes, plasmonic near-field ablation to create parallel grooves on the fringe, and subsequent excitation of surface plasmon polaritons to regulate the groove intervals at 1/3 or 1/4 of the fringe period over the whole irradiated area. Calculation for a model target having a thin oxide layer on the metal substrate reproduces well the observed groove periods and explains the mechanism for the nanograting formation.

Fabrication of 50-nm period gratings on GaN in air through plasmonic near-field ablation induced by ultraviolet femtosecond laser pulses

We demonstrate the formation of a homogeneous nanograting with 50-nm period on GaN in air, using ultraviolet femtosecond (fs) laser pulses at 266 nm in the recently developed two-step ablation technique. The experimental results have shown that the ablation technique successfully controlled the spatial mode of surface plasmon polaritons (SPP) excited on the target surface and decreased the grating period in accordance with the short wavelength of fs laser pulses. Calculation for a model target reproduces well the laser-wavelength dependent periods, being in good agreement with the observed, and supports the mechanism for nanostructuring.

Nanograting fabricated with femtosecond laser pulses

Intense femtosecond laser pulses are a common tool in precision processing of a variety of materials, which are needed in the fabrication of, for example, electro-optical devices and microelectromechanical systems. The advantage of using femtosecond laser pulses for material processing is the ultrafast deposition of high-density energy onto the target and the resulting suppression of undesirable thermal and mechanical effects in the lightmatter interaction. In addition, observations of self-organized, surface nanostructure formations in femtosecond-laser ablation experiments have found the size of these structures is typically 1/10–1/5 of the laser wavelength ,1–4 which suggests that femtosecond lasers have the potential to sculpt, or ‘nanoprocess,’ structures smaller than the diffraction limit. To find a new route to femtosecond-laser nano-processing, we focused our attention on the nanoscale, ultrafast light-matter interaction responsible for nanostructuring. We explored in particular the use of superimposed multiple shots of low-fluence femtosecond laser pulses. Our preliminary studies with dielectric and semiconductor materials have shown that the laserinduced near-field plays a fundamental role in the nanoscale ablation of a corrugated surface,5, 6 and the origin of periodicity can be attributed to the excitation of surface plasmon polaritons (SPPs) in the surface layer.7, 8 Based on the model of nanostructuring, we successfully fabricated a nanograting with a uniform period on a crystalline gallium nitride (GaN) surface. We performed our experiment in air (as opposed to vacuum) using linearly polarized 800-nm, 100-fs laser pulses from a Ti:sapphire laser system operated at a repetition rate of 10Hz. In the first-step of a two-step process, we split the femtosecond laser output into two beams. As schematically illustrated in Figure 1(a), Beam 1 is normally incident on the target while Beam 2 is incident at an angle with respect to the normal. The two beams overlap on the target surface to create interference fringes in the direction perpendicular to the laser Figure 1. (a) This schematic of the optical configuration shows how Beam 1 (normal incidence) overlaps with Beam 2 (incidence angle ) on the target surface. (b) The interference from the two beams (coming from a single femtosecond pulse) forms a periodic structure on a gallium nitride (GaN) surface, as seen in this scanning probe microscope (SPM) image. The fluence in each beam is F D 400mJ/cm2.

Probing molecular structure with alignment-dependent high-order harmonic generation

High-order harmonic generation (HHG) from nonadiabatically aligned molecules has been demonstrated to have new abilities to probe molecular dynamics and structures with femtosecond (fs) temporal resolutions [1–4]. In the recent study of HHG from coherently rotating molecules, the present authors have shown that HHG properties strongly depend on the degree of molecular alignment, and a low rotational temperature Trot and resulting high degree of alignment is strongly desired in the HHG experiment to retrieve the image of electronic distribution or orbital structure in a molecule [3].