Takuro Tomita

Effect of surface roughening on femtosecond laser-induced ripple structures

The effect of surface roughness was studied on the formation of femtosecond laser-induced ripple structures for 4H-SiC. Of the two types of ripple structures, fine and coarse, the threshold fluence for the formation of the fine ripple was remarkably reduced on the surface roughened by intentional rubbing, while the threshold for the formation of the coarse ripple was not affected. The result suggests that the initial surface roughness plays a significant role only for the formation of fine ripples. A brief explanation will be given on the qualitative basis for the specific effect of surface morphology to the fine ripple formation.

Temperature dependence of near ultraviolet photoluminescence in ZnO/(Mg, Zn)O multiple quantum wells

We report on temperature dependence of excitonic photoluminescence (PL) from ZnO/(Mg, Zn)O multiple quantum wells (MQWs). Two kinds of MQWs having different barrier heights grown by laser molecular-beam epitaxy showed significantly different temperature dependences of PL spectra; in ZnO/Mg0.27Zn0.73O MQWs, the PL peak energy at 50–200 K was a monotonically increasing function of temperature, which was opposite to that ascribed by band gap shrinkage. Moreover, spectra taken at 95–200 K encompassed two peaks, both of which originated from recombination of localized excitons. The temperature-induced shift (redshift-blueshift-peak duplication-redshift) at 5–300 K is caused by a change in the exciton dynamics with increasing temperature due to inhomogeneity and the exciton localization effect. On the other hand, the corresponding dependence in ZnO/Mg0.12Zn0.88O MQWs (lower barrier height) was similar to that in bulk II–VI semiconductors.

Observation of laser-induced surface waves on flat silicon surface

We found that irradiation by a single femtosecond laser pulse produced a surface wave structure on a (100) silicon surface. The surface wave structure bearing a concentric shape was produced only in the edge regions in the direction of the electric field from the center. The increase in the number of irradiating pulses allowed the concentric structure gradually to align perpendicularly to the laser beam’s electric field. The period of the surface wave structure was nearly equal to the wavelength of the irradiating laser pulse. In this accord, we propose that this structure is a precursor of ripple formation.

Single-shot picosecond interferometry with one-nanometer resolution for dynamical surface morphology using a soft X-ray laser

Using highly coherent radiation at a wavelength of 13.9 nm from a Ag-plasma soft X-ray laser, we constructed a pump-and-probe interferometer based on a double Lloyds mirror system. The spatial resolutions are evaluated with a test pattern, showing 1.8-mum lateral resolution, and 1-nm depth sensitivity. This instrument enables a single-shot observation of the surface morphology with a 7-ps time-resolution. We succeeded in observing a nanometer scale surface dilation of Pt films at the early stage of the ablation process initiated by a 70 fs near infrared pump pulse.

Laser microfabrication and rotation of ship-in-a-bottle optical rotators

We have fabricated optical rotators inside a silica substrate and rotated them by a laser trapping technique. The fabrication method used was femtosecond laser-assisted etching, i.e., modification of the host material by irradiation with femtosecond laser pulses along a predesigned pattern, followed by selective chemical etching. The rotators, which consist of the same material as the substrate, can move inside the microcavity but cannot get out. The rotation speed was proportional to the trapping laser power, and the maximum achieved was about 100rpm. Such rotators will be applicable to micro-total-analysis systems and microfluidics.

AF–FRI metamagnetic transition in itinerant Mn2−xCoxSb system: high-field and high-pressure effects

Abstract The magnetization of Mn2−xCoxSb system with 0⩽x⩽0.35 has been measured under high magnetic fields up to 100 T and high pressure up to 12 kbar . Mn 2 Sb is a ferrimagnet and substitution of Co for Mn at 0.18⩽x⩽0.35 results in the appearance of the spontaneous first-order magnetic phase transition from ferrimagnetic (FRI) to antiferromagnetic (AF) state at T=Tt with decreasing temperature at a critical Co concentration. Below Tt a first-order field-induced AF–FRI transition is observed at a critical field Bc. The critical field Bc is found to decrease with increasing pressure applied at T=4.2 K . The AF state which appeared for the compound with the critical concentration x=0.18 can be transformed to the FRI one by the application of the high pressure of about 16 kbar by increasing the amount of FRI phase. Using the experimental data, the magnetic phase diagrams Bc–T–x and Bc–p–x are determined. The observed magnetic properties and magnetic phase diagrams of Mn2−xCoxSb are explained by the itinerant magnetism of d-electrons and by peculiarities of electronic structure. The itinerant nature of AF–FRI transition is confirmed by specific heat measurements in AF- and field-induced FRI states.

Formation of periodic strained layers associated with nanovoids inside a silicon carbide single crystal induced by femtosecond laser irradiation

We observed the formation of subwavelength periodic strained layers associated with nanovoids in the cross section of femtosecond laser-irradiated lines written inside 4H-SiC single crystals. Both conventional and high-voltage transmission electron microscopies were carried out for microstructural analyses. The cross section of the irradiated lines consists of four to six groups of fine periodic structures. Each group is composed of strained layers with a typical spacing of 150 or 300 nm. The layers extend along the irradiated lines, aligned parallel to the electric field of the laser light. Tiny voids approximately 20 nm in diameter are found in the layers.

Inter-conduction band electron relaxation dynamics in 6H–SiC

The ultrafast inter-conduction band carrier dynamics in 6H–SiC was observed by using pump and probe transient absorption technique. Probe wavelength dependence of the bleaching was compared with the steady-state absorption profiles for polarizations parallel and perpendicular to the c axis, and these bleachings were ascribed to the decrease of electron populations in the lowest conduction band. The relaxation time from the higher to the lowest conduction band due to the inter-conduction band electron–phonon scattering is 1.25 ps.

Experimental verification of femtosecond laser ablation schemes by time-resolved soft x-ray reflective imaging

Pump and probe reflective imaging using a soft x-ray laser probe was applied to the observation of the early stage of femtosecond laser ablation process on platinum. In strongly excited area, drastic and fast reflectivity drop was observed. In moderately excited area, the decay of the reflectivity is slower than that in the strongly excited area, and the reflectivity reaches its minimum at t = 160 ps. In weakly excited area, laser-induced reflectivity change was not observed. In addition, the point where the reflectivity dip was observed at t = 10 ps and t = 40 ps, coincides with the position of the edge of reflectivity drop at t = 160 ps. These results give the critical information about the femtosecond laser ablation.

Enhancement of local electrical conductivities in SiC by femtosecond laser modification

Enhancement of local electric conductivities induced by femtosecond laser modification in silicon carbide was studied. Current-voltage (I-V) characteristics of the laser-modified regions were measured between the ion-implanted metal contacts. Interestingly, the resistance sharply decreased in the fluence range from 5.0 to 6.7 J/cm2. The resistance at the irradiation fluence of 53 J/cm2 decreased by more than six orders of magnitude compared with the nonirradiated one. From the I-V characteristics and the scanning electron microscope observations, we conclude that the phase separation associate with the formation of classical laser induced periodic structure causes the drastic increase in electric conductivity.