Masaaki Sakakura

Ultrafast Manipulation of Self-Assembled Form Birefringence in Glass

Ultrashort pulse lasers have allowed probing of molecular dynamics in real time on the femtosecond time scale, with exotic behavior ranging from alignment of molecules and clusters, structural deformation, phase transitions on solid, and electron localization in magnetic materials. A recent progress in high power ultrashort pulse lasers has opened new frontiers in physics and technology of light-matter interactions from X-ray generation, nuclear fusion, laser surgery, integrated and fiber optics, optical data storage, to 3D micro- and nano-structuring. An intriguing phenomenon that currently attracts a lot of interest is the self-assembly of periodic nanostructures in the direction perpendicular to the light polarization. Uniaxial birefringence observed after femtosecond laser irradiation of silica glass has been explained by induced nanogratings and referred as self-assembled form birefringence. Self organization process has been interpreted in terms of the interference of electron plasma waves resulting in electron concentration modulation, followed by freezing of the interference pattern by structural change in glass. However, the mechanism including dynamics of self-organized nanostructures formation is still not fully understood. Recently, a double-pulse pump-probe configuration was used to enhance ablation in fused silica and silicon. In similar experiments molecular ensembles with an oriented angular momentum were produced. Here, we describe the ultrafast writing dynamics of form birefringence produced by self-organized nanogratings in double pulse experiments. Rewritable five-dimensional (5D) optical data storage using self-assembled form birefringence was demonstrated.

Simultaneous Tailoring of Phase Evolution and Dopant Distribution in the Glassy Phase for Controllable Luminescence

Construction of an active composite with multicolor visible and broadband near-infrared luminescence is of great technological importance for various applications, including three-dimensional (3D) display, broadband telecommunication, and tunable lasers. The major challenge is the effective management of energy transfer between different dopants in composite. Here we present an in situ strategy for controlling energy transfer between multiple active centers via simultaneous tailoring of the evolution of phases and the distribution of dopants in the glassy phase. We show that the orderly precipitation of Ga(2)O(3) and LaF(3) nanocrystals and the selective incorporation of Ni(2+) and Er(3+) into them can be achieved. The obtained composite shows unique multicolor visible and broadband near-infrared emission. Possible mechanisms for the selective doping phenomenon are proposed, based on thorough structural and optical characterizations and crystal-field calculation results. Moreover, the strategy can be successfully extended to accomplish space-selective control of multicolor luminescence by employing the modulated stimulation field. The results suggest that the strategy could be applied to fabricate a multifunctional light source with a broad range of important host/activator combinations and to construct various types of three-dimensional active microstructures.

Temperature distribution and modification mechanism inside glass with heat accumulation during 250 kHz irradiation of femtosecond laser pulses

Heat accumulation by high repetition rate femtosecond laser irradiation inside glass generates a much larger modification than that by a single pulse. In this study, we determined the temperature distribution due to heat accumulation and the characteristic temperature for heat modification inside a soda lime glass by analyzing the relationship between the radius of modification and glass temperature. The validity of the analysis was confirmed by reproducing the modification due to two-beam irradiation. The determined characteristic temperature suggested that the temperature distribution and the spatial dependence of the stress relaxation are important in the mechanism of heat modification.

Observation of pressure wave generated by focusing a femtosecond laser pulse inside a glass

The pressure (or stress) wave generated by focusing a femtosecond laser pulse inside a glass has been considered one of the important factors in determining structures created in the laser focal region. In this paper, a method of the transient lens (TrL) analysis was proposed to characterize the pressure wave. Experimentally, the TrL signal exhibited damping oscillation within 2 ns. Simulations of the TrL signal showed that the shape of the oscillating signal depended on the width and amplitude of the pressure wave. Comparing the observed TrL signal with the simulated one, we estimated these properties of the pressure wave generated after femtosecond laser focusing inside a soda-lime glass.

Ultrashort-pulse laser calligraphy

Control of structural modifications inside silica glass by changing the front tilt of an ultrashort pulse is demonstrated, achieving a calligraphic style of laser writing. The phenomena of anisotropic bubble formation at the boundary of an irradiated region and modification transition from microscopic bubbles formation to self-assembled form birefringence are observed, and the physical mechanisms are discussed. The results provide the comprehensive evidence that the light beam with centrosymmetric intensity distribution can produce noncentrosymmetric material modifications.

Mechanism of heat-modification inside a glass after irradiation with high-repetition rate femtosecond laser pulses

Accumulation of thermal energies by highly repeated irradiation of femtosecond laser pulses inside a glass induces the heat-modification whose volume is much larger than that of the photoexcited region. It has been proposed that the heat-modification occurs in the region in which the temperature had overcome a threshold temperature during exposure of laser pulses. In order to understand the mechanism of the heat-modification, we investigated the temperature distribution during laser exposure and the threshold temperature by analyzing the volume of the modification based on a thermal diffusion model. We found that the threshold temperature becomes lower with increasing laser exposure time. The dependence of the threshold temperature on the laser exposure time was explained by the deformation mechanism based on the temperature-dependent viscosity and viscoelastic behavior of a glass under a stress loading by thermal expansion. The deformation mechanism also could simulate a tear-drop shape of a heat-modific...

Directionally controlled 3D ferroelectric single crystal growth in LaBGeO5 glass by femtosecond laser irradiation

Laser-fabrication of complex, highly oriented three-dimensional ferroelectric single crystal architecture with straight lines and bends is demonstrated in lanthanum borogermanate model glass using a high repetition rate femtosecond laser. Scanning micro-Raman microscopy shows that the c-axis of the ferroelectric crystal is aligned with the writing direction even after bending. A gradual rather than an abrupt transition is observed for the changing lattice orientation through bends up to approximately 14 degrees. Thus the single crystal character of the line is preserved along the bend through lattice straining rather than formation of a grain boundary.

Heating and rapid cooling of bulk glass after photoexcitation by a focused femtosecond laser pulse.

To investigate the energy dissipation process after focusing a femtosecond laser pulse inside a zinc borosilicate glass, the time-dependent lens effect in the laser focal region was observed by a transient lens (TrL) method. We found that the TrL signal after 100 ns can be explained clearly by thermal diffusion. By fitting the observed signal, we obtained the phase change due to temperature increase, the initial diameter of the heated volume and the thermal diffusivity. On the basis of the results, the temperature increase and the cooling rate were estimated to be about 1800 K and 1.7X10(8) Ks(-1), respectively. We have also observed the signal change on a 100 ns scale, which can not be explained by the thermal diffusion model. This change was attributed to the relaxation of the heated material.

Fabrication of three-dimensional 1 x 4 splitter waveguides inside a glass substrate with spatially phase modulated laser beam.

Multiple light spots can be generated by modulating the spatial phase distribution of laser beam with a spatial light modulator (SLM). In this paper, we demonstrate the fabrication of three-dimensional 1 x 4 splitter waveguides inside a glass by focusing multiple light spots of femtosecond (fs) laser pulses, which can be controlled by switching spatial phase distributions on an SLM. In the conventional fs laser writing technique, a highly precise positioning of a substrate is essential for fabricating a branched waveguide in a splitter. Using the technique proposed in this paper, a continuously branched waveguide can be produced easily by translating a glass substrate only one time; therefore this technique can eliminate the need for a high precision in positioning of a substrate and save a fabrication time.

Periodic metallo-dielectric structure in diamond

Intense ultrashort light pulses induce three dimensional localized phase transformation of diamond. Photoinduced amorphous structures have electrical conducting properties of a maximum of 64 S/m based on a localized transition from sp(3) to sp(2) in diamond. The laser parameters of fluence and scanning speed affect the resultant electrical conductivities due to recrystallization and multi-filamentation phenomena. We demonstrate that the laser-processed diamond with the periodic cylinder arrays have the characteristic transmission properties in terahertz region, which are good agreement with theoretical calculations. The fabricated periodic structures act as metallo-dielectric photonic crystal.