When femtosecond laser pulses are focused inside a single crystal, anisotropic structural changes such as dislocation and
cleavage occur along specific orientations. It can be interpreted that the anisotropic structural changes should be induced
by transient stress after photoexcitation, such as a thermal stress and stress wave. To elucidate the mechanism of the laser
induced structural changes inside crystals, we developed a novel time-resolved polarization imaging system, in which
circularly polarized laser pulse was used as a probe light. The system enabled us to observe laser-induced transient stress
distribution as well as the orientation after focusing fs laser pulses inside MgO and LiF single crystals. Based on the
observation, we elucidated the relation between laser-induced transient stress distribution and anisotropic structural change
inside the crystals.
Localized phase separation was induced inside a glass, the composition of which is not immiscible, by femtosecond
laser-induced compositional modification. The glass composition was changed locally from a miscible composition to an
immiscible one with high-repetition femtosecond laser irradiation. The phase separation was confirmed by analyzing the
composition of the irradiated area with confocal Raman spectroscopy and by observing the co-continuous structure due
to phase separation with scanning electron microscopy. The compositional change seems to be related to
thermomigration, which is the migration of atoms or ions by the temperature gradient, because the sharp temperature
gradient is caused with a high-repetition femtosecond laser. With this method, we can obtain nanoscale co-continuous
structure, which would have high surface area, on a glass surface. Moreover, we can control the morphology of the
structure by heat treatment while avoiding phase separation in the entire glass because the composition of the non-modified
region is not immiscible.
Femtosecond laser has been widely used in a light source for materials processing when high accuracy and small
structure size are required. When a transparent material e.g. glass is irradiated by a tightly focused femtosecond laser, the
photo-induced reaction is expected to occur only near the focused part of the laser beam inside the glass due to the
multiphoton processes based on the ultrashort interaction time and the ultrahigh light intensity. We proposed a research
idea of "induced structure" which means spatially modified micro- and nanostructures in a transparent material by the
femtosecond laser irradiation. In this paper, we review our recent investigations on the three-dimensional nanostructure
self-organization composed of oxygen deficiencies inside fused silica, the space-selective silicon structures formation in
silicate glass based on thermite reaction triggered by femtosecond laser pulses, and diffusion of elements constituting
glass based on thermal accumulation by high repetition rate femtosecond laser pulses. We also discuss the mechanisms
and possible applications of the observed phenomena.
We show a novel method of fabricating a periodic nanovoid structure inside commercial borosilicate glass using
femtosecond laser irradiation. The aligned voids are formed spontaneously with a period of micrometer length along the
propagation direction of the fs laser beam. In addition, structure parameters of the void array, i.e., the period between
voids, the number of voids, and the entire length of the aligned structure, can be controlled by adjusting the laser
irradiation conditions. In addition, cross-shaped pattern due to local dislocations can be formed spontaneously inside
MgO single crystal using fs laser irradiation. The ability to easily fabricate such controllable periodic void or crossshaped
pattern guarantees applicability in optoelectronics areas, such as 3D photonic crystals and polarizer.
We have fabricated silicon structure in silicate glass prepared with metallic aluminum in the starting material, using femtosecond laser irradiation and subsequent annealing. Small Si-rich structures such as oxygen-deficiency (O-deficiency) defects or Si clusters transform into nano-sized Si particles by the focusing irradiation of the laser. Then the Si-rich structures grow into micro-size particles due to the thermite reaction promoted by heat treatment. We determine the effect of focused laser pulse on the Si deposition process by using a time-resolved transient lens method with a sub-picosecond laser pulse. Localized high-temperature, high-pressure, and the generation of shock waves appear to be very important in forming the Si-rich structures that ultimately grow into Si particles. The diffusion of oxygen by shock waves and the existence of Al-rich structures help form Si-rich structures as Si-O bonds continuously break under high temperature. The focusing irradiation of femtosecond lasers is very useful for fabricating Si structures inside glass.