Observation of laser-induced stress waves and mechanism of structural changes inside rock-salt crystals.

The structural changes inside rock-salt crystals after femtosecond (fs) laser irradiation are investigated using a microscopic pump-probe technique and an elastic simulation. The pump-probe imaging shows that a squircle-shaped stress wave is generated after the fs laser irradiation as a result of the relaxation of thermal stress in the photoexcited region. Pump-probe crossed-Nicols imaging and elastic simulation elucidate that shear stresses and tensile stresses are concentrated in specific regions during the propagation of the stress wave. The shear stresses and tensile stresses observed in this study can explain the characteristic laser-induced structural changes inside rock-salt crystals.

Formation mechanism of element distribution in glass under femtosecond laser irradiation.

We report on the formation mechanism of element distribution in glass under high-repetition-rate femtosecond laser irradiation. We simultaneously focused two beams of femtosecond laser pulses inside a glass and confirmed the formation of characteristically shaped element distributions. The results of the numerical simulation in which we considered concentration- and temperature-gradient-driven diffusions were in excellent qualitative agreement with the experimental results, indicating that the main driving force is the sharp temperature gradient. Since the composition of a glass affects its refractive index, absorption, and luminescence property, the results in this study provide a framework to fabricate a functional optical device such as optical circuits with a high-repetition-rate femtosecond laser.

Fabrication and characterization of silicon antireflection structures for infrared rays using a femtosecond laser.

We focus on IR sensors with lower reflection for the wavelength around 10 µm, strongly awaited for detecting human bodies. A concave structure was designed as a more suitable reflection-free structure for IR light, and an optical system with a femtosecond laser was employed for verification of the effectiveness of the structure. The microstructures prepared through this process were fabricated and optically measured using SEM, FT-IR, and Raman spectroscopy. The measurement revealed that good reflection-free structures were realized for IR sensors with lower reflection for the wavelength of around 10 µm.

Silicon precipitation via photoinduced reaction using femtosecond laser.

Silicon precipitation inside a glass is an important technique for silicon photonics. We successfully precipitated silicon inside silicate glasses containing an Al metal film using femtosecond laser irradiation. First, the Al-inserted sandwiched glass was fabricated by the direct bonding method. The results of a tensile test indicated that the adhesive strength of the sandwich structure reached approximately 4 MPa. Next, femtosecond laser pulses were focused at the Al/glass interface in the sandwich structure. A transmission electron microscopy photograph at the focus of the laser showed that the Al particles were dispersed into the glass substrate to a depth of approximately 2 microm from the initial Al layer. In addition, Raman spectra indicated that silicon had formed at the interface between the glass and Al film after the laser irradiation. The morphology or the particle size of the precipitated silicon was successfully modified by changing the repetition rate or the pulse energy of the laser.

Composition dependence of photoinduced second-order nonlinearity of germanosilicate glasses.

The dependence of the photoinduced second-order nonlinearity of germanosilicate glasses on the GeO(2)-SiO(2) composition was investigated using photoinduced second-harmonic generation (SHG). The photoinduced SHG was encoded by coherent superposition of the 800-nm fundamental and the 400-nm second-harmonic light of a femtosecond laser. Experimental results showed that the photoinduced second-order optical nonlinearity increased with increasing concentrations of GeO(2) in the glasses, and the concentration dependence was mainly due to the dependence of the photoinduced internal dc electric field of the glass on the GeO(2)-SiO(2) composition.

Periodic nanovoid structures via femtosecond laser irradiation.

We have observed periodically aligned nanovoid structures inside a conventional borosilicate glass induced by a single femtosecond (fs) laser beam for the first time, to our knowledge. The spherical voids of nanosized diameter were aligned spontaneously with a period along the propagation direction of the laser beam. The period, the number of voids, and the whole length of the aligned void structure were controlled by changing the laser power, the pulse number, and the position of the focal point.