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.Ultrashort pulsed lasers such as femtosecond (fs) lasers are considered powerful tools for inducing nonlinear optical effects or microscopic modifications inside transparent materials because of their high power density of more than 10 TW/cm 2 and their ultrashort pulse width. 1-6 Using a focusing fs laser, it is possible for one to induce the multiphoton reduction of Au 3+ or Ag + ions to their corresponding metal, the increase in refractive index, and the formation of void at the focal point inside the glass.However, a laser interference technique using fs laser pulses is applicable for the fabrication of periodic microstructures at a wide space of transparent materials because it produces a periodically modulated optical intensity with the size of the order of its wavelength. 7-10 Photonic crystals are materials in which the dielectric constant or refractive index is modulated periodically on a length scale comparable to the desired wavelength of the operation. 8,11 It is highly desired that periodic microstructures fabricated by a laser interference technique function as photonic crystals because the fabrication time can be shortened greatly. The interference of two beams produced by splitting a single laser beam can create one-dimensional (1D) periodic structures at the surface of transparent bulk materials. 7 Two-dimensional (2D) and 3D periodic structures can be fabricated using three and fourbeam interference, respectively. 8-10 Recently, we also reported the fabrication of periodic microstructures in azodyedoped polymers by multibeam interference of fs laser pulses in which a refractive index contrast (∆n) of about 5.5 × 10 -4 was induced inside the polymer.9,10 The value is large enough to induce the Bragg diffraction, but a larger ∆n value is needed in order for the periodic structures to function as photonic crystals. One method of achieving large ∆n values for photonic crystals is to make periodically aligned void structures in transparent materials. However, it is difficult to form a periodical void array using the laser interference technique in transparent materials such as glass.In this paper, we report a novel method for fabricating periodic nanosized voids inside a glass sample using a focused fs laser beam. In the conventional method, a focused fs laser beam produces a single void at the focal point. 4 Therefore, the fabrication of a periodic void array is achieved by precisely translating the glass sample.12 Our method does not require such a procedure. Surprisingly, periodically aligned voi...

We explain the occurrence of ion exchange and an index profile around the focal point inside a commercial crown glass formed by femtosecond laser irradiation. The index profile in the photoinduced area has a ring-shaped pattern, which indicates that local densification occurred in the glass. An irregular surface reflecting the density distribution is formed around the focal point by dry etching process using a focused ion beam. By the irradiation of femtosecond laser pulses, the effect of ion exchange between the focal point and the surrounding area is also observed in the area in which local densification occurred.Micromachining using femtosecond laser pulses has attracted considerable attention because it is extremely useful for the fabrication of waveguides, 1,2 couplers, 3 and microvoids 4,5 and for the precipitation of gold nanoparticles 6,7 inside transparent materials. When femtosecond laser pulses are focused inside glass or polymer via an objective lens, a photoinduced reaction occurs only near the focal point due to multiphoton absorption, resulting in a change in refractive index or the formation of microvoids at the focal point. In addition, when high-intensity laser pulses are focused inside specific transparent materials, filamentation is caused by nonlinear pulse propagation due to a dynamic competition between self-focusing and defocusing of electron plasma. 8 In a previous paper, we have reported the spontaneous formation of a periodical array composed of nano-sized spherical voids in commercial borosilicate glass, under femtosecond laser irradiation at a low repetition rate of 1 kHz. 9 This method can also be used to form dense dislocations with a cross-shaped pattern in a specific area in MgO single crystal. 10 These formations are considered to be caused by shock waves near the focal point that is exposed to femtosecond laser pulses.Femtosecond laser irradiation at a high repetition rate of over ϳ200 kHz tends to produce not only shock waves but also causes a heat accumulation effect around the focal point. 11 Recently, it has been demonstrated that the latter is highly effective in writing a waveguide with a low optical attenuation 12 and precipitating a ␤-BaB 2 O 4 ͑Barium borate͒ crystal inside the glass. 13 If heat transfer, also known as thermomigration, occurs inside the glass due to laser irradiation, the constituent elements of the glass will either disperse or agglomerate in a specific area because of the formation of a thermal gradient in a small area. Then, not only migration but also local distortion may occur. After the laser irradiation, the heated area in the glass is immediately quenched by rapid cooling. Investigating the relationship between ion behavior and the distortion is important for clarifying the reason for the change in refractive index or formation of voids using the laser irradiation technique; however, this relationship has not yet been fully understood.In this study, we analyzed the ion distribution in glass by using a dry etching process and an electron probe mi...

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.

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