Periodic Nanovoid Structures via Femtosecond Laser Irradiation

Abstract

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...

Formation and applications of periodic structures in transparent materials induced by single fs laser beam

“…However, periodic nanovoid array was observed in transparent materials along the propagation direction of single fs laser beam. [1] By adopting a physical model which combines the nonlinear propagation of fs laser pulses with the spherical aberration (SA) effect at the interface of two mediums of different refractive indices, reasonable agreements between the simulation results and the experimental results were obtained. By comparing the fluence distributions of the case with both SA and nonlinear effects included and the case with only consideration of SA, we suggest that SA, which results from the refractive index mismatch between air and fused silica glass, is the main reason for the formation of the self-organized void array.…”

Femtosecond Laser Induced Nanowire Technique and Its Applications

“…Their periodicity is about half of the wavelength of the laser beam and is explained by the interference effect [5][6][7][8].…”

Self-organized voids revisited: Experimental verification of the formation mechanism

“…As an example, self-organized void array, first reported by S. Kanehira, [14] are expected to be used as photonic crystals, [15] diffraction devices [6] and optical storage units, [7] but the structure paramenter (e.g. void-void spacing, void diameter uniformity, and total length) of the void array still cannot be accurately changed because of the lack of generally-accepted formation mechanism.…”