Recently developed lasers with short output pulses in the femtosecond range may be used in many different applications, including three-dimensional binary data storage, photonic crystals, optical waveguides, waveguide splitters, and waveguide amplifiers.1–3 The advantage of femtosecond lasers is that all their energy is crammed into about ∼100fs. As such, they have very high peak intensity, more than 1014Wcm−2. Current investigations using these laser pulses have led to significant progress in physics, chemistry, and laser micro-machining. Moreover, they have provided new insight into the interactions of many kinds of material with light.
Photonic crystals are periodic optical nanostructures that affect the propagation of electromagnetic waves. Their fabrication requires the precise formation of voids with a periodic arrangement inside transparent materials. In the conventional method, a focused femtosecond laser beam produces a single void at the focal point. A periodic void array is fabricated by precisely translating the sample many times, but this process is time-consuming.
To the formation of periodic void arrays, we have applied the spontaneous filament formation that accompanies intense femtosecond-pulse propagation in transparent media.4 The filament phenomenon results from the dynamic competition between the self-focusing and defocusing of electron plasma. At powers near the critical value for self-focusing, the balance between the two can give rise to a long-range filament path. With a single pulse, this process can create multiple voids with a micrometer period along the filament line in the direction of laser beam propagation. Furthermore, structural parameters of the void array, including the number of voids, their period, and the length of the aligned structure, can be controlled by adjusting the laser irradiation conditions.
Figure 1. (a) An optical microscopy photograph showing a cross section of borosilicate glass 0.9mm thick after irradiation with a 1kHz femtosecond laser beam with a pulse energy of 10μJ for 1/4s. The 250 pulses were focused at a depth of 750μm from the entrance surface. (b) A scanning electron microscopy photograph of the periodic void structure.
In our experiments, we used commercially available glasses of 12mm × 6mm × 0.9mm in size and a regeneratively amplified 800nm Ti:sapphire laser that emits 120fs, 1kHz, mode-locked pulses. The glass sample was placed on an XYZ stage that was controlled by a personal computer. Then a 5mm-diameter laser beam, in Gaussian mode, was focused through a 100× objective lens with a numerical aperture of 0.9 into the interior of the glass sample. Figure 1(a) shows a side view optical microscopy photograph of the inside of a glass sample exposed to the femtosecond beam at a pulse energy of 10μJ. Below the focal point is an aligned void structure along the propagation direction of the femtosecond laser beam. The voids are nearly spherical, and neighboring voids are independent of each other. No micro cracks or catastrophic collapses occurred around the voids or the focal point. As depicted in Figure 1(a), a void with a diameter of 1.6μm formed at the focal point. The distance between this void and the next one is 7.3μm. Both the diameter of the voids and the interval between neighboring voids decreases gradually as the filament approaches the bottom surface of the glass sample. A periodic section occurs at a distance of ∼90μm from the bottom surface. The void size and intervoid separation in the periodic section are 380nm and 1.7μm, respectively.
This method of void formation is advantageous because periodic multiple voids form spontaneously with a period of micrometer length along the propagation direction of the laser beam without translating the focal point. Other patterns can also be formed inside a variety of single crystals using this method. The ability to easily fabricate such controllable periodic void structures guarantees this technique's applicability in optoelectronics areas such as 3D photonic crystals.
International Innovation Center
Shingo Kanehira is an assistant professor in the International Innovation Center at Kyoto University. His research focuses on nano- and micro-structure control as well as fabrication of optical devices in transparent materials using femtosecond lasers.
1. E. N. Glezer, M. Milosavljevic, L. Huang, R. J. Finlay, T.-H. Her, J. P. Callan, E. Mazur, Three-dimensional optical storage inside transparent materials, Opt. Lett. 21, pp. 2023-2025, 1996.