Thermal accumulation effect in three-dimensional recording by picosecond pulses

Abstract

In-bulk recording without cracking in borosilicate glass by high repetition rate (80 MHz) 12 ps pulses at 355 nm wavelength is demonstrated and discussed. The theoretical model of a "hot-line" scan and thermal accumulation qualitatively well describes the experimental results. The analytical expression of a thermally induced stress was obtained. Recent advances in microfabrication, in which ultrafast lasers with pulse durations of less than 1 ps are implemented, show the potential ability to record sub-µm structures inside transparent materials. 1,2 The nonlinear mechanism of absorption allows the photomodification to be localized within the volume 2 or on the surface 3 with a cross section smaller than the wavelength, . The diffraction limit of the focusing optics can be overcome. However, for some three-dimensional (3D) microstructuring tasks, a high repetition rate (over 200 kHz) is required to achieve partial softening/melting of glass. 4 Obviously, longer pulses (from picoseconds to nanoseconds) could be considered for this type of recording, however, they usually cause crack formation in glass. Recently, it was demonstrated that brittle materials can also demonstrate a ductile response before onset of crack formation at a specific crossover length of order 10 nm in glass. 5 Control of the local temperature and its distribution inside transparent materials at the focus allows one to tune a thermal stress for a desired application, e.g., a 3D dicing (a controlled cleaving) would require a controlled crack propagation, while the optical memory and waveguide recording would benefit from prevention of the crack formation by material softening/ melting.The aim of this work was to investigate the effect of thermal accumulation on 3D laser recording by 12 ps duration, 355 nm wavelength pulses at a high (80 MHz) repetition rate inside glass. The experimental results are compared with those of theoretical modeling.A Vanguard (Spectra Physics) laser was used for 3D recording inside borosilicate slide glass (Corning 2947). Recording was carried out by beam scanning with galvano mirrors at a constant speed of 5-10 cm/s using -lens focusing, which fixed the focal depth along the line of recording inside the 1-mm-thick samples (aimed at the center). The pulse duration was 12 ps, the repetition rate 80 MHz, the maximum pulse energy 50 nJ, and the wavelength 355 nm. In-bulk laser fabrication is possible at this wavelength because the main absorption mechanism is a nonlinear two-photon absorption (TPA). The TPA coefficient of slide glass ␤ = 25± 7 cm/ GW was directly measured at 355 nm using by the transmission method applicable when linear absorption is negligible. 6 The edge of glass absorption was at approximately 320 nm wavelength where the decadic optical density ͑OD͒ Ͼ 0.1 for a 1-mm-thick glass; while the linear absorption coefficient at 355 nm was only ␣ 0 = 0.47 cm −1 .For recording, we used focusing defined by f-number ͑f # ͒ = 10, at which a cost-efficient diffraction-limited performance of lens can be achieved. ...

Femtosecond versus picosecond laser machining of nano-gratings and micro-channels in silica glass

“…Despite picosecond lasers are rarely employed for in-bulk processing of wide-bandgap dielectrics, there is a growing body of evidence that this intermediate regime of material modification brings advantages to some specific applications. Juodkazis and Misawa highlighted how thermal induced stress in borosilicate glass irradiated by 12 ps pulses could be exploited for crack-free in-bulk recording [5]. Additionally, investigation of optical waveguides fabricated by laser direct writing with a wide 50-fs to 5-ps range of pulse duration showed the lowest loss of 0.2 dB/cm for 1 ps pulses [6,7].…”

Three-Dimensional Micro- and Nano-Structuring of Materials by Tightly Focused Laser Radiation

“…The photo-structuring of materials carried out in our studies was made using a basic DLW setup which with some modifications has been presented in our earlier work. [21][22][23][24][25][26][27][28][29][30][31][32][33][34] Its optical layout is shown in Figure 2. The same setup can be used for spectroscopic characterization of the processes within small focal volumes whose cross-sections are smaller than several micrometers.…”

Ion migration assisted inscription of high refractive index contrast waveguides by femtosecond laser pulses in phosphate glass

“…For phosphate glasses, thermal diffusivities are in the range of 1-6 × 10 −3 cm 2 ∕s. As a consequence, in our experimental conditions, the effective cooling time, tc [13], is much larger than the time between laser pulses (2 μs). Heat-accumulation effects [14] are thus considerable.…”