Abstract
We present time-resolved studies of glass densification created by an acoustic phenomenon: collision of the transverse and longitudinal sound waves inside glass. Localization of the permanent denisfied region has a lateral cross section ∼ 0.4 μm and is approximately half of the wavelength of femtosecond laser pulses which were used to generate breakdown and launched shock waves inside glass. Controlled time delay between two closely spaced irradiation spots reveals dynamics and relaxation (electronic, thermal, stress) of glass after excitation. The observed phenomenon is important for femtosecond direct laser writing and recording of waveguide couplers using multiple beams.
Highlights
Acoustic and thermal phenomena occurring on the scales from sub-micrometer to tens of micrometers is an active research field spanning nano-/micro-mechanics, fluidics, light trapping [1], thermal transport and heat management [2]
Modifications of materials by acoustic and shock waves have been reported on surfaces, where tensile stresses generated by shock reflection caused spallation [6]
Optical modification of materials by fs-laser pulses is already a well established field of optical memory and waveguide engineering [12,13,14,15,16,17,18], while, acoustical and thermal effects triggered by fs-laser pulses are active field of fundamental research [19,20,21,22,23,24] important to practical applications in laser micro-fabrication, scribing and surface nano-structuring [25, 26]
Summary
Acoustic and thermal phenomena occurring on the scales from sub-micrometer to tens of micrometers is an active research field spanning nano-/micro-mechanics, fluidics, light trapping [1], thermal transport and heat management [2]. For thermal and acoustic research on micro- and nano-scale, ultra-short laser pulses are useful tools to generate instantaneous heating and build up thermoelastic stresses applicable in cell surgery and generation of coherent phonons in solid state materials [10,11]. We have demonstrated development of an interferometric pump-probe timeresolved imaging technique capable to characterize modifications in glass induced by fs-laser pulses [28, 29] in both lateral and axial cross sections at the conditions of dielectric breakdown with sub-wavelength resolution. Pump-probe techniques are required to resolve ultra-fast phenomena related to free carrier generation and self-trapping [30, 31], lattice instability in crystalls [27] which lead to formation of amorphous regions, stress, and defects [17, 18, 32, 33]. Temporal and spatial analysis reveals that such modifications can strongly alter direct laser writing of waveguide couplers when laser spots are scanned in close proximity
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