3.2 Solids and Nanostructures: Electrons, Spins, and Phonons
Project coordinator(s): C. von Korff Schmising, M. Wörner

T2: Material modification with femtosecond laser pulses

Laser-induced periodic surface structures (LIPSS or ripples) are a universal phenomenon observed after irradiation of solids using linearly polarized laser pulses. The advantage of this technology, which makes it very interesting for many applications, is the ability to generate a nanostructured surface (on the sub 100 nm scale) in a single process step. For femtosecond irradiation, two different types of LIPSS have been reported: so-called low spatial frequency LIPSS (LSFL) with a periodicity (Λ) close to the irradiation wavelength λ (ΛLSFL∼λ) and high spatial frequency LIPSS (HSFL) with periods significantly smaller than λ (ΛLSFL<<λ) (Fig. 1).

Figure 1: Scanning electron micrograph of a crystalline SiO2 sample after irradiation with N=10 pulses at a peak fluence of F0 = 7.15 J/cm².

The formation of laser-induced periodic surface structures (LIPSS or ripples) upon irradiation of metals (Ti), semiconductors (Si), and dielectrics (SiO2) with multiple irradiation sequences, consisting of linearly polarized Ti:sapphire femtosecond single pulses is studied experimentally. Additionally multiple double-fs-irradiation sequences of laser pulse pairs (pulse duration 50 - 150 fs, 800 nm) according to the experimental scheme in Fig. 2 were generated to get deeper insights into the dynamics and the formation mechanisms of LIPSS. The temporal pulse delay between the individual cross-polarized fs-laser pulses can be varied from -40 to +40 ps with a temporal resolution better than 0.2 ps. The surface morphology of the irradiated surface areas is characterized by means of scanning electron and scanning force microscopy.

Figure 2: Michelson interferometer based experimental setup for the generation of double-fs-pulse irradiation sequences. BS: beam splitter, Δt: temporal delay, λ/4: quarter-wave plate.

Bulk microstructuring

Ultrashort pulses offer the unique possibility to induce permanent structural changes inside the bulk of transparent materials. Upon tight focusing (numerical aperture > 0.4), the energy deposition region is confined down to a micrometric volume, enabling precise bulk micromachining applications. Those applications include laser direct write of micro- photonic structures such as waveguides, couplers, splitters, bragg gratings, long term data storage, or lab-on-chip fabrication. The goal of our group is to study the fundamental mechanisms underlying the laser-matter interaction, such as nonlinear propagation, energy coupling and energy relaxation into the bulk of transparent materials. As an investigation tool, we developed a time-resolved phase contrast microscopy (PCM) technique [Rev. Sci. Instrum. 82, 033703 (2011)].

Figure 3: Time-resolved microscopy apparatus developed to investigate laser-induced transient refractive index changes up to 1 ns after laser excitation with a subpicosecond temporal resolution (a). For longer time delays, we employ an external Nd:YAG laser as an illumination source (b). The numerical aperture of the focusing objective is 0.45. Pump beam characteristics: τ=180 fs, λ=800 nm.

Time-resolved PCM delivers snapshots of the laser-induced refractive index changes with a temporal resolution of 300 fs and a submicrometer spatial resolution. In PCM, a positive refractive index change translates into a lower intensity on the camera sensor and conversely, a negative refractive index change translates into a higher intensity than the backgrouund. Our setup can also be employed as a time-resolved optical transmission microscopy (OTM) apparatus. The OTM mode provides insights into the laser-induced transient absorption. In amorphous fused silica, our experimental results show a nonuniform energy deposition into the focal region [see Fig. 4(a)], resulting into the onset of hot spots visible on the optical axis. Those hot spots can be explained by studying the nonlinear pulse propagation. At equilibrium, hots spots result in strongly scattering centers of lower refractive index than the pristine bulk. We also demonstrated that the energy relaxation lasts up to the microsecond timescale. Figure 4(b) shows that 10 microseconds after the laser excitation, a region of high refractive index persists. We associate this transient refractive index increase to a state of material compression subsequent to hoop stress release [Phys. Rev. B 77, 104205 (2008)]. This suggests that high repetition rate micromachining (>100 kHz) is more efficient, as structural defects (e.g. E' centers) generation is facilitated in densified materials.

Figure 4: Time-resolved imaging of bulk amorphous fused silica upon fs laser irradiation. (a) Laser-induced transient absorbance 400 fs after the laser excitation [taken from Proc. SPIE 8247, 82470Q (2012)]. The corresponding Abel transform of the absorbance map and the aspect of the permanent laser-induced phase object in PCM are also shown. (b) OTM and PCM snapshots taken several microseconds after laser exposure. Refractive index change [taken from Proc. SPIE 7925, 79250R (2011)]. The laser comes from the left side of the picture, the laser pulse energy is about 4 µJ.