Temporal-contrast imperfections as drivers for ultrafast laser modifications in bulk silicon

Despite recent successes in three-dimensional ultrafast laser writing inside silicon and other narrow gap materials, strong discrepancies remain in the identified narrow experimental windows to exceed material breakdown thresholds. In an experiment in which we irradiate silicon with perfectly synchronized femtosecond, picosecond, and nanosecond pulses, we show that the temporal contrast of the ultrashort pulses is a critical driving parameter, even more than the achievable peak vacuum intensity. We identify by appropriate pulse combinations breakdown channels seeded by pre-ionization and thermal runaway from the simulated contrast imperfections. The introduction of this multi-timescale control of the interactions allows bulk writing inaccessible otherwise. It also permits a comprehensive review of previous reports using infrared laser technologies at different characteristic contrast performances. The quantitative analysis of the problem reveals an extremely high sensitivity with modification ignition at intensity contrasts down to ${10}^{-6}$ and below. This sensitivity level is common for high-intensity physics experiments but has no equivalent in any other laser material processing application.

Figure 1

Comparison of modification attempts in bulk Si using subpicosecond laser pulses. The empty symbols indicate unsuccessful attempts while solid symbols show conditions for which permanent modification is reported. (a) Outcome of the tested conditions as function of NA and pulse energy. The symbol colors refers to the different type of laser technologies. The colored regions (see also arrows) show the strong difference between the inappropriate parameter regions depending on technologies. (b) Same data but sorted by technologies. Symbol colors refers to the applied pulse energy level and the results are displayed as function of NA and expected temporal contrast depending on technologies. The introduction of contrast solves the apparent contradictions in (a). Data are extracted from: our previous works ($\circ$) [4, 5, 17, 18], and Kononenko et al. ($△$) [3] using optical parametric amplification (OPA), from Shimotsuma et al. ($\square$) [7] using a Cr:F amplified laser (RA), and from Nolte et al. ($▽$) [8, 10] and Pavlov et al. ($\diamond$) [9] using Er:doped fiber lasers.

Figure 2

Simplified schematic of the experimental arrangement used for multi-time scale control of infrared pulses. An amplified Ytterbium femtosecond laser is used as master source. The beam pumps an optical parametric amplifier to generate pulses of about 190 fs at 1550-nm wavelength. A beam splitter is used to create a separated picosecond laser beam with a grating-based stretcher arrangement. A motorized delay line (stage 1) is implemented for synchronization of the femtosecond and picosecond pulses. In the stretcher, one grating and retro-reflector can be moved (Staged 2) to continuously change the pulse duration from 4 to 21 ps. A synchronization signal from the femtosecond laser is used to trigger an erbium-based fiber laser emitting 1550-nm pulses of 5-ns duration at delay that can be adjusted electronically. The combination of all pulses using thin-film optics create a new beam directed toward the bulk-Si interaction setup (not shown).

Figure 3

Autocorrelation (AC) measurements for pulses prepared with different contrasts (a). The black solid line shows experimental results and the red solid line is a theoretical curve assuming the combination of two Gaussian pulses with durations of 190 fs and 11 ps, a perfect synchronization (zero delay) and the intensity ratio mentioned on each graph. (b) Autocorrelation measurement results extracted from Ref. [22] for the fiber system used in several studies reporting bulk modification in Si [8, 10, 13]. The data are compared to the same fitting procedure identifying a 800-fs main pulse and a 4-ps pulse pedestal.

Figure 4

Bulk Si modifications obtained after irradiation with 190-fs pulses and picosecond prepulse contrasts of different energies on separated sites. The duration of the prepulse degrading the contrast is 10 ps. The impacts are made 300-$\mu \mathrm{m}$ below the surface and observed with infrared microscopy. The blue circles indicates irradiations without modifications and crosses are cases in the matrix that have not been tested because of laser energy limitations. We note the incapacity of achieving modification with high-contrast femtosecond pulses and a contrast-dependent modification threshold given by the transition between the last circle and the first modification on each line. Scale bar is 50 $\mu \mathrm{m}$.

Figure 5

Measured energy threshold for ultrafast laser induced modification in bulk silicon as function of prepulse contrast. The contrast is given by the peak intensity ratio of a 11-ps prepulse and the main 190-fs delayed by 20 ps. In the upper part of the figure, blue and grey bars represent the applied energy with the picosecond and femtosecond pulse respectively. The relative contribution of the femtosecond pulse leading to modification, obtained by subtracting the threshold for the pure picosecond case shown in red to the measured threshold, is shown with linear scale in the lower part.

Figure 6

Measured energy threshold for ultrafast laser induced modification in bulk silicon as function of prepulse contrast. The contrast is given by the peak intensity ratio of a main 190-fs pulse and 11-ps postpulse delayed by 20 ps. The blue and grey bars represent the applied energy with the picosecond and femtosecond pulse respectively. The measured threshold for pure picosecond irradiation is shown in red. The red crosses represent no modification with the maximum tested energy.

Figure 7

Measured energy threshold for ultrafast laser induced modification in bulk silicon as function of pedestal contrast (yellow) and comparisons with satellite pre- and postpulse cases in blue and black respectively. The contrast dependence evidences a general dominating postpulse contribution in energy deposition. The red crosses represent no modification with the maximum tested energy.

Figure 8

Energy thresholds for ultrafast laser-induced modification in bulk silicon as function of nanosecond contrast. The contrast is given by the peak intensity ratio of a main 190-ps pulse and a synchronized 5-ns pulse. As reference the red line correspond to threshold for modification in the ‘pure’ nanosecond case. Comparison of the measurement(marked by blue triangle) to predictions(marked by black dashed line) assuming the absence of contribution from the femtosecond pulse indicates nanosecond background driven modification and negligible cross-action between components independently from the delay as shown by the inserted graph.