Propagation dynamics of the solid–liquid interface in Ge upon ns and fs laser irradiation

Abstract

Monitoring the laser-induced melting and solidification dynamics of Ge upon laser irradiation is an enormous challenge due to the short penetration depth of its liquid phase. In this work, real-time pump-probe experiments in combination with finite element calculations have been employed to investigate the melting and solidification dynamics of germanium upon ns and fs laser pulse irradiation (λ=800nm). Excellent agreement between experiments and simulations allowed us to indirectly determine additional time- and depth-dependent information about the transformation dynamics of germanium, including the thickness evolution of the molten layer, as well as its melting and solidification velocities for the two pulse durations for different fluences. Our results reveal considerable differences in the maximum thickness of the molten Ge superficial layers at sub-ablative fluences for ns and fs pulses, respectively. Maximum melt-in velocities of 39m/s were obtained for ns pulses at high fluences, compared to non-thermal melting of a thin layer within 300fs for fs pulses already at moderate fluences. Maximum solidification velocities were found to be 16m/s for ns pulses, and up to 55m/s for fs pulses. Weak signs of amorphization were observed for fs excitation, suggesting that the lower limit of solidification velocities for a complete amorphization is above 55m/s. In addition, we show high precision measurements of the melt-in velocities over the first 20 nm by means of fs microscopy with sub-ps temporal resolution. Here, differences of the melt-in process of several orders of magnitude were observed, ranging from virtually instantaneous melting within less than 2 ps even for a moderate peak fluence up to 200 ps for fluences close to the melting threshold.

Real‐Time 3D Visualization of the Formation of Micrograting Structures Upon Direct Laser Interference Patterning of Ge

“…We base this conclusion on the fact that real time reflectivity measurements are in general very sensitive to the presence of ablation, strongly distorting RTR transients. [ 24 ] Moreover, ablation is generally also accompanied by a significant decrease of the final reflectivity level, something which we did not observe in the fluence range studied in this work.…”

“…This strong increase in reflectivity can be attributed to the formation of the molten phase of Ge. [ 24 ] As can be seen by comparing Figure 4a,b, the relative reflectivity increases with laser fluence, and the time of resolidification (indicated by vertical dashed lines) is delayed. It is worth stressing that this time is not reached when the reflectivity reaches the initial level ( R(t) = 1), but when reaching the local minimum that corresponds to the reflectivity of the solid phase at the melting temperature.…”

“…In order to determine these values, we have modeled the melting and solidification dynamics in Ge upon ns pulse irradiation via finite element method (FEM), described in detail for the case of excitation employing ns pulses at λ = 800 nm and large Gaussian spots. [ 24 ] The adaptation of the model to the present case of sinusoidal excitation provided by DLIP can be found in Section S2 (Supporting Information), together with the results h m (t) and ΔT(t) for different fluence values.…”

On the Melting Thresholds of Semiconductors under Nanosecond Pulse Laser Irradiation

“…One of the basic parameters for laser material processing that involves surface patterning, modification, crystallization and ablation, is the melting (damage) threshold. This key parameter is usually determined in the experimental studies of laser material processing as a reference point for controlling the laser modification process and for testing theoretical models developed with the aim of better understanding the fundamental processes at laser-matter interaction [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. For detection of the phase change, several approaches are used, such as time-resolved reflectivity (TRR) [9,11,[19][20][21][22], acoustic [23] and electrical conductivity [24] measurements and pump-probe microscopy [25], or combined techniques as in [13], where time-of-flight velocity distributions together with the evaporation rate and reflectivity were measured and analyzed.…”

Diffraction-based approach for real-time monitoring of nanosecond direct laser interference patterning structure formation on stainless steel

“…This can enable a better understanding of the dynamics involved in the process as well as permitting a better control of the topographies that can be produced. The performed experimental work has been so far dedicated to semiconductors (Si and Ge) which were processed with a two-beam DLIP setup with 20 ns pulses 30,31 and 8 ns 32 . In addition, different simulation models and ex-situ measurement have been performed for metals, in order to estimate for instance the amount of molten and vaporized material as well as estimating temperature distributions 33 .…”