Structural Relaxation and De-Relaxation Phenomena in Amorphous Ge Films upon Irradiation with Short and Ultrashort Laser
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Time (ns) Figure 1: Time evolution of the reflectivity of as-deposited a-Ge films upon irradiation with 10 ps laser pulses. The pulse fluence is indicated in each transient and t=0 corresponds to the temporal position of the pump pulse. melt duration. The experimental setup used consists of a synchronously pumped dye laser (Rhodamine 6G) whose output is amplified by a pulsed dye amplifier (Kitton Red-620). The beam (elliptical) is focused at the sample site to a size ranging between 600 /m and 1 mm. The fluences at the maximum of the intensity distribution are in the 0-200 mJ/cm 2 range and are determined within 10%. The evolution of the reflectivity of the irradiated surface is measured in real time with a resolution of about 1 ns by means of a HeNe probe laser (633 nm) focused, at nearly normal incidence, at the center of the irradiated region to a size of approximately 50 ptm. The ratio between the size of the irradiation and the probe beam ensures that the reflectivity is measured over an homogeneously irradiated region. RESULTS Figure 1 shows the time evolution of the reflectivity of the as-deposited a-Ge films upon irradiation with 10 ps laser pulses of several fluences. Each irradiated region receives only one pulse. The arrival of the irradiation pulse is followed by an abrupt reflectivity increase to a maximum value whose height depends on the pulse fluence. For fluences up to approximately 50 mJ/cm 2 , the maximum is followed by an abrupt decrease and by a more gradual evolution which is completed after approximately 200 ns (not shown in the plot). In this case, the reflectivity of the surface before and after irradiation remains nearly unchanged. For fluences above this value, the decrease after the maximum shows the presence of a shoulder in which the reflecivity remains nearly constant for a few ns. After the shoulder, the reflectivity decreases again but at a slower rate and the final reflectivity value observed is always clearly smaller than the initial one. Surface ablation occurs for fluences higher than approximately 80 mJ/cm 2 (dashed line transient). The observed time evolution of the reflectivity is related both to the dependence of the optical properties on temperature and to the presence of melting and rapid solidification. The optical reflectivity at 633 nm of the amorphous material at the melting temperature has been previously measured to be t 0.56 (13-14% higher than the room temperature value) [7]. Besides, it is well known that liquid Ge is metallic [11] and its bulk reflectivity is about
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50% higher than that of the solid material. Therefore any transient reflectivity increase above 0.56 can be interpreted in terms of surface melting. Figure 2a shows the maximum value of the reflectivity observed in the transients as a function of the laser pulse fluence. The maximum transient reflectivity increases sharply to reach in a short fluence interval a nearly constant value. For fluences above q 20 mJ/cm 2 the maximum transient reflectivity is above 0.56, ev
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