Infrared (IR) radiometry and time resolved reflectivity (TRR) methods can be used for investigation of laser
pulse effects on materials in nanosecond time scale. The methods in combination are capable to quantify
object temperature and detect phase transformations in the solid state, melting and plasma formation from
vapour. Measurements with different laser pulse energy densities provide threshold of the transformation. The
melt duration can be also determined. The experimental system is described. It contains KrF excimer laser
with homogenizer and variable attenuator, fast IR detector for radiometry, continuous probing laser with Si
photodiode for reflectivity measurement and UV detector for pump laser pulse reflection measurement. The
system was applied to investigation of responses to laser light of silicon and different pure metals and alloys. The
range of energy densities used was 1-5500 mJ.cm-2 and measurements were done with temporal resolution of 6
ns for radiometry and 1 ns for reflectivity.
A high number of papers were published on the simulation of laser/surface interaction at the level of nanosecond scale.
Several assumptions on thermal properties data, laser spot homogeneity, were assumed for describing as well as possible
the boundary conditions, the mathematical writing and finally the numerical or the analytical results. A few tentative of
surface temperature monitoring during laser processing were proposed for the numerical validation. Also, simulation of
the melting kinetics is rarely directly compared to in situ experiments. It is very hard to determine the time duration of a
melting pool by in situ experiments. It should be the same for the surface temperature.
A new method to plot the thermal history of the surface by using a combination of the Time Resolved Reflectivity (TRR)
and the Pulsed Photo-Thermal (PPT) or Infrared Radiometry (IR) methods is proposed in this paper. Surface
temperature, melting kinetics, threshold of melting and threshold of plasma formation are determined in the case of KrF
laser spot in interaction with several materials. In the first step, the experimental setup including fast detectors (IR, UV,
Vis.) and related optical devices is described. In the second step, typical results (TRR and IR spectra) for monocrystaline
silicon are presented and discussed. Namely, phase change transitions (melting and resolidification) are detected versus
fluence change and number of laser shots change. TRR and IR spectra of metallic surfaces (Cu, Mo, Ni, Stainless steel
15330 and 17246, Sn, Ti), are measured. For each sample the surface temperature during heating, the threshold of
melting, melting duration and the threshold of plasma formation are directly deduced.
Phase transitions induced by laser irradiation in CdTe wafers have been studied in situ and experimental and computer simulation methods. The samples were irradiated by ruby laser with pulse duration 80 ns in energy density range from 0.02 to 0.5 J/cm2. Time-resolved reflectivity (TRR) measurements were carried out at the wavelengths of λ1=1.064 μm and λ2=0.532 μm. Dynamics of transmissivity was studied at λ1. Photoluminescence of CdTe, excited by the ruby laser single pulse, was also investigated. The character of TRR transients changes with the increase of irradiation energy density. The changes are more considerable at λ1 than at λ2. The time dependencies of reflectivity are explained by the changes of optical parameters of CdTe in course of laser-induced melting, solidification and evaporation. The experimental data obtained from transmissivity and photoluminescence measurements correlate with those from TRR transients. Laser-induced melting, crystallization and evaporation processes were studied on the basis of the computational solution of a two-phase moving boundary problem with two moving interfaces. The calculated dependency of melt duration on energy density is in a reasonable agreement with experimental data. From our investigation it follows that in the molten state CdTe is characterized by constant or weakly changing reflectivity in the temperature range from Tm to 3000 K.