Absorptiometry

 

Principle

Schematic structure to determine the absorption of USP lasers radiation from irradiated materials Copyright: Chair for Laser Technology LLT Schematic structure to determine the absorption of USP lasers radiation from irradiated materials

Due to the pulse durations less than 10 picoseconds, focused ultrashort laser pulses have extremely high intensities ranging from approximately 1013 to 1016 W/cm². If a sample is irradiated with such high intensities, the absorption of laser radiation from the material cannot be described with classical, linear models any longer. The electric field of the laser radiation is so strong, that the electronic potentials of the atoms are extremely distorted and further interaction and absorption processes become relevant. Due to this, a different absorption of short laser pulses (100 fs) compared to long laser pulses (10 ps) at a comparable pulse energy is expected. The absorption for non-transparent materials is determined by measurements of the reflection of the laser radiation during the irradiation of the surface within an integrating sphere using a photodiode.

 

Applications

Determined absorption of graphite for different pulse durations. Copyright: Chair for Laser Technology LLT Determined absorption of graphite for different pulse durations.

As part of a project funded by the German Research Foundation (DFG), the absorption of graphite upon irradiation with ultrashort laser pulses with pulse durations of 100 fs to about 4 ps was determined (see Manufacture of graphene with USP-laser radiation). Using the Fresnel formulas, an absorption of 71% is expected for a wavelength of 800 nm for graphite. This value is measured experimentally for pulse durations longer than 1 ps. For shorter pulses, the absorption reduces significantly to about 64% for 100 fs. One possible explanation for this is the stimulation of a significantly larger number of free electrons for the shortest pulse durations. Consequently, the electrons do reflect a greater amount of the incident pulse.

 

Publications

M. Reininghaus, 2014: Dynamics of ultrashort pulsed laser radiation induced non-thermal ablation of graphite