Laser-induced thermal expansion

Second, the rapid laser-induced thermal expansion causes [103] a pressure rise, given by the product of the Griineisen coefficient, T, and the absorbed energy density... 

If the system under consideration is chemically inert, the laser excitation only induces heat, accompanied by density and pressure waves. The excitation can be in the visible spectral region, but infrared pumping is also possible. In the latter case, the times governing the delivery of heat to the liquid are those of vibrational population relaxation. They are very short, on the order of 1 ps this sort of excitation is thus impulsive. Contrary to a first impression, the physical reality is in fact quite subtle. The acoustic horizon, described in Section VC is at the center of the discussion [18, 19]. As laser-induced perturbations cannot propagate faster than sound, thermal expansion is delayed at short times. The physicochemical consequences of this delay are still entirely unknown. The liquids submitted to investigation are water and methanol. 

If effective absorbance and photothermal conversion efficiency are independent on the fluence, absorbed energy should be proportional to the latter and the slopes can be discussed simply in terms of thermal expansion coefficient of polymer Aims. The coefficient of linear expansion of rubber and glass states, ttg and Of, below and above glass-rubber transition temperature (Tg) of PMMA is (2.5-2.7) X 10- K and (5.6-5.S) X W K , respectively 3S). We consider that the one-dimensional expansion is induced along the perpendicular direction to the film, since polymer film outside the area irradiated by the excimer laser is of course hard. Then, the volume expansion could be replaced by the linear one along the thickness, so that corresponds to the change in the slope below... 

Another method is photothermal excitation. In the method, a cantilever is coated with an Au thin film. A laser beam with a power modulated at /o is irradiated to the cantilever. The modulated laser beam causes thermal expansion of the Si (or SisN4) cantilever and the Au film. Owing to the difference in the thermal expansion coefficient between the two materials, the laser beam generates a stress at the cantilever-film interface, which induces the cantilever vibration S3mchronized with the laser power modulation. In this case, the cantilever is directly driven by the laser beam so that the amplitude and phase curves are free from the influence of the spurious resonances. 

A laser-induced change in the temperature of an isotropic liquid crystal can modify its refractive index in two ways, very much as in the nematic phase. One is the change in density dp due to thermal expansion. This is the thermal absorptive component discussed before [Eq. (9.18) for p ] this term may be written as (0n/0p) p. The other is the so-called internal temperature change dT which modifies the spectral dependence of the molecular absorption-emission process we may express this contribn-tion as (0n/07)p dT. A pnie density change effect arises from the electrostrictive component p, which contribntes a change in the refractive index by (0u/0p) p . 

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