Boundary Condition Effect On Photoacoustic Response Of Absorbing Liquid To Modulation Of Laser Intensity

The  photoacoustic  (PA)  pressure  response  of  the absorbing  liquid  (water, ethanol)  with  free  and confined  surface  to  laser  pulse  with  modulated intensity is studied.  Modulation  amplitude  of  the PA  signal  demonstrates  non-monotonous behavior during laser pulse action which can be explained as interference  between thermo-acoustic  and vaporization  mechanisms  of  pressure  generation. The phase behavior of modulated part of PA signal also  gives  important  information  on processes  in the  irradiated  matter.  Doppler  photoacoustic monitoring  (DPM)  is used  to  determine  surface movement  during  laser  irradiation  (wavelength  – 2.94 um,  pulse  duration  ~  200-300  ns,  fluence  ~ 0.3-1.6  J/cm2).  In  the  case  of free  surface  this movement  is  possible  into  the  upward  and downward  directions while  for  the  constrained irradiated surface only one movement is supposed. This supposition  is  in  agreement  with  the experiment.  However  the  total  behavior  of PA response  is  more  complicated  due  to  probably vapor  cavity  formation between liquid  and  solid surfaces.  In  particular,  despite  the  imposed constraint PA response behavior resembles the free surface case

Explosive Boiling of Transparent Liquids on Absorbing Targets Heated by Short Laser Pulses

Explosive boiling induced by sub-nanosecond laser pulses  is  theoretically  analyzed in  the  case  of transparent liquids on metal targets. In this case the variation  of boiling  start  times  due  to inhomogeneous  distribution  of  laser  intensity through the  irradiation  spot  can  be  reduced  up  to laser pulse duration and registered explosive boiling pressure signals become less  distorted. The results are  compared with  experimental  investigations  of photoacoustic (PA) signals induced in metal target under  transparent  liquid  layer  irradiated  by  laser pulses of about 100 ps duration and wavelength 532 nm