PULSED-LASER INDUCED MODE-MISMATCHED CROSSED-BEAM THERMAL LENS MEASUREMENTS

In this article, an accurate Fresnel diffraction model for a pulsed la ser induced thermal lens effect in mode-mismatched crossed-beam config uration is presented. The effect of a lateral offset between the two b eams is included in the model. This model is used to calculate the dif fraction field of the probe beam at the detection plane from the very near- to the far-field, therefore the evolution of the intensity profi le of the probe beam from the near- to the far-field is clearly illust rated. Numerical calculations predict and experimental results demonst rate that the thermal lens (TL) signal maximum is obtained when the pr obe beam propagates about a characteristic distance away from the samp le position, while the characteristic distance is determined by the ra dius of the excitation beam in the sample and the wavelength of the pr obe beam. At large size mismatch of two beams and near-field detection cases, the TL signal is approximately independent on the sample locat ion relative to the waist of the probe beam. The theoretical and exper imental results are of great importance to configuration optimization of an extremely sensitive and compact TL instrument, which is the tren d in the development of analytical instruments.