Pulsed laser induced mode-mismatched crossed-beam thermal lens measurements

Abstract

In this article, an accurate Fresnel diffraction model for a pulsed laser induced thermal lens effect in mode-mismatched crossed-beam configuration is presented. The effect of a lateral offset between the two beams is included in the model. This model is used to calculate the diffraction field of the probe beam at the detection plane from the very near- to the far-field, therefore the evolution of the intensity profile of the probe beam from the near- to the far-field is clearly illustrated. Numerical calculations predict and experimental results demonstrate that the thermal lens (TL) signal maximum is obtained when the probe beam propagates about a characteristic distance away from the sample position, while the characteristic distance is determined by the radius of the excitation beam in the sample and the wavelength of the probe beam. At large size mismatch of two beams and near-field detection cases, the TL signal is approximately independent on the sample location relative to the waist of the probe beam. The theoretical and experimental results are of great importance to configuration optimization of an extremely sensitive and compact TL instrument, which is the trend in the development of analytical instruments.