Absorption coefficient of nearly transparent liquids measured using thermal lens spectrometry

Optical absorption in bulk materials is usually determined by measuring the beam transmission using a commercial spectrophotometer [1] with low precision and a calorimetric method [2] with higher precision. For measuring low absorption coefficients multi-pass cells [3] and cavity integration methods [4] have been also developed. The sensitivity of these methods is of the order of 10 − 10 cm. An alternative technique for measuring low absorption values is the thermal lens (TL) method. TL spectrometry is a photo-thermal technique which has been widely used for the determination of absorption of different materials with high sensitivity and versatility [5–10]. Since its discovery in 1964 the method has been used for the intracavity measurements of absorption of nearly transparent materials [5,6]. When a beam of light with a Gaussian intensity profile propagates in an absorbing medium, the heat generated, as a consequence of optical absorption, causes the sample temperature to rise. Because the refraction index depends on the temperature, a spatial distribution of the refraction index of similar extension is generated in the absorbing medium (TL stationary generation). The thermal diffusion expands this lens over a volume much larger than the volume defined by the pump beam within the sample. The TL induces phase shifts in the beam wave-fronts of the probe beam. In this two-beam experiment, the TL signal is usually defined as the relative probe transmission change through a small aperture located at the far field and on the optical beam axis.