J. Phys. IV France 125 (2005) 229-232 Ó EDP Sciences, Les Ulis DOI: 10.1051/jp4:2005125054
High-sensitivity absorption coefficients measurements using thermal lens spectrometry C. Jacinto* and T. Catunda Instituto de Física de São Carlos, Universidade de São Paulo - USP, CEP 13560 - 970, São Carlos, SP, Brazil Abstract. We report on measurements of absorption coefficients (Ae) with high-sensitivity (as small as 10-8 cm-1) in liquid and solid samples, using a simple pump-probe thermal lens experiment. Thermooptical properties required for Ae determination, such as thermal conductivity K, thermal diffusivity D, and temperature coefficients of refractive index dn/dT and optical path length ds/dT, were also obtained. We measure an absorption coefficient at 457 nm of (5.3 ± 0.4)x10-3 cm-1 and at 514 nm of (2.50 ± 0.05)x10-4 cm-1 for undoped silica glass and Ethylene Glycol, respectively.
1. INTRODUCTION Measurement of absorption coefficient with high sensitivity is important by several reasons, for instance, for the identification of the constituents of a given sample. At low absorption, the optical transmission decreases exponentially with the absorption path length following Beer’s law. Based on this law, optical techniques can measure changes in the transmission of about 10-3% [1], where to increase the sensitivity in the absorption measurement, multipass cells have been used [2]. The transmission technique is not the only method for the absorption determination. The thermal lens spectrometry (TLS), first reported by Gordon et al [3], has been developed as alternative way for the characterization of the absorption in optical samples. Snook and Lowe have presented a review on TLS and its applications to chemical measurements in solid, liquid and gaseous phases [4]. Recently, Marcano et al. have shown sensitivity to determine absorbance as low as 10-9 in liquid and solid samples using the TL Z-scan technique, which is an experiment very sensitive to the setup, since the two beams need be very well overlaped [5,6]. For Ae determination using TLS, essential parameters, such as temperature coefficient of refractive index (dn/dT) or optical path length (ds/dT) and thermal conductivity K [7], need be known. In this work, using mode-mismatched pump-probe TLS and one prism refractometer, we measured the absorption coefficients (Ae) with high-sensitivity in Ethylene Glycol and undoped silica sample. We also determined their thermo-optical properties, such as thermal conductivity K, thermal diffusivity D, and polarizability temperature coefficient f. 2. THEORETICAL MODEL AND EXPERIMENT The TL technique is based on the induced TL effect in a partially transparent medium, when an excitation laser beam passes through the sample and the absorbed energy converted into heat changes the optical path length (s) for solid sample and refractive index (n) for liquid one. The propagation of a probe beam through the TL results in either a spreading (ds/dT, dn/dT < 0) or a focusing (ds/dT, *
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Article published by EDP Sciences and available at http://www.edpsciences.org/jp4 or http://dx.doi.org/10.1051/jp4:2005125054
JOURNAL DE PHYSIQUE IV
230
dn/dT > 0) of the beam. The TL effect can be treated through the calculation of the temporal evolution of the sample temperature profile DT(r,t), caused by a gaussian intensity distribution of the excitation beam. The transient signal amplitude is proportional to its phase shift, q, which normalized by the excitation power, P, is given by q = (Ae Leff P
æ 1 ds ö ÷ ÷ è Kl p dT ø
)çç
(1)
where Ae is the absorption coefficient at the excitation wavelength, Leff = [1- exp(-AeL)]/Ae, L the sample length (for AeL
