Absorption coefficient of nearly transparent liquids measured using thermal lens spectrometry
We use an optimized pump-probe mode-mismatched thermal lens scheme to determine the optical absorption coefficient and thermal diffusivity of ethanol, benzene, acetone, methanol, toluene and chloroform. In this scheme the excitation beam is focused in the presence of a collimated probe beam. The a...
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Інститут фізики конденсованих систем НАН України
2006
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| Назва видання: | Condensed Matter Physics |
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| Цитувати: | Absorption coefficient of nearly transparent liquids measured using thermal lens spectrometry / H. Cabrera, A. Marcano, Y. Castellanos // Condensed Matter Physics. — 2006. — Т. 9, № 2(46). — С. 385–389. — Бібліогр.: 11 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| id |
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irk-123456789-1213282017-06-15T03:03:09Z Absorption coefficient of nearly transparent liquids measured using thermal lens spectrometry Cabrera, H. Marcano, A. Castellanos, Y. We use an optimized pump-probe mode-mismatched thermal lens scheme to determine the optical absorption coefficient and thermal diffusivity of ethanol, benzene, acetone, methanol, toluene and chloroform. In this scheme the excitation beam is focused in the presence of a collimated probe beam. The agreement between experimentally obtained results and values reported in the literature is good. 2006 Article Absorption coefficient of nearly transparent liquids measured using thermal lens spectrometry / H. Cabrera, A. Marcano, Y. Castellanos // Condensed Matter Physics. — 2006. — Т. 9, № 2(46). — С. 385–389. — Бібліогр.: 11 назв. — англ. 1607-324X PACS: 78.20.Nv, 78.20.Ci, 42.25.Bs, 42.62.Fi DOI:10.5488/CMP.9.2.385 http://dspace.nbuv.gov.ua/handle/123456789/121328 en Condensed Matter Physics Інститут фізики конденсованих систем НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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English |
| description |
We use an optimized pump-probe mode-mismatched thermal lens scheme to determine the optical absorption
coefficient and thermal diffusivity of ethanol, benzene, acetone, methanol, toluene and chloroform. In this
scheme the excitation beam is focused in the presence of a collimated probe beam. The agreement between
experimentally obtained results and values reported in the literature is good. |
| format |
Article |
| author |
Cabrera, H. Marcano, A. Castellanos, Y. |
| spellingShingle |
Cabrera, H. Marcano, A. Castellanos, Y. Absorption coefficient of nearly transparent liquids measured using thermal lens spectrometry Condensed Matter Physics |
| author_facet |
Cabrera, H. Marcano, A. Castellanos, Y. |
| author_sort |
Cabrera, H. |
| title |
Absorption coefficient of nearly transparent liquids measured using thermal lens spectrometry |
| title_short |
Absorption coefficient of nearly transparent liquids measured using thermal lens spectrometry |
| title_full |
Absorption coefficient of nearly transparent liquids measured using thermal lens spectrometry |
| title_fullStr |
Absorption coefficient of nearly transparent liquids measured using thermal lens spectrometry |
| title_full_unstemmed |
Absorption coefficient of nearly transparent liquids measured using thermal lens spectrometry |
| title_sort |
absorption coefficient of nearly transparent liquids measured using thermal lens spectrometry |
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Інститут фізики конденсованих систем НАН України |
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2006 |
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http://dspace.nbuv.gov.ua/handle/123456789/121328 |
| citation_txt |
Absorption coefficient of nearly transparent liquids measured using thermal lens spectrometry / H. Cabrera, A. Marcano, Y. Castellanos // Condensed Matter Physics. — 2006. — Т. 9, № 2(46). — С. 385–389. — Бібліогр.: 11 назв. — англ. |
| series |
Condensed Matter Physics |
| work_keys_str_mv |
AT cabrerah absorptioncoefficientofnearlytransparentliquidsmeasuredusingthermallensspectrometry AT marcanoa absorptioncoefficientofnearlytransparentliquidsmeasuredusingthermallensspectrometry AT castellanosy absorptioncoefficientofnearlytransparentliquidsmeasuredusingthermallensspectrometry |
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2025-07-08T19:39:50Z |
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2025-07-08T19:39:50Z |
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1837108920036884480 |
| fulltext |
Condensed Matter Physics 2006, Vol. 9, No 2(46), pp. 385–389
Absorption coefficient of nearly transparent liquids
measured using thermal lens spectrometry
H.Cabrera1,2, A.Marcano 1, Y.Castellanos3
1 Centro de Fı́sica, Instituto Venezolano de Investigaciones Cientı́ficas,
Caracas 1020A, Apartado 21827, Venezuela
2 Instituto de Ciencia y Tecnologı́a de Materiales (ICTM),
U.H, CP 10400, Ciudad Habana, Cuba
3 Universidad Central de Venezuela, Apartado Postal 20513,
Caracas 1020 A, Venezuela
Received October 24, 2005, in final form April 17, 2006
We use an optimized pump-probe mode-mismatched thermal lens scheme to determine the optical absorption
coefficient and thermal diffusivity of ethanol, benzene, acetone, methanol, toluene and chloroform. In this
scheme the excitation beam is focused in the presence of a collimated probe beam. The agreement between
experimentally obtained results and values reported in the literature is good.
Key words: thermal lens, absorption coefficient, transparent liquids, thermal diffusivity
PACS: 78.20.Nv, 78.20.Ci, 42.25.Bs, 42.62.Fi
1. Introduction
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 10−3 −
10−4 cm−1. 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.
Marcano et al. have developed a simple calibrated TL method for measuring low absorption
values based on a pump-probe scheme with a collimated probe beam and a focused pump beam [7–
9]. The method not only optimizes the TL response but also allows a simple calibration and simple
data interpretation. In this work we use this method for obtaining the values of the absorption
coefficient and the thermal diffusivities of a series of nearly transparent organic solvents. Our results
are in good agreement with previously reported values.
c© H.Cabrera, A.Marcano , Y.Castellanos 385
H.Cabrera, A.Marcano, Y.Castellanos
2. Theoretical considerations
We consider that the pump beam and the probe beam are Gaussians with Rayleigh parameters
ze and zp, waist positions ae and ap and wavelength λe and λp for the pump and probe beam
respectively. The method supposes that the pump beam is focused and the probe beam is collimated
giving a beam spot constant and more than two orders of magnitude larger that the pump beam
spot. Due to the presence of the TL generated by the pump beam the spatial distribution of the
probe beam changes. The signal can be recorded by measuring the transmission of the probe light
power passing through a small aperture located at the far field. In this regard the signal is defined as
S(z, t) =
[T (z, t) − T0]
T0
, (2.1)
where T0 is the probe light transmittance through the aperture after passing the sample cell with
no pump beam present and T (z, t) is the probe beam transmittance in the presence of the pump
beam, z is the position of the sample cell and t is the time. When scanning the sample around
the excitation beam focal point, a single peak (Z-scan) is obtained if zp � ze. In the stationary
situation (t → ∞) we obtain a simple equation for determination of the TL phase amplitude Φ0 [9]
Smax = πΦ0/2, (2.2)
Φ0 =
α l P0(dn/dT )
κλp
, (2.3)
where Smax is the maximal value of the TL signal which is obtained for position z = 0, (excitation
beam waist position), P0 is the impinging pump light power, α is the absorption coefficient, dn/dT
is the thermal gradient of the refraction coefficient,κ is the thermal conductivity and l is the sample
length.
The described equation (2.2) provides a simple relation between the maximum value of the
signal and TL phase amplitude, which value is used for absorption determination if the rest of the
photo-thermal parameters are known.
Shen et al. have pointed out that it takes longer time to carry out steady-state measurements
when the mode mismatched between pump and probe beams is bigger [11]. We report a similar
effect for the proposed configuration with collimated probe beam and focused excitation beam
[9]. From the practical point of view this time is of the order of several seconds (7 to 10). This
time-span can be easily achieved in a real experimental situation using appropriate mechanical
shutter.
The thermal diffusivity is D = κ/ρCν , where ρ is the sample density and Cν is its heat capacity.
The thermal diffusivity can be determined by fitting the time dependence of the TL signal using
the theoretical model previously described in [7,8].
3. Experimental method
The experimental set-up is similar to the ones described previously [7–9]. The excitation beam is
generated by a 500-mW CW green diode pumped Nd–YAG laser (λe = 533 nm, Intelite Inc.)). The
probe light is coming from a 4-mW CW He-Ne laser (632 nm, PMS Electrooptics). The resulting
probe power at the sample position is 0.5 mW. This light passes through 1-cm and 30-cm focal
length lenses resulting in a near collimated probe beam of 6-millimeter diameter. The excitation
beam is directed collinearly with the probe using a dichroic beam splitter. This light is focused
onto the sample (1-cm path-length) using a converging lens with 40-cm focal length. Behind the
sample the excitation beam is canceled by using interference filters and a dichroic mirror. The
probe beam is directed toward a 100-microns aperture and a diode detector (Melles Griot) located
at a distance of 80 cm measured from the position of the excitation beam waist. The signal from
the detector is then pre-amplified using a current amplifier (SR570 Stanford Research Systems)
and then directed toward a digital oscilloscope (TDS3052 Tektronix) for recording.
386
Thermal lens spectrometry
To determine the absolute value of the TL signal the use of a shutter (Sh) is required. The
shutter (846 Newport Corporation) is put on for a period of 10 seconds, after which the signal
reaches a maximum. Then, it is turned off for another ten seconds for allowing the system to relax.
The cycle is repeated 30 times and the averages are computed. The time evolution of the signal
is used to determine the sample thermal diffusivity. Using this method we determine the absolute
value of the TL signal according to the definition given by equation (2.1). We use different samples
such as: ethanol, benzene, acetone, methanol, toluene, and chloroform. The liquids are contained
in a quartz cell of 1-cm path-length.
4. Results and discussion
In figure 1a we show the TL signal for ethanol as a function of time as measured for an excitation
power of 46 mW. Superimposed we show the theoretical fitting estimated using the model described
(a)
(b) (c)
Figure 1. (a) Experimental TL signal for ethanol calculated using equation (2.1) (z = 0); (b)
Z-scan curve for ethanol; (c) Excitation power(mW) versus maximum of TL signal.
in [7], a diffusivity coefficient of 0.89× 10−3 cm2/s and a phase amplitude value Φ0 = −0.20. The
rest of the parameters for this fitting are λe=533 nm, λp=632 nm, zp = 10000 cm, ze = 0.25 cm,
387
H.Cabrera, A.Marcano, Y.Castellanos
z = 0, L = 80 cm, ae = 0, ap = 0, and t = 10 s. The value of the thermal diffusivity of ethanol
agrees well with previous reports [10].
We also perform Z-scan measurements for the value of the excitation beam Rayleigh parameter
obtained. The results are plotted in figure 1b. The solid line corresponds to the theoretical fitting
performed using the same parameters as in figure 1a. We have measured the TL signal at the
maximum for different values of the excitation power. The result is shown in figure 1c. The result
is a linear dependence as corresponds to a one-photon absorption process. From the slope of this
dependence and from equations (2.2) and (2.3) we obtain for the absorption coefficient of the
ethanol
α =
0.0026
10−3
W−1
κλp
l(dn/dT )
. (4.1)
Using the values of the photo-thermal parameters for ethanol dn/dT = −4 × 10−4 ◦C−1 and
κ = D/Cν = 1.67 × 10−3 W cm−1◦C−1, we obtain for the absorption coefficient α=(6.8 ± 0.5) ×
10−4 cm−1.
Performing the same procedure we determine the absorption coefficient and thermal diffusivity
of the rest of the liquids, the results are shown in table 1.
Table 1. Measured optical absorption coefficient (α) and thermal diffusivity (D) of several liq-
uids.
Parameters Ethanol Benzene Acetone Methanol Toluene Chloroform
α × 104 cm−1 6.8 ± 0.5 4.0 ± 0.4 1.89 ± 0.10 5.9 ± 0.5 2.8 ± 0.2 1.52 ± 0.10
D×103(sec−1cm2) 0.89 ± 0.01 0.90 ± 0.01 1.09 ± 0.01 1.04 ± 0.01 0.92 ± 0.01 0.81 ± 0.01
5. Conclusions
This work demonstrates the usefulness of the optimized method for measurement of optical
and thermal properties of nearly transparent liquids. Results obtained agree quite well with those
reported in the literature. We have measured the absorptivity of six organic liquids at 532-nm.
The absorptivities do not exceed 10−4 cm−1. The accuracy of this experiment (5.3 to 10%) is
only limited by the fluctuations of the excitation light power. We have also measured the thermal
diffusivity of these liquids with high precision.
6. Acknowledgements
The authors would like to thank Fondo Nacional para Ciencia y la Tecnoloǵıa, Fonacit, Caracas,
Venezuela (Grant G97000593), the Latin American Center of Physics (CLAF) and the ICTP.
388
Thermal lens spectrometry
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PACS: 78.20.Nv, 78.20.Ci, 42.25.Bs, 42.62.Fi
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