HIGH-SENSITIVITY ABSORPTION MEASUREMENTS IN LIQUIDS ...

XXIX ENFMC - Annals of Optics 2006
HIGH-SENSITIVITY ABSORPTION MEASUREMENTS IN
LIQUIDS AND SOLIDS
R. A. Cruz, C. Jacinto, and T. Catunda.
Instituto de Física de São Carlos, Universidade de São Paulo, USP, C.P. 369, CEP 13560-
970, São Carlos, São Paulo, Brasil.
A. Marcano O.
Centro de Física, Instituto Venezolano de Investigaciones Científicas, Apartado 21827,
Caracas 1020 A, Venezuela.
renato@if.sc.usp.br
Abstract
We report on a mode-mismatched pump-probe thermal lens experiment performed to measure
low absorption. The use of a collimated probe beam in the presence of a focused excitation
beam optimizes the thermal lens experiment. The signal becomes independent on the Rayleigh
parameters and waists positions of the beams. We apply this method to study BK7 optical glass
and pure water. Linear optical absorption coefficient spectra in the range of 457-528 nm were
determined.
Introduction
Measurement of absorption coefficient with high sensitivity is important by several reasons, for instance, for
the identification of constituents such as impurity 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 , where to increase the sensitivity in
the absorption measurement multipass cells have been used. The transmission technique is not the only
method for the absorption determination. The thermal lens (TL) spectrometry [1,4] has been developed as
alternative way for the characterization of absorption in optical samples. In this work, we applied the modemismatched
pump-probe TL method, which optimizes the TL experiment [12], for determination of linear
optical absorption spectra in the range of 457-528 nm in water and BK7 glass.
The TL spectrometry is characterized by the incidence of a Gaussian beam in a medium. The energy of the
laser beam produces heating in the illuminated region, and as the intensity of the beam is bigger in its center,
it generates a temperature radial distribution ∆T(r, t). The refractive index is given by n(r) = n o +(dn/dT)·∆T(r),
whose profile follows the temperature one. Therefore, the Gaussian profile of the incident beam is transferred
to the refractive index. So, depending on the dn/dT signal the material behaves like a convergent (dn/dT > 0)
or divergent (dn/dT < 0) lens. The TL signal is defined as the normalized change of the transmission of the
probe field through an aperture of radius, r o , much smaller than the radius of the probe beam, centered at the
beam axis and located at a distance much larger than the pump Rayleigh parameter. Shen et al. have derived
an expression for the TL signal using a diffraction approximation for Gaussian beams, which is given by: [8]
I ( Z,
t)
= I
0
1
−θ
tan
−1
2m
⋅V
2 2 tc
2
[(1
+ 2m)
+ V ] + 1+
2m
+ V
2t
(1)
where
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XXIX ENFMC - Annals of Optics 2006
Pe
⋅α
⋅l
θ = −
Kλ
p
ds
dT
(2)
t
c
2
we
= (3)
4D
in which, λ p is the wavelength of the probe beam, D and K are the thermal diffusivity and conductivity,
respectively, L is the detector position, P e is the excitation beam power, α is the absorption coefficient, and l
is the sample thickness. The ds/dT parameter can be considered constant for the changes in temperature
typically observed in a TL experiment (< 0.01 K), and it is equal to dn/dT in liquid samples, m and V are
geometrical parameters.
Experimental Setup
The experimental setup is shown in Fig. 1. A HeNe cw laser operating at 632.8 nm and P e = 1 mW generates
the probe beam, which is collimated and directed to the sample before it is enlarged, and subsequently
collected onto a silicon photodetector (Thorlabs, DET 310). At the detector plane, the probe beam has a
diameter of 4 cm. In front of the detector, an aperture of radius 0.1 cm is placed centered on the axis of the
probe beam. The pump beam is provided by an Ar + cw laser (Coherent, Innova 90 Plus), which generates
different wavelengths between 457 and 528 nm. This beam is focused onto the sample with a 30 cm focal
length achromatic lens resulting in a Rayleigh parameter of ~1.7 cm. The collimated probe beam diameter is ∼
1 mm. Optical filters were used to block the pump beam in the photodetector. The samples are distilled and
deionized water, contained in a quartz cell 1 cm thick, and a BK7 optical glass plate 5.7 mm thick. To
determine the nonlinear optical phase shift, the TL signal is measured at the position where its value is
maximum (sample at position of the excitation beam waist). For negative (positive) photothermal coefficients,
the TL signal is negative (positive).
Osciloscópio
Amplificador
D
Ch
Laser de
Excitação
P
L 1
Laser de
Prova
B
S
A
F
M
L 2
FIG. 1. Mode-mismatched dualbeam
experimental setup. Ch is
the optical chopper, P the prism, L
the lens, B the beam-splitter, F
optical filters, L the defocusing
lens, D the photodetector, A the
current preamplifier, and O the
digital oscilloscope.
Results and Discussions
Figure 2 shows a typical TL signal from distilled water and BK7 glass plate. The solid lines are results of
theoretical fits obtained using Eq. (1), from where the phase shifts were achieved. The following parameters
were used: D = 1.44·10 -3 cm 2 /s, dn/dT = 99·10 -6 K -1 , and K = 6·10 -3 W/cmK for water; and D = 5.2·10 -3 cm 2 /s,
ds/dT = 6·10 -6 K -1 and K = 11.1·10 -3 W/cmK for BK7 glass [10,11]. From these phase shifts the linear
absorption coefficients were calculated. The result for water is in good agreement with previous
measurements based on integrating cavity methods [7]. The obtained spectra for water and BK7 glass are
presented in the Figure 4.
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XXIX ENFMC - Annals of Optics 2006
TL Signal (ua)
1.06
1.05
1.04
1.03
1.02
1.01
BK7 Optical Glass
λ e
= 514 nm
P e
= 1 W
TL Signal (ua)
1.000
0.995
0.990
0.985
0.980
Pure Water
λ e
= 514 nm
P e
= 1.3 W
1.00
0.975
0 1 2 3 4 5
t (s)
0 100 200 300 400 500
FIG. 2. Transient TL signal for (a) distilled water and (b) BK7 glass. The solid lines are fits with TL equation
from the experimental data.
t (ms)
6
5
BK7 Glass
This work
Schott catalogue (1995)
4
3
Pure Water
α (10 -3 cm -1 )
4
3
2
α (10 -4 cm -1 )
2
1
1
450 460 470 480 490 500 510 520
0
450 465 480 495 510 525
λ (nm)
λ (nm)
FIG. 4. Absorption spectra for (a) pure water and (b) BK7 glass, in the region of 457 - 514 nm.
Conclusions
A particular configuration of the TL technique, where the pump beam is focused in the presence of a
collimated probe beam, has been used for the measurement of low absorption coefficient of transparent
materials. Absorption spectra, in the range of 457 to 528 nm, were obtained for distilled water and BK7 glass.
References
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(1982).
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