High reflector absorptance measurements by the surface thermal lensing technique Page: 4 of 10
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(Fig. la). In the PTD set-up, the probe beam size is much smaller than that of the lateral dimension of the deformed area.
Thus after reflection from the deformed surface, the probe beam experiences a change in direction which is proportional to the
slope of the surface deformation. For the STL set-up, in contrast to PTD, a probe beam with a size similar to or larger than
the lateral dimension of the thermally deformed area is reflected from the sample surface (Fig. lb). The deformed area on the
sample surface acts as a lens which diffracts the probe beam. The shape of the surface deformation is thus recorded in the
diffraction pattern of the reflected probe beam, which can be analyzed by using either a CCD camera or a scanning
photodetector.
pump
probe laser probe pump
laser laser laser
,-- --
a s a m p le bs m l
Figure 1 Comparison of a conventional photothermal deflection technique (PTD) and the surface thermal lensing (STL)
method. The PTD requires two small diameter laser beams aligned close to each other. The probe beam diameter must be
smaller than the lateral dimension of the thermally-induced deformation. The STL technique uses a probe beam diameter that
is greater than the lateral dimension of the deformation.
STL offers two advantages over the conventional PTD configuration. STL has the same high sensitivity but avoids the
critical alignment requirements of PTD. STL is easier to align because the probe beam is larger than the pump beam. The
probe beam must overlap the pump beam, a diffraction-induced signal is obtained, and the signal may then be optimized. In
the present work, both the pump and probe beam are in the visible region and observable to the naked eye. Furthermore, if a
CCD camera is used for detecting the diffraction pattern, STL obtains the full field information of the surface deformation.
This can be an important advantage over PTD, where the probe beam samples only a small spot of the deformed area. Less
time is required to map the shape of the deformation.
The experimental setup for the absorptance measurements is illustrated in Figure 2. An Ar ion laser at 514.5 nm is used as
the pump source. The beam is split so that the power can be measured, modulated at 12 Hz with a mechanical chopper wheel
synchronized to a lock-in amplifier, and focused onto the sample at near normal angle of incidence with a beam diameter about
100 pm. The probe laser beam is from a 10 mW He-Ne laser, at the wavelength of 632.8 nm. It is focused onto the sample
surface, coincident with the pump laser beam. For this particular set-up, the He-Ne laser beam diameter is focused to a
diameter of about 500 pm to achieve optimum sensitivity of this set-up. The detector is located 60 cm from the sample. To
detect the STL signal, a pinhole is placed in front of the photodetector. In this way the detector monitors intensity changes at
the center of the probe beam, which is proportional to the optical absorptance of the sample. To convert the PTD signal into
absorptance data, the traditional calibration method for PTD is used, where the calibration coefficient is achieved by measuring
the PTD signal of a calibration thin film sample with known absorptance [2,3,7].
The coatings for the absorptance measurements are high reflectors designed to reflect wavelengths of 511 and 578 nm. All the
coatings are made by a reactive e-beam process and deposited onto superpolished fused silica substrates. The substrates are
76.2 mm in diameter x 15 mm thick. The coatings are made by different manufacturers, and so have various material
combinations and layer thicknesses.
The absorptance were measured by the STL technique described above and by a radiometric technique using a custom-built
copper vapor laser (CVL) as the pump source [8]. The CVL is a high-average-power (200 watts), 4.4 kHz repetition rate, 50
ns pulse-width system [9]. The CVL beam is filtered to 511 nm light and focused approximately into a 2 mm diameter spot
at 10 degrees angle of incidence onto the coating. An Inframetrics (model 525) infrared camera senses the temperature change
with respect to the rest of the surface. The temperature change is converted into an absorptance value using an empirically
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Chow, R.; Taylor, J. R.; Wu, Z. L.; Krupka, R. & Yang, T. High reflector absorptance measurements by the surface thermal lensing technique, article, November 1, 1996; California. (https://digital.library.unt.edu/ark:/67531/metadc687686/m1/4/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.