International Forum for Green Asia 2020

Measuring Multiphoton Absorption Spectrawith Thermal Lens Detection under A Crossed-Beam Configuration
Aira Kamito, Miki Isoda, and Akira Harata
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University
Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan.

Abstract
An experimental setup of excitation wavelength-tunable thermal lens detection under a crossedbeam configuration is constructed to measure multiphoton absorption spectra of molecules in aliquid solution. Dependence of the thermal lens signal intensity on light intensity for an aqueous
solution of auramine O was nonlinear and showed one- and two-photon absorption. The thermallens spectrum of the solution at 0.2 mJ/pulse of light intensity was measured for 410-600 nm.
1. Introduction
Molecules can absorb some photons
simultaneously to be excited to a high energy
state when the photon density becomes large.
Even the visible and infrared light can excite
relatively small molecules that haveabsorption wavelength in the UV region. The
process is known as multiphoton
absorption[1] and is frequently applied to twophoton excitation of fluorescence.
Thermal lens (TL) spectroscopy[2] isapplied to analyze various materials using achange in a refractive index by thermal
expansion after non-radiative relaxation ofphotoexcitation. Highly sensitive detection of
non-fluorescent molecules in a liquid
solution is one of the excellent applications of
the method. Multiphoton excitation spectraof non- or less-fluorescent molecules are notwidely investigated. Thus, an experimental
setup of excitation-wavelength tunable
thermal lens detection under a crossed-beamconfiguration is constructed to measure
multiphoton absorption spectra of molecules
in a liquid solution.

2.2. Experimental Setup
Excitation beam
OPO laser( λex= 210-750 nm)

HWP
Prism

Oscilloscope
Lock-in amplifier

PBS

Focusing lens(fex=100 mm)

Photo diodeBand-pass filter
Sample cell
(1 cm)

Focusing lens(fpr=120 mm)

Probe beam
He-Ne laser( λpr= 632 nm)

MirrorFigure 1. Experimental setup of the thermallens spectroscope under a crossed-beamconfiguration for measuring multiphoton
absorption spectra.
HWP, half-wavelength plate.
PBS, polarizing beam splitter.

Figure 1 is a schematic illustration of
the experimental setup of the thermal lens
spectroscope under a crossed-beam configuration. An output from a nanosecond-pulsed
optical parametric oscillator (OPO. NT23050-SH/SF-2H-3H, EXSPLA, repetition
frequency, 50 Hz; pulse width, 5 ns) was used
as an excitation beam, and the wavelength istunable from 210 to 2006 nm.
The excitation beam was sequentially
set a wavelength from 410 nm to 600 nm formeasuring absorption spectra with the TL
detection. The laser intensity was adjusted
using a half-wavelength plate and a

2. Experimental Methods
2.1. Materials
Aqueous solutions of auramine O
were selected as the sample. The solution
concentrations were adjusted to 1.0×10-6 M
for UV-Vis spectrophotometer and 1.0×10-4
M for TL spectrometer. The absorption peaks
were 370 nm and 423 nm in the visible region.
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International Forum for Green Asia 2020
polarizing beam splitter. One lens with a focal
length of fex=100 mm focused the excitation
beam. The beam changed a refractive index
to cause a TL effect. A He-Ne laser (LGK7654-08, LASOS) was used as a probe beamof 632 nm wavelength, focused with anotherlens with a focal length of fpr =120 nm. Theprobe beam and the excitation beam crossed
in a sample cell.

absorption signal was observed. However,measurement of the two-photon absorption
spectrum suffered from the intensity instabilityof the OPO laser depending on the wavelength
at the present stage.

3. Results and Discussion
Figure 2 shows the dependence of thethermal lens signal intensity on light intensity(LI) for an aqueous solution of Auramine Oat 432 nm wavelength. The graph shows anonlinear behavior and seems to be divided
into three regions. The low LI (0 - 0.2mJ/pulse) region was caused by one-photon
absorption, and the high LI (0.45 - 0.5mJ/pulse) region was caused by two-photon
absorption. The intermedial LI (0.2 - 0.45mJ/pulse) region was caused by both oneand two-photon absorption. The LI of 0.2mJ/pulse was selected to get a one-photon
absorption spectrum.

Figure 3. Absorption spectra of aqueous
solutions of auramine O. (Top) UV-Vis
spectrum; (Bottom) thermal lens spectrum.

4. Conclusion
An experimental setup of excitationwavelength tunable thermal lens detection
under a crossed-beam configuration isconstructed using a nanosecond-pulsed
optical parametric oscillator as the excitation
light source molecules in a liquid solution.
Nonlinear behavior of excitation-power
dependence of thermal lens signal intensity
for an aqueous solution auramine O wasobserved. The thermal lens spectrum of thesolution at 0.2 mJ/pulse of light intensity wasmeasured.

Figure 2. Dependence of the thermal lens signal
intensity on light intensity for an aqueous
solution of auramine O.
Figure 3 shows the two types ofabsorption spectrum. The top one is the UV-Vis
spectrum, and the bottom one is the TL
spectrum at the LI of 0.2 mJ/pulse. The two
spectra show a similar shape, and this TL
spectroscope obtained a one-photon absorption
spectrum was measured with the TL
spectroscope constructed, and a two-photon

References[1] Joseph W. Perry, “Two-photon absorption: Anoverview of measurements and principles”,
Adv. In optics and Photonics,2010[2] A.Marcano, OK.Williams N.Melikechi,
Optics communications, 2008, 2595, 281
kamito.aira.753@s.kyushu-u.ac.jp
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