Thermal Lens Spectroscopy
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Thermal Lens Spectroscopy and Thermal Lens Effect
Figure 1 Schematic illustration of
thermal lens effect
The coaxial beams of the excitation and the probe beam are focused into the liquid sample (Fig.1).
The excitation beam is absorbed by the molecules in the sample, and the molecules undergo a transmission to excited states. The excited-state molecules subsequently decay back to ground state causing localized temperature increases to occur in the sample.
Since the refractive index of the medium depends on the temperature, the ensuing spatial variation of refractive index produces an effect which appears equivalent to the formation of a lens with in the medium. For most liquids, the temperature coefficient of refractive index is negative and consequently, the insertion of a liquid in the laser beam produces a concave lens. This effect which the refractive index of the material is modified in such a way that the medium behave as a lens by temperature increase of the material is called "Thermal Lens Effect".
Since the size of thermal lens linearly depends on the amount of the absorbed molecules, quantitative analysis can be carried out by measuring probe beam divergency to which a concave thermal lens effects. In a practical measurement, the excitation beam was modulated by a mechanical chopper, and a signal from probe beam monitored by photodiode was synchronously detected by lock-in amplifier.
Development of Thermal Lens Microscope
Figure 2 Schematic illustration of
Thermal Lens Microscope
Dr. T. Kitamori (Professor, School of Engineering, The University of Tokyo) introduced a microscope to thermal lens spectroscopy setup (Fig.2), consequently excitation and probe beam are coaxially concentrated to about 2 μm beam spot by the objective lens. Spatial resolution is also improved to about 2 μm. Furthermore, we can easily investigate where focused beam is irradiated through ocular during measurement. This setup is called "Thermal Len Microscope".
The thermal lens microscope is highly suitable for ultratrace quantitative analysis in small volume samples such as microchip analysis, especially for non-fluorescent samples.
Downsizing to the portable
The thermal lens microscope is promising measurement method not only for analytical chemistry but for biological, material and environmental fields. But thermal les microscope setup is relatively large as compared with a micro chemical chip. This means that even if microchip should be small, total analysis setup may become large.
We have developed a portable size instrument of thermal lens spectroscopic analysis by making use of our fiber optical communication technology.
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