Thermal Lens Effect

Alfheim JA, Langford CH. 1985. Detennination of formaldehyde with the thermal lens effect. Anal Chem 57 861-864. 

Thermal lens effects occur in the air above the sample surface and within any layer of the sample that is not optically... 

Figure 2. Schematic depiction of physical processes affecting the laser probe beam for an opaque homogeneous sample including thermoelastic deformation of the air-sample interface and thermal lens effects in the air above the sample.

For air at 1 atm and O C, e = 1.1 x 10 / C, and thus the ratio of thermal lens effect to surface deflection effect for Si is 0.6 and for Al is 0.05. However, nonlinear effects due to higher temperatures (to be discussed later) will tend to reduce these ratios. Figure 3 presents comparisons with experiments for a complete calculation (that includes nonlinearities) under vacuum, where there is no thermal lens effect (dashed curves), and in air, where there is a thermal lens effect (solid curves). The agreement between theory and experiment is excellent. Note that the somewhat stronger dependence on frequency predicted for the thermal lens effect is observed experimentally in that its contribution to the total measurement decreases with increasing frequency. 

FIGURE 45.5 Schematic illustration of the principle of the thermal lens effect in TLM. (Reprinted from Tokeshi, M., et al., Electrophoresis, 24, 3583, 2003. With permission.)... 

In case of small detector areas fluctuations of the local position of the laser spot on the detector will cause fluctuations of the measured output signal. These beam instabilities can be caused by thermal effects of the laser resonator which might result in a small tilt of the resonator mirrors, or by inhomogeneous heating of the active laser medium, which leads to thermal lens effects and affects the stability of the laser beam. 

A model has been proposed to describe the thermal lens effect in which the signal intensity is expressed by... 

The thermal lens effect has been used for LC detection with a small-volume sensor cell. Basically, it consists of a heating laser, the light from which is passed directly through the sample, and another laser which passes light through the cell in the opposite direction. When an absorbing solute arrives in the cell, a thermal lens is produced that... 

Similar to the thermal lens effect is exploitation of the fact that a narrow probe laser beam exhibits deflection as a consequence of a refractive index gradient again, the amount of deflection can be measured using a position-sensitive photodetector. 

The dashed lines show the beam divergence due to the thermal lens effect... 

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