K. Kato, A. Takatsu, N. Matsuda, and Y. Sugitani
[Chem. Lett., Vol.1997, pp583-584, 1997]

A new detection technique of photothermal signal is demonstrated. In this technique photothermal (PT) signal is generated on the slab optical waveguide (SOWG). Photothermal deflection of probe laser beam transmitted in the core layer of the SOWG was detected as a PT signal. Dependence of signal intensity and phase change on distance between the excitation and probe beam positions was measured.
The optical waveguide (single-mode at 532nm) was prepared by the ion exchange of a slide glass by dipping it in fused potassium nitrate. As for the sample specimen 10 muL rhodamine-6G solution (100 mumol/dm³) was dropped and dried on the waveguide surface. Ar laser beam chopped at 120 Hz was led through an optical fiber and was irradiated downward upon the optical waveguide as an excitation beam where the probe laser (He-Ne) beam was propagating. The deflection of the probe beam was detected using a slit and a photo-diode.
When the position of excitation beam was swept across the probe beam, two large peaks are found in the signal intensity. The phase signal change shows the opposite phase for the two peaks. The phase difference for the two peaks is exactly 180 degrees, and the turning position is exactly the instant when the excitation beam is near the center of the probe beam. These effects show that the modulation of probe beam is caused by the photothermal beam deflection effect. The proportional relationship between the intensity of the beam deflection of probe laser and the power of the irradiating beam is seen, as is considered to be supporting that the ordinary photothermal phenomena is observed. Background signals without the sample and without the excitation beam were 3.8 muV and 3.5 muV. The standard deviations were 0.03 muV and 0.09 muV, respectively. About 0.4 pmol of rhodamine-6G is calculated to be in the spot of the excitation beam. The detection limit was about 10fmol (S/B = 2 Conclusively, the photothermal effects have been successfully observed in the optical waveguide and this method presented here using SOWG can detect ultra-low amount of substance on the waveguide surface.