Measurement of refractive-index change at a liquid-solid interface close to the critical angle

We measured the refractive-index change on a liquid sample, using the refle ction of a polarized Gaussian laser beam close to the angle of total re ref lection. We applied this technique to a solution of nickel (II) phthalocyan ine tetrasulfonated (NiPTS) in water-ethanol (1/1 v/v), in which the nonlin earity of the refractive index is due to optically induced thermal effects. We show that close to the angle of total reflection the sensitivity of thi s technique is four times bigger than at normal incidence. (C) 2000 Optical Society of America OCIS codes: 190.4350, 120.6810,350.6830.