Voltage-dependent coupling of light into ITO-covered waveguides

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Abstract
Indium tin oxide (ITO)-covered waveguides incorporating a grating and immersed in a fluid have been used to study the incoupling of light as a function of a voltage applied between the ITO and an electrode parallel to the waveguide and also immersed in the fluid. The optical set-up is typical for adsorption and binding studies of (bio)chemical material, but the voltage dependence is a new feature. Applied voltages, positive at the ITO surface, shifted the incoupling angle of linearly polarized laser light. Voltages below 1 V are sufficient; above, the effect saturates. dc and ac protocols have been applied. Weak dependence on solution composition and strong asymmetry with respect to the polarity of the applied potential imply that the ITO itself is involved in the effect. It is proposed to use it as a reference or compensation method for standard biosensor applications. It could considerably simplify them, avoiding high-precision goniometers or high-resolution position-sensitive devices for determining waveguide-effective refractive index shifts, and using a simple voltage sweep instead.

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