The existence of evanescent optical waves is usually demonstrated by the observation of transmitted light in frustrated total-internal-reflection experiments that make use of two closely spaced prisms. The main characteristic of a monochromatic evanescent plane wave is the exponential decay of its amplitude in the direction perpendicular to its surface of generation. This decay, however, is not what is seen in the usual experiments when the gap between the prisms is small. Only when the gap is sufficiently large does it gradually approach the exponential dependence. We use a different technique that uses a local probe to reveal the presence of an evanescent wave. The results come closer to the ideal of the exponential decay of the wave amplitude, and the presence of the evanescent wave can be seen directly, making it a suitable demonstration for pedagogical purposes.

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