The evanescent field

We usually think of light in terms of either travelling waves or standing waves. There is however another way that light can behave. If light impinges on the interface between two dielectric media under an angle bigger than the critical angle as predicted by Snell's law, all of the light will be reflected and thus no light will be transmitted through the interface. This is the so-called total internal reflection. However, it turns out that light does extend across the interface in the form of an exponentially decaying electromagnetic field. This is referred to as the evanescent field.

The first experiment that proved the existence of an evanescent field was performed by Newton. To study the effect of total internal reflection, he brought a lens very close to the surface and noticed that light propagated out of the lens. Somehow, the light 'escaped' from the prism into the lens, because the lens was brought into the evanescent field of the prism.

Near-field microscopy

This effect proves that it is possible to study the evanescent field by immersing a probe into the field. In near-field microscopy, instead of using a lens, a sub-wavelength probe is used. Usually these probes are fabricated by heating an optical fiber and pulling it until a sharp tip is formed. It is common practice to evaporate a metal coating onto the tip. One can then cut the tip to obtain the desired aperture.

In order for the probe to detect enough light, one needs to be able to keep the tip extremely close to the surface (<50 nm). This is achieved using a piezo actuator and a clever feedback loop. Two more piezo actuators are used to scan the tip over the surface, to obtain 2 dimensional images of the electromagnetic field on the surface.

Our setup

The difference between our setup and other near-field microscopes, is that we do the near-field measurements in a phase-sensitive way. As sketched in the figure, the light is split into two branches. In one branch, the light passes through the sample and is picked up by the tip. In the other branch, the light passes through two acousto-optical modulators (AOM's) and a delay line. The two AOM's are used to frequency shift the reference branch by 40 kilohertz. The two branches are subsequently mixed on a photo diode and the resulting detector signal is sent through a lock-in amplifier, using the 40kHz difference frequency as a reference. This allows us to obtain the full amplitude and phase information of the light captured by the tip. In a CW measurement, we can use this information to reconstruct the dispersion of light in the surface. In a pulsed measurement, we can use it to monitor the evolution of the pulses as they propagate through the surface.