In near-field scanning optical microscopy (NSOM), the very high resolution is determined by the diameter of an aperture, typically on the order of a few tens of nanometers. However, in the tapered optical fibers often used for NSOM applications, the single mode fiber ceases to function as a single mode waveguide when the tip diameter decreases below the nominal core diameter. This results in multimode performance, and much of the light is lost in the taper before reaching the probe aperture. The optical transmission T of a sub-wavelength, nanometer-scale aperture in even a flat metal film is extremely small when its diameter d is significantly smaller than l, the wavelength of the incident light, and is expected to follow T/f ~ (d/l)4, where f is the fractional area occupied by the aperture. In combination, these effects cause the transmission of tapered-tip NSOM probes to be very low--typically in the range of 10-5 to 10-4.
We have demonstrated a very large enhancement of the transmission of light through sub-wavelength apertures in metallic films when the metal surface surrounding the aperture is corrugated. The enhancement results from a resonant coupling of the incident radiation field to surface plasmon polaritons at the corrugated metal surface. The optimal corrugation geometry is a set of concentric circular grooves around the aperture; transmission as large as T/f ~ 3 has been observed, i.e. three times more light is transmitted than is directly incident on the aperture, despite the fact that for d << l the transmission through the aperture is evanescent. The wavelength of the resonance may be tuned by the design of the corrugation geometry, as well as the index of refraction of the adjacent medium.
For this reason, plasmon-enhanced devices are highly promising for use as NSOM probes. We report on the fabrication of plasmon-enhanced devices at the face of a tapered fiber tip, in which the fiber core is preserved. We show that the enhanced probes provide superior performance, exhibiting both extremely high transmission as well as high spatial resolution. Applications for these plasmon enhanced NSOM probes will be discussed.
Figure 1. Transmission spectrum of single aperture (d = 440 nm) enhanced by a surface corrugation with circular symmetry and with sinusoidal cross section (see inset). The spectrum exhibits resonances due to two distinct surface plasmon modes at l = 760 nm and l = 800 nm. For comparison, the dashed line shows the transmission spectrum of a similar aperture in a smooth metal film. The peak transmission is nearly 3 times higher than the fractional area of the hole.