Nanoantennae Show Promise for Subwavelength Optical Microscopy

Lauren I. Rugani

Researchers have fabricated optical nanoantennae on the end face of a near-field probe. Using single fluorescent molecules, they measured the antennae’s resonance, excitation conditions and field localization, showing that the constructs are similar to the radio-frequency monopole antenna.

The scientists, from Institut de Ciencies Fotoniques in Barcelona, Spain, and from the University of Twente in Enschede and the Institute for Atomic and Molecular Physics in Amsterdam, both in the Netherlands, created aluminum antennae using gallium ion beam milling on the end face of the glass fiber probe next to a 100-nm-diameter aperture. They conducted simulations of perfect electrical conductor (radio wave) antennae that have radii of 2 and 20 nm to investigate the effect of material choice and antenna thickness on resonance.

The 2-nm perfect electrical conductor antenna exhibited resonance at 115 nm in length — slightly shorter than a λ/4 of the 514-nm excitation wavelength — whereas the antenna with the larger radius experienced a shift and demonstrated resonance at ~100 nm. The aluminum antenna, which had a 20-nm radius, showed a larger resonance shift of down to 70 nm while retaining characteristic field distributions.

The local field of the antenna was mapped with single fluorescent molecules, which were scanned below the probe with a near-field scanning optical microscope. The fluorescence was separated from the excitation light from an argon laser and detected by a photon-counting avalanche photodiode.

When the excitation polarization was parallel to the direction of the antenna, the fluorescence response showed narrow patterns coupled with larger weak spots that resulted from background illumination. When the polarization was perpendicular to the antenna, only the large weak spots were visible. Various patterns showed that the local field consisted of components in all directions and could excite molecules of different orientations.

The investigators formed antennae between 30 and 140 nm long to excite the molecules and characterize each antenna’s resonance. They found that those between 70 and 90 nm had the highest efficiencies, as measured by the intensity ratio of the narrow patterns to the larger features. Antennae that were 75 nm in length exhibited maximum efficiency and had a strong field at the apex, with a spatial resolution of 25 nm.

This resonance length, which was shorter than the λ/4 value of the radio-frequency monopole antenna, and the spatial resolution demonstrate the significance of nanoantennae for nanometer-resolution optical microscopy.

Nano Letters, January 2007, pp. 28-33.