Nanometer-scale optical antennas resonate

A radio-wave antenna converts radiation into a local oscillation in its so-called feed gap, which is the region between the antenna arms where contact is made to antenna wires or a waveguide. In reverse, the antenna converts electromagnetic energy produced in the feed gap into emitted radiation.

But unlike a radio-wave antenna, the overall length of an optical resonant antenna (ORA) should be about half the wavelength to work efficiently; the feed gap is always much smaller than the wavelength. Researchers at the University of Basel (Basel, Switzerland) and the Swiss Federal Institute of Technology Lausanne (Lausanne, Switzerland) have fabricated nanometer-scale gold dipole antennas that are resonant at optical frequencies, and have demonstrated strong field enhancement in the feed gaps of these antennas that leads to white-light-supercontinuum (WLSC) generation.¹ Effective field localization in the nanometer regime should allow these ORAs to be used for manipulation of nanostructures, subwavelength imaging, interaction with single quantum emitters, and optical information processing.

In addition to the generation of WLSC in the feed gap when these gold dipole antennas are illuminated with picosecond laser pulses, two-photon photoluminescence (TPPL) is also generated in the antenna arms. These emissions are more than 1000 times stronger for resonant antennas than that from solid gold stripes of the same dimensions but without a feed gap.

Ion-beam milled

The researchers fabricated the antennas using focused-ion-beam milling. Initially, sets of stripes with a length of 190 to 400 nm and width of 45 nm were cut from 40 nm thick, micrometer-size rectangular gold patches arranged on a 10‑nm-thick indium tin oxide (ITO)-coated glass cover slide. For complete gold removal, the ion beam had to cut slightly into the substrate, leaving an approximately 20-nm shallow depression around the stripes. Then, half of the stripes were converted into ORAs by cutting a narrow groove through the centers of the stripes and leaving the 20-nm wide feed gap between the arms.

The WLSC spectra generated by the application of picosecond laser pulses to ORAs and gold stripes of different dimensions was obtained by mounting the sample in an inverted optical microscope modified for confocal operation in reflection, and by focusing the horizontally or vertically polarized laser pulses to a diffraction-limited spot on the sample (see figure). Using laser pulses with center wavelength of 830 nm, a repetition rate of 80 MHz, a pulse length of 8 ps at the sample, and maximum average power of 150 µW, WLSC-emission maps were generated by scanning the sample with a single-photon-counting avalanche-diode detector in combination with a bandpass filter (450 to 750 nm) for different laser intensities. Sizable WLSC generation was found only for ORAs of a certain length range and with orientation along the pump polarization axis.

The researchers concluded from analysis of the data that, as a function of ORA or stripe length, high-power excitation (40 µW or more) corresponded to a dominance of WLSC generation, while low-power excitation (40 µW or less) corresponded to a dominance of TPPL. And, in direct comparison to the stripes, the ORA emission was 10 times as strong at lower powers and 100 times as strong at higher powers. The researchers also noted that strong field concentration in the feed gap of the ORAs suggests that WLSC is generated mainly in the underlying ITO/glass substrate, and possibly in water that might condense inside the gap; they expect that a reduction in the feed gap width may further enhance WLSC emission.

From the experiment, the researchers identify a peak in WLSC generation for an antenna length of approximately 255 nm. “We think that the demonstration of the antenna concept at optical frequencies is a significant step in the development of optics on the nanometer scale,” says researcher Bert Hecht. “Optical antennas provide the missing link between the diffraction-limited free-space propagating light and highly confined and enhanced stationary or guided optical near-fields, opening the road for many yet unforeseen applications. When Heinrich Hertz demonstrated the antenna concept in 1888 he never imagined the importance of his finding for telecommunications-the impact of optical antennas may be of similar significance.” (For more on subwavelength optics, see “Subwavelength optics come into focus,” Laser Focus World, September 2005, p. 86.)

REFERENCE

1. P. Mühlschlegel et al., Science308, 1607 (June 10, 2005).

2. Graphic from Mühlschlegel et al., Science308, 1607 (June 10, 2005). Reprinted with permission from AAAS; www.sciencemag.org.