λ/4 Resonance of an Optical MonopoleAntenna Probed by Single MoleculeFluorescenceTim H. Taminiau,†Robert J. Moerland,‡Frans B. Segerink,‡Laurens Kuipers,‡,§and Niek F. van Hulst*,†,|ICFOsInstitut de Ciencies Fotoniques, Mediterranean Technology Park,08860, Castelldefels (Barcelona), Spain, Applied Optics Group,MESA+ Institute for NanoTechnology, UniVersity ofTwente, P.O. Box 217,7500AE Enschede, the Netherlands, and FOM Institute for Atomic and MolecularPhysics (AMOLF), Kruislaan 407, 1098 SJ Amsterdam, the NetherlandsReceived July 25, 2006; Revised Manuscript Received November 9, 2006ABSTRACTWe present a resonant optical nanoantenna positioned at the end of a metal-coated glass fiber near-field probe. Antenna resonances, excitationconditions, and field localization are directly probed in the near field by single fluorescent molecules and compared to finite integrationtechnique simulations. It is shown that the antenna is equivalent to its radio frequency analogue, the monopole antenna. For the right antennalength and local excitation conditions, antenna resonances occur that lead to an enhanced localized field near the antenna apex. Directmapping of this field with single fluorescent molecules reveals a spatial localization of 25 nm, demonstrating the importance of such antennasfor nanometer resolution optical microscopy.Antennas play an essential role as transmitters and receiversin our modern wireless society. The charge movements theysupport efficiently link propagating electromagnetic fieldsto localized fields. Radiation from a distant source can beconcentrated in a local volume, vice versa a localizedexcitation can be efficiently coupled into directed radiation.The efficiency of an antenna depends critically on itsresonances, as dictated by the operation frequency and bythe shape, material, and dimensions of the antenna. Onlyrecently antennas are experimentally explored in the opticaldomain, where they are crucial to surpass the fundamentaldiffraction limit. Properly engineered antennas will play adecisive role in the manipulation of light on the nanometerscale, such as control and optimization ofemissive systems,1-3nanofabrication,4optical manipulation and characterization,5-7integrated optoelectronic devices, and, as shown in thisLetter, subwavelength optical microscopy.8,9So far most research has focused on optical antennasfabricated on planar substrates, restricting their applications.Resonances and coupling effects ofsuch antennas have beeninvestigated thoroughly by linear far-field spectroscopy10-12and by exploiting nonlinear responses,13such as two-photonluminescence.14-16Unfortunately these characterization tech-niques do not allow a direct local measurement ofthe electricfield. More insight can be obtained using single emitters,such as fluorescent molecules, which are well-defined anddirect “point” probes of both local field amplitude anddirection.9,17To accomplish this, either the emitter or theantenna has to be mounted on a scanning probe. In near-field scanning optical microscopy (NSOM) scattering typeprobes are used, which exploit the local field enhancementnear a sharp tip.18-20Such probes could be labeled antennas;however, their infinite size does not support geometricalresonances.21Only a few examples offinite-size probe-basedantennas have been demonstrated, a gold nanoparticleattached to a glass probe22-24and an aluminum bowtieantenna.2All antennas mentioned are illuminated from the far fieldin a large, at best diffraction limited, focus. The extendedilluminated area adds a background contribution to theantenna response. The resulting background problem can beovercome by detection of nonlinear processes19,25or bymodulation techniques.20However, the large illuminated areais unacceptable in fluorescence detection where photobleach-ing is a primary drawback. Conventional NSOM apertureprobes26provide a background-free localized excitationvolume, but the obtainable field confinement is limited inpractice by the low throughput of subwavelength apertures.Frey et al.27combined scattering and aperture probes by* Corresponding author: Niek.vanHulst@ICFO.es.†ICFOsInstitut de Ciencies Fotoniques.‡Applied Optics Group, MESA+.§FOM Institute for Atomic and Molecular Physics (AMOLF).|Also at ICREAsInstitucio ´ Catalana de Recerca i Estudis Avanc ¸ats,08015 Barcelona, Spain.NANOLETTERS2007Vol. 7, No. 128-3310.1021/nl061726h CCC: $37.00Published on Web 12/02/2006© 2007 American Chemical Society
