Nano-emitters such as single molecules are many magnitudes smaller than the wavelength of visible light. Therefore, the selection of single nano-emitters and the control of light–molecule interaction processes — including fluorescence efficiency — is desirable. One approach is to use optical nano-antennas to enhance electric fields and thus improve emission, but achieving reliable fluorescence enhancements is still an ongoing challenge.

Antennas are most familiar as key components in radio communication and broadcasting systems, where they have two roles. First, they convert local oscillating currents, and their associated local fields, into electromagnetic waves that propagate to the far-field. Second, they convert electromagnetic waves arriving at the antenna into oscillating currents. It is also currently thought that they operate at maximum efficiency when their size is approximately half the wavelength of the signal they are designed to emit or receive. Antennas that operate at much higher frequencies than radio waves were demonstrated in 1977, at infrared wavelengths1. The availability of precise nanofabrication technology such as electron- and ion-beam lithography has allowed antenna technology to move to the visible region of the spectrum. Much like other antennas, these 'optical antennas', can produce significant enhancements to the electric field when illuminated at resonant wavelengths. So far, most work has involved optical antenna configurations that use sharp metal tips2,3,4,5, small apertures5 and metal nanoparticles6,7. One of the best designs — for both electric field localization and enhancement — is the metal 'bowtie' configuration8. A bowtie antenna has two triangular metal regions that face each other, tip to tip, with a small dielectric gap between them. The precise triangular shape is not as important as the close proximity of the two sharp metallic tips. When the closely spaced tips are illuminated by a uniform electric field, for example by a broad laser beam, negative charge accumulates on one tip and effective positive charges (electrons are pushed away) accumulate on the other. Thus, owing to the Coulomb attraction between the opposite charges across the dielectric gap, both the electric field and the steady-state amplitude of the charge are enhanced.

A fluorophore placed in the gap can couple to the antenna structure in a way similar to the excitation of a radiofrequency antenna at its feed point. On page 654 of this issue9, Anika Kinkhabwala et al. describe a method that places single molecules in the gap of a bowtie antenna (Fig. 1), giving a 1,340-fold fluorescence enhancement — an improvement ten times larger than achieved previously using other schemes. This results both from an enhanced electric field in the resonant gap, and also from an increased radiative emission rate — caused by an enhancement in intrinsic quantum efficiency — despite additional Ohmic losses to the metallic antenna structure.

Figure 1: Bowtie antenna with a fluorophore (red dot) situated in the gap.
figure 1

Incident light (blue) excites the fluorophore and antenna, following which the antenna enhances the fluorescence emission (red lines).

Full size image

The bowtie antennas are made using evaporated 20-nm-thick gold films on quartz cover slips coated with 50 nm of indium tin oxide. By using electron-beam lithography, an array of gold bowtie antennas with gaps that varied from about 80 nm to around 15 nm were fabricated on a single substrate. A low quantum efficiency near-infrared dye (TPQDI) in a dilute solution of PMMA was spun to a thickness of 30 nm on the substrate and fabricated nano-antennas. This ensured that at least some of the bowtie gaps contained one or more molecules.

The nano-antennas were illuminated with a 780-nm diode laser, and fluorescence from the bowtie gap regions was collected in a confocal arrangement that allowed the fluorescence from individual antennas to be examined. Kinkhabwala et al. were able to demonstrate their fluorescence enhancement by comparing the fluorescence of molecules in bowties with molecules in substrate regions.

The fluorescence from single molecules was identified by examining its time dependence (the fluorescence tends to decay over time because excitation radiation bleaches the fluorophores). Fluorescence from a single molecule shows a discrete, sudden drop-off as the molecule is bleached, and this 'blinking' phenomenon allows individual molecules to be identified. It also allows the observation of varying degrees of fluorescence enhancement because the dye molecules are placed randomly in the bowtie gap. Some of these molecules will be in the optimal position for fluorescence enhancement, where the combination of enhanced electric field and reduced quenching in a location not too near the metal of the antenna produce the overall largest enhancement.

It has been known for a long time that the placement of a fluorophore near a metal surface, or even near a dielectric boundary, affects its fluorescence10. There are competing phenomena that influence whether the fluorescence is enhanced or suppressed11. All fluorophores have a quantum efficiency that describes the ratio of spontaneous emission (a radiative loss process) to all loss processes — including non-radiative ones — through which an excited fluorophore loses energy. Close proximity to a metal can significantly affect the quantum efficiency, for example by modifying radiative and non-radiative loss processes12. Depending on the geometry of the metal and the distance of the fluorophore from the boundary, the local electric field may be enhanced, for example in a bowtie resonator, leading to more energy absorption by the fluorophore and perhaps more spontaneous emission. The interaction between excitation light, fluorophore and metal can be described classically, unless the fluorophore is very close to an isolated metal subnanoscale particle. The dipole moment of the fluorophore couples to the metal to produce spatial oscillations of electron density, which may be either radiating surface plasmon polaritons (SPPs) or localized plasmons, depending on the geometry. All of these local excitations are subject to damping. SPPs cannot be excited directly on a flat metal surface in air because it is not possible to match the momentum of the incoming photons in the plane to the larger momentum of SPPs. If the metal surface is not flat but is structured on the nanoscale (as with a metal nanoparticle or bowtie antenna), the excitation light can excite SPPs or localized plasmons directly, through the spatial frequencies provided by the particular profile of the surface. These plasmons can produce a significant enhancement to the local electric field and increase the excitation of the fluorophore. If the enhanced excitation is greater than any increased non-radiative damping (caused by close proximity to the metal), then an increase in fluorescence will be observed. A fluorophore close to a smooth metal surface is generally quenched because energy couples to plasmons that cannot re-radiate, and Ohmic losses — electrons dissipating energy from the electromagnetic oscillations by heating the metal — increase the non-radiative loss rate. If the fluorophore is more than approximately one wavelength away from the surface, there is negligible quenching of its fluorescence.

Kinkhabwala et al. have demonstrated significantly enhanced emission from fluorescent molecules by placing them in the gap of a bowtie nano-antenna. By using dye molecules with low intrinsic quantum efficiency, they were able to show a fluorescence enhancement much larger than that possible if a dye with high quantum efficiency was used. The bowtie enhances emission in more than one way. First, the excitation light produces a strong field enhancement in the bowtie gap. Second, the bowtie functions as an optical antenna to enhance radiative emission from the fluorophore. This process is strongly mediated by plasmons, which enhance emission from the bowtie.

The ability to fabricate a large number of bowties on a single substrate allows many individual molecules to be observed. In this study, fluorophores were randomly distributed on the surface and some, at least, were optimally placed within the bowtie gaps. In the future, controlled placement of fluorophores such as quantum dots will allow arrays of controlled single-photon sources to be fabricated. Such structures also have potential as biological and chemical sensors.