"Our results indicate that mid-infrared photons can be focused to ultrasmall volumes – a factor of 105 times smaller than the volume of light that can be focused by an ordinary optical lens," team leader Hatice Altug told nanotechweb.org.

The team, which includes researchers from the University of Cambridge, UK, fabricated 100 × 100 µm arrays of metallic coaxial rectangular apertures in optical films 100 nm thick. Since it is not possible to directly couple light to a volume much smaller than the diffraction limit, Altug and colleagues first converted the mid-infrared light incident on the array into "surface plasmon excitations" (electromagnetic waves trapped in the metal-dielectric interface). Next, the researchers funnelled these surface plasmons into coaxial apertures, leading to enhanced electric fields in the small cavities with sub-wavelength dimensions.

"Once the surface plasmons reach the other side of the metallic film, they are converted back into photons that are picked up by a detector, revealing information about the shape and the electromagnetic characteristics of the cavity," explained Altug.

Although the nanoapertures are nearly 100 times smaller than the incident mid-infrared wavelength, almost 50% of light is transmitted through the optically thick metallic films. This is possible because, when the incident light is converted into surface plasmonic excitations, they are effectively funnelled through the nano-sized openings. Once the excitations reach the second surface, each hole acts like a point source by converting the plasmons back to light. An outgoing plane wave is then reconstructed as a result of these sources interfering.

The strength of this interference, and thus the transmission efficiency and its polarization characteristics, strongly depend on the aperture geometry. This is because surface plasmons created on the incidence surface can only channel to the second surface via the nanoapertures by coupling the aperture's localized plasmons, says Altug.

This "extraordinary transmission" effect from nanoapertures can lead to large field enhancement and strong field localization in small volumes, which is promising for increasing non-linear effects and enhancing vibration-based signals in techniques such as infrared and Raman spectroscopy. Enhanced light transmission phenomena could also come in useful in fields ranging from near-field optics, by exploring the effect of hole shapes in SNOM tips, to optoelectronics, by increasing the speed of detectors, enhancing the efficiency of light sources and solar-powered cells, adds Altug.

The team is now also employing "MIR plasmonic nanoapertures" for biomolecule detection through absorption spectroscopy. "We plan to enhance fingerprint absorption bands of the biomolecules through our plasmonic aperture arrays," Altug revealed.

The researchers reported their work in Applied Physics Letters.