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Dimple lens goes beyond the diffraction limit

Photonics Spectra
Nov 2009
Marie Freebody, mariefreebody@physics.org

LOS ANGELES – University of California scientists have created a new type of lens that can focus light down to a sub-100-nm spot. This record-breaking focusing power could open up new areas of research in biological sciences, nonlinear optics and near-field scanning optical microscopy (NSOM) techniques.

Until now, conventional optics could focus light to a spot size of only around half a wavelength. By exploiting the novel properties of surface plasmonic modes, the UCLA team has not only surpassed the diffraction limit but has also built the first plasmonic dimple lens, paving the way to widespread applications.

Heat-assisted magnetic recording is a much-anticipated technology that will be used in next-generation magnetic hard disks. In the device, a sub-50-nm optical near-field source would serve to heat up the magnetic domains of the hard disk, and a dimple lens would be an ideal candidate for the job.

TNdimple_fig1.jpg
The plasmonic dimple lens uses a grating coupler to couple free-space photons into surface plasmons. These plasmons are focused into a nanoscopic volume by a 3-D dimple lens. Image courtesy of UCLA.

“Our dimple lens is best compatible with the industry-established fabrication process sequence of the magnetic hard disks,” explained co-inventor Shantha Vedantam. “What’s more, the smallest critical dimension of our dimple lens is not limited by the standard available lithographic techniques.”

Another important application that could benefit from the strong near-field light created by the dimple lens is NSOM, which is a scientifically important tool in material, physical and biological sciences.

“As far as NSOM is concerned, our dimple lens design scores over the commercially available pulled metal-coated aperture fiber probes both in terms of achievable spot size as well as energy/field concentration,” Vedantam added. “Of course, the fabrication process sequence for this application would be slightly different from that of the [heat-assisted magnetic recording] application.”

The dimple lens, which is described in the September issue of Nano Letters, is a semicircular tapered metal-insulator-metal structure containing a grating in-coupler. When incident light reaches the grating, surface plasmons are generated. These surface plasmons travel radially along the tapered metal-insulator-metal stack and are eventually focused to a sharp point at the center of the semicircular dimple.

A precise measurement of the final spot size produced by the dimple lens was not possible due to the resolution limit of the pulled-fiber NSOM used in the experiment. But what the team does know is that the lens is capable of focusing light to spot sizes smaller than the aperture of commercially available pulled-fiber NSOM probes – in this case less than 100 nm.

According to Vedantam, the next step for the UCLA team is to identify new applications and fabricate the dimple lens structures accordingly. “We are also working toward improving our experimental measurement setup to allow us to measure not only the spot-size but also the electric field enhancement of our dimple lens for future nanostructural designs,” Vedantam concluded.


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