Plasmonics is a new branch of photonics that exploits surface plasmon polaritons, which arise from the interaction of light with the electrons that oscillate at a metal's surface. A few years ago, scientists discovered that plasmonic structures composed of nanowaveguides (slits) in metal films could focus light. However, these plasmonic microlenses have proved difficult to make because they have very complicated geometries. The structures can also only focus single wavelengths of light and cannot easily be scaled up.

Teri Odom and colleagues have now made a new type of microlens based on finite 2D arrays – patches – of circular holes that overcomes the limitations of both conventional diffractive lenses (for example, Fresnel lenses) and plasmonic slit lenses. Unlike either of these types of lenses, the patches do not require "phase-front engineering" for them to be able to focus light. The same patch can also focus single wavelengths over a range that spans from 500 to 780 nm – something that is extremely unusual for a diffractive lens.

Scaling up And that's not all: the lenses can easily be scaled up using parallel nanofabrication techniques and can be arranged in very dense arrays. Such miniature optical devices are crucial for micro-optoelectronics, from coupling light into single-mode waveguides to collimating emission from laser diodes, to displaying images in 3D, explains Odom.

Calculations and modelling by the team revealed that both optical diffraction and surface plasmon effects are needed for focusing to occur. The patches produce focal spots based on constructive interference of in-phase electromagnetic waves transmitted through the nanoholes. Essentially, the surface plasmons enhance the throughput and ensure that light from every hole has the same phase.

Odom's team first prototyped their plasmonic microlenses using focused ion beam milling by drilling patches of nanoholes into optically thick gold films. The patches were designed to have different-sized holes, lattice spacings and lattice symmetries. Next, and very importantly, the researchers used soft nanolithography to simultaneously fabricate millions of microlenses that all have the same light focusing properties.

Different from other lenses
Thanks to their planar structure, the plasmonic lenses could be used in integrated optical circuits, says Odom. "The broadband focusing capabilities of the patches sets them apart from other diffractive and refractive microlenses and opens up possibilities in multicolour stereo imaging, broadband light collection and multichannel optical communication," she told "Being able to fabricate these microlenses in parallel will also drive the development of high-throughput sensing, photolithography and imaging."

The researchers now plan to improve the optical throughput, or transmission, of their microlenses. "We are also testing the lens arrays in new types of label-free imaging," revealed Odom.

The work was published in Nano Letters.