Integrated optical devices need to focus light on very small length scales. This is typically achieved using optical microlenses, but the ultimate size reduction for these lenses is generally limited by the technological precision that can be realized at such small scales. Ann Roberts, Ling Lin and co-workers from the University of Melbourne in Australia have now constructed planar plasmonic lenses with a much smaller footprint than conventional lenses.1

The resonances of electromagnetic waves on the surfaces of metals, known as plasmonics, have been investigated for a number of applications because they allow light to be controled and modified on a nanometer scale. The researchers have now used plasmonic effects in a thin metal film to construct a planar plasmonic lens. “Plasmonic lenses can serve as compact passive components, such as optical couplers, connectors and collimators, for high-density integrated optics and telecommunications,” says Roberts. Fabrication of the lenses is straightforward, and their flat geometry makes them particularly easy to integrate into photonic devices.

Fig. 1: Schematic of the plasmonic lens design.

The planar plasmonic lens is a thin metal film perforated with cross-shaped apertures on a glass substrate (Fig. 1). The variation in the length of aperture crossbars across the device determines the parameters of the lens, such as focal length. Typically, the crossbars are between 100 and 300 nm in length, so the overall diameter of the lenses is only a few micrometers.

An unpolarized red laser beam directed on the lens could be focused at micrometer-scale focal lengths consistent with theoretical predictions. Although the focusing achieved so far is not perfect, the researchers are confident that improvements in device design — such as incorporating other plasmonic effects — will solve problems such as diffraction effects caused by the small dimensions of the lens.

The researchers envision that enhanced lens designs will lead to more sophisticated beam control. “We are currently investigating the use of phase effects in a broader range of compact plasmonic devices to manipulate not only the intensity but also the polarization and phase of transmitted fields in three-dimensions,” says Lin. Planar plasmonic lenses are a promising addition to the ever-growing arsenal of surface-plasmon-based photonic devices.