Holographic lithography boosts plasmonic nanogap array production

Ashley N. Paddock, ashley.paddock@photonics.com

A technique based on methods from holographic lithography has been developed to demonstrate a new approach for scaling up the fabrication of plasmonic nanogap arrays while simultaneously reducing costs and infrastructure. A team led by Dr. Stefan Strauf, director of the NanoPhotonics Laboratory at Stevens Institute of Technology, developed the process to create uniform arrays of metallic nanostructures. Existing nanogap array production methods have low throughput and are expensive and time-consuming to produce.

Plasmonic nanogap arrays have small air gaps between them. By producing strongly confined electrical fields in optical illumination, the air gaps allow the arrays to be used in a number of applications, including ultrasensitive sensing and photonic circuit miniaturization. Such sensors could be used in high-resolution microscopy or to determine the presence of specific chemicals or proteins at a single-molecule level. Nanophotonic circuits, which can transfer huge amounts of data, are crucial to future-generation computing power and to bringing about the exaflop processing era.

The original goal was to make photonic crystals with compound lattices to be used in nanophotonics applications, according to Strauf and doctoral candidate Xi Zhang. But it became clear early on that the lattices’ twin motive shapes enable creation of metallic nanogaps with tiny features that are not limited by the laser wavelength used. These metallic nanogaps could be applied to surface-enhancement Raman scattering (SERS) applications, the investigators say.

Holographic lithography, an optical method that produces periodic patterns using interference patterns of laser beams, previously was used to create subwavelength features. The team upgraded the process by coupling the compound lattice concept and four-beam interference to produce tunable twin motive patterns into a polymer template, yielding metallic nanostructures with air gaps as low as 7 nm, approximately 70 times smaller than the wavelengths of the blue laser light used to write the features.

Bottom left shows a four-beam interference lithography setup using blue light; upper left, the resulting 3-D pattern. When the phase of one beam is changed, the trianglular lattice splits into a compound lattice featuring a twin motive whose relative spacing can be tuned using beam polarization or exposure time. At upper right is a scanning electron microscope image of the resulting nanogap template written into a photoactive polymer, which can be backfilled with gold, resulting in the plasmonic nanogap array (lower right). Courtesy of Xi Zhang and Stefan Strauf, Stevens Institute of Technology.

The team extended the utility of the process to form gaps with yields equivalent to that of painstaking serial fabrication methods including focused-ion-beam milling and electron-beam lithography. The new method not only is easier and economical, but also does not need a cleanroom, according to the researchers.

So far, the technique has yielded 90 percent consistency in the array pattern, providing the foundation for high-quality, large-scale superior-quality arrays, they say.

The work was detailed in the July issue of Nano Letters (doi: 10.1021/nl200994k).

The team is focusing on implementing the SERS experiments using different analytes and determining the enhancement factors on various modifications to the nanogap array design, Strauf and Zhang report.

The most pressing goal is to improve the fabrication technique for SERS templates; after that, if funding is available, they hope to scale the array size from 1-mm to 4-in. wafers.