Inkjet-printed droplets on photonic-crystal template

A team of researchers at Cambridge’s Cavendish Lab, U.K., has developed a method in which photonic-crystal cavities are created by using a high-resolution inkjet printer to deposit femtoliter-scale droplets on a generic photonic-crystal substrate. The droplets form nanocavities that can serve as loci of light trapping and concentration.

Two-dimensional photonic-crystal cavities—optical nanocavities with strikingly high quality factors—form part of the bedrock of nanophotonics, serving in applications ranging from low-threshold lasers to single-photon sources to sensors. Recent advances in deep-UV photolithography have even raised the prospect of mass-producing high-quality photonic-crystal cavities using CMOS-compatible methods.

Now, scientists from Cambridge’s Cavendish Laboratory, U.K., have suggested an alternative approach for creating the cavities: use an inkjet printer (Adv. Mater., doi: 10.1002/adma.201704425).

The Cambridge team’s technique involves writing tiny, femtoliter droplets of low-refractive-index, organic inks onto a generic 2-D inorganic photonic-crystal template, using an electrohydrodynamic-jet printer. The droplets then serve as loci of light confinement sitting onboard the crystal substrate. The method, the team believes, raises the prospect of expanding design horizons, by allowing on-the-fly post-processing of nanophotonic devices onto a mass-produced, generic (and reusable) template, and by opening up cavity design to a range of unconventional materials.

Off-the-shelf equipment

The idea of creating photonic-crystal cavities by depositing a low-refractive-index polymer on a photonic-crystal waveguide dates back about 10 years (Opt. Express, doi: 10.1364/OE.15.017248). But these earlier efforts tended to rely on relatively complex, material-specific electron-beam or UV-exposure techniques. And while conventional inkjet printers have made vast strides in recent years as a microfabrication technology, their resolution has been far too coarse to put them to use in nanophotonics.

Those ostensibly dim prospects brightened a bit at Cavendish lab when one of the new paper’s authors, the optoelectronics researcher Vincenzo Pecunia, got his hands on an electrohydrodynamic-jet printer. While still definitely qualifying as commercially available, off-the-shelf equipment, the printer—which pushes organic inks through a one-micron nozzle on demand, via the application of a voltage differential between the ink reservoir and the substrate—can deliver droplets between 1 and 10 femtoliters in volume.

Sweet spot

That exceptionally high resolution raised the prospect of using the printer as a platform for nanophotonic fabrication, which Pecunia began to explore with his lead coauthor on the study, Frederic Brossard, also of the Cavendish Lab. The surface-to-volume ratio of the femtoliter-scale droplets turned out to hit the sweet spot for use on a photonic-crystal substrate; somewhat larger droplets, while potentially usable to create photonic-crystal cavities, underwent “dewetting” that limited their ability to stick to the crystals.

Armed with this tool, the team began playing with writing tiny patterns on photonic-crystal membranes, including membranes already containing waveguide structures and completely blank photonic-crystal templates. The templates were mounted on a programmable stage that enabled the ink to be deposited with micrometer accuracy. The ink used by the team was a custom-made material mixing a small molecule and a polymer, dissolved in dichlorobenzene, an organic solvent.

Multiple passes

The team found that it was able to create “reproducible and structurally tunable high-Q cavities with small modal volumes” using the technique. The researchers also stress that, while they used a specific ink for their experiments, the method is in principle material-agnostic. And they found that the thickness of the inks creating the cavities could be controlled, to a resolution of less than 10 nm, merely by varying the number of passes of the printer jet over the structure. That raises the prospect of creating stacked combinations of materials that could serve as midwives to complex light-matter interactions, making such interactions potentially easier to set up and study.

The Cambridge scientists envision a workflow in which a generic, 2-D photonic-crystal template, mass produced by deep-UV lithography, would serve as a reusable foundation for on-demand deposition of structures via the inkjet printer. In a press release, Brossard suggested that such a workflow could be used to create, for example, “a high density of highly sensitive sensors to detect minute amounts of biomolecules such as viruses and cancer cells,” or other applications such as compact optical circuits and lasers.

Brossard’s coauthor, Pecunia, likewise sees strong prospects for the marriage of nanophotonics and inkjet printers. “Previous efforts to combine these two areas had bumped into the limitations of conventional inkjet printing technology, which cannot directly deposit anything small enough to be comparable to the wavelength of light,” he said. “But through electrodynamic inkjet printing we've been able to move beyond these limitations.”