The thin-film device uses subwavelength dielectric structures to color-selectively steer white light, and thereby to create images that can be seen only from a certain direction. The device, which is compatible with mass-production techniques, could find use in anti-counterfeiting applications, optical security, biosensing and more. [Image: Adapted with permission from Quaranta et al., ACS Photon., doi: 10.1021/acsphotonics.7b00232. Copyright 2017, American Chemical Society.]
Scientists from two Swiss institutions have used subwavelength resonant waveguide gratings (RWGs) to create a single-layer, thin-film beam-steering device that can color-filter and redirect a low-coherence white light source, such as light from a smartphone camera’s flash (ACS Photon., doi: 10.1021/acsphotonics.7b00232). The RWG-based design, according to the researchers, makes the device “relevant for high-volume production,” in a range of applications in optical security, flat-lens optics, biosensing and near-eye/AR displays.
Scaling up planar diffractive optics
Planar optics has seen a range of developments in recent years, with new designs for flat lenses and other flat diffractive elements coming especially out of the study of highly engineered “metasurfaces.” Many of these surfaces, however, work best with coherent light sources in restricted wavelength bands. And even those devised to work with lower-coherence and broadband visible sources, according to the research team behind the new study, tend to require relatively complex, low-throughput fabrication processes that are difficult to scale up to large sample sizes and high-volume production.
To find a more scalable alternative for beam steering from a thin film, the Swiss research team—including scientists from EPFL Lausanne and the Swiss Center for Electronics and Microtechnology (CSEM)—turned to RWGs. These subwavelength structures, with a pedigree stretching back to the early 1990s, consist of periodic arrangements of dielectric materials that can resonantly couple to an incoming beam and can selectively filter it by color and steer the forward-propagating beam energy in a new direction. And, the authors note, RWGs have already proved themselves suitable for mass production through high-throughput processes such as roll-to-roll nanoimprint lithography (NIL).
The EPFL-CSEM team put these ideas into practice in a number of prototype devices. To do so, they started with a numerical model of the grating structure, and then used electron-beam lithography to fashion a mastering template for that structure on a silicon wafer. Next, a single NIL pass from the master transferred the subwavelength structure to a curable sol-gel material on a glass substrate. Finally, using physical vapor deposition, the team dropped a single, high-refractive-index layer of zinc sulfide onto the sol-gel material, to create the RWG guiding layer.
Anti-counterfeiting applications
The result was a number of devices that could embed into a surface a pattern that was invisible in most light, but became visible when illuminated from a specific direction using the flash on a smartphone camera—a functionality demonstrated in a video accompanying the paper. That functionality, according to the paper’s lead author, CSEM/EPFL Ph.D. student Giorgio Quaranta, could have practical relevance in anti-counterfeit labeling and other optical-security applications, in biosensing, in see-through optical combiners for near-eye (e.g., augmented-reality) displays, and in other applications requiring beam steering in a compact setup.
But the structure’s “greatest novelty,” Quaranta says, is its “compatibility with up-scalable fabrication processes.” The e-beam lithography used in creating the master template, the paper notes, allows for the cost-effective creation of patterns larger than 1 cm2, and turning that master pattern into an actual beam-steering film requires only a single NIL replication step and one thin-film-layer deposition. Those attributes makes the device, in Quaranta’s words, “relevant for high-volume and affordable fabrication.”