Researchers developed a technique called photonic origami that can fold ultrathin glass sheets into microscopic 3D photonic structures directly on a chip. [Image: Tal Carmon, Tel Aviv University]
Researchers have reportedly devised a way to use a laser to fold sub-micrometer-thick glass sheets directly on a chip (Optica, doi: 10.1364/OPTICA.560597). Discovered by accident, the technique could enable the creation of microscopic and complex optical devices for data processing, sensing and experimental physics.
A serendipitous discovery
Constructing high-quality silica photonic structures at the nanoscale with existing 3D-printing techniques is challenging because surface roughness—a result of 3D printing—can cause scattering losses, drastically degrading optical performance. This challenge was not front of mind, however, when Tal Carmon, Tel Aviv University, Israel, asked his graduate student Manya Malhotra to point a laser at glass, with increasing power, until the spot glowed.
Carmon was only hoping to identify the exact location where the invisible laser light was hitting the surface, but then the glass folded. “It was exciting to see the folding silica under the microscope,” said Carmon.
Folding glass
With the new technique, which is reminiscent of origami, the team is able to manipulate ultrasmooth silica on silicon chips with 20-nm alignment accuracy into polylines and helices. To prepare the glass for folding, Carmon, Malhotra and colleagues thermally grow a very smooth (down to 0.5 μm) amorphous silica layer on top of a silicon chip with cleanroom-grade silicon and oxygen. They then release the silica from the silicon with dry xenon difluoride silicon etching. Now, the glass surface is ready to be folded.
Manya Malhotra discovered photonic origami by chance while trying to locate an invisible laser beam. [Image: Tal Carmon, Tel Aviv University]
The researchers leverage interfacial tension at the border between the liquid and gas phases for the photonic origami. They focus an 11-µm-wavelength CO2 laser at crosshairs projected onto the silica surface from a top-view microscope. The laser heats the contact point to 3000 K, starting to vaporize it. The side of the silica opposite the laser contact point reaches a temperature of 1500 K, which is slightly above the transition temperature of glass, so it begins to liquify and exhibit viscosity. The tension formed between the hotter and cooler sides of the silica—one in liquid phase and the other gas—causes it to bend against gravity. This folding process, which the team monitors with a side-view microscope, occurs in under 1 ms.
Carmon, Malhotra and colleagues can precisely (to the 0.1 microradian) achieve different angles and shapes by manipulating either the laser or silica surface while performing the folding. A slow train of lower-power pulses can result in a specific angle, and monotonically moving the glass in relation to the laser focus will produce a circular or helical shape. The researchers can also fold a single sheet of silica multiple times. “The level of control we had over 3D microphotonic architecture came as a pleasant surprise—especially given that it was achieved with a simple setup involving just a single laser beam focused on the desired fold,” Carmon said.
Broken records and next steps
Using the on-chip glass origami method, the researchers were able to create record length-to-thickness ratio structures, measuring 3 mm long and 0.5 μm thick. These structures, which include concave micromirrors and microresonators, are also ultrasmooth so can reflect light without distortion.
Additionally, they believe that their new technique can be applied to materials other than silica, based on similar surface tension and viscosity characteristics at liquid–phase boundaries observed in other materials. Examples of such materials include dielectrics, like silicon nitride and aluminum oxide, and compound semiconductors, like amorphous gallium arsenide.
The team says the approach could enable the transformation of planar electro-opto-mechanical circuits into high-quality 3D configurations. “High-performance, 3D microphotonics had not been previously demonstrated,” said Carmon. “This new technique brings silica photonics—using glass to guide and control light—into the third dimension, opening up entirely new possibilities for high-performance, integrated optical devices.”