Two sheets of boron nitride crystals are dynamically twisted with respect to each other. At certain angles, the incoming laser light (orange beam) can be efficiently converted to higher energy light (pink beam) as a result of micromechanical symmetry breaking. Image: Nathan R.Finney and Sanghoon Chae/Columbia Engineering.Nonlinear optics, a study of how light interacts with matter, is critical to many photonic applications. These range from the green laser pointers we're all familiar with to the intense broadband (white) light sources for quantum photonics, which find use in optical quantum computing, super-resolution imaging, optical sensing, and more. Through nonlinear optics, researchers are discovering new ways to use light, from getting a closer look at ultrafast processes in physics, biology and chemistry to enhancing communication and navigation, solar energy harvesting, medical testing and cybersecurity.
Researchers at Columbia Engineering now report their development of a new, efficient way to modulate and enhance an important type of nonlinear optical process – optical second harmonic generation, where two input photons are combined in a material to produce one photon with twice the energy. As the researchers report in a paper in Science Advances, they did this with 2D hexagonal boron nitride, through micromechanical rotation and multilayer stacking.
"Our work is the first to exploit the dynamically tunable symmetry of 2D materials for nonlinear optical applications," said James Schuck, associate professor of mechanical engineering, who led the study along with James Hone, professor of mechanical engineering.
A hot topic in the field of 2D materials has been exploring how twisting or rotating one layer relative to another can change the electronic properties of the layered system – something that can't be done with 3D crystals because their atoms are bound tightly together. This has led to a new research area termed 'twistronics'. In this new study, the researchers showed that concepts from twistronics can also apply to optical properties.
"We are calling this new research area 'twistoptics'," said Schuck. "Our twistoptics approach demonstrates that we can now achieve giant nonlinear optical responses in very small volumes – just a few atomic layer thicknesses – enabling, for example, entangled photon generation with a much more compact, chip-compatible footprint. Moreover, the response is fully tunable on demand."
Most of today's conventional nonlinear optical crystals are made of covalently bonded materials such as lithium niobate and barium borate. But because these materials have rigid crystal structures, it is difficult to engineer and control their nonlinear optical properties. For most applications, though, some degree of control over a material's nonlinear optical properties is essential.
The group found that van der Waals multilayer crystals provide an alternative solution for engineering optical nonlinearity. Thanks to the extremely weak interlayer force, the researchers could easily manipulate the relative crystal orientation between neighboring layers of hexagonal boron nitride by micromechanical rotation.
With the ability to control symmetry at the atomic-layer limit, the researchers could demonstrate both precise tuning and giant enhancement of optical second harmonic generation using micro-rotator devices and superlattice structures, respectively. For the superlattices, the team first used layer rotation to create 'twisted' interfaces between layers of hexagonal boron nitride, thereby yielding an extremely strong nonlinear optical response, and then stacked several of these 'twisted' interfaces on top of one another.
"We showed that the nonlinear optical signal actually scales with the square of the number of twisted interfaces," said Kaiyuan Yao, a postdoctoral research fellow in Schuck's lab and co-lead author of the paper. "So this makes the already large nonlinear response of a single interface orders of magnitude stronger still."
The group's findings have several potential applications. Tunable second harmonic generation from micro-rotators could lead to novel on-chip transducers that couple micromechanical motion to sensitive optical signals by turning mechanical motion into light. This is critical for many sensors and devices, including atomic force microscopes.
Their findings could also offer a new way to manufacture efficient nonlinear optical crystals with atomic precision. These crystals could find use in a broad range of laser, optical spectroscopy, imaging and metrology systems. And perhaps most significantly, they also could provide a compact means for generating entangled photons and single photons for next-generation optical quantum information processing and computing.
"We hope," Schuck said, "that this demonstration provides a new twist in the ongoing narrative aimed at harnessing and controlling the properties of materials."
This story is adapted from material from Columbia Engineering, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.