Nonlinear optics, a study of how light interacts with matter, is critical to many photonic applications, from the green laser pointers we're all familiar with to intense broadband light sources for quantum photonics that enable optical quantum computing, super-reso­lution imaging, optical sensing and ranging, 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. Columbia Engi­neering researchers report that they developed 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 the material to produce one photon with twice the energy – from 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 , Wang Fong-Jen 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 in 3D crystals because the atoms are bond so tightly together in a 3D network. Solving this challenge has led to a new research area termed "twistronics." In this new study, the team used concepts from twis­tronics to show that they 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 enabling, for example, entangled photon generation with a much more compact, chip-compatible foot print. 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 they 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 relative crystal orien­tation between neighboring layers by micro­mechanical rotation. With the ability to control symmetry at the atomic-layer limit, they demons­trated precise tuning and giant enhancement of optical second harmonic generation with micro­rotator devices and super­lattice structures, respectively. For the super­lattices, the team first used layer rotation to created twisted interfaces between layers that yield 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. “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 such as atomic force micro­scopes.

Stacking multiple boron nitride thin films on top of each other with controlled twist angle demons­trated greatly enhanced nonlinear response. This could offer a new way to manu­facture efficient nonlinear optical crystals with atomic precision. These could be used in a broad range of laser, optical spectro­scopy, imaging, and metrology systems. And perhaps most signi­ficantly, they 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.” (Source: Columbia U.)

Reference: K. Yao et al.: Enhanced tunable second harmonic generation from twistable interfaces and vertical superlattices in boron nitride homostructures, Sci. Adv. 7, eabe8691 (2021); DOI: 10.1126/sciadv.abe8691

Link: Nanolight Lab., Dept. of Mechanical Engineering, Columbia University, New York, USA • Theory Dept., Max Planck Institute for the Structure and Dynamics of Matter, Hamburg, Germany