Columbia engineers introduce metasurfaces to 2D materials

In January, a team led by Jim Schuck, professor of mechanical engineering at Columbia Engineering, developed a method for creating entangled photon pairs, a critical component of emerging quantum technologies, using a crystalline device just 3.4 micrometers thick. 

Now, in a paper published in Nature Photonics in October, Columbia Engineers have shrunk nonlinear platforms with high efficiency down to just 160 nanometers by introducing metasurfaces: artificial geometries etched into ultrathin crystals that imbue them with new optical properties.

“We’ve established a successful recipe to pattern ultrathin crystals at the nanoscale to enhance nonlinearity while maintaining their sub-wavelength-thickness,” said corresponding author Chiara Trovatello. Trovatello is currently an assistant professor at Politecnico di Milano and was a Marie Skłodowska-Curie Global Fellow at Columbia working with Schuck. 

From stacks to surfaces 

The Schuck lab works with a class of crystals called the transition metal dichalcogenides (TMDs), which can be peeled into atom-thin layers and assembled into customizable stacks. Despite their large nonlinearity, these ultrathin layers had proved too thin to efficiently generate photons with new frequencies compared to conventional nonlinear crystals, like those you’ll find in devices like laser pointers. 

“Size doesn’t matter for something like a laser pointer that you’ll hold in your hand. But for quantum technologies, like quantum processors, size becomes crucial,” explained Trovatello. These devices operate on quantum bits, or qubits. Today’s state-of-the-art qubit sources have footprints of several centimeters, and hundreds, if not thousands, of them are needed. Current quantum devices occupy entire rooms in buildings, like in the early days of classical computers. “To make quantum technologies scalable, we need to shrink the size of our qubit sources,” she said. 

In their January Nature Photonics paper, the team used a technique called periodic poling to generate the photons that could one day power qubits. That involved layering a TMD called molybdenum disulfide in alternating directions to optimize its optical output. The carefully arranged layers ensured that the light waves traveling through each layer were phase-matched and did not interfere with one another. 

They now detail a complementary platform: TMDs with highly tunable, etched metasurfaces.

A simple technique enhances optical output

Metasurfaces allow researchers to introduce new properties that wouldn’t otherwise exist.  Unmodified crystals have a naturally repeating pattern of atoms that determines their optical properties. To make a metasurface, sections of atoms are strategically removed. This creates new periodic geometries, which, in turn, yield new optical properties that can be dictated by the design. 

In the current work, PhD student and first author Zhi Hao Peng, who developed the nanofabrication technique needed, etched a series of repeating lines onto a flake of molybdenum disulfide. “Our design enhances the nonlinear effects much more than traditional linear optical optimization techniques, and therefore achieves strong enhancement not previously possible," said Peng.

Their metasurface enhanced what’s known as second-harmonic generation by almost 150 times compared to unpatterned samples. In second-harmonic generation, two photons merge into one, with double the frequency and half the wavelength of the original light particles. With that process optimized, the team will now focus on running it in reverse: splitting one photon into two entangled ones. 

Notably, Peng’s approach requires fewer steps that are easier and cheaper to execute than prior patterning approaches. “Nonlinear crystals have been key to a lot of photonic technologies, but these materials can be brittle and have been notoriously difficult to shape and fabricate,” explained Schuck. “Peng figured out a technique that is deceptively simple. We can now make increasingly complex patterns using standard cleanroom etching technologies.”

From theory, towards on-chip quantum photonics

Theoretical collaborators Andrea Alu from the CUNY Advanced Science Research Center and his former postdoctoral researcher, Michele Cortufo, worked with the Columbia team to determine the metasurface pattern needed to boost the nonlinear response of TMDs and achieve macroscopic efficiencies across ultrathin path lengths. “We showed that such nontrivial behavior can be obtained with a remarkably simple modification to the sample: rather than working with flat flakes, we pattern them with a periodic arrangement of lines with alternating widths,” said Cortufo, now an assistant professor at the University of Rochester. 

Metasurfaces have been at the forefront of the photonics field for about a decade now, but Peng and his collaborators' device is one of the first to combine them with a 2D crystal to such potent effect. “This work demonstrates how engineered nonlocalities in metasurfaces can unlock unprecedented nonlinear efficiencies when combined with 2D materials,” said Alu. “It is exciting to see how concepts we’ve been developing in nanophotonics and metamaterials can enable compact, integrable platforms for nonlinear optics and light generation.”

That light is at telecommunications-range wavelengths, which should make the photons they produce easily integrable with current networks and telecommunications devices, achieved at a nanometer scale. “This could be one of the most compact sources of entangled photons at that wavelength range. With our footprint, we can really start to think about fully on-chip quantum photonics,” said Schuck.

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