Modern electronics run on a single property of the electron: the charge. Every email exchange and streamed blockbuster film comes down to charge shuttled through billions of transistors, a scheme straining against the limits of power and heat. Yet charge is not all an electron carries.
In certain semiconductors just one atom thick, electrons can settle into one of two energy pockets known as valleys—a natural binary pair that could encode digital information. The catch is that writing, steering, and reading valley information have so far required bulky optical set-ups sprawled across lab benches. Pieces of the puzzle had been demonstrated individually, but never together on one chip.
A multinational team of researchers has now done just that. Associate Professor Dong Zhaogang from the Singapore University of Technology and Design, in collaboration with Professors Haoran Ren and Stefan A. Maier of Monash University, Professor Qingdong Ou of Macau University of Science and Technology, Professor Andreas Tittl of Ludwig-Maximilians-Universität München, and researchers from Singapore’s Agency for Science, Technology and Research (A*STAR), demonstrated a circuit that generates, routes, and electrically reads valley information entirely on chip, at room temperature. Their findings were published in the paper “ An on-chip programmable valley optoelectronic nanocircuit ” in Nature Photonics .
“The main challenge lies in bringing together several fundamentally different physical processes into one compact device,” said Assoc Prof Dong.
The device is built around a monolayer of tungsten disulphide, in which left- and right-handed circularly polarised light each address one of the two valleys.
“You can think of them as two independent information channels, similar to the binary states 0 and 1 in conventional digital electronics,” he explained.
Photoluminescence, the conventional way to read valleys with light, falters at room temperature: thermally agitated electrons hop between valleys before emitting, and the valley signal drops to roughly 10 percent. The team turned instead to second-harmonic generation (SHG), a nonlinear process in which two infrared photons merge into a single photon at twice the frequency.
“SHG is governed by the crystal symmetry and nonlinear optical selection rules of the material rather than by the lifetime of excited carriers,” added Assoc Prof Dong. “Our device can produce much cleaner valley signals without requiring cryogenic cooling.”
Beneath the monolayer sits a metasurface, an array of rotated nanoscale silicon pillars on a silicon nitride waveguide that converts a photon’s polarisation into a direction of travel.
“Simply put, the metasurface functions like an optical traffic controller,” he said. “Depending on the polarisation state of the incoming photons, it automatically sends them either to the left or to the right side of the chip.”
At each end of the waveguide, atomically thin tungsten diselenide photodetectors convert the arriving photons into electrical current, registering the upconverted SHG photons while staying blind to the infrared pump laser. Tested on its own, the metasurface sorted light with a polarisation selectivity of 0.97. In the fully assembled circuit, photocurrent selectivity settled at 0.63—a drop the team traced to the added layers reshaping the local optical field rather than any fundamental limit, yet still ample for reliable decoding.
To prove the point, the researchers encoded two 256-by-256-pixel images, a kangaroo and a koala, onto opposite valley states and sent both simultaneously through the circuit. The detectors reconstructed their respective images electrically, with signal-to-noise ratios of roughly 30:1 and 60:1.
“This demonstrates that valley information can function as an additional information channel within integrated photonic circuits,” noted Assoc Prof Dong, comparing it to the wavelength multiplexing that lets a single optical fibre carry many data streams at once.
Valley multiplexing could squeeze more capacity out of photonic chips without enlarging them—a boon for low-power optical interconnects and programmable processors for artificial intelligence. Because valley states are quantum mechanical in origin, the platform may also become a building block for quantum communication and sensing circuits.
Still, SHG is an intrinsically weak process, and boosting its efficiency by coupling the monolayer to optical resonators or photonic cavities is a priority. The photodetectors’ response speed is limited by interface defects, and practical systems will need wafer-scale growth of high-quality two-dimensional materials as well as electrically driven valley light sources in place of external lasers.
“Our work demonstrates that valley information can now be generated, manipulated, and detected entirely on chip,” said Assoc Prof Dong. “We hope this will stimulate further research towards compact, scalable valleytronic devices that complement existing electronic and photonic technologies.”
Nature Photonics