Silicon nanospheres amplify second-harmonic generation while preserving valley polarization

Mar 25, 2026

Silicon nanospheres amplify second-harmonic generation from monolayer semiconductors over 40-fold while retaining valley-polarization information.

(Nanowerk News) A team of researchers has demonstrated that silicon nanospheres placed on monolayer tungsten disulfide can amplify second-harmonic generation more than 40-fold without disrupting the circular polarization that encodes valley information. Published in Nano Letters (“Simultaneous Enhancement and Preservation of Valley-Polarized Second-Harmonic Generation in Monolayer WS2 via Mie Resonances”), the study establishes design rules for nanoscale nonlinear light sources that retain polarization fidelity, a requirement for devices that use the valley degree of freedom as an information carrier.

Key Findings

Monolayer transition-metal dichalcogenides such as tungsten disulfide (WS2) possess a crystal symmetry that ties circular polarization directly to the electronic valley index. Second-harmonic generation, the nonlinear process that converts light to twice its original frequency, therefore acts as a direct readout of valley information: the polarization state of the frequency-doubled output reveals which valley carried the signal. Exploiting this link is central to valleytronics, where the valley index serves as a binary information carrier for quantum computing and optical communications. A single atomic layer, however, produces only a faint SHG signal. Metallic and dielectric nanostructures can concentrate optical fields to amplify the output, but previous implementations scrambled the circular polarization in the process, erasing the valley signature the measurement was meant to capture. Signal strength and polarization fidelity traded off against each other with no clear path to achieving both simultaneously. Schematic of the resonant structure. Right: High degree of circular polarization is preserved after enhancement. (Image: Keisuke Shinokita, Institute for Molecular Science) Numerical simulations by the research group, led by Associate Professor Keisuke Shinokita at the Institute for Molecular Science and including collaborators at Kyoto University and Kobe University, revealed that this trade-off originates in the interplay between the electric and magnetic Mie resonance modes of the enhancing structure. When the two mode amplitudes remain comparable, the local electromagnetic field preserves the symmetry conditions required to maintain circular polarization. A large imbalance between modes distorts the polarization state. This relationship serves as a predictive rule for selecting nanostructure geometries that maximize enhancement without degrading polarization. Guided by this principle, the team deposited silicon nanospheres onto monolayer WS2. Silicon nanospheres support strong Mie resonances while introducing negligible resistive energy loss, making them well suited as low-loss optical antennas. Two sphere diameters were tested, 200 nm and 241 nm, and both delivered SHG enhancements exceeding a factor of 40 relative to bare monolayer WS2. The enhancement arose from coupling between the excitation light and the Mie resonance modes concentrated near the sphere surface. Circularly polarized SHG measurements confirmed that the valley signature remained intact. With 200 nm spheres, the degree of circular polarization held near 80% across the enhanced spectral range. Switching to the 241 nm diameter shifted the balance between signal strength and polarization retention, demonstrating that sphere size offers a practical lever for tuning both quantities to suit a given application. Because silicon nanospheres are achiral, the measured polarization reflects only the intrinsic valley properties of the monolayer, free from artifacts that a structurally chiral enhancer might introduce. The spheres function as a non-destructive add-on and can be placed onto any monolayer transition-metal dichalcogenide or van der Waals heterostructure without damaging the underlying material. This versatility makes the approach applicable across the broad family of two-dimensional semiconductors used in valleytronics research, opening a route toward integrated devices that encode information in polarization for quantum computing and optical communications.