Release Summary Text: In atomically thin semiconductors, enhancing the frequency-doubled light signal has come at the cost of losing polarization information tied to the valley degree of freedom. Researchers now show that silicon nanospheres can break this dilemma, providing design guidelines for next-generation valley photonic devices.
A research group led by Associate Professor Keisuke Shinokita at the Institute for Molecular Science (IMS), National Institutes of Natural Sciences, and The Graduate University for Advanced Studies (SOKENDAI), together with collaborators at Kyoto University and Kobe University, has demonstrated that silicon nanospheres can strongly enhance second-harmonic generation (SHG) from an atomically thin semiconductor while preserving the circular polarization information tied to its valley degree of freedom. The study, published in Nano Letters, provides design guidelines for efficient, polarization-preserving nonlinear light sources at the nanoscale.
SHG is a nonlinear optical process that converts light to twice its original frequency. Monolayer transition-metal dichalcogenides (TMDs) such as tungsten disulfide (WS2) possess valley-dependent optical selection rules that link circular polarization directly to the electronic valley index, making the SHG polarization state a direct readout of valley information. To harness the valley degree of freedom as an information carrier in valleytronics, it is essential to enhance the SHG signal while preserving its circular polarization. However, the atomic-scale thickness of monolayer TMDs severely limits conversion efficiency, and previous approaches using nanostructures to boost the signal have disrupted the valley-polarization information -- a dilemma of "enhance the signal, lose the polarization."
To overcome this dilemma, the team turned to silicon nanospheres, which support Mie resonances with negligible ohmic loss. By placing nanospheres of different diameters (200 nm and 241 nm) on monolayer WS2, SHG enhancement of more than 40-fold was achieved, driven by coupling between the excitation light and the Mie resonance modes of the nanospheres.
Crucially, circularly polarized SHG measurements revealed that the valley-polarization information was well preserved through the enhancement process. With 200 nm nanospheres, a degree of circular polarization (DOCP) of approximately 80% was maintained in the enhanced spectral regime. By tuning the nanosphere diameter, the balance between enhancement magnitude and polarization retention can be controlled.
Numerical simulations revealed that the key to polarization preservation lies in the balance between the electric and magnetic Mie modes of the nanosphere. When the two modes remain comparable in amplitude, both enhancement and high polarization retention coexist. This framework provides a universal design guideline for predicting and optimizing the relationship between enhancement and polarization preservation.
Because silicon nanospheres are achiral, the measured polarization faithfully reflects the intrinsic valley information without contamination from structural chirality. The nanospheres can also be applied as a non-destructive add-on to any monolayer TMD or van der Waals heterostructure, offering a versatile tool for valleytronics research. These results open a pathway toward integrated valleytronic technologies that exploit polarization as an information carrier for quantum computing, optical communications, and beyond.
