In the rapidly evolving field of valleytronics, the ability to manipulate and read out valley-specific information encoded in two-dimensional materials stands as a significant challenge and opportunity. Recently, a breakthrough led by Associate Professor Keisuke Shinokita at the Institute for Molecular Science, together with collaborators from Kyoto University and Kobe University, has demonstrated a novel approach to simultaneously enhance and preserve valley-polarized second-harmonic generation (SHG) signals in atomically thin semiconductors. Their findings, published in Nano Letters, not only overcome longstanding technical hurdles but open pathways for next-generation quantum information technologies relying on valley degrees of freedom.
Second-harmonic generation refers to a nonlinear optical process where photons interacting with a nonlinear material are effectively combined to produce new photons at twice the energy—thus double the frequency—of the original light. Monolayer transition-metal dichalcogenides, such as tungsten disulfide (WS₂), have attracted intense interest as platforms for valleytronic applications due to their strong spin-valley coupling and unique valley-dependent optical selection rules. Photons emitted or absorbed in these materials carry valley-specific circular polarization, turning the polarization of emitted SHG light into a direct fingerprint of valley occupation and electronic dynamics.
However, a persistent dilemma has large impeded practical applications: while enhancing the SHG signal is crucial for device integration and readout efficiency, traditional methods of signal enhancement invariably degrade the crucial valley polarization information. The ultrathin nature of monolayer WS₂ limits nonlinear conversion efficiency, and prior approaches involving plasmonic or metallic nanostructures typically induce ohmic losses and break the delicate polarization balance, thus scrambling or diminishing intrinsic valley signatures.
To elegantly resolve this conundrum, the research team turned to silicon nanospheres, which act as low-loss dielectric nanoresonators supporting strong Mie resonances. Unlike their metallic counterparts, silicon nanospheres avoid detrimental ohmic losses, enabling pronounced light-matter interactions while maintaining high fidelity to intrinsic optical properties. By strategically placing silicon nanospheres with diameters tuned between 200 nm and 241 nm on monolayer WS₂, the researchers reported more than a 40-fold increase in SHG intensity driven by resonant coupling to the nanospheres’ electric and magnetic Mie modes.
Crucially, the enhanced SHG signals retained nearly pristine circular polarization characteristics. Experiments with 200 nm diameter nanospheres demonstrated a remarkable degree of circular polarization (DOCP) of approximately 80% within the enhanced spectral window, a level of polarization retention unprecedented in previous enhancement schemes. This balance signifies that information encoded in valley pseudospin is preserved even after photonic signal amplification, overcoming the “enhancement-versus-polarization” tradeoff that has stymied valleytronic signal transduction for years.
Numerical simulations underpinning the experimental results illuminate the physical mechanism at play. The team discovered that the preservation of polarization hinges on the relative amplitudes of electric and magnetic Mie resonances within the silicon nanospheres. When these dipolar resonances maintain comparable magnitudes, the resonant enhancement and polarization conservation coexist. This insight provides a universal design principle: by engineering the size and optical environment of all-dielectric resonators to maintain balanced Mie modes, future photonic devices can achieve strong nonlinear enhancement without compromising valley indexing.
An additional advantage of the silicon nanosphere approach lies in its achirality. Unlike chiral plasmonic nanostructures that can mix intrinsic valley polarization with extrinsic structural chirality, the silicon spheres faithfully reflect the true valley-polarized state of the underlying semiconductor, enabling uncontaminated optical readout. Moreover, their non-destructive and additive nature means they can be integrated with virtually any two-dimensional transition-metal dichalcogenide or van der Waals heterostructure, offering unprecedented versatility for engineering valley-photonic devices.
The implications of this research reach far beyond just improved SHG signals. Valley pseudospin has been proposed as an information carrier in quantum computing architectures, secure optical communications, and spintronics. By providing a scalable and controllable route to enhance valley-polarized nonlinear optical signals, the silicon nanosphere platform lays foundational technology for devices capable of encoding, manipulating, and reading valley information at depths and speeds unattainable with current photonic components.
This work also pioneers a promising direction for integrated photonics in two dimensions, combining the atomically thin world of monolayer semiconductors with sophisticated all-dielectric nanophotonics. Such synergy could lead to compact on-chip valleytronic light sources, detectors, and modulators, transforming how information is processed at the nanoscale. The demonstrated design rules stemming from the interplay of Mie resonances provide a roadmap for engineering other nonlinear and spin-valley phenomena with high efficiency and fidelity.
In conclusion, the team’s application of silicon nanospheres as tailored Mie resonators surmounts a critical bottleneck in valleytronics by generating a strong nonlinear optical signal that preserves valley-dependent circular polarization. Their work not only resolves the longstanding tradeoff between signal enhancement and polarization loss but also offers universal design principles to extend this approach to a range of materials and device geometries. The research presents new opportunities for quantum technologies harnessing the valley degree of freedom and enriches the toolkit of nanophotonics with a scalable, high-fidelity method of nonlinear signal amplification.
As the demand for ultrafast, low-energy, and quantum-resilient information processing technologies grows, the capability to encode information in valley pseudospin and efficiently read it out with enhanced, polarization-preserved nonlinear optics marks a milestone toward practical valleytronic circuits and devices. This breakthrough ushers the field closer to realizing the promise of valley-based quantum computing, secure optical communication channels, and next-generation photonic components integral to future technological landscapes.
Subject of Research:
Not applicable
Article Title:
Simultaneous Enhancement and Preservation of Valley-Polarized Second-Harmonic Generation in Monolayer WS2 via Mie Resonances
News Publication Date:
18-Mar-2026
Web References:
DOI: 10.1021/acs.nanolett.6c00297
Image Credits:
Keisuke Shinokita, Institute for Molecular Science
Keywords
Valleytronics, second-harmonic generation, monolayer WS2, Mie resonances, silicon nanospheres, nonlinear optics, circular polarization, spin-valley coupling, nanophotonics, 2D semiconductors, valley-polarized light, quantum information
