Overview
This study experimentally demonstrates and theoretically describes high-harmonic generation (HHG) using a single dielectric subwavelength resonator. The resonator is shown to function as a direction-selective high-harmonic source, capable of controlling multiple harmonic orders. This control is achieved through the excitation and hybridization of Mie modes within the resonator. A key finding is the observation of pronounced forward-backward asymmetry in the generation of the third, fifth, and seventh harmonics, which is attributed to the structural asymmetry of the resonator along the propagation direction and its effect on mode coupling under opposite illumination.
Research Context
High-harmonic generation is a process associated with producing attosecond light pulses and table-top sources of coherent extreme-ultraviolet and soft X-ray radiation. Historically, HHG has been linked primarily with gases and plasma. However, nanostructured solids are emerging as alternative sources, offering potential for both the enhancement and control of HHG phenomena. This research contributes to the expanding field of strong-field light-matter interactions, particularly with Mie-resonant nanophotonics, by proposing bianisotropic subwavelength resonators as a platform for flexible asymmetric generation of high harmonics.
Approach
The research employed both experimental demonstration and theoretical description. The core component of the investigation was a single dielectric subwavelength resonator. This resonator had a geometrical volume of $0.12 \lambda^3$ and an optical mode volume of $0.03 \lambda^3$ at its pump wavelength. The methodology focused on the excitation and hybridization of Mie modes within this resonator. The structural asymmetry of the resonator along the propagation direction was a specific design feature investigated for its influence on mode coupling. The generation of higher harmonics, specifically the third, fifth, and seventh harmonics, was measured and analyzed under opposite illumination directions to observe any directional asymmetry.
Findings
- A single dielectric subwavelength resonator can act as a direction-selective high-harmonic source.
- Control over multiple harmonic orders is possible through the excitation and hybridization of Mie modes within the resonator.
- The resonator's geometrical volume was $0.12 \lambda^3$, with an optical mode volume of $0.03 \lambda^3$ at its pump wavelength.
- Structural asymmetry of the resonator along the propagation direction results in different mode coupling depending on the illumination direction.
- This difference in mode coupling leads to pronounced forward-backward asymmetry in the generation of the third, fifth, and seventh harmonics.
- Bianisotropic subwavelength resonators are established as a platform for flexible asymmetric generation of high harmonics.
Why This Matters
This work expands the available tools for controlling strong-field light-matter interactions. By demonstrating a precise method to achieve direction-selective high-harmonic generation from nanoscale structures, it offers new avenues for engineering light sources with specific directional properties in fields requiring attosecond pulses or coherent extreme-ultraviolet and soft X-ray radiation.