A new silicon metasurface capable of rapidly controlling wide-bandwidth light pulses could help advance compact technologies for optical communications, computing, LiDAR and photonic information processing. The approach combines broad spectral operation with picosecond-scale switching, addressing a longstanding trade-off between modulation strength and bandwidth in ultrafast optical devices.
Optical metasurfaces consist of arrays of structures smaller than the wavelength of light, enabling precise control over transmitted amplitude and phase while offering a compact alternative to conventional optical components. Semiconductor metasurfaces are particularly promising for all-optical modulation because their resonances can enhance ultrafast optical effects. However, strong modulation has traditionally depended on narrow resonances, restricting the range of wavelengths that can be controlled simultaneously.
The new device tackles this limitation with an engineered kink-like transmission spectrum. Instead of relying on the shift of a single narrow resonance, its asymmetric spectral profile allows a resonance shift to influence a much broader wavelength range. The metasurface is fabricated from 100-nanometre-thick crystalline silicon on sapphire, with its nanoscale geometry optimized to produce the required transmission response.
The effect arises from the interaction between a dark toroidal dipole mode and a bright electric dipole mode, producing the asymmetric optical response needed for broadband modulation. When illuminated by femtosecond green laser pulses at 520 nm, free carriers are generated in the silicon, rapidly changing its refractive index and shifting the metasurface resonance. The initial spectral shift occurs in less than 0.5 picoseconds, while the free-carrier dynamics provide a characteristic switching time of approximately 25 picoseconds.
The metasurface achieves 28% absolute transmission modulation across a 14 nm bandwidth for y-polarized light and 10% modulation across 49 nm for x-polarized light. The broader 49 nm operating range is sufficient to support the switching of sub-50-femtosecond laser pulses, demonstrating how the kink-shaped spectral response can overcome the conventional bandwidth limitations of resonant optical modulation.
The combination of ultrafast speed, broadband operation and compact metasurface design could provide a useful foundation for future spatiotemporal pulse shaping, ultrafast LiDAR, optical communications, laser mode locking and photonic neuromorphic computing, where rapid and efficient control of short optical pulses is increasingly important.