Ultrafast acousto-optic modulation at the near-infrared spectral range by interlayer vibrations.

The acousto-optic modulation over a broad near-infrared (NIR) spectrum with high speed, excellent integrability, and relatively simple scheme is crucial for the application of next-generation opto-electronic and photonic devices. This study aims to experimentally demonstrate ultrafast acousto-optic phenomena in the broad NIR spectral range of 0.77-1.1 eV (1130-1610 nm). Hundreds of GHz of light modulation are revealed in an all-optical configuration by combining ultrafast optical spectroscopy and light-sound conversion in 10-20 nm-thick bismuth selenide (Bi₂Se₃) van der Waals thin films. The modified optical transition energy and the line shape in the NIR band indicate phonon-photon interactions, resulting in a modulation of optical characteristics by the photoexcited interlayer vibrations in Bi₂Se₃. This all-optical, ultrafast acousto-optic modulation approach may open avenues for next-generation nanophotonic applications, including optical communications and processing, due to the synergistic combination of large-area capability, high photo-responsivity, and frequency tunability in the NIR spectral range.