High-Speed Acousto-Optic Modulation at Optical Communication Band by Ultrafast-Laser-Induced Hypersonic Vibrational Coherence

All-optical control of electronic/optical properties in materials with collective lattice vibrations is a feasible approach to manipulate device performances at a high speed [1]. Recent studies of topological insulators with tensile strain for controlling electronic band structures and bandgaps motivated us to ultrafast control of band structure in Bi
2
Se
3
by employing ultrafast lasers for exciting photoinduced strain waves [2]. Apart from the uniqueness of the topological surface state in Bi
2
Se
3
, insulating bulk states with a small bandgap (0.3 eV) indicate attractive interest for infrared (IR) photonics owing to its sufficient optical absorption and photocurrent response in a broad wavelength range including optical communication band around 1300~1650 nm [3]. Here, we present active control and hypersonic modulation of optical properties of Bi
2
Se
3
thin films by interlayer vibrations, initiated by ultrafast laser pulse excitation. It includes that the optical activity such as reflectance and absorption of the sample can be modulated by the modulation in the interlayer distance.