Ultrafast laser pulse repetition rate dependent switching of nonlinearity in water

Tailoring the nonlinear optical properties of materials through extrinsic parameter controls is advantageous for many applications. In this article, ultrafast laser pulse repetition rate induced switching of reverse saturable absorption to saturable absorption of water is reported. For the investigations, a conventional open aperture z-scan measurement system equipped with a variable repetition rate ultrafast laser source operating at the wavelength of 1030 nm was employed. A combination of weak saturable absorption and strong reverse saturable absorption induced by three-photon absorption is observed at low and moderate repetition rates. At repetition rates above 5 kHz, there is a significant variation in nonlinear absorption, where the reverse saturable absorption starts to diminish and saturable absorption increases. For repetition rates beyond 100 kHz, the nonlinearity gets reversed, where water tends to behave more like a saturable absorber with a weak three-photon absorption component.