An international team led by researchers from the Skolkovo Institute of Science and Technology (Skoltech) has shown that the nonlinear optical response of carbon nanotubes — specifically, saturable absorption — can be controlled by electrochemical gating. The researchers applied their discovery to the design of a device for controlling laser pulse duration. Saturable absorption is a nonlinear optical effect where the absorption coefficient decreases with the increasing power of incident light. Thus, the material becomes more transparent under intense laser radiation. “We showed that [the] magnitude of the nonlinear transparency can be controlled by placing the material in an electrochemical cell,” researcher Yuriy Gladush said. It is known that nanotubes can accumulate a considerable amount of electrical charge on their surface when they are placed in an electrochemical cell. According to Gladush, what scientists have not known, until now, is that the charge accumulation leads to a significant change in the material’s nonlinear optical response and, in particular, to a reduction in saturable absorption. A drop of electrolyte on a transparent film of carbon nanotubes. Courtesy of Skoltech. One broad use for saturable absorption is in laser systems to generate femtosecond light pulses, where a saturable absorber with predetermined parameters is placed in the laser cavity. When the team began to investigate potential applications for a material with a controlled nonlinear response, they surmised that by adjusting the material’s nonlinear response, they could control the laser generation regime. The researchers built an electrochemical cell with carbon nanotubes placed on the optical fiber surface and integrated the cell into a fiber optic laser cavity. “We discovered that by applying voltage to the device, one can switch from [a] continuous laser generation regime to [a] pulsed regime in the femtosecond and microsecond ranges,” professor Albert Nasibulin said. The researchers used a pump-probe technique to show that under an applied voltage below 2 V the photobleaching of the material could be gradually reduced and even turned to photoinduced absorption. They showed that the pulse generation regime could be reversibly switched between femtosecond mode-locking and microsecond Q-switching using different gate voltages. The discovery could advance the development of nanotube optical devices with tunable nonlinearity. “This invention paves the way for universal laser systems with a controllable pulse duration that can be used in laser processing of materials, laser surgery, and aesthetic medicine,” Nasibulin said. Skoltech scientists worked with colleagues from the Fiber Optics Research Center of the Russian Academy of Sciences, Novosibirsk State University, and the University of Warwick on this research. The research was published in Nano Letters (https://doi.org/10.1021/acs.nanolett.9b01012).