Nanotubes Control Laser Pulses

A drop of electrolyte on a transparent film of carbon nanotubes. (Source: Skoltech)

An inter­national team of scientists led by researchers from the Laboratory of Nano­materials at the Skoltech Center for Photonics and Quantum Materials (CPQM) has shown that the nonlinear optical response of carbon nanotubes can be controlled by electro­chemical gating. This approach enabled designing a device for controlling the laser pulse duration.

Optical phenomena that we encounter in our everyday life, such as reflection, refraction or absorption of light, do not depend on the intensity of incident light. However, at very high radiation inten­sities, a new class of phenomena arises, that causes changes in the refraction index, self-focusing of light or emergence of radiation at new wave­lengths. These and other phenomena that are dependent on the intensity of light are studied by a section of physics called nonlinear optics. Normally, the efficiency of nonlinear optical response is material’s invariable charac­teristic determined by its structure.

Using nano­materials as an optical nonlinear medium opens up new perspectives for nonlinearity control thanks to the fact that the majority of its atoms are exposed to the surface. This enables controlling a material’s electronic structure and thus changing its nonlinear optical response. Skoltech scientists in colla­boration with their colleagues from the Fiber Optics Research Center of RAS, Novo­sibirsk State Uni­versity and the University of Warwick (UK) have proposed a method for controlling the saturable absorption of carbon nanotubes using electro­chemical gating.

Saturable absorption is a nonlinear optical effect when the absorption coefficient decreases with increasing power of incident light. Thus, the material gets more transparent under intense laser radiation. “We showed that magnitude of the nonlinear trans­parency can be controlled by placing the material in an electro­chemical cell. It has been known that, if placed in the electro­chemical cell, nanotubes can accumulate a considerable amount of electrical charge on their surface. What has not been known thus far is that the charge accu­mulation leads to a signi­ficant change in the material’s nonlinear optical response and, in particular, a reduction in saturable absorption,” says Yuriy Gladush.

Also, the scientists have looked into one of the potential practical appli­cations of a material with a controlled nonlinear response. Saturable absorption is widely used in laser systems to generate femto­second light pulses. All you have to do is place a saturable absorber with given parameters in the laser cavity. “We assumed that the laser generation regime can be controlled by adjusting the material’s nonlinear response. To do so, we built an electro­chemical cell with carbon nano­tubes placed on the optical fiber surface and integrated it into the fiber optic laser cavity. We discovered that by applying voltage to the device, one can switch from continuous laser generation regime to pulsed regime in the femto­second and microsecond ranges. 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,” explains Albert Nasibulin, Head of Skoltech’s Laboratory of Nano­materials. (Source: Skoltech)

Reference: Y. Gladush et al.: Ionic Liquid Gated Carbon Nanotube Saturable Absorber for Switchable Pulse Generation, Nano Lett. 19, 5836 (2019); DOI: 10.1021/acs.nanolett.9b01012

Link: Skolkovo Institute of Science and Technology, Moscow, Russia

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