Ultrafast excitation of nonlinear optical absorption and refraction in dual-emitting carbon dots at visible and NIR wavelengths

Nonlinear optical properties of nanomaterials have attracted significant attention due to their critical role in advancing photonics, optoelectronics, and laser technologies. Carbon dots, a novel class of carbon-based nanomaterials, exhibit exceptional nonlinear optical behaviour, attributed to their unique structure and versatile surface functionalities. Unlike traditional semiconductors, carbon dots offer excellent photostability, tunable emission, environmentally friendly characteristics, and good biocompatibility, making them promising candidates for nonlinear optical studies. In this paper, we investigate the third-order nonlinear optical response of solution-dispersed dual-emitting carbon dots under ultrafast laser excitation at visible and near-infrared wavelengths using the single-beam z-scan technique. At an excitation wavelength of 515 nm, the dual-emitting carbon dots exhibit saturable absorption at low pulse energies and a combination of saturable absorption and two-photon absorption at higher pulse energies. Conversely, at an excitation wavelength of 1030 nm, the nonlinear optical response is dominated by two-photon absorption. Additionally, studies on the nonlinear refractive properties revealed that the sample exhibited positive nonlinear refraction (self-focusing) at both excitation wavelengths. Furthermore, we show that the nonlinear absorptive and refractive properties of these carbon dots are strongly solvent-dependent, with the strength of the nonlinear processes varying significantly based on the solvent polarity. These findings highlight the potential of synthesised dual-emitting carbon dots for broadband and tunable nonlinear optical applications. Since the same carbon dots behave differently with different dispersing solvents, they can be used as saturable absorbers, ultrafast optical limiters, and contrast agents for various bioimaging technologies.