Nonlinear optical properties of SnSe₂ quantum dot composite gel glass

Objective Quantum dots derived from typical two-dimensional layered materials have attracted widespread attention due to their unique optoelectronic properties. Compared with two-dimensional layered nanomaterials, quantum dots generally have better photochemical stability, higher solubility, and enhanced nonlinear optical (NLO) effects. However, most of the reported NLO performance of quantum dots prepared based on two-dimensional layered materials focused on liquid-phase matrices. Although liquid matrix can quickly restore quantum dots after laser excitation, providing convenience for the study of nonlinear optical properties and mechanisms, which is not conducive to the material and device development of nonlinear optical materials. In addition, prolonged exposure of quantum dot suspensions to air inevitably leads to aggregation and structural damage, resulting in a decrease in their NLO performance. Therefore, if quantum dots can be introduced into the solid matrix, they can effectively prevent aggregation and isolate contact with air, thus effectively solving the above problems. In this paper, the two-dimensional layered tin diselenide (SnSe₂) crystal is used as raw material, and SnSe₂ quantum dots with uniform size and homogeneous dispersion were synthesized using liquid phase exfoliation method. Subsequently, the resulted SnSe₂ quantum dots were introduced into the silica gel glass matrix with good physical and chemical properties and optical transparency by sol-gel wet chemical technology, and obtained SnSe₂ quantum dot composite gel glass. And the potential application of the SnSe₂ quantum dot composite gel glass in the field of nonlinear optics was explored.

Methods The morphology, composition, structure and linear optical properties of the SnSe₂ quantum dot composite gel glass were systematically characterized by transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), Raman spectroscopy and ultraviolet visible absorption spectroscopy (UV/Vis). The nonlinear optical properties of SnSe₂ quantum dot composite gel glass under nanosecond and picosecond laser pulses were studied by open-apertures Z-scan technique. In addition, the ultrafast carrier dynamics of SnSe₂ quantum dot composite gel glass was explored by pump-probe technique and transient absorption spectroscopy. The NLO parameters and relaxation time of SnSe₂ quantum dot composite gel glass were deduced using Eq.(2)-(6) and Eq.(7), respectively.

Results and Discussions SnSe₂ quantum dots with uniform size and homogeneous dispersion were successfully introduced into silica gel glass by sol-gel method (Fig.3(c) and Fig.4), and the resulted composite gel glass with uniform doping and high transmittance (Fig.3(a)). Interestingly, when the laser pulse width changes from picosecond to nanosecond, the NLO absorption of SnSe₂ quantum dots composite gel glass changes from saturated absorption (SA) to reverse saturated absorption (RSA) (Fig.6), and the SA performance of SnSe₂ quantum dots in gel glass matrix under picosecond and RSA response under nanosecond is improved as compared to SnSe₂ quantum dots suspension (Fig.7). By experimentally fitting the obtained kinetic curve with a double exponential decay function, two different time constants of carrier relaxation processes were obtained. The relaxation time of τ₁ and τ₂ are 4.9-28.66 ps and 288.64-726.28 ps, respectively, in the wavelength of 500-640 nm (Fig.8(d)). The rapid recovery process ( τ₁) may be due to electron relaxation in the conduction band, while the slow recovery process (τ₂) may come from the recombination of electron hole pairs. The NLO parameters (Tab.1) and relaxation time (Tab.2) of SnSe₂ quantum dot composite gel glass are comparable to that of reported low-dimensional materials, confirming the prepared composite gel glass are a promising material for use in photonic devices.

Conclusions SnSe₂ quantum dots with uniform size and uniform dispersion were introduced into silica gel glass by sol-gel method, and composite gel glass with uniform doping and high transmittance was prepared. And the morphology, composition, structure, and linear optical properties of the synthesized composite gel glass were systematically characterized. SEM, EDX and Raman spectra results confirmed that SnSe₂ quantum dots have been successfully introduced into gel glass matrix. The nonlinear optical effects of SnSe₂ quantum dot composite gel glass with ps and ns pulse widths were investigated under 532 nm laser pulses. The results show that when the laser pulse width changes from ps to ns, the nonlinear absorption of SnSe₂ quantum dot composite gel glass undergoes a transition from SA to RSA. The mechanisms of SA and RSA were also explained in detail. In addition, compared with SnSe₂ quantum dot suspension, the SA and RSA response enhance after incorporating into solid state matrix. The mechanism of enhancing nonlinear optical effects is also explained by the matrix effect. The ultrafast carrier dynamics of SnSe₂ quantum dot composite gel glass was further studied by pump probe technique and transient absorption spectroscopy. The calculated relaxation time is equivalent to that of reported low-dimensional materials, which proves that SnSe₂ quantum dot composite gel glass shows great potential application prospects in the field of ultrafast optoelectronics. Our work would provide theoretical and material basis for the preparation of novel composite nonlinear optical materials with excellent performance.