Exceptional ultrafast nonlinear optical response of functionalized silicon nanosheets

The present work reports on the ultrafast saturable absorption (SA), optical limiting (OL), and the nonlinear refractive response of hydride-terminated silicon nanosheets (SiNS-H) differently functionalized with styrene and tert-butyl methacrylate (tBuMA), namely, SiNS-styrene and SiNS-tBuMA, using 50 fs, 400 nm and 70 fs, 800 nm laser pulses. SiNS-styrene and SiNS-tBuMA exhibit dramatically enhanced nonlinear optical (NLO) responses compared to SiNS-H, with their absorptive nonlinearity strongly dependent on the laser excitation wavelength. More specifically, the studied functionalized SiNSs reveal strong SA behavior under 400 nm laser excitation, with NLO absorption coefficients, saturable intensities, and modulation depths comparable to various two-dimensional (2D) materials, known to exhibit strong SA, such as graphene, black phosphorous (BP), some transition metal dichalcogenides (TMDs), and some MXenes. On the other hand, under 800 nm laser excitation, SiNS-styrene and SiNS-tBuMA show highly efficient OL performance with OL onset values of about 0.0045 and 0.0065 J cm^-2, respectively, which are significantly lower than those of other 2D nanostructures. In addition, it is shown that both SiNS samples have great potential in already existing Si-based optoelectronic devices for optical-switching applications since they exhibit very strong NLO refraction comparable to that of bulk Si. The results of the present work demonstrate that the chemical functionalization of SiNSs provides a highly efficient strategy for the preparation of 2D Si-based nanostructures with enhanced NLO response in view of several optoelectronic and photonic applications, such as OL, SA, and all-optical switching.