We had proved that a kind of graphene nanomaterial “carboxyl-functionalized graphene oxide (GO-COOH)” possessed nonlinear saturable absorber (SA) property. The modulation depth of a GO-COOH water solution was measured as ∼8%. Moreover, a GO-COOH-based SA device was made and applied in an erbium-doped fiber laser. In this fiber laser, Q-switching pulses and mode-locked pulses were both obtained. With an increase in the pump power, the pulse width of Q-switching pulses decreased from 9.05 to 2.49 μs. The mode-locked pulse width was 844 fs, and the corresponding spectral bandwidth was 3.64 nm. Moreover, polarization adjusting or control was not needed during the whole process of mode locking. It illustrated that the proposed fiber laser incorporating GO-COOH could endure bigger intracavity birefringence. Our results indicated that the GO-COOH nanomaterial was a promising SA for generating high-performance pulse lasers.
The negative refractive index characteristics of one-dimensional photonic crystal consisted by Fibonacci multi-layer films has been studied by numerical method. The refractive indices for two materials, which are used to construct the Fibonacci multi-layer films, are 0.920 and 0.999, respectively. The calculation result shows that, on one hand, there are several negative refractive index zones for this kind of photonic crystal even if the refractive indices are very small; on the other hand, the difference is very large for the frequency between the zones. As an example, a kind of transmission-type plano-concave lens is designed. The simulation of the electromagnetic field distribution for the lens demonstrates that the lens can focus the incoming X-ray radiation. At the same time, the calculation of the absorption strength and refractive indices for real materials shows that not only there are large differences for the absorption strength with different materials, but also the refractive indices for real materials are different in X-ray band. Obviously, the characteristics above support a kind of transmission-type lens using in X-ray.