Saturable absorption of silver nanoparticles in glass for femtosecond laser pulses at 400 nm

The commercially available glass BK7 was chosen to prepare silver-rich layer within the glass by the silver (Ag) ion exchange method in order to enhance its nonlinear optical properties. The glass substrates were doped with Ag at 320 degrees C by a 50-minute Ag+ double left right arrow Na+ ion exchange in a melt of NaNO3 and KNO3 containing 14 mol% of AgNO3. Then, one piece of glass BK7 containing Ag ions (as-exchanged sample) was annealed in air for 1 h at 600 degrees C to prepare the as-annealed sample. The thickness of the layer containing Ag in the as-exchanged glass BK7 was measured to less than 1 mu m. It was found that during the post-exchanged annealing most silver diffused deeply into the glass substrate leading to increase the thickness of the layer to 110 mu m. However, some amount of Ag nucleated during the annealing, thus creating small nanoparticles (NPs) with an average size of 2 nm. The linear and nonlinear optical properties of the as-exchanged and as-annealed glass BK7 were studied. The as-exchanged sample showed no absorption in the visible spectral region whereas, as-annealed glass BK7 showed a peak absorption at 420 nm which can be attributed to the presence of Ag NPs formed in this glass. The nonlinear optical behavior of the samples was investigated using 800 nm and 400 nm femtosecond pulses. Strong saturable absorption was observed in the as-annealed sample at 400 nm. This feature promises an efficient way for designing all optical switching devices appropriate for the second harmonic of the Ti-sapphire laser. Using nonlinear transmission (NLT) method the saturation intensity for the as-annealed sample was determined 8.5 x 10(11) W.cm(+2). An intensity dependent switching from saturable absorption (SA) to reverse saturable absorption (RSA) was also observed when the as-annealed sample was irradiated simultaneously by 800 nm and 400 nm femtosecond pulses. (C) 2015 Elsevier B.V. All rights reserved.