Educação matemática pela arte
Gusmão, Lucimar Donizete
2013-08-28
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Comment: Submitted December 28, 2006
The dynamics of the light-induced, reflectivity switching at a silica–gallium interface prepared by ultrafast pulsed laser deposition, is studied for the first time in the blue–green spectral region. A considerable interface reflectivity increase is seen at fluencies of about 1 mJ/cm2. The effect peaks at temperatures just below the gallium melting point (29.8 °C), while the reflectivity recovery times increase critically.
The gallium/silica interface optical nonlinearity associated with a light-induced structural phase transition from alpha-gallium to a more reflective, more metallic phase shows an exceptionally broadband spectral response. It allows 40% deep nanosecond/microsecond cross-wavelength intensity modulation between signals at 1.3 and 1.55µm.
Transient pump–probe measurements of circular anisotropy in nickel films induced by 38-fs optical pulses show an instantaneous response that is related to the optical orientation of the spins of free electrons. Measurements in a sample of variable thickness, performed in both transmission and reflection, revealed that the surface significantly influences the degenerate cubic optical nonlinearity of the nickel films to a depth of approximately 4–5 nm into the bulk
A new technique for UV direct writing of Bragg gratings in planar silica is presented. In this method the Bragg gratings and the channels are defined simultaneously, conferring advantages in flexibility of design and grating performance. Photosensitive germanium-doped silica-on-silicon produced by flame hydrolysis deposition was used.
The reflectivity of a gallium/silica interface formed on an optical flat or at the tip of a cleaved optical fiber can be reduced in a reversible fashion when the interface is excited by a few milliwatts of laser power. This phenomenon occurs at temperatures just below gallium's melting point. We believe that the effect can be attributed to light-induced structuring at the interface.
Transient pump-probe optical reflectivity measurements of the nano- to microsecond dynamics of a fully reversible, light-induced, surface-assisted metallization of gallium interfaced with silica are reported. The metallization leads to a considerable increase in the interface's reflectivity when solid α-gallium is on the verge of melting. The reflectivity change was found to be a cumulative effect that grows with light intensity and pulse duration. The reflectivity relaxes back to that of α-gallium when the excitation is withdrawn in a time that increases critically at gallium's melting point. It is shown that thermal processes cannot account for the effect and so a mechanism based on a nonthermal light-induced structural phase transition is proposed.
We use the Direct Grating Writing technique, based on Direct UV writing, to define planar Bragg channel waveguides and accurately probe the refractive index to characterise thermal waveguide erasure and material properties.
We discovered a new type of light-induced structural surface assisted phase transition, which occurs in the vicinity, but below the bulk melting point (29.6C) of metallic alpha-gallium interfacing with a dielectric such as silica or glass. The transition is not a thermal effect, but is provoked by an optical destabilisation of the covalent bonding structure of alpha-gallium. The transition is to a more reflective, metastable 'metallic' like phase. The light penetrates only about 25 nm in gallium, with only several tens of atomic layers of the metal being affected by the phase transition. This is enough to change the interface reflectivity significantly, on several tens of percent. The light-induced phase has not been precisely identified yet, but several metastable phases of crystalline gallium and quasi liquid state are potential cand...
