Phase transformations of crystalline Ge 2 Sb 2 Te 5 films upon pulsed laser irradiation have been studied using in situ reflectivity measurements with temporal resolution. Two different configurations allowed point probing with nanosecond temporal resolution and imaging with subpicosecond temporal and micrometer spatial resolution. The role of the pulse duration and laser fluence on the dynamics of the phase change and the degree of amorphization is discussed. Several advantageous features of femtosecond compared to nanosecond laser-induced amorphization are identified. Moreover, a high-resolution study of the amorphization dynamics reveals the onset of amorphization at moderate fluences to occur within ϳ100 ps after arrival of the laser pulse. At high fluences, amorphization occurs after ϳ430 ps and the molten phase is characterized by an anomalously low reflectivity value, indicative of a state of extreme supercooling.

Spectroscopic ellipsometry together with an effective medium model is used to determine simultaneously the effective refractive index, thickness, and metal volume fraction of thin nanocomposite films. The films are formed by Bi nanocrystals embedded in amorphous matrices, either semiconducting ͑Ge͒ or dielectric (Al 2 O 3 ). For the Bi:Ge films ͑metal in an absorbing host͒, the values obtained for both the real and imaginary parts of the refractive index vary continuously from that of Ge to that of Bi. The metal contents determined from the ellipsometry analysis are in excellent agreement with those obtained from direct measurements of the composition. For the Bi:Al 2 O 3 films ͑metal in a nonabsorbing host͒, the extinction coefficient (k) exhibits a maximum around 360 nm which is related to the metal plasmon resonance frequency of Bi nanocrystals. The metal content determined from the ellipsometry analysis in this case is underestimated, probably due to interaction of the Bi crystals with the Al 2 O 3 host.

Bismuth films are grown at room temperature onto Si substrates by pulsed laser deposition ͑PLD͒. Some films are deposited by dc sputtering ͑DCS͒ for comparison. PLD films are smooth and show micron-sized laminar grains as opposed to DCS films which are rough and show nanometer-sized columnar grains. The optical properties of the films have been studied by means of spectroscopic ellipsometry in the 300-800 nm wavelength interval over a period of six months. The results show that the refractive index of PLD Bi films is independent of the film thickness, thus providing reliable data of the optical constants of Bi films. In addition, it is shown that PLD films exhibit an improved stability to oxidation probably related to their favorable microstructural properties. The thickness dependence of the refractive index in DCS films is related to their surface roughness, the results being improved by pulsed laser melting of these films.

Abstract:The polarization characteristics of unpolarized light passing through a double wedge depolarizer are studied. It is found that the degree of polarization of the radiation propagating after the depolarizer is uniform across transverse planes after the depolarizer, but it changes from one plane to another in a periodic way giving, at different distances, unpolarized, partially polarized, or even perfectly polarized light. An experiment is performed to confirm this result. Measured values of the Stokes parameters and of the degree of polarization are in complete agreement with the theoretical predictions.

Time-resolved measurements of the spectral dielectric function reveal new information about ultrafast phase transitions induced by femtosecond laser pulses in Sb-rich amorphous GeSb films. The excitation generates a nonthermal phase within 200 fs. The dielectric function of this phase differs from that of the crystalline phase, contrary to previous suggestions of a disorder-to-order transition. The observed dielectric function is close to that of the liquid phase, indicating an ultrafast transition from the amorphous phase to a different disordered state.