Déformation photoinduite dans les films minces contenant des ...

Chapter 2. Experimental setup and sample preparation 36frequency of 10 µV . The two structures at the resonant frequencies of the piezotube and of the fiber tip are present, with a wider amplitude for the first structure.(e) Feedback electronics on - closed loop The (x, y, x) piezo tube is powered andthe feedback electronics is input a DC signal that simulates the shear-force signal.In this condition the average noise amplitude is moderately wider, the peak correspondingto the piezo tube second harmonic is more evident. However, the noiseamplitude remains approximately of 10 µV at the working frequency.2.1.4 Scanning Near-field Optical Microscope (SNOM)pastel-00527388, version 1 - 19 Oct 2010The photo-activation of our samples is achieved by means of two coherent blue 5 laserbeams, that we will call the interfering beams, whose wavelength is close to the sampleabsorption peak 6 , and of a third incoherent homogeneous green 7 laser beam, that wewill call the assisting beam, whose wavelength corresponds to an absorption 30% weaker.These are combined in several different ways, obtaining a variety of local intensity andpolarization geometries and dynamics.At the sample surface, we measure the transmitted optical field by a scanning near-fieldoptical microscope. We use a tapered optical fiber tip that has the double function ofdiffracting the local electromagnetic field (aperture-less SNOM) and also of collectingthe light through its small aperture of ≃ 50 nm (aperture SNOM). The near-field imageis measured by means of a photomultiplier and recorded using a Labview TM routine,then stored into a 256 × 256 matrix in a binary file, as previously described for theshear-force microscopy.2.1.4.1 Optical setupThe optical experimental setup is depicted in Figure 2.9(a). It is constituted by twooptical paths.The first one leads the green assisting laser beam, through a linearpolarizer, in normal incidence on the surface, by means of two mirrors. The laser outputpolarization is ≃ 40 ◦ with respect to the y-axis, so a linear polarizer is sufficient to obtains or p linear polarizations. The second path leads the blue laser out-front beam througha half-wave plate and a beam-splitter, which separates the beam into two beams, ofsame intensity. These two beams recombine on the sample surface, with an incidenceangle θ = 16.5 ◦ . A polarizer on each of these beam paths define the orientation of thelight polarization. The blue laser out-front beam is initially linearly polarized along the5 λ = 473 nm.6 See Figure 2.18.7 λ a = 532 nm.