Soft Report - Dipartimento di Fisica - Sapienza

FacilitiesBrillouin Light Scattering Laboratory – UdR RomaCCD based Brillouin spectrometer DMDP2000. In this set up the incoming laser beam, 500 mW at 5514.5nm, from a Coherent INNOVA Ar-ion laser operating in single mode owing to an intracavity ethalon and inpower stabilized mode, is focused onto the sample. The polarization of the incident beam can be either verticalor horizontal with respect to the scattering plane. The light, scattered at an average angle of 90° or 180°, iscollected by a field lens (20 cm focal length, 4 cm diameter). After selecting the polarization by a polaroid film,the scattered light is focused on the entrance slit of the SOPRA DMDP2000 mono-chromator. The latter iscomposed of a couple of two meter focal length grating mono chromators, in Fastie–Ebert mounting, each onewith entrance and exit slits and coupled in additive dispersion by an external 1:1 telescope. Themonochromator can opearte in the single pass/double monochromator or double pass/double monochromatorconfigurations at the 11th diffraction order, corresponding to a transmissions of 25% (4%) and a totaldispersing power on the exit slit plane equal to 1 cm-1/mm (0.5 cm-1/mm). In a conventional experiment allthe slits are closed according to the desired resolution width, a photomultiplier is placed in front of the exit slit,and frequency scans of scattered light are obtained by simultaneous rotation of the two gratings. In a CCDbased operating mode the two intermediate slits and the exit slit of the monochromator are fully open and theimage formed in the final plane slit is focused via of a singleachromat doublet (focal length 75 mm, magnification 1:10) onthe surface of a CCD camera. The camera is a HiResIII camera(DTA, Pisa, Italy) mounting a SITe 11003330 pixel (totaldimension 2 mm x 0.6 mm) back-illuminated CCD sensor(quantum efficiency of 80%) cooled via a Peltier element and afluid close circuit system based on an HAAKE 75 refrigerator. Inthis setup, the spectral range covered by the CCD sensor is 2cm-1, i.e., 0.002 cm-1/pixel, that correspond to ten points oneach spectral resolution as dictated by the width of themonochromator entrance slit. The image of the camera istransferred to a PC, where the BLS spectrum is obtained byintegrating the image in vertical (nondispersing) dimension.Figure: Three-dimensional image of the intensity measured bythe CCD detector (exposure time 10 s) on a sample of toluene.Time Resolved Spectroscopy Laboratory – UdR RomaImpulsive Stimulated Scattering (ISTS). Twointerfering pulsed laser beams interfere in the samplevolume stimulating a density grating whose time evolutionis probed by monitoring the intensity of a reflected cwprobe beam. Transient grating dynamics providesinformations on the non-equilibrium thermodynamics andrelaxation processes in complex fluids. Our setup consistsof an infrared mode-locked pulsed laser (100 ps pulses of1064 nm wavelength with a 10 Hz repetition rate) actingas the pump and diode-pumped intracavity-doubled Nd-YVO (CW 532 nm) providing the probe beam. A dichroicmirror is used to sent colinearly both the excitation andthe probing beams on a phase grating. The diffractedbeams are collected and refocused on the sample by twoachromatic doublets. One of the two probe beams isattenuated and adjusted in phase to serve as a referencelocal field for heterodyne detection. The reflected portionof the other probe beam is mixed with the local field on theactive area of an ultrafast amplified photodiode whosevoltage output is recorded by a 4 GHz oscilloscope. Therelaxation of a sinusoidal density fluctuation can be somonitored on a nanoscond to millisecond time windowgiving access to a broad range of relaxation phenomena(acoustic waves, structural relaxation in supercooled fluids,thermal diffusion).SOFT Scientific Report 2004-0622