Embedded Files
for more information on these topics contact: R.Torre ; torre@lens.unfi.it
Dynamic Ligth Scattering
Dynamic Ligth Scattering
We recently developed an experimental line of Dynamic Light Scattering for the measurement of Photon Correlation Imaging. This apparatus enables the detection in real time of speckle images with a temporal resolution of 1 microsecond, using an ultrafast camera characterized by a maximum frame rate of 5x10^5 frame/sec.
The Time-Domain Spectroscopy with THz pulses (TDS-THz)
The Time-Domain Spectroscopy with THz pulses (TDS-THz)
High Intensity - Low Repetition
High Intensity - Low Repetition
We generate the THz pulse using the non-linear phenomena in the plasma and ... (to be continued)
Dr. Stefano Benci working on the THz set-up present at LENS.
Low Intensity - High Repetition
Low Intensity - High Repetition
In our THz-TDS set-up the femtosecond laser pulses (about 20 fesc) generate picosecond THz pulses by the conversion in a photo-conductive antenna . The THz pulses are characterized by broad frequency band spanning from 0.1 to 5 THz ( 3.3 cm-1 to 165 cm-1). The THz detector is a second photo-coductive antenna switch which is fed with an optical pulse trigger coming from the same excitation laser source. The temporal shape of the broadband THz wave is then measured in femtosecond time-resolution by an optical delay line provided by a motorized stage. By Fourier-transformation of the time-domain data, the terahertz spectra is obtained .
An overview of the THz emission and detection mechanism in our THz-TDS is presented.
Time-resolved Optical Kerr Effect
Time-resolved Optical Kerr Effect
In a Hetorodyne Detected OKE experiment a laser pulse induces a transient birefringence in a medium by means of a non resonant and non-linear effect. The induced birefringence can be probed by a second pulse spatially superimposed within the sample with the pump pulse and having different polarization. The measurement of its polarization changes as a function of the delay time between the pulses gives information about the non-linear response of the studied material. The signal is characterized by an instantaneous electronic contribution plus a decaying contribution, which constitutes the more interesting part because it contains information about the relaxation and vibrational response of the molecules in the sample.
The time window probed in a OKE experiment can be very broad, extending from tens of femtoseconds to hundreds of picoseconds, this makes OKE a very powerful technique, capable of revealing very different dynamic regimes.
The Transient Grating experiment (TG)
The Transient Grating experiment (TG)
In a TG experiment, two infrared laser pulses, obtained dividing a single pulsed laser beam, interfere within the sample and produce an impulsive spatially periodic variation of the dielectric constant. The spatial modulation is characterized by a wave vector q which is given by the difference of the two pump wave vectors k1- k2 . The relaxation toward equilibrium of the induced modulation can be probed by measuring the Bragg scattered intensity of a second cw laser beam. The time evolution of the diffracted signal supplies information about the dynamic of the relaxing TG and, consequently, on the dynamical properties of the studied sample. TG experiments fall within the framework of the four-wave-mixing theory. The heterodyne detected TG signal measures directly and linearly the relaxation processes defined by the tensor components of the response function. By selecting different polarizations of the fields, different elements of the response function tensor are probed.
The infrared pump pulses have a 1064nm wavelength, a temporal length of 20 ps and repetition rate of 10 Hz. They are produced by an amplified regenerated oscillator (Nd-YAG EKSPLA PL2143). The typically used pump pulse energy was 5 mJ. The probing beam, instead, is a continuous-wave laser at 532nm produced by a diode-pumped intracavity-doubled Nd-YVO (Verdi-Coherent).
The two laser beams are collinearly sent to a phase grating (PG) to get the two pump pulses and the probe and local field beams. These are obtained taking the +1 and .1 diffraction orders of the infrared and green lasers. A couple of confocal achromatic lenses, AL1 and AL2, collects all the four beams and focuses them on the sample. The phase grating directly supplies a probe at the right Bragg angle and a local field exactly collinear with the scattered field and phase locked with the probe. The HD-TG signal is optically filtered and measured by a fast avalanche silicon photodiode with a bandwidth of 1 GHz (APD, Hamamatsu). The signal is then amplified by a DC-800MHz AVTECH amplifier and recorded by a digital oscilloscope with a 7GHz bandwidth and a 20 Gs/s sampling rate (Tektronix). The instrumental function of our setup has a temporal full width half maximum of 1 ns and is mainly determined by the bandwidth of the detector and its amplifier.
Page updated
Report abuse