Our 10-50 kHz amplified laser system was designed for low noise operation, and this allows us to do pump-probe experiments with excellent S/N. The fundamental output of this amplifier has a shot to shot energy fluctuation that is < 0.1% RMS, we use it to pump a TOPAS -White, to get a broadband visual pulse (200nm bandwidth, 20 fs).As a consequence also after the many non-linear conversions that are required to generate broadband UV light the noise is <<1%. The output of the TOPAS is frequency doubled using Achromatic doubling, to get a 100 nm broad UV probe pulse .
Ultrabroadband femtosecond two-dimensional ultraviolet transient absorption. G. Auboeck, C. Consani, F. van Mourik and M. Chergui. Optics Letters 37, p. 2337-2339, 2012.
Femtosecond pump/supercontinuum-probe setup with 20 kHz repetition rate. Gerald Auböck, Cristina Consani, Roberto Monni, Andrea Cannizzo, Frank van Mourik, and Majed Chergui
Rev. Sci. Instrum. 83, 093105 (2012)
Multi-dimensional coherent spectroscopy
To study ultrafast electronic coherences in molecular crystals, as well as biological systems, we developed a setup for ultra-broadband two-dimensional Fourier transform spectroscopy. The output of a 6 – 10 kHz Ti:Sa femtosecond laser is broadened by self-phase modulation inside an argon filled hollow core fiber, to produce continuum (500 – 1000 nm) laser pulses that are compressed to sub-10 fs, using a set of chirped mirrors (Ultrafast Innovations). We use the compressed laser pulses to perform photon echo spectroscopy in a passively phase stabilized BOXCARS geometry. Three excitations pulses with time delays τ and T stimulate a third order non-linear signal (the photon echo) that is emitted after a third time delay t (shown in the figure) and heterodyne detected with a local oscillator field (not shown). Fourier transformation along τ and t yields two dimensional correlation spectra of the excitation and detection frequencies at a certain population time T, allowing us to extract information that are “hidden” in classical transient absorption spectroscopy.
A. Al Haddad, A. Chauvet, J. Ojeda, C. Arrell and F. van Mourik et al. Set-up for broadband Fourier-transform multidimensional electronic spectroscopy, in Optics Letters, vol. 40, num. 3, p. 312-315, 2015.
UV/vis/NIR Femtosecond Fluorescence experiments
The lay out of the upconversion set-up is shown above. A first femtosecond pulse excites the dye molecule and sets the time t=0 of the dynamics. The fluorescence emission from the dye is collected and focussed on a non-linear BBO crystal, where it is mixed with a second femtosecond pulse at 800 nm (called the gate pulse) which arrives at a given time delay. The mixing of the fluorescence light and with the gate pulse gives rise to the sum frequency of the two, which can be imaged with a spectrograph and a CCD camera. The delay of the gate with respect to the pump pulse can be continuously tuned which allows to record the temporal evolution of the dye fluorescence.
A special feature in our set-up is the polychromatic detection scheme. At each time-delay the BBO is rotated over the range of phase-matching angles to convert the whole fluorescence spectrum which is recorded with the CCD camera. In a single time-scan we obtain the whole 2D- (spectral and temporal) evolution of the fluorescence spectrum over a spectral range of >200nm with a temporal resolution of <100 fs.
Of special interest is the combined coverage of a very wide spectral range with time-resolution we obtained in the UV spectral range
Time-resolved photoelectron spectroscopy