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Energy level diagram and interaction scheme for coherent anti-Stokes Raman scattering process with spectrally shaped excitation fields.
Experimental 2D CARS spectrogram (log scale), obtained by measuring the spectrum of anti-Stokes pulses (horizontal axis), while scanning the probe pulse delay (vertical axis). -step phase shaping has been applied to the probe’s spectrum, resulting in sharp vertical lines around the positions of Raman resonances (782, 1000, and ). These sharp interference fringes serve as “frequency markers” for the XFROG retrieval algorithm.
Experimental setup. Ti:sapphire regenerative amplifier generates 40 fs, 3 mJ pulses at 1 kHz repetition rate. Pump beam is spectrally narrowed down to . Two OPAs are used to produce Stokes and probe pulses at 862 and 720 nm, respectively. Both OPA beams are shaped by means of the pulse shapers (PSs), as described in the text. Pump, Stokes, and probe pulses are overlapped in BOXCARS geometry and focused into cuvette with liquid toluene. CARS signal is coupled into spectrometer and detected by cooled CCD camera.
Amplitude (thin) and phase (thick) of the spectral shaping applied to Stokes excitation field. In the time domain, such shaping results in a train of two phase-locked Stokes pulses. Raman modes of toluene at 782, 1000, and are indicated by dashed lines.
Amplitude (thin) and phase (thick) of the vibrational response of Toluene in the frequency (a) and time (b) domains. Two amplified Raman lines around 1000 and are clearly visible in (a), while the line around indicated by vertical dashed marker, is suppressed. Plot (b) shows two strong excitation peaks, corresponding to the pair of Stokes pulses. First pulse at −0.8 ps excites all three Raman lines, and is followed by the fast beating at the frequency difference between 1000 and . Second pulse at 0.8 ps de-excites Raman vibration at , and is followed by the low frequency beating between the modes at 1000 and .
Calculated 2D CARS spectrograms with high temporal (a) and spectral (b) resolution. The spectrograms were calculated by numerically convolving experimentally retrieved vibrational response (Fig. 5) with a model 35 fs (a) and 1 ps (b) reference pulse [Eq. (3)].
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