Cross-correlation frequency-resolved optical gating coherent anti-Stokes Raman scattering with frequency-converting photonic-crystal fibers

Photonic-crystal fibers provide a high efficiency of frequency upconversion of regeneratively amplified femtosecond pulses of a Cr: forsterite laser, permitting the generation of subpicosecond anti-Stokes pulses with a smooth temporal envelope and a linear positive chirp, ideally suited for femtosecond coherent nonlinear spectroscopy. These pulses from a photonic-crystal fiber were cross correlated in our experiments with the femtosecond second-harmonic output of the Cr: forsterite laser in toluene solution, used as a test object, in boxcars geometry to measure the spectra of coherent anti-Stokes Raman scattering (CARS) of toluene molecules (XFROG CARS).

Figure 1

Diagram of femtosecond CARS spectroscopy with the use of ultrashort pulses frequency-upconverted and chirped in a photonic-crystal fiber. The inset shows the cross-section view of the central part of the photonic-crystal fiber.

Figure 2

The spectrum of the anti-Stokes signal generated in a photonic-crystal fiber by $1.24\text{−}\mu \mathrm{m}$ $90\text{−}\mathrm{fs}$ Cr: forsterite-laser pulses with an input energy of (solid line) $170\mathrm{nJ}$, (dashed line) $220\mathrm{nJ}$, and (dash--dotted line) $270\mathrm{nJ}$. The insets show (1) the intensity of the sum-frequency signal generated in a BBO crystal by the second-harmonic pulse from the Cr: forsterite laser and the anti-Stokes pulse from the photonic-crystal fiber as a function of the wavelength and the delay time $\tau$ between the second-harmonic and anti-Stokes pulses, (2) theoretical fit of the XFROG trace, and (3) pulse envelope and the phase of the anti-Stokes pulse providing the best fit.

Figure 3

(a) A diagram of femtosecond CARS spectroscopy with chirped pulses is shown on the left. The first pulse (frequency ${\omega }_1$) is transform limited. The second pulse (frequency ${\omega }_2$) is linearly chirped. The linear chirp of the second pulse maps the delay time between the pulses on the frequency, allowing the frequency difference ${\omega }_1-{\omega }_2$ to be scanned through Raman resonances $\Omega$ by tuning the delay time $\tau$. (b) The intensity of the CARS signal generated in a noncoplanar boxcars geometry from toluene solution as a function of the wavelength and the delay time $\tau$ between the second-harmonic and anti-Stokes pulses used as a biharmonic pump. (c) A model of the Raman spectrum, including a doublet of Lorentzian lines and a frequency-independent nonresonant background, used in theoretical fit. (d) Theoretical fit of the XFROG CARS spectrogram.