Source: https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-2-1226
Timestamp: 2019-04-21 20:12:29+00:00

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The spectral phase shift of broadband amplified pulses, induced by population inversion, was measured in Ti:Sapphire at different pump fluence values. The measurement was performed for two orthogonal polarization directions and at two different crystal temperatures of 296 K and 30 K. Zero shifts and sign changes were observed in the spectral phase, which are connected to the gain spectrum of the crystal. The electronic refractive index changes were also numerically calculated by the Kramers-Kronig theory. The results are highly important for achieving sub-10 fs pulse duration and phase stability in the next generation of Ti:Sapphire-based laser systems.
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Fig. 1 Schematic layout of the experimental setup, highlighting the Jamin interferometer with more details in the bottom left inset. I1, I2 and I3 are irises, PH is a pinhole, Sa is a sapphire plate, FS is a fused silica plate, SF + SPM are spatial filtering and self-phase modulation, GP’s are Glan polarizers, HWP is a broadband halfwave plate, BSP1 and BSP2 are beam splitter plates, D is a beam damper, VC is the vacuum chamber for cryogenic environment, SaC is a sapphire crystal, FW is reflective neutral density filter wheel, SM is spectrometer, while W1 and W2 are vacuum windows.
Fig. 2 Typical spectral interference fringe (green) and spectral phase error of a single measurement.
Fig. 3 Electronic phase shifts for different pump fluence values for π- (a) and σ-polarized (b) pulses at 296 K temperature. Color coding refers to the pump fluence in J/cm2.
Fig. 4 Electronic phase shifts for different pump fluence values for π- (a) and σ-polarized (b) pulses at 30 K temperature. Color coding refers to the pump fluence in J/cm2, which is identical for the two polarizations.
Fig. 5 Electronic RIC in the function of the inverted population density for π- (a,c) and σ-polarized (b,d) pulses at room and 30 K temperatures, respectively. The inversion density range corresponds to absorbed pump fluence values from 0 to 2 J/cm2.
Fig. 6 Slope of linear fit to the RICs along the inverted population density in the investigated spectral range, for different polarizations and temperatures.
Fig. 7 The cross-sections for emissions σ em π,σ minus the exited state absorption cross sections σ abs ESA,π,σ (solid curves, blue for π-polarized transitions and red for σ- polarized transitions); and the cross-sections for the ground-state absorptions σ ab gr,π,σ (dashed curves, blue for π-polarized transitions and red for σ- polarized transitions). These cross-sections were used for numerical calculations of the electronic RICs.
Fig. 8 The numerically calculated parameter Cπ,σ in the investigated spectral range for room temperature, where KK refers to the Kramers-Kronig theory.

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