For the first time, with the use of I-scan and z-scan techniques, the five-photon absorption cross-section in a CaF 2 sample has been measured to be σ 5 = 5 × 10 −148 cm 10 s 4 at 473 nm. The applicability of the technique and the contribution to the measured absorption of an avalanche and tunnel ionization are discussed.
Transmission versus intensity in 100 μm CaF2 and UV fused silica samples at a wavelength of 473 nm for different durations of the initial pulse was studied. Fitting the experimental data with a model based on the solution of the nonlinear Schrödinger equation and taking into account multiphoton absorption and absorption by the plasma, as well as the Kerr nonlinearity, diffraction and dispersion of the medium, made it possible to obtain the values of multiphoton (k-number of photons) σk
and plasma absorption (inverse bremsstrahlung) σ cross sections. The obtained values are σ
4 = 2.5 × 10−117 cm8 s3, σ = 3.6 × 10−18 cm2 for CaF2 and σ
4 = 4.4 × 10−116 cm8 s3, σ = 3 × 10−18 cm2 for UV fused silica.
Broadband frequency downconversion of a 90 fs 744 nm Ti:sapphire laser pulse into the mid-infrared (IR) was demonstrated via its filamentation-induced self-frequency shift in air and subsequent intra-pulse difference frequency generation in a
L
i
G
a
S
2
crystal. The filamentation of the laser pulse in air provided its continuous spectral broadening to the Stokes wing with spectral humps separated by
∼
1000
c
m
−
1
that was appropriate for the laser pulse difference frequency conversion into the mid-IR. The difference frequency emission spectrum spanned from 8.5 to 13.5 µm at the
e
−
2
level. The transform limited pulse duration of the mid-IR pulse was 47 fs, which corresponded to a 1.3-cycle laser pulse. Energy conversion efficiency was up to
10
−
4
and
5
⋅
10
−
4
without and with chirp compensation, respectively.
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