We use time-resolved degenerate four-wavemixing (DFWM) with
femtosecond pulses in the wavelength
range 0.74−1.7 μm to measure both phase and amplitude of all
nonvanishing elements of the electronic
third-order nonlinear optical susceptibility tensor
c
ijkl
(−ω,ω,ω,−ω) of
a 10 μm amorphous C60 film on a
CaF2 substrate. Linear absorption is found to be less
than 1% in this range. We find a single resonance in
DFWM, the amplitudes and phases of which are fit well by a Lorentzian
model of a two-photon resonance
to a level 2.7 ± 0.1 eV above the ground level, with width 0.25 eV.
The peak two-photon absorption coefficient
is 0.02 cm/MW, essentially the same peak value as for bulk gallium
arsenide, one of the strongest and most
widely studied of the two-photon absorbers. Our results show there
is only one two-photon allowed transition
below 3.4 eV (as well as below the first one-photon transition), an
unambiguous signature which is expected
from theory. Theory assigns the symmetry Hg to this
lowest lying two-photon state. We see a clearly
nonresonant long-wavelength limit for the third-order optical
susceptibility tensor which is 250 ± 70 times
the known long-wavelength limit for fused quartz (our nonlinear
standard). This result is at least an order-of-magnitude larger than any of several theoretical
predictions.
Room temperature Ho:YAG and Ho:LuAG lasers pumped by a Tm:YLF laser demonstrated a 3.4 mJ threshold and 0.41 slope efficiency, incident optical to laser output energy. Results for numerous rod lengths, Ho concentrations, and output mirror reflectivities are presented. OCIS Code: (140.3580) Lasers, solid state, Summary: Ho:YAG and Ho:LuAG lasers pumped by a Tm:YLF laser were evaluated to empirically optimize the Ho concentration, laser rod length, and laser output mirror reflectivity. Ho:YAG, with concentrations of 0.005, 0.010, and 0.020 were evaluated using 4 rod lengths and 5 different output mirrors. A threshold of 3.4 mJ and a slope efficiency of 0.41 were achieved with the Ho:YAG laser at room temperature. Ho:LuAG with 0.01 concentration was also evaluated. Quoted thresholds and slope efficiencies are incident optical to laser output energy. Laser optimization is a compromise of absorption efficiency, Ho:Ho up conversion, mode volume to pumped volume overlap, lower laser level population, and losses. Although performance is calculable, uncertainty in some parameter accuracy, makes empirical optimization useful.Tm laser pumped Ho lasers are appealing because Ho has a much larger emission cross section than Tm, a complete absence of Ho:Tm up conversion, as well as the ability to store energy and efficiently deliver a single Q-switched pulse. Because the emission cross section of Tm is small, fluences required for an efficient extraction from laser amplifiers often exceed fluences associated with laser induced damage. A much larger emission cross section, makes efficient Ho laser amplifiers practical. Although Ho:Tm lasers demonstrated notable efficiency [1], they
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