This paper reports measurements of the room-temperature laser transition cross section of
Nd+3 in yttrium aluminum garnet at 1.06 \ensuremath{\mu}. One method used is based on the measured upper-state lifetime and a determination of the fluorescence branching ratio for this state. The value thus determined is 3.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}19}$
cm2. The second method involves measurement of the ratio of fluorescent intensities from the upper laser level to the ground state and to the lower laser level, and measurement of the cross section in absorption of the transition which involves the ground state. The second method yields a value of 2.7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}19}$
cm2. The difference represents the contribution of fluorescence to high-lying levels (not included in the determination of branching ratio) and nonradiative transitions. The probable experimental accuracy in both methods is \ifmmode\pm\else\textpm\fi{}15%.
An investigation was made of the absorption spectrum of Cr 3+ ions in the ground ( 4 A 2 ) and metastable ( 2 E) states, and of the luminescence spectrum of the same ions in Y 3 Al 5 O 12 crystals at 300 and 77°K. The effective absorption cross sections were determined for the 4 A 2 state. The luminescence deexcitation time was found to be 1.5 and 9 msec at 300 and 77°K, respectively. The absorption spectrum of the 2 E state had four wide absorption bands with maxima at 16,700, 23,300, 28,600, and 40,500 cm –1 . The theoretical energy level scheme of Cr 3+ was calculated for Dq = 1725 cm –1 , G = 640 cm –1 , C = 3200 cm –1 , v = 495 cm –1 , and v' = 259 cm –1 , making an allowance for the mixing of the Γ trig S states. The experimentally observed absorption bands were attributed to specific transitions and the theoretical and experimental intensities were compared.
The Raman relaxation rates for
Ni2+ and
Cr3+ ions in MgO crystals have been calculated using a shell model of phonons which is in good agreement with the results of neutron scattering experiments. These shell-model results are compared with those obtained with the Debye model for the phonons and, also, with the available experimental results for
Ni2+ and
Cr3+ in MgO. In the case
Ni2+, 50\ifmmode^\circ\else\textdegree\fi{}K is the highest temperature for which the relaxation time is sufficiently long to be directly measureable. As might be expected, in this low-temperature region the results based on the shell model and the Debye model of phonons are not significantly different. However, the values of relaxation rates at 123 and 136\ifmmode^\circ\else\textdegree\fi{}K obtainable from the recent line-shape measurements for
Ni2+ in MgO are in good agreement with those calculated on the basis of the shell model of phonons. In the case of
Cr3+ ions, the relaxation time can be measured directly up to temperatures as high as 200\ifmmode^\circ\else\textdegree\fi{}K, and in this range of temperature the two sets of results differ considerably. With the shell model, we obtain good agreement with the experimental results on the functional dependence of the relaxation rate
T11 on temperature. However, the absolute values of
T11 for
Cr3+ obtained with this phonon model are on the average less than the experimental measurements by a factor of 5. The possible sources which might be important in bridging the gap between the theoretical and experimental results are discussed.
Stimulated emission due to the 4 I 11/2 – 4 I 13/2 transition (λ = 2.936 μ) in yttrium aluminum garnet (Y 3 Al 5 O 12 ) doped with Er 3+ ions (30 wt%) was observed for the first time at 300°K. The output energy from crystals of 50 × 3 mm dimensions was 0.3 J under free-oscillation conditions and 0.05 J in the case of Q switching. The lifetime of the 4 I 11/2 level was determined for crystals with 3–30 wt% Er 3+ . The energy from the higher Er 3+ levels relaxed in a cascade manner through 4 I 11/2 to 4 I 13/2 .
Coherent dynamic property of neodymium yttrium aluminum garnet (Nd:YAG) crystal at 77 K is studied via the conventional absorption, the femtosecond fringe-resolved wave packet interferometry, and the related difference-phase spectrum. The recorded interferogram exhibits beatings in subpicosecond time scale arising from the interferences among various weakly split 4f-electronic states and the coupled vibronic optical phonon sidebands. The electron–phonon coupling in Nd:YAG can be well described by multiple Brownian oscillators model involving in each individual electronic transition. The parameters for characterizing material coherence and relaxation are determined via the theoretical modelings of both the frequency and the time-domain experimental signals.
Accumulated photon echoes have been used to investigate the mechanisms of optical dephasing in CaF2 crystals activated by Nd3+ ions. Tunable picosecond laser radiation, which permits the selective excitation of various Nd3+ optical centers in the 4 I 9/2→4 G 5/2, 2 G 7/2 transition, is used. The optical phase relaxation times measured at temperatures from 9 to 50 K permit determination of the homogeneous widths of the transitions between the low-lying 4 I 9/2 Stark level and three excited 4 G 5/2, 2 G 7/2 levels, and calculation of the constants of the inter-Stark relaxation transitions in the ground and excited multiplets for the rhombic N and M Nd3+ centers in CaF2 crystals. An analysis of the temperature dependence of the homogeneous linewidth of the transitions between low-lying Stark levels of the ground and excited states shows that the mechanism of optical dephasing in the crystals investigated is described well by direct relaxation processes with resonant inter-Stark absorption of one phonon in the ground and excited states. At T=9 K, the homogeneous linewidth Γh in CaF2 crystals is almost an order of magnitude smaller than Γh in disordered CaF2-YF3 crystals. This difference can be attributed to the significantly greater spectral phonon density of states in disordered crystals.