Optically pump-induced athermal and nonresonant refractive index changes in the reference Cr-doped laser materials: Cr:GSGG and ruby
Optically pump-induced athermal and nonresonant refractive index changes in the reference Cr-doped laser materials: Cr:GSGG and ruby
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参考掺铬激光材料中光泵引起的无热和非谐振折射率变化：Cr:GSGG 和红宝石
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发表时间：
2012
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通讯作者：
T. Catunda
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作者：
T. Godin;R. Moncorgé;J. Doualan;M. Fromager;K. Ait;Renato Antonio Cruz;T. Catunda
大多数离子掺杂材料的折射率随着激发态布居而增加。这种效应在许多激光材料中进行了研究，特别是那些掺杂Cr 3+和稀土离子，使用几种技术，如干涉测量，波混合，和Z扫描。这种折射率变化是非热的（具有电子起源），并且与Cr 3+离子在其激发态和基态的极化率差Δαp有关。Cr ~（3+）在可见光区的光跃迁是电偶极禁戒的，并且具有较低的振子强度。因此，对Δαp的主要贡献已被分配给UV中的电荷转移带（CTB）的允许跃迁，其强度高出约3个数量级。虽然这个CTB模型定性地解释了主要的观察结果，但它从未被定量地测试过。为了进一步研究掺Cr ~（3+）晶体中Δαp的物理来源，对Cr：Al_2O_3（红宝石）和Cr：GSGG晶体进行了激发态吸收（ESA）和Z扫描测量。选择Cr：GSGG是因为其E2和T24发射能级的接近性，因此可以通过比较低温和室温数据来探索ESA光谱中自旋选择规则的作用以及由此产生的极化率变化，这在以前从未报道过。另一方面，选择Cr：Al 2 O3（红宝石）是因为它是唯一能够从基态和激发态获得CTB吸收数据的晶体，因此可以更准确地检查CTB模型。由于这些更准确和更完整的数据，我们得出了第一个结论，即自旋选择规则在E2-T24能量失配的极化率变化中不起任何重要作用。我们还发现，在红宝石的情况下使用CTB模型将导致负Δαp值，这与所有折射率变化（包括Z扫描）测量相反。
The refractive index of most ion-doped materials increases with the excited state population. This effect was studied in many laser materials, particularly those doped with Cr3+ and rare earth ions, using several techniques, such as interferometry, wave mixing, and Z-scans. This refractive index variation is athermal (has an electronic origin) and is associated with the difference in the polarizabilites of the Cr3+ ion in its excited and ground states, Δαp. The Cr3+ optical transitions in the visible domain are electric-dipole forbidden, and they have low oscillator strengths. Therefore, the major contribution to Δαp has been assigned to allowed transitions to charge transfer bands (CTBs) in the UV with strengths ∼3 orders of magnitude higher. Although this CTB model qualitatively explains the main observations, it was never quantitatively tested. In order to further investigate the physical origin of Δαp in Cr3+-doped crystals, excited state absorption (ESA) and Z-scan measurements were thus performed in Cr:Al2O3 (ruby) and Cr:GSGG. Cr:GSGG was selected because of the proximity of its E2 and T24 emitting levels, and thus the possibility to explore the role of the spin selection rule in the ESA spectra and the resulting variations in polarizability by comparing low and room temperature data, which were never reported before. On the other hand, Cr:Al2O3 (ruby) was selected because it is the only crystal for which it is possible to obtain CTB absorption data from both ground and excited states, and thus for which it is possible to check the CTB model more accurately. Thanks to these more accurate and more complete data, we came to the first conclusion that the spin selection rule does not play any significant role in the variation of the polarizability with the E2–T24 energy mismatch. We also discovered that using the CTB model in the case of ruby would lead to a negative Δαp value, which is contrary to all refractive index variation (including Z-scan) measurements.