Fürst J, Sturman B, Buse K, Breunig I
Opt Express. 2016 Sep 5;24(18):20143-55. doi: 10.1364/OE.24.020143.
Axial symmetry is the cornerstone for theory and applications of high-Q optical whispering gallery resonators (WGRs). Nevertheless, research on birefringent crystalline material persistently pushes towards breaking this symmetry. We show theoretically and experimentally that the effect of broken axial symmetry, caused by optical anisotropy, is modest for the resonant frequencies and Q-factors of the WGR modes. Thus, the most important equatorial whispering gallery modes can be quantitatively described and experimentally identified. At the same time, the effect of broken axial symmetry on the light field distribution of the whispering gallery modes is typically very strong. This qualitatively modifies the phase-matching for the χ nonlinear processes and enables broad-band second harmonic generation and optical parametric oscillation. The effect of weak geometric ellipticity in nominally symmetric WGRs is also considered. Altogether our findings pave the way for an extensive use of numerous birefringent (uniaxial and biaxial) crystals with broad transparency window and large χ coefficients in nonlinear optics with WGRs.
轴对称性是高品质光学回音壁模式谐振器（WGRs）理论与应用的基石。然而，对双折射晶体材料的研究一直在不断推动打破这种对称性。我们通过理论和实验表明，由光学各向异性引起的轴向对称性破缺对WGR模式的谐振频率和品质因数的影响较小。因此，最重要的赤道回音壁模式可以进行定量描述并通过实验识别。同时，轴向对称性破缺对回音壁模式光场分布的影响通常非常强烈。这定性地改变了χ非线性过程的相位匹配，并实现了宽带二次谐波产生和光学参量振荡。我们还考虑了名义上对称的WGR中弱几何椭圆率的影响。总的来说，我们的研究结果为在WGR非线性光学中广泛使用具有宽透明窗口和大χ系数的众多双折射（单轴和双轴）晶体铺平了道路。
