Gurwich I, Sivan Y
Unit of Electro-optics Engineering, Faculty of Engineering Sciences, Ben-Gurion University of the Negev, Israel.
Phys Rev E. 2017 Jul;96(1-1):012212. doi: 10.1103/PhysRevE.96.012212. Epub 2017 Jul 14.
We show that the standard perturbative (i.e., cubic) description of the thermal nonlinear response of a single metal nanosphere to intense continuous-wave (CW) illumination is sufficient only for a temperature rise of up to 100 degrees above room temperature. Beyond this regime, the slowing down of the temperature rise requires a nonperturbative description of the nonlinear response, even though the permittivity is linearly dependent on the temperature and despite the deep subwavelength effective propagation distances involved. Using experimental data, we show that, generically, the increase of the imaginary part of the metal permittivity dominates the increase of the host permittivity as well as the resonance shift due to the joint changes to the real parts of the metal and host. Thus, the main nonlinear effect is a decrease of the quality factor of the resonance. We further analyze the relative importance of the various contributions to the temperature rise and thermal nonlinearity, compare the nonlinearity of Au and Ag, demonstrate the potential effect of the nanoparticle morphology, and show that although the thermo-optical nonlinearity of the host typically plays a minor role, its thermal conductivity and its temperature dependence is important. Finally, we discuss the differences between CW and ultrafast thermal nonlinearities.
我们表明，对于单个金属纳米球对强连续波（CW）光照的热非线性响应，标准的微扰（即三次方）描述仅适用于比室温高100度以内的温度升高情况。超过这个范围，尽管介电常数与温度呈线性相关，且涉及极深的亚波长有效传播距离，但温度上升的减缓仍需要对非线性响应进行非微扰描述。利用实验数据，我们表明，一般来说，金属介电常数虚部的增加主导了主体介电常数的增加以及由于金属和主体实部的联合变化导致的共振偏移。因此，主要的非线性效应是共振品质因数的降低。我们进一步分析了对温度升高和热非线性的各种贡献的相对重要性，比较了金和银的非线性，展示了纳米颗粒形态的潜在影响，并表明尽管主体的热光非线性通常起次要作用，但其热导率及其温度依赖性很重要。最后，我们讨论了连续波和超快热非线性之间的差异。
