Department of Physics, Colorado State University, Fort Collins, Colorado 80523, USA.
St. Petersburg Electrotechnical University, 197376 St. Petersburg, Russia.
Phys Rev Lett. 2018 Sep 7;121(10):107204. doi: 10.1103/PhysRevLett.121.107204.
Exact fractals of nonlinear waves that rely on strong dispersion and nonlinearity and arise spontaneously out of magnetic media were observed for the first time. The experiments make use of a microwave to excite a spin wave in a quasi-one-dimensional magnonic crystal. When the power of the input microwave (P_{in}) is low, the output signal has a power-frequency spectrum that consists of a single peak. When P_{in} is increased to a certain level, new side modes are generated through modulational instability, resulting in a comblike frequency spectrum. With a further increase in P_{in}, each peak in the frequency comb can evolve into its own finer comb through the modulational instability. As P_{in} is increased further, one can observe yet another set of finer frequency combs. Such a frequency-domain fractal manifests itself as multiple layers of amplitude modulation in the time-domain signal.
首次观察到了依赖强色散和非线性、自发产生于磁介质中的非线性波的精确分形。实验利用微波在准一维磁振子晶体中激发自旋波。当输入微波的功率 (P_{in}) 较低时，输出信号的功率-频率谱由单个峰值组成。当 P_{in}增加到一定水平时，新的边带模式通过调制不稳定性产生，导致梳状频谱。随着 P_{in}的进一步增加，每个频率梳的峰值都可以通过调制不稳定性演变成自己的更精细的梳。随着 P_{in}的进一步增加，可以观察到另一组更精细的频率梳。这种频域分形在时域信号中表现为多层幅度调制。
