JARA-FIT and II. Institute of Physics, RWTH Aachen University, 52074 Aachen, Germany.
Huygens-Kamerlingh Onnes Laboratory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands.
Nat Commun. 2015 Mar 10;6:6444. doi: 10.1038/ncomms7444.
Heterodyne detection schemes are widely used to detect and analyse high-frequency signals, which are unmeasurable with conventional techniques. It is the general conception that the heterodyne signal is generated only by mixing and that beating can be fully neglected, as it is a linear effect that, therefore, cannot produce a heterodyne signal. Deriving a general analytical theory, we show, in contrast, that both beating and mixing are crucial to explain the heterodyne signal generation. Beating even dominates the heterodyne signal, if the nonlinearity of the mixing element (mixer) is of higher order than quadratic. The specific characteristic of the mixer determines its sensitivity for beating. We confirm our results with both a full numerical simulation and an experiment using heterodyne force microscopy, which represents a model system with a highly non-quadratic mixer. As quadratic mixers are the exception, many results of previously reported heterodyne measurements may need to be reconsidered.
外差探测方案被广泛用于探测和分析传统技术无法测量的高频信号。一般认为，外差信号仅由混频产生，而拍频可以完全忽略，因为它是一种线性效应，因此不能产生外差信号。通过推导出一个通用的分析理论，我们表明，拍频和混频对于解释外差信号的产生都是至关重要的。如果混频元件（混频器）的非线性高于二次方，那么拍频甚至会主导外差信号。混频器的具体特性决定了其对拍频的灵敏度。我们通过全数值模拟和使用外差力显微镜的实验验证了我们的结果，后者代表了一个具有高度非二次方混频器的模型系统。由于二次方混频器是例外，因此以前报道的许多外差测量结果可能需要重新考虑。
