Nonreciprocal coupling to microstrip resonators

Inducing nonreciprocal wave propagation is a fundamental challenge across a wide range of physical systems in electromagnetics, optics, and acoustics. Linear, time-invariant systems are always reciprocal, but reciprocity can be broken through the action of a bias that is asymmetric under time-reversal. Magnetic fields are the most common bias used to produce nonreciprocal devices. However, nonreciprocal devices using magnetic fields are difficult to integrate into larger systems that may be sensitive to magnetic fields. To overcome this challenge, recent efforts to create nonreciprocal devices have instead exploited momentum-based techniques such as coherent spatiotemporal modulation of resonators and waveguides. One such technique, indirect interband scattering, uses a traveling wave bias to scatter light between two modes which differ in frequency and momentum. Due to momentum conservation, this process is inherently nonreciprocal --- light traveling in different directions will be scattered differently by the traveling wave bias. This thesis extends the method of indirect interband scattering in two separate domains. Indirect interband scattering has so far been demonstrated only between co-propagating traveling modes, both in waveguides and whispering-gallery-mode resonators. Here, indirect scattering between a waveguide and standing-mode resonator is described and demonstrated experimentally. There are several capabilities and advantages unique to this type of indirect scattering, which we term "nonreciprocal coupling". Additionally, while indirect scattering has so far mainly been explored in optical systems, the experiments in this thesis occur in the microwave frequency domain