In “standard” systems, waves (electromagnetic, acoustic) propagate reciprocally – if we exchange the locations of the source and the measurement probe, the measured field will not change. However, when we violate the conditions for this, for example by applying a static magnetic field bias or motion to our system, this doesn’t hold anymore, and nonreciprocal propagation is made possible. The most extreme version of it is the “one-way” or “sector way” propagation, where waves can only propagate into certain directions in space, even though they are excited by a source that does not make this directional distinction.
Allowing systems to change in time can “enhance” them with various properties – wave propagation is no longer bound to be reciprocal, effective gain and loss of power can be applied, etc. Applying both space and time modulation can create intriguing phenomena. An interesting question is whether or not a “moving” time of modulation, such as a traveling wave one, is equivalent to actual motion. Also, can space-time modulation induce in system properties that it does not have when static?
Wave interaction with complex particles and structures
Light interaction with complex structures, such as particle arrays that have a complex unit-cell structure or respond to both electric and magnetic fields, or chiral and anisotropic structures, allows for interesting wave propagation effects to occur such as:
Nonreciprocal and one-way wave propagation under specific requirements, that can be characterized using general group theory considerations.
Asymmetric wave propagation that can be leveraged to obtain a completely unidirectional propagation when combined with the right excitation scheme without the use of a magnetic field, time modulation, or nonlinear media.
Extreme dispersion features such as hyperbolic-like features in cylindrical structures.
And many more…