Geometry, associated with three dimensions: width, length and height, is an important concept for electromagnetic and optical engineers. We have been successful to manipulate the light and the electromagnetic radiation by changing the geometry of for example antennas, scatterers, and etc. However, time is the fourth dimension which must be also employed in electromagnetic and optical engineering to control over radiation and realize novel devices.
In this talk, we will present the recent results in our group on this research area. First, we will show that nonreciprocal metasurfaces can be engineered as tunable and multifunctional devices. Our idea is to exert nonharmonic spatiotemporal modulation functions on the meta-atoms. Non-harmonic modulation provides additional freedoms for the control of nonreciprocity in metasurfaces. It is demonstrated that a variety of nonreciprocal devices like isolators, gyrators and circulators could be incorporated into a uniform hardware platform by changing only the modulation functions. Then, we will show that spatial modulation is not an essential requirement for achieving nonreciprocity in time-modulated metasurfaces. If the metasurfaces are bianisotropic, modulating such structures only in time can also induce strong nonreciprocity.