Radiative transfer of electromagnetic energy and momentum between objects is one of fundamental and ubiquitous processes in nature. The ability to control it is crucial for various optoelectronic and optomechanical engineering applications including optical sensing, photovoltaics, and object manipulations. The radiative energy and momentum transfer between objects can be controlled by engineering electromagnetic states in the objects and in their environment. Although this can be achieved by modifying materials’ geometrical shapes, optical properties, and system configurations, the extent of this control is restricted when using naturally occurring materials. In this thesis, we explore two strategies of engineering electromagnetic states to control radiative transfer of energy and momentum.One strategy is to use photonic nanostructures and we study two systems. First, we examine a metal mirror with periodic metal-dielectric-metal nanoslit structures, in which each slit supports a single surface plasmon mode that propagates normal to the mirror surface. We numerically demonstrate that complete resonant absorption at multiple wavelengths in the range from the visible to mid-infrared can be achieved. We also discuss that engineering interactions between the surface plasmon modes excited at the mirror surface and within the slits allow to independently control spectral and angular responses of this system. Second, we develop a planar multilayered photonic-plasmonic structure supporting optical Tamm plasmon states. The resonant near-perfect absorption of incident light owing to coupling to these states is accompanied by a singular behavior of the phase of a reflected electromagnetic field. We use this singular behavior to achieve highly sensitive sensing and experimentally demonstrate remote temperature measurements. Our simulation and experiments form the basis for simple and robust planar sensing platforms with tunable spectral characteristics.Most photonic materials and systems obey the Lorentz reciprocity theorem, by which detailed balance of emission and absorption of an electromagnetic mode is satisfied. To bring these systems into fundamentally new regime for controlling radiative transfer, the other strategy we pursue is to engineer electromagnetic states via magnetization-induced reciprocity breaking. We predict near-complete violation of Kirchhoff’s law of radiation without external magnetic field by resonantly exciting nonreciprocal surface plasmon polaritons at the interface between a dielectric material and magnetic Weyl semimetals, which we identify as a promising reciprocity-breaking material. We also investigate radiative momentum transfer between two and three magnetic Weyl semimetal spheres both in thermal equilibrium and nonequilibrium situations. We derive a formalism of Casimir forces between an arbitrary number of spheres in thermal nonequilibrium based on fluctuational electrodynamics and scattering theory without the assumption of Lorentz reciprocity. We predict that lateral Casimir forces can arise in thermal nonequilibrium situations due to non-zero angular momentum of thermal radiation from magnetic Weyl semimetal spheres. We also show that the Casimir energy in thermal equilibrium depends on the static magnetization directions in the spheres and that the lateral Casimir force will act between the spheres to relax the system into the minimum energy state without transferring net energy and momentum to the environment. Our work on engineering light-matter interactions in nonreciprocal systems points a path towards improving efficiency of radiative energy conversion devices and extending capability for optomechanical manipulation of objects in nonreciprocal systems.Ph.D.