  Recent interest in developing fast spintronic devices and laser-controllable
magnetic solids has sparked tremendous experimental and theoretical efforts to
understand and manipulate ultrafast dynamics in materials. Studies of spin
dynamics in the terahertz (THz) frequency range are particularly important for
elucidating microscopic pathways toward novel device functionalities. Here, we
review THz phenomena related to spin dynamics in rare-earth orthoferrites, a
class of materials promising for antiferromagnetic spintronics. We expand this
topic into a description of four key elements. (1) We start by describing THz
spectroscopy of spin excitations for probing magnetic phase transitions in
thermal equilibrium. While acoustic magnons are useful indicators of spin
reorientation transitions, electromagnons that arise from dynamic
magnetoelectric couplings serve as a signature of inversion-symmetry-breaking
phases at low temperatures. (2) We then review the strong laser driving
scenario, where the system is excited far from equilibrium and thereby subject
to modifications to the free energy landscape. Microscopic pathways for
ultrafast laser manipulation of magnetic order are discussed. (3) Furthermore,
we review a variety of protocols to manipulate coherent THz magnons in time and
space, which are useful capabilities for antiferromagnetic spintronic
applications. (4) Finally, new insights on the connection between dynamic
magnetic coupling in condensed matter and the Dicke superradiant phase
transition in quantum optics are provided. By presenting a review on an array
of THz spin phenomena occurring in a single class of materials, we hope to
trigger interdisciplinary efforts that actively seek connections between
subfields of spintronics, which will facilitate the invention of new protocols
of active spin control and quantum phase engineering.

Optics;Mesoscale and Nanoscale Physics