  Owing to its high brilliance, infrared and terahertz synchrotron radiation
(IR/THz-SR) has emerged as a powerful tool for spectroscopy under extreme
(i.e., technically more difficult) experimental conditions such as high
pressure, high magnetic field, high spatial resolution, and a combination of
these. The methodologies for pressure- and magnetic-field-dependent
spectroscopy and microscopy using IR/THz-SR have advanced rapidly worldwide. By
applying them in strongly correlated electron systems (SCESs), many
experimental studies have been performed on their electronic structures and
phonon/molecular vibration modes under extreme conditions. Here, we review the
recent progress of methodologies of IR/THz-SR spectroscopy and microscopy, and
the experimental results on SCESs and other systems obtained under extreme
conditions.
