  Induced transparency is a common but remarkable effect in optics. It occurs
when a strong driving field is used to render an otherwise opaque material
transparent. The effect is known as electromagnetically induced transparency in
atomic media and optomechanically induced transparency in systems that consist
of coupled optical and mechanical resonators. In this work, we introduce the
concept of photothermally induced transparency (PTIT). It happens when an
optical resonator exhibits non-linear behavior due to optical heating of the
resonator or its mirrors. Similar to the established mechanisms for induced
transparency, PTIT can suppress the coupling between an optical resonator and a
traveling optical field. We further show that the dispersion of the resonator
can be modified to exhibit slow or fast light. Because of the relatively slow
thermal response, we observe the bandwidth of the PTIT to be $2\pi\times15.9$
Hz which theoretically suggests a group velocity of as low as $5$ m/s.
