  We present a method to search for scalar field ultralight dark matter
directly interacting with gravitational-wave interferometers via a modulation
of the fine structure constant and the electron mass. This modulation induces
an effective strain in solid materials at a frequency determined by the mass of
the dark matter particle. We study the prospects for looking for such an effect
in the LIGO detectors by using the solid cavity which is nominally used for
pre-stabilizing the laser frequency and we project upper limits. We
contextualize them with previous limits from GEO600, possible limits from a
similar strain in the LIGO beamsplitter, and with potential limits from
upcoming experiments like LISA, Cosmic Explorer and from an upgraded solid
cavity. We find that with the sensitivity of Advanced LIGO, competitive upper
limits on DM coupling can be placed at the level of $\left\vert
d_{m_e}+d_e\right\vert \sim 0.2$ for $m_\text{DM} \sim
10^{-13}\,\mathrm{eV}/\mathrm{c}^2$ with a combination of two searches using
the solid cavity and the beamsplitter in LIGO; future experiments could reduce
this upper limit to $\sim10^{-3}$.
