  We demonstrate that the Unruh-DeWitt harmonic-oscillator detectors in (1+1)
dimensions derivative-coupled with a massless scalar field can mimic the atom
mirrors in free space. Without introducing the Dirichlet boundary condition to
the field, the reflectivity of our detector/atom mirror is dynamically
determined by the interaction of the detector's internal oscillator and the
field. When the oscillator-field coupling is strong, a broad frequency range of
the quantum field can be mostly reflected by the detector mirror at late times.
Constructing a cavity model with two such detector mirrors, we can see how the
quantum field inside the cavity evolves from a continuous to a quasi-discrete
spectrum which gives a negative Casimir energy density at late times. In our
numerical calculations, the Casimir energy density in the cavity does not
converge until the UV cutoff is sufficiently large, with which the two internal
oscillators are always separable.
