Fig. S2.

Cubic cavity under screw-like deformations. (A) Sketch of the geometry: open cubic cavity of side $=1\hspace{0.17em}{\lambda }_p$, immersed in an unbounded vacuum space, containing a dielectric sphere of permittivity ${\epsilon }_p=11.7$ and resonant radius $r_p=0.2091\hspace{0.17em}{\lambda }_p$, as wells as two “screws” consisting of dielectric blocks of square cross-section with side $s=0.2\hspace{0.17em}\lambda {\hspace{0.17em}}_p$ and variable height $d$. The cavity is otherwise assumed to be filled with a nonmagnetic substance characterized by relative permittivity following a Drude model, ${\epsilon }_2\left(\omega \right)=1-{\omega }_p^2/\left({\omega }^2+i\omega {\omega }_c\right)$ with ${\omega }_c={10}^{-4}{\omega }_p$. A QE, modeled as a two-level system whose transition frequency equals the plasma frequency of the host medium filling the cavity, ${\omega }_0={\omega }_p$, is positioned at the center of the dielectric sphere. (B) Simulated spectral density, $g\left(\omega \right)$, normalized to the maximal computed value, for different screw heights: $d=0\hspace{0.17em}{\lambda }_p$ (no screws), $d=0.2\hspace{0.17em}{\lambda }_p$, and $d=0.275\hspace{0.17em}{\lambda }_p$. The figure confirms that screw-like deformations induce no significant detuning but significantly impact the peak value of the spectral density.