  Towards a spin-based quantum microwave-optical photon transducer, we
demonstrate a low-loss dielectric microwave resonator with an internal quality
factor of $2.30\times10^4$ while accommodating optical apertures with a
diameter of $8\, \mathrm{mm}$. The two seemingly conflicting requirements, high
quality factor and large optical apertures, are satisfied thanks to the large
dielectric constant of rutile ($\mathrm{TiO_2}$). The quality factor is limited
by radiation loss, and we confirmed by numerical simulation that this
dielectric resonator can achieve a quality factor exceeding $10^6$ by extending
the height of the resonator enclosure. Using this resonator, we performed both
continuous-wave (cw) and pulse electron spin resonance (ESR) spectroscopy on
2,2-diphenyl-1-picrylhydrazyl (DPPH) crystalline powder and P1 centers in a
diamond crystal in a dilution refrigerator. The cw ESR spectroscopy
demonstrated high-cooperativity and strong spin-resonator coupling with the
DPPH and P1 centers respectively, while the pulse ESR spectroscopy successfully
measured longitudinal and transverse relaxation times.
