  A patterned structure of monolithic hexagonal boron nitride (hBN) on a glass
substrate, which can enhance the emission of the embedded single photon
emitters (SPEs), is useful for onchip single-photon sources of high-quality.
Here, we design and demonstrate a monolithic hBN metasurface with quasi-bound
states in the continuum mode at emission wavelength with ultrahigh Q values to
enhance fluorescence emission of SPEs in hBN. Because of ultrahigh electric
field enhancement inside the proposed hBN metasurface, an ultrahigh Purcell
factor (3.3*10^4) is achieved. In addition, the Purcell factor can also be
strongly enhanced in most part of the hBN structure, which makes the hBN
metasurface suitable for e.g. monolithic quantum photonics.
