High quality-factor diamond-confined open microcavity

Abstract

With a highly coherent, optically addressable electron spin, the nitrogen vacancy (NV) centre in diamond is a promising candidate for a node in a quantum network. However, the NV centre is a poor source of coherent single photons owing to a long radiative lifetime, a small branching ratio into the zero-phonon line (ZPL) and a poor extraction efficiency out of the high-index host material. In principle, these three shortcomings can be addressed by resonant coupling to a single mode of an optical cavity. Utilising the weak-coupling regime of cavity electrodynamics, resonant coupling between the ZPL and a single cavity-mode enhances the transition rate and branching ratio into the ZPL. Furthermore, the cavity channels the light into a well-defined mode thereby facilitating detection with external optics. Here, we present an open Fabry-Perot microcavity geometry containing a single-crystal diamond membrane, which operates in a regime where the vacuum electric field is strongly confined to the diamond membrane. There is a field anti-node at the diamond-air interface. Despite the presence of surface losses, quality factors exceeding 120 000 and a finesse F = 11 500 were observed. We investigate the interplay between different loss mechanisms, and the impact these loss channels have on the performance of the cavity. This analysis suggests that the "waviness" (roughness with a spatial frequency comparable to that of the microcavity mode) is the mechanism preventing the quality factors from reaching even higher values. Finally, we apply the extracted cavity parameters to the NV centre and calculate a predicted Purcell factor exceeding 150.

Widely-tunable, doubly-resonant Raman scattering on diamond in an open microcavity

“…Next, we determine the quality factor of the pump mode of the cavity, Q p , following the method reported in Ref. [36]. To extract the cavity linewidth, we keep the laser frequency ν pump fixed while scanning the cavity length, monitoring the reflected light on a photodiode.…”

“…For these simulations, we use the exact mirror structure obtained from fitting the mirror stopband (Fig. 1 (c) [33,36]), and sweep the width of the air-gap t a and the diamond thickness t d for fixed wavelengths λ cav p and λ cav S . The double resonance condition is met whenever the modes for λ cav p cross the modes for λ cav S .…”

“…We employ 14 (15) λ c /4 layers of a SiO 2 /Ta 2 O 5 distributed Bragg reflector for the curved top (planar bottom) mirrors. From a white-light transmission measurement [33,36,71], the center of the stopband of the top mirror is determined to be λ c = 625 nm (Fig. 1 (c)).…”

“…The bottom mirror is mounted on a stack of xyzpiezoelectric nanopositioners (attocube, 2×ANPx51 and ANPz51) and placed inside a homebuilt titanium "cage"; the top mirror is rigidly attached to the top of the cage [36]. By applying a voltage to the nanopositioners, the bottom mirror can be moved in all three dimensions with respect to the top mirror, offering both spatial and spectral tunability [46].…”

“…Such microcavities, sometimes referred to as open microcavities, offer high Q/V ratios and thereby promote strong light-matter interactions [34,35]. We recently demonstrated that high Qfactors at visible wavelengths can be achieved when embedding a high-quality diamond membrane into the cavity [36]. In comparison to ring resonators, there are no limitations due to bending losses and surface roughness is less detrimental.…”