Hybrid Nanocavity Resonant Enhancement of Color Center Emission in Diamond

Resonantly enhanced emission from the zero-phonon line of a diamond nitrogen-vacancy (NV) center in single crystal diamond is demonstrated experimentally using a hybrid whispering gallery mode nanocavity. A 900 nm diameter ring nanocavity formed from gallium phosphide, whose sidewalls extend into a diamond substrate, is tuned onto resonance at a low temperature with the zero-phonon line of a negatively charged NV center implanted near the diamond surface. When the nanocavity is on resonance, the zero-phonon line intensity is enhanced by approximately an order of magnitude, and the spontaneous emission lifetime of the NV is reduced by as much as 18%, corresponding to a 6.3X enhancement of emission in the zero photon line.

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In photonics-based networks for quantum information processing, photons serve as flying qubits, capable of carrying quantum information and mediating communication between stationary qubits. For such applications, single photons emitted from the nitrogen-vacancy center in diamond—an atomic-scale defect consisting of a substitutional nitrogen atom adjacent to a carbon vacancy—hold particular promise. One of the challenges of implementing quantum networks with these defects is to enhance their photon emission at preferential wavelengths, another is to direct the emitted photons into on-chip components that can be integrated into photonic circuits. In this experimental paper, we present a semiconductor-diamond hybrid nanophotonic structure that addresses both of these challenges.

In our hybrid structure, a semiconductor (gallium phosphide) ring, with an outer diameter just below 1 μm, is fabricated on top of a similarly sized ring machined from a single crystal diamond chip. The semiconductor ring, which serves as an optical cavity, traps photons within a wavelength-scale volume and increases the time and the strength of the interactions between photons and nitrogen-vacancy centers located in the diamond. The result is then a factor-of-6 enhancement of the emission of desired wavelengths from a single nitrogen-vacancy center. This degree of enhancement is similar to what has been achieved in another diamond ring structure of a different construction that requires a more demanding fabrication process. The new structure not only allows for flexibility in design that can take advantage of the properties of the semiconductor material, but also provides a chip-based photonics platform well suited for collecting and routing light between distant centers. We envision that photonic circuits consisting of elements such as these hybrid nanocavities will be useful for future quantum information processing applications, ultralow power optical information processing and switching, and on-chip, high-resolution magnetometry.

Figure 1

(a) Scanning electron microscope image of a hybrid GaP-diamond whispering gallery mode nanocavity. (b) Top-view and (c) cross-section of the dominant electric field component $E_r$ of the lowest order TE-like standing-wave mode supported by the device in (a), with resonance wavelength ${\lambda }_o\sim 637\mathrm{nm}$. The fields are calculated using finite difference time domain simulations. The field in (b) is plotted in the $x\mathrm{\text{−}}y$ center-plane of the GaP layer.

Figure 2

(a) Spectrum of the nanocavity studied in (c) and (d) prior to tuning the nanocavity modes. (b) High-resolution PL spectrum of the nanocavity mode in (a) closest to the ${\mathrm{NV}}^{-}$ ZPL. The dashed line is a fit to the data of two incoherently added Lorentizian lineshapes. (c) Nanocavity PL spectrum as a function of cavity tuning cycles. Each tuning cycle corresponds to releasing a fixed volume of Xe gas into the cryostat. (d) Nanocavity PL spectra when the nanocavity is on resonance with the ZPL of NV1 and NV2 (spectra A and B, respectively), and off resonance from any NV ZPL (spectra C). Spectra A, B, and C are measured after the tuning cycles indicated by the dashed lines in (c).

Figure 3

(a) Time resolved photoluminescence of NV1 excited with a pulsed green source, when the NV1 ZPL is on (blue) and off (red) resonance with the nanocavity ${\lambda }_{-}$ mode. The time origin is chosen $\sim 3.0\mathrm{ns}$ after the excitation pulse peak (see inset) so that fast decaying nanocavity background does not influence the fits (solid lines). Nanocavity spectra under (b) CW and (c) pulsed 532 nm excitation, when NV1 ZPL is on (blue) and off (red) resonance with the nanocavity mode. The green shaded regions indicate the monochromator spectral window used for the lifetime measurements in (a). The dashed line in (c) is a fit to the data consisting of two incoherently superimposed Lorentzian lineshapes. (d)–(f) Data analogous to (a)–(c), with the nanocavity ${\lambda }_{-}$ mode tuned on and off resonance with the NV2 ZPL.