Sub-to-super-Poissonian photon statistics in cathodoluminescence of color center ensembles in isolated diamond crystals

Abstract

Impurity-vacancy centers in diamond offer a new class of robust photon sources with versatile quantum properties. While individual color centers commonly act as single-photon sources, their ensembles have been theoretically predicted to have tunable photon-emission statistics. Importantly, the particular type of excitation affects the emission properties of a color center ensemble within a diamond crystal. While optical excitation favors non-synchronized excitation of color centers within an ensemble, electron-beam excitation can synchronize the emitters excitation and thereby provides a control of the second-order correlation function g ₂(0). In this letter, we demonstrate experimentally that the photon stream from an ensemble of color centers can exhibit g ₂(0) both above and below unity, thereby confirming long standing theoretical predictions by Meuret et al. [S. Meuret, L. H. G. Tizei, T. Cazimajou, et al., "Photon bunching in cathodoluminescence," Phys. Rev. Lett., vol. 114, no. 19, p. 197401, 2015.]. Such a photon source based on an ensemble of few color centers in a diamond crystal provides a highly tunable platform for informational technologies operating at room temperature.

Keywords: cathodoluminescence; color center; impurity-vacancy centers in diamond; photon antibunching; photon bunching; single-photon emitter.

Figure 1:

Electron beam-based correlation spectroscopy of color centers in a diamond. (a) Focused electron beam (orange) excites cathodoluminescence of color centers in an isolated diamond crystal (blue) in an SEM chamber. The generated photons (red) are collected by a parabolic mirror and sent to a spectrometer and a TCSPC system. (b) Monte Carlo simulation of primary electron trajectories in diamond for a 5 kV incoming electron beam with a spot size of 5 nm. (c) Penetration depth of electrons in diamond (energy loss of 75 %) at increasing accelerating voltages. The superimposed shading serves as a guide to the eyes.

Figure 2:

Photon bunching in CL of GeV⁻ and SiV⁻ ensembles with large number of color centers (N > 100). (a and b) CL spectra of diamond crystals with ensembles of GeV⁻ and SiV⁻ centers. The fit (red) reveals the ZPL at 606 nm (green) for the GeV⁻ ensemble and 739 nm (blue) for the SiV⁻ ensemble over the background diamond emission. The gray-shaded spectral range indicates the transmission of a band-pass filter used to collect intensity-correlation histograms. The insets show SEM images of the investigated crystals. The scale bars denote 100 nm. (c) Photon statistics of GeV⁻ (green) and SiV⁻ (blue) emitter ensembles measured in CL at low and high electron-beam currents. The shaded satellite peaks in the SiV⁻ histogram (indicated by arrows) are afterglow artifacts of detectors (see [21] and Supporting Information therein). (d and e) Emission statistics over 17 crystals with GeV⁻ centers and 11 crystals with SiV⁻ centers. (f) g ₂(0) is inversely proportional to the electron-beam current I (red lines) and converges to 1 at high currents.

Figure 3:

Single-photon emission in CL of an isolated GeV⁻ center. Secondary electron (SE) image of a diamond crystal (a) and its corresponding CL intensity map (b) revealing the position of a GeV⁻ center (40 pA, 5 kV). The scale bars denote 50 nm. (c) CL spectrum of the localized GeV⁻ center with ZPL at 605 nm (acquired at 40 pA). (d) CL intensity time trace measured at 40 pA presents stable and bright emission at $\sim 10^6$ counts per second. (e) CL intensity saturation of the GeV⁻ center. (f) Intensity-correlation histograms of the GeV⁻ center reveal the single-photon nature of generated emission at all currents. (g) Single-photon purity g ₂(0) = 0.06 ± 0.01 is independent of the electron-beam current.

Figure 4:

Cathodoluminescence of an ensemble with two color centers. (a) CL spectrum of a nano-diamond with few GeV⁻ centers emitting at 606 nm. The inset shows the SE image of the nano-diamond. The scale bar denotes 100 nm. (b) Second-order intensity-correlation histograms g ₂(τ) measured at increasing electron-beam currents, revealing the transition from super to sub-Poissonian statistics. Note the different vertical scale for the low and high electron-beam currents. (c) Extracted g ₂(0) values from panel b are plotted as a function of applied electron-beam current I. The I-dependency is fitted with Eq. (1) (solid red) converging to g ₂(0) = 0.5, indicating that the emitting ensemble consists of N = 2 color centers. To illustrate the certainty in extracting N, the red dashed lines show the expected I-dependency in the cases of N = 3 and N = 100 emitters.