Single-photon emission in the near infrared from diamond colour centre
Introduction
Single photons on demand can be efficiently produced by suited pulsed excitation of a single emitting dipole, such as a single atom, molecule, quantum dot or diamond colour centre [1]. Since it takes a single emitting dipole a finite time to go through a full cycle of excitation–emission–reexcitation before it emits a second photon, a sufficiently short and intense excitation pulse will enable the single dipole to emit only one photon per excitation pulse [2]. Among various solid-state sources, single defects in diamond are one of the potential candidates due to their unsurpassed efficiency and photostability. The nitrogen-vacancy (NV) colour centre is one of the best known optically active defects in diamond [3], [4], [5], [6]. It consists of a substitutional nitrogen atom (N) and a vacancy (V) in an adjacent lattice site, emitting around 670 nm with a zero phonon line (ZPL) at 637 nm. A reliable triggered single-photon source was recently built, based on the pulsed, optically excited photoluminescence of a single NV colour centre in diamond nanocrystal [7]. This single-photon source was applied to the observation of single-photon wavefront-splitting interference [8] and the realization of open-air quantum key distribution (QKD) experiments [9], [10]. However, in the latter application, broadband emission of the NV colour centre (FWHM≈70 nm at room temperature) precludes day-light operation of the QKD set-up due to difficulties in filtering the transmitted single photons from day-light background.
Recently, the nickel–nitrogen NE8 impurities in the diamond arouse the researchers’ interest for their intense narrow emission band in the near infrared [11], [12]. In this article, we report on the observation of similar diamond colour centres also tentatively associated to nickel–nitrogen impurities. Photoluminescence of these single emitters shows perfect photostability at room temperature. Compared to NV colour centre emission, it has several striking features. (i) A narrow band emission around 780 nm which is almost entirely concentrated in a ZPL even at room temperature. (ii) A short photon emission lifetime, around 2 ns. (iii). Linearly polarized photoluminescence from the single defect.
Section snippets
Experimental set-up
The experimental set-up is sketched in Fig. 1. Confocal microscopy is employed to select the single defects in the sample. The sample is a type IIa natural bulk diamond wafer 2×2 mm2 in size and 0.32 mm in thickness (Element 6, The Netherlands). A commercially available laser diode emitting at 687 nm provides the continuous-wave (CW) excitation source. The beam is focused on the sample about 4 μm below its surface using a microscope objective with a magnification of 100 and a numerical aperture of
Results and discussion
To identify well-isolated photoluminescent emitters, we first raster scan the sample using CW excitation. For each photoluminescent spot, the unicity of the emitter is then checked by observation of antibunching in the photoluminescence intensity. Since after the emission of a first photon it takes a finite time for a single emitter to be excited again and then to spontaneously emit a second photon, the antibunching effect appears as a dip around zero delay in the normalized intensity
Conclusion
In conclusion, we have studied photoluminescence properties at the single emitter level of optically active colour centres based on nickel–nitrogen impurities in natural diamond samples. Under CW excitation at room temperature, these colour centres reveal narrow band emission around 782 nm, the short photon emission lifetime about 2 ns and fully polarized photoluminescence. Thanks to narrow spectral emission, it will be easy to pick up the useful photoluminescence while removing stray light.
Acknowledgements
This work was partly supported by “AC Nanosciences” grant from Ministère de la Recherche, by Institut d’Alembert (ENS de Cachan IFR 121) and by Institut Universitaire de France.
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