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Single photon emission from N-V colour centres in a diamond nanocrystal

par Loïc Rondin - publié le , mis à jour le


Due to their considerable photostability, defects in solids are particularly interesting emitters as complementary and analogous to fluorescent molecular systems. In particular, more than 500 optical centers in diamond have been reported based on its absorption and emission spectra. We consider the emission of a single nitrogen-vacancy (N-V) colour centre in diamond, a system which has an unsurpassed photostability even at room temperature.


 


The N-V centres consist of a substitionnal nitrogen atom (N) and a vacancy (V) in an adjacent lattice site. They are created by irradiation of a diamond sample with high-energy electrons followed by annealing at 800°C. At small electron exposure doses, the N-V centre density is small enough so that single N-V colour centres can be spatially isolated and detected using standard confocal microscopy. Their fluorescence then appears as bright spots when the sample is scanned with strongly focused green laser radiation (figure 1). The fluorescence spectrum of the colour centre (figure 2) consists of a narrow zero phonon line (ZPL) at approximatively 1.945 eV (wavelength 637.7 nm) and a broad phonon wing with a width of about 300 meV (wavelength of about 100 nm FWHM).


 


Significant limitation of defect photoemission in diamond arises from the high index of refraction of the bulk material (n=2.4), which makes difficult an efficient extraction of the emitted photons. Refraction at the sample interface leads to a small collection solid angle, limited by total internal refraction, and to optical aberrations. An efficient way to circumvent these problems is to consider the emission of defects in diamond nanocrystals, which size is much smaller than the wavelength of the radiated light. The sub-wavelength size of the nanocrystals renders refraction irrelevant. One can then simply assimilate the colour defect as a point source radiating in air. Furthermore, the small volume of diamond excited by the pumping laser yields very low background light. Such property is also of crucial importance for single photon emission, since residual background light will contribute to a non-vanishing probability of having more than two photons within the emitted light pulse.


NV PL
Figure 1:Raster scan of the sample showing the bright and stable fluorescence spot of an N-V colour centre inside a diamond nanocrystal.



 


Figure 2 : Fluorescence spectrum of a single N-V colour centre in a diamond nanoparticle. The spectrum is obtained with 532-nm nanosecond pulsed excitation. It shows the principal emission characteristics of N-V colour centres : a zero-phonon line (ZPL) at wavelength of 637 nm, as well as a Raman diffusion line (R) characteristic of the diamond matrix.


Nanostructured samples are prepared by starting with type Ib synthetic diamond powder (de Beers, Netherlands). The diamond nanocrystals are size-selected by centrifugation, yielding a mean diameter of about 90 nm. A polymer solution containing selected diamond nanocrystals is deposited by spin-coating onto the surface of a dielectric mirror, resulting in a 30-nm-thick polymer layer holding the nanocrystals. The ultra-low fluorescing dielectric mirrors (Layertec, Germany) are optimized to reflect efficiently the emission of an N-V colour centre centered on 690 nm. Note that background fluorescence around the emission of a single N-V colour centre is strongly reduced by photobleaching after a few hours of illumination, while the the N-V colour centre emission properties remain unaffected.


 


 



 


Figure 3 : (a) Triggered single photon emission from a fluorescent four-level emitter. (b) Experimental setup for emitting triggered single-photon from an N-V colour centre.


Under pulsed excitation with a pulse duration shorter than the excited-state lifetime, a single dipole emits photon one by one (figure 3a). To excite the N-V colour centre in such conditions, we use a home-built pulsed laser at a wavelength of 532 nm. The laser system delivers 800 ps pulses with energy 50 pJ. The energy per pulse is high enough to ensure efficient pumping of the defect centre in its excited state. The repetition rate, synchronized on a stable external clock, is set at between 2 to 6 MHz so that successive fluorescent decays are well separated in time. Single photons are thus emitted by the N-V colour centre at predetermined times within the accuracy of the emission lifetime of its excited state, which is about 40 ns.


The experimental setup is depicted on Figure 3b. Green excitation light is tightly focused on the nanocrystals by a metallographic objective of high numerical aperture NA=0.95. Fluorescence light emitted by the colour defect is collected through the same objective and spectrally filtered. Following a standard confocal detection scheme, collected light is then focused onto a 100 microns diameter pinhole. Such optical configuration restricts axial extend of the sample which could lead to scattering background.