Fibre Integrated Diamond Based Single Photon Source

After successful assembly of the fibre integrated single photon source from a fibre and a nanodiamond (see Figure 6.6 (b) for an AFM image), optical characterisation of the system is carried out. For this, three different experimental configurations are implemented, as sketched in Figure 6.6 (a). In all configurations, the excitation light is a green laser and the detected fluorescence light is filtered by a 650 nm long pass filter. Configuration I is a confocal configuration where excitation and detection are done free-space using an objective lens with a numerical aperture of 0.9. In configuration II, the excitation is still performed via free-space, but the fluorescence light is collected through the fibre and in configuration III, excitation as well as detection are done through the fibre. Images acquired by scanning the excitation laser over the fibre facet in configurations I and II can be found in Figure 6.6 (e,f). Increased fluorescence at the nanodiamond’s position is found in both configurations, indicating that the NV centre’s emission is visible from both

6.2. Fibre Integrated Single Photon Source

a

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2 µm 250 nm

b c

f e

d

Figure 6.5.: Nanodiamonds on fibre cores. (a) shows an AFM cantilever chip (black object in the middle) clamped to a glass block approaching an optical fibre which sits in a brass mount. The black arrow indicated where the fibre emerges from the its mount. (b-f) show AFM images of fibre facets of cloverleaf (b,c) and wagon-wheel (d,e,f) microstructured fibres. (b,d) give an overview of the core regions while (c,e,f) are zooms to the white squares in (b,d). While in (c,f) the nanodiamond (indicated by an arrow) is already put on the core, in (e) the fibre’s core prior to coupling the nanodiamond is shown. For an AFM image of the fibre used in the fibre integrated diamond based single photon source see Figure 6.6.

((b-f) adapted from [229])

sides. When looking at the emission spectrum through the fibre (Figure 6.6 (c)), large lines from Raman scattering and other background light from the fibre is visible (black line), but after filtering with the 650 nm filter, nearly all of this light is filtered out.

For a functional single photon source, the photon statistics (see Section 2.1.3) is of great importance. The g⁽²⁾(0) (cf. Equation 2.16) has to be sufficiently low.

Only if its value is below 0.5, the main contribution to the photons stems from a single emitter. Ideally, a single photon source has a g⁽²⁾(0) = 0 which is a value that can not be reached in presence of any background. Using continuous wave excitation light, measurements of the g⁽²⁾-function are shown in Figure 6.7 (d,e) for the confocal configuration (I) and detected through the fibre (configuration II), respectively. The g⁽²⁾(0)-values (deduced from a fit to a model found for example in Jelezko et al. [89]) areg⁽²⁾(0) = 0.45 at an excitation power of 40µW in config-uration I and g⁽²⁾(0) = 0.36 at an excitation power of 49µW in configuration II.

It has to be noted that no background correction has been applied to any of these data, since for a single photon source all the photons emitted play a role.

After it is shown that the source emits single photons, the next important quan-tity is the rate of emitted photons when the emitter is fully saturated. The corre-sponding measurements are shown in Figure 6.7 (a,b) for configurations I and II, respectively. A fit (red lines) yields maximum count rates of R_inf = 52.6 kcts/s in configuration I and Rinf = 43.2 kcts/s in configuration II. Here, background correction for a linear background is applied to the data, so in contrast to the g⁽²⁾ -functions in Figure 6.7 (d,e), only photons stemming from the NV centre contribute to these values.

By comparison of the photon count rate collected through the NA=0.9 microscope objective and the count rate collected through the fibre, it is possible to estimate the effective numerical aperture NA_{ef f} of the fibre, which should be higher than the nominal NA of the fibre due to near-field interaction at the dielectric-air interface at the fibre facet. Assuming a uniform emission of the NV centre, this estimate is NA_{ef f} = 0.82. This value is similar to values of usual high-NA optics, but with an integrated and alignment-free way of coupling.

For most applications, synchronisation of different parts is required. Hence, pulsed single photon sources are needed. It is straightforward to implement a pulsed laser as excitation source in the integrated single photon source. By exploit-ing the pulsed nature of the laser, another type of filterexploit-ing can be implemented:

time gating. Here, only photons from defined time intervals are evaluated and oth-ers are excluded in the TCSPC (for TCSPS see Section 2.4.1). In Figure 6.7 (c), an antibunching measurement using time gating is shown. Photons arriving in a time span of 3.5 ns after a laser pulse are discarded in the evaluation. In this way, all the Raman scattering and fast decaying fluorescent background light does not con-tribute while, due to its long lifetime, photons from the NV centre are still detected.

6.2. Fibre Integrated Single Photon Source

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Figure 6.6.: Characterisation of the fibre-coupled single photon source. (a) is a sketch of the three different excitation and detection schemes used. Either exci-tation and detection happens in confocal manner (I), the diamond is excited from top and the photons are collected through the fibre (II), or both, excitation and detection happen through the fibre (III). (b) is an AFM micrograph of the assem-bled system with the white arrow indicating the diamond nanocrystal hosting a single NV centre. An optical spectrum of the emission collected through the fibre is shown in (c) as black line. The red line shows the spectrum after adding a 650 nm long pass filter. The inset is a zoom to a part of the recorded spectrum. (d) is an overlay from scanning electron beam and confocal image while (e) is the confocal measurement only (configuration (I)). (f) is a micrograph measured in configuration (II). Clearly, the fluorescence is highest at the position of the NV centre. (Figure adapted from [229])

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Figure 6.7.: Optical characterisation of the fibre-coupled single photon source.

(a) and (b) show measurements of the saturation behaviour for the configurations I and II, respectively. The red line is a fit to a three level model system. Satura-tion count rates are Rinf = 52.6 kcts/s in configuration I and Rinf = 43.2 kcts/s in configuration II. (c-e) show autocorrelation measurements in different configura-tions under continuous wave excitation (d,e) and under pulsed excitation with time gating applied (c,f). Red lines are fits to a theoretical model yielding g⁽²⁾(0) = 0.21 in (c),g⁽²⁾(0) = 0.45 in (d), g⁽²⁾(0) = 0.36 in (e), and g⁽²⁾(0) = 0.23 in (f). (Figure adapted from [229])

6.2. Fibre Integrated Single Photon Source

The measured g⁽²⁾(0)-value in configuration II is with g⁽²⁾(0) = 0.21 significantly smaller than without the time gating.

With time gating, it is also possible to measure in configuration III, where excita-tion as well as detecexcita-tion is performed through the fibre. Exciting through the fibre means that all the green excitation light has to go through the fibre. Furthermore, it means that the light is less focused at the nanodiamond’s location, because the mode field diameter of the fibre is larger than the laser focus used in configurations I and II, what makes higher excitation powers necessary. This leads to increased background, which then is suppressed by time gating. An autocorrelation function measured in configuration III is shown in Figure 6.7 (f). It yieldsg⁽²⁾(0) = 0.23. An interesting aspect of this configuration is that no direct optical access to the fibre facet is needed – the source is completely fibre coupled. Also, this configuration can be used as a sensor when the environment of the NV centre is changed.

Chapter Summary: Nanoassembled Hybrid Photonic Structures

In this chapter, two implementations of nanoassembeld hybrid photonic structures were reported. The first structure was built in order to enhance the zero phonon line of a NV centre resonantly by coupling it to a photonic crystal cavity. An enhancement factor of 12.1 was observed. The second structure was a directly integrated and efficient single photon source, consisting of a nanodiamond with NV centre and a photonic crystal fibre. There are many interesting devices, which can be nanoassembled with the pick-and-place technique and more structures can be found in References [114, 250]. An alternative way to build quantum hybrid devices without nanoassembling techniques will be shown in the next chapter.