Generation of Single SPPs on a Nanowire

surface plasmons [313].

8.1.9. Applications of SPPs

In recent years, there has been a great interest in SPPs in many different fields of physics and technology. Their ability to be guided on volumes far below the optical diffraction limit makes them possible candidates for highly integrated optical information processing and communication [309, 314]. This is especially true in the near-infrared, where their losses (see Section 8.1.5) become more tolerable than at visible frequencies, but are still a major hurdle. To date, practical devices are still missing.

The ability of SPPs to focus and confine energy at the nanoscale, in contrast, has found a lot more of applications. Highly concentrated SPPs at the end of metal tips can be used in near field microscopy to have very localised excitation [315]. Emit-ters can be interfaced very efficiently via SPPs, what, possibly in combination with a photonic to SPP converter (see Section 8.3) can be used in devices like a proposed single photon transistor [290]. Another, and maybe the most important, applica-tion of SPPs lies in sensing. Informaapplica-tion on an analyte is obtained by measuring the coupling condition of a light beam to SPPs (see Section 8.1.6). A change in refractive index of the analyte will shift this condition and from the corresponding shift information on the sample is obtained [316, 317].

Here, only some applications of propagating SPPs, which are the ones most important in this thesis, are reviewed, but it has to be noted that there are many other applications of plasmonics. Reviews on plasmonics can be found, for example, in References [284, 318, 319].

8.2. Generation of Single SPPs on a Nanowire

In this section, as a very fundamental experiment for quantum plasmonics, con-trolled generation, waveguiding, and splitting of single SPPs is shown. Even though SPPs involve collective oscillations of many electrons, it is possible to have single SPPs with properties similar to the properties described for photons in Chapter 2.

A basic demonstration of this is shown in the following, where single SPPs are ex-cited on a silver nanowire. Parts of this have also been published inOptics Express with the title Single defect centers in diamond nanocrystals as quantum probes for plasmonic nanostructures [168].

The nitrogen vacancy centre in diamond (NV centre, see Chapter 3) can create a well defined optical excitation which can then be launched into a nearby system under investigation [320]. The generation of a single photon from a NV centre can thus be regarded as an ultimate quantum limit of a pump pulse in a pump-probe experiment.

a b c

Figure 8.6.:Coupling of a diamond nanocrystal with a single NV centre to a silver nanowire and excitation of single surface plasmon polaritons. (a) is a scheme of the experimental setup used. (b) shows an AFM image of the diamond nanocrystal (indicated by the dashed circle) positioned at the side of a bent silver nanowire. (c) shows a microscope photoluminescence image of the same configuration. Positions 1 and 3 indicate the ends of the nanowire, while position 2 marks the location of the diamond nanocrystal. (Figure adapted from [168])

Excitation of SPPs by a single NV centre and guiding along a metal nanowire was demonstrated using structures which were randomly assembled by deposition of metal nanowires and quantum emitters on a sample surface [321–324]. In these experiments, on-demand positioning and change of a once assembled configuration was not possible.

Here, a setup consisting of a confocal microscope combined with an atomic force microscope (AFM, see Figure 8.6 (a) and also Section 5.2) is used for a more con-trolled experiment. This combination allows both, optical detection of photons and nanomanipulation of the nanodiamond [170, 206]. Photon correlation mea-surements are performed with a Hanbury Brown and Twiss configuration of two avalanche photo diodes with a quantum efficiency at 700 nm of approximately 30 %.

A diamond nanocrystal containing a single NV defect centre is first optically char-acterised on a cover slip and then individually picked up by the AFM tip (see Section 5.3 for details of this process). Then, the nanocrystal is transferred to a cover slip with chemically synthesised silver nanowires and placed on-demand near a previously selected wire which serves as SPP waveguide. With this technique, one can be sure that there is exactly one diamond containing exactly one single NV defect centre on the whole sample. Hence, there is no possibility of accidentally measuring photons coming from multiple diamond crystals or a diamond contain-ing more than one defect centre. Subsequent nanomanipulation with the AFM tip allows for fine-positioning of the nanocrystal and launching of a single excitation at arbitrary positions along the wire.

8.2. Generation of Single SPPs on a Nanowire

Figure 8.6 (b) shows an AFM image of the nanowire used in this experiment.

Its diameter is approximately 80 nm. A sharp bend separates the nanowire into two arms of 1.9µm and 0.7µm length. Under continuous wave laser excitation of the NV centre (at a wavelength of 532 nm), photoluminescence directly from the NV centre (position 2 in Figure 8.6 (c)) as well as light emerging from the bend and from the ends of the nanowire (positions 1 and 3 in Figure 8.6 (c)) is visible. Since there is a strong fluorescence background emerging from the bend of the nanowire while exciting the diamond, for further measurements the diamond was placed at another position, so that the nanowire bend is no longer in the excitation spot (position 2 in Figure 8.7 (c)). Already this repositioning of the nanocrystal shows the advantage of nanomanipulation. Accidental inconvenient configurations can be corrected and experiments can be repeated under otherwise unchanged conditions. At the same time, the ability to reposition is crucial in more complex structures, since slight changes of the position of an emitter with respect to a plasmonic structure may already modify its emission as well as the structure’s plasmonic properties significantly. Despite the possibility to perform near-field simulations, under experimental conditions an a-priori prediction of an optimum position for an emitter or nanoprobe is often impossible [325].

With the new position of the diamond nanocrystal, there is now an additional fluorescent spot observable on the nanowire. One (position 2 in Figure 8.7 (b)) corresponds to the emission of fluorescence directly from the nanodiamond. An autocorrelation measurement on this spot shows a pronounced antibunching be-haviour (Figure 8.7 (c)) confirming the single photon character of emission from a single NV centre in the nanodiamond. The other spots correspond to three output ports for single excitations launched into the wire via the nanodiamond.

In order to prove the quantum nature of the excitations, cross-correlation mea-surements between the light directly emitted from the nanodiamond (position 2 in Fig. 8.7 (b)) and the light emitted from both ends of the nanowire (positions 1 and 3 in Figure 8.7 (b)) are performed. Again, a clear antibunching is observed, proving that indeed light from the single NV centre is converted into single SPPs which propagate to the output ports. The nanowire thus represents a plasmonic beamsplitter with three output ports, a key building block for quantum plasmonic elements.

Recently, this technique of launching single SPPs with a NV centre has been used in a number of further experiments, including pulsed excitation [172] and extensive nanomanipulation of the waveguiding silver wire [211]. In this thesis, in Section 9.3.2, the plasmonic properties of silver nanowires are further investigated.

1

3 2

a b c

d e

Figure 8.7.: Nanodiamond with single NV centre launching single plasmonic ex-citations. (a,b) are AFM and fluorescence image showing a silver nanowire and a nanodiamond with a single NV centre. The bright spot between position 2 and 3 in (b) emerges from the bend of the nanowire. (c) shows the autocorrelation of the photons from the diamond measured at point 2. (d,e) are cross-correlations between the photons emitted from position 2 and from positions 1 and 3, respectively. The antibunching dip in the curves clearly reveals the non-classical properties. (Figure adapted from [168])