Progress in Optics (eBook)

10 Plasmonic Sources

Many proposals have been made to use surface plasmon polaritons (SPPs) coupled to single-photon sources (Bulu, Babinec, Hausmann, Choy, & Loncar, 2011; Chang, Chen, et al., 2006; Gan, Hugonin, & Lalanne, 2012; Tame et al., 2008).

Surface plasmons, or SPPs, are propagating excitations of charge-density waves and their associated electromagnetic fields on the surface of a conductor. These collective electronic excitations can produce strong electric fields localized to subwavelength distance scales, which make SPPs interesting for several applications. Remarkably, these collective electron states preserve many key quantum mechanical properties of the photons used to excite them, including entanglement (Altewischer, Van Exter, & Woerdman, 2002; Fasel et al., 2005) and sub-Poissonian statistics (Akimov et al., 2007).

A strong theoretical activity started in the first years of 2000s promoting the use of coupled plasmonic structures with single-photon sources. The advantages would be an increased emission probability, due to the enhancement of the electric field produced by the plasmon, and a better extraction. Chang, Chen, et al. (2006) described a method that enables strong, coherent coupling between individual emitters and electromagnetic excitations in conducting nanostructures at optical frequencies, via excitation of guided plasmons localized to nanoscale dimensions. The authors show that under realistic conditions, optical emission can be almost entirely directed into the plasmon modes. The method, that is discussed only theoretically, can be used to create efficient single-photon sources.

Tame et al. (2008) discussed theoretically single-photon excitation of SPPs, considering the quantum description and finding that remarkably high quantum efficiencies can be reached for photon-to-surface plasmon polaritons transfer. The single photon to SPP transfer process may be assessed by measuring the correlation function g(2)(0) which is shown to be unaffected by losses.

Mallek-Zouari et al. (2010) investigated the fluorescence properties of single CdSe/CdS nanocrystals deposited close to a semicontinuous gold film, reporting a reduction by a factor ten of the monoexcitonic state decay rate. A large fraction of the plasmons are converted in the far-field single photons.

Naiki, Masuo, Machida, and Itaya (2011) obtained single-photon emission from isolated CdSe/ZnS QDs exhibiting enhanced fluorescence by interacting with silver nanoparticles.

Bulu et al. (2011) proposed to use plasmonic resonators for enhanced diamond NV-center single-photon sources. They showed theoretically that in an optimized geometry, pumping can be improved by a factor of 7, the spontaneous emission rates can be enhanced up to 50 times over NV centers in a bulk crystal, and collection efficiencies can be as high as 40%, more than 10-fold increase over the bulk.

Gan et al. (2012) discussed a compact solid-state III–V semiconductor single-plasmon source. A sketch of the proposal for the semiconductor single-plasmon source and its coupled-wave model is shown in Figure 43. An SPP channel waveguide with the attached nanocavity is placed near to the QD. The QD first excites the cavity mode, which in turn decays into the SPP.

Akimov et al. (2007) were among the first to experimentally obtaining generation of single optical plasmons in metallic nanowires coupled to quantum dots.

When a single CdSe quantum dot is optically excited in close proximity to a silver nanowire, emission from the quantum dot couples directly to guided surface plasmons in the nanowire, causing the wire's ends to light up (Figure 44a). Photon correlation demonstrated the generation of single quantized plasmons. Moreover, the efficient coupling was accompanied by more than 2.5-fold enhancement of the quantum dot spontaneous emission. The light emission at the nanowire end is a result of single, quantized surface plasmons scattering off the ends of the nanowire. This is demonstrated by the dip at t = 0 in the photon coincidence between the free space fluorescence of the quantum dot and emission from the wire end.

Schietinger, Barth, Aichele, and Benson (2009) presented a controlled coupling of a single NV center to a plasmonic structure. With the help of an atomic force microscope, a single nanodiamond containing a single NV center and two gold nanospheres was assembled step by step. The excitation rate and the radiative decay rate are enhanced by about one order of magnitude, while the single-photon character of the emission is maintained.

Kolesov et al. (2009) showed that a single-photon source coupled to a silver nanowire is able to excite single SPPs that exhibit wave and particle properties, similar to those of single photons. To generate the SPP, single color centers in diamond nanocrystals were coupled to the metallic waveguide at room temperature (Figure 45). Figure 45b shows the wide-field image of a silver plasmonic waveguide when the laser beam is focused on a single NV defect coupled to the wire. Both ends of the wire show detectable fluorescence, indicating the propagation of plasmons along the wire. To check that single SPPs are excited by the single-photon source, measurements of the second-order correlation function were carried out between the two ends of the wire (Figure 45c). The lowering of the peak at zero delay time provides experimental proof that the plasmon originates from the fluorescence of a single quantum system. It is remarkable that even though a surface plasmon involves a large number of electrons, it behaves like a single quantum particle. A control was made, measuring coincidences between the defect emission and the end of the wire which correspond to the joint detection of a photon and a plasmon. Near perfect anticorrelation was observed. Also a single-plasmon self-interference experiment was successfully done.

Choy et al. (2011) presented a high-yield approach to directly embed single NV centers into metallic nanostructures, leading to a reduction in the spontaneous emission lifetime of the enclosed NV center. They considered plasmonic apertures consisting of cylindrical diamond nanoposts surrounded by silver (Figure 46).

The device was realized and antibunching was observed.

A source of polarization-entangled photon pairs based on a cross-shaped plasmonic nanoantenna driven by a single quantum dot has been suggested by Maksymov, Miroshnichenko, and Kivshar (2012).

Enhancement of two-photon emission in the vicinity of metallic nanoparticles due to surface plasmon resonances was discussed by Poddubny, Ginzburg, Belov, Zayats, and Kivshar (2012).

11 Application to Quantum Information Processing

The challenge for the development of quantum information technologies is having reliable and efficient sources to produce, distribute, and detect entangled states (see, for example, Walmsley & Raymer, 2005). Although sources of entanglement have been described and demonstrated in many branches of physics, so far the most common way to distribute entanglement is by means of pairs of photons. The most reliable source of entanglement between photons is the spontaneous parametric down-conversion process as discussed previously. Of great importance is to have an accurate description of the distribution of the generated entangled states.

In realistic applications, where pure entangled states become mixed states it is crucial to discriminate the nature of different types of noise introduced by the emission process itself, due to imperfections in the collections or in the transmission of the entangled pairs, as discussed by Bovino (2013b), where a method to discriminate the different noise contribution in a Quantum Key...