Extreme Multiexciton Emission from Deterministically Assembled Single Emitter Subwavelength Plasmonic Patch Antenna

Plasmonic antennas are attractive optical structures for many applications in nano and quantum technologies. They have the ability to confine light on highly sub-wavelength volumes to improve significantly light-matter interaction between single quantum emitters and plasmons. By providing enhanced interaction between a nanoemitter and light, they efficiently accelerate and direct spontaneous emission. One challenge, however, is the precise nanoscale positioning of the emitter in the structure [2]. Here we present the realization of a patch plasmonic antenna [2] consisting in a single colloidal core/shell CdSe/CdS deterministically positioned with three-dimensional nanoscale control between a thick gold layer and a thin gold-subwavelenth size-nano disk [3].


Introduction.
The interaction between an emitter and its local electromagnetic field can be engineered by increasing the local density of states for applications in quantum information [1] and single photon generation [2]. This has been widely explored with various dielectric or plasmonic environments [3,4,5,6,7] and a large variety of solid-state emitters such as selfassembled [8] or colloidal [9] quantum dots, single molecules [10], and defects in diamond [11].
Optimizing the coupling between the emitter and the nanostructure makes it possible to control the emission directivity, and the dynamics of spontaneous emission. The latter is quantified by the Purcell factor F p , which scales as the inverse of the electromagnetic confinement volume provided by the photonic structure [12,13,4]. Plasmonic nano-antennas [14] exhibit very low volume and wide spectral resonance, and therefore are excellent structures for obtaining high Purcell factors F p with broadband emitters.
Achieving intense light-matter interaction using extremely low volume plasmonic structures [9,15] necessitates very precise, hence challenging, spatial positioning of the emitter at the nanometer scale. Many works so far have relied on randomly positioned emitters to demonstrate high acceleration of spontaneous emission of a single quantum dot (QD) or a cluster of QDs coupled to a plasmonic antenna [16,17,18]. The challenge of deterministically positioning individual colloidal QDs inside nanophotonic structures with three-dimensional nanoscale control has to be overcome for realizing efficient devices working at room temperature and benefiting from highly optimized light-matter interaction regimes [19,16].
A promising approach for the deterministic positioning of emitters in dielectric cavities or plasmonic antennas is in-situ optical lithography [20], which consists in measuring the emitter position optically through emission mapping with subwavelength accuracy and defining the photonic structure around the emitter during a single optical lithography step. It has been shown to allow the fabrication of efficient dielectric cavity based single photon source operating at cryogenic temperature [21], as well as to couple single self-assembled quantum dots [22] and small clusters of colloidal quantum dots in plasmonic nanostructures [6]. A similar approach has been developed using in-situ e-beam lithography [23] to create self-assembled QD microlens structures. Thus far, these technologies have not been transferred to chemically synthetized emitters such as colloidal QDs that have strong potential for cheap and room-temperature applications. The main reason for this is the exacerbated sensitivity of these emitters to technological processing. Because of the strong sensitivity of the emission process to surface states, preserving their properties during the technological protocols, involving, for example, ebeam or laser exposure, dry or wet etchings and solvents, is highly challenging.
Here, we report on a non-destructive in-situ far-field laser etching lithography technique that Plasmonic patch antenna: design and operation. Figure 1(a) depicts the system that we explore in this work: a single emitter coupled to a plasmonic patch antenna. The antenna consists of a thin dielectric layer (typically 30-40 nm) that is sandwiched between an optically thick bottom layer of gold, and a thin gold patch on top; the patch thickness is about 20 nm and its diameter is in a range of 0.2-2.5 μm. This system has been both theoretically and experimentally shown to be an excellent tool for accelerating and directing fluorescence emission [24,25]. Optimal positioning of the emitter inside the antenna couples its radiation to surface plasmon polaritons (SPPs) at both nano-spaced metal-dielectric interfaces. These SPPs at the two interfaces further couple and create strong confinement of the electromagnetic field around the emitter. The SPPs generated in the thin plasmonic metal patch (thinner than the skin depth) lead to the emission of photons [24] as depicted in Figure 1(a). The antenna operation depends on the dipole orientation of the emitter (stronger acceleration of spontaneous emission for vertical dipole orientation), the patch size (large patches are more directive but small patches show stronger resonances), and the dielectric spacer [24]. In this work, we insert chemically synthetized [26] relatively large quasi-spherical individual CdSe/CdS semiconductor core/shell colloidal QDs [27,28] in the antenna. They have CdSe cores of about 3 nm in diameter, which are encapsulated by slowly grown 6-8 nm thick CdS shells (Figure 1(b)), which make them almost non-blinking [29]. Due to their high absorption cross-section, they show bright fluorescence at room temperature. Under ultra-violet (UV) excitation at room-temperature, they emit at 633 nm with a spectral width of 30 nm. We characterize these QDs in the low excitation limit, where Auger processes are efficient enough to lead to single photon emission 1 .
In-situ subwavelength laser etching on fragile emitters. The proposed technological protocol is illustrated in Figure 2. It consists of deterministic and non-destructive in-situ laser etching that allows positioning a single QD within an antenna with a 3 nm vertical and 50 nm lateral precision (see Supplementary Information). We use a low luminescence bi-layer polymer [30] to be able to locate single emitters by mapping their luminescence.
The process starts by evaporating a thick layer of gold (200 nm) on a Si substrate using an intermediate adhesion layer of Ti/Cr. A thin layer of PMMA (10 nm) is then spin-coated above it.
Then a layer of spatially well-isolated single QDs is spin-coated using a dispersion of QDs in hexane. To create optimized spacing around the single emitters, and to protect them from direct dielectric vapor deposition, a 35 nm smooth PMMA film is spin-coated, which embeds them in a dielectric layer. This protects the emitters in the proceeding lithographic steps. A bi-layer consisting of a lift-off resist (LOR) and PMMA is then spin-coated. The low luminosity of this bilayer permits the detection of the emission of single QDs embedded beneath and excited by a low intensity spectrally filtered broadband supercontinuum laser emitting at 473-478 nm ( Figure   2(a)). As in in-situ lithography methods [20,23], we map QD fluorescence with nanoscale accuracy. The laser wavelength is then tuned to 550-605 nm, which corresponds to a wavelength range where the laser light is absorbed substantially more by the lift-off resist than by the QD.
The excitation power can thus be chosen to burn the resist bi-layer locally and create a hole directly above the selected QD without photobleaching it (Figure 2(b)). We note in Figure 2(f) that the emission lifetime of a QD does not change after the resist burning-this demonstrates that the laser etching process does not photodegrade the QD. After chemical development ( Figure   2(c)), gold is deposited by evaporation (Figure 2(d)), and a lift-off is performed to obtain patch antennas embedding a single emitter (Figure 2

(f) Emission lifetime of a QD before (blue) and after (red) the laser etching.
Emission from a subwavelength size antenna. We study the emission pattern of a very small antenna (Figures 3(a) and 3(b)) coupled to a single QD. Such subwavelength plasmonic patch antennas have been predicted to show very strong Purcell effect, and directive emission [24]. A striking feature of the measured radiative pattern is its high symmetry in the far-field. Indeed, if the emitter is slightly off-centered with respect to the antenna patch, the radiation pattern rapidly shows angular asymmetry [6]. The symmetry in the lobes of Figure 3  Acceleration of spontaneous emission. Figure 4(a) displays the fluorescence decay of a given QD before and after it was placed inside the patch antenna of Figure 3. Before the deposition of the upper gold patch, when the QD is embedded in a PMMA layer and is 10 nm above the gold surface, its emission decay has monoexponential character, that is indicative of single exciton recombination with a decay of X ref = 36 ns. As proximity to gold shortens the emitter lifetime [35,36,37,38], we estimate that, on average, this lifetime is 3 times shorter than the decay time of the same colloidal QDs in a homogeneous PMMA layer, i.e., X homogeneous X ref

=3 (see Supplementary
Information), where X homogeneous and X ref are the exciton lifetimes of the emitter in homogeneous medium (infinite dielectric medium of index 1.5) and reference medium (inside the same dieletric media but 10 nm above gold). After the deposition of a gold patch centered on the QD, the emission lifetime is considerably reduced to X Antenna . The Purcell factor is calculated using the exciton decay rate, and is given as Note that the acceleration of spontaneous emission observed in Figure 4(a) is so strong that we are limited by the response time of the measuring instrument from precisely quantifying F p . By analyzing the instrument response function, we find that emission acceleration is greater than 24 with respect to the lifetime of the same QD before the gold patch deposition, which results in F p > 72 (see Supplementary Information), when comparing with the QD embedded in a homogeneous dielectric medium. We remark that the stated Quenching of Auger processes and multiexciton emission. As the consequence of such a significant Purcell effect, the electromagnetic decay channels are accelerated, making radiative multiexciton recombination more efficient than the Auger non-radiative channels. The photon correlation curves of Figure 4(b) shows the measured second order intensity correlation of the QD emission before it was placed in the antenna, exhibiting single photon emission with g (2) (0)=0.2. When inserted in the antenna, this value rises to g (2) (0)=1 (Figure 4(c)), confirming the radiative multiexciton recombination. Detected photon rate (red) and corresponding photon emission rate after detection efficiency correction (green). The blue line denotes the pulsed laser repetition rate.

Figure 4 | Emission characteristics of a highly accelerated antenna. (a)
Inside the plasmonic antenna, the radiative decay rates are accelerated by the Purcell factor F P , not only at the exciton but at the multiexciton level as well. As a result, Auger processes that are very efficient at the multiexciton level become relatively inefficient in the high Purcell factor regime, which facilitates radiative recombination of multiexcitons [40,41,42]. This is confirmed by the loss of single photon emission in Figure 4 kHz. Given the 25% detection efficiency of our experimental setup, the photon emission rate from the QD is estimated to be about 136 kHz. Therefore, for each laser pulse, the single emitter in the antenna emits about 4-5 photons. This demonstrates extremely efficient quenching of Auger processes for at least 5 levels of radiative multiexciton recombination. It is very probable that the relaxation of the QD inside the antenna included many more higher multiexciton levels for which the non-radiative Auger recombination was not completely quenched, and which therefore did not contribute to radiation.
While high Purcell effect in plasmonic antennas has often been associated to low brightness due to the predominance of non-radiative channels [43,44,45,46], the structures developed here display very high fluorescence enhancement. Figure 5  We also note the high optical quality and stability of the fabricated antenna. At this excitation, the antenna could sustain pulses with an instantaneous power of 120 mW for more than 10 minutes before it photobleached-exhibiting the quality of the QDs under study and their preservation during the laser etching protocol reported here. We finally note that the onset of multiexciton emission depends strongly on the antenna size and the Purcell effect it provides. In general, the appearance of multiexciton emission depends on the relative weight of the radiative decay of the antennas, the onset of multiexciton emission is typically observed at larger excitation power than for smaller (hence faster) antennas. This effect can be used in applications that demand single photon to multiphoton emission switching [47].