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In this work we detail the fabrication method of a hybrid metal-dielectric nanoantenna with a single nanocrystal quantum dot positioned in its center. We have recently shown in [Nano Lett. 16, 2527 (2016)] that this device efficiently directs photons from the nanocrystal emission into a small divergence angle perpendicular to the nanoantenna surface. The fabrication method presented here is robust and can be fine-tuned by only a few parameters to achieve high yield of such nanostructures.
© 2017 Optical Society of America
1. Introduction
In recent years, there has been a great effort in shaping the emission pattern from colloidal Nanocrystal Quantum Dots (NQDs) to utilize them as efficient light sources down to the single photon level. The advantage of NQDs in this matter stems from their high quantum yield, the possibility to change the emission wavelength by changing the nanocrystal size and composition, the Stokes shift between the excitation wavelength and the emission wavelength preventing re-absorption of the emission, and most importantly, their ability to emit single photons even at room temperature. In order to exploit the promising features of NQDs for single photon sources (SPS), one needs to overcome their intrinsic isotropic emission pattern, to allow an efficient collection of single photons from single NQDs. The enhanced collection efficiency is essential for achieving deterministic single photon emission at room-temperature which is desirable for various quantum technology applications such as Quantum Key Distribution (QKD) [1] ,quantum bits (qubits) and quantum metrology [2,3].
The various methods for shaping the emission pattern include patch antennas [4] , split ring resonators [5], Yagi-Uda nanoantennas [6], and plasmonic lenses [7–9]. All these works have focused on enhancing the directivity and collection efficiency by exploiting plasmonic resonances or by multipolar interference. In these works the NQDs are positioned in close proximity to the metallic nanostructure in order to efficiently direct the emission. However, the proximity to the metal can introduce quenching of the photoluminescence (PL) which reduces the quantum yield of the single photon emission. In addition, it has been shown that the proximity to the metal enhances the biexciton emission rate compared to the exciton emission rate [10, 11]. This in return degrades the performance of the SPS as there will be a non-negligible probability of emission of two simultaneous photons.
Another obstacle is the positioning of a single NQD at an exact location on the nanoantenna to achieve the desirable performance. This requires nanometric accuracy. One method for accurate positioning is nano-manipulation with an AFM tip [12]. This has been shown to be efficient for moving nanoparticles on a flat surface. In the case of a plasmonic nanoantenna it is difficult to manipulate the NQD due to the structure roughness. This method is also limited for surface positioning, and does not allow positioning inside a host material. Furthermore, it is a rather cumbersome procedure which is probably not adequate for mass production.
Other very sophisticated methods have shown the possibility of attaching nanoparticles using plasmonics optical tweezing which is assisted by the hotspots of a plasmonic bowtie antenna [13]. Still, it is not yet a simple task to attach and stick such tiny single NQDs so they will remain at the center of the nanoantenna. Furthermore, the single NQD is positioned very close to the metal which would lead to enhanced biexciton emission probability which degrades the single photon emitter properties of the NQD as discussed above.
Other type of structures which have been proven to be efficient for directing emission of nano-emitters are dielectric nanoantennas [14–17], nano-pillars [18] and photonic crystals [19]. These structures have been used with self-assembled quantum dots and with color centers in diamonds. The advantage of dielectric nanoantennas comes from the high directionality along with low loss, which is appealing compared to metallic nanoantennas. The disadvantage of dielectric nanoantennas, regarding room-temperature NQDs, is the high quality factor which limits the operation bandwidth, and the large Purcell factor which can reduce the efficiency of Auger recombination and limit the fidelity of a dielectric single photon source. In addition, dielectric nanoantennas require a very demanding fabrication to overcome the substantial emission into the substrate, and it is unclear how to incorporate NQDs accurately into such epitaxially grown complex multi-layers structures.
Our recent efforts for achieving a well defined collimated beam of single photons emitted from NQDs embedded in a hybrid metal-dielctric nanoantenna were presented in Refs. [7,8,20]. In Ref. [20] we were able to demonstrate collimation of single photon emission from a single NQD in such a device, so it can be efficiently collected using low numerical aperture (NA) optics, such as that of a single mode fiber (SMF). In this paper we present a detailed method of incorporating a single NQD at the center of a hybrid metal-dielectric circular nanoantenna similar to the device presented in Ref. [20]. We also present the different characterization steps to evaluate the success of the process. The ability to efficiently couple the photons from the single NQD into such an NA can be utilized for a direct coupling of the device with a SMF without the need of any additional optics, which can allow a great deal of compacting and simplifying of future on-chip, integrated SPS. We fully describe here all the stages, starting from the principle of operation, simulations and design, through the layer-by-layer e-beam lithography process and various chemical processes, and finally, the optical characterization and verification of the fabricated device. Such a device was recently shown as a prototype of an efficient SPS operating at room temperature [20].
2. Device design
A cross section of the hybrid metal-dielectric nanoantenna with a single NQD in its center defining all the geometrical parameters is presented in Fig. 1(a). The hybrid nanoantenna consists of an Ag substrate layer with an Ag bulls-eye nanostructure above (see Fig. 2(a)), and a PMMA layer in which a single NQD is embedded at a distance of 250 − 350nm from the Ag substrate.
Fig. 1 (a) A cross section of the proposed hybrid nanoantenna with the relevant geometrical parameters (described in detail in the main text). The red arrows depict the physical operation mechanism of the nanoantenna–the single NQD emits photons into the waveguide mode which are scattered and diffracted constructively by the slits in a direction perpendicular to the surface. (b) The simulated angular emission pattern as a function of the polar angle θ of a single NQD located at a distance of d = 300nm from the surface of the metallic nanoantenna. This is the average distance for the NQDs embedded in the designed hybrid nanoantenna. The shaded areas correspond to collection into NA = 0.12 (pink area) and NA = 0.65 (blue area). (c) The simulated collection efficiency as a function of the distance d from the surface for NA = 0.12 (red curve) and NA = 0.65 (blue curve).
To understand the principal design rules, we describe the physical mechanism of the hybrid nanoantenna. The single NQD (purple circle in Fig. 1(a)), emits photons (red arrows in Fig. 1(a)) preferentially into the PMMA layer which serves as a single mode slab waveguide. These photons are scattered from the circular slits of the bulls-eye structure such that the scattered fields interfere constructively mostly in a direction perpendicular to the hybrid nanoantenna (Fig. 1(a)). Therefore, the first requirement of the sample should be matching between the waveguide propagation k-vector (β), which is determined primarily by the PMMA layer thickness and the emission wavelength λ, to the grating Bragg vector. This implies:
so the bulls-eye period (Λ) and the thickness of the PMMA layer (h) are adjusted to fulfill Equation 1. This requirement is not enough to ensure the efficient extraction of the PL from the hybrid nanoantenna. The location of the emitter (d) is also very important, as it strongly influences the efficiency of photon emission into the waveguide mode, and is investigated in the simulations, as is described below.The geometrical dimensions of the hybrid nanoantenna are optimized using extensive COM-SOL simulations. There are several crucial geometrical parameters that need to be adjusted correctly. Fig. 1(b) shows the numerically simulated normalized angular emission pattern for a single emitter emitting at 753nm, located at a distances d = 300nm from the substrate (corresponding to the average distance of the NQDs in the fabricated device described below). This calculation shows the emission polarized perpendicular to the plane of Fig. 1(a), which was found to be the most prominent polarization. The emission into a narrow angle around θ = 0 is evident though side lobes are visible at larger angles. These side lobes are sensitive to the distance d from the substrate. This sensitivity results from the different coupling efficiencies of the emission into the waveguide mode for different d. Additionally, for very small d, the coupling to the metal surface modes compete with the coupling to the waveguide modes, which reduces the overall efficiency of the device.
In Fig. 1(c) we present how different vertical locations (d) of the NQD result in different collection efficiencies for different numerical aperture (NA). The NA used for this calculation are NA = 0.12 and NA = 0.65 which correspond to the NA of a standard SMF and the objective used in Ref. [8], respectively. It is evident that when locating the NQD at d ≃ 300nm the collection efficiency between the two different NA’s is similar. This is seen in Fig. 1(b) which shows that the side lobes are rather low for d = 300nm. It is quite clear that locating the NQD at d ≃ 300nm should result in high collection efficiency for NA = 0.12 of almost 40% of the total emission. As a result, the dimensions of the specific bulls-eye structure that will be presented here are a period of Λ = 690nm, slit width of a = 150nm, slit depth of w = 100nm, NQD distance of d ≃ 300nm, SiO2 layer thickness of s = 250nm and PMMA layer thickness of h = 410nm, as determined by the optimization procedure described above.
3. Device fabrication
After obtaining the desired geometrical dimensions of the hybrid nanoantenna from the simulation, we move to describe the multi-step fabrication process of a device having a single NQD in the center of the hybrid nanoantenna. The different stages are depicted in Fig. 2. The substrate of the nanostructure is a 100nm thick evaporated Ag on Si (< 100 > plane). On the substrate we fabricate an array of 100 bulls-eye nanostructures using standard e-beam lithography (see also SEM image in Fig. 3(a)). Alignment marks are written down along with each bulls-eye structure for an accurate re-alignment of an additional e-beam lithography stage that will be described later. The first e-beam lithography is followed by e-beam evaporation of 100nm Ag followed by a lift-off procedure that results in a metallic bulls-eye nanostructure (Fig. 2(a)). The sample is then covered by a 250nm thick SiO2 layer grown by PECVD. This is a crucial step as we have found that without a SiO2 buffer layer, the NQDs tend to stick to the metal substrate. The thickness of the SiO2 layer also helps defining the distance of the NQD layer from the surface d. After this stage, we prepare a PMMA solution with multiple NQDs [25]. PMMA was chosen to host the NQDs as it is a standard polymer for nanofabrication methods and is a positive resist which implies that the NQDs are not damaged where the resist is not exposed to the e-beam irradiation. It is also highly transparent at the emission wavelength range of the NQDs used, and its index of refraction is quite similar to that of the SiO2 layer. The NQDs are initially dispersed in Decane. Mixing the NQDs solution with PMMA causes the NQDs to sink to the bottom of the solution, due to the low solubility of the NQDs in Anisole, which is the standard solvent of PMMA. This problem is solved by dilution of the PMMA in Toluene. Toluene is a good solvent for both the NQDs and the PMMA and prevents aggregation of the NQDs that is detrimental for achieving only a singly dispersed NQDs in the polymer matrix. The ratio of Toluene and PMMA A11 is 20 : 1. PMMA A11 stands for the percent of solid PMMA dissolved in Anisole. Therefore, when using a different PMMA concentration, the ratio of Toluene:PMMA should be recalibrated in order to achieve the desired PMMA thickness.
Fig. 2 (a) A schematic drawing of the Ag bulls-eye nanostructure fabricated by e-beam lithography and liftoff procedure. (b) The hybrid nanoantenna after deposition of 250nm SiO2 and spin-coating of 100nm thick PMMA+NQDs solution. (c) The hybrid nanoantenna after a second e-beam lithography using alignment marks. The cylinder of PMMA+single NQD is on top of the SiO2 layer. (d) The final device. After removing the SiO2 layer around the PMMA cylinder with BHF, the sample is spin-coated with an additional PMMA layer. The cylinder in the image is the remaining SiO2 cylinder that was protected by the PMMA cylinder in (c).
Fig. 3 (a) An SEM image of the bulls-eye nanostructure (corresponds to Fig. 2(a)). (b) An AFM scan of the device after the second e-beam lithography step (corresponds to Fig. 2(c)) In this case a 5μm cylinder was created for better visibility. (c) A radial averaging of the AFM image of Fig. 3(b). The area of the cylinder (the edge is represented by the vertical black line) is higher by ∼ 50nm from the sample outside the cylinder. (d) An AFM scan of the final device. The surface is optically flat (roughness< λ/10). (e) A radial average of Fig. 3(d). The edge of the cylinder (which is 2μm in diameter) is marked by the black vertical line which shows that the difference in the height of the cylinder compared to the bulls-eye nanostructure is reduced from ∼ 250nm to ∼ 40nm. The red dashed line is the average of the surface roughness outside the cylinder and shows variations of only a few nm, having the grating period.
The mixed solution is spin-coated at a speed of 2000 RPM on top of the SiO2 layer to achieve a thin layer of less than 100nm (Fig. 2(b)). The PMMA layer is baked for 2 minutes at 180°C without any damage to the NQDs (this was verified from second order photon correlation measurements [20] before and after the baking). The concentration of the NQD in the PMMA is tuned such that we have an average of 0.5NQD/μm2 in the baked thin film.
In order to remove all NQDs except for the ones at the centers of the nanoantennas, another step of e-beam lithography is performed where we remove the PMMA from the whole bulls-eye nanostructure except for a cylinder with a diameter of 2μm exactly at the center of the bulls-eye (Fig. 2(c)). Several works have already shown the possibility to attach a single NQD to different nanoantennas using patterning with e-beam exposure [5, 6, 26], although no single NQD emission was shown in these systems as far as we know. The 2μm diameter is chosen with respect to the diluted PMMA+NQDs solution to achieve only a single NQD on average in the remaining PMMA cylinder, while it is still wide enough to prevent any damage to the NQDs from both the exposure to the electron beam and from the backscattered electrons. The accurate positioning of the cylinder at the center of the nanostructure is done by a realignment procedure using the alignment marks written in the first e-beam lithography with accuracy better than 20nm. The accuracy of the realignment is verified by an AFM measurement scan (Fig. 3(b)–3(c)).
The next step is selective removal of ≃ 70nm from the SiO2 buffer layer using Buffer Hydro-Fluoric acid (BHF). As mentioned above, without the SiO2 layer, residual NQDs from the PMMA+NQD layer tend to attach to the patterned metal substrate during the development process, thus contaminating the antenna with unwanted NQDs outside the central cylinder. In order to avoid this, a large fraction of the SiO2 layer is etched along with those residual NQDs. This procedure leaves a sample with a diluted NQD+PMMA cylinder intact at the center (BHF does not etch PMMA). A large fraction of fabricated nanoantennas has cylinders that contain only a single NQD in it. At this stage, the sample consists of Ag bulls-eye nanostructures, each has a cylinder SiO2 below a cylinder of PMMA+NQD at its center, and some of those antennas have only a single NQD embedded in the cylinder. The PMMA and the SiO2 have an optical index of 1.49 [27] and 1.45 [28] respectively at 800nm. This index matching between the two dielectric materials corresponds to the design optimization of the hybrid nanoantenna with an NQD embedded in a homogeneous dielectric layer with a refractive index of ∼ 1.47, as described above. Next, another PMMA layer is spin-coated on top of the hybrid nanoantenna to complete a waveguide layer thickness of 410nm, so the full hybrid metal-dielectric nanostructure is achieved (Fig. 2(d)). The second PMMA layer is also baked for 2 minutes at 180°C without any damage to the NQD. An AFM measurement reveals that the surface of the final hybrid nanoantenna is optically smooth (< λ/10) with the highest variation around the cylinder (Fig. 3(d)–3(e)).
An important point to address is the selected thickness of the PMMA+NQDs layer (∼ 100nm). In principle, to have the best vertical accuracy and the least variation of the NQD positioning above the metal structure, the PMMA+NQD layer should be as thin as possible. However, as the size of the NQDs is approximately 21nm [25], it is crucial to have the NQDs in a PMMA layer much thicker than the NQDs diameter as the BHF can damage the NQDs even when they are embedded inside the PMMA. This is seen in Fig. 3(b), which shows an AFM image where the bright dots on top of the PMMA+NQDs cylinder are NQDs that are exposed to the BHF and are consequently damaged. On the contrary, the NQDs deep inside the PMMA layer are not damaged and therefore are still optically active.
4. Device characterization
The ultimate goal of the hybrid nanoantenna is to direct the emission of single photons from a single NQD [20]. First we verify that we indeed have only a single NQD in the center of the hybrid nanoantenna. This is done by measuring the blinking behavior of the NQD on the final device, by recording the emission intensity from the device as a function of time using a nonresonant CW laser excitation. A telegraphic intensity distribution with two distinct signal levels which correspond to an “OFF” state (low intensity) and an “ON” state (high intensity) is a signature of a single NQD. In Fig. 4(a) we present the intensity time trace from the device, showing indeed a telegraphic behavior of a single NQD with “ON and “OFF” states.
Fig. 4 (a) Time-trace of the emission from the hybrid nanoantenna device. The telegraphic blinking is a good evidence of a single NQD. (b) An angular emission measurement of a single NQD emitting from the center of the hybrid metal dielectric nanoantenna. (c) A 2nd order normalized intensity correlation g(2) measurement of the hybrid nanoantenna device. The measurement shows the normalized signal per pulse with a value of 0.37 (less than 0.5) around zero time delay which is another indication of a single NQD emission. (d) The normalized emission from the metallic bulls-eye structure (red curve) overlaps the emission spectrum of the NQDs (blue curve). The emission from the metal was taken from a hybrid nanoantenna without any NQDs.
As discussed, the goal was to direct the emission into a well defined angle. In Fig. 4(b) we show the angular emission measurement of a single emitting NQD. The white lines correspond to the constant values of the azimuthal angle φ (see Refs. [7, 8, 20] for more details). The emission is directed to a small solid angle around 8°.
A single NQD has a strong non-classical light emission, with a sub-Poisson statistics. This is manifested by an anti-bunching behavior when measuring second-order photon correlations, using a Hanbury-Brown and Twiss (HBT) setup [29]. To check this behavior, the sample is excited with a pulsed laser at 405nm with a pulse duration of ∼ 55ps and a repetition rate of 1MHz (corresponding to a delay between pulses τp = 1μs). In Fig. 4(c) we show results of the second order correlation in an HBT measurement. A clear anti-bunching around zero delay, i.e, g(2)(0) < 0.5 is evident which indicates a single NQD emission from the hybrid nanoantenna device.
It is important to note a limitation when using a metal in designing nanoantennas for single photon sources. As can be seen from Fig. 4(c), g(2)(0) > 0 which means a non-vanishing probability of a simultaneous emission of more than one photon. The residual peak at g(2)(0) is associated with the simultaneous emission of a photon from the NQD and a photon emitted from the metallic bulls-eye nanostructure itself. This metal emission results from the excitation of the rough and corrugated surfaces of the metallic antenna by the non-resonant laser excitation (intended to excite the NQD). Non-resonant excitation of corrugated metals results in a much more metal emission than excitation of flat metals [30, 31]. Figure 4(d) shows the spectrum of the PL of NQDs ensemble along with the spectrum from the bare metal. The spectrum from the metal was taken by illuminating a hybrid nanoantenna without any NQDs (no PL of NQDs from this specific nanoantenna) thus the PL is indeed from the metallic nanostructure. This limitation was discussed in detail in Ref. [20] were we showed that indeed most of the residual emission comes from the excitation of the corrugated metal structure and not from an enhancement of the biexciton emission. We showed that the probability for emission of only a single photon per excitation pulse in such a device was slightly greater than 70% [20]. Several modified designs approaches can be taken in order to further improve this value. In one approach, we propose to use sinusoidal grooves rather than rectangular ones in the metal, using a stripping-template method [32]. This would decrease the roughness of the metallic nanostructure thus reducing the direct optical excitation of the metal part. Another new design, which seems to show promising preliminary results (not shown) is a bulls-eye structure with an extended flat center. In such a design, the laser excitation is focused only on the flat metal part, thus the efficient excitation of the corrugated parts is avoided, strongly reducing the intensity of the metallic residual emission.
5. Conclusions
In this work we show a new approach of embedding a single NQD into a hybrid metal-dielectric nanoantenna for directional emission of single photons. This new design preserves the advantages of plasmonic nanoantennas of easy and robust fabrication on one hand, and avoids the associated ohmic losses, quenching, and enhancement of the biexciton emission rate on the other hand. This work shows that the fabrication process of the hybrid metal-dielectric nanoantennas are successful in preserving the single NQD optical properties with an antenna design intended for strongly enhancing the collection efficiency of the single photon emitter by tailoring its angular emission [8, 20]. The design and fabrication process should enable the development of scalable, high efficiency room temperature operating single photon sources based on NQDs or other nanoemitters of single photons at room temperature.
Funding
The Einstein Foundation Berlin; The U.S. Department of Energy: Office of Basic Energy Sciences, Division of Materials Sciences and Engineering; The European Cooperation in Science and Technology through COST Action MP1302 Nanospectroscopy; The Ministry of Science and Technology, Israel.
Acknowledgments
MGH and NL contributed equally to this work.
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