Letter pubs.acs.org/NanoLett
Unidirectional Emission of a Site-Controlled Single Quantum Dot from a Pyramidal Structure Sejeong Kim, Su-Hyun Gong, Jong-Hoi Cho, and Yong-Hoon Cho* Department of Physics and KI for the NanoCentury, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea S Supporting Information *
ABSTRACT: Emission control of a quantum emitter made of semiconductor materials is of significance in various optical applications. Specifically, the realization of efficient quantum emitters is important because typical semiconductor quantum dots are associated with low extraction efficiency levels due to their high refractive index contrast. Here, we report bright and unidirectional emission from a site-controlled InGaN quantum dot formed on the apex of a silver-coated GaN nanopyramidal structure. We show that the majority of the extracted light from the quantum dot is guided toward the bottom of the pyramid with high directionality. We also demonstrate that nanopyramid structures can be detached from a substrate, thus demonstrating great potential of this structure in various applications. To clarify the directional radiation, the far-field radiation pattern is measured using Fourier microscopy. This scheme will pave the way toward the realization of a bright and unidirectional quantum emitter along with easy fabrication and large-area reproducibility. KEYWORDS: Unidirectional emission, single-quantum emitter, efficient single emitter, self-aligned coupling
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It has been reported that apex QDs can be not only sitecontrolled but also linearly polarized, with a high polarization ratio of up to 0.93.15,16 Light sources with high polarization ratios are indispensable for quantum computing applications.17 The polarization of light from a QD has been originated from the fine structure splitting of the exciton and influenced by anisotropic strain and shape and the size of a QD.18,19 For instance, strongly polarized light from apex QDs was recently demonstrated by controlling predefined elongated direction of a pyramid.20 Given their unique characteristics, apex QDs represent a promising light source for quantum information applications. Since the first demonstration of the formation of InGaAs QDs on GaAs pyramids,21 numerous studies based on diverse semiconductor materials have been conducted.22 For example, single exciton peaks were observed from InAs QDs on a GaAs pyramid structure,23 from InGaN QDs on a GaN pyramid,24−27 and from GaN QDs on an AlGaN pyramid structure.28 Singlephoton emissions from QDs on pyramid structures were also reported using InGaAs QDs21 and InGaN QDs.27 More recently, single-photon emissions were confirmed from GaN− AlGaN QDs on pyramidal structures at temperature up to 350 K.28,29
emiconductor quantum dots (QDs) are a promising building block for the implementation of solid-state quantum information processing.1−6 Semiconductor QDs have many advantages, including high stability, a fast decay time,7,8 and good wavelength tunability.9 They also enable unique systems such as electrically driven QDs or optical resonator combined QDs.10−14 Despite these advantages, semiconductor QDs typically have random positions, preventing them from being utilized in numerous applications. Single QDs formed on the apex of a pyramidal structure (which will be referred to as “apex QD”) have recently received much attention because they create site-controlled QDs under natural conditions. By virtue of the well-defined positions of the apex QDs, they facilitate the deterministic coupling between single quantum emitters and micro- or nanocavities,10,15 thus expanding the possible application areas to cavity quantum electrodynamics. A metallic cavity using plasmonic mode is especially advantageous because it provides easy spectral matching between cavity resonance and QD emission frequency. The strong Purcell effect as a consequence of a single QD coupled to an optical cavity is a highly desirable feature, as the acceleration of the spontaneous emission rate would increase the repetition rate, extend operation conditions close to room temperatures, and reduce dephasing from a solidstate environment. Recent works clearly show that coupling between optical modes and QDs is easily realized using apex QDs.10,15 © 2016 American Chemical Society
Received: June 8, 2016 Revised: September 12, 2016 Published: September 14, 2016 6117
DOI: 10.1021/acs.nanolett.6b02331 Nano Lett. 2016, 16, 6117−6123
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Nano Letters
Figure 1. Quantum dot (QD) on the apex of pyramidal structure. (a) Schematic of the silver-coated nanopyramidal structure. A single dipole source is located at the apex of the pyramid structure. (b) Dimensions of the nanopyramid structure for the FDTD simulation and the monitor positions are displayed, with the inset showing a cross-sectional TEM image of a nanopyramid. The scale bar in the TEM image is 50 nm. Calculated far-field radiation pattern toward the upper and lower hemisphere at a wavelength of 520 nm for single dipole in (c) a pyramidal structure and (d) a planar structure. The maximum intensity values of the upper hemispheres in the scale bar, which are 0.03 and 0.09, respectively, are normalized by that of the lower hemispheres. Normalized radiation intensity versus angle θ for (e) a pyramid structure and (f) a planar structure.
directional emission. As a result, highly directional emission (beam divergence angle corresponding to 28°) from single QDs in inverted nanopyramid structures is experimentally demonstrated using Fourier microscopy. The second-order photon correlation histogram was also obtained using the Hanbury Brown−Twiss (HBT) setup to prove single-photon emission from the apex QDs. A schematic design of a silver-coated nanopyramid made of GaN (n = 2.42) is shown in Figure 1a. The GaN pyramidal structure is composed of a hexagonal pyramid, a post, and a substrate. A linearly polarized single dipole source is located at the position of the apex QD. The detailed parameters of the sample and monitor positions are shown in Figure 1b. The inset shows a cross-sectional transmission electron microscope (TEM) image of a nanopyramid. The hexagonal pyramid has a height of 180 nm, and the post has a diameter of 180 nm and a height of 70 nm. These structural dimensions can be controlled by changing the size of the hole opening size and the growth conditions. A detailed description of the growth mechanism is explained in our previous work.15,32 The GaN-based pyramidal structure is specifically investigated in this study; however, the simulation results are applicable to other types of materials having comparable dimensions. Our study is mainly focused on metal-coated pyramidal structures to make use of Purcell enhancement stemming from the three-dimensional (3D) plasmonic cavity;15 however, simulation data for the case without a metal film is also included in Figure S3 to present the effect of solely a pyramidal-shaped structure. In this work, the GaN nanopyramid is especially conformally coated by silver film with a thickness of 40 nm to realize efficient light coupling toward the bottom direction and to prevent light from escaping toward the top or side directions. Apex QDs in plasmonic cavities experience strong enhancements of the spontaneous emission rate, thus shortening the decay time, which is a key function for deterministic single-photon emitters. A detailed
In addition to the site controllability and the high polarization ratio, controlling the directions of photons with small beam divergence toward a predesigned optical waveguide or a free-space measurement setup is important for high-quality single photons. Light emitted toward a vertical direction with narrow beam divergence can enhance the light collection efficiency when using a practical measurement system with a numerical aperture (NA) of less than 1. Among various photonic structures containing a quantum emitter, efficient light collection has long been discussed with regard to nanowire architectures and microcavities such as photonic wires,3 photonic crystals, 30 or distributed Bragg reflectors (DBRs).11,31 In pyramidal architectures with apex QDs, however, manipulating the emission direction remains to be discussed, representing an inevitable prerequisite to achieving high collection efficiency levels. Furthermore, the pyramidal array with site-controlled apex QDs are easy to fabricate because they are formed by a bottom-up approach, as opposed to earlier methods, which mostly use top-down approaches with randomly distributed individual QDs, making stable directionality comparatively easy to realize. Here, we report a method by which pyramidal structures can be used to control the emission direction of site-controlled QDs. With this structure, 93% of the extracted light emitted from apex QDs is guided toward the bottom of the pyramidal structure, as confirmed using the finite-difference time-domain (FDTD) method. The guided light experiences reflection at the substrate−air surface, still harvesting more than 91% of the extracted light through the bottom hemisphere. More precisely, the strongly directional emission facilitates a high collection rate of 50.8% using an objective lens with a NA of 0.7 in the free space. These apex QDs, which naturally emit most of their photons toward the bottom, are detached and inverted using an ultraviolet (UV)-curable optical adhesive material to provide various applications and to facilitate the easy collection of 6118
DOI: 10.1021/acs.nanolett.6b02331 Nano Lett. 2016, 16, 6117−6123
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Figure 2. Calculated far-field radiation patterns of the pyramidal structure at different wavelengths (500, 520, and 540 nm). The upper spheres (a−c) indicate light propagation toward the top of the pyramid structure, while the lower spheres (d−f) indicate the light after it passes through a finite thickness of the substrate. Percentage values represent the integrated radiation power percentage to each (upper or lower) hemisphere when the total power radiated toward the outside of the structure (i.e., far-field) is 100%. Here, the thickness of the substrate is assumed to be much greater than the wavelength, as in a practical case. The white circles represent θ = 30°.
displayed inside the middle of Figure 1c,d. In the metal-coated pyramid case, the transmitted power to the upper and the lower direction is 0.595 and 8.09, respectively. This indicates that 93% of the light extracted from the dipole source goes toward the substrate in a normal direction, i.e., unidirectional emission. The detailed power calculation result is shown in Table S1. The metal-coated pyramidal structure not only provides good directionality but also radiates light with the Gaussian shaped radiation profile to the lower hemisphere, which may contribute to the efficient coupling to a fundamental mode of the optical fiber. This type of directional emission is also observed from a bare pyramid structure (i.e., without a silver film), as shown in Figure S3, and is a distinctive feature compared to the dipole emitter in a planar substrate (e.g., self-assembled QDs), as described in Figure 1d. Figure 1d shows that most of the light propagates toward the substrate but with a highly diffracted angle. Total transmitted power to the lower hemisphere corresponding 1.02 is much weaker than that of the metalcoated pyramid case. To compare the relative intensity between the lower and the upper hemispheres, the far-field radiation patterns in panels c and d of Figure 1 are replotted in Figure 1e,f using polar coordinates in which r and θ correspond to intensity and the radiation angle, respectively. Both plots are normalized by the maximum intensity of each lower hemisphere. For the nanopyramid structure (Figure 1e), the unidirectional emission toward the lower hemisphere in the normal direction is clearly observed, while weak and diffracted emission appears at 42° toward the upper hemisphere. For the planar structure (Figure 1f), it shows bidirectional emission toward the lower hemisphere at an angle of 35°, leading to the total internal reflection at the GaN−air interface. The far-field radiation patterns of pyramidal structures are displayed at various wavelengths in Figure 2. Here, we considered the case that the guided light toward the substrate direction in Figure 1c eventually escapes from the GaN substrate and undergoes reflection and refraction at the GaN−
study of the spontaneous emission rate enhancement, including numerical modeling and experimental evidence, was described in a previous paper.15 Due to the enhanced spontaneous emission from the dipole emitter in the metallic cavity, light collection through the lower hemisphere is greater than that without the metal pyramidal structure despite the metal absorption taking place. In detail, 66% of the light from the dipole source in a metal-free GaN pyramid is guided toward the lower direction. However, only 18.9% of dipole power is radiated through the lower hemisphere due to metal absorption (79.7% of the light is absorbed by metal). However, the dipole power in a metal-coated GaN pyramid is enhanced by approximately 644 times due to the 3D-focused plasmonic mode in comparison to the case without a metal film. Therefore, the total radiated power toward the lower hemisphere is 180 times brighter than that for the case without metal. These calculations are based on a dipole wavelength of 520 nm. A method that can be used to attain bright and unidirectional emissions from an apex QD with a silver-coated pyramid structure is explained below. The far-field radiation patterns of the emission wavelength at 520 nm are plotted in Figure 1c. On the one hand, as shown in Figure 1c, the light emitted from a dipole source toward the air (the upper hemisphere) is diffracted from its normal direction. On the other hand, we observed highly directional emission toward the substrate (i.e., the lower hemisphere). In this case, the far-field radiation pattern to the lower hemisphere is plotted at the position of monitor 1, as described in Figure 1b, indicating that the light propagates through the GaN medium. The far-field radiation intensities are normalized to their maximum values at each hemisphere to show the shape of each radiation patterns clearly. The maximum value of the upper hemisphere scale bar is normalized by that of the lower hemisphere. Transmitted power to the upper and the lower hemispheres, assuming that the dipole power in homogeneous medium of GaN equals 1, is 6119
DOI: 10.1021/acs.nanolett.6b02331 Nano Lett. 2016, 16, 6117−6123
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Figure 3. Collection efficiencies. (a) Collection efficiency of the silver-coated pyramid structure with a finite substrate measured from the lower and the upper hemispheres depending on the numerical aperture of the detection system. (b) A graph to present the collection efficiency ratio, which corresponds to the collection efficiency to the lower hemisphere divided by that to the upper hemisphere.
Figure 4. Fabrication process of inverted nanopyramids. (a) Schematic and SEM image of the as-grown silver-coated nanopyramid array. (b) Illustration and SEM image of inverted nanopyramids embedded in a UV-curable polymer. (c) Illustration and SEM image of the substrate after the transfer of the nanopyramid. (d) Calculated far-field radiation pattern for the inverted pyramid at a dipole wavelength of 480 nm. (e) Normalized radiation intensity versus angle θ.
In this work, we detached and inverted the nanopyramid array using the transparent UV-optical adhesive material NOA63 (Norland Optical Adhesive). This approach can facilitate a wide range of applications, such as QDs on a transparent substrate or QDs on a heterogeneous substrate. The detailed steps of this process are explained below. First, nanosized silver-coated GaN pyramidal structures grown on a GaN−sapphire substrate are prepared as shown in Figure 4a. In the next step, NOA63 was cast onto a GaN−sapphire pyramid template and cured using UV light. After the NOA63 stiffened, it was heated with a hot plate for easy detachment from the GaN−sapphire substrate. The nanopyramidal structures were removed from the GaN−sapphire substrate, leaving circular traces of the nanopyramids. The scanning electron microscope (SEM) image of the NOA63 surface in Figure 4b shows that the nanopyramids are successfully attached. We also examined a GaN−sapphire substrate to check if the pyramids could be removed. As shown in Figure 4c, the nanopyramids were removed, and the circular traces remained. The flipped nanopyramid array structures, i.e., detachment from the asgrown substrate and embedding in a UV-curable polymer (Figure 4b), were referred to as an “inverted nanopyramid” compared to an as-grown (upright) nanopyramid (Figure 4a). The far-field radiation pattern is also analyzed for the inverted nanopyramids using the FDTD method, as shown in Figure 4d. Directional emission toward the bottom of the pyramid structure (i.e., toward the air in this inverted sample, as
air interface. Therefore, the far-field radiation patterns in Figure 2 are the results that can be obtained at the position of monitor 2, as shown in Figure 1b. These radiation patterns depict what transpires during the free-space measurement setup. Three representative wavelengths are selected, all of which are within the InGaN photoluminescence (PL) spectrum (450−550 nm).15 The intensity profiles in the Fourier images over the wavelength range also show directional emission toward the lower hemisphere in the normal direction. However, it shows a broadened line-width of the angular distribution due to the refraction of the light at the interface. The far-field radiation patterns of the upper hemisphere in Figure 2 are also clearly presented with the scale bar normalized to its maximum value. The NA versus the collection efficiency from far-field result in Figure 2 is also plotted at λ = 520 nm in Figure 3 as the light collected through a certain NA is important in practical experiments. In Figure 3a, the blue triangles denote the outcoupling to the lower hemisphere, while the black circles represent the collection through the upper radiation. Here, the collection efficiency refers to the collected light out of the total radiated light from the dipole emitter. This result shows that the most of the light is collected through the lower hemisphere, with the collection efficiency reaching 50.8% when the NA is 0.7. The collection efficiency ratio between the upper and lower hemispheres is shown in Figure 3b. The result indicates that collecting the light emitted from the apex QD at the lower hemisphere can be done very much efficiently. 6120
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Nano Letters shown in Figure 4e) remains even after the inversion process. Figure 4e shows the radiation intensity as a function of angle θ, which presents a corresponding beam divergence of 71°. This beam divergence tends to vary as the dipole wavelength changes. It is important to emphasize here that this highly directional emission from the free-standing, inverted nanopyramid structures can facilitate numerous opportunities for integration with other optical systems. Far-field measurements were taken followed to observe the directional emission from the inverted nanopyramids. A microPL (μ-PL) setup was used at 5 K with optical pumping using a continuous-wave laser diode at a wavelength of 405 nm. The optical pump source is focused to a diameter of approximately 1 μm using an objective lens with a NA of 0.5. The focused spot simultaneously excites six or seven nanopyramids, which explains the multiple sharp peaks in the PL spectra. The QDs in both the as-grown pyramid and the inverted pyramid structures are randomly selected and investigated. As-grown silver-coated nanopyramids were initially analyzed with the emission to the top of the pyramid structures (upper hemisphere). A sharp emission line from the upright nanopyramid is observed at a wavelength of 509 nm (red arrow), as shown in Figure 5a, and broad emissions are observed due to quantum wells at the sidewall of the pyramids. QD emission is linearly polarized along the axis, as reported in a previous paper.15 The inset in Figure 5a shows the simulation results of the far-field radiation pattern at λ= 509 nm within θ= ± 30°, normalized to its maximum value. The back focal plane of the objective lens is imaged at the slit of a monochromator to measure the radiation pattern of the QDs experimentally.33 Spectrally resolved radiation patterns were constructed after a line scan of a back focal image (see the Methods section for details). Figure 5b shows that the measured far-field radiation pattern at the wavelength of the QD (λ = 509 nm) consists of two lobes, as shown in the simulated results in Figure 5a. These two main lobes corresponds to θ= 18° in the simulation and θ= 15° in the measured data. The PL spectrum from the inverted pyramid structure embedded in NOA63 is shown in Figure 5c. It shows QD emission at a wavelength of 458 nm, while Figure 5d shows the measured far-field radiation pattern of the QD emitter, which is also in good agreement with the simulated farfield pattern at the emitter’s wavelength (shown in the inset of Figure 5c). The small shift of the origin of the polar coordinates can be attributed to misalignment of the optical axis with respect to the normal axis of the sample (i.e., the surface of the sample possibly being slightly tilted inside the cryogenic system). The cross-sectional intensity versus θ from Figure 5b,d is plotted in Figure 5e. The gray area corresponds to the area out of the NA of the objective lens (NA = 0.5). It is important to note that QDs from the upright nanopyramid structure mainly radiate photons into two main lobes, whereas QDs from the inverted structure emit photons toward the normal direction (θ= 0°) with small beam divergence angle of 28°. Finally, we undertook photon correlation measurements to demonstrate the generation of a single photon from an inverted nanopyramid structure. Figure 5f shows the second-order correlation histogram as measured by a HBT interferometer. The graph is fitted with the function of G(2)(τ) = A[1−(1− g(2)(0)) exp(−|τ|/τ0)] and with the fitting parameters of A, g(2)(0), and τ0.15 The fitting result of was 0.39, demonstrating antibunching behavior. The nonzero value of g(2)(0) stems from both the background emission of the ternary InGaN quantum well (with large In composition inhomogeneity)
Figure 5. Far-field and HBT measurement. (a) Micro-PL (μ-PL) spectrum from an as-grown nanopyramid structure with a silver film. The red triangle denotes the spectrum from a single QD. (Inset) Calculated far-field radiation pattern toward the detection system. (b) Measured far-field radiation pattern of a single QD (λ = 509 nm) from the objective lens of a microscope with an NA of 0.5. (c) μ-PL spectrum from an inverted nanopyramid structure with a silver film. (d) Measured far-field radiation pattern of a single QD (λ = 458 nm) from the objective lens of a microscope with an NA of 0.5. (e) Normalized cross-sectional intensity depending on the angle of emission from as-grown and inverted nanopyramid structures. (f) Antibunching in the photons emitted from an inverted nanopyramid structure, as is revealed by second-order autocorrelation measurement.
formed at the side walls of the pyramid structure and from the limited time resolution of the detection system used here. Therefore, we clearly confirm the single-photon emission characteristic from a single apex QD in the inverted nanopyramid structure. In this work, we observed highly directional emission from apex QDs in silver-coated nanopyramid structures. An optical analysis by means of the FDTD method was conducted to 6121
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Nano Letters determine the directional far-field radiation patterns. We experimentally demonstrated that the nanopyramid structure could be detached and inverted using a UV-curable polymer (NOA63 in this study) and that strongly directional radiation from apex QDs is possible by virtue of the inverted pyramidal geometry. Far-field measurements of the back-focal image of an objective lens were taken. The results were found to be in good agreement with the simulated data, and single-photon emission was also confirmed. Note that this high directionality of single photons could be obtained using bottom-up, site-selective approach with a high fabrication yield. Given that a QD formed on an exact pyramid apex provides good site controllability, this represent a promising candidate for future single-photon sources and is applicable to quantum information purposes. Furthermore, we emphasize here that detached QDs in nanopyramid structures, having directional emission toward the bottom direction, can be easily be integrated with a photonic waveguide system with high coupling efficiency. Methods. Numerical Calculation. Numerical modeling was performed by solving Maxwell equations using the 3D FDTD method. The GaN hexagonal pyramid has a bottom diameter of 260 nm and a height of 180 nm. The post has a diameter of 180 nm, and the height corresponds to 70 nm. A single dipole source is located at the position of the QD, which is 11 nm below the tip of the pyramid. It has strong linear polarization along the direction due to the anisotropy potential of the QD on the pyramid structure. The far-field radiation patterns depending on the wavelength are calculated using the Fourier transform from the near-field profile approximately 70 nm above and below the pyramid structures. The far-field radiation patterns toward the lower hemisphere are plotted for both cases when the light propagates inside the GaN substrate and when the light escapes from the substrate and propagates through air. The latter case considers Fresnel’s and Snell’s laws at the interface of the GaN and air, as described in Figure S1. Experimental Measurements. Both the as-grown nanopyramid structures and the inverted nanopyramid structures were mounted in a closed-cycle cryostat system at a temperature of 5 K. A microscope with an objective lens with a high numerical aperture (NA = 0.5) was used to excite the nanopyramids with a spot diameter of ∼1 μm. Luminescence with angle information within ±30 degrees was collected using the same objective lens. Back-focal (Fourier) images of the luminescence from the nanopyramid structures were transferred to the entrance slit of a monochromator using a 4-f system. The Fourier images were selected using a 50 μm entrance slit of the monochromator, and these images were spectrally dispersed using a 150 g/mm grating to obtain the E− ky curve at each selected kx-space range. After the Fourier image was scanned horizontally using a motorized stage with a step of 50 μm, wavelength-resolved Fourier images were constructed. A second-order correlation function was obtained using the HBT setup with two rapid avalanche photodiodes (APD, temporal resolution of 40 ps, ID Quantique). With a timecorrelated single-photon counting (TCSPC, Picoharp 300; Picoquant) system, a coincidence histogram of the delay times between two detected photons was conducted and investigated.
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Figures showing schematics of a planar, a metal-free pyramid and a silver-coated pyramid structure, far-field simulation results for a planar and a metal-free pyramidal structure. A table showing the calculated power at each monitor. (PDF)
AUTHOR INFORMATION
Corresponding Author
*Tel: +82-42-350-2549; fax: +82-42-350-5549; e-mail: yhc@ kaist.ac.kr. Author Contributions
S.K. and S.-H.G. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Prof. Jong-Hwa Shin (KAIST) for the discussion about the simulation results. This work was supported by the National Research Foundation (NRF2016R1A2A1A05005320) of the Korean government (MSIP), the Future Semiconductor Device Technology Development Program (10044735) funded by MOTIE (Ministry of Trade, Industry and Energy) and the KSRC (Korea Semiconductor Research Consortium), and the Climate Change Research Hub of KAIST (grant no. N11160013).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b02331. 6122
DOI: 10.1021/acs.nanolett.6b02331 Nano Lett. 2016, 16, 6117−6123
Letter
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DOI: 10.1021/acs.nanolett.6b02331 Nano Lett. 2016, 16, 6117−6123
