The single-photon source has had a significant impact on the development of multiple fields, encompassing quantum information, quantum computing, and quantum communication[
Journal of Semiconductors, Volume. 42, Issue 7, 072901(2021)
Fiber coupled high count-rate single-photon generated from InAs quantum dots
In this work, we achieve high count-rate single-photon output in single-mode (SM) optical fiber. Epitaxial and dilute InAs/GaAs quantum dots (QDs) are embedded in a GaAs/AlGaAs distributed Bragg reflector (DBR) with a micro-pillar cavity, so as to improve their light emission extraction in the vertical direction, thereby enhancing the optical SM fiber’s collection capability (numerical aperture: 0.13). By tuning the temperature precisely to make the quantum dot exciton emission resonant to the micro-pillar cavity mode (Q ~ 1800), we achieve a fiber-output single-photon count rate as high as 4.73 × 106 counts per second, with the second-order auto-correlation g2(0) remaining at 0.08.
1. Introduction
The single-photon source has had a significant impact on the development of multiple fields, encompassing quantum information, quantum computing, and quantum communication[
In this work, we optimize previously-adopted experimental conditions[
2. Epi-structure and experimental setup
The epitaxial structures were grown on semi-insulating (100) GaAs substrates by means of solid-source molecular beam epitaxy (Veeco Gen930). The Epi-structure of the sample is shown in Fig. 1.
Figure 1.(Color online) The epi-structure of the sample.
We adopted the subcritical indium deposition technique, together with a gradient indium flux on the static GaAs substrate, to form InAs QDs with top 15 and bottom 25 pairs of GaAs/Al0.9Ga0.1As, in order to obtain a high-Q DBR cavity. Although the thickness of the GaAs layer is designed to be 62.57 nm, and the thickness of the Al0.9Ga0.1As layer is designed to be 74.7 nm, we employed phase-matching growth[
3. Coupling step
Firstly, we inscribe a stripe with a narrow ditch onto the substrate, along the gradient indium flux direction, and divide it into four small pieces (see Fig. 2(b)), and measure each piece at low temperature (T = 6 K), to locate a QD in a low-density area. If C (A, B, C, D) is a low-density area, the yellow area as shown in Fig. 2(c) is also a low-density area, and the blocks numbered 3 and 4 will be used for the micro-pillar process.
Figure 2.(Color online) (a) Inscribing a stripe with a narrow ditch on the substrate along the gradient indium flux direction. (b) Dividing the stripe into four small parts (A, B, C, D). (c) Illustrations of selected areas for etching.
16 single-mode fibers G657, core/cladding diameters: 9 μm/125 μm are embedded in V-grooves with an interval of 127 μm, as shown in Figs. 3(a)–3(c), to form an optical fiber array, designed to be coupled to micropillars. By means of sputtering SiO2 (as a hard mask), photolithography, and ICP etching, we were able to form micropillars with a diameter of 3 μm, as shown in Fig. 3(d), and an interval of 12 μm.
Figure 3.Schematic diagrams of (a) fiber coupling, (b) fiber array, (c) cross-section of optical fiber array. (d) SEM image of micropillar array.
The substrate with micropillar array was then cut into smaller rectangles. A drop of ultraviolet-curable epoxy (Norland Optical Adhesive 61) was placed on the fiber array facet, then the front side of the substrate was pasted onto the glue, and aligned with the coupling fiber row.
The device is shown in Fig. 4(a): Firstly, one drop of ultraviolet curable epoxy was dripped onto the fiber surface, and a substrate with micropillars was attached to the fiber’s surface. Secondly, we used a needle, pressing on the back of the substrate, to reduce the inclination angle between the front of the substrate and the optical fiber. A strong ultraviolet laser (365 nm) pointer then irradiated the coupling for 5–10 s. Once the ultraviolet curable epoxy was cured, the coupling fiber from the pressing device was removed. Thirdly, a few drops of ultraviolet curable epoxy were dripped onto the back of the substrate. The substrate was then placed under an ultraviolet lamp for 4–5 h to consolidate the coupling between the substrate and the optical fiber. Once the ultraviolet curable epoxy was completely cured, we used a metal holder, as shown in Fig. 4(b), to carry and fix the coupling optical fiber. The inside of the metal holder was coated with thermally conductive glue, then the coupling optical fiber was inserted into the thermally conductive glue, and finally fixed with screws.
Figure 4.(Color online) (a) Auxiliary coupling device. (b) The coupled fiber array is fixed onto the metal holder.
We used the JANIS CCS-100 device for our preliminary test. This device can measure the PL spectrum of 16 fibers at a time, but its temperature control accuracy is low, and the temperature cannot be changed arbitrarily. At a temperature of about 35 K, we located QDs of superior intensity and quality, as shown in Fig. 5(a). They exhibit good monochromaticity, with no influence from other nearby QDs. In contrast, Fig. 5(b) shows a bad coupling position, where photons from many QDs are coupled to the same fiber.
Figure 5.(a) Single photon coupled by the SM fiber at preliminary testing. (b) Multiple photons, coupled by one fiber.
Finally, the higher-quality coupling groups are tested separately on a more accurate temperature control platform, with the same experimental setup[
4. Results and discussion
We find that a change in temperature has an obvious effect on the matching degree of the cavity mode, as well as single QD excitation (X). The cavity-mode (CM) matching requires precise temperature control. The fitting results for the PL spectra under different temperature conditions are shown in Fig. 6.
Figure 6.(Color online) (a) Cavity modes are mismatched at 27.4 K (with p200 attenuation), using the Lorentz function fit for the PL spectrum. (b) Cavity modes matched at 33.6 K (with p200 attenuation), fitting the PL spectrum.
Fig. 6(a) shows the fitting for cavity-mode mismatching, at a temperature of 27.4 K. Fig. 6(b) shows the fitting for cavity-mode matching, at a temperature of 33.6 K.
Fig. 7 depicts a three-dimensional graph, based on measuring and fitting a set of variable-temperature PL-spectrum data. The figure clearly shows that when the cavity mode overlaps the single QD excitation (X), the luminosity efficiency reaches its maximum.
Figure 7.(Color online) (a) Three-dimensional PL spectrum with variable temperature from 27.4 to 40.4 K.
As shown in Fig. 8, when the cavity is matched with single QD excitation (X), by changing the excitation power, measuring the intensity of a single photon and cavity mode reveals the linear relationship (I ∝ Pn). This demonstrates the emission characteristics of the QD.
Figure 8.Intensity of single QD excitation (X) as the excitation power changes, where
In order to prove that the single-photon source has good anti-beam properties, the second-order correlation function is calculated by means of the Hanbury Brown–Twiss (HBT) experiment. We measured cavity-mode resonance and cavity-mode mismatch respectively, as shown in Fig. 9.
Figure 9.(a) Spectrum when the sample temperature is 32.4 K (with
In the case of cavity-mode mismatching (T = 32.4 K), the single-channel APD received count rate is 59 000. After deconvolution fitting, g2(0) = 0.0817. Fig. 9(a) shows the spectrum corresponding to a temperature of 32.4 K, and Fig. 9(c) corresponds to the HBT measurement result. In the case of cavity-mode matching (T = 33.6 K), the single-channel APD count rate reaches 78 000, g2(0) = 0.0795. The results show that our single-photon source has a high single-photon purity. Fig. 9(b) corresponds to the spectrum of the emitted light at a temperature of 33.6 K, and Fig. 9(d) shows the corresponding HBT result.
Lastly, we calculated various optical path losses (including an optical fiber HBT optical path efficiency of 10%, and an APD detection efficiency of 33%) when measuring the HBT, finally estimating that the single photon count-rate transmitted in the coupled fiber could be as much as 4.73 × 106 (78 000 × 2 / 0.10 / 0.33 = 4.73 × 106 cps).
5. Conclusions
We have achieved the transmission of a high count-rate single-photon source in SM optical fiber by optimizing QD growth conditions and measurements via precise temperature control. The single photon count rate at the fiber end reaches 4.7 × 106 cps, and the second-order autocorrelation coefficient, g2(0), is 0.08 for the resonance of the QD exciton and cavity mode. This coupling method is scalable, and has the potential for significant further improvement.
Acknowledgements
This work is supported by the Key-Area Research and Development Program of Guangdong Province (Grant No. 2018B030329001), the National Key Technologies R&D Program of China (2018YFA0306101), The Scientific Instrument Developing Project of the Chinese Academy of Science (YJKYYQ20170032), the National Natural Science Foundation of China (61505196), and the Program of Beijing Academy of Quantum Information Sciences (Grant No.Y18G01).
[1] M Kroutvar, Y Ducommun, D Heiss et al. Optically programmable electron spin memory using semiconductor quantum dots. Nature, 432, 81(2004).
[2] N I Cade, H Gotoh, H Kamada et al. Optical characteristics of single InAs/InGaAsP/InP(100) quantum dots emitting at 1.55
[3] Y M He, Y He, Y J Wei et al. On-demand semiconductor single-photon source with near-unity indistinguishability. Nat Nano, 8, 213(2013).
[4] A Faraon, I Fushman, D Englund et al. Coherent generation of non-classical light on a chip via photon-induced tunnelling and blockade. Nat Phys, 4, 859(2008).
[5] K Sebald, P Michler, T Passow et al. Single-photon emission of CdSe quantum dots at temperatures up to 200 K. Appl Phys Lett, 81, 2920(2002).
[6] G C Shan, Z Q Yin, C H Shek et al. Single photon sources with single semiconductor quantum dots. Front Phys, 9, 170(2014).
[7] O Gazzano, S M de Vasconcellos, C Arnold et al. Bright solid-state sources of indistinguishable single photons. Nat Commun, 4, 1425(2013).
[8] S Buckley, K Rivoire, J Vučković. Engineered quantum dot single-photon sources. Rep Prog Phys, 75, 126503(2012).
[9] P Lodahl, A F van Driel, I S Nikolaev et al. Controlling the dynamics of spontaneous emission from quantum dots by photonic crystals. Nature, 430, 654(2004).
[10] Y Ma, P E Kremer, B D Gerardot. Efficient photon extraction from a quantum dot in a broad-band planar cavity antenna. J Appl Phys, 115, 023106(2014).
[11] S Fischbach, A Schlehahn, A Thoma et al. Single quantum dot with microlens and 3D-printed micro-objective as integrated bright single-photon source. ACS Photonics, 4, 1327(2017).
[12] T Miyazawa, K Takemoto, Y Sakuma et al. Single-photon generation in the 1.55-
[13] K Takemoto, M Takatsu, S Hirose et al. An optical horn structure for single-photon source using quantum dots at telecommunication wavelength. J Appl Phys, 101, 081720(2007).
[14] E M Purcell. Spontaneous emission probabilities at radio frequencies. Phys Rev, 69, 681(1946).
[15] L Rayleigh. CXII. The problem of the whispering gallery. Lond Edinb Dublin Philos Mag J Sci, 20, 1001(1910).
[16] A Kors, K Fuchs, M Yacob et al. Telecom wavelength emitting single quantum dots coupled to InP-based photonic crystal microcavities. Appl Phys Lett, 110, 031101(2017).
[17] W H Chang, W Y Chen, H S Chang et al. Efficient single-photon sources based on low-density quantum dots in photonic-crystal nanocavities. Phys Rev Lett, 96, 117401(2006).
[18] J Claudon, J Bleuse, N S Malik et al. A highly efficient single-photon source based on a quantum dot in a photonic nanowire. Nat Photon, 4, 174(2010).
[19] B Ma, Z S Chen, S H Wei et al. Single photon extraction from self-assembled quantum dots via stable fiber array coupling. Appl Phys Lett, 110, 142104(2017).
[20] S L Li, Y Chen, X J Shang et al. Boost of single-photon emission by perfect coupling of InAs/GaAs quantum dot and micropillar cavity mode. Nanoscale Res Lett, 15, 145(2020).
Get Citation
Copy Citation Text
Yao Chen, Shulun Li, Xiangjun Shang, Xiangbin Su, Huiming Hao, Jiaxin Shen, Yu Zhang, Haiqiao Ni, Ying Ding, Zhichuan Niu. Fiber coupled high count-rate single-photon generated from InAs quantum dots[J]. Journal of Semiconductors, 2021, 42(7): 072901
Category: Research Articles
Received: Dec. 17, 2020
Accepted: --
Published Online: Jul. 14, 2021
The Author Email: