Abstract
A pronounced high count rate of single-photon emission at the wavelength of 1.3 μm that is capable of fiber-based quantum communication from InAs/GaAs bilayer quantum dots coupled with a micropillar (diameter ~3 μm) cavity of distributed Bragg reflectors was investigated, whose photon extraction efficiency has achieved 3.3%. Cavity mode and Purcell enhancement have been observed clearly in microphotoluminescence spectra. At the detection end of Hanbury-Brown and Twiss setup, the two avalanched single-photon counting modules record a total count rate of ~62,000/s; the time coincidence counting measurement demonstrates single-photon emission, with the multi-photon emission possibility, i.e., g 2(0), of only 0.14.
Background
Optical fiber-based quantum information requires real single-photon sources (SPSs) at telecom band to replace the traditional pseudo-SPSs based on strongly decayed pulse lasers. Self-assembled individual quantum dots (QDs) are potential to emit real single photons and thus have attracted great interest [1,2,3,4]. The integration of a distributed Bragg reflector (DBR) cavity to a single QD will enhance its directional emission. Compared to InAs QDs grown on InP substrate emitting at ~1.55 μm with lattice-matched indium-rich materials grown at a low temperature as DBR [5, 6], InAs QDs grown on GaAs substrate are advantageous on the easy integration of lattice-matched high-quality GaAs/Al0.9Ga0.1As DBR. To realize InAs/GaAs QD SPSs at telecom band, their emission wavelength must extend from the usual one ~0.9 to 1.3 or 1.55 μm and their density must keep as low as 107–108 cm−2 to realize single QDs in a microregion. To fabricate low-density InAs QDs by molecular beam epitaxy (MBE), some constructive schemes have been proposed, such as ultralow growth rate [3], high growth temperature [7,8,9], and precise control of deposition amount [10] of QDs and the isolation of QDs by growth on a mesa/hole-patterned substrate [11] or etching into micropillars [12, 13]. To extend their emission wavelength, several techniques have been developed, such as strain engineering of QDs [14], metamorphic structures [2], and strain-coupled bilayer QD (BQD) structure [15,16,17]. BQD structure on GaAs substrate is effective to achieve emission above 1.3 μm. High-density BQDs have been applied in laser diodes at ~1.5 μm operating at room temperature [15, 16]. Since it avoids the use of metamorphic layer and ultralow growth rate in the active layer, which might deteriorate the crystal quality [2], the BQD structure is also desired to grow low-density QDs in telecom wavelength. Low-density InAs/GaAs BQDs emitting at 1.3 μm have been obtained in our previous work [18]. To achieve a high count rate of single photons at 1.3 μm for fiber-based applications [2, 19], the photon extraction efficiency from single QDs must be improved. In this letter, by further optimizing the growth conditions of BQD structure and fabricating a micropillar structure, we improve the photon extraction from single InAs/GaAs BQDs emitting at 1.3 μm greatly. The single-photon count rate has reached 62,000 counts/s at the InGaAs single-photon counting module or 3.45 M counts/s at the first objective lens considering the photon collection efficiency of the confocal microscope spectroscopy setup. This is the first time to report a high count rate of single-photon emission at telecommunication wavelength by using InAs/GaAs BQDs. The emission intensity can be further enhanced by introducing an n-type δ-doped layer adjacent to the BQD layer to produce electron charged excitons [13].
Methods
The investigated sample was grown by solid-source MBE (VEECO Gen930 system) on semi-insulating (100) GaAs substrate. The sample structure consists of, in sequence, a 300-nm-thick GaAs buffer layer, a 25.5-pair wavelength-matched Al0.9Ga0.1As (113.7 nm)/GaAs (98.6 nm) bottom DBR, a one λ-thick undoped GaAs cavity, and an 8-pair Al0.9Ga0.1As/GaAs upper DBR with the same period. In the center of the GaAs cavity, the active layer for telecom emission, i.e., BQD structure with InGaAs strain-reducing layer, was grown at 470 °C in the Stranski-Krastanov growth mode, which was lower than the temperature used in our previous work. More growth details are reported in Ref. [18]. In this work, specially, micropillar arrays are fabricated on the DBR cavity-coupled BQD samples by photolithography and inductive coupled plasma (ICP) etching with chlorine (Cl2) and argon (Ar) mixture gas. As shown in the scanning electron microscope (SEM) image in Fig. 2a, the micropillars are in diameter of ~3 μm and height of 7.75 μm, with very smooth sidewalls. The sample was cooled in a cryogen-free bath cryostat with the temperature finely tuned from 4 to 50 K and excited by a He-Ne laser at wavelength of 633 nm. The confocal microscope setup with an objective (NA, 0.65) focuses the laser into a spot in a diameter of 2 μm and collects the luminescence effectively into a spectrograph, which enables a scanning of microregion to search single QD exciton spectral lines. Microphotoluminescence (μPL) spectrum was detected by a 0.3-m-long focal length monochromator equipped with a liquid-nitrogen-cooled InGaAs linear-array detector for spectrograph. For reflectivity measurement, a spectrophotometer (PerkinElmer 1050) was used with a scanning step of 2 nm and light spot of 3 mm × 3 mm. To investigate the radiative lifetime of the exciton, a time-correlated single-photon counting (TCSPC) board and a Ti:Sapphire pulsed laser (pulse width, ~100 fs; repetition frequency, 80 MHz; wavelength, 740 nm) were used for time-resolved μPL measurement. To measure the second-order autocorrelation function g (2)(τ), the QD spectral line luminescence was sent to a fiber-coupled Hanbury-Brown and Twiss (HBT) setup [20] and detected by two InGaAs avalanched single-photon counting modules (IDQ 230; time resolution, 200 ps; dark count rate, ~80 counts/s; dead time, 30 μs) and a time coincidence counting module.
Results and Discussion
Figure 1a, b shows AFM images of BQDs grown at 480 and 470 °C, respectively. For 480 °C sample, the BQDs are in a mean diameter of 61 nm and a height of about 10 nm. For 470 °C sample, the mean diameter is 75 nm and the height is 13 nm, taller and larger than that grown at 480 °C. The lower temperature contributes to the increased QD size and aspect ratio [21]. To enhance the photon collection efficiency, the BQDs were embedded in a λ-thick GaAs cavity and sandwiched between 25.5 lower and 8 upper DBR stacks. All are the same for the two samples, only except the growth temperature of BQDs. As shown in Fig. 1c, the brightest BQDs in the two samples we observed are quite different in PL spectrum. The PL intensity was greatly enhanced at the lower growth temperature, which can be attributed to the reduced strain relaxation and dislocation around BQDs [21]. Figure 1d shows the measured reflectivity spectrum of the bottom DBR, with a value about 99% at a range of 1310–1380 nm, demonstrating a good mirror to reflect QD emission.
Figure 2 shows the SEM image of the micropillar and the μPL spectra of a typical BQD embedded in it. Figure 2d shows the μPL spectra as a function of temperature. The emission from the BQD reaches its maximal intensity at 30 K, suggesting a cavity resonance; also see Fig. 2c. The quality factor (Q) of the micropillar cavity is estimated to be about 361. The low Q is attributed to the small reflectivity offset between GaAs and Al0.9Ga0.1As in the telecom wavelength, and a fewer DBR pairs were used here than the conventional DBRs coupled to QDs emitting at <1 μm [12, 22].
The excitation power-dependent μPL spectra of InAs/GaAs BQDs in a micropillar was studied by using a continuous wave (cw) He-Ne laser for above-band excitation, as Fig. 3a shows. They show the exciton line (X) at 1325.6 nm and charged exciton line (X*) at 1327.1 nm. The identification of these emission lines is supported by their various power dependences. In Fig. 3b, the integrated PL intensity of X line at 1325.6 nm showed a linear dependence upon the excitation power in the low power region and saturated at a high excitation power. The solid lines are linear fitting to the data in a double-logarithmic plot. The X* line at 1327.1 nm shows a non-saturated excitation power dependence [23]. The followed investigations were performed on the X line.
The time-resolved PL measurements were carried out to determine the Purcell enhancement. The spontaneous emission decay of the BQD X line at QD-cavity resonance and at far detuning are shown in Fig. 4a. The fitted radiative lifetime is 0.66 ns for resonance and 1.25 ns for far detuning, corresponding to a Purcell enhancement factor of 1.9. In order to confirm the single-photon emission of the X line at 1325.6 nm, we measured the second-order correlation function g (2)(τ) with a HBT setup under cw citation and saturated pulse excitation. Figure 4b shows the measured second-order correlation function of the X line as a function of the delay time τ under cw excitation. The data could be fitted with the following expression: g (2)(τ) = 1 − [1 − g (2)(0)]exp(−|τ|/T) [24]. The fitting results in g 2(0) = 0.14, proving a single-photon emitter with a strong suppression of the multi-photon emission at zero time delay. The count rate measured on the detectors is presented in Fig. 4c, as a function of the pump power. It shows a linear dependence in the weak pump regime and becomes saturated in the strong pump regime. At saturation, the count rate is around 62,000 counts/s from two InGaAs single-photon detectors, also including the dark counts of the two detectors. To deduce the corresponding number of photons collected in the first lens, we calibrate all the optical loss by using a cw laser at 1320 nm. Transmission loss including microscope objective, long-pass filter, mirrors, and lens and the efficiency of monochromator, lens, and connectors between fibers was 10.46 dB. The detection efficiency and dark count rate of the InGaAs detector with dead times of 30 μs are 18% and ~150 counts/s, respectively. Based on the count rate on InGaAs single-photon detectors and corrected photon count rate by the factor of [1−g (2)(0)]1/2 [25], we estimate the net single-photon detection rate after compensating the contribution of multi-photon emission and dark count rate is 3.45 × 106 counts/s at the saturated pump power at the first objective lens. To evaluate the photon extraction efficiency of micropillar structure, the measurement under pulsed excitation was also performed. In Fig. 4d, e, we observe a count rate of 48,000/s on the single-photon detectors at the saturated pump power with g 2(0) = 0.19, under 80 MHz repetition rate laser excitation, which gives a photon extraction efficiency of 3.3% after compensating the contribution of multi-photon emission and considering the efficiency of the detection setup. In our opinion, due to the non-resonant excitation process [12, 26] and low-detection efficiency and long dead time of the InGaAs detector, the observed count rate of single photons may be underestimated.
Conclusions
In conclusion, we have presented a bright single-photon source at 1325.6 nm by using a single strain-coupled bilayer InAs/GaAs QD in a micropillar Al0.9Ga0.1As/GaAs DBR cavity. The single-photon emission has really been enhanced by optimizing QD growth temperature and fabricating micropillar structure. The detected single-photon rate reaches 62,000 counts/s, corresponding to a single-photon emission rate of 3.45 MHz at the first objective lens. The photon extraction efficiency is estimated to be about 3.3%, with a Q ~300 micropillar cavity. The second-order autocorrelation measurement with InGaAs single-photon counting modules yielded g (2)(0) = 0.14, demonstrating single-photon emission even at high count rate. This is the first time to report so high rate of single-photon emission in the telecom band by using a single InAs/GaAs bilayer QD.
Abbreviations
- AFM:
-
Atomic force microscopy
- BQD:
-
Bilayer QD
- cw:
-
Continuous wave
- DBRs:
-
Distributed Bragg reflectors
- HBT:
-
Hanbury-Brown and Twiss
- ICP:
-
Inductive coupled plasma
- MBE:
-
Molecular beam epitaxy
- QDs:
-
Quantum dots
- SEM:
-
Scanning electron microscope
- SPSs:
-
Single-photon sources
- TCSPC:
-
Time-correlated single-photon counting
- μPL:
-
Microphotoluminescence
References
Cadeddu D, Teissier J, Braakman FR, Gregersen N, Stepanov P, Gérard J-M, Claudon J, Warburton RJ, Poggio M, Munsch M (2016) A fiber-coupled quantum-dot on a photonic tip. Appl Phys Lett 108:011112
Muñoz-Matutano G, Barrera D, Fernández-Pousa CR, Chulia-Jordan R, Seravalli L, Trevisi G, Frigeri P, Sales S, Martínez-Pastor J (2016) All-optical fiber Hanbury Brown and Twiss interferometer to study 1300 nm single photon emission of a metamorphic InAs quantum dot. Sci Rep 6:27214
Xu XL, Brossard F, Hammura K, Williams DA, Alloing B, Li LH, Fiore A (2008) “Plug and play” single photons at 1.3 μm approaching gigahertz operation. Appl Phys Lett 93:021124
Hidekazu K, Takumi H, Ikuo S, Hideaki N, Takashi K, Takaaki M, Kazuaki S, Satoru O, Hirotaka S (2016) Stable and efficient collection of single photons emitted from a semiconductor quantum dot into a single-mode optical fiber. Appl Phys Express 9:032801
Benyoucef M, Yacob M, Reithmaier JP, Kettler J, Michler P (2013) Telecom-wavelength (1.5 μm) single-photon emission from InP-based quantum dots. Appl Phys Lett 103:162101
Benyoucef M, Yacob M, Reithmaier JP, Kettler J, Michler P (2013) Telecom-wavelength (1.5 μm) single-photon emission from InP-based quantum dots. Appl Phys Lett 103:162101–162101-4
Jie S, Peng J, Zhan-Guo W (2004) Extremely low density InAs quantum dots realized in situ on (100) GaAs. Nanotechnology 15:1763
Sung-Pil R, Nam-Ki C, Ju-Young L, Hye-Jin L, Won-Jun C, Jin-Dong S, Jung-Il L, Yong-Tak L (2009) Effect of modified growth method on the structural and optical properties of InAs/GaAs quantum dots for controlling density. Jpn J Appl Phys 48:095506
Masato O, Takuya K, Kousuke T, Takuji T, Hiroyuki S (2008) Formation of ultra-low density (≤104 cm−2) self-organized InAs quantum dots on GaAs by a modified molecular beam epitaxy method. Appl Phys Express 1:061202
Ward MB, Karimov OZ, Unitt DC, Yuan ZL, See P, Gevaux DG, Shields AJ, Atkinson P, Ritchie DA (2005) On-demand single-photon source for 1.3 μm telecom fiber. Appl Phys Lett 86:201111
Schneider C, Heindel T, Huggenberger A, Niederstrasser TA, Reitzenstein S, Forchel A, Höfling S, Kamp M (2012) Microcavity enhanced single photon emission from an electrically driven site-controlled quantum dot. Appl Phys Lett 100:091108
Ding X, He Y, Duan ZC, Gregersen N, Chen MC, Unsleber S, Maier S, Schneider C, Kamp M, Höfling S, Lu C-Y, Pan J-W (2016) On-demand single photons with high extraction efficiency and near-unity indistinguishability from a resonantly driven quantum dot in a micropillar. Phys Rev Lett 116:020401
Heindel T, Schneider C, Lermer M, Kwon SH, Braun T, Reitzenstein S, Höfling S, Kamp M, Forchel A (2010) Electrically driven quantum dot-micropillar single photon source with 34% overall efficiency. Appl Phys Lett 96:011107
Shimomura K, Kamiya I (2015) Strain engineering of quantum dots for long wavelength emission: photoluminescence from self-assembled InAs quantum dots grown on GaAs (001) at wavelengths over 1.55 μm. Appl Phys Lett 106:082103
Majid MA, Childs DT, Shahid H, Chen S, Kennedy K, Airey RJ, Hogg R, Clarke E, Howe P, Spencer PD (2011) Toward 1550-nm GaAs-based lasers using InAs/GaAs quantum dot bilayers. IEEE J Sel Top Quantum Electron 17:1334–1342
Clarke E, Spencer P, Harbord E, Howe P, Murray R (2008) Growth, optical properties and device characterisation of InAs/GaAs quantum dot bilayers. J Phys Conf Ser 107:012003
Liu Y, Liang B, Guo Q, Wang S, Fu G, Fu N, Wang ZM, Mazur YI, Salamo GJ (2015) Electronic coupling in nanoscale InAs/GaAs quantum dot pairs separated by a thin Ga(Al)As spacer. Nanoscale Res Lett 10:1–6
Chen Z-S, Ma B, Shang X-J, He Y, Zhang L-C, Ni H-Q, Wang J-L, Niu Z-C (2016) Telecommunication wavelength-band single-photon emission from single large InAs quantum dots nucleated on low-density seed quantum dots. Nanoscale Res Lett 11:1–7
Intallura PM, Ward MB, Karimov OZ, Yuan ZL, See P, Shields AJ, Atkinson P, Ritchie DA (2007) Quantum key distribution using a triggered quantum dot source emitting near 1.3 μm. Appl Phys Lett 91:161103–161103–3
Hanbury Brown R, Twiss RQ (1956) The question of correlation between photons in coherent light rays. Nature 178:1447–1448
Howe P, Le Ru EC, Clarke E, Abbey B, Murray R, Jones TS (2004) Competition between strain-induced and temperature-controlled nucleation of InAs/GaAs quantum dots. J Appl Phys 95:2998–3004
Gazzano O, Michaelis de Vasconcellos S, Arnold C, Nowak A, Galopin E, Sagnes I, Lanco L, Lemaître A, Senellart P (2013) Bright solid-state sources of indistinguishable single photons. Nat Commun 4:1425
Yu Y, Li MF, He JF, Zhu Y, Wang LJ, Ni HQ, He ZH, Niu ZC (2012) Photoluminescence study of low density InAs quantum clusters grown by molecular beam epitaxy. Nanotechnology 23:065706
Kako S, Santori C, Hoshino K, Gotzinger S, Yamamoto Y, Arakawa Y (2006) A gallium nitride single-photon source operating at 200 K. Nat Mater 5:887–892
Pelton M, Santori C, Vuc̆ković J, Zhang B, Solomon GS, Plant J, Yamamoto Y (2002) Efficient source of single photons: a single quantum dot in a micropost microcavity. Phys Rev Lett 89:233602
Kettler J, Paul M, Olbrich F, Zeuner K, Jetter M, Michler P (2016) Single-photon and photon pair emission from MOVPE-grown In(Ga)As quantum dots: shifting the emission wavelength from 1.0 to 1.3 μm. Appl Phys B 122:48
Acknowledgements
The authors are grateful to Bao-quan Sun and Yong-zhou Xue for their optical measurements. This work is supported by the National Key Basic Research Program of China (Grant No. 2013CB933304), the National Natural Science Foundation of China (Grant Nos. 61505196 and 91321313), Strategic Priority Research Program B of Chinese Academy of Sciences (Grant No. XDB01010200), and Beijing Key Discipline Foundation of Condensed Matter Physics.
Authors’ Contributions
Z-SC grew the samples; carried out the alignment; took part in discussions and in the interpretation of the results; and wrote the manuscript. X-JS and B M participated in the design of the study and discussions of the results. X-JS and J-LW have supervised the writing of the manuscript. H-QN and Z-CN supervised the writing of the manuscript and the experimental part. All the authors have read and approved the final manuscript.
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The authors declare that they have no competing interests.
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Chen, ZS., Ma, B., Shang, XJ. et al. Bright Single-Photon Source at 1.3 μm Based on InAs Bilayer Quantum Dot in Micropillar. Nanoscale Res Lett 12, 378 (2017). https://doi.org/10.1186/s11671-017-2153-2
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DOI: https://doi.org/10.1186/s11671-017-2153-2