Single photon emission from droplet epitaxial quantum dots in the standard telecom window around a wavelength of 1.55 μ m
SSingle photon emission from droplet epitaxial quantum dots in the standardtelecom window around a wavelength of 1.55 µ m Neul Ha, a) Takaaki Mano, Samuel Dubos, Takashi Kuroda, b) Yoshiki Sakuma, and Kazuaki Sakoda National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan (Dated: 22 January 2020)
We study the luminescence dynamics of telecom wavelength InAs quantum dots grown on InP(111)A bydroplet epitaxy. The use of the ternary alloy InAlGaAs as a barrier material leads to photon emission inthe 1.55 µ m telecom C-band. The luminescence decay is well described in terms of the theoretical interbandtransition strength without the impact of nonradiative recombination. The intensity autocorrelation func-tion shows clear anti-bunching photon statistics. The results suggest that our quantum dots are useful forconstructing a practical source of single photons and quantum entangled photon pairs.This is the version of the article before peer review, assubmitted to Applied Physics Express . The final pub-lished version will be available as an Open Access articlefrom the journal’s site.A source of single photons and quantum entangledphoton pairs is a key device in vast quantum technolo-gies. Semiconductor quantum dots are expected to serveas photon sources that can be operated very efficientlyand deterministically. Numerous efforts have alreadybeen made to develop a practical quantum dot photonsource. However, photon emission in the standard tele-com band, particularly around a wavelength of 1.55 µ m,which is the maximum transmission window of silica op-tical fibers, is a material challenge. Careful growth op-timization is required to achieve a 1.55 µ m emission .Nevertheless, the well-known quantum dot growth basedon the Stranski-Krastanow mode leads to an asymmet-ric dot shape, which is not favorable for entangled pairgeneration.The problem is ideally solved by using droplet epi-taxy, which offers considerable freedom regarding thechoice of materials and substrates . The applicationof a C v symmetric (111)A surface to the growth sub-strate results in the creation of almost perfectly symmet-ric quantum dots, which can work in both the visiblewavelength region and the infrared telecom wave-length region . Recently, the emission wavelength hasbeen extended beyond 1.5 µ m for InAs dots embedded inInAlGaAs on InP(111)A . However, the previous sam-ples were not sufficiently optimized: the dot density wastoo high for a single quantum dot to be isolated usingstandard micro optics. Moreover, the dot size distribu-tion is relatively large so that careful dot selection is re-quired to find a dot that emits at 1.55 µ m.Here, we extend the droplet epitaxy scheme to achievea purely 1.55 µ m photon emission. We introduce thehigh temperature crystallization protocol, which has re-cently been applied to the GaAs material system , tothe InAs/InP material system in order to improve the a) Electronic mail: [email protected] b) Electronic mail: [email protected]
FIG. 1. (Color online) (b) The luminescence spectra of alarge ensemble of InAs quantum dots in In . Al . Ga . As/InP(111)A at 12 K at different excitation powers. (b) Theluminescence spectra of a single isolated InAs dot with a cwexcitation of 40 nW. We focus on this dot in our time-resolvedstudy. The inset shows an atomic force microscope image ofthe dot surface. dot morphology property. The use of a state-of-the-artsuperconducting photon detector, together with an effi-cient dot sample, allows us to investigate single photonemission dynamics in the standard telecom C-band.The quantum dot sample is grown on Fe-doped semi-insulating InP(111)A using a solid source molecular beamepitaxy machine. After depositing a lattice-matchedIn . Al . Ga . As barrier with a thickness of 200 nmat 490 ◦ C, we grow 0.5 of a monolayer (ML) of InAs at490 ◦ C. We then supply 0.25 ML of indium at a growthrate of 0.16 ML/s at 400 ◦ C, which leads to the forma-tion of indium droplets on InAlGaAs(111)A. Next, anAs flux (9 × − Torr) is supplied at 400 ◦ C to crys-tallize InAs dots from indium droplets. After in vacuo annealing at 450 ◦ C for 5 min, the dots are capped byIn . Al . Ga . As with a thickness of 100 nm. A no-table point in this sequence is the crystallization tem-perature, which we set higher than that of the standardprotocol. The small diffusion length of group-III adatomssuppresses the transformation from dots to layers even at400 ◦ C, leading to the formation of dots with a high crys-talline quality. a r X i v : . [ c ond - m a t . m e s - h a ll ] J a n We measure the stationary- and time-resolved re-sponses of photoluminescence from single InAs dots. Forthe stationary study we use a semiconductor laser diodeat a wavelength of 980 nm as a cw excitation source. Forthe time-resolved study we use a ps mode-locked titaniumsapphire laser whose wavelength is tuned to 900 nm as apulsed source. The laser light is focused on the sampleusing a microscope objective lens with a numerical aper-ture of 0.65 (Olympus LCPlan50xIR). The luminescencesignal is collected by the same lens, passed through adichroic beam splitter, and coupled to a single mode op-tical fiber that has a mode field diameter of 9 µ m at awavelength of 1.3 µ m. The fiber output is fed into a 50 cmspectrometer that consists of a 600 line/mm grating.The luminescence signal is spectrally analyzed usinga cooled InGaAs photodiode array (Andor iDus 491)and temporally resolved using a superconducting single-photon detector (Single Quantum Eos) with a fast-response time-to-digital converter (PicoQuant PH300).The polarization state of the input light is adjusted tomaximize the detection efficiency. Note that we attach ahigh-index hemispherical lens ( n = 2) to the sample sur-face to increase the light collection efficiency . Thanksto the efficient setup together with the bright sample weachieve a maximum count rate as high as ∼
50 kHz un-der saturation conditions. The sample is cooled using aclosed cycle cryostat. All the experiments are performedat 8 K unless otherwise noted.The inset in Fig. 1(b) shows an atomic force microscopeimage of the quantum dot surface. It reveals the forma-tion of nearly circular dots without significant elongation.The dot isotropy arises due to the use of the C v symmet-ric { } surface as a growth substrate. The quantumdots have a disk-like shape with a diameter of 48 ± . ± . . The dot density is ∼ × cm − thus making it easy to isolate a single dot without anypost-growth processing such as the fabrication of smallmesas or apertures.Figure 1(a) shows the photoluminescence spectra of thequantum dot ensemble. They were measured using stan-dard long focus optics. For low excitation, the spectrumhas a Gaussian-like single peak centered at a wavelengthof 1,550 nm. Its full width at half maximum is ∼
100 nm,which is more than two times smaller than that of ourprevious sample targeting a 1.55 µ m emission . Hence,the majority of the dots in the present sample can emitin the telecom C-band. For high excitation, the spec-trum shows another broad band that originates from theexcited states, as well as an additional peak at 1,350 nmdue to carrier recombination in the barrier layer.Figure 1(b) shows a typical luminescence spectrum fora single isolated dot. The observed split lines are at-tributed to neutral excitons (X, 1,572.4 nm), positivelycharged excitons (X + , 1,573.8 nm), and neutral biexci-tons (XX, 1,578.6 nm). The spectral assignment wasbased on the large number statistics of the multiexciton FIG. 2. (Color online) The luminescence decay of the neu-tral exciton line of an InAs quantum dot emitting at a wave-length of 1,572 nm with an excitation power of 4 nW (redcircles). The black square line is the luminescence decay ofthe neutral exciton line of a GaAs quantum dot embedded inAl . Ga . As(111)A, with an emission wavelength of 674 nm.The inset shows the decay rate dependence on the emissionfrequency for the measured GaAs and InAs dots, and the the-oretical prediction of Eq. (1) for n = 3 . p /m = 20 eV. binding energies in InAs/InAlAs droplet dots . Notethat the present sample frequently shows X + , but rarelyshows a negatively charged line. This implies that oursample is slightly p -doped possibly due to the residualpresence of carbon impurities.Figure 2 shows the luminescence decay signal of theX line shown in Fig. 1(b) after short pulsed excitation.The decay signal of a GaAs quantum dot embedded inAl . Ga . As(111)A, which we studied previously , isalso shown for comparison. Both decay curves are wellapproximated by straight lines in the semilogarithmicplot, which implies that they follow single exponent func-tions. The decay time constant of the InAs dot is esti-mated to be 1.56 ns, which is significantly longer thanthat of the GaAs dot (0.56 ns). The large difference inthe luminescence decay times arises due to the frequencydispersion of the photonic density of states, as discussedbelow.The spontaneous emission rate for atomic transitions(Einstein’s A coefficient) is expressed as τ − = (cid:18) µµ n (cid:19) e ω | ¯ p | π(cid:15) ¯ hm c , (1)where (cid:15) and (cid:15) ( µ and µ ) are the vacuum and relativepermittivities (permeabilities), respectively, n is the re-fractive index given by (cid:112) (cid:15)µ/(cid:15) µ , ω is the angular fre-quency of emitted light, and p is the matrix element ofthe momentum operator (cid:98) p = − i ¯ h ∇ . The above formulawas deduced by including the interaction Hamiltonian H int = ( A · (cid:98) p + (cid:98) p · A ) e/ m , where A is the quantizedvector potential, and the three-dimensional photonic den-sity of states ρ ( ω ) = V ω /π c , where V is the normal-ization volume, to the Fermi’s golden rule.Note that the matrix element p serves as a band mixingsource in the k · p perturbation theory, and it is more orless constant for most group-IV, III-V, and II-VI semi- FIG. 3. (Color online) (a) Comparison of the transient re-sponses of the luminescence signals of the X line for differentexcitation powers. (b) Model calculation results for the lumi-nescence transients. N is the average initial exciton number. conductors, with the Kane energy 2 p /m ≈
20 eV .Consequently, the material dependence in Eq. (1) ap-pears only in the ω -proportional factor if we assume aconstant n value. The solid line in the inset of Fig. 2is the model dependence of the emission decay rate onthe photon frequency, where we assume that n = 3 . p /m = 20 eV. The ω -linear dependence agrees withthe measured decay rates of telecom wavelength InAsdots and visible wavelength GaAs dots. Thus, the radia-tive process of our quantum dots is purely described bythe atomic description in Eq. (1) free from the impact ofnonradiative recombination.Figure 3(a) shows the luminescence transients for dif-ferent excitation powers. The signal observed at 4 nW,i.e., the lowest excitation condition, is identical to thatshown by the semilogarithmic plot in Fig. 2. Hence, thedecay curve follows a single exponent. When the exci-tation power is increased to 20 nW, the signal deviatesfrom a monotonic decay, and reveals a significant rise af-ter t = 0. The rise signature is more evident for 80 nW,where the intensity maximum is substantially delayed bymore than 1 ns after excitation. The observed power-dependent evolution arises due to the multiexciton re-laxation cascade. We analyze the single exciton spectralline, which is generated only when a single electron andhole pair remains in the dot, following the recombinationof all the other pairs. The same luminescence behavioris reported in Refs. 23 and 24. Figure 3(b) shows thenumerical simulation results, where we assume a Poisso-nian distribution for the initial number of excitons . Forsimplicity, we deal only with the cascade evolution fromXX to X, and we assume that XX decays twice as fast asX, i.e., XX decays like noninteracting two excitons. Thesimple model reproduces the measured behavior qualita-tively.Figure 4 shows the intensity autocorrelation function g (2) ( t ) of the X luminescence line in Fig. 1(b). Here, weadopt the Hanbury Brown and Twiss setup to measure FIG. 4. (Color online) The intensity autocorrelation functionof the X line for different excitation powers. The coincidencenumber was integrated with a time bin of 256 ps for 6 hours(4 nW), 80 min (20 nW), and 20 min (80 nW). The simulationresults are also plotted by the gray broken line. the coincidence of two photons as a function of delaytime . The sample is illuminated by cw light. With lowexcitation at 4 nW, the signal shows a clear antibunch-ing dip, which yields nearly no probability of emittingtwo photons at the same time. As the delay time is in-creased the signal recovers from ∼ τ = 1 .
56 ns). The solid line shows a model ∝ − exp( −| t | /τ ), which agrees with the observed sig-nal. With increasing excitation power, the dip width isobserved to decrease, and the signal quickly recovers tothe equilibrium level. This is due to the acceleration ofthe X population recovery for strong excitations . Notethat several researchers have reported the emergence ofpositive bunching correlations superimposed on the an-tibunching dip . However, we do not observe sucha signature possibly due to the lower population of neu-tral XX in our sample. Nevertheless, the value of g (2) (0)is lower than the classical limit of 0.5 at least over thepresent excitation range, supporting the emission of sin-gle photons from this dot.In conclusion, we used droplet epitaxy to fabricateInAs quantum dots that emit single photons at wave-lengths around 1.55 µ m. Careful growth optimizationenabled us to reduce the dot size distribution, and so themajority of the dots could emit photons in the telecomC-band. The exciton lifetime was nearly the same as thetheoretically ideal value free from the impact of nonra-diative recombination. The use of a trigonally symmetricInP(111)A substrate led to the formation of nearly cir-cular dots. Thus, our dots can be expected to serve asbright single photon and entangled photon pair sourcesthat will be useful for practical quantum communicationapplications. ACKNOWLEDGMENTS
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