Experiments towards a periodically pumped single-photon source are presented. The lateral piezoelectric field of a surface acoustic wave dissociates laser-generated two-dimensional excitons into electrons and holes. These carriers are separated by the wave potential and are transported over macroscopic length scales without recombining. When reaching a stress-induced quantum dot in the quantum well they periodically populate the zero-dimensional states and recombine, emitting single photons periodically in time according to the surface acoustic-wave frequency. We have successfully reduced the number of pumped quantum dots down to 100 and have detected a strong blinking photoluminescence signal. By further reducing the number of quantum dots down to 1 a periodically pumped single photon source could be realized.Peer reviewe

Bödefeld, C.

Ebbecke, J.

Lipsanen, Harri

Sopanen, Markku

Toivonen, J.

Wixforth, A.

Aaltodoc Publication Archive

This is an electronic reprint of the original article.This reprint may differ from the original in pagination and typographic detail.Author(s): Bödefeld, C. & Ebbecke, J. & Toivonen, J. & Sopanen, M. &Lipsanen, Harri & Wixforth, A.Title: Experimental investigation towards a periodically pumpedsingle-photon sourceYear: 2006Version: Final published versionPlease cite the original version:Bödefeld, C. & Ebbecke, J. & Toivonen, J. & Sopanen, M. & Lipsanen, Harri & Wixforth,A. 2006. Experimental investigation towards a periodically pumped single-photon source.Physical Review B. Volume 74, Issue 3. P. 035407/1-5. ISSN 1098-0121 (printed). DOI:10.1103/physrevb.74.035407.Rights: © 2006 American Physical Society (APS). http://www.aps.org/All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, andduplication or sale of all or part of any of the repository collections is not permitted, except that material maybe duplicated by you for your research use or educational purposes in electronic or print form. You mustobtain permission for any other use. Electronic or print copies may not be offered, whether for sale orotherwise to anyone who is not an authorised user.Powered by TCPDF (www.tcpdf.org)Experimental investigation towards a periodically pumped single-photon sourceC. Bödefeld,1 J. Ebbecke,2 J. Toivonen,3 M. Sopanen,3 H. Lipsanen,3 and A. Wixforth21Sektion Physik der Ludwig-Maximilians-Universität and Center for NanoScience (CeNS), Geschwister-Scholl-Platz 1, 80539 Munich,Germany2Institut für Physik der Universität Augsburg, Experimentalphysik I, Universitätsstrasse 1, 86135 Augsburg, Germany3Optoelectronics Laboratory, Helsinki University of Technology, Tietotie 3, FIN-02150 Espoo, FinlandReceived 23 March 2006; published 10 July 2006Experiments towards a periodically pumped single-photon source are presented. The lateral piezoelectricfield of a surface acoustic wave dissociates laser-generated two-dimensional excitons into electrons and holes.These carriers are separated by the wave potential and are transported over macroscopic length scales withoutrecombining. When reaching a stress-induced quantum dot in the quantum well they periodically populate thezero-dimensional states and recombine, emitting single photons periodically in time according to the surfaceacoustic-wave frequency. We have successfully reduced the number of pumped quantum dots down to 100 andhave detected a strong blinking photoluminescence signal. By further reducing the number of quantum dotsdown to 1 a periodically pumped single photon source could be realized.DOI: 10.1103/PhysRevB.74.035407 PACS numbers: 73.63.Kv, 78.20.Hp, 78.70.GqSingle-photon sources SPS’s are of great interest in thevast growing field of information technology.1 As the emittedphotons of SPS are antibunched, applications become fea-sible that would be often impossible to realize with classicallight sources. The first important area to mention at this pointis quantum cryptography.2 To transfer information by codingthem in a certain way that no one else is able to understand istopic of actual research and has also been a desire for manycenturies. If the information has been transmitted by electro-magnetic waves, many photons have been carrying the dataso far. Utilizing single-photon sources to transfer coded dataeither by the state of polarization or in time packages a newpossibilty of securing information is realizable. Due to thenoncloning theorem of a single quantum state, nobody caneavesdrop on a single photon without changing his quantumstate or even annihilating it. Therefore the actual receptor ofthe information will know that the secure transmittance ofdata has been attacked. That is in principle impossible byclassical information transmittance.There are several technological approaches of realizingSPS’s with different grades of success. One of the first real-ized devices that has shown antibunched signals is the so-called single-photon turnstile device.3 Multiple-quantum-well structures and alternating tunneling of electron andholes are used to realize the SPS. The disadvantage of thisCoulomb-blockade-controlled mesoscopic p-n junction is anextremely low operation temperature in the lower-millikelvinregion. This is, on the other hand, the advantage of SPS’srealized by nitrogen-vacancy centers in diamond operating atroom temperature.4 The simplified model of the luminescentdefect is a four-level structure with two nonradiating and onephoton emitting transition. After laser excitation antibunchedphotons are emitted.Besides the realizations of SPS’s by the use of self-organized InAs quantum dots in optically excited5 and elec-trically pumped structures6 also one realization of a SPS bythe use of surface acoustic waves SAW’s has beenproposed.7 The basic idea of this device is the use of asingle-electron transport mediated by the SAW’s towards alateral p-n junction. The superposition of the piezoelectricSAW field and the potential of the one-dimensional channelleads to the creation of dynamic quantum dots QD’s inwhich single electrons can be transported through the chan-nel. If these electrons are transported towards a lateral p-typeregion, they can recombine with the existing holes and singlephotons can be emitted. Even though no working device withsingle electrons has been realized so far,8 this idea representsthe realization of a SPS.In this paper, we report on experimental progress on real-izing a different approach towards a periodically pumpedSPS mediated by a SAW.9 The basic mechanism is sketchedin Fig. 1. Electron-hole pairs are excited by laser radiation inan empty quantum well a in Fig. 1. The lateral piezoelec-tric potential of a SAW dissociates the excitons and alsoleads to a periodic modulation of the semiconductor bandstructure. Therefore the electrons relax to the local minima ofthe conduction band and the holes relax to the local maximaof the valence band b in Fig. 1. The resulting decrease inwave function overlap increases the lifetime of the electronsand holes drastically. Then the charges are transported later-ally in the semiconductor structure by the propagating SAWc. If a sufficiently deep single QD potential is superim-posed to the band structure, one single electron and onesingle hole relax successively into the QD potential per wavecycle d+ e in Fig. 1, resulting in a determining emissionFIG. 1. A simpified schematic picture of a single-photon esca-lator: laser-generated electron-hole pairs are dissociated by a sur-face acoustic wave and transported towards a single quantum dotwhere they recombine and single photons are emitted.PHYSICAL REVIEW B 74, 035407 20061098-0121/2006/743/0354075 ©2006 The American Physical Society035407-1of single photons with a repetition rate given by the SAWfrequency.Measurements of two almost identical samples are pre-sented in this paper. Based on a GaAs substrate first a125-nm-thick GaAs buffer and then a 7-nm In0.2Ga0.8Asquantum well are grown by metal organic chemical vaporphase epitaxy followed by an undoped GaAs cap layer of20 nm thickness, leaving the quantum well empty of chargecarriers. Finally, at the surface of the sample self-organizedInP islands are grown in the Stranski-Krastranov growthmode10,11 3 monolayers. The islands have an average diam-eter of 80 nm, a height of 22 nm, and a density of 1–2109 cm−2 see close-up part of Fig. 2. Due to a 37% latticemismatch of InP and GaAs, the local strain underneath theself-organized islands induces potential cavities in the In-GaAs quantum well. These stressor-induced quantum dotsSTIQD’s are deliberately made for emitting single photons.In Fig. 2 the layout of sample A is shown. In most parts ofthe surface the InP islands have been removed chemicallyusing 10% HCl for 60 s, leaving the undoped GaAs layer atthe surface. Then an interdigital transducer IDT with a pe-riod of 5 m has been processed 5 nm NiCr, 100 nm Al,5 nm NiCr on the etched surface and with a given wavevelocity of v2865 m/s on GaAs a SAW with a frequencyof 580 MHz can be launched. A laser diode with a wave-length of 680 nm is used to create two-dimensional electron-hole pairs in the InGaAs quantum well in the area betweenthe IDT and InP islands.First a nitrogen-cooled charge-coupled device CCD hasbeen used to perform energy-resolved measurements in orderto characterize the sample without a SAW present. Twosingle measurements of the photoluminescence PL inten-sity of sample A are presented in Fig. 3. First the part ofsample A with the InP islands has been investigated Fig.3a. Here the most prominent PL intensity peak is at1.39 eV which corresponds to the ground state of thestressor-induced quantum dots see Fig. 1d. Beside thismost dominant peak also the first and second excited statesof the quantum dots and weakly the PL of the quantum wellare visible. In Fig. 3b a spectrum of the etched region ofsample A is shown. In contrast to Fig. 3a here the mostdominant PL peaks can be allocated to the quantum wellwhereas almost no signal from the stress-induced QD’s canbe detected. In Fig. 4 a whole series of the PL intensitymeasurements is presented as a gray scale coded plot as afunction of the lateral position on the sample and photonenergy. A schematic drawing of the sample layout is addedon the right side for clarity. In the area where the InP QD’shave been chemically removed x= 0,800 m a signal atE1.45 eV has been detected resulting from recombinationof electrons and holes in the InGaAs quantum well, whereasin the area with InP QD’s between x=800 m andx=1500 m the main PL signal is shifted towards a lowerenergy of E1.39 eV caused by primary recombinations inthe STIQD’s. The measurements shown in Figs. 3 and 4 areproof that the InP dots can be removed completely by the wetFIG. 2. Sample A: GaAs/ InGaAs heterostructure with InP is-lands at the surface. In most parts of the sample the InP islands havebeen removed chemically and an interdigital transducer to launch aSAW has been processed. At the bottom an AFM picture of theetched-unetched transition is shown.FIG. 3. Photoluminescence spectra of sample A: a PL spec-trum of the GaAs/ InAs heterostructure with InP islands at the sur-face showing dominating peaks from the induced quantum dots. bSpectrum from a part of the sample where the islands have beenchemically removed, exhibiting the remaining quantum well peaks.FIG. 4. Gray-scale-coded plot of the PL intensity as a functionof lateral position on sample A and photon energy. In the area wherethe InP QD’s have been removed chemically there is a PL signalfrom the InGaAs quantum well where in the other area the electronsand holes primarily recombine in the InP QD’s.BÖDEFELD et al. PHYSICAL REVIEW B 74, 035407 2006035407-2etching process without modifying the InGaAs quantumwell, causing an abrupt change in emission energy at theetched-nonetched transition.In the remaining part of the paper measurements are pre-sented where an intensified CCD 4 picos, Stanford Com-puter Optics has been used in order to get a better lateral andtime resolution. Due to the fact that this measurement setupwith the ICCD has no energy resolution, both the PL of thequantum well and of the QD’s contribute to the ICCD signal.Therefore the investigations shown in Fig. 4 were necessary.Figure 5 presents a lateral- and time-dependent measurementwith the use of a SAW. The top panel shows the direct PL ofthe sample under continuous illumination and with no SAWpresent. This panel is used to elucidate the sample geometryin this experiment. The following panels represent the resultsof a time-resolved experiment with a SAW present on thesample. A SAW is launched on the left side of the sampleat t=0 s, and next to it in the etched area the laser diode isused in a pulsed mode to excite electron-hole pairs. The sig-nal from direct recombination processes of surface states canclearly be seen in the first frame for t=50 ns at the spotwhere the laser was focused to. To improve contrast, theintensity range is twice from white low intensity to blackhigh intensity. The laser excited surface states have a longlifetime which can be seen in Fig. 5 because there is still aPL signal identifiable at the excitation site for t=450 ns. Asdescribed in Fig. 1 the SAW potential dissociates the laser-generated electron-hole pairs and transports them laterallytowards the STIQD’s. This mechanism emerges at the timeinterval t= 300 ns,400 ns and can be seen in Fig. 5 as asmall vertical black line at the etched-nonetched transition.This time period of 100 ns of PL from the QD’s correspondswell with the SAW pulse duration of 100 ns. Also thetime delay of 300 ns conforms with the time a SAW needsto propagate the 900 m towards the InP QD’sv2865 m/s.In order to gain more information about the microscopicprocesses leading to this photon emission from the STIQD’s,a PL intensity evolution with high temporal resolution hasbeen performed. In Fig. 6, the normalized PL intensity isshown as a function of time for a certain area of sample A.This region is highlighted as an inset and consists ofa 203 m2 area directly after the transition etched part andInP islands. Taken into account a density of 2109 islandsper cm2, Fig. 6 represents the averaged PL intensity of 1200STIQD’s. The ICCD signal exhibits an oscillation of ±10%with a period of 1.8 ns, meaning a frequency off =566 MHz, corresponding well with the SAW frequency offSAW=580 MHz. These periodic changes in PL intensity pro-vides the first hint that the photon emission is intentionallywith a frequency defined by the SAW frequency.An ideal single-photon source would consist of one singleemitting QD. As a first attempt towards this final goal asecond sample has been processed where the interdigitaltransducer has been replaced by a focusing one in order tominimize the number of emitting STIQD’s. Focusing IDT’sare a special form of transducers that are able to focus theemitted SAW onto a small area. First layouts with sphericallyshaped IDT’s have already shown resonable performance12but it has been shown13 that a parabolic layout taking theslowness curve of the substrate into account increases theperformance to focus the power towards one single spot. Dueto limitations of producing curved geometries by the usede-beam lithography setup, the ideal layout had to be approxi-mated by ten linear segments as shown in Fig. 7. Sample Bonly deviates from the layout of sample A by the differentshape of the IDT.FIG. 5. Lateral and temporal PL signal presentation showingthe acousto-optic pumping of QD’s black vertical lines att= 300 ns,400 ns.FIG. 6. Temporal intensity evolution integrated over an area of203 m2. An oscillation of the signal with a periodicity given bythe SAW frequency is visible.EXPERIMENTAL INVESTIGATION TOWARDS A¼ PHYSICAL REVIEW B 74, 035407 2006035407-3A picture of sample B is presented at the top panel of Fig.8 where the parabolic shape of the IDT is clearly visible. Forclarity, also the SAW path and the region with the remainingInP QD’s are highlighted. In Fig. 8, a similar temporal evo-lution here with sample B as presented in Fig. 5 is shown.For the complete time series between t= 100 ns,500 nsan ICCD signal is visible next to the focusing IDT where thesample has been illuminated by a short laser pulse. Again,this signal has its origin in recombination processes of laser-excited surface states. But in contrast to the measurementspresented in Fig. 5 where a vertical line of PL signal hasbeen detected here in Fig. 8 just one single spot of PL signalhas been measured for t=500 ns. Again, the dissociatedelectrons and holes are transported by the SAW towards theSTIQD’s but with sample B the region of recombination hasbeen decreased significantly by the use of the focusingIDT’s.In Fig. 9 a detailed time evolution of this single spot ispresented. For these measurements the lateral resolution hasbeen increased, resulting in a focus of 22.5 m2, mean-ing an averaged signal over only 100 STIQD’s. Now, theICCD signal exhibits a clear oscillation of ±30% with a fre-quency of f =480 MHz, which corresponds within experi-mental uncertainties with the SAW frequency of sample B offSAW=508 MHz. The exciton lifetime in semiconductorquantum dots has been found to be in the order of=1.3 ns,14,15 and by using a resonant cavity it can be re-duced to 0.3 ns, which is lower than the repetition fre-quency given by the SAW, 1/ f2 ns, ensuring the emissionof single photons.In summary, we have performed experiments of acousto-optic photon emission. The laser-generated electron-holepairs were dissociated by the piezoelectric SAW potentialand laterally transported towards stressor-induced QD’s in anInGaAs quantum well. By the use of a focusing IDT we wereable to reduce the number of emitting QD’s down to 100dots. By further selection processes—e.g., e-beam litho-graphically defined wet-etching techniques—it can be envis-aged that the number of participating QD’s can be reduced to1. Then a periodically pumped single-photon source wouldhave been realized.This work has been partially funded by the DeutscheForschungsgemeinschaft DFG SFB 348. The authors liketo thank F. Haake, A. O. Govorov, and J. P. Kotthaus forvaluable discussions.FIG. 7. Layout of the processed interdigital transducer onsample B where a focusing SAW can be launched with.FIG. 8. Lateral and temporal PL signal presentation showing theacousto-optic pumping of a small number of QD’s using sample B.A picture of this sample is added at the top for assistance.FIG. 9. Temporal intensity evolution integrated over a distanceof 2 m. An enhanced beating of the signal with a periodicity givenby the SAW frequency is visible.BÖDEFELD et al. PHYSICAL REVIEW B 74, 035407 2006035407-41 D. Bouwmeester, A. Ekert, and A. Zeilinger, The Physics of Qua-tum Information Springer, Berlin, 2000.2 W. Tittel, G. Ribordy, and N. Gisin, Phys. World 11, 41 1998.3 J. Kim, O. Benson, H. Kan, and Y. Yamamoto, Nature London397, 500 1999.4 C. Kurtsiefer, S. Mayer, P. Zarda, and H. Weinfurter, Phys. Rev.Lett. 85, 290 2000.5 P. Michler, A. Imamoglu, M. D. Mason, P. J. Carson, G. F.Strouse, and S. K. Buratto, Nature London 406, 968 2000.6 Z. Yuan, B. E. Kardynal, R. M. Stevenson, A. J. Shields, C. J.Lobo, K. Cooper, N. S. Beattie, D. A. Ritchie, and M. Pepper,Science 295, 102 2002.7 C. L. Foden, V. I. Talyanskii, G. J. Milburn, M. L. Leadbeater,and M. Pepper, Phys. Rev. A 62, 011803 2000.8 M. Cecchini, V. Piazza, F. Beltram, D. G. Gevaux, M. B. Ward,A. J. Shields, H. E. Beere, and D. A. Ritchie, Appl. Phys. Lett.86, 241107 2005.9 C. Wiele, F. Haake, C. Rocke, and A. Wixforth, Phys. Rev. A 58,R2680 1998.10 M. Sopanen, H. Lipsanen, and J. Ahopelto, Appl. Phys. Lett. 66,2364 1995.11 H. Lipsanen, M. Sopanen, and J. Ahopelto, Phys. Rev. B 51,13868 1995.12 W. Sauer, M. Streibl, T. H. Metzger, A. G. C. Haubrich, S.Manus, A. Wixforth, J. Peisl, A. Mazuelas, J. Härtwig, and J.Baruchel, Appl. Phys. Lett. 75, 1709 1999.13 F. W. Beil, private communications.14 R. M. Thompson, R. M. Stevenson, A. J. Shields, I. Farrer, C. J.Lobo, D. A. Ritchie, M. L. Leadbeater, and M. Pepper, Phys.Rev. B 64, 201302R 2001.15 G. S. Solomon, M. Pelton, and Y. Yamamoto, Phys. Rev. Lett. 86,3903 2001.EXPERIMENTAL INVESTIGATION TOWARDS A¼ PHYSICAL REVIEW B 74, 035407 2006035407-5

Experimental investigation towards a periodically pumped single-photon source

Ebbecke, Jens

Wixforth, Achim

OPUS - Augsburg University Publication Server

OPUS Augsburg

Experimental investigation towards a periodically pumped single-photon sourceC. Bödefeld,1 J. Ebbecke,2 J. Toivonen,3 M. Sopanen,3 H. Lipsanen,3 and A. Wixforth21Sektion Physik der Ludwig-Maximilians-Universität and Center for NanoScience (CeNS), Geschwister-Scholl-Platz 1, 80539 Munich,Germany2Institut für Physik der Universität Augsburg, Experimentalphysik I, Universitätsstrasse 1, 86135 Augsburg, Germany3Optoelectronics Laboratory, Helsinki University of Technology, Tietotie 3, FIN-02150 Espoo, FinlandReceived 23 March 2006; published 10 July 2006Experiments towards a periodically pumped single-photon source are presented. The lateral piezoelectricfield of a surface acoustic wave dissociates laser-generated two-dimensional excitons into electrons and holes.These carriers are separated by the wave potential and are transported over macroscopic length scales withoutrecombining. When reaching a stress-induced quantum dot in the quantum well they periodically populate thezero-dimensional states and recombine, emitting single photons periodically in time according to the surfaceacoustic-wave frequency. We have successfully reduced the number of pumped quantum dots down to 100 andhave detected a strong blinking photoluminescence signal. By further reducing the number of quantum dotsdown to 1 a periodically pumped single photon source could be realized.DOI: 10.1103/PhysRevB.74.035407 PACS numbers: 73.63.Kv, 78.20.Hp, 78.70.GqSingle-photon sources SPS’s are of great interest in thevast growing field of information technology.1 As the emittedphotons of SPS are antibunched, applications become fea-sible that would be often impossible to realize with classicallight sources. The first important area to mention at this pointis quantum cryptography.2 To transfer information by codingthem in a certain way that no one else is able to understand istopic of actual research and has also been a desire for manycenturies. If the information has been transmitted by electro-magnetic waves, many photons have been carrying the dataso far. Utilizing single-photon sources to transfer coded dataeither by the state of polarization or in time packages a newpossibilty of securing information is realizable. Due to thenoncloning theorem of a single quantum state, nobody caneavesdrop on a single photon without changing his quantumstate or even annihilating it. Therefore the actual receptor ofthe information will know that the secure transmittance ofdata has been attacked. That is in principle impossible byclassical information transmittance.There are several technological approaches of realizingSPS’s with different grades of success. One of the first real-ized devices that has shown antibunched signals is the so-called single-photon turnstile device.3 Multiple-quantum-well structures and alternating tunneling of electron andholes are used to realize the SPS. The disadvantage of thisCoulomb-blockade-controlled mesoscopic p-n junction is anextremely low operation temperature in the lower-millikelvinregion. This is, on the other hand, the advantage of SPS’srealized by nitrogen-vacancy centers in diamond operating atroom temperature.4 The simplified model of the luminescentdefect is a four-level structure with two nonradiating and onephoton emitting transition. After laser excitation antibunchedphotons are emitted.Besides the realizations of SPS’s by the use of self-organized InAs quantum dots in optically excited5 and elec-trically pumped structures6 also one realization of a SPS bythe use of surface acoustic waves SAW’s has beenproposed.7 The basic idea of this device is the use of asingle-electron transport mediated by the SAW’s towards alateral p-n junction. The superposition of the piezoelectricSAW field and the potential of the one-dimensional channelleads to the creation of dynamic quantum dots QD’s inwhich single electrons can be transported through the chan-nel. If these electrons are transported towards a lateral p-typeregion, they can recombine with the existing holes and singlephotons can be emitted. Even though no working device withsingle electrons has been realized so far,8 this idea representsthe realization of a SPS.In this paper, we report on experimental progress on real-izing a different approach towards a periodically pumpedSPS mediated by a SAW.9 The basic mechanism is sketchedin Fig. 1. Electron-hole pairs are excited by laser radiation inan empty quantum well a in Fig. 1. The lateral piezoelec-tric potential of a SAW dissociates the excitons and alsoleads to a periodic modulation of the semiconductor bandstructure. Therefore the electrons relax to the local minima ofthe conduction band and the holes relax to the local maximaof the valence band b in Fig. 1. The resulting decrease inwave function overlap increases the lifetime of the electronsand holes drastically. Then the charges are transported later-ally in the semiconductor structure by the propagating SAWc. If a sufficiently deep single QD potential is superim-posed to the band structure, one single electron and onesingle hole relax successively into the QD potential per wavecycle d+ e in Fig. 1, resulting in a determining emissionFIG. 1. A simpified schematic picture of a single-photon esca-lator: laser-generated electron-hole pairs are dissociated by a sur-face acoustic wave and transported towards a single quantum dotwhere they recombine and single photons are emitted.PHYSICAL REVIEW B 74, 035407 20061098-0121/2006/743/0354075 ©2006 The American Physical Society035407-1of single photons with a repetition rate given by the SAWfrequency.Measurements of two almost identical samples are pre-sented in this paper. Based on a GaAs substrate first a125-nm-thick GaAs buffer and then a 7-nm In0.2Ga0.8Asquantum well are grown by metal organic chemical vaporphase epitaxy followed by an undoped GaAs cap layer of20 nm thickness, leaving the quantum well empty of chargecarriers. Finally, at the surface of the sample self-organizedInP islands are grown in the Stranski-Krastranov growthmode10,11 3 monolayers. The islands have an average diam-eter of 80 nm, a height of 22 nm, and a density of 1–2109 cm−2 see close-up part of Fig. 2. Due to a 37% latticemismatch of InP and GaAs, the local strain underneath theself-organized islands induces potential cavities in the In-GaAs quantum well. These stressor-induced quantum dotsSTIQD’s are deliberately made for emitting single photons.In Fig. 2 the layout of sample A is shown. In most parts ofthe surface the InP islands have been removed chemicallyusing 10% HCl for 60 s, leaving the undoped GaAs layer atthe surface. Then an interdigital transducer IDT with a pe-riod of 5 m has been processed 5 nm NiCr, 100 nm Al,5 nm NiCr on the etched surface and with a given wavevelocity of v2865 m/s on GaAs a SAW with a frequencyof 580 MHz can be launched. A laser diode with a wave-length of 680 nm is used to create two-dimensional electron-hole pairs in the InGaAs quantum well in the area betweenthe IDT and InP islands.First a nitrogen-cooled charge-coupled device CCD hasbeen used to perform energy-resolved measurements in orderto characterize the sample without a SAW present. Twosingle measurements of the photoluminescence PL inten-sity of sample A are presented in Fig. 3. First the part ofsample A with the InP islands has been investigated Fig.3a. Here the most prominent PL intensity peak is at1.39 eV which corresponds to the ground state of thestressor-induced quantum dots see Fig. 1d. Beside thismost dominant peak also the first and second excited statesof the quantum dots and weakly the PL of the quantum wellare visible. In Fig. 3b a spectrum of the etched region ofsample A is shown. In contrast to Fig. 3a here the mostdominant PL peaks can be allocated to the quantum wellwhereas almost no signal from the stress-induced QD’s canbe detected. In Fig. 4 a whole series of the PL intensitymeasurements is presented as a gray scale coded plot as afunction of the lateral position on the sample and photonenergy. A schematic drawing of the sample layout is addedon the right side for clarity. In the area where the InP QD’shave been chemically removed x= 0,800 m a signal atE1.45 eV has been detected resulting from recombinationof electrons and holes in the InGaAs quantum well, whereasin the area with InP QD’s between x=800 m andx=1500 m the main PL signal is shifted towards a lowerenergy of E1.39 eV caused by primary recombinations inthe STIQD’s. The measurements shown in Figs. 3 and 4 areproof that the InP dots can be removed completely by the wetFIG. 2. Sample A: GaAs/ InGaAs heterostructure with InP is-lands at the surface. In most parts of the sample the InP islands havebeen removed chemically and an interdigital transducer to launch aSAW has been processed. At the bottom an AFM picture of theetched-unetched transition is shown.FIG. 3. Photoluminescence spectra of sample A: a PL spec-trum of the GaAs/ InAs heterostructure with InP islands at the sur-face showing dominating peaks from the induced quantum dots. bSpectrum from a part of the sample where the islands have beenchemically removed, exhibiting the remaining quantum well peaks.FIG. 4. Gray-scale-coded plot of the PL intensity as a functionof lateral position on sample A and photon energy. In the area wherethe InP QD’s have been removed chemically there is a PL signalfrom the InGaAs quantum well where in the other area the electronsand holes primarily recombine in the InP QD’s.BÖDEFELD et al. PHYSICAL REVIEW B 74, 035407 2006035407-2etching process without modifying the InGaAs quantumwell, causing an abrupt change in emission energy at theetched-nonetched transition.In the remaining part of the paper measurements are pre-sented where an intensified CCD 4 picos, Stanford Com-puter Optics has been used in order to get a better lateral andtime resolution. Due to the fact that this measurement setupwith the ICCD has no energy resolution, both the PL of thequantum well and of the QD’s contribute to the ICCD signal.Therefore the investigations shown in Fig. 4 were necessary.Figure 5 presents a lateral- and time-dependent measurementwith the use of a SAW. The top panel shows the direct PL ofthe sample under continuous illumination and with no SAWpresent. This panel is used to elucidate the sample geometryin this experiment. The following panels represent the resultsof a time-resolved experiment with a SAW present on thesample. A SAW is launched on the left side of the sampleat t=0 s, and next to it in the etched area the laser diode isused in a pulsed mode to excite electron-hole pairs. The sig-nal from direct recombination processes of surface states canclearly be seen in the first frame for t=50 ns at the spotwhere the laser was focused to. To improve contrast, theintensity range is twice from white low intensity to blackhigh intensity. The laser excited surface states have a longlifetime which can be seen in Fig. 5 because there is still aPL signal identifiable at the excitation site for t=450 ns. Asdescribed in Fig. 1 the SAW potential dissociates the laser-generated electron-hole pairs and transports them laterallytowards the STIQD’s. This mechanism emerges at the timeinterval t= 300 ns,400 ns and can be seen in Fig. 5 as asmall vertical black line at the etched-nonetched transition.This time period of 100 ns of PL from the QD’s correspondswell with the SAW pulse duration of 100 ns. Also thetime delay of 300 ns conforms with the time a SAW needsto propagate the 900 m towards the InP QD’sv2865 m/s.In order to gain more information about the microscopicprocesses leading to this photon emission from the STIQD’s,a PL intensity evolution with high temporal resolution hasbeen performed. In Fig. 6, the normalized PL intensity isshown as a function of time for a certain area of sample A.This region is highlighted as an inset and consists ofa 203 m2 area directly after the transition etched part andInP islands. Taken into account a density of 2109 islandsper cm2, Fig. 6 represents the averaged PL intensity of 1200STIQD’s. The ICCD signal exhibits an oscillation of ±10%with a period of 1.8 ns, meaning a frequency off =566 MHz, corresponding well with the SAW frequency offSAW=580 MHz. These periodic changes in PL intensity pro-vides the first hint that the photon emission is intentionallywith a frequency defined by the SAW frequency.An ideal single-photon source would consist of one singleemitting QD. As a first attempt towards this final goal asecond sample has been processed where the interdigitaltransducer has been replaced by a focusing one in order tominimize the number of emitting STIQD’s. Focusing IDT’sare a special form of transducers that are able to focus theemitted SAW onto a small area. First layouts with sphericallyshaped IDT’s have already shown resonable performance12but it has been shown13 that a parabolic layout taking theslowness curve of the substrate into account increases theperformance to focus the power towards one single spot. Dueto limitations of producing curved geometries by the usede-beam lithography setup, the ideal layout had to be approxi-mated by ten linear segments as shown in Fig. 7. Sample Bonly deviates from the layout of sample A by the differentshape of the IDT.FIG. 5. Lateral and temporal PL signal presentation showingthe acousto-optic pumping of QD’s black vertical lines att= 300 ns,400 ns.FIG. 6. Temporal intensity evolution integrated over an area of203 m2. An oscillation of the signal with a periodicity given bythe SAW frequency is visible.EXPERIMENTAL INVESTIGATION TOWARDS A¼ PHYSICAL REVIEW B 74, 035407 2006035407-3A picture of sample B is presented at the top panel of Fig.8 where the parabolic shape of the IDT is clearly visible. Forclarity, also the SAW path and the region with the remainingInP QD’s are highlighted. In Fig. 8, a similar temporal evo-lution here with sample B as presented in Fig. 5 is shown.For the complete time series between t= 100 ns,500 nsan ICCD signal is visible next to the focusing IDT where thesample has been illuminated by a short laser pulse. Again,this signal has its origin in recombination processes of laser-excited surface states. But in contrast to the measurementspresented in Fig. 5 where a vertical line of PL signal hasbeen detected here in Fig. 8 just one single spot of PL signalhas been measured for t=500 ns. Again, the dissociatedelectrons and holes are transported by the SAW towards theSTIQD’s but with sample B the region of recombination hasbeen decreased significantly by the use of the focusingIDT’s.In Fig. 9 a detailed time evolution of this single spot ispresented. For these measurements the lateral resolution hasbeen increased, resulting in a focus of 22.5 m2, mean-ing an averaged signal over only 100 STIQD’s. Now, theICCD signal exhibits a clear oscillation of ±30% with a fre-quency of f =480 MHz, which corresponds within experi-mental uncertainties with the SAW frequency of sample B offSAW=508 MHz. The exciton lifetime in semiconductorquantum dots has been found to be in the order of=1.3 ns,14,15 and by using a resonant cavity it can be re-duced to 0.3 ns, which is lower than the repetition fre-quency given by the SAW, 1/ f 2 ns, ensuring the emissionof single photons.In summary, we have performed experiments of acousto-optic photon emission. The laser-generated electron-holepairs were dissociated by the piezoelectric SAW potentialand laterally transported towards stressor-induced QD’s in anInGaAs quantum well. By the use of a focusing IDT we wereable to reduce the number of emitting QD’s down to 100dots. By further selection processes—e.g., e-beam litho-graphically defined wet-etching techniques—it can be envis-aged that the number of participating QD’s can be reduced to1. Then a periodically pumped single-photon source wouldhave been realized.This work has been partially funded by the DeutscheForschungsgemeinschaft DFG SFB 348. The authors liketo thank F. Haake, A. O. Govorov, and J. P. Kotthaus forvaluable discussions.FIG. 7. Layout of the processed interdigital transducer onsample B where a focusing SAW can be launched with.FIG. 8. Lateral and temporal PL signal presentation showing theacousto-optic pumping of a small number of QD’s using sample B.A picture of this sample is added at the top for assistance.FIG. 9. Temporal intensity evolution integrated over a distanceof 2 m. An enhanced beating of the signal with a periodicity givenby the SAW frequency is visible.BÖDEFELD et al. PHYSICAL REVIEW B 74, 035407 2006035407-41 D. Bouwmeester, A. Ekert, and A. Zeilinger, The Physics of Qua-tum Information Springer, Berlin, 2000.2 W. Tittel, G. Ribordy, and N. Gisin, Phys. World 11, 41 1998.3 J. Kim, O. Benson, H. Kan, and Y. Yamamoto, Nature London397, 500 1999.4 C. Kurtsiefer, S. Mayer, P. Zarda, and H. Weinfurter, Phys. Rev.Lett. 85, 290 2000.5 P. Michler, A. Imamoglu, M. D. Mason, P. J. Carson, G. F.Strouse, and S. K. Buratto, Nature London 406, 968 2000.6 Z. Yuan, B. E. Kardynal, R. M. Stevenson, A. J. Shields, C. J.Lobo, K. Cooper, N. S. Beattie, D. A. Ritchie, and M. Pepper,Science 295, 102 2002.7 C. L. Foden, V. I. Talyanskii, G. J. Milburn, M. L. Leadbeater,and M. Pepper, Phys. Rev. A 62, 011803 2000.8 M. Cecchini, V. Piazza, F. Beltram, D. G. Gevaux, M. B. Ward,A. J. Shields, H. E. Beere, and D. A. Ritchie, Appl. Phys. Lett.86, 241107 2005.9 C. Wiele, F. Haake, C. Rocke, and A. Wixforth, Phys. Rev. A 58,R2680 1998.10 M. Sopanen, H. Lipsanen, and J. Ahopelto, Appl. Phys. Lett. 66,2364 1995.11 H. Lipsanen, M. Sopanen, and J. Ahopelto, Phys. Rev. B 51,13868 1995.12 W. Sauer, M. Streibl, T. H. Metzger, A. G. C. Haubrich, S.Manus, A. Wixforth, J. Peisl, A. Mazuelas, J. Härtwig, and J.Baruchel, Appl. Phys. Lett. 75, 1709 1999.13 F. W. Beil, private communications.14 R. M. Thompson, R. M. Stevenson, A. J. Shields, I. Farrer, C. J.Lobo, D. A. Ritchie, M. L. Leadbeater, and M. Pepper, Phys.Rev. B 64, 201302R 2001.15 G. S. Solomon, M. Pelton, and Y. Yamamoto, Phys. Rev. Lett. 86,3903 2001.EXPERIMENTAL INVESTIGATION TOWARDS A¼ PHYSICAL REVIEW B 74, 035407 2006035407-5

English

Toivonen, Juha

Sopanen, M.

Lipsanen, H.

Physical Review B

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Experimental investigation towards a periodically pumped single-photon source

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