We report on the realization of polariton quantum boxes in a semiconductor
microcavity under strong coupling regime. The quantum boxes consist of mesas
that confine the cavity photon, etched on top of the spacer of a microcavity.
For mesas with sizes of the order of a few micron in width and nm in depth, we
observe quantization, caused by the lateral confinement, of the polariton modes
in several peaks. We evidence the strong exciton-photon coupling regime through
a typical/clear anticrossing curve for each quantized level. Moreover the
growth technique is of high quality, which opens the way for the conception of
new optoelectronic devices

Baas, Augustin

Brantut, Jean-Philippe

Daïf, Ounsi El

Deveaud, Benoit

Guillet, Thierry

Idrissi-Kaitouni, Reda

Morier-Genoud, Francois

Staehli, Jean-Louis

English

arXiv

O. El Daïf

A. Baas

T. Guillet

J.-P. Brantut

R. Idrissi Kaitouni

J. L. Staehli

F. Morier-Genoud

B. Deveaud

Crossref

Polariton quantum boxes in semiconductor microcavities

El Daïf, Ounsi

Brantut, J.-P.

Idrissi Kaitouni, Reda

Staehli, J. L.

Infoscience - École polytechnique fédérale de Lausanne

APPLIED PHYSICS LETTERS 88, 061105 2006Polariton quantum boxes in semiconductor microcavitiesO. El Daïfa and A. BaasInstitut de Photonique et d’Électronique Quantiques, École Polytechnique Fédérale de Lausanne (EPFL),CH-1015 Lausanne, SwitzerlandT. GuilletGroupe d’étude des Semiconducteurs (GES), Université de Montpellier II, Place Eugène Bataillon,F-34095 Montpellier cedex 5, FranceJ.-P. Brantut, R. Idrissi Kaitouni, J. L. Staehli, F. Morier-Genoud, and B. DeveaudInstitut de Photonique et d’Électronique Quantiques, École Polytechnique Fédérale de Lausanne (EPFL),CH-1015 Lausanne, SwitzerlandReceived 8 August 2005; accepted 21 December 2005; published online 7 February 2006We report on the realization of polariton quantum boxes in a semiconductor microcavity understrong coupling regime. The quantum boxes consist of mesas, etched on the top of the spacer of amicrocavity, that confine the cavity photon. For mesas with sizes of the order of a few microns inwidth and nanometers in depth, we observe quantization of the polariton modes in several states,caused by the lateral confinement. We evidence the strong exciton-photon coupling regime througha typical anticrossing curve for each quantized level. Moreover, the growth technique permits oneto obtain high-quality samples, and opens the way for the conception of new optoelectronicdevices. © 2006 American Institute of Physics. DOI: 10.1063/1.2172409Confining semiconductor structures allows the study ofvarious fundamental effects, ranging from the Purcell effectto the full quantum confinement. Such confinement is alsoused for applications in many fields, from optoelectronics toquantum information. Previous works have focused on dif-ferent aspects: on the one hand, on the matter part, with theconfinement of the excitonic resonances in quantum wells,quantum wires, and quantum dots. On the other hand, envi-ronment for the electromagnetic field has been modified byoptical confinement in different types of cavities. Addition-ally since the middle of the 90s, low dimensional deviceshave been realized in the strong coupling regime.1 Confine-ment can enhance the interactions, modify the real andimaginary parts of the resonance’s energy, or open access tonew interaction processes. It is also often considered as apossible way to obtain a condensed phase of bosons insemiconductors,2 but so far the fermionic nature of excitonshas always become dominant upon increasing density. In thissense, polaritons are of great interest as, despite their exci-tonic content, they have a very small effective mass in com-parison to the exciton thanks to their photonic component,which theoretically increases their temperature of condensa-tion above 0.1 K.3 The peculiar trap shape of the lowermicrocavity polariton dispersion curve has motivated severalrelaxation experiments towards the bottom of this “trap”4,5but no clear evidence for the formation of spontaneous co-herence formation has been given yet.Zero-dimensional 0D Polariton confinement can beachieved either through their excitonic or through their pho-tonic component. Recently, evidence for 0D polaritons hasbeen given with single quantum dots in micropillars,6 photo-nic nanocavities,7 or microdisks,8 and for a large number ofexcitations in micropillar structures.9–11 Here we consider anovel system under strong coupling regime, where 0D con-finement is achieved through the photonic part of polaritonsaElectronic mail: ounsi.eldaif@epfl.ch0003-6951/2006/886/061105/3/$23.00 88, 06110Downloaded 08 Feb 2006 to 128.178.83.77. Redistribution subject to in high Q cavities. Our original structure contains polaritonquantum boxes, constituted by mesas in the spacer layer of asemiconductor microcavity, allowing to keep the strong cou-pling regime. Each mesa, by acting on the two degrees offreedom of the photonic component of the two-dimensional2D cavity polaritons, does create a localized photonic boxin the microcavity.The main specificity of our technique is that the semi-conductor microcavity around the mesas is in no way alteredby the creation of the box, contrary to the case of etchedmicrocavities. This brings about a number of advantages: ithe presence of the 2D cavity restricts the lateral losses forthe confined mode, ii the number of confined levels in thequantum box is controlled by the height of the mesa, iiiinteraction between the 0D and the surrounding 2D polari-tons is possible, iv the technique also allows to create pat-terns at will, for example polariton quantum wires,12 whichwill allow studying possible interactions between 0D, one-dimensional, and 2D bosons.By using an original technique, we were able to fabricatemicrocavities with mesas etched on the spacer layer. Themesa lengthens the cavity and thus lowers the cavity modeenergy see Fig. 1. This local difference creates a potentialFIG. 1. Color online a Scheme of a circular mesa. Only the first fourlayers of the Bragg mirrors are represented. b Scheme of a potential trapwith two confined levels: the ground level G and one excited state E1.c AFM image of a 9-m-diameter circular mesa on the surface of the topmirror. The lateral scales are in microns and the vertical scale is innanometers.© 2006 American Institute of Physics5-1AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp061105-2 El Daïf et al. Appl. Phys. Lett. 88, 061105 2006trap for the confined photons. The lateral shape and size ofthe mesas are defined by photolithography. The height of themesas is determined by the etching process and can bechanged at will. It has to be large enough to allow the exis-tence of at least one confined level. Here, it has also beenchosen small enough to obtain mesas and planar cavity pho-ton modes close to the excitonic resonance. Thus we manageto observe mixed exciton-photon states inside and outsidethe mesas on the same part of the sample. Such a method ofdefining the structure, by a step of a few nanometersonly, avoids any lateral losses as observed in the micropillarstructure previously published.10The final structure, schematically represented with a cir-cular mesa on Fig. 1a, is prepared in the following way.First, we grow 22 AlAs/GaAs pairs of distributed Braggreflectors, then the GaAs  spacer, with a single embeddedInGaAs/GaAs quantum well. The 80 Å quantum well withabout 4% indium in gallium arsenide, is chosen to allowmeasurements in transmission through the wafer below thegap of GaAs.13 Then we perform photolithography definingthe pattern of the mesas, and finally the etching, which de-termines their height. We can design various patterns for themesas on a single sample, but the etching process defines acommon height for all of them. Let us stress that the exci-tonic mode is absolutely not affected by the etching process,whatever the pattern of the mesa. Finally the regrowth of theupper 21 pairs is realized again by molecular beam epitaxy,after an in situ hydrogen cleaning which guarantees a perfectregrowth interface.The high quality of the sample can be appreciated byatomic force microscopy AFM measurements. Figure 1cshows a 9 m-diameter circular mesa on the surface of thecomplete structure. Amazingly enough, the 6 nm step heightof the etched mesas is kept even after the regrowth of the2.5-m-thick top Bragg mirror. The abruptnesses of the stepsas a function of the crystal orientation is correlated to themobility of the elements during growth. The profile along011 is much steeper 0.5 m width than along 0113 m width, which corresponds to a slower mobility of theatoms. This gives rise to a surface asymmetry along the dif-ferent crystalline orientations, with a smoothing of the etchedfaces that increases proportionally to the overgrowth thick-ness. The step height corresponds to the change of the pho-ton mode energy that we calculated by transfer matrix simu-lations. According to the AFM measurement on the topmirror of a 6 nm step, we compute an energy difference of9 meV, in very good agreement with the photoluminescenceexperiment results.Let us first consider the properties of the microcavity, theeffect of etching and regrowth. For this we measure the an-ticrossing curve of the resonances, resulting from the strongcoupling regime of the 2D polaritons and on large squaremesas of 300 m, where no confinement effect is expected,but where the photonic resonance is redshifted due to thelarger length of the cavity. The anticrossing curve is acces-sible using the wedge of about 4% in this sample. This thick-ness variation corresponds, for a  cavity, to an energy varia-tion for the photonic resonance of about 50 meV across thewhole sample. In the zone of interest the variation is about2.4 meV/mm. As the quantum well’s resonance is lessaffected by the thickness, its energy variation is below3 meV across the whole sample. The spectral properties aremeasured in a photoluminescence experiment. The sample,Downloaded 08 Feb 2006 to 128.178.83.77. Redistribution subject to cooled to about 10 K, is pumped by an Argon laser=532 nm, at low pump intensity see later. The lumines-cence is analyzed with a 25 eV resolution spectrometer.The microcavity features a Rabi splitting energy of 3.5 meV,and a full width at half the maximum of 220 eV for thephoton mode, corresponding to a quality factor of Q7103, and of 500 eV for the quantum well exciton. Wethen performed the same anticrossing measurement on thelarge square mesas of 300 m, disposed all along the wedge,separated by 300 m one from the other. Finally, we ob-tained a double anticrossing curve not shown. The two pho-ton modes—on the mesas and around the mesas—are sepa-rated by 9 meV. The photon mode shows the same linewidthon both domains. The two anticrossings have exactly thesame characteristics, which guaranties that the etchingprocess just shifts the cavity mode, as expected.We now focus on the effect of the quantum confinementof the polaritons in cylindrical mesas of 3 m in diameterother sizes give similar results. The laser spot diameterbeing 17 m, the spectra display both the confined polari-tons of the mesa and the 2D polaritons of the planar cavityoutside the mesa, displaced towards higher energies. Let usnote that the light emission from one single quantum box isvery intense, which allowed us to perform all experiments onsingle polariton boxes, well below the nonlinear regime, incontrast to the case of pillar microcavities.10,11 We considerfirst the photoluminescence spectrum of a mesa relatively farfrom the excitonic resonance on Fig. 2a. The lower polar-iton mode of the mesa is thus quasiphotonic and probes thequantum confinement of the photonic box. It is split intoseveral peaks. For sake of clarity, we will only consider theground level G and the first excited level E1, which is adoublet. This doublet corresponds to a degeneracy lifting thatresults from the asymmetry of the mesas at the level of thespacer.14 This asymmetry is already present on the photoli-thography mask and is not linked to any growth or etchingprocess. The quantized photonic peaks have a spectral widthof about 90 eV, a factor of 2 smaller than observed on thenonconfined cavity photon mode. The smallest value mea-sured reached 70 eV, indicating a quality factor ofQ2.1104 for the confined photon modes better than forthe 2D photon modes, thanks to the additional lateral con-finement. The energy of the ground level is 1.5 meV abovethe energy of the bottom of the trap as observed on the largemesas at the same position on the wedge.To demonstrate the strong coupling even in the smallestmesas and the polariton nature of the trapped states, we sys-tematically measured the spectra on mesas disposed alongthe wedge, i.e., with different detunings. For the 2D polari-tons, we are always rather far from the position for whichexciton and 2D cavity photon are degenerate, and the lowerpolariton mode stays at the exciton energy on the wholerange of positions shown in Fig. 3. For the confined states,strong coupling is very nicely demonstrated by the anticross-ing curve for the emission of each of the two confined levelsof the mesas’ polaritons. They present a Rabi splitting ofaround 3.35 meV. The degeneracy lifting of the first excitedlevel is no longer visible for the lower polariton modes whentheir excitonic component is too important. It is not at allvisible for the upper polariton modes, due to additional re-laxation channels, such as the coupling to continuum statesor lower energy modes. Figure 2b displays the photolumi-nescence spectrum very close to zero detuning in the trap,AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp061105-3 El Daïf et al. Appl. Phys. Lett. 88, 061105 2006where lower and upper polariton modes confined in thequantum box are nicely resolved. Despite the relatively largelinewidth of the 2D exciton, the high Q factor and thevery efficient relaxation to the quantum boxes allow one toobserve the confinement for both the lower and upperpolariton states.In summary, we have reported the realization of a semi-conductor microcavity in strong coupling regime with em-bedded polariton quantum boxes. The trap consists of a mesain the spacer layer, obtained by a well controlled technique.Quantum confinement is resolved simultaneously for bothupper and lower polariton modes, and the anticrossing isresolved separately for all the quantized polaritons. Experi-mental and theoretical work on the dispersion of the quan-tized modes is being performed. The coexistence in the samesample of 2D and 0D polaritons, as well as the possibility tostudy a single quantum box, will allow original studies suchas resonant parametric scattering or trapping by spatial relax-ation. Wide possibilities in the transverse shape of the mesasalso open the way to the realization of various experimentalFIG. 2. Photoluminescence spectra of a circular 3 m diameter mesa at twodifferent positions: a x=1.04 and b x=4.42 see Fig. 3. The origin of thehorizontal axis corresponds to the exciton’s energy. Grey circles: lower 2DLP and upper 2D UP 2D polaritons around the mesa. Dark circles: lowerLG and upper UG polaritons of the ground level of the mesa. Triangles:lower and upper polaritons of the first excited level E1 of the mesa. a LGand LE1 polariton states are quasiphotonic. b The excitonic and photoniccomponents are comparable for the confined polariton states. Other confinedlevels are visible on both spectra but are not labeled for sake of clarity.configurations for the bosonic polariton states, which energy,Downloaded 08 Feb 2006 to 128.178.83.77. Redistribution subject to wave function, and dimensionality can beengineered on demand.The authors would like to acknowledge fruitful discus-sions with C. Ciuti, P. Lugan, M. Saba, and V. Savona. Theyare thankful for the strong financial support from the SwissNCCR research program Quantum Photonics.1C. Weisbuch, M. Nishioka, A. Ishikawa, and Y. Arakawa, Phys. Rev. Lett.69, 3314 1992.2S. A. Moskalenko and D. W. Snoke, Bose-Einstein Condensation ofExcitons and Biexcitons and Coherent Nonlinear Optics with ExcitonsCambridge University Press, Cambridge, 2000.3A. Kavokin, G. Malpuech, and F. P. Laussy, Phys. Lett. A 306, 1872003.4J. J. Baumberg, P. G. Savvidis, R. M. Stevenson, A. I. Tartakovskii, M. S.Skolnick, D. M. Whittaker, and J. S. Roberts, Phys. Rev. B 62, R162472000.5M. Richard, J. Kasprzak, R. Romestain, R. André, and L. S. Dang, Phys.Rev. Lett. 94, 187401 2005.6J. P. Reithmaier, G. Sek, A. Löffler, C. Hoffmann, S. Kuhn,S. Reitzenstein, L. V. Keldysh, V. D. Kulakovskii, T. L. Reinecke, andA. Forchel, Nature London 432, 197 2004.7T. Yoshie, A. Scherer, J. Hendrickson, G. Khitrova, H. M. Gibbs,G. Rupper, C. Ell, O. B. Shchekin, and D. G. Deppe, Nature London432, 200 2004.8E. Peter, P. Senellart, D. Martrou, A. Lemaître, J. Hours, J.-M. Gérard, andJ. Bloch, Phys. Rev. Lett. 95, 067401 2005.9J. Bloch, F. Boeuf, J.-M. Gérard, B. Legrand, J.-Y. Marzin, R. Planel,V. Thierry-Mieg, and E. Costard, Physica E Amsterdam 2, 915 1998.10M. Obert, J. Renner, A. Forchel, G. Bacher, R. André, and D. L. S. Dang,Appl. Phys. Lett. 84, 1435 2004.11G. Dasbach, M. Schwab, and A. Forchel, Phys. Rev. B 64, 201309 2001.12G. Dasbach, A. A. Dremin, M. Bayer, V. D. Kulakovskii, N. A. Gippius,and A. Forchel, Phys. Rev. B 65, 245316 2002.13R. P. Stanley, R. Houdré, U. Osterle, M. Gailhanou, and M. Ilegems, Appl.Phys. Lett. 65, 15 2004.14They have an elliptical shape with an absolute difference of about0.75 m between the small and the large axis. This asymmetry is given byFIG. 3. Energy of the photoluminescence peaks as a function of the spotposition along the wedge. Rabi splitting of 3.5 meV 3.35 meV for the 2Dpolaritons 0D polaritons. a and b indicate the positions at which thespectra in Fig. 2 have been taken. Zero detuning for G E1 around x=4.6 cm x=3.05 cm.the pattern we designed on the photolithography mask.AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Polariton quantum boxes in semiconductor microcavities

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