p s scurrent topics in solid state physics

c

statu

s

soli

di

www.pss-c.comph

ysi

ca

REPRINT

0&+1$2# %"%()$,(1""'.*!3*%23,($&(1/,)' *&"1$,)&- "1#%$#

.: ,#2;()4#!"-" 3: 1: .6%%282#!"-" 1: '4;#(6;(2;282#-" 9: /6()4$4*54#-"&" 6;8 0: ,: 3+5+76;4#!"-

!

J-7$P>-;O E)>$9)$ -9& D$)@98;8A0 '$S-P7:$97( C9>3$PO>70 8Z *P$7$( GRHR ,81 55."( NT..6 Y$P-<;>89( !P$$)$

"

J>)P8$;$)7P89>)O F$O$-P)@ !P84S( #HFDYLW%EK( GRHR ,81 TX5N( NTTT. Y$P-<;>89( !P$$)$

#

G@0O>)O '$S-P7:$97( C9>3$PO>70 8Z *P$7$( GRHR ,81 55."( NT..6 Y$P-<;>89( !P$$)$

F$)$>3$& 6. I83$:+$P 5..N( P$3>O$& 56 J-0 5.."( -))$S7$& 5" J-0 5.."

G4+;>O@$& 89;>9$ 5V E$S7$:+$P 5.."

#!$" V5RXXRE-( NTR6MRQ)( "TR.XR%-

B$ SP$O$97 SP$;>:>9-P0 $1S$P>:$97-; P$O4;7O 89 - &8S$&?',F

:>)P8)-3>70 &>8&$ O-:S;$( &$O>A9$& Z8P $;$)7P>)-;;0?S4:S$&

8S$P-7>89 -7 7@$ $;$3-7$& 7$:S$P-74P$O -P849& 5.. UR K82

7$:S$P-74P$ P$Z;$)7>3>70 :$-O4P$:$97O -O - Z49)7>89 8Z >9)>?

&$9)$ -9A;$ @-3$ O@829 );$-P $3>&$9)$ 8Z S8;-P>789O >9 7@$

O7P89A )84S;>9A P$A>:$ >9 7@>O O-:S;$R J8P$83$P( P$Z;$)7>3>70

OS$)7P- P$)8P&$& -7 3-P>84O 7$:S$P-74P$O +$72$$9 T"X -9&

5TN U @-3$ &$:89O7P-7$& 7@-7 7@$ O7P89A )84S;>9A P$A>:$

S$PO>O7O -7 7@$O$ $;$3-7$& 7$:S$P-74P$OR H4P P$O4;7O -P$ 3$P0

SP8:>O>9A Z8P 7@$ P$-;>/-7>89 8Z S8;-P>789>) &$3>)$O 49&$P

$;$)7P>)-; >9=$)7>89R

phys. stat. sol. (c) 5, No. 12, 3594–3596 (2008) / DOI 10.1002/pssc.200780210

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

p s scurrent topics in solid state physics

c

statu

s

soli

di

www.pss-c.comph

ysi

caphys. stat. sol. (c) 5, No. 12, 3594–3596 (2008) / DOI 10.1002/pssc.200780210

Towards electrically-pumped microcavity polariton lasers

S. Tsintzos*1,2, P. G. Savvidis1,2, G. Konstantinidis2, Z. Hatzopoulos2,3, and N. T. Pelekanos1,2

1 Materials Science and Technology Department, University of Crete, P.O. Box 2208, 71003 Heraklion, Greece 2 Microelectronics Research Group, FORTH/IESL, P.O. Box 1527, 71110 Heraklion, Greece 3 Physics Department, University of Crete, P.O. Box 2208, 71003 Heraklion, Greece

Received 30 November 2007, revised 23 May 2008, accepted 28 May 2008

Published online 24 September 2008

PACS 42.55.Sa, 71.36.+c, 81.05.Ea

* Corresponding author: e-mail simostsi@physics.uoc.gr

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction The strong coupling of resonant pho-tons with confined excitons inside a submicron high-Q mi-crocavity produces new quasiparticles, called exciton-polaritons, with strongly modified dispersion relations and drastically new optical properties. In the recent years, the unique properties of polaritons in the strongly coupled re-gime have attracted much attention. Recent work on mi-crocavities has shed light on the bosonic properties of po-laritons in the strong coupling regime, such as stimulated scattering [1, 2], parametric amplification [3-5], polariton lasing [6, 7], condensation [8-10] and superfluidity [11, 12] under optical pumping. Until now, polariton lasing and nonlinearities have only been demonstrated in optical ex-periments, which have shown the potential of polariton la-sers for reduced lasing thresholds by two orders of magni-tude compared to conventional semiconductor lasers [13]. A number of key issues such as resistivity of the p-type DBR mirrors at low temperatures, cavity losses related to doping of the mirrors, and polariton stability under electri-cal injection have thwarted so far the realization of any electrically pumped polariton emitting semiconductor de-vice. We note here that electrical injection in an organic microcavity LED in the strong coupling regime has been reported recently [14], where the much larger exciton bind-

ing energies and enhanced Rabi splittings allow operation at room temperature. However, the electroluminescence spectra are superposed with emission from the hole-injection TPD layer and localized excitonic states that do not strongly couple to cavity photons. In this work, as a first step towards electrically-pumped polariton emitters, we report optical characterization results from a doped-DBR microcavity diode, showing strong coupling up to 220 K. 2 Experimental The microcavity of our interest con-sists of a p-i-n diode grown by molecular beam epitaxy on n+ GaAs (001) substrate. It comprises a 5λ/2 cavity sand-wiched between two doped GaAs/AlAs Bragg reflectors. To obtain high-Q cavity, the p-type DBR consists of 17 pe-riods and the n-type of 21 periods. Three pairs of 10 nm In0.1Ga0.9As/GaAs quantum wells (QWs) are placed at the antinodes of the electromagnetic field, as shown in Fig. 1. Two types of reflectivity experiments were used in or-der to characterize the above microcavity and decipher whether it can operate in the strong-coupling regime: an-gle-dependent reflectivity measurements performed at 20 K, and reflectivity spectra recorded as a function of tem-perature in the range of 185-217 K, at an incidence angle of θ = 10°.

We present preliminary experimental results on a doped-DBR

microcavity diode sample, designed for electrically-pumped

operation at the elevated temperatures around 200 K. Low

temperature reflectivity measurements as a function of inci-

dence angle have shown clear evidence of polaritons in the

strong coupling regime in this sample. Moreover, reflectivity

spectra recorded at various temperatures between 185 and

217 K have demonstrated that the strong coupling regime

persists at these elevated temperatures. Our results are very

promising for the realization of polaritonic devices under

electrical injection.

phys. stat. sol. (c) 5, No. 12 (2008) 3595

www.pss-c.com © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Contributed

Article

Figure 1 (Coloured online) Electric field and refractive index

distribution inside the microcavity structure. The QWs are posi-

tioned at the maxima of electromagnetic field to increase light

matter interaction.

3 Results and discussion The microcavity sample was designed and grown to exhibit strong coupling regime at elevated temperatures. However the initial characteriza- tion experiments have been performed at low temperatures to assess the strong coupling regime in such doped struc- tures. This puts the cavity and exciton modes out of reso- nance, which can be brought back into resonance by resort- ing to reflectivity measurements at higher angles. In Fig. 2a, reflectivity spectra at 20 K are shown. The spectra were recorded at different incident angles in the range between 43° and 67°. At this temperature the condition of zero de- tuning is achieved at an angle of 54°.

Figure 2 (a) Angle-resolved reflectivity spectra at 20 K. The

spectra are shifted from each other for clarity. (b) Extracted re-

flectivity peak positions as a function of incidence angle. The

solid lines running through the data points are theoretical fits.

The energy dispersion curves for the upper and low po- lariton branches are shown in Fig. 2b. The circular points are extracted peaks from the reflectivity spectra of Fig. 2a. The solid lines are theoretical fits which are generated us- ing a coupled harmonic oscillator model [14],

with E± being the energy of the upper and lower polari- tons,

xE and phE the bare exciton and cavity modes re-

spectively, and Ω� the Rabi splitting. Normal mode Rabi splitting of 4.6 meV at 54° is obtained. For this value of ,Ω� excellent fitting of the experimental data is ob- tained as presented in Fig. 2b. Another important issue concerns the cavity losses re- lated to the doping of the DBRs [15]. To estimate the in- fluence of doping on the photon confinement, we have fab- ricated a similar undoped microcavity structure, and com- pared the FWHM linewidths of the cavity modes in the two samples. Fig. 3 shows reflectivity spectra from the doped and undoped samples at an incidence angle of 5° at 20 K. The linewidth of the cavity mode in the doped sam- ple measures 1.1 meV, while it is 0.7 meV for the undoped one. The respective Q factors are estimated at 1300 and 2000, approximately. Despite this reduction in the cavity Q factor, caused by doping of the DBR mirrors, the strong coupling regime remains intact, as shown in this work.

Figure 3 Cavity mode comparison between the doped and un-

doped microcavity samples described in the text.

The next step of the sample characterization was to-wards the observation of strong coupling at elevated tem-peratures. For this purpose, a set of reflectivity spectra were recorded at temperatures between 185 and 217 K, at an incidence angle of 10°, as shown in Fig. 4. By changing the temperature of the sample, we are able to tune the exciton resonance through the cavity mode, which changes at a much slower rate. The anti-crossing behaviour, characteristic of the strong coupling regime be-tween the cavity and the exciton modes, is clearly visible in the spectra of Fig. 4. The polariton energy dispersion curves, extracted from Fig. 4, are plotted in Fig. 5. The solid lines are theoretical fits, generated by the coupled harmonic oscillator model mentioned above, using a Rabi splitting of 4.1 meV at 196 K. It should be mentioned that in the model we have assumed temperature independent exciton oscillator strength, an assumption which appears to work rather well in our case. The significance of these re-sults stems from the fact that it is the first demonstration of

40 45 50 55 60 65 70

1.390

1.395

1.400

1.405

1.410

E-

E+

En

erg

y (

eV

)

Angle (degrees)

Exciton

Cavity

(b)

T=20K

1.38 1.39 1.40 1.41 1.42

E-

Refl

ecti

vit

y (

a.u

)

Energy (eV)

T=20 K

670

630

610

580

530

500

470

430

(a)

E+

1.38 1.39 1.40 1.41

Refl

ecti

vit

y (

a.u

)

Energy (eV)

T=20K

undoped

doped

( ) ( )221,

2 2

x ph

x ph

E EE E EΩ

±+

= ± + −�

3596 S. Tsintzos et al.: Towards electrically-pumped microcavity polariton lasers

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

ph

ysic

ap s sstat

us

solid

i c

polariton existence at such high temperatures using a doped microcavity sample which can be readily processed into an LED structure for electrical injection of polaritons.

Figure 4 (Coloured online) Reflectivity spectra of the doped mi-

crocavity sample as a function of temperature at an incident angle

of 10°. The spectra are presented with an offset for clarity and the

red solid lines are guides to the eye.

Figure 5 Polariton energy dispersion curves. Circles and squares

are extracted reflectivity peak positions from Fig. 4. Solid lines

are fits for the upper and lower polaritons and the dashed and dot-

ted lines show the bare exciton and cavity modes, respectively.

4 Conclusion In this paper, we reported preliminary optical results which prove strong coupling in a GaAs-based microcavity with doped DBRs up to 217 K. As far as we know, this is the first demonstration of polaritons existence in such a microcavity. Comparing the FWHM of the cavity mode between the doped and a similar undoped microcavity structure, we confirm that the influence of the doped mirrors to the strong coupling is not significant. Finally, we note that, following submission of this manuscript, we have demonstrated polariton LED emission at 235 K [16] using the structures of the present work.

Acknowledgements Support by the PENED projects

03ΕΔ841 and 03ΕΔ816 (funded 25% by national funds and 75%

by EC) is gratefully acknowledged.

References

[1] Le Si Dang, D. Heger, R. Andre, F. Boeuf, and A.

Romestain, Phys. Rev. Lett. 81, 3920 (1998).

[2] P. Senellart, J. Bloch, B. Sermage, and J. Y. Marzin, Phys.

Rev. B 62, R16263 (2000).

[3] P.G. Savvidis, J.J. Baumberg, R. M. Stevenson, M. Skolnick,

D.M. Whittaker, and J.S. Roberts, Phys. Rev. Lett. 84, 1547

(2000).

[4] C. Ciuti, P. Schwendimann, B. Deveaud, and A. Quattro-

pani, Phys. Rev. B 62, R4825 (2000).

[5] M. Saba, C. Ciuti, J. Bloch, V. Thierry-Mieg, R. Andre, L.S.

Dang, S. Kundermann, A. Mura, G. Bongiovanni, J.L. Stae-

hli, and B. Deveaud, Nature 414, 731 (2001).

[6] D. Porras, C. Ciuti, J.J. Baumberg, and C. Tejedor, Phys.

Rev. B 66, 085304 (2002).

[7] G. Malpuech, A. Di Carlo, A. Kavokin, J.J. Baumberg, M.

Zamfirescu, and P. Lugli, Appl. Phys. Lett. 81, 412 (2002).

[8] J. Kasprzak, M. Richard, S. Kundermann, A. Baas, P. Jeam-

brun, J. M. J. Keeling, F. M. Marchetti, M. H. Szymanska,

R. André, J. L. Staehli, V. Savona, P. B. Littlewood, B. De-

veaud, and Le Si Dang, Nature 443, 409 (2006).

[9] H. Deng, D. Press, S. Gotzinger, G. S. Solomon, R. Hey, K.

H. Ploog, and Y. Yamamoto, Phys. Rev. Lett. 97, 146402

(2006).

[10] L. Butov, Nature 447, 540 (2007).

[11] I. Carusotto and C. Ciuti, Phys. Rev. Lett. 93, 166401

(2004).

[12] A. Kavokin, G. Malpuech, and F. P. Laussy, Phys. Lett. A

306, 187 (2003).

[13] H. Deng, G. Weihs, D. Snoke, J. Bloch, and Y. Yamamoto,

PNAS 100, 15318 (2003).

[14] J.R. Tischler, M.S. Bradley, V. Bulovic, and J.H. Song, and

A. Nurmikko, Phys. Rev. Lett. 95, 036401 (2005).

[15] C. Asplund, S. Mogg, G. Plaine, F. Salomonson, N. Chitica,

and M. Hammar, J. Appl. Phys. 90, 794 (2001).

[16] S.I. Tsintzos, N.T. Pelekanos, G. Konstantinidis, Z. Hat-

zopoulos, and P.G. Savvidis, Nature 453, 372 (2008).

1,33 1,34 1,35 1,36

217K

215K

213K

205K

210K

207K

200K197K

193K

190K

187 K

Refl

ecti

vit

y (

a.u

)

Energy (eV)

185 K

203K

Cavity

Exciton

180 185 190 195 200 205 210 215 220

1.335

1.340

1.345

1.350

1.355

1.360

E-

En

erg

y (

eV

)

Temperature (K)

Cavity

Exciton

E+