V.V. Chaldyshev , A.S. Shkolnik, V.P. Evtikhiev, Ioffe Institute, St.Petersburg, Russia,

1. Optical spectroscopy of a semi-insulating GaAs/AlGaAs multiple quantum well system near double exciton-polariton and Bragg resonance V.V. Chaldyshev, A.S. Shkolnik, V.P. Evtikhiev, Ioffe Institute, St.Petersburg, Russia, T. Holden Brooklyn College of the City University of New York, USA

2. Outline • Introduction • Periodic systems of quantum wells (QWs) • Theory • Optical reflection from resonant Bragg structures • Structure design • Experiment • Optical reflection spectra • Contactless electroreflection spectroscopy • Conclusions • Materials issue

3. Short-period system of QWs Tunneling between QWs. Minibands. Conduction. Long-period system of QWs Probability of tunneling is low. QWs are insulated from each other.

4. d e(x) = e(x+d) e1 e2 e1 e2 e1 e2 e1 e2 e1 e2 e1 e2 Resonant Bragg structures upper polariton branch Photons Excitons lower polariton branch w band gap 0 π/d Textbooks: Joannopoulos et al.; Cardona and Yu;

5. Optical reflection spectra for different numbers of QWs: Impact of damping Schematic representation of light reflection from anN-QW structure. Spectral dependence of the reflection coefficient RNfrom N-QW structure with the matched dielectric constants of compositional materials A and B. The calculation is performed for the background refractive index nb = 3.45, the exciton resonance frequency and radiative damping rate defined by ћω0 = 1.533 eV, ћΓ0 = 50 μeV, b′ = (a/2) + b and the nonradiative damping rate ћΓ = 0 (a) and ћΓ = 100 μeV (b).

6. Optical reflection spectra for different numbers of QWs: Impact of dielectric contrast Reflection spectra from Bragg QW structureswith the dielectric contrast between the compositional materialsA and B. The calculation is performed for ћΓ = 100μeV, a = 120 Ǻ, na = 3.59 and nb= 3.45. Curves are calculatedfor six structures containing different number, N, ofwells indicated at each curve. The symbol ∞ corresponds tothe structure with infinite N.

7. Structure design 36x Bragg resonance Exciton-polariton resonance Double resonance

8. Structure design 32x 4x Bragg resonance Exciton-polariton resonance Double resonance

9. AlGaAs QW-GaAs AlGaAs 36 periods AlGaAs QW-GaAs AlGaAs QW-GaAs AlGaAs GaAs (001) buffer and substrate Experimental Bragg structure(AlGaAs/GaAs/AlGaAs)36 Period = 119 nm, QW = 15 nm, DQW = 20 nm 32xQW 4xDQW AlGaAs = SPSL (3ML-AlAs/7ML-GaAs)n

10. photodetector Lightsource sample inrefrigerator Optical reflection spectra for different angles of incidence θ = var, Т = const θ Т Reflection spectra recorded with s-polarization at temperatureof 7К for GaAs/AlGaAs MQW with 36 periods. Angles of incidence are 70o и 0o. Arrows indicate photonic band gaps due to dielectric contrast between the well and barrier materials. Inset shows the excitonic features.

11. photodetector Lightsource sample inrefrigerator Optical reflection spectra at different temperatures θ = const, Т = var θ Т Reflection spectra for s-polarization and the light incidence angle of 43°. Arrows mark the features originating from exciton-polaritons in QWs. Spectra recorded at 80К, 50К и 17К are shifted up for better view.

12. photodetector Lightsource sample inrefrigerator Spectra of contactless electroreflection (CER) at different temperatures ~300 V Angle of incidenceis 43°, s-polarization. Arrows mark the features originating from exciton-polaritons in QWs and in bulk GaAs..

13. Line shape analysis for CER at 17 K 3 2 1 4 5

14. Line shape analysis for CER at 50 K 3 2 1 5 4

15. Line shape analysis for CER at 80 K 1 3 2 5 4

16. Parameters of the CER features

17. Parameters of the CER features

18. Conclusions • We have designed and grown a periodic GaAs/GaAlAs QW structure, in which the electromagnetic Bragg resonance coincides with the exciton-polariton resonance. • Different contribution to the optical reflection spectra were revealed and separated by using contactless electroreflection technique. • Quantitative analysis of the CER line shapes accompanied by theoretical calculations allowed us to determine the energies and broadening parameters of the exciton-polaritons in the periodic QW system. • Roughness of the GaAs/AlAs interfaces seems to be the main source of inhomogeneous broadening. Acknowledgement We appreciate discussions and collaboration with E.L.Ivchenko, A.N.Poddubny, A.B.Pevtsov, and A.V.Selkin (the Ioffe Institute); A.A.Lisyansky, L.I.Deich (Queens College of CUNY) and financial support from the Russian Foundation for Basic Research and Russian Academy of Sciences.