Itoh Laboratory Masataka Yasuda

1. Controlled Spontaneous Lifetime in Microcavity Confined InGaAlAs/GaAs Quantum DotsL. A. Graham et al, Appl. Phys. Lett., 72, 1670 (1998) Itoh Laboratory Masataka Yasuda

2. Abstract Control of spontaneous lifetime of microcavity including quantum dots About this paper • Advantage of using quantum dots as light emitter • Relation between luminescence wavelength and lifetime • Factor to decide lifetime • Comparison between measurements and calculated value

3. Contents • Introduction • Cavity QED • Microcavity • Distributed Bragg Reflector • Purpose • Experimental • Results and Discussion • Summary

4. Cavity QED Introduction Spontaneous emission was an uncontrollable phenomenon. But it is possible to control it by using the resonator of the size about wavelength. Example: Spontaneous emission can be reinforced to a specific direction. Lifetime of reinforced spontaneous emission is shortened. Cavity QED (Quantum Electrodynamics):共振器量子電磁力学

5. Spontaneous emission Application Introduction Flash lamp Semireflecting mirror Mirror Laser medium Stimulated emission Laser medium:レーザー媒質 Semireflecting mirror:半反射鏡

6. Microcavity Introduction Microcavity is a resonator of the size about wavelength. Mirror Light is confined here Mirror http://www.shef.ac.uk/eee/nc35t/new_research/microcavity_pillars_etched_using.html

7. Distributed Bragg Reflector Introduction Incidence light Wavelength: Bragg’s law Refractive index Merits …… • Reflectivity is nearly equal to 100%. • is changed by controlling . DBR (Distributed Bragg Reflector):分布ブラッグ反射鏡

8. Structure of microcavity Introduction Ex) AlAs/GaAs DBRs, 30 pairs DBR Optical path length: Spacer DBR Substrate Wavelength of cavity resonance Merit The resonator can be miniaturized.

9. Low dimensional structures Introduction Quantum well Quantum wire Quantum dot (QD) DOS DOS DOS discrete stepwise energy energy energy dephasing high low

10. Purpose • To measure the spontaneous lifetimes in the microcavity confined InGaAlAs/GaAs QDs structure at various wavelengths. • To compare the results of lifetime dependence with calculated predictions.

11. Electron-beam deposition 3 pair MgF/ZnSe DBRs 680Å GaAs layer 80Å graded layer 600°C DBR 360Å AlGaAs layer 100Å GaAs layer 520°C 6 monolayers of In0.5Ga0.35Al0.15As(QD) spacer Sample Molecular beam epitaxy 1300Å GaAs layer 15.5 pair AlAs/GaAs DBRs 600°C 5000Å GaAs buffer layer GaAs substrate

12. Reflectivity spectrum Cavity resonance at 956nm without MgF/ZnSe DBRs. QDs are placed close to the upper interface of the spacer. antinode of electric field

13. Experimental setup • Wavelength:735nm • Pulse width :200fs • Repetition rate:76MHz Ti:Sapphire laser Temporal resolution:350ps Temporal separation:130ns Pulse picker Single photon counting module Cryostat Grating spectrometer Silicon avalanche-photodiode Microscope objective Sample

14. Photoluminescence decay (a) (b) Cavity resonance peak is 9514Å with MgF/ZnSe DBRs. Spontaneous lifetimes between (a) and (b) are differed.

15. Calculated emission intensity (a) Spontaneous emission is not reinforced. →Lifetime is increased. (b) Reinforced at 12 degrees →Lifetime is decreased.

16. Spontaneous emission pattern (d) without cavity (c) layout (e) (f) (g)

17. Decay rates Decay rates change rapidly near cavity wavelength. Between measured and calculated lifetime changes is good agreement.

18. Summary • Cavity resonance of the microcavity is tuned to PL wavelength of InGaAlAs/GaAs quantum dot. • The spontaneous lifetimes are different on the boundary of the wavelength of cavity resonance. • It is possible to control lifetimes by optimizing the QD positioning and the cavity layer thickness.