Controlled Spontaneous Lifetime in Microcavity Confined InGaAlAs/GaAs Quantum Dots L. A. Graham et al, Appl. Phys. Lett., 72, 1670 (1998) Itoh Laboratory Masataka Yasuda 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 Contents • Introduction – Cavity QED – Microcavity – Distributed Bragg Reflector • • • • Purpose Experimental Results and Discussion Summary Introduction Cavity QED 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 Introduction Application Flash lamp Semireflecting mirror Mirror Laser medium Stimulated emission Spontaneous emission Laser medium:レーザー媒質 Semireflecting mirror:半反射鏡 Introduction Microcavity 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 Introduction Distributed Bragg Reflector 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 Introduction Structure of microcavity Ex) AlAs/GaAs DBRs, 30 pairs 100 DBR Spacer Reflectivity [%] Optical path length: 80 60 40 20 DBR 0 800 850 900 950 1000 Wavelength [nm] Substrate Wavelength of cavity resonance Merit The resonator can be miniaturized. Introduction Low dimensional structures Quantum dot (QD) DOS Quantum wire DOS DOS Quantum well discrete stepwise energy high energy dephasing energy low 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. Sample Electron-beam deposition 3 pair MgF/ZnSe DBRs 600°C 680Å GaAs layer 80Å graded layer DBR 360Å AlGaAs layer 520°C 6 monolayers of In0.5Ga0.35Al0.15As(QD) 1300Å GaAs layer 15.5 pair AlAs/GaAs DBRs 600°C 5000Å GaAs buffer layer GaAs substrate 100Å GaAs layer spacer Molecular beam epitaxy 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 Experimental setup Ti:Sapphire laser Temporal Pulse picker separation:130ns • Wavelength:735nm • Pulse width :200fs • Repetition rate:76MHz Temporal resolution:350ps Single photon counting module Cryostat Grating spectrometer Sample Microscope objective Silicon avalanchephotodiode Photoluminescence decay (a) (b) Cavity resonance peak is 9514Å with MgF/ZnSe DBRs. Spontaneous lifetimes between (a) and (b) are differed. Calculated emission intensity (a) Spontaneous emission is not reinforced. →Lifetime is increased. (b) Reinforced at 12 degrees →Lifetime is decreased. Spontaneous emission pattern (d) without cavity (c) layout (e) (f) (g) Decay rates Decay rates change rapidly near cavity wavelength. Between measured and calculated lifetime changes is good agreement. 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.
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