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
• Advantage of using quantum dots as light emitter• Relation between luminescence wavelength and lifetime• Factor to decide lifetime• Comparison between measurements and calculated value
About this paper
Contents
• Introduction– Cavity QED– Microcavity– Distributed Bragg Reflector
• Purpose
• Experimental
• Results and Discussion
• Summary
Cavity QED
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
Spontaneous emission was an uncontrollable phenomenon.
But it is possible to control it by using the resonator of the size about wavelength.
Introduction
Application
Semireflectingmirror
Mirror
Flash lamp
Laser medium
Laser medium: レーザー媒質Semireflecting mirror: 半反射鏡
Introduction
Spontaneous emissionStimulated emission
Microcavity
Microcavity is a resonator of the size about wavelength.
Mirror
Mirror
Light is confined here
http://www.shef.ac.uk/eee/nc35t/new_research/microcavity_pillars_etched_using.html
Introduction
Distributed Bragg Reflector
DBR (Distributed Bragg Reflector): 分布ブラッグ反射鏡……
Bragg’s law
• Reflectivity is nearly equal to 100%.
• is changed by controlling .
Merits
Incidence lightWavelength:
Refractive index
Introduction
Substrate
Structure of microcavity
Spacer
DBR
DBR
The resonator can be miniaturized.
Merit
Ex) AlAs/GaAs DBRs, 30 pairs
Optical path length:
Wavelength of cavity resonance
800 850 900 950 10000
20
40
60
80
100
Ref
lect
ivity
[%]
Wavelength [nm]
Introduction
Low dimensional structuresD
OS D
OS D
OS
energy energy energy
Quantum well Quantum wire Quantum dot (QD)
stepwisediscrete
Introduction
dephasinghigh low
Purpose
• To measure the spontaneous lifetimes in t
he microcavity confined InGaAlAs/GaAs Q
Ds structure at various wavelengths.
• To compare the results of lifetime depende
nce with calculated predictions.
Sample
GaAs substrate
5000Å GaAs buffer layer
15.5 pair AlAs/GaAs DBRs
1300Å GaAs layer
600°C
spacer Molecular beam epitaxy
6 monolayers of In0.5Ga0.35Al0.15As(QD)520°C 100ÅGaAs layer
360Å AlGaAs layer
680Å GaAs layer
3 pair MgF/ZnSe DBRs
600°C 80Ågraded layer
Electron-beam deposition
DBR
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
Grating spectrometer
Sample
Cryostat
• Wavelength:735nm• Pulse width :200fs• Repetition rate:76MHz
Single photon counting module
Microscope objective
Temporal resolution:350ps
Silicon avalanche-photodiode
Pulse pickerTemporal separation:130ns
Photoluminescence decay
Cavity resonance peak is 9514Å with MgF/ZnSe DBRs.
Spontaneous lifetimes between (a) and (b) are differed.
(a)
(b)
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.