controlled spontaneous lifetime in microcavity confined ingaalas/gaas quantum dots l. a. graham et...

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.