High-temperature ultrafast polariton parametric amplification in semiconductor

microcavitiesM. Saba et al. Nature 414, 731-735 (2001)

Itoh Lab. M1

Masataka YASUDA

Contents

• Introduction

– Cavity Polariton

– Microcavity

– Polariton-Polariton Parametric Scattering

• Experimental

• Results and Discussion

• Summary

Introduction

Wide and rapid spread of the optical networking

Rapid increase of information capacity

Optical switchesUltrafast responseLarge nonlinearity Small size

Development of devices using new optical phenomena

For example : Cavity Polariton

Polariton

Exciton (e : electron, h : hole)

Light

Introduction

Strong coupling wave of electromagnetic field and polarization field (such as phonon, exciton and plasmon)

Exciton

Light

The anticrossed dispersion curves appear by the interaction of light and exciton.

Exciton PolaritonExciton Polariton

eh

eh

eh

Cavity Polariton

Cavity Polariton 共振器ポラリトンQuantum Well ; QW 量子井戸Parametric Scattering パラメトリック散乱

Introduction

To observePolariton-polariton parametric scattering

The exciton energy is constant by introducing the quantum well structure.

This paperThis paper

Strong bonding state of photons in cavity mode and excitons in quantum well

Distributed Bragg Reflector

Distributed Bragg Reflector ; DBR 分布ブラッグ反射器

Refractive index

Optical path length of each layer ;Reflectance is very high.

is controlled by changing the thickness.

Fixed endFree end

Introduction

Incident lightWavelength

Microcavity

Spacer

Substrate

DBR

DBR

Introduction

1450 1500 1550 1600 1650 1700 1750 18000

20

40

60

80

100

Tra

nsm

ittan

ce [%

]

Wavelength [nm]

Cavity mode (Light is confined.)

AlAs/GaAs DBRs, 30 pairs

Optical Parametric Amplification

Optical Parametric Amplification ; OPA 光パラメトリック増幅

Pump

Signal

Idler

Nonlinear optical crystal

Introduction

OPA occurs when phase-matching condition is satisfied.

Energy

Polariton-Polariton Parametric ScatteringIntroduction

Ek

E0 E2k

Probe

Pump

Idler

Microcavity

2Ek = E0 + E2k

Energy conservation

Momentum conservation2Pk = P0 + P2k

EnergyEk

Ek

E0

E2k

P0

Pk

P2k

Motivation

• Observe efficient light amplification by polari

ton-polariton parametric scattering in microca

vity at high temperatures.

• Explore the material that can realize the room

temperature operation of the parametric scatte

ring.

Samples

AlAs spacer

36 GaAs QWs(9 stacks of 4 wells)

AlAsGa0.8Al0.2As

Cd0.75Mn0.25Te

Cd0.4Mg0.6Te

Cd0.4Mg0.6Te spacer

24 CdTe QWs(6 stacks of 4 wells)

… … … …

AlAs spacer

12 GaAs QWs(3 stacks of 4 wells)

… …

AlAs

GaAlAs-based microcavity(36QWs)

Polariton splitting : 20meV

Ga0.8Al0.2As

GaAlAs-based microcavity (12QWs)Polariton splitting : 15.3meV

CdTe-based microcavityPolariton splitting : 25meV

Polariton Splitting

Rabi splitting (Coupling strength between exciton and photon)

Varying sample position

Incident light

●:Polariton energy measured from reflectivity spectra○:Cavity(C) and Exciton(X) modes extracted from the experimental data by using a two coupled oscillators model

Anticrossing

Angle-resolved Pump-probe Configuration

Light source : Ti:Sapphire laserFWHM of pulse : 250fsRepetition rate : 80MHz

FWHM ; Full Width at Half Maximum 半値全幅

Probe spot is spatially selected by pin-hole.

Gain Spectra of Samples

12QWs

Higher temperature:Peaks become smoother, broader and weaker.

Polariton mode becomes broader because of the thermal dephasing of exciton.

Gain reached to about 5000.

pulse pump without intensity Probepulse pump withintensity Probe

Gain

Angular Resonance

Inflection point of the lower polaritonMaximum gain

Energy and momentum conversionMost easily satisfied

Angular resonance of CdTe is broader than that of GaAlAs.

It is related to the gain spectral line width.

Power Dependence of Gain

l0 = 1013 photons・ cm-2・ pulse-1

Gain increases in proportion to pump intensity to the power of 5.7.

Near the threshold

77K

150K

90K

Gain is reduced by raising probe power. (inset)

Gain shows a threshold by raising pump power.

Temperature Dependence of Gain

Cut-off

Cut-off temperatures almost constant.

• The gain falls to 1.• Intrinsic parameter of the material

Almost twice large(Difference of polariton splitting is only 25%.)

Exciton Binding Energy vs. Cut-off Temperature

Cut-off temperature seems to be proportional to exciton binding energy.

Room temperature operation is expected.

Exciton binding energy is very different between CdTe (25meV)

and GaAlAs (13.5meV).

Ultrafast Dynamics of Gain

Pump polaritons escape from the cavity within a few ps.

Repetition rate of the device is the THz range.

CdTe

Summary

• Efficient light amplification by porariton-poratiron parametric scattering was observed by using GaAlAs-based microcavity.

• High temperature amplification was achieved by using CdTe-based microcavity.

• Cut-off temperature is increased in proportion to the exciton binding energy.

• The materials with the large exciton binding energy are expected to achieve the room temperature operation of the parametric scattering.