1. Itoh Lab. M1 Masataka YASUDA
High-temperature ultrafast polariton parametric amplification in semiconductor microcavities M. Saba et al. Nature 414, (2001)
Itoh Lab. M1
Masataka YASUDA

2. Contents Introduction Experimental Results and Discussion Summary
Cavity Polariton
Microcavity
Polariton-Polariton Parametric Scattering
Experimental
Results and Discussion
Summary

3. Introduction Optical switches For example : Cavity Polariton
Wide and rapid spread of the optical networking
Rapid increase of information capacity
Ultrafast response
Large nonlinearity
Small size
Optical switches
Development of devices using new optical phenomena
For example : Cavity Polariton

4. Polariton Exciton Polariton
Introduction
Strong coupling wave of electromagnetic field and
polarization field (such as phonon, exciton and plasmon)
Exciton Polariton
Exciton (e : electron, h : hole)
Light
Light e e e h h h
The anticrossed dispersion curves appear
by the interaction of light and exciton.
Exciton

5. Cavity Polariton This paper
Introduction
Strong bonding state of photons in cavity mode
and excitons in quantum well
The exciton energy is constant by
introducing the quantum well structure.
This paper
To observe
Polariton-polariton parametric scattering
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

6. Distributed Bragg Reflector
Introduction
Fixed end
Free end
Incident light
Wavelength
Refractive index
Optical path length of each layer ;
Reflectance is very high.
is controlled by changing the thickness.
Distributed Bragg Reflector ; DBR 分布ブラッグ反射器

7. Microcavity Spacer Substrate AlAs/GaAs DBRs, 30 pairs DBR DBR
Introduction
AlAs/GaAs DBRs, 30 pairs
DBR
Spacer
DBR
Substrate
Cavity mode (Light is confined.)

8. Optical Parametric Amplification
Introduction
Nonlinear optical crystal
Signal
Pump
Idler
Energy
OPA occurs when phase-matching
condition is satisfied.
Optical Parametric Amplification ; OPA 光パラメトリック増幅

9. Polariton-Polariton Parametric Scattering
Introduction
Idler
P2k Pk
Probe P0
Microcavity
Pump
Momentum conservation
2Pk = P0 + P2k
Energy E0 Ek Ek E0
E2k
E2k Ek
Energy conservation
2Ek = E0 + E2k

10. Motivation
Observe efficient light amplification by polariton-polariton parametric scattering in microcavity at high temperatures.
Explore the material that can realize the room temperature operation of the parametric scattering.

11. Samples GaAlAs-based microcavity (12QWs) Polariton splitting : 15.3meV
CdTe-based microcavity
Polariton splitting : 25meV
AlAs spacer
Cd0.4Mg0.6Te spacer
AlAs … … … …
Ga0.8Al0.2As
Cd0.4Mg0.6Te
12 GaAs QWs
(3 stacks of 4 wells)
24 CdTe QWs
(6 stacks of 4 wells)
Cd0.75Mn0.25Te
AlAs spacer
Ga0.8Al0.2As
AlAs
GaAlAs-based microcavity
(36QWs)
Polariton splitting : 20meV … …
36 GaAs QWs
(9 stacks of 4 wells)

12. Polariton Splitting
Rabi splitting (Coupling strength between exciton and photon)
Incident light
Varying sample position
Anticrossing
●:Polariton energy measured from reflectivity spectra
○:Cavity(C) and Exciton(X) modes extracted from the
experimental data by using a two coupled oscillators model

13. Angle-resolved Pump-probe Configuration
Light source : Ti:Sapphire laser
FWHM of pulse : 250fs
Repetition rate : 80MHz
Probe spot is spatially
selected by pin-hole.
FWHM ; Full Width at Half Maximum 半値全幅

14. Gain Spectra of Samples
12QWs
Gain reached to about 5000.
Higher temperature:
Peaks become smoother, broader and weaker.
Polariton mode becomes broader
because of the thermal dephasing of exciton.

15. Angular Resonance Inflection point of the lower polariton
Maximum gain
Angular resonance of CdTe is broader
than that of GaAlAs.
Energy and momentum conversion
Most easily satisfied
It is related to the gain spectral line width.

16. Power Dependence of Gain
150K
Gain shows a threshold
by raising pump power.
77K
Near the threshold
Gain increases in proportion to pump intensity to the power of 5.7.
90K
Gain is reduced by raising
probe power. (inset)
l0 = 1013 photons・cm-2・pulse-1

17. Temperature Dependence of Gain
Cut-off temperatures
almost constant.
The gain falls to 1.
Intrinsic parameter of the material
Cut-off
Almost twice large
(Difference of polariton splitting is only 25%.)

18. Exciton Binding Energy vs. Cut-off Temperature
Exciton binding energy is very
different between CdTe (25meV)
and GaAlAs (13.5meV).
Cut-off temperature seems to
be proportional to exciton
binding energy.
Room temperature operation is expected.

19. Ultrafast Dynamics of Gain
CdTe
Pump polaritons escape from
the cavity within a few ps.
Repetition rate of the device is
the THz range.

20. 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.