Itoh Lab. M1 Masataka YASUDA High-temperature ultrafast polariton parametric amplification in semiconductor microcavities M. Saba et al. Nature 414, 731-735 (2001) Itoh Lab. M1 Masataka YASUDA

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

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

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

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

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

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.)

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

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

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.

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)

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

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

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.

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.

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

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%.)

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.

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

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.