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 – 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 switches Ultrafast response Large nonlinearity Small size Development of devices using new optical phenomena For example : Cavity Polariton Introduction Polariton 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 h e h h The anticrossed dispersion curves appear by the interaction of light and exciton. Exciton Introduction Cavity Polariton 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 Introduction Distributed Bragg Reflector 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 Introduction Microcavity 100 AlAs/GaAs DBRs, 30 pairs DBR Spacer Transmittance [%] 80 60 40 20 DBR 0 1450 1500 1550 1600 1650 1700 1750 Wavelength [nm] Substrate Cavity mode (Light is confined.) 1800 Introduction Optical Parametric Amplification 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 Introduction Polariton-Polariton Parametric Scattering Idler P2k Pk Probe P0 Microcavity Pump Momentum conservation 2Pk = P0 + P2k Energy Ek Ek E0 Ek E0 E2k Energy conservation 2Ek = E0 + E2k 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 Cd0.4Mg0.6Te spacer AlAs spacer … AlAs … … Ga0.8Al0.2As 12 GaAs QWs (3 stacks of 4 wells) … 24 CdTe QWs (6 stacks of 4 wells) Ga0.8Al0.2As AlAs spacer GaAlAs-based microcavity … (36QWs) Polariton splitting : 20meV Cd0.4Mg0.6Te Cd0.75Mn0.25Te AlAs … 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 Probe intensity with pump pulse Gain Probe intensity without pump pulse 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 Angular resonance of CdTe is broader than that of GaAlAs. Inflection point of the lower polariton Maximum gain It is related to the gain spectral line width. Energy and momentum conversion Most easily satisfied 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.
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