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Intraband transitions in semiconductor nanocrystals P. Guyot-Sionnest and M. A. Hines Appl. Phys. Lett. 72, 686 (1998) and P. Guyot-Sionnest et al/ Phys. Rev. B 60, R2181 (1999) ITOH Lab. Hiroaki SAWADA (references)

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Abstract These report Intraband transition of one-electron confined in CdSe quantum dots has been observed by infrared pump-probe spectroscopy. The transition energy depends on dot-size. The time profile of transient absorption is influenced by surface modifications of the quantum dots.

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Contents Introduction Quantum dot, Quantum-size effect Motivation Experiment (1)Size-dependence of transient absorption (2)Time evolution of transient absorption Summary

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Quantum dot A quantum dot is a nanometer-sized semiconductor. It consists of 10 3 ~10 6 atoms. Quantum effects appear due to three dimensionally confined electrons. The energy levels are discrete. E E E DOS E bulk Quantum well Quantum dot Quantum wire bulk well wire dot

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Quantum size effect Confined electrons have higher energy than those in bulk crystal, and it depends on dot size. The energy shift is derived by size energy n: principal quantum number h: Planck constant m: effective mass a: dot radius.

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Quantum confinement effect Weak confinementStrong confinement Center-of-mass motion is confined. Motions of electron and hole are confined individually. aBaaBaaBaaBa electron hole a B Bohr radius a:dot radius Consider the effect on an exciton in a spherical dot. lowest state excited state lowest state excited state lowest state excited state 2a (exciton:an electron-hole pair combined by Coulomb force)

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Applications of quantum dot Quantum dots show interesting optical properties and are expected to be used for many optical devices. 1.Quantum dot laser For example Electron-hole pair confinement leads to the efficient recombination. Superior lasing efficiency over existing devices 2. Optical switch The network communication carrier shifts from electric to optical. Large optical nonlinearity of quantum dot realizes optical switch.

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Motivation Authors clarify the details of intraband dynamics of electrons in quantum dots. In these reports Infrared laser etc. Intraband transition energy (1S-1P etc) in quantum dot exists in infrared region, and the energy depends on the dot size. Intraband transition electronic transition in conduction band (1S-1P transition etc)

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Transient infrared absorption Pump-probe spectroscopy To study intraband transition in quantum dots Transient infrared absorption by pump-probe spectroscopy is useful. lowest state excited state pump beam probe beam valence band conduction band By using two beams, we can observe the intraband transition that cannot be observed with the single beam.

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Colloidal CdSe quantum dot CdSe colloids are the best characterized semiconductor quantum dots in the strong confinement regime. CdSe quantum dot is produced by colloid method. colloid method (ref) C. B. Murray et al/ J.Am.chem.Soc. 115,8706(1993)