Ultrafast nonlinear gain dynamics in semiconductor nanocrystals

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Abstract
II - VI semiconductor (CdS, CdSe) nanocrystals with an average size of approximately one bulk exciton Bohr radius are embedded in a glass matrix. Due to the quantum confinement effect, they act as a quasi zero-dimensional system (quantum dots). Under strong nanosecond and femtosecond optical excitation, these quantum dots exhibit optical amplification (gain). We investigate the ultrashort gain dynamics of the strongly confined CdSe quantum dots by femtosecond pump-probe spectroscopy. From multiple-beam pump-probe measurements, we conclude that the gain mechanism is governed by biexciton to exciton transitions. Femtosecond dephasing measurements reveal a constant scattering rate across the gain region and confirm the two-electron-hole pair gain model. Nanosecond pump-probe measurements on CdS quantum dots in sol-gel glasses show optical gain up to room temperature. In all cases, the gain region is broad and stretches below the fundamental absorption of the nanocrystals. The reason is the multitude of transitions involved in the gain formation. No phonon bottleneck did prevent any quick relaxation or any fast scattering that would replenish the spectral holes in the gain region. The relaxation rates are on the order of 10ps. The luminescence decay time is nonexponentiai with a 40–100 ps component for the stimulated emission.