THz waves generated from the optical beats between longitudinal mode of a multimode cw laser daide. Efficiency of optical beats in a chaotically oscillating la-ser is confirmed comparing that of free running CW laser using a highly efficient plasmonic photomixer. The great potential of chaotically oscillating lasers is verified for THz systems.
We successfully demonstrated the manipulation of CdSe/ZnS quantum dots with a diameter of nearly 6 nm in organic solvent at room temperature by applying an intense electric field (1.7 MV/m) under resonant optical excitation (the wavelength 532nm). From the time-variation of the quantum dot distribution monitored by fluorescence imaging, it was experimentally found that the quantum dots gather around the local maximum of the electric field intensity and the potential energy applied on the quantum dot under the optical excitation is estimated to be nearly 400 K, which is approximately 20 times larger than that expected with conventional dielectrophoresis. Such a large potential energy is considered to be due to the Stark effect of the exciton created in the optically excited quantum dot.
To realize optical manipulation and measurement for isolated quantum dots (QDs) in gaseous phase, we are developing experimental apparatus for dispersing QDs into a gas by using droplets of organic solvents. The droplet was generated with a nebulizer using a piezoelectric element and was monitored by observing the scattered light. The time variation of the QD density was also monitored by observing the fluorescence from the QDs. In the case of the diethyl ether solution of CdSe/ZnS core-shell type QDs, it was confirmed that the evaporation of the droplet was enhanced with a liquid-nitrogen trap and its typical lifetime was roughly 30 s, which was considered to be determined from the descent by the force of the gravity, the evaporation, and the diffusion.
The emission spectra of CdSe/ZnS core-shell dots have been monitored after the dilution of their toluene solution with organic
solvents (toluene, n-hexane, diethyl ether, acetone, ethanol, and methanol). In addition to the well-known difference of the emission
efficiency according to the solvent, we found their time variation depending on the solvent. From the discussion based on the solubility
of the capping organic ligand, hexadecylamine (HDA), to each solvent it is suggested that the observed time variation is caused by the
liquation of the capping molecules form the dot surface and the resulting change of the number of the trap site for charges in the
quantum dot.
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