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  • Molecularly Bridged Quantum Dots Transfer Spin Information

Photonics Spectra
Sep 2003
Using a two-color time-resolved Faraday rotation measurement technique, researchers at the University of California, Santa Barbara, have observed the transfer of electron spin information between molecularly linked CdSe quantum dots at room temperature. The work, which they reported July 31 online on Sciencexpress, suggests that such structures may be suitable for the fabrication of spintronic devices for applications such as quantum information processing.
To produce the quantum-dot assemblies, Min Ouyang and David D. Awschalom immersed functionalized fused silica substrates in a solution of 3.4- or 7.0-nm-diameter quantum dots and toluene, dipped the coated substrate in a dithiol bath and immersed it in another solution of quantum dots. By alternately exposing the surface to the dithiol and the quantum-dot solutions, they produced multilayer samples of various sizes of quantum dots linked by the conjugated molecular bridges.
To investigate spin transfer in the samples, the scientists employed a pump-probe technique using two optical parametric amplifiers. The first system produced 150-fs pulses of circularly polarized radiation, which they tuned to excite spin in one size of quantum dot. It also pumped the other optical parametric amplifier, which produced tunable, linearly polarized probe pulses that enabled them to determine which size of dot remained spin-excited after a time delay.
Ouyang and Awschalom observed spin transfer from the 7.0- to the 3.4-nm dots, which they attribute to a process that enables electrons to pass across the molecule from one quantum dot to the other without losing energy or phase information. Important for potential spintronics applications, the efficiency of the transfer process increased with temperature, from 11.8 percent at 4.5 K to more than 20 percent at room temperature.

faraday rotation
The effect discovered by Faraday in 1845 whereby nonoptically active materials or substances become capable of rotating the polarization plane of polarized radiation (light) passed through them when placed into a strong magnetic field with a component in the direction of rotation. One of the most familiar optical instruments utilizing this effect is the Faraday rotator; one well-known present-day application is in the protective devices used to prevent the destruction of high-power laser...
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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