Silver Metamaterial Engineered with Negative Refraction and Low Loss at Telecom Wavelengths
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RESEARCH/RESEARCHERS
Optical Diode Based on Core–Shell Quantum Dots Demonstrated With sizes in the range of just a few nanometers, quantum dots have become an attractive choice for biological labeling and novel optics. These semiconductor nanocrystals emit light in a particular narrow-wavelength band after an outside source, such as an ultraviolet light, excites the electrons in them. By simply changing the size of the semiconductor core, the emission wavelength can be tuned and particles can be created that fluoresce different colors. By utilizing these properties of CdSe/ZnS core–shell quantum dots, D. Alexander and coworkers at the University of Nebraska– Lincoln have demonstrated a device with the properties of an optical diode. For CdSe/ZnS core–shell quantum dots, sizes ranging from 3 nm to 6 nm can cover emission wavelengths from 490 nm to 620 nm. As they reported in the July 1 issue of Optics Letters (p. 1957), the researchers used two different sizes of quantum dots, and while the first type of particles (green) emitted at 540 nm, the other type (red) not only emitted at 620 nm but also absorbed the emission of green particles efficiently. The dried green quantum dots were loaded into one end of a plastic capillary fiber splice, which had an inner diameter of 140 μm, and red quantum dots were loaded into the opposite end. Each end of the splice was then coupled to a fiber with a core diameter of 99 μm and a cladding diameter of 140 μm. When a 488-nm argon ion laser with a power of 10 mW was focused into the fiber, the spectrometer output varied with not only the thicknesses of the two quantum dot layers, but also with which layer was illuminated first by the pump laser, demonstrating properties of an optical diode. For example, when the film thicknesses of both red and green particles were the same, the emission of the device was dominated by the red particles with a peak at 640 nm, regardless of which quantum dot film layer was illuminated first. This is because the red dots absorbed all the pump power and also the emission from the green dots, the researchers said. But when the thickness of the green quantum dot film was twice that of the red dot film, the emission spectrum was dominated by the red dots only when the input laser was incident on the red layer first, said the researchers. If the input laser was incident on the green layer first, then the emission spectrum exhibited a strong fluorescence peak of green dots at 565 nm, with a narrow peak at 488 nm depicting a significant amount of unabsorbed pump light. The researchers attributed the MRS BULLETIN • VOLUME 31 • AUGUST 2006
observed asymmetry to the effects of absorption saturation in the red dots, which can absorb both the 488-nm pump light and the fluorescence from the green dots. According to the researchers, this shows that the output of the device can be either the incoming laser wavelength or the fluorescence wavelength of one of the quantum dots. The researchers said that such devices could have useful applications in optical communica
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