Quantum dot is now becoming hottest subject at CES2015 for making new gen TVs. A quantum dot is a semiconductor nanostructure whose photonic properties such as polarization and wavelength can be controlled. The quantum dot technology is also used in high speed communication replacing slower electronics communication. The preferred method to have a highly secure and high-speed Internet is to have something called quantum Internet, where photons in particular quantum state or transmitted to generate a secure cryptography key. There is a technology called fiber in-line required to develop such distributed quantum networks.
Japanese researcher Kohzo Hakuta, Director of the Center for Photonic Innovations at the University of Electro-Communications (UEC) supported by his colleagues and other researchers at Vietnam, India, China and New Zealand have developed such fiber in-line technology.
They have developed a combination of technologies which includes fabrication of tapered nanofiber, a method to pick and place single quantum dots from colloidal solution and deposit it on the nanofiber by using a computer controlled pico-liter liquid dispenser combined with an inverted microscope and precision translation stages. Researchers claim they could deposit single quantum dot with spatial accuracy better than 3μm, and importantly, the maximum photon channelling efficiency is measured to be 22.0% as predicted from the theory.
"We have been working with our industrial partner Ishihara Sangyo Inc. to develop equipment for producing tapered nanofibers," explains Hakuta. "The resulting 400 nm diameter tapered fibers have 99% light transmission."
Hakuta and colleagues have used cavity structures to increase the photon channelling efficiency. They are developing two methods. "On one hand, we can produce photonic crystal nanofibers with an array of thousands of highly ordered nano-craters using femto second lasers" explains Hakuta. "We were surprised to find highly periodic craters produced on the shadow sides of the nanofibers. Promptly we understood, it is due to the lensing effect of the nanofiber itself. On the other hand we are developing composite nanofiber cavities with external nano-grating structures". Using these composite nanofiber cavities they have demonstrated cavity QED with single quantum dots.
Researchers say the outcome of this research creates a new paradigm and forms the basis for quantum internet and other applications. Femto-second laser fabricated photonic crystal nanofiber cavities coupled with cold atoms can realize various manipulation methods of single photons which offer the basic tools for the next generation of internet communications, claimed by researchers.
SEM image of a photonic crystal nanofiber fabricated with femto-second laser. Nanofiber diameter is around 600 nm. Thousands of equidistant nano-craters are fabricated on a nanofiber by irradiating a single femtosecond laser pulse.
Composite photonic crystal cavity formed by combining an optical nanofiber and a nanofabricated grating. The PL intensity spectra of single quantum dots show strong enhancement at the cavity resonance demonstrating the cavity QED effect.
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