Fireflies emit flashes of light allowing them to locate each other. At the nanoscale, quantum dots behave the same way. Through the work of a team from the University of Copenhagen, it is now possible to handle with tongs low optical power. A quantum dot is a tiny nanocristal occupying only the space of ten atoms. The very small size of quantum dots gives them different physical properties of the material they are made. Thus, as the atoms, quantum dots are characterized by discrete energy levels. The passage of an excited level at the basic level of a quantum dot is then the issue of a

small flash light. Quantum dots, often compared this to artificial atoms, are being extensively studied for about 25 years. M But if the very small size of quantum dots placed in the spotlight, it also made it extremely difficult so far to handle. A study published in the prestigious journal Nano Letters by Lene Oddershede of the Niels Bohr Institute at Copenhagen University shows that it is possible to capture a single quantum dot with an optical trap continuous moderate intensity. Optical tweezers is a highly focused laser beam, which in its ordinary use, capture microparticles. Since Kepler, who in 1619 described the deformation of the tails of comets under the influence of sunlight, it is known that light can exert forces and thus to transmit an impulse. Thus, a particle that reflects or refracts a light beam will be subjected to any forces capable of moving. Under certain conditions, the optical tweezers can build a true optical trap where the bodies are attracted to areas of high light intensity. He however was not until the advent of the laser to be able to concentrate enough light in a point and imparting forces of light intense enough to trap a microparticle. The problem was further complicated when it comes to nanoparticles. Manipulating quantum dots or other tiny objects requires in fact to oppose their Brownian movement, random movement of all microscopic immersed in a fluid. The larger particles seen decreases, the challenge is important because the intensity of light forces applied to the particle decreases at the same time that the Brownian movement is growing! Before the publication of Lene Oddershede, we felt it was necessary to have a beam of 20 watts concentrated on a square micrometer to manipulate optical box. Such a power flux is 20 terrawatts per square meter, ie about 20 million times the power light that we receive from the Sun at noon! The team of "Niels Bohr Institute has successfully manipulate a quantum dot laser with a continuous CW only 500 milliwatts. This performance opens the door to understanding the unexpected properties of quantum dots placed in an electromagnetic field. It also provides innovative knowledge on their physical characteristics. The manipulation of quantum dots by optical tweezers could also give rise to multiple applications. The researchers could for example be used to manipulate objects far too small to be moved individually. Thus, biomolecules and molecular motors responsible for movement of cells and intracellular transport are studied since the 90s with optical tweezers. But the size and complexity of biomolecules from being caught by the light beam. The researchers were thus linked to a large particle, of the order of micrometers, to manipulate and use a fluorochrome for the observer. By attaching a quantum dot, it is now possible to manipulate and to observe simultaneously.