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Quantum dot ink will make lasers printable

EuroPhotonics
Oct 2010
Dr. Jörg Schwartz, joerg.schwartz@photonics.com

CLAYTON SOUTH and MELBOURNE, Australia, & PADUA, Italy – Printing techniques that already have been incorporated into solar cell manufacturing could prove useful for the production of lasers and other emitting optoelectronic devices, according to researchers from Australia and Italy.

Quantum dots are semiconductors whose conduction characteristics are closely related to the size and shape of the individual crystal. This means that with decreasing crystal size, the bandgap gets larger. The bandgap determines the energy needed to excite the dot – or the energy that is released when the crystal returns to its resting state. In fluorescent dye applications, this equates to higher frequencies of light emitted after excitation of the dot as the crystal size grows smaller, resulting in a color shift from red to blue in the light emitted. The big advantage in using quantum dots as semiconductors is that, because of the high level of control over the size of the crystals produced, it is possible to have very precise control of conduction and emission properties.


Quantum dot inks make different colors, depending on the size of the crystals. Images courtesy of Raffaella Signorini, University of Padua.


This led the scientists to set out making cheaper, more versatile lasers. “Creating cheaper lasers relies heavily on progress in materials science,” said researcher Jacek Jasieniak of CSIRO, the Australian Commonwealth Scientific and Industrial Research Organization. “At present, lasers are manufactured using expensive materials and production techniques. To make them more cost-effective, we have focused on developing materials that are cheap, function well as lasers, and can be printed. Quantum dots meet all these requirements.”


Conventional lasers cannot be used for printable nanolasers because they require large optical cavities, so the researchers used a patterned surface for the printing (a). Repetitive grooves on the surface act as little resonators (b).


However, to make a laser, both an active medium and a cavity are required. Making lasers that can be spread over wide areas takes many small cavities. “Conventional lasers use large optical cavities which make them impossible to use for printable … nanometer-size lasers,” Jasieniak said. Therefore, a patterned surface is used for the printing, and the repetitive grooves in this surface, reflecting the light at the interfaces, become little resonators. A major benefit of these nano-structured optical cavities is that they can be produced during the printing process by controlled indentation or scratching of the material’s surface.


A prototype quantum lasing device.


Jasieniak’s work, performed jointly with his colleagues at CSIRO and the University of Melbourne in Australia and at the University of Padua in Italy, was recently presented for the first time in public through Fresh Science, a communication boot camp for early-career scientists held in Melbourne.

The potential of printable optoelectronic devices could be tremendous, reaching from flat emissive lighting panels to new types of televisions and displays, and to a wide range of fields in computers, electronics and sensors. For this vision to be realized, however, a little more work is needed. Currently, the printable lasers are optically pumped, and the big goal is to make them electrically driven instead.


GLOSSARY
cavity
In a laser, the optical resonator formed by two coaxial mirrors, one totally and one partially reflective, positioned so that laser oscillations occur.
conduction band
A partially filled or empty energy band through which electrons can move easily. The material can therefore carry an electric current. The term is usually applied to semiconductors.
crystal
A solid with a structure that exhibits a basically symmetrical and geometrical arrangement. A crystal may already possess this structure, or it may acquire it through mechanical means. More than 50 chemical substances are important to the optical industry in crystal form. Large single crystals often are used because of their transparency in different spectral regions. However, as some single crystals are very brittle and liable to split under strain, attempts have been made to grind them very...
exciton
A moving, electrically neutral, excited condition of holes and electrons in a crystal. One example is a weakly bound electron-hole pair. When such a pair recombines, with the electron "falling" into the hole, the energy yielded is the bandgap decreased by the binding energy of the pair.
solar cell
A device for converting sunlight into electrical energy, consisting of a sandwich of P-type and N-type semiconducting wafers. A photon with sufficient energy striking the cell can dislodge an electron from an atom near the interface of the two crystal types. Electrons released in this way, collected at an electrode, can constitute an electrical current.
valence band
In a crystalline substance, the spectral range of states of energy that contains the crystal's binding valence electrons.
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