Light Squeezed From QDs

Light has successfully been amplified with semiconductor nanocrystals suspended in another medium, or colloidal quantum dots (QDs). The technology had been seen as a dead-end but could now lead to advances in lasers, telecommunications and optical computing.

Over the last 15 years, repeated quantum dot research efforts failed to deliver on expected improvements in amplification, and many researchers started to believe that an unknown but insurmountable law of physics was blocking their path. Essentially, they said, quantum dots would simply never work well for one of their primary applications.

However, after extensive research, professor Patanjali Kambhampati and colleagues at McGill University's department of chemistry determined that colloidal quantum dots do indeed amplify light as promised. The earlier disappointments were due to accidental roadblocks, not by any fundamental law of physics, the researchers said. Their results were published in the March 2009 issue of Physical Review Letters.

Vials of highly luminescent water-soluble quantum dots span a wavelength range of 500 to 600 nm. Each vial contains the same material, the color emitted depends on the size of the dots. (Photo: NIST)

Colloidal quantum dots can actually be painted directly on to surfaces, and this breakthrough has enormous potential significance for the future of laser technology, and by extension, for telecommunications, next-generation optical computing and an innumerable array of other applications.

Like sound, radio waves or electricity, laser signals gradually lose power over distance and must be passed through an amplifier to maintain signal strength. Until now, the best available amplification technology was the quantum well, a thin sheet made of semiconductor material which confines electrons to a one-dimensional plane, and consequently amplifies light. Colloidal quantum dots perform a similar function, but in a 3-D box-like structure instead of a flat sheet.

"Everyone expected this little box to be significantly better than a thin sheet," Kambhampati said. "You'd require less electrical power, and you wouldn't need to use arrays of expensive cooling racks. The idea was to make the lasing process as cheap as possible. But the expected results were not really there. So people said 'let's forget about the quantum dot' and they tried rods or onion shapes. It became a game of making a whole soup of different shapes and hoping one of them would work.

"In our view," he continued, "no one had figured out how the simple, prototypical quantum dot actually worked. And if you don't know that, how are you going to rationally construct a device out of it?"

In the end, Kambhampati and his colleagues discovered that the major problem lay in the way researchers had been powering their quantum dot amplifiers.

"We discovered that there was nothing fundamentally wrong with the dots. If you weren't careful in your measurements, when powering the quantum dot, you would accidentally create a parasitic effect that would kill the amplification," he said. "Once we understood this, we were able to take a quantum dot that no one believed could amplify anything, and turned it into the most efficient amplifier ever measured, as far as I know."

For more information, visit: www.mcgill.ca