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In Search of the Dipole Moment

Photonics Spectra
Aug 2003
Anne L. Fischer

Turning quantum dots on and off sounds as simple as flipping a switch, but it can't be accomplished until we understand how much excitation is required to complete the task. There has been a lot of confusion in the literature, according to Kevin L. Silverman, of the National Institute of Standards and Technology in Boulder, Colo.

resDipole.jpg
InGaAs/GaAs quantum dots were imaged on a 500 x 500-nm field using transmission electron microscopy.

"A lot of techniques were indirect and uncertain," Silverman said, so his group teamed with researchers from the National Renewable Energy Laboratory in Golden, Colo., to find a way to accurately measure the amount of laser light needed to shift the electron in a quantum dot between low and high states, or the "dipole moment."

The researchers studied self-assembled quantum dots because they are common in semiconductor lasers and present an interesting system in which to investigate quantum information. Using an optical parametric oscillator that puts out pulses in the 1.1- to 1.3-µm range, they measured the dipole moment by enclosing the quantum dots in a waveguide cavity, allowing the pulsed light to pass over them repeatedly. The light from the laser diminished each time as the dots absorbed energy. They then averaged the absorption level coefficient of various ensembles of InGaAs/GaAs quantum dots.

To achieve accurate measurements, the researchers time-resolved the output. The unique aspect of the technique, Silverman said, is that, because waveguide coupling has an equal effect on each exiting pulse, the uncertainties do not prevent accurate measurement.

The only challenges were difficulty in characterizing the background loss of the waveguide and in finding the density of the quantum dots.

To characterize the background loss, they made waveguides identical to those they were studying, but without quantum dots. When they measured those, they could determine the background loss and compensate. To help find the density of the quantum dots, they used transmission electron microscopy.

pulses with an approximate bandwidth of 12 nm
Experimental setup shows pulses with an approximate bandwidth of 12 nm. The nonlinear crystal is 1.5 mm of beta-barium borate.

The scientists discovered that the dipole moment of single-layer InGaAs/GaAs quantum dots is between 26 and 33 Debye. Silverman said that he believes that this value will enable researchers to more easily calculate the absorption and amplification in quantum dot lasers along with strength of excitation logic options. The next step is to perform measurements on vertically coupled quantum dots.


GLOSSARY
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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