For Quantum Dots, One Laser Does Two Jobs

Hank Hogan

To paraphrase an old saying, one researcher’s problem is another’s solution. That lesson has been reinforced by recent work by investigators from Max Planck Institute for Solid State Research in Stuttgart, from Paderborn University and from IFW Dresden, all in Germany. The group demonstrated that a laser could both probe and heat single optical microcavities and self-assembled quantum dots, enabling postprocess engineering of these micro- and nanodevices.

Using a laser at low power as a probe and at a higher power as a heat source, researchers brought separated quantum dots into energy-state resonance. On the upper left is a scanning electron microscopy image of a GaAs microdisk containing InGaAs quantum dots with an inset of an atomic force microscopy image of the self-assembled particles. In the upper right is a diagram of photoluminescent intensity as a function of energy and laser power. On the lower right, spectra show a redshift as a result of heating, which enables measurement of the local temperature of the disk. On the lower left is a calculated temperature profile of the disk. Reused with permission of the American Institute of Physics.

However, the group’s initial tries at postprocessing using quantum dots alone were disappointing. There was no effect with a few hundred milliwatts of laser power. At higher intensities, the emission disappeared entirely. Then team member Armando Rastelli discovered something in the literature. “For microdisks, a commonly encountered problem is the disk heating — and consequent emission quenching — at moderate laser powers. That ‘problem’ was the solution to our problem.”

The researchers put this behavior to use to correct one of the difficulties that plague self-assembled quantum dots. The particles can be considered artificial atoms, but, unlike natural atoms, particles are rarely identical. That matching is important, however, if the particles are to interact coherently — a necessity for quantum information processing. Achieving matched particles requires bringing the energy states of separate particles into resonance.

In developing a process to accomplish this, the investigators constructed microdisk resonators — or square-shaped mesas — atop thin posts rising from a GaAs substrate. They embedded self-assembled InGaAs quantum dots inside the structure. The particles were at such a low density that a disk might contain only one or a few. Finally, the scientists deposited a SiO_x layer on top of everything to limit arsenic desorption during the heating process.

After cooling the samples to cryogenic temperatures, the researchers used a 532-nm laser from Coherent Inc. as both a probe and a heat source. Focusing the light to a 1.5-μm-diameter spot on the microcavity, they probed the microdisks with the power level set at a few nanowatts. They captured the resulting photoluminescent spectra with a Roper Scientific spectrometer equipped with Princeton Instruments detectors.

They increased the power level to a few milliwatts in 100 s, leading to an emission redshift that they used as a local thermometer. From previous work, the researchers knew that heating would produce a controlled blueshift of the quantum dot emission without affecting the quality. When they measured the emission after a multistep process that involved heating followed by measurement, they found that they could bring the energy states of the spatially separated particles into resonance.

Rastelli acknowledged that much more work must be done before the particles can be put to use. However, he also noted that the probe-and-heat technique can be used for more than quantum dots. “The method itself is general and can be employed as a postfabrication tool to fine-tune the properties of nano- and microstructures.”

Applied Physics Letters, Feb. 12, 2007, Vol. 90, 073120.