QUANTUM ELECTRONICS: Researchers demonstrate quantum-dot lasers

Scientists at Fujitsu Laboratories Ltd. (Atsugi, Japan) have reported the first current-injection laser oscillation from indium gallium arsenide quantum dots at 80 K. The dots, which emit at 1300 nm, were grown on GaAs substrates by atomic layer epitaxy (ALE) and demonstrate three-dimensional quantum confinement.

According to Fujitsu Laboratories' Hajime Shoji, the main feature of a quantum-dot laser is the potential reduction of threshold current due to enhanced quantum effects and reduced volume of the active region. Shoji hopes quantum-dot lasers will be used in optical systems that require very low power consumption.

To form the laser structure, the Fujitsu scientists grew an n-type InGaP cladding layer, a nondoped InGaAs/ GaAs active layer by ALE, and a p-type AlGaAs cladding layer on an n-type GaAs substrate. The quantum dots, which were 20 nm in diameter and 10 nm in height, were randomly organized in the active layer. Aerial coverage of the quantum dots in the active layer was about 10%. Using the laser structure, they fabricated 900-µm-long broad-area contact lasers.

Pulsed laser oscillation was achieved at the threshold current of 1.1 A. Observed electroluminescence spectra, however, predict extremely low threshold-current operation for quantum-dot lasers, with a possible threshold-current density of 40 A/cm². To confirm that the lasing is from the subband of the quantum dots, the researchers tested the diamagnetic shift. In three-dimensional quantum-confined dots, the diamagnetic shifts of the lasing wavelength would be smaller than those of the quantum well.

The results with a reference laser proved that the observed laser oscillation is from a high-order sublevel of the quantum dots. Quantum-dot density was, however, too low so the operating temperature was 80 K. Hence, Fujitsu researchers are now trying to increase dot density to achieve room temperature-operation at 1300 nm. Other tasks include increasing the uniformity of dot size. Shoji also hopes that research into the quantum dots will lead to the experimental development of new devices that use the enhanced optical nonlinearity of quantum dots.

In Japan, research into quantum dots is also being done at NTT Corp (Atsugi, Japan), where the dots, which are larger than Fujitsu's, are called "quantum disks." And, recently, the NEC Basic Research group (Tsukuba, Japan) has started investigating quantum dots. In Germany, a group of researchers at the Technical University of Berlin has achieved laser oscillation from such structures, but quantum effects have apparently not been clearly observed.