Research Focus 2

Comments & Views on Current Projects at NTT Laboratories

Quantum Wire Microcavity Laser

What happens when the size of a semiconductor particle becomes very small? This simple question was the starting point for our recent research on quantum wire microcavities. Our idea was to study how light was emitted from nanometer-sized semiconductor wires completely embedded in micrometer-sized optical microcavities. We wanted to do this for two reasons. In nanometer-sized semiconductor wires, electrons can be confined to a size less than their DeBroglie wavelength (10 nm). Because of this confinement, the optical emission strength of the quantum wire is concentrated into a narrow frequency range. The other reason was that in wavelength-sized optical microcavities all the light is channeled into a single emission mode.This is different from the usual waveguide type laser where the light energy is distributed over tens of thousands of emission modes. We hoped that by completely integrating semiconductor quantum wires with optical microcavities we could achieve highly efficient light emission.

Up to now the only way to make one-dimensional quantum wires has been to grow two-dimensional quantum wells first, and then to use sophisticated process technology to define the wires. One of the drawbacks of the conventional approach has been that the processing can easily introduce impurities or unwanted damage into the structures.

We used a different approach. When a GaAs crystal is cut slightly off one of its crystallographic orientations, a staircase-like pattern of regularly spaced nanometer-size steps naturally forms. We were able to grow layers of aluminum arsenide (AlAs) and gallium arsenide (GaAs) crystals, less than a monolayer in thickness, on top of the steps using chemical vapor deposition (CVD) growth technology. Using this technique, we succeeded in producing nanometer-order GaAs quantum wires. We call structures grown in this way fractional-layer superlattices.The deposition technique is unique in that it can create densely-packed quantum wires that can be precisely integrated into microcavity laser structures. The figure shows how the quantum wires are formed and how they fit into the microcavity laser structure.

Using an optical pumping technique to generate optical gain in the quantum wires, we were able to see strong monochromatic emission with a sharp threshold even at room temperature. This is the first time that lasing has ever been achieved in a quantum wire microcavity [Applied Physics Letters, vol.64, p.1759 (1994)]. The laser light exhibits polarization properties unique to the one-dimensional nature of the quantum wires. With further optimization of the laser structure, it will be possible to reduce the threshold current as well to demonstrate new switching capabilities which use polarization. We expect that quantum wire microcavity lasers will find wide application as light sources for optical data processing as well as optical communications.

Arturo Chavez-Pirson,
NTT Basic Research Laboratories
e-mail: chavez@wave.ntt.jp