Electrically-Activated Nanocavity Photonic Crystal Laser

The goal of this work is to design and fabricate semiconductor lasers with novel quantum dot layers in the active region with the goal of incorporating these active regions within nanocavity photonic crystal lasers. Quantum-dot (QD) heterostructure lasers are a type of semiconductor laser that utilize quantum dots as the active media within the light-emitting region. Quantum dots are semiconductor nanocrystals of narrow band-gap material that are embedded in a wider band-gap material. The use of molecular beam epitaxy for the growth of highly lattice-mismatched III-V semiconductor materials has made the self-assembly of these structures possible. Due to the strong three-dimensional carrier confinement, devices that employ quantum dots have unique capabilities that are otherwise practically unachievable with bulk semiconductors, or even with quantum well structures.

One of the significant benefits of exploiting quantum effects in QD semiconductor lasers is to achieve lower threshold current densities. A reduction in the threshold current density is a direct result of the reduction in the translational degrees of freedom of the charge carriers (electrons and holes) thereby leading to an increase in the density of states of the charge carriers near the band edges. Another important benefit is that the threshold current density in QD lasers is unaffected by temperatures up to about 300K since the carriers can only be thermally excited to a very limited number of these well-spaced energy levels within the quantum dots.

To evaluate the use of quantum dots within the active region of a photonic crystal laser, a separate confinement heterostructure laser with 7 layers of quantum dots is being fabricated. A SCH laser provides electrical confinement via the energy band structure while optical confinement is achieved by means of ridge waveguides and index contrast. The semiconductor laser consists of epitaxially-grown layers of both doped and undoped semiconductor on n-GaAs substrate.

The photonic crystal laser design incorporates one-dimensional (1-D) photonic crystals patterned on two crossing waveguides. The nanocavity, located at the two waveguide’s intersection, creates a high-Q optical resonator with the 1-D photonic crystals acting as highly reflective mirrors. By removing some of the holes in one of the photonic crystals, one can control the direction of the emitted light. The top GaAs-based waveguide is doped p-type, while the bottom, InGaAlP-based waveguide, is doped n-type, so that a p-n diode exists only in the area where the waveguides overlap. The top GaAs waveguide contains a quantum dots-in-a-well structure, which serves as the active material for the laser.

The quantum dot and nanocavity photonic crystal lasers will be characterizated both in the CW and pulsed mode of operation at a variety of temperatures. The use of interchangeable InGaAs and PbS detectors will allow for a good sensitivity for emission wavelengths less than 2.5microns.