Aug 1, 2007
Stranski-Krastanov quantum dots
Self-assembled semiconductor quantum dots (QDs) formed using the Stranski-Krastanov growth technique have potential applications in quantum information processing and quantum computation. In this technique, the lattice mismatch between the substrate and the QD material leads to the formation of strained islands. Electronic and optical properties of these quantum dots are significantly altered due to strain. Further, the optical response of the nanostructures can be controlled by applying a magnetic field.
In the article published in Nanotechnology, researchers from Shri Govindram Seksaria (SGS) Institute of Technology and Science, Indore, India have theoretically analysed the effect of strain and magnetic fields on the spectral characteristics and coherent optical response of a single self-assembled semiconductor QD. The QD material that was considered is CdSe grown on a ZnSe wetting layer, which is gaining importance due to the wide bandgap and deep confinement potential offered by this combination. Luttinger Hamiltonian formalism was used to account for the complex valence-band structure consisting of light and heavy holes. The energy levels and the wavefunctions were obtained by numerical diagonalization of the Hamiltonian. The absorption spectra are analyzed using these energy eigenvalues and eigenfunctions. The transient optical response was calculated following the effective semiconductor Bloch equation approach.
The researchers observed that the strain effects incorporated in terms of deformation potential lead to a substantial blue shift of the energy levels and also cause a relative shift of heavy and light hole energy levels. Multiple oscillations arising due to quantum interference are seen in the temporal characteristics of the induced polarization; the frequency of these oscillations can be enhanced by increasing the magnetic field.
Understanding coherent oscillations in semiconductor nanostructures is an extremely useful technique to achieve coherent optical control of excited states and minimizing decoherence, which forms the basis for quantum computation.
About the author
Pranay K Sen is the professor and head of the applied physics department, SGS Institute of Technology and Science, Indore, India. His group is currently working in semiconductor nano-optics and nonlinear optics. The other authors are members of this group. Dr J Kumar is a lecturer in the same department, while Dr S Kapoor has been a Young Scientist at the Department of Science and Technology, Government of India. S K Gupta completed his PhD work and recently joined the Birla Institute of Technology, Ranchi, India as a lecturer.