PhD dissertation defense/Zhigang Xie (Feb 23, wed, 3:30pm)

Zhigang Xie zxie at snowmass.Stanford.EDU
Fri Feb 18 21:32:44 PST 2005

I apologize if you receive multiple copies.


Zhigang Xie
Research Advisor: Professor James S. Harris

Integration of Quantum Dots with Microdisk Cavities

February 23, 2005 @ 3:30pm in CISX auditorium (CISX 101)


Self-assembled quantum dots growth is a strain-driven phenomenon that can
take place in lattice-mismatch epitixial growth. Electrons and holes
confined in these nano-scale structures exhibit discrete optical emission
that in many ways is similar to atoms, yet with the richness of many-body
interactions. In the spontaneous emission regime, this emission can be
used in quantum information processing if the optical extracting
efficiency is high enough to allow for error correction algorithms and
privacy amplification. In contrast, lasing from QDs can be seen at very
low threshold powers, possibly providing a unique low power source. Both
spontaneous emission and lasing from QDs can be made possible by embedding
the QDs into a micro-scale optical cavity.

In this talk, I will discuss spontaneous emission and lasing of QDs in a
microdisk cavity. When the QD dipole is properly coupled to the cavity
mode, the spontaneous emission can be enhanced. For lasing the cavity
provides an enhanced photon density. However, in a microdisk cavity the
cavity mode is isolated at the disk parameter and for efficient coupling
the QDs should be located in this region. To achieve this, we have
developed a regrowth technique to place QDs close to the anti-node of
microdisk whispering-gallery modes. We observe a decreasing QD perimeter
density with decreasing disk diameter: on big disk of 30um, the linear
density will saturated to 6/um; while on small disk of 3~4um, there
usually will only 1~3 QDs present. Our micro-PL shows well-defined single
QD emission signature, as well as a large splitting for neutral excitons
emitting with opposite linear polarization. This regrowth technique can
provide sharp exciton peaks as well as a cavity Q of 2,000 for the small
disk of 3~4 um.

I will compare this technique with the normal microdisk fabrication made
without regrowth. Using this technique, I will discuss QD lasing in the
smallest microdisk (1.8um) reported, with a cavity Q of 10,000. From the
cavity mode lines tuning through QD exciton lines, it approves near single
QD lasing, usually a Pucell factor of 80 is required to achieve this goal.
When the QD is at resonance with a cavity mode, a lasing threshold as low
as 10W/cm2, or 300nW for each disk is estimated.

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