Wednesday 14th January 2004 University of Glasgow
The impetus to develop quantum dot lasers came from the numerous performance
advantages predicted for a zero dimensional system, including low threshold
current density and low temperature dependence of threshold, zero linewidth
enhancement factor and extremely high differential gain [1,2]. Dot lasers have
delivered improved performance relative to quantum well lasers in terms of threshold
current density [3], linewidth enhancement factor [4] and other parameters important
for lasers and amplifiers [5]. However, the gain and recombination mechanisms
in quantum dot active regions are more complicated than originally envisaged.
In a real system of self assembled quantum dots the dots cannot be treated as
isolated zero dimensional states due to the proximity of higher lying dot states
and the two dimensional wetting layer within which the dots are formed. This
can have a profound affect on the distribution of carriers among the various
states and their interaction and ultimately on laser performance. Here the fundamental
active material characteristics are studied, with reports of measurements of
gain, internal optical mode loss, spontaneous emission and carrier distribution
using a single pass, multi-segment method. A series of quantum dot structures
are examined, including single and multi layers of dots focussing on the distribution
of carriers and the origin of the low value of maximum saturated gain observed
by many groups worldwide [6]. The significance of these effects and the means
to reduce their impact is examined.
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[1] Y. Arakawa and H. Sakaki, Applied Physics Letters, 40, 939 (1982)
[2] M. Asada, Y. Miyamoto and Y. Suematsu, IEEE Journal of Quantum Electronics,
QE-22, 1915 (1986)
[3] P.G. Eliseev, H. Li, G.T. Liu, A. Stintz, T.C. Newell, L.F. Lester, K.J.
Malloy, I IEEE Journal of Quantum Electronics, 7, 135 (2001)
[4] T.C. Newell, D.J. Bossert, A. Stintz, B. Fuchs, K.J. Malloy and L.F. Lester,
IEEE Photonic Tech. Lett. 11 1527 (1999).
[5] for a recent review see D. Bimberg and N. Ledentsov, Journal of Physics:
Condensed Matter, 15, R1063 (2003)
[6] L.V. Asryan, M. Grundmann, N.N. Ledentsov, O. Stier, R.A. Suris, D.Bimberg,
Journal of Applied Physics, 90, 1666, (2001) and refs therein.