Biomedical Engineering Reference
In-Depth Information
i.e. stacked in the growth direction where each layer is grown differently [
46
,
47
].
Broadband SLDs find applications in various fields, the most prominent of which
is possibly as the broadband source at the heart of optical coherence tomography
(OCT) [
48
]. The resolution of images acquired from OCT is inversely proportional
to the luminescent linewidth (FWHM) of the NIR source [
49
]. We proposed and
demonstrated that lateral QDMs are well suited as an active material for broadband
devices and systems. The main advantage over conventional QDs or QWs-based
chirped structures is the broader FWHM for the same number of stacked layer,
or smaller number of stacked layers for the same FWHM. This section describes
chirping schemes based on lateral QDMs bi-layers as the active material. The bi-
layers comprise four nominally different sub-ensembles; the PL in each of which
has a predictable temperature dependency, allowing easy design and optimization
of structures with a greater number of stacks.
3.5.1
Chirped Bi-Layers
A single layer of lateral QDMs exhibits two GS energies: a low-energy, narrow
emission from cQDs and a high-energy, broad emission from sQDs. A lateral QDMs
bi-layer thus exhibits four GS energies which, in order to maximize the FWHM for
broadband applications, can be designed to overlap in accordance with one of the
three schemes shown in Fig.
3.9
a-c [
23
].
The straddled scheme or Type-I chirp depicted in Fig.
3.9
a makes use of a wide
separation between the two GS energies of one layer (cQD
1
and sQD
1
in the figure,
unshaded curves) to straddle or sandwich the narrow separation of the other (cQD
2
and sQD
2
, shaded). The wide separation can be achieved by a relatively thick
capping and regrowth, a condition where cQDs are filled but sQDs are forming
and still far from saturation. The 1.8/25/1.5 QDMs described in Sect.
3.4.2
,for
example, meet this criteria. The spectrum shown in Fig.
3.5
b indicates GS separation
as wide as 145 meV. The narrow separation, on the other hand, can be achieved by
a relatively thin capping and thick regrowth. The 2/6/1.4 QDMs described in Sect.
3.4.1
with PL spectrum in Fig.
3.4
b, for example, show the separation of almost
zero as the cQD and sQD GS peaks are unresolved. Alternatively, a single cQD
peak can be employed. By growing a bi-layer of 2/26/2 QDM
1
and 2/26/1.4 QDM
2
,
a Type-I chirp spectrum can be obtained. (Subscripts 1 and 2 indicate, respectively,
the lower and upper QDM layers in the growth sequence.) Figure
3.9
dshowsthePL
spectra of the bi-layer (upper spectrum) with respect to the controlled, single layer
2/25/1.4 QDMs. The latter, previously shown as a linear plot in Fig.
3.4
b, exhibits
a single PL peak since the deep nanoholes have yet to be saturated. The former
exhibits three GS peaks: the minimum at 1.048 eV and the maximum at 1.214 eV
are from the 2/26/2 QDM
1
, whereas the intermediate peak at 1.086 eV is from the
2/26/1.4 QDM
2
. Multi-Gaussian function fits (dashed lines) show that the lower two
peak energies are narrow, indicative of cQDs-based origin, and the high peak energy
is broad, indicative of sQDs-based origin. The intermediate peak energy from the
