Nanocrystal quantum dots: building blocks for tunable optical amplifiers and lasers Page: 4 of 7
This article is part of the collection entitled: Office of Scientific & Technical Information Technical Reports and was provided to UNT Digital Library by the UNT Libraries Government Documents Department.
Extracted Text
The following text was automatically extracted from the image on this page using optical character recognition software:
EXPERIMENTAL DETAILS
CdSe NQDs were fabricated as shown in figure 1 [5]. NQD solids were prepared
by drop casting films from hexane/octane solutions; solvent evaporation yielded
amorphous dry solid films of close-packed NQDs. Transient absorption (TA)
measurements in the visible were performed using a femtosecond (fs) pump-probe
experiment as previously described [6]. Briefly, the samples were pumped at 3.1 eV by
frequency doubled 100 fs pulses from an amplified Ti-sapphire laser. Pump-induced
absorption changes were measured using time-delayed broad-band probe pulses of a fs
white light continuum generated in a sapphire window.
RESULTS/DISCUSSION
Solvent/interface dependent optical gain dynamics in CdSe NODs
In an attempt to observe gain in NQD CdSe solutions, we studied the TA pump
dependence for NQD CdSe of various sizes in hexane (figure 2). The gain threshold
corresponds to -Aa/ao = 1, where ao is a linear absorption coefficient and Au a pump-
induced absorption change. None of the samples in figure 2 shows crossover from
absorption to gain even at very high pump densities corresponding to Neh> 10, where Neh
is the number of electron-hole pairs per dot. Rather, a "universal" size-independent TA
pump-intensity dependence, showing a saturation of the iS absorption bleaching before
crossover to gain, is observed. The universal curve shows a linear growth below Neh = 1
and saturation at a level -Aa/ao 1 above Neh = 1.
These data are compared with the pump dependence expected for state filling in a
system for which the 1S optical transition couples a populated electron state to either a
populated (dashed line) or an unpopulated (solid line) hole state. The data are consistent
with the latter, suggesting fast hole relaxation to a state that is lower in energy than that
involved in the iS transition [7]. We recently studied this relaxation process using an
ultrafast PL experiment [8]. Fs PL data indicate a sub-ps hole relaxation from the state
responsible for the iS absorption ("absorbing" state) to a lower energy state involved in
+ O2eh/NQD IS abs
Gain threshold 4
1 08 .
. 06 " "
0 1 ..TOPO-capped 04-
"X 2.33 nm 02- +++ +++ " 6Gae
y Q 1.73 nm -,G+++ ++$+""+*1+
A A 1.17nm o 0
001 -ZnS-capped
n X 1.17 nm -o 2 " NN""
SPA
22 23 24 25 26 27
001 01 1 10 Energy (eV)
Neh
Figure 3. Pump dependence of 1S bleaching
Figure 2. Pump dependence of the normalized 1S (Aa<0) and photoinduced absorption (Aa>0) in a
absorption changes in a NQD-CdSe/hexane sample. NQD-CdSe/hexane sample.
Upcoming Pages
Here’s what’s next.
Search Inside
This article can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Article.
Mikhailovsky, A. A. (Alexander A.); Malko, A. V. (Anton V.); Klimov, V. I. (Victor I.); Leatherdale, C. A. (Catherine A.); Eisler, H-J. (Hans-J); Bawendi, M. (Moungi) et al. Nanocrystal quantum dots: building blocks for tunable optical amplifiers and lasers, article, January 1, 2001; United States. (https://digital.library.unt.edu/ark:/67531/metadc925859/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.