|
Polymer Nanowire Lasers
We have employed the method of melt-assisted
template wetting to synthesise semi-crystalline polyfluorene (PFO) nanowires. A
scanning electron microscope (SEM) image of an array of PFO wires confirmed that
aligned forests of close-packed PFO nanowires may be successfully generated by
this method (~ 109 nanowires per template); see Figure 1a. The tips
of many wires exhibited regular features, being cleanly fractured with both
terraced and planar end facets. An epi-fluorescence micrograph of a random PFO
nanowire mat is shown in Figure 1b. The wires exhibited characteristic blue
emission with bright photoluminescence (PL) spots at the wire tips and
comparatively weaker emission from the wire bodies, i.e., following
optical excitation of a wire, resulting PL emission propagated to the ends of
the wire where bright spots arose due to out-coupling of PL by
scattering.
|
|
Figure 1. (a) Scanning electron microscopy (SEM) image of a
PFO nanowire array following template removal.
(b) Epi-fluorescence microscopy image of a PFO nanowire mat on glass.
|
Isolated PFO nanowires were uniformly excited by
the 355 nm output of a 0.7 ns, 1.25 kHz pulsed Nd:YVO laser and PL spectra were
acquired from both body and tips; see Figure 2a. Periodic intensity variations
were observed in tip emission spectra of single wires, especially around the 0-1
peak. These variations were not apparent in body spectra suggesting that they
were due to Fabry-Pérot type cavity resonances. To confirm that the wires
operated as axial Fabry-Pérot microcavities, mode spacing at 460 nm was plotted
versus inverse nanowire length for 14 wires and shown to exhibit a linear
dependence; see Figure 2b.
|
|
|
Figure 2. (a) Emission spectra collected from the tip (blue) and body (gray) of an
isolated PFO nanowire under uniform pulsed excitation (1.4 nJ). Inset:
emission image of an excited wire. Scale bar, 2 µm. (b) Plot of mode spacing measured at 460 nm versus inverse nanowire
length for 14 different nanowires. Black squares, experimental data points;
green triangle, extrapolation to infinite length; dashed blue line, linear
fit to the data. Inset: schematic depiction of a nanowire with well-defined
end facets acting as a Fabry-Pérot microcavity.
|
Single isolated PFO nanowire microcavities were
then uniformly excited (355 nm, 0.7 ns, 1.25 kHz) while tip emission spectra
were collected as a function of pump energy; see Figure 3a. At lower pump
energies, tip spectra exhibited typical nanowire microcavity emission with
pronounced Fabry-Pérot modes apparent at the 0-1 peak. Above an energy
threshold (typically 100 nJ), a single spectrally narrowed emission peak
developed by preferential gain in a single Fabry-Pérot mode and the onset of
lasing. Below threshold, emission intensity increased linearly with excitation
energy; see Figure 3b. Above threshold, a kink in emission output was followed
by a super-linear increase due to optical gain (emission intensity showed an
approximate quadratic dependence on pump energy). Also, the emission peak
width narrowed from 19.6 nm to 1.4 nm (instrument limited) at threshold,
indicating the high Q of the nanowire cavity.
|
|
|
Figure 3. (a) Emission spectra collected from the tip of an isolated
PFO nanowire
under uniform excitation as a function of increasing pump energy from 13 nJ
(lowest spectrum) to 230 nJ (highest spectrum) at room temperature. Inset:
emission image of the wire. Scale bar, 5 µm. (b) Plot of the emission peak intensity (blue squares) and
full-width-at-half-maximum (red triangles) versus pump energy for the data
shown in a. The intensity of emission from the nanowire body (green squares)
is low and almost linear with pump energy. Solid symbols correspond to
experimental data points and lines are guides to the eye.
|
Publications
|
O'Carroll, D.; Lieberwirth, I. & Redmond, G.
Microcavity Effects and Optically Pumped Lasing in Single Conjugated
Polymer Nanowires.
Nat. Nanotech. 2,
180 (2007). (Cover Article). |
 |
|
