Ring microlasers from conducting polymers
S. V. Frolov, M. Shkunov, and Z. V. Vardeny
Department of Physics, University of Utah, Salt Lake City, Utah 84112
K. Yoshino
Department of Electronic Engineering, Osaka University, Yamada-Oka 2-1 Suita, Japan
Received 18 June 1997
We demonstrate pulsed, photopumped multimode laser emission in the visible spectral range from cylindri-
cal microcavities formed by conducting polymer thin films deposited around optical fibers. The laser is char-
acterized by narrow emission lines ( 1.5 cm
-1
), a well-defined excitation threshold, anisotropic emission in
both polarization and azimuthal intensity distribution, and high Q( 3000), which leads to a low excitation
threshold of order 1 nJ/pulse. We observed two different sets of laser modes; these are waveguided ring modes
in the polymer film and whispering gallery modes close to the optical fiber surface. S0163-18299753032-3
Recent advances in the synthesis of luminescent conduct-
ing polymers LCP followed by a better understanding of
their optical and electronic properties, have provided evi-
dence of optical stimulated emission SE in LCP solutions
and thin films.
1–4
SE occurs at a relatively high excitation
intensity when the optical gain exceeds optical losses due to
self-absorption, light scattering caused by imperfections, and
other factors. SE is usually accompanied by a dramatic spec-
tral narrowing, as has recently been observed in various con-
ducting polymer films.
2–4
Even though spectral narrowing is
one of the main characteristic properties of lasing,
1–2
other
phenomena of high optical gain, such as superfluorescence
and amplified spontaneous emission.
2–4
may also produce
analogous spectral narrowing. Laser oscillations, on the other
hand, require a cavity to provide optical feedback.
5
The reso-
nant nature of the laser cavity establishes a well-defined
spectral mode structure and an associated highly anisotropic
i.e., directional emission pattern.
In this paper we investigate photopumped lasing in LCP
using cylindrical microcavity structures of thin films depos-
ited on glass fibers. Microcavities provide excellent coupling
of spontaneous emission into lasing modes, and a high-cavity
quality factor Q which consequently leads to low lasing
thresholds.
6
Two LCP have been tested: derivatives of
polyp -phenylene-vinylenePPV with lasing in the red,
and disubstituted polyacetylene DPA with lasing in the
green. Multimode, narrow laser lines ( 1.5 cm
-1
) with low
threshold intensities 1 nJ/pulse have been demonstrated in
these structures using pulsed photoexcitation. We note that
low lasing threshold is an important criterion in developing
electrically pumped plastic lasers.
In our studies we used soluble derivatives of PPV and
DPA as gain media; including DOO-PPV Ref. 3 and
PDPA-Si( i Pr)
3
,
7
respectively. Cylindrically shaped thin
polymer films were prepared by dipping commercially avail-
able optical fibers into saturated chloroform solutions. Thin
polymer rings were consequently formed around the glass
cylindrical core following fast drying in the air. The esti-
mated thickness d of the deposited polymer film was about
2–3 m. The self-assembled polymer microstructures in the
form of cylindrical microrings are similar to those previously
reported in Ref. 8, with deposited films of laser dyes blended
in transparent polymers. In the present paper, however, we
have exclusively used pristine conducting polymers as lasing
media. The excitation source was a Nd: yttrium aluminum
garnet YAG regenerative laser amplifier producing 100 ps
pulses with a repetition rate of 100 Hz. This laser light was
either frequency doubled 532 nm, or tripled 355 nm for
pumping the DOO-PPV and PDPA-Si( i Pr)
3
films, respec-
tively. The pump beam was focused using a cylindrical lens
into a 100 m5 mm stripe perpendicular to the polymer
coated fiber ( 100– 200 m); thus only a small fraction of
the pump light was absorbed by the polymer film. The pump
beam polarization could be rotated to be either parallel or
perpendicular to the fiber axis. The light emitted from the
excited polymer ring was collected in the plane of the ring
with a round lens and spectrally analyzed using a 0.6 m triple
spectrometer and a charge-coupled device array with maxi-
mum spectral resolution of about 1.5 cm
-1
.
Figure 1 shows the emission spectra of DOO-PPV films
coated onto two different optical fibers with outside diam-
eters D of 200 m a and 125 m b, respectively, ob-
tained at pumping intensities of order 1 nJ/pulse. In the insets
of Fig. 1a we show the DOO-PPV polymer repeat unit and,
for comparison, the much broader ( 2500 cm
-1
) photolu-
minescence PL spectrum. The sharp emission lines seen in
Fig. 1 appear only above a threshold excitation intensity I
0
.
Figure 1b shows three emission spectra measured below,
at, and above I
0
. Since the sharp lines are absent at low
intensity, they cannot be due to a quantum electrodynamic
enhancement of PL in a cavity.
9
We therefore attribute
these lines to laser modes associated with the cylindrical
cavity formed by the thin polymer film. The linewidth of
the narrow laser lines was limited by the spectral resolution
of the apparatus (1.5 cm
-1
). The inset of Fig. 1b also
shows that the spectrally integrated emission intensity de-
pends nonlinearly on the excitation intensity, and this is an-
other characteristic feature of lasing. We note that I
0
, ex-
pressed in terms of absorbed energy/pulse, is of order 1 nJ.
This I
0
is lower by about three orders of magnitude than that
of the spectral narrowing previously observed in flat thin
DOO-PPV films.
2
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PHYSICAL REVIEW B 15 AUGUST 1997-II VOLUME 56, NUMBER 8
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