S. V. Frolov

Cylindrical microlasers and light emitting devices from conducting polymers

Substantially improved, photopumped polymer lasers are demonstrated using microrings and microdisks of various diameters D ranging from 5 to 200 μm. Various cavity-dependent laser modes were observed, which for D<10 μm were dominated by a single longitudinal mode with linewidth of less than 1 A. These microlasers were also characterized by Q of order 5000, low threshold excitation energy of order 100 pJ/pulse for pulse duration ranging from 100 ps to sub-μs, and an abrupt increase in the emission directionality and polarization degree. Light emitting diodes with cylindrical geometry, fully compatible with these microlasers are also demonstrated.

Mirrorless Lasing in Conducting Polymer poly(2,5-dioctyloxy-p-phenylenevinylene) Films

We report mirrorless lasing in thin films of poly(2,5-dioctyloxy- p-phenylenevinylene) (DOO-PPV), that we identify as superradiance. We have measured the characteristic properties associated with this phenomenon including spectral narrowing, polarization memory, thickness and intensity dependencies. From the measurements of picosecond transient gain and loss in DOO-PPV we conclude that DOO-PPV films are excellent candidates for solid state lasers.

Plastic microring lasers on fibers and wires

Photopumped, pulsed, narrow line laser emission is demonstrated using cylindrical microcavities formed by π-conjugated polymer thin films wrapped around thin glass optical fibers and metal wires with various diameters D. A variety of cavity-dependent resonant laser mode structures were observed, which for D<10 μm contain a single resonant spectral line of less than 1 A in width. The microring lasers are also characterized by a well-defined, very low threshold excitation intensity, at which beam directionality and polarization degree dramatically increase. These findings open up the fields of lasers and fiber optics to organic materials.

Optical properties and electroluminescence characteristics of polyacetylene derivatives dependent on substituent and layer structure

Abstract Intense photoluminescence (PL) is observed in di-substituted polyacetylene derivatives even though solitonic mid-gap absorption is observed upon doping, contrary to non-substituted trans-polyacetylene and mono-substituted polyacetylene in which strong PL is not observed. Intense green and blue electroluminescence (EL) is realized utilizing poly(diphenylacetylene) derivatives and poly(1-alkyl-2-phenylacetylene) derivatives, respectively. Greenish-blue emission is also observed in poly (1-chloro-2-phenylacetylene) derivatives. The dependence of wavelength and intensity of PL and EL on the molecular structure of substituents is clarified in detail. The effects of molecular alignment and layer structure on the EL characteristics are also discussed. Upon intense light excitation, remarkable spectral narrowing due to stimulated emission is also observed in these di-substituted polyacetylene derivatives.

Picosecond photophysics of luminescent conducting polymers from excitons to polaron pairs

Abstract We present a study of picosecond(ps) time-resolved photo-modulation(PM) spectroscopy of DOOPPV in solution, pristine, photo oxidized and C 60 -doped DOOPPV films, and thin films of P3BT. We find that the photophysics of pristine luminescent polymers is determined by excitons, which in the singlet manifold are characterized by a PM spectrum consisting of two photoinduced absorption(PA) bands and a stimulated emission band. Excitons in the triplet manifold, on the contrary, can be identified by a PM spectrum consisting of a single PA band. After photo-oxidation or doping, a new type of photo-excitations is produced — intrachain polaron pairs, characterized by a broad PA band and much slower recombination dynamics compared to that of singlet excitons

Spectral narrowing of emission in di-substituted polyacetylene

We studied spontaneous and stimulated optical emission in films of di-substituted trans-polyacetylene (t-PA). We found that di-substitution of t-PA drastically changes the polymer optical emission properties, so that photogeneration of exciton and photoluminescence (PL) quantum yields become much higher than those measured in unsubstituted and mono-substituted t-PA. The strong PL is accompanied by high optical gain and emission spectral narrowing at high excitation intensities; the emission spectrum changes from a broad PL band of 100 nm at low intensities to a narrow stimulated emission band of ~ 8 nm at high intensities. These properties show that di-substituted t-PA films can be good candidates for laser medium in the green and blue spectral ranges.

Double-modulation electro-optic sampling for pump-and-probe ultrafast correlation measurements

We describe a novel electro-optic double-modulation (DM) sampling technique for ultrafast transient spectroscopy, which is characterized by a superior signal-to-noise ratio compared to that of a regular single-modulation technique. DM is achieved by a combined effect of a radio-frequency modulation, which eliminates most of the low-frequency noise, and an audio-frequency modulation, which makes use of a high-performance, low-frequency lock-in amplifier. The DM sensitivity is comparable to that of the more sophisticated schemes involving electrical mixing and the A–B noise reduction method. We show that the DM technique offers superior performance in two-beam transient pump-and-probe correlation measurements compared to the regular single frequency modulation technique and is an ideal scheme for three-beam picosecond correlation measurements.

FEMTOSECOND TIME-RESOLVED LASER ACTION IN POLY(P-PHENYLENE VINYLENE) FILMS: STIMULATED EMISSION IN AN INHOMOGENEOUSLY BROADENED EXCITON DISTRIBUTION

Abstract Femtosecond time-resolved studies of photoluminescence (PL) and stimulated emission (SE) have been conducted in neat thin films of poly( p -phenylene vinylene) derivatives. For high excitation intensity I , we found that the SE spectral narrowing is accompanied by a dramatic lifetime shortening from ∼250 to ∼3 ps, which enhances energy migration within the inhomogeneously broadened exciton distribution. In addition, the SE is delayed with respect to the PL onset by a few picoseconds, depending on I and the illumination stripe length. In some cases, a second delayed SE is formed in the film due to the continuing energy migration into exciton states with superior radiative coupling.

Picosecond to millisecond photoexcitation dynamics in blends of C60 with poly(p-phenylene vinylene) polymers

Abstract We compare photoexcitation dynamics in films of pristine and C 60 -mixed 2,5-dioctyloxy poly( p -phenylene vinylene) (DOO–PPV) using ps transient and cw photoinduced absorption (PA), and PA-detected magnetic resonance (PADMR). We conclude that C 60 doping is relatively inefficient in DOO–PPV. Consequently, the instantaneous exciton photogeneration yield remains high (close to 1) even at high C 60 concentrations. However, as the C 60 concentration increases up to 10 molar percent, the stimulated emission lifetime and cw photoluminescence intensity decrease together by about an order of magnitude due to dissociation of the singlet excitons at C 60 related defect centers into intrachain polaron pairs, which were identified by PADMR.

Observation of Superradiance in DOO-PPV films;towards ultrafast lasers.

We report the observation of superradiance in thin films of DOO-PPV. We also measure the characteristic properties associated with the phenomenon. Comparison between the transient gain and loss in DOO-PPV leads us to conclude that this is an excellent candidate for a laser gain medium.