Cooperative emission in ∏-conjugated polymer thin films

Journal ArticlePicosecond dynamics of exciton emission and absorption have been studied in neat thin films of a variety of poly (phenylene vinylene) derivatives. We found that the stimulated emission band of 120 nm width and ~1 ns duration, which is observed at low exciton density n, collapses at n > 1017 cm-3 into a much narrower band of 7 nm width and lifetime r « 10 ps. Based on its excitation intensity dependence, polarization, lifetime, illuminated area, and film thickness dependencies, we assign this narrow band to superfluorescence rather than to amplified spontaneous emission

Ring microlasers from conducting polymers

Journal ArticleWe demonstrate pulsed, photopumped multimode laser emission in the visible spectral range from cylindrical microcavities formed by conducting polymer thin films deposited around optical fibers. The laser is characterized 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 wave-guided ring modes in the polymer film and whispering gallery modes close to the optical fiber surface

Excited-state relaxation in ∏-conjugated polymers

Journal ArticleWe study ultrafast relaxation processes of odd- (Bu) and even-parity (Ag) exciton states in poly(p-phenylene vinylene) derivatives. The Bu states are studied using a regular two-beam pump-and-probe spectroscopy, which can monitor vibronic relaxation and exciton diffusion. In order to observe the Ag states, a three-beam femtosecond transient spectroscopy is developed, in which two different excitation pulses successively generate odd-parity (1Bu) excitons at 2.2 eV and then reexcite them to higher Ag states. We are able to distinguish two different classes of Ag states: one class (mAg) experiences ultrafast internal conversion back to the lowest singlet exciton, whereas the other class (kAg) in violation of the Vavilov-Kasha's rule undergoes a different relaxation pathway. The excitons subsequently dissociate into long-lived polaron pairs, which results in emission quenching with the action spectrum similar to that of the intrinsic photoconductivity. We conclude that the Ag states above 3.3 eV (kAg) are charge-transfer states, that mediate carrier photogeneration

Cooperative and stimulated emission in poly(p-phenylene-vinylene) thin films and solutions

Journal ArticleWe discuss cooperative and stimulated emissions and separate their respective contributions to the emission spectral narrowing in thin films and solutions of poly(p-phenylene-vinylene) derivatives. Whereas cooperative radiation is favored in films with poor optical confinement, directional stimulated emission is dominant in dilute solutions and thin films with superior optical confinement. Spectral narrowing in the latter case could be achieved by increasing either the excitation length or excitation intensity, from which we determined the optical gain and loss coefficients at the 0-1 emission band assuming a simple amplified spontaneous emission model. We found that the threshold excitation intensity for stimulated emission in neat films is mainly determined by self-absorption and that the gain saturates at emission intensities of about 107 W/cm2

Lasing and stimulated emission in ∏-conjugated polymers

Journal ArticleRecent studies of lasing and stimulated emission in luminescent -conjugated polymers performed by our group are presented. Optical properties of cylindrical high- polymer microcavities are discussed. The emission spectra of plastic microring and microdisk lasers are measured and analyzed. Light-emitting polymer microdiodes are demonstrated as possible candidates for electrically driven plastic lasers. In addition, two unusual regimes of stimulated emission characterized by narrow laser-like spectral lines are demonstrated in thin waveguiding polymer films. These regimes may be associated with random optical feedback introduced by light scattering inside the polymer films and amplified Raman scattering, respectively

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

Journal ArticleWe 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

Plastic microring lasers on fibers and wires

Journal ArticlePhotopumped, pulsed, narrow line laser emission is demonstrated using cylindrical microcavities formed by p-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 mm contain a single resonant spectral line of less than 1 Å 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

Stimulated emission in high-gain organic media

Journal ArticleStimulated emission was studied in various scattering gain media, including thin p-conjugated polymer films, organic dyes-doped gel films, and opal crystals saturated with polymer and laser dye solutions. We found that at high excitation intensities a featureless amplified spontaneous emission band transforms into a finely structured spectrum having features as narrow as 0.1 nm. Stimulated emission in this regime is directional and characterized by a linear laserlike intensity dependence. We speculate that this phenomenon is due to optical feedback caused by light scattering inside the inhomogeneous gain media

Laser action in conducting polymers

Journal ArticleWe discuss both cooperative radiation and stimulated emission and consider their role in spectral narrowing of luminescent conducting polymers. We argue that cooperative radiation is favored in films with poor optical confinement. On the other hand, directional stimulated emission can be observed in dilute solution and thin films with superior optical confinement. Spectral narrowing in this case can be achieved by increasing either the excitation length or excitation intensity. The optical gain and loss coefficients are measured. Narrow line (~1.5 cm-1) laser emission is observed in cylindrical microcavities formed by thin polymer films coated around glass fibers in the red and green spectral ranges. The cavity quality factors of these plastic lasers are mainly determined by self-absorption and estimated to be about 5000

Ultrafast spectroscopy of even-parity states in ∏-conjugated polymers

Journal ArticleRelaxation dynamics of even parity (Ag) states in poly(p-phenylene vinylene) derivatives are studied using a novel fsec transient spectroscopy, in which two different excitation pulses successively generate odd parity (1 1Bµ) excitons at 2.2 eV and then reexcite them to higher Ag states. For reexcitation energies hω, 1.1 eV the decay occurs in a nonemissive state identified as a polaron pair, showing that the Ag states above 3.3 eV mediate charge transfer