Unidirectional laser emission from ∏-conjugated polymer microcavities with broken symmetry

Journal ArticleWe report unidirectional laser emission from ∏-conjugated polymer microcavities with broken symmetry geometries such as spiral and microdisk containing a "line defect," in comparison with plain microdisk cavity having isotropic emission. We found that the laser emission directionality contrast ratio is 8-10 and far field lateral divergence angle is 12°-15° for both broken symmetry geometries, with no significant increase in the laser threshold intensity. Fourier transform analysis of the laser emission spectra shows that unlike microdisks with line defect, the variation of light trajectories in the spiral microcavities leads to less defined laser modes

Ultralow threshold laser action from toroidal polymer microcavity

Cataloged from PDF version of article.We report laser action from a toroidal microcavity coated with pi-conjugated polymer. An ultralow threshold value of similar to 200 pJ/pulse is achieved by free space excitation in ambient conditions. This is the lowest threshold energy obtained in microtoroid lasers by free space excitation. The effective refractive index of the polymer, extracted from Fourier analysis of emission spectra, is 1.787, which is very close to measured value of 1.790 indicating that laser modes are located around the circumference of the cavity as whispering gallery resonances

Threshold excitation statistics and imaging of random lasers in ?-conjugated polymer films; evidence for random resonators

ManuscriptRandom lasing is laser action that consists of a sequence of narrow, coherent spectral lines observed when stimulated emission in a disordered gain medium is excited above threshold excitation intensity, Ith. We studied three Ith distribution functions, F(n)(Ith) in ?-conjugated polymer films, where n (= 1-3) is the number of lasing random resonators in the excited film area; including their evolution with respect to the excitation area size, and polymer mixture with various concentrations of TiO2 nanoparticles. Optical images of the excited film area support the Ith measurements; since the sharp laser lines in the emission spectrum are accompanied by the appearance of lasing random resonators in the picture. At Ith, where a single dominant random lasing resonator appears in the accompanying picture, we found good agreement between the cavity diameter extracted from the emission power Fourier transform analysis, and the cavity size in the picture. This was directly confirmed by a novel technique of space/spectrum cross-correlation of the laser emission lines with 10x10 ?m2 spatial resolution; where we show that the laser modes are indeed localized within the respective lasing resonator. We found that F(1)(Ith) has both a marked asymmetry with respect to the mean threshold, (Ith)ave, and decreases with the area size. We also found that upon increasing the TiO2 nanoparticles density, ? in the polymer/TiO2 mixture the light mean free path decreases, F(1)(Ith) broadens, and (Ith)ave linearly increases with ?, indicating that the addition of extrinsic scatterers to the polymer matrix is not beneficial to random lasing. Our results provide strong evidence for the model of random resonators in the gain medium for explaining random lasing in ?-conjugated polymer films

Laser action studies of φ-conjugated polymer microcavities

Unidirectional laser emission was observed from φ-conjugated polymer microcavities with spiral geometry, and from microdisks containing line defects. For both type of cavities directionality contrast of the laser emission was found to be ̃ 10, with far-field lateral divergence angle of ̃ 15°. No significant increase in the laser threshold intensity was observed compared with simple microdisk cavities. In addition we also studied the emission spectra of microring polymer lasers fabricated on nylon microfibers upon application of uniaxial stress with strain up to ̃ 12%. Substantial change in the laser emission spectrum was observed when stress was applied; showing enhanced optical sensitivity to stress larger than the changes induced in the emission spectrum caused by the microfiber diameter change. We explain the enhanced sensitivity to stress as due to the induced change in the polymer refractive index dispersion, in particular at the laser emission wavelength (̃ 635 nm). ©2008 IEEE

Studies of polymer microring lasers subject to uniaxial stress

The emission spectra of microring lasers fabricated from π -conjugated polymer films casted on nylon microfibers with diameters in the range of 35-90 μm were studied upon application of uniaxial stress with strain up to ∼12%. The laser emission spectra substantially change with the applied stress, showing enhanced sensitivity to stress over changes induced in the fiber diameter alone. This is explained as due to the induced change in the polymer refractive index spectrum upon stress, causing an unexpected increase in the refractive index dispersion and, consequently, also in the effective refraction index for lasing at emission wavelengths. © 2007 American Institute of Physics

Unidirectional laser emission from π-conjugated polymer microcavities with broken symmetry

We report unidirectional laser emission from π-conjugated polymer microcavities with broken symmetry geometries such as spiral and microdisk containing a "line defect," in comparison with plain microdisk cavity having isotropic emission. We found that the laser emission directionality contrast ratio is 8-10 and far field lateral divergence angle is 12°-15° for both broken symmetry geometries, with no significant increase in the laser threshold intensity. Fourier transform analysis of the laser emission spectra shows that unlike microdisks with line defect, the variation of light trajectories in the spiral microcavities leads to less defined laser modes. © 2007 American Institute of Physics

Generation of new frequencies in toroid microcavities

Microtoroid cavities with ultra high Q-factor have been fabricated using a combined process of photolithography and reflow technique for observing non-linear effects such as generation of new frequency components. For this purpose SiO2 material was used to form the toroidal cavity shape, and chalcogenide material of As2S3 maintaining very high nonlinear refractive index was thermally evaporated on top with varying thicknesses. Simulation results of the fabricated structure have exhibited new frequency components around the excitation window of 1520 nm produced by non-linear interactions. ©2008 IEEE

Solid-state emissive BODIPY dyes with bulky substituents as spacers

Bright fluorescence of the BODIPY dyes, just like most other fluorophores, is quenched in the solid state due to reabsorption and self-quenching. However, introduction of bulky ferf-butyl substituents on the meso-phenyl groups result in more spaced packing in the solid state, resulting in highly luminescent powders and films. © 2009 American Chemical Society

Impact of viral replication inhibition by entecavir on peripheral T lymphocyte subpopulations in chronic hepatitis B patients

<p>Abstract</p> <p>Background</p> <p>To investigate dynamic fluctuations of serum viral load and peripheral T-lymphocyte subpopulations of chronic hepatitis B patients and their correlation during entecavir therapy.</p> <p>Methods</p> <p>Fifty-five patients received entecavir 0.5 mg/d therapy. Serum HBV DNA load was measured by Real-Time-PCR, and the levels of peripheral T-lymphocyte subpopulations by flow cytometry biweekly, every four weeks and every eight weeks during weeks 1–12, 13–24 and 24–48, respectively. Multilevel modelling was used to analyse the relationship between these variables.</p> <p>Results</p> <p>Of the 55 patients, all HBeAg positive and with detectable HBV DNA, the majority (81.8%) had serum levels of HBV DNA over 10⁷copies per milliliter. HBV viral load dropped sharply during the first two weeks. In 28 and 43 patients, the level became undetectable from week 24 and 48, respectively. Using pre-therapy level as the reference, a significant decrease in CD8⁺T cells and increase in CD4⁺T cells were found from week 12. Both parameters and CD4⁺/CD8⁺ratio steadily improved throughout the 48 weeks. Multilevel analyses showed that the level of decrement of HBV DNA was associated with the increment of T-lymphocyte activities only in the later period (4–48 week). After 4 weeks of therapy, for each log₁₀scale decrement of HBV DNA, the percentage of CD4⁺lymphocyte was increased by 0.49 and that of CD8⁺decreased by 0.51.</p> <p>Conclusion</p> <p>T-lymphocyte subpopulations could be restored partially by entecavir treatment in patients with chronic hepatitis B concurrently with reduction of viremia.</p

Assessment of the requisites of microbiology based infectious disease training under the pressure of consultation needs

<p>Abstract</p> <p>Background</p> <p>Training of infectious disease (ID) specialists is structured on classical clinical microbiology training in Turkey and ID specialists work as clinical microbiologists at the same time. Hence, this study aimed to determine the clinical skills and knowledge required by clinical microbiologists.</p> <p>Methods</p> <p>A cross-sectional study was carried out between June 1, 2010 and September 15, 2010 in 32 ID departments in Turkey. Only patients hospitalized and followed up in the ID departments between January-June 2010 who required consultation with other disciplines were included.</p> <p>Results</p> <p>A total of 605 patients undergoing 1343 consultations were included, with pulmonology, neurology, cardiology, gastroenterology, nephrology, dermatology, haematology, and endocrinology being the most frequent consultation specialties. The consultation patterns were quite similar and were not affected by either the nature of infections or the critical clinical status of ID patients.</p> <p>Conclusions</p> <p>The results of our study show that certain internal medicine subdisciplines such as pulmonology, neurology and dermatology appear to be the principal clinical requisites in the training of ID specialists, rather than internal medicine as a whole.</p