No abstract

A U-shape probe for microwave resonance in plasma was investigated in a range of high pressure (10-760 Torr). The electromagnetic field finite difference time domain-simulation of the probe revealed that, in the resistive resonance at high pressure, the resonant frequency f r approaches the dipole-antenna resonance frequency in vacuum, with almost no dependence on the electron density n e . However, the resonance width f is sensitive to the pressure, showing the maximum value at ∼20 Torr. The normalized resonance width ( f/f r ) corrected by the one of vacuum value (no plasma) is linearly proportional to n e , and inversely proportional to ν m (electron-molecule collision frequency) for high-pressure plasmas. These simulation results are successfully explained in a model of two-conductor transmission line in cold collisional plasma. According to the resonance analysis, the methods for measuring the electron density and the electron collision frequency at high pressures (ν 2 m ω 2 , ω 2 p ) were proposed.

This paper reports state-of-the-art fluorene-based yellow-green conjugated polymer blend gain media using Förster resonant-energy-transfer from novel blue-emitting hosts to yield low threshold (≤7 kW cm −2 ) lasers operating between 540 and 590 nm. For poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) (15 wt%) blended with the newly synthesized 3,6-bis(2,7-di([1,1′-biphenyl]-4-yl)-9-phenyl-9H-fluoren-9-yl)-9-octyl-9H-carbazole (DBPhFCz) a highly desirable more than four times increase (relative to F8BT) in net optical gain to 90 cm −1 and 34 times reduction in amplified spontaneous emission threshold to 3 µJ cm −2 is achieved. Detailed transient absorption studies confirm effective exciton confinement with consequent diffusionlimited polaron-pair generation for DBPhFCz. This delays formation of host photoinduced absorption long enough to enable build-up of the spectrally overlapped, guest optical gain, and resolves a longstanding issue for conjugated polymer photonics. The comprehensive study further establishes that limiting host conjugation length is a key factor therein, with 9,9-dialkylfluorene trimers also suitable hosts for F8BT but not pentamers, heptamers, or polymers. It is additionally demonstrated that the host highest occupied and lowest unoccupied molecular orbitals can be tuned independently from the guest gain properties. This provides the tantalizing prospect of enhanced electron and hole injection and transport without endangering efficient optical gain; a scenario of great interest for electrically pumped amplifiers and lasers.

H-shaped oligofluorenes as gain media exhibit excellent photo- (large robustness against oxidation) and thermal stabilities in ambient atmosphere for large σe and low-threshold (0.22 nJ pulse(-1) ) deep blue distributed feedback (DFB) lasers. Their amplified spontaneous emission (ASE) thresholds increase less than 3-fold and the emission spectra exhibit almost no shift with film samples annealed up to 200 °C in open air.