Zhengguo Zhu

Sensitivity gains in chemosensing by lasing action in organic polymers

Societal needs for greater security require dramatic improvements in the sensitivity of chemical and biological sensors. To meet this challenge, increasing emphasis in analytical science has been directed towards materials and devices having highly nonlinear characteristics; semiconducting organic polymers (SOPs), with their facile excited state (exciton) transport, are prime examples of amplifying materials. SOPs have also been recognized as promising lasing materials, although the susceptibility of these materials to optical damage has thus far limited applications. Here we report that attenuated lasing in optically pumped SOP thin films displays a sensitivity to vapours of explosives more than 30 times higher than is observed from spontaneous emission. Critical to this achievement was the development of a transducing polymer with high thin-film quantum yield, a high optical damage threshold in ambient atmosphere and a record low lasing threshold. Trace vapours of the explosives 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (DNT) introduce non-radiative deactivation pathways that compete with stimulated emission. We demonstrate that the induced cessation of the lasing action, and associated sensitivity enhancement, is most pronounced when films are pumped at intensities near their lasing threshold. The combined gains from amplifying materials and lasing promise to deliver sensors that can detect explosives with unparalleled sensitivity.

How doping a cholesteric liquid crystal with polymeric dye improves an order parameter and makes possible low threshold lasing

Lasing conditions in a dye-doped cholesteric liquid crystal (ChLC) have been studied in view of optical modes for the light propagating in ChLCs using a polymeric dye with the transition dipole moment parallel to the local director of the ChLC host. We found that lasing always occurs at the lower-energy edge of the photonic gap. This is because that the optical eigen mode at the lower-energy gap is linearly polarized parallel to the director, while it is perpendicular at the higher-energy gap. Because of this well-defined lasing condition, low-threshold lasing was successfully achieved.

Advantages of Highly Ordered Polymer-Dyes for Lasing in Chiral Nematic Liquid Crystals

We have studied lasing in dye-doped chiral nematic liquid crystals (N*LCs). To demonstrate the advantages of using a polymer dye, that is highly aligned along the local director of N*LCs, over commercial small molecule dyes such as 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostryl)-4H-pyran (DCM), comparative studies of the fluorescence spectra, lasing conditions and order parameters have been made using polymer-dye doped N*LC and DCM-doped N*LC cells. Right- and left-circularly polarized fluorescence spectra for both dyes were accurately simulated by taking account of their density of modes and order parameters. These results indicate that the greater alignment afforded by the polymer dye in N*LCs provides ideal conditions for lasing.

Polarization characteristics of phase retardation defect mode lasing in polymeric cholesteric liquid crystals

Abstract We have studied the lasing characteristics of a dye-doped nematic layer sandwiched by two polymeric cholesteric liquid crystal (CLC) films as photonic band gap (PBG) materials. The nematic layer acts as a defect layer, the anisotropy of which brings about the following remarkable optical characteristics: (1) reflectance in the PBG region exceeds 50% due to the retardation effect, being unpredictable from a single CLC film; (2) efficient lasing occurs either at the defect mode wavelength or at the photonic band edge; and (3) the lasing emission due to both the defect mode and the photonic band edge mode contains both right- and left-circular polarizations, while the lasing emission from a dye-doped single CLC layer with a left-handed helix is left-circularly polarized.

Energy migration in conjugated polymers: the role of molecular structure

Conjugated polymers undergo facile exciton diffusion. Different molecular structures were examined to study the role of the excited state lifetimes and molecular conformations on energy transfer. There is a clear indication that extended fluorescence lifetimes give enhanced exciton diffusion as determined by fluorescence depolarization measurements. These results are consistent with a strong electronic coupling or Dexter-type energy transfer as the dominating mechanism. The control of polymer conformations in liquid crystal solvents was also examined and it was determined that more planar conformations gave enhanced energy transfer to emissive low band-gap endgroups.

Chemosensory lasing action for detection of TNT and other analytes

We demonstrate that attenuated luminescence and lasing in optically excited organic thin films is a sensitive probe to vapours of explosives, such as trinitrotoluene (TNT). The combined chemosensing gains from organic amplifying materials and the lasing action, promise to deliver sensors that can detect explosives with unparalleled sensitivity.

Lasing characteristics of a dye-doped nematic liquid crystal layer sandwiched by two polymeric cholesteric liquid crystals

We have studied the lasing characteristics from a dye-doped nematic layer sandwiched by two polymeric cholesteric liquid crystal (PCLC) films as photonic band gap materials. The nematic layer possessing birefringence brings about the following remarkable optical characteristics; (1) reflectance in the photonic band gap (PBG) region exceeds 50% due to the retardation effect, being unpredictable from a single CLC film, (2) efficient lasing occurs either at the notch of PBG or at the photonic band edge, (3) the lasing emisions contain both right- and left-circular polarizations, and (4) tunable lasing can be achieved by the reorientation of nematic liquid crystal molecule under the application of an electric field.