Aimee Rose

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.

All-in-Fiber Chemical Sensing

A new all-in-fiber trace-level chemical sensing approach is demonstrated. Photoconductive structures, embedded directly into the fiber cladding along its entire length, capture light emitted anywhere within the fibers hollow core and transform it directly into an electrical signal. Localized signal transduction circumvents problems associated with conventional fiber-optics, including limited signal collection efficiency and optical losses. This approach facilitates a new platform for remote and distributed photosensing.

Enhanced chemiluminescent detection scheme for trace vapor sensing in pneumatically-tuned hollow core photonic bandgap fibers

We demonstrate an in-fiber gas phase chemical detection architecture in which a chemiluminescent (CL) reaction is spatially and spectrally matched to the core modes of hollow photonic bandgap (PBG) fibers in order to enhance detection efficiency. A peroxide-sensitive CL material is annularly shaped and centered within the fibers hollow core, thereby increasing the overlap between the emission intensity and the intensity distribution of the low-loss fiber modes. This configuration improves the sensitivity by 0.9 dB/cm compared to coating the material directly on the inner fiber surface, where coupling to both higher loss core modes and cladding modes is enhanced. By integrating the former configuration with a custom-built optofluidic system designed for concomitant controlled vapor delivery and emission measurement, we achieve a limit-of-detection of 100 parts per billion (ppb) for hydrogen peroxide vapor. The PBG fibers are produced by a new fabrication method whereby external gas pressure is used as a control knob to actively tune the transmission bandgaps through the entire visible range during the thermal drawing process.

Postdeposition reduction of noble metal doped ZnO films

Insulating ZnO (wurtzite phase) films containing 1% to 2% Pt, Au, Pd, Ga, or Al dopants have been deposited onto silica, silicon, and aluminum substrates under oxidizing conditions. Substantial enhancement in film conductivity is promoted by postdeposition reduction in hydrogen above a critical temperature or by room temperature cathodic reduction in an electrochemical cell. Film reduction requires the presence of atomic hydrogen, formed at the film surface either by dissociative adsorption of gaseous H2 or its electrochemical generation from a buffered aqueous solution. As deposited and reduced films have been characterized using x-ray photoemission spectroscopy, Raman spectroscopy, optical transmission measurements, spectroscopic ellipsometry, voltammetry, chronopotentiometry, and four-point conductivity measurements. Results indicate that film deposition parameters and postdeposition reduction alter the oxidation states of both the zinc and the resident dopant cations. The zinc reduction reaction appea...

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.

Localized deposition of zinc oxide films by automated fluid dispensing method

Abstract Optically clear Au- and Ga-doped zinc oxide films have been locally deposited from solution precursors on silicon and silica substrates as micrometer size dots using an automated fluid dispensing method. Dot dimensions have been shown to be strongly dependent on the solution composition and substrate temperature. Solution-dispensed films have been characterized by means of optical transmission and reflectance spectroscopy, spectroscopic ellipsometry, and profilometry measurements. Trivalent Au- and Ga-doped zinc oxide features about 500 μm in diameter were deposited on chemically-sensitive field-effect transistors (CHEMFET). Contrasting behavior in measured work function and film resistance was found for Au and Ga doped films upon exposure to hydrogen and ammonia.

Optimization of TNT sensory polymers

Our group has been involved in the design and synthesis of ultra-sensitive fluorescence sensory materials for the detection of 2,4,6 trinitrotoluene (TNT) and 2,4 dinitrotoluene (DNT). These schemes make use of a novel energy migration mechanisms to amplify the fluorescence response and have led to systems capable of rapid detection of these analytes at sub part-per-billion levels. In an effort to optimize the amplification and specificity, we have examined the nature of energy migration in our polymers systems because it is inherent in achieving amplification. polarization measurements and energy transfer studies between polymers were conducted in order to evaluate and maximize energy migration and hence TNT sensory response. The correlation of photo physical properties with molecular structure guided the synthesis of novel polymers with more discriminant optical responses. These synthetic efforts have yielded a library of sensory polymers with varying sensitivities to different analytes.

Pulsed laser irridation of isothermally heated titania films

Stoichiometric titanium dioxide films comprised of either amorphous or anatase phases have been deposited onto cleaned silica substrates by means of a low pH sol-gel processing method. Coated optics were then exposed to pulsed 1064 nm light from a Nd:YAG laser. Irradiations were performed on samples heated in an oven held at temperatures between 300 and 450 K. A marked decrease in the threshold for catastrophic damage was observed at increased film temperature for both Q-switched and non-Q-switched pulses which is likely due to a thermally induced increase in film residual stress. However, continued irradiation of damaged regions with non-Q-switched pulses at slightly higher temperatures was found to heal the damaged regions. Subsequent irradiation at somewhat higher temperatures generated additional damage sites which could not be healed upon additional irradiation at still higher temperatures. Films were characterized by means of optical transmission measurements, Raman spectroscopy, and optical microscopy. Results demonstrate the influence of ambient film temperature on irradiation damage phenomena.