Erich M. See

A Fiber Bragg Grating Temperature Sensor for 2–400 K

We demonstrate fiber optic, multiplexible temperature sensing using a fiber Bragg grating (FBG) with an operational range of 2-400 K, and a temperature resolution better than 10 mK for temperatures <;12 K. This represents a significant reduction in the lowest usable temperature as well as a significant increase in sensitivity at cryogenic temperatures compared with previously reported multiplexible solutions. This is accomplished by mounting the section of the fiber with a FBG on a polytetrafluoroethylene coupon, which has a non-negligible coefficient of thermal expansion down to <;4 K. The sensors exhibit a good stability over multiple temperature cycles and acceptable sensor-to-sensor repeatability. Possible applications for this sensor include distributed temperature sensing across superconducting elements and cryogenic temperature measurements in environments where electrical measurements are impractical or unsafe.

Enhanced Multiphoton-Induced Luminescence in Silver Nanoparticles Fabricated with Nanosphere Lithography

We study multiphoton-absorption-induced luminescence (MAIL) in triangular silver nanoprisms fabricated with nanosphere lithography. We observe strong MAIL when the exciting infrared fs laser pulses overlap the main surface plasmon resonance of the particles, with significantly less signal from off-resonance structures present on the same substrate. The MAIL signal partially bleaches during imaging, but at higher illumination intensities, certain particles undergo a sudden multiplefold increase in MAIL brightness, which we show is associated with the deformation or melting of the particles into spheroids. The brightening may be due to the reduction of surface silver oxide into strongly luminescent silver nanoclusters or to the formation or activation of emission centers on the particle surface triggered by the melting of the particle. Regardless of the exact nature of the emission, silver oxide plays an important role in the process, evidenced by a twofold or threefold increase in MAIL luminosity from the triangular particles when their surface oxide is chemically reduced during imaging.

Two-photon activation of o-nitrobenzyl ligands bound to gold surfaces

We report on the use of two-photon absorption to photocleavage o-nitrobenzyl-based ligands bound to a gold surface with thiol groups. Ablation of ligands occurs at high power densities, but this can be largely avoided by reducing the optical power level, at which point two-photon mediated reactions still occur on a time scale of tens of seconds. This means that photoactive ligands can be activated at wavelengths where plasmon resonances in gold and silver nanoparticles can easily be achieved, which will allow the surface properties at the hot spots on plasmonic nanostructures to be chosen differently from the rest of the structure, with possible applications in high-efficiency Surface-Enhanced Raman Spectroscopy and bottom-up nanoassembly.