pH Sensor Fabricated from Single-Walled Carbon Nanotubes
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RESEARCH/RESEARCHERS
IR-to-UV Upconversion Occurs through Simultaneous Three-Photon Absorption in Ce3+:Lu2Si2O7 Single Crystals New materials for IR-to-UV frequencyupconversion luminescence are currently sought. Most research has focused on organic compounds, semiconductors, nanocrystals, and optical fibers performed with nanosecond- or picosecond-pulsed lasers. Recently, however, Q. Ren of Shanghai Jiao Tong University, China, and J. Xu and co-researchers at the Chinese Academy of Sciences used a femtosecondpulsed laser to demonstrate three-photonexcited violet upconversion luminescence in single crystals of Ce3+:Lu2Si2O7. As reported in the July 15 issue of Optics Letters (p. 2175), Ren, Xu, and coresearchers used an established technique to grow Ce3+:Lu2Si2O7 single crystals and then sliced 2-mm-thick disks for laser radiation and spectral measurements at room temperature. Focusing their 800-nm Ti:sapphire laser on the sample, the researchers easily observed with the naked eye strong violet emission, which suggested to them that violet upconversion fluorescence could be excited with an IR source without UV optics. The researchers said that the Ce3+ substitutes for Lu3+ in the Lu2Si2O7 host lattice and acts as the luminescence center. The emission spectra for optical excitation at 800 nm and at 267 (i.e., 800/3) nm were nearly identical. Furthermore, for both UV and IR excitation, the fluorescence yield ratios and corresponding spectral distributions were essentially the same, indicating to the researchers that emission occurs from the same state independent of the excitation wavelength. The researchers showed that the emission intensity depends on the cube of the laser power, thereby demonstrating a three-photon excitation process. Noting that there is no linear absorption at 800 nm, the researchers discounted a mechanism consisting of one-photon absorption followed by two-photon absorption. The researchers also precluded two-photon absorption followed by one-photon absorption because this requires an intermediate state corresponding to a 400-nm absorption, which was not observed. However, the researchers expect simultaneous three-photon absorption, because the three-photon energy of the 800-nm radiation falls into a strong UV absorption band. In addition, the researchers excluded the possibility of other upconversion mechanisms for rare-earthdoped materials, including energy transfer upconversion, excited state absorption, cooperative upconversion, and photon avalanche. The three-photon cross section MRS BULLETIN • VOLUME 31 • SEPTEMBER 2006
was determined to be 2.44 × 10–77 cm6 s2. The researchers said that their result “extends the application of Ce3+:Lu2Si2O7 single crystals and suggests opportunities for a three-photon process in frequencyupconversion lasers, optical communication, high-density three-dimensional optical data storage, display, IR quantum counters, and three-dimensional fluorescence imaging.” STEVEN TROHALAKI
pH Sensor Fabricated from Single-Walled Carbon Nanotubes Monitoring the pH of bio
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