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Development of fluorescence near-field optics thermal nanoscopy using photonic crystal fiber for high sensitivity temperature measurement


T. Fujii1, Y. Taguchi1, T. Saiki1 and Y. Nagasaka1

1Keio University, Japan

Keywords: near-field optics, thermometry
property: fluorescence lifetime
material: photonic crystal fiber, quantum dot

We have developed a novel nanoscale temperature measurement method using fluorescence in near-field, namely Fluor-NOTN (Fluorescence Near-filed Optics Thermal Nanoscopy)[1]. Fluor-NOTN enables nanoscale materials to measure temperature distribution in vivo / in situ.

In order to achieve a nanoscale spatial resolution, a near-field light generated in a proximity region of a small aperture was utilized. In Fluor-NOTN, the excitation laser diode (wavelength: 473 nm) coupled into the Au-coated near-field optical fiber probe, and then the near-field light is generated at the small aperture of probe tip. For the temperature measurement, fluorophores (Cd/Se quantum dots, Qdot) are modified on the sample surface. Fluorophores are excited to some higher vibrational level by the near-field light, and then the vibrational level of fluorophore relaxes to the ground state in average time of 10-8 second (fluorescence lifetime) with the emission of the light (wavelength: 655 / 705 nm)[2]. Fluor-NOTN can measure temperature by detecting fluorescence lifetime because the fluorescence lifetime is dependent on the sample temperature. Thus Fluor-NOTN has the capability of nanoscale temperature measurement by detecting fluorescence in near-field.

For the high sensitivity temperature measurement, an auto-fluorescence emitted from an optical fiber should be decreased. Auto-fluorescence is emitted from the dopant of either fiber core or cladding of the near-field optical fiber probe, and this is considered to be the most critical noise in Fluor-NOTN. In order to decrease the noise, we have fabricated a novel near-field optical fiber probe by fusion splicing photonic crystal fiber (PCF) and single-mode fiber (SMF). PCF has the pure-silica core and the air-cladding as a substitute for doped-silica cladding; therefore PCF emits extremely low auto-fluorescence compared to conventional optical fibers. The validity of the novel fiber probe is experimentally assessed by evaluating auto-fluorescence spectrum of PCF and fabrication parameters. Due to the decrease of auto-fluorescence, fivefold increase of S/N in the near-field fluorescence detection is achieved by newly fabricated near-field fiber probe.

References
  1. Y. Taguchi, T. Oka, T. Saiki, Y. Nagasaka, Nanosc Microsc Therm (2009) doi: 10.1080/15567260902820821

  2. J.R. Lakowicz, Principles of Fluorescence Spectroscopy, 2nd edn. (Kluwer Academic / Plenum Publishers, New York, 1999), pp. 1-22

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