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Proceedings Paper

Force spectroscopy and two photon excited luminescence in an optical tweezers system
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Paper Abstract

Up to now optical spectroscopies have analyzed the scattered light or the heat generated by absorption as a function of the wavelength to get information about the samples. Among the light matter interaction phenomena one that has almost never been used for spectroscopy is the direct photon momenta transfer. Probably because the forces involved are very small, varying from hundreds of femto to tens of pico Newtons. However, the nowadays very popular Optical Tweezers can easily accomplish the task to measure the photon momenta transfer and may be the basis for the Optical Force Spectroscopy. We demonstrate its potential as such a tool by observing more than eight Mie resonance peaks of a single polystyrene microsphere, and showed the capability to selective couple the light to either the TE, TM or both microsphere modes depending of the beam size, the light polarization and the beam positioning. The Mie resonances can change the optical force values by 30-50%. Our results also clearly show how the beam polarization breaks the usually assumed azimuthal symmetry by Optical Tweezers theories. We also obtained the spectrum from the two photon excited luminescence using the Optical Tweezers to hold a single bead suspended and a femtosecond Ti:sapphire laser for the non-linear excitation. This spectrum shows the pair of peaks due to both TE and TM spherical cavity modes. We have been able to observe more than 14 Mie resonance peaks in the TPE luminescence. Our results are in good agreement with optical force calculations using Maxwell stress tensor and partial wave decomposition of the incident beam approximated to a 3th order gaussian beam.

Paper Details

Date Published: 18 August 2005
PDF: 7 pages
Proc. SPIE 5930, Optical Trapping and Optical Micromanipulation II, 59300C (18 August 2005); doi: 10.1117/12.620744
Show Author Affiliations
Adriana Fontes, UNICAMP, Campinas (Brazil)
Antonio Alvaro Ranha Neves, UNICAMP, Campinas (Brazil)
Wendel Lopes Moreira, UNICAMP, Campinas (Brazil)
Andre Alexandre de Thomaz, UNICAMP, Campinas (Brazil)
Luiz Carlos Barbosa, UNICAMP, Campinas (Brazil)
Ana Maria de Paula, Univ. of Southampton (United Kingdom)
Katsuhiro Ajito, NTT Corp. (Japan)
Carlos Lenz Cesar, UNICAMP, Campinas (Brazil)

Published in SPIE Proceedings Vol. 5930:
Optical Trapping and Optical Micromanipulation II
Kishan Dholakia; Gabriel C. Spalding, Editor(s)

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