In Situ Accurate Analysis of Colloidal Nanoparticles via Four Wave Mixing
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In Situ Accurate Analysis of Colloidal Nanoparticles via Four Wave Mixing Jian Wu1, Dao Xiang1, Ching-Chung Hsueh2, Jörg Rottler2, Reuven Gordon1 1
University of Victoria, Canada University of British Columbia, Canada
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ABSTRACT
Four-wave mixing (FWM) is used to measure the vibrational modes of nanoparticles in solution. The vibrations give information about the particle size, material properties and shape. This method has been used for in-situ monitoring of the growth of nanoparticles with high accuracy, as confirmed by electron microscopy analysis. We observe a threshold in the FWM signal which we believe is from a cavity forming around the nanoparticles that reduces viscous damping. We have observed this effect in molecular dynamics simulations as well.
Introduction: Here we report a highly accurate method for the analysis of colloidal nanoparticles by means of four wave mixing (FWM). Other optical methods exist to analyse nanoparticles in solution, such as extinction and dynamic light scattering. Extinction is widely used in plasmonic nanoparticle analysis; however, it is not very sensitive to particle size, even though I t is quite sensitive to particle shape. For accurate sizing and shape characterization, usually transmission electron microscopy is used as alternative measure. Previously we developed an optical tweezer method to measure individual nanoparticles, including proteins and DNA [1,2]. This method interfered two lasers at the trapping site to create a beat signal with high frequency that excited the vibration modes of the trapped nanoparticle. The vibration resonance was measured indirectly via increased motion of the trapped nanoparticle. We developed the FWM technique to analyse many nanoparticles in solution, instead of individually.
Experimental approach In FWM, two laser beams are interfered with a slight frequency difference (in the 10 GHz – 10 THz range). The setup is based on an early degenerate FWM configuration [3]. This drives oscillations in the nanoparticles via electrostriction. When the oscillation frequency matches a natural vibration resonance of the nanoparticles, extremely strong four-wave mixing is observed by scattering of a third beam off of a dynamic grating induced by the electrostriction force. Presently, the method is not automated so a typical measurement takes 10-30 minutes per scan. It is anticipated with automation that the typical acquisition time will be 1 minute or less.
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Results The vibration resonances allow for accurate sizing and size distribution information. For example, 2 nm gold nanoparticles give a resonance at 1.5 THz. The resonance frequencies allow for precise determination of nanoparticle size and shape, as has been verified by electron microscopy measurements. We have also demonstrated that thi
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