Three-dimensional motion detection of a sub-50 nm gold nanoparticle

Abstract

We demonstrated three-dimensional position measurements of a gold nanoparticle using an in-line low-coherence digital holographic microscope with twilight-field method (TFM) and a regulation of temporal coherence of light source. An intensity of scattering light from a nanoparticle is very weak, and therefore, in order to get interference fringes with higher contrast, the TFM regulates only an intensity of the reference light to be close to an intensity of the object light using a low-frequency attenuation filter in the in-line digital holographic microscope. Coherence of light relates to get interference fringes with higher contrast on a wide aria. High coherence makes a lot of undesired coherent noises, therefore the fringes derived from nanoparticles is masked by them. Too low coherence makes fringes with low contrast and corresponding low signal-to-noise ratio. Consequently an adequate regulation of the coherence of light source gives the best performance in the position measurements. Uses of these methods allowed us to reach the minimum diameter of 20 nm in the position measurement of a gold nanoparticle.