Three-dimensional motion detection of a 20-nm gold nanoparticle using twilight-field digital holography with coherence regulation.

In the twilight-field method for obtaining interference fringes with high contrast in in-line digital holography, only the intensity of the reference light is regulated to be close to the intensity of the object light, which is the ultra-weak scattered light from a nanoparticle, by using a low-frequency attenuation filter. Coherence of the light also strongly affects the contrast of the interference fringes. High coherence causes a lot of undesired coherent noise, which masks the fringes derived from the nanoparticles. Too-low coherence results in fringes with low contrast and a correspondingly low signal-to-noise ratio. Consequently, proper regulation of the coherence of the light source, in this study the spectral width, improves the minimum detectable size in holographic three-dimensional position measurement of nanoparticles. By using these methods, we were able to measure the position of a gold nanoparticle with a minimum diameter of 20 nm.