Low-coherence Digital Holographic Microscope Research Articles

Holographic three-dimensional motion detection of an optically trapped sub-100 nm gold nanoparticle

We demonstrated, for the first time, three-dimensional (3D) motion detection of a gold nanoparticle held in optical tweezers in water using holographic microscopy. Motion detection was performed with an in-line, low-coherence digital holographic microscope. The nanoparticle diameter was 60nm, which is the smallest reported particle whose 3D motion can be measured. The motion of the optically-trapped nanoparticle had an axial variation of 7.6nm (standard deviation) when a 1070nm laser beam with an intensity of more than 27MW/cm2 was focused with a 1.25 NA objective lens. From a comparison of a gold nanoparticle fixed on a glass substrate and a gold nanoparticle trapped by optical tweezers with a sufficient light intensity, we found that most of the variation was caused by noise in the experimental setup, and not the motion itself. The lateral variations for the fixed and trapped nanoparticles were the same, but the axial variation for the trapped nanoparticle was slightly larger. This suggested that the optically trapped nanoparticle had a motion with a single-nanometer order in the standard deviation only along the axial direction when the laser intensity was sufficiently large for trapping, which in this experiment was more than 27MW/cm2.

Three-dimensional subpixel estimation in holographic position measurement of an optically trapped nanoparticle

We propose three-dimensional (3D) subpixel estimation in the position measurement of a nanoparticle held in optical tweezers in water by using an in-line, low-coherence digital holographic microscope. The 3D subpixel estimation was performed with the addition of axial subpixel estimation to the lateral subpixel estimation introduced in our previous work [Appl. Opt.50, H183 (2011)]. The axial subpixel estimation allowed the step length in the diffraction calculation of a hologram to be increased to ~20 nm while keeping the axial resolution of ~3 nm. This drastically decreased the computation time of the diffraction calculation to less than 10% of the two-dimensional subpixel estimation.

Dual wavelength full field imaging in low coherence digital holographic microscopy

A diffractive optical element (DOE) is presented to simultaneously manipulate the coherence plane tilt of a beam containing a plurality of discrete wavelengths. The DOE is inserted into the reference arm of an off-axis dual wavelength low coherence digital holographic microscope (DHM) to provide a coherence plane tilt so that interference with the object beam generates fringes over the full detector area. The DOE maintains the propagation direction of the reference beam and thus it can be inserted in-line in existing DHM set-ups. We demonstrate full field imaging in a reflection commercial DHM with two wavelengths, 685 nm and 794 nm, resulting in an unambiguous range of 2.494 micrometers.