Digital in-line holography for biological applications

Scheme to improve the reconstructed image in parallel quasi-phase-shifting digital holography

We propose a scheme to improve the reconstructed image in parallel quasi-phase-shifting digital holography. Parallel quasi-phase-shifting digital holography is a technique capable of noiseless instantaneous measurement of three-dimensional objects, and it implements four kinds of phase shifting at a time with an array of 2 x 2 phase-shifting devices located in the reference wave. In the phase-shifting calculation in the reconstruction process of the technique, the scheme assigns the 2 x 2 cell configuration for each pixel in the vertical direction and for each 1-pixel interval in the horizontal direction of the hologram recorded by the image sensor. We conduct both a numerical simulation and a preliminary experiment. The results show that the proposed scheme can improve the quality of the reconstructed image calculated by the conventional scheme of parallel quasi-phase-shifting digital holography we previously proposed, and then the effectiveness of the proposed scheme is verified.

Digital holographic microscopy with dual-wavelength phase unwrapping

We apply the techniques of digital holography to obtain microscopic three-dimensional images of biological cells. The optical system is capable of microscopic holography with diffraction-limited resolution by projecting a magnified image of a microscopic hologram plane onto a CCD plane. Two-wavelength phase-imaging digital holography is applied to produce unwrapped phase images of biological cells. The method of three-wavelength phase imaging is proposed to extend the axial range and reduce the effect of phase noise. These results demonstrate the effectiveness of digital holography in high-resolution biological microscopy.

Lensless imaging of magnetic nanostructures by X-ray spectro-holography

Our knowledge of the structure of matter is largely based on X-ray diffraction studies of periodic structures and the successful transformation (inversion) of the diffraction patterns into real-space atomic maps. But the determination of non-periodic nanoscale structures by X-rays is much more difficult. Inversion of the measured diffuse X-ray intensity patterns suffers from the intrinsic loss of phase information, and direct imaging methods are limited in resolution by the available X-ray optics. Here we demonstrate a versatile technique for imaging nanostructures, based on the use of resonantly tuned soft X-rays for scattering contrast and the direct Fourier inversion of a holographically formed interference pattern. Our implementation places the sample behind a lithographically manufactured mask with a micrometre-sized sample aperture and a nanometre-sized hole that defines a reference beam. As an example, we have used the resonant X-ray magnetic circular dichroism effect to image the random magnetic domain structure in a Co/Pt multilayer film with a spatial resolution of 50 nm. Our technique, which is a form of Fourier transform holography, is transferable to a wide variety of specimens, appears scalable to diffraction-limited resolution, and is well suited for ultrafast single-shot imaging with coherent X-ray free-electron laser sources.

Iterative reconstruction of in-line electron holograms

In low energy electron point source (LEEPS) microscopy, electrons emerge from a point source, propagate as spherical waves, and arrive at a screen. Some electrons scatter off an object, i.e. a cluster of atoms, placed between the source and the screen; others arrive at the screen without scattering. The interference pattern on the screen, an electron hologram, is used to reconstruct the object by calculating and analyzing a function K(r) in the region occupied by the object. We present an iterative method that uses the original reconstruction K(o)(r) to determine the atomic configuration of the object. No knowledge of the object, except for the vicinity in which the object is located, is used in the iterative scheme. In particular, no knowledge of the atomic structure is used. The method uses K(o)(r) to make a test object that in turn gives another reconstruction K(1)(r); K(1)(r) and K(o)(r) are used to modify the test object and obtain K(2)(r). The iteration is repeated until it converges on a final object that gives a reconstruction K(f)(r) that is very similar to K(o)(r). The final object gives an atomic structure that is close to the atomic structure of the original object. Results for several idealized objects are presented and discussed.

Tomographic reconstruction of multiple in-line electron holograms of realistic objects

In theoretical low energy electron point source microscopy, simulated holograms are made and used to reconstruct atomic clusters. In previous investigations, simple test clusters were used for convenience. In this paper we explore more realistic structures composed of a single type of atom such as diamond, graphite and Buckminsterfullerene--all of which consist of carbon atoms. We also examine clusters with two or more distinct atom types, including TiO(2), CuO, Pt(3)O(4), Ga(3)Pt(5), WC, MgCO(3), and MgO. Reconstructions from a single hologram do not give atomic resolution, and consist instead of 'spurious peaks' that are not located at atomic sites, as well as 'atomic peaks' close to atomic sites. Here we apply methods we developed previously to remove all spurious peaks. We show final reconstructions that consist only of atomic peaks located very close to atomic sites.

The three-dimensional flow field generated by a feeding calanoid copepod measured using digital holography

Digital in-line holography is used for measuring the three-dimensional (3-D) trajectory of a free-swimming freshwater copepod Diaptomus minutus, and simultaneously the instantaneous 3-D velocity field around this copepod. The optical setup consists of a collimated He-Ne laser illuminating a sample volume seeded with particles and containing several feeding copepods. A time series of holograms is recorded at 15 Hz using a lensless 2Kx2K digital camera. Inclined mirrors on the walls of the sample volume enable simultaneous recording of two perpendicular views on the same frame. Numerical reconstruction and matching of views determine the 3-D trajectories of a copepod and the tracer particles to within pixel accuracy (7.4 microm). The velocity field and trajectories of particles entrained by the copepod have a recirculating pattern in the copepod's frame of reference. This pattern is caused by the copepod sinking at a rate that is lower than its terminal sinking speed, due to the propulsive force generated by its feeding current. Consequently, the copepod sees the same fluid, requiring it to hop periodically to scan different fluid for food. Using Stokeslets to model the velocity field induced by a point force, the measured velocity distributions enable us to estimate the excess weight of the copepod (7.2x10(-9) N), its excess density (6.7 kg m(-3)) and the propulsive force generated by its feeding appendages (1.8x10(-8) N).

Tracking particles in four dimensions with in-line holographic microscopy

We describe a simple holographic method that has enabled us to capture as a single data set the trajectories of micrometer-sized objects suspended in water. By subtracting consecutive holograms of a particle suspension and then adding these difference holograms, we constructed a final data set that contains the time evolution of the particle trajectories free from spurious background interference effects. The method is illustrated by a recording of the motion of 5-10-microm diameter algae in water.

Digital in-line holography of microspheres

We have used digital in-line holography (DIH) with numerical reconstruction to image micrometer-sized latex spheres as well as ferrimagnetic beads suspended in gelatin. We have examined in detail theoretically and experimentally the conditions necessary to achieve submicrometer resolution of holographic reconstructions. We found that both transparent and opaque particles could be imaged with a resolution that was limited only by the wavelength of the light used. Simple inspection of intensity profiles through a particle allowed an estimate to be made of the particle's three position coordinates within an accuracy of a few hundred nanometers. When the derivative of a second-order polynomial fitted to the intensity profiles was taken, the X, Y, Z position coordinates of particles could be determined within +/-50 nm. More-accurate positional resolution should be possible with the help of more-advanced computer averaging techniques. Because a single hologram can give information about a large collection of distributed particles, DIH offers the prospect of a powerful new tool for three-dimensional tracking of particles.