J. Gass

Phase imaging without 2π ambiguity by multiwavelength digital holography

We present a phase-imaging method with an axial range that can in principle be arbitrarily large compared to the wavelength and does not involve the usual phase unwrapping by detection of phase discontinuity. The method consists of the generation and combination of two phase maps in a digital holography system by use of two separate wavelengths. For example, we reconstructed the surface of a spherical mirror with approximately 10-nm axial resolution and an axial range of approximately 3 microm.

Magnetization and magnetocaloric effect in ball-milled zinc ferrite powder

Enhancement of magnetocaloric effect (MCE) in nanostructured materials is important for refrigeration applications such as spot cooling in microelectromechanical system devices. Here we report the first investigation of MCE properties in ball-milled ZnFe2O4 particles. The MCE was obtained by measuring a family of M-H curves at set temperature intervals and calculating the entropy change (ΔS) for this system using the Maxwell relation. The surface structure of zinc ferrite particles is sensitive to ball milling conditions and we observed that these surface effects greatly impact the MCE and our observations could provide a route for its potential enhancement by controlled surface modification.

Microscopic three-dimensional imaging by digital interference holography

Digital interference holography (DIH) is being developed as a novel method of microscopic 3-D imaging. A number of holo- graphic fields are numerically calculated from optically recorded ho- lograms using a range of wavelengths. Numerical superposition of these fields results in a tomographic representation of the object with narrow axial resolution. We demonstrate the practicality of DIH in microscopic imaging with a set of preliminary experiments, yield- ing approximately 3 mm of lateral resolution and 10 mm of axial resolution, using a simple optical setup and straightforward numeri- cal algorithms.

Large Magnetocaloric Effect, Moment, and Coercivity Enhancement after Coating Ni Nanoparticles with Ag

We observe a large magnetocaloric effect in monodisperse Ni and Ni(core)Ag(shell) nanoparticles in the superparamagnetic region. The organically passivated Ni nanospheres show a large magnetic entropy change of 0.9 J kg(-1)  K for a 3 T magnetic field change. In comparison to the surfactant-coated Ni nanoparticles, the Ni(core)Ag(shell) nanoparticles show an enhanced coercivity, magnetization, and magnetocaloric effect (1.3 kg K for a 3 T magnetic field change). The coercivity at 10 K increases from 360 Oe for Ni nanoparticles to nearly 610 Oe for Ni(core)Ag(shell) particles. This large enhancement is attributed to the enhanced inter-particle interaction, which is mediated by the metallic shell, over the relatively weaker dipolar interaction in the surfactant-coated Ni nanoparticles, and to modification of the surface spin structure.