School of Physics - Research Publications
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ItemThree-dimensional imaging of microstructure in Au nanocrystals.(American Physical Society (APS), 2003-05-02)X-ray diffraction using a coherent beam involves the mutual interference among all the extremities of small crystals. The continuous diffraction pattern so produced can be phased because it can be oversampled. We have thus obtained three-dimensional images of the interiors of Au nanocrystals that show 50 nm wide bands of contrast with [111] orientation that probably arise from internal twinning by dynamic recrystallization during their formation at high temperature.
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ItemBlue five-level frequency-up conversion system in rubidium(OPTICAL SOC AMER, 2006-04-01)We demonstrate production of continuous coherent blue laser light by using a five-level system in rubidium vapor. Two low-power lasers, at 780 and 776 nm, induce strong atomic coherence in the 5S-5P-5D states. The atoms decay to the 6P excited state, from which stimulated emission produces a coherent blue (420 nm) beam. We have coupled both ground-state hyperfine levels, effecting coherence between four levels. The coherent blue output is enhanced by several mechanisms, including stronger coupling to a larger fraction of the atomic population, operation at a detuning such that the vapor is nominally transparent to the 780 nm pump field, reduced losses owing to optical pumping, and optimal phase matching. We report experimental findings and compare them with results from a semiclassical Maxwell-Bloch model.
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ItemMotion Workshop: tracking motion in an on-line environment(Apple University Consortium Academic and Developers Conference, 2001)
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ItemPrecision measurement of the electromagnetic fields in the focal region of a high-numerical-aperture lens using a tapered fiber probe(OPTICAL SOC AMER, 2002-08-01)We present a measurement of the intensity around the focus of a N.A.-0.95 lens using a tapered optical fiber probe. An asymmetry introduced by the vector nature of the incident polarized light is evident, although it is inconsistent with that predicted theoretically by considering the magnitude squared of the electric field. The sensitivity of the probe to different components of the electromagnetic field is considered, and it is shown that the measurement is consistent with vector diffraction theory when the probe properties are taken into account.
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ItemFabrication of long-period fiber gratings by use of focused ion-beam irradiation(OPTICAL SOC AMER, 2001-06-01)Long-period gratings have been made in nonphotosensitive optical fibers by irradiation of the core of a fiber with a focused beam of high-energy protons. The irradiated fibers exhibit relatively low loss, even before thermal annealing, and possess strongly wavelength-dependent transmission. The absence of a mask provides the opportunity to tailor the grating to a desired profile, and a variety of grating profiles were explored. The profile most resembling a sinusoid was found to produce the cleanest transmission spectra.
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ItemUnique phase recovery for nonperiodic objects(AMERICAN PHYSICAL SOC, 2003-11-14)It is well known that the loss of phase information at detection means that a diffraction pattern may be consistent with a multitude of physically different structures. This Letter shows that it is possible to perform unique structural determination in the absence of a priori information using x-ray fields with phase curvature. We argue that significant phase curvature is already available using modern x-ray optics and we demonstrate an algorithm that allows the phase to be recovered uniquely and reliably.
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ItemRefractive index profiling of axially symmetric optical fibers: a new technique(OPTICAL SOC AMER, 2005-05-02)We present a new technique for determining the refractive index profiles of axially symmetric optical fibers based on imaging phase gradients introduced into a transmitted optical field by a fiber sample. An image of the phase gradients within the field is obtained using a new non-interferometric technique based on bright field microscopy. This provides sufficient information to reconstruct the refractive index profile using the inverse Abel transform. The technique is robust, rapid and possesses high spatial resolution and we demonstrate its application to the reconstruction of the refractive index profiles of a single-mode and a multimode optical fiber.
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ItemNondestructive imaging of a type I optical fiber Bragg grating(OPTICAL SOC AMER, 2003-05-15)Nondestructive images of refractive-index variation within a type I fiber Bragg grating have been recorded by the differential interference contrast imaging technique. The images reveal detailed structure within the fiber core that is consistent with the formation of Talbot planes in the diffraction pattern behind the phase mask that had been used to fabricate the grating.
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ItemX-ray phase contrast tomography with a bending magnet source(AMER INST PHYSICS, 2005-08-01)X-ray radiography and x-ray tomography are important tools for noninvasive characterization of materials. Historically, the contrast mechanism used with these techniques has been absorption. However, for any given sample there are x-ray energies for which absorption contrast is poor. Alternatively, when good contrast can be obtained, radiation damage from an excessive dose may become an issue. Consequently, phase-contrast methods have in recent years been implemented at both synchrotron and laboratory facilities. A range of radiographic and tomographic demonstrations have now been made, typically utilizing the coherent flux from an insertion device at a synchrotron or a microfocus laboratory source. In this paper we demonstrate that useful results may be obtained using a bending magnet source at a synchrotron. In particular we show that the same beamline can be used to make and characterize a sample made by x-ray lithographic methods.