Biomedical subjects

George Barbastathis

Publications and source records attributed to George Barbastathis.

14 recordsLinked to original sources

PubMed2006

Time-domain frequency-selective processing in nuclear magnetic resonance: a spatial-spectral holographic perspective.

The fundamental nuclear magnetic resonance (NMR) imaging equation can be derived from a spatial-spectral holographic wavefront reconstruction formulation similar to that in quantum optics. A spatial-spectral holographic interpretation arises naturally in NMR from the inhomogeneous linewidth broadening due to either an imposed set of linear orthogonal gradient fields or from the intrinsic chemical anisotropy of the spin system. We can thus think of NMR k-space as a spatial-spectral holographic grating. The spatial holographic component arises from dielectric effects at high field strength (>4 T) when the excitation wavelength is less than or commensurate with the size of the imaging sample. The holographic properties of storage, time-reversal, recognition, and triple correlations are experimentally demonstrated in an inhomogeneously broadened NMR sample. This holographic NMR interpretation has additional implications on selective radio-frequency pulse design, microscopy imaging, and the use of conjugate imaging for field inhomogeneity corrections using the time-reversed component of the readout, to be the subject of a subsequent paper.

Algorithms↗

PubMed2006

Reduced-complexity representation of the coherent point-spread function in the presence of aberrations and arbitrarily large defocus.

We introduce a method to analyze the diffraction integral for evaluating the point-spread function. Our method is based on the use of higher-order Airy functions along with Zernike and Taylor expansions. Our approach is applicable when we are considering a finite, arbitrary number of aberrations and arbitrarily large defocus simultaneously. We present an upper bound for the complexity and the convergence rate of this method. We also compare the cost and accuracy of this method with those of traditional ones and show the efficiency of our method through these comparisons. In particular, we rigorously show that this method is constructed in a way that the complexity of the analysis (i.e., the number of terms needed for expressing the light disturbance) does not increase as either defocus or resolution of interest increases. This has applications in several fields such as biological microscopy, lithography, and multidomain optimization in optical systems.

Journal Article↗

PubMed2005

High-resolution volume holographic profilometry using the Viterbi algorithm.

We use the Viterbi decoding algorithm to resolve depth features beyond the nominal resolution limit of a volume holographic profilometry system. The formulation treats the truncated point-spread function as an intersymbol interference and uses surface constraints and transition constraints to reduce the computational complexity. A factor-of-5 improvement in resolution was obtained in our experimental demonstration.

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PubMed2005

Rainbow volume holographic imaging.

We present a new form of volume holographic imaging with rainbow illumination. High depth resolution is obtained because each quasi-monochromatic band of the rainbow acts as a depth-selective confocal slit. The color slits work in parallel to achieve a wide field of view (FoV) and so the need to scan in one lateral dimension is eliminated. Our experiments demonstrated <250-micro m depth resolution over an approximately equal to 15 degree FoV at a 50-mm working distance.

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PubMed2005

Photon-sieve lithography.

We present the first lithography results that use high-numerical-aperture photon sieves as focusing elements in a scanning-optical-beam-lithography system [J. Vac. Sci. Technol. B 21, 2810 (2003)]. Photon sieves are novel optical elements that offer the advantages of higher resolution and improved image contrast compared with traditional diffractive optics such as zone plates [Nature 414, 184 (2001)]. We fabricated the highest-numerical-aperture photon sieves reported to date and experimentally verified their focusing characteristics. We propose two new designs of the photon sieve that have the potential to significantly increase focusing efficiency.

Journal Article↗

PubMed2005

Diffraction from deformed volume holograms: perturbation theory approach.

We derive the response of a volume grating to arbitrary small deformations, using a perturbative approach. This result is of interest for two applications: (a) when a deformation is undesirable and one seeks to minimize the diffracted field's sensitivity to it and (b) when the deformation itself is the quantity of interest and the diffracted field is used as a probe into the deformed volume where the hologram was originally recorded. We show that our result is consistent with previous derivations motivated by the phenomenon of shrinkage in photopolymer holographic materials. We also present the analysis of the grating's response to deformation due to a point indenter and present experimental results consistent with theory.

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PubMed2004

N-ocular volume holographic imaging.

Volume holographic imaging utilizes Bragg selectivity to optically slice the object space of the imaging system and measure four- (three spatial and one spectral) dimensional object information. The N-ocular version of this method combines multiple-volume holographic sensors and digital postprocessing to yield high-resolution three-dimensional images for broadband objects located at long working distances. We discuss the physical properties of volume holography pertinent to imaging performance and describe two computational algorithms for image inversion based on filtered backprojection and least-squares optimization.

Journal Article↗

PubMed2004

Broadband volume holographic imaging.

We demonstrate transmission geometry volume holograms working under broadband illumination. We show that increased illumination bandwidth enhances the lateral field of view of planar reference holograms. We exploit this phenomenon to design volume holographic spectrum analyzers and present results from an experimental prototype. Furthermore, we show that there is a trade-off involved, because an improvement in the field of view results in a reduction of image contrast as a function of depth. We experimentally demonstrate this trade-off and discuss possible ways to overcome it.

Journal Article↗

PubMed2004

Volume holographic hyperspectral imaging.

A volume hologram has two degenerate Bragg-phase-matching dimensions and provides the capability of volume holographic imaging. We demonstrate two volume holographic imaging architectures and investigate their imaging resolution, aberration, and sensitivity. The first architecture uses the hologram directly as an objective imaging element where strong aberration is observed and confirmed by simulation. The second architecture uses an imaging lens and a transmission geometry hologram to achieve linear two-dimensional optical sectioning and imaging of a four-dimensional (spatial plus spectral dimensions) object hyperspace. Multiplexed holograms can achieve simultaneously three-dimensional imaging of an object without a scanning mechanism.

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PubMed2004

Coherence patterns originating from incoherent volume sources.

We derive the complex degree of coherence that originates from generalized incoherent two- and three-dimensional sources. Further, we find the locus of maximum coherence and analyze the dependence of the decay of coherence on source thickness.

Journal Article↗

PubMed2004

Volume holographic imaging in transmission geometry.

We address the performance of transmission geometry volume holograms as depth-selective imaging elements. We consider two simple implementations using holograms recorded with spherical and plane beams. We derive the point-spread function (PSF) of these systems using volume diffraction theory and use the PSF to estimate depth resolution. Furthermore, we show that appropriately designed objective optics can significantly improve the depth resolution or the working distance of plane-wave reference holographic imaging systems. These results are confirmed experimentally and demonstrated for objects with millimeter axial features, imaged from the 5- to 50-cm range.

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PubMed2004

Cross talk in resonant holographic memories.

We analyze cross talk in resonant holographic memories and derive the conditions under which resonance improves storage quality. We also carry out the analysis for both plane-wave and apodized Gaussian reference beams.

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PubMed2003

Analog tunable gratings driven by thin-film piezoelectric microelectromechanical actuators.

We present a microfabricated grating whose period can be tuned in analog fashion to within a fraction of a nanometer. The tunable angular range is more than 400 microrad in the first diffracted order. The design concept consists of a diffractive grating defined onto a 400-nm membrane, with the membrane subsequently strained in the direction perpendicular to the grating grooves by thin-film piezoelectric actuation. The strain-tuned grating device was fabricated with microelectromechanical processes, utilizing both surface and bulk micromachining. The fabricated piezoelectric film achieved a measured dielectric constant of 1200. Device characterization yielded grating period changes up to 8.3 nm (0.21% strain in the membrane) at 10 V and a diffracted angular change of 486 microrad, in good agreement with the theory. Uniformity across the actuated grating and out-of-plane deflections are characterized and discussed.

Journal Article↗

PubMed2002

Axial imaging necessitates loss of lateral shift invariance.

We conjecture that the lateral shift invariance of an imaging system must be limited if axial imaging capability is desired. We develop shift-invariance and depth-resolution metrics and demonstrate the trade-off in simple representative systems.

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