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Biomedical subjects

C M Bowden

Publications and source records attributed to C M Bowden.

10 recordsLinked to original sources
PubMed2003

Dynamics of counterpropagating pulses in photonic crystals: enhancement and suppression of stimulated emission processes.

Using numerical methods, we study the propagation of counterpropagating pulses in finite photonic crystals. We show that linear interference and localization effects combine to either enhance or suppress stimulated emission processes, depending on the initial phase difference between the input pulses. We consider the example of second harmonic generation, where we find a maximum contrast of three orders of magnitude in nonlinear conversion efficiency as a function of the input phase difference between incident pulses. We interpret these results by viewing the photonic crystal as an open cavity, with a field-dependent, electromagnetic density of modes sensitive to initial and boundary conditions.

Journal Article↗
PubMed2002

Third-harmonic generation and self-channeling in air using high-power femtosecond laser pulses.

It is shown, both theoretically and experimentally, that during laser pulse filamentation in air an intense ultrashort third-harmonic pulse is generated forming a two-colored filament. The third-harmonic pulse maintains both its peak intensity and energy over distances much longer than the characteristic coherence length. We argue that this is due to a nonlinear phase-locking mechanism between the two pulses in the filament and is independent of the initial material wave-vector mismatch. A rich spatiotemporal propagation dynamics of the third-harmonic pulse is predicted. Potential applications of this phenomenon to other parametric processes are discussed.

Journal Article↗
PubMed2001

Simultaneously phase-matched enhanced second and third harmonic generation.

Second and third harmonic generation via a chi((2)) three-wave mixing process can occur with high conversion efficiency in a one-dimensional photonic band gap structure. We find that it is possible to simultaneously achieve enhancement and exact phase-matching conditions of second harmonic and sum frequency generation, omega+2 omega-->3 omega. It is also remarkable that high conversion efficiencies persist under tuning conditions that correspond to a phase mismatch. While these conditions are quite unusual and cannot be achieved in any known bulk material, we show that they can be easily obtained in finite layered structures by using and balancing an interplay between material dispersion and the geometrical dispersion introduced by the structure.

Journal Article↗
PubMed2001

Photonic band edge effects in finite structures and applications to chi 2 interactions.

Using the concept of an effective medium, we derive coupled mode equations for nonlinear quadratic interactions in photonic band gap structures of finite length. The resulting equations reveal the essential roles played by the density of modes and effective phase matching conditions necessary for the strong enhancement of the nonlinear response. Our predictions find confirmation in an experimental demonstration of significant enhancement of second harmonic generation near the photonic band edge. The measured conversion efficiency is in good agreement with the conversion efficiency predicted by the effective-medium model.

Journal Article↗
PubMed2001

Group velocity, energy velocity, and superluminal propagation in finite photonic band-gap structures.

We have analyzed the notions of group velocity V(g) and energy velocity V(E) for light pulses propagating inside one-dimensional photonic band gap structures of finite length. We find that the two velocities are related through the transmission coefficient t as V(E)=/t/(2)V(g). It follows that V(E)=V(g) only when the transmittance is unity (/t/(2)=1). This is due to the effective dispersive properties of finite layered structures, and it allows us to better understand a wide range of phenomena, such as superluminal pulse propagation. In fact, placing the requirement that the energy velocity should remain subluminal leads directly to the condition V(g)<or=c//t/(2). This condition places a large upper limit on the allowed group velocity of the tunneling pulse at frequencies of vanishingly small transmission.

Journal Article↗
PubMed1999

Dispersive properties of finite, one-dimensional photonic band gap structures: applications to nonlinear quadratic interactions.

We discuss the linear dispersive properties of finite one-dimensional photonic band-gap structures. We introduce the concept of a complex effective index for structures of finite length, derived from a generalized dispersion equation that identically satisfies the Kramers-Kronig relations. We then address the conditions necessary for optimal, phase-matched, resonant second harmonic generation. The combination of enhanced density of modes, field localization, and exact phase matching near the band edge conspire to yield conversion efficiencies orders of magnitude higher than quasi-phase-matched structures of similar lengths. We also discuss an unusual and interesting effect: counterpropagating waves can simultaneously travel with different phase velocities, pointing to the existence of two dispersion relations for structures of finite length.

Journal Article↗
PubMed1993

Investigation into the histogenesis of congenital epulis of the newborn.

Five previously unreported cases of congenital epulis of the newborn are presented. All five cases were on the anterior maxillary alveolar ridge. Four were removed at 2 days of age and one at 7 weeks. Light microscopy demonstrated large eosinophilic granular cells within vascular fibrous connective tissue. Immunohistochemical studies revealed a positivity for vimentin and neuron specific enolase. Cytogenetic evaluation performed on one case was normal. Estrogen and progesterone receptors were absent in the one case so studied. Electron microscopy demonstrated tumor cells that were filled with autophagosomes. Cellular organelles were significantly reduced and inversely related to the number of cytoplasmic autophagosomes. Many of the autophagosomes contained collagen precursors. Poorly formed junctional complexes were seen. Occasional tumor cells demonstrated long processes that contained contractile microfilaments, pinocytosis, and areas of exocytosis. These studies suggest the tumor cells represent early mesodermal cells that express pericytic and myofibroblastic features that undergo cytoplasmic autophagocytosis.

Collagen↗
PubMed1975

Reimplantation of freeze-treated mandibular bone.

Mandibular bone devitalized by freezing after removal from the body and subsequent reimplantation was observed for its potential to act as a framework of new bone formation. Clinical union and bony replacement occurred when the reimplanted bone was covered by healthy, well-vascularized soft tissue. New bone initially formed subperiosteally along the devitalized bone. Later specimens showed increased production of endosteal bone. The results suggest that a resected mandible subjected to freezing might be immediately reimplanted as a graft when the mandible is involved as part of a radical operative procedure. Further studies are indicated to evaluate this potential when the periosteal covering is removed.

Animals↗