103 research outputs found
Control of superluminal transit through a heterogeneous medium
We consider pulse propagation through a two component composite medium (metal
inclusions in a dielectric host) with or without cavity mirrors. We show that a
very thin slab of such a medium, under conditions of localized plasmon
resonance, can lead to significant superluminality with detectable levels of
transmitted pulse. A cavity containing the heterogeneous medium is shown to
lead to subluminal-to-superluminal transmission depending on the volume
fraction of the metal inclusions. The predictions of phase time calculations
are verified by explicit calculations of the transmitted pulse shapes. We also
demonstrate the independence of the phase time on system width and the volume
fraction under specific conditions.Comment: 21 Pages,5 Figures (Published in Journal of Modern Optics
Superluminal optical pulse propagation in nonlinear coherent media
The propagation of light-pulse with negative group-velocity in a nonlinear
medium is studied theoretically. We show that the necessary conditions for
these effects to be observable are realized in a three-level -system
interacting with a linearly polarized laser beam in the presence of a static
magnetic field. In low power regime, when all other nonlinear processes are
negligible, the light-induced Zeeman coherence cancels the resonant absorption
of the medium almost completely, but preserves the dispersion anomalous and
very high. As a result, a superluminal light pulse propagation can be observed
in the sense that the peak of the transmitted pulse exits the medium before the
peak of the incident pulse enters. There is no violation of causality and
energy conservation. Moreover, the superluminal effects are prominently
manifested in the reshaping of pulse, which is caused by the
intensity-dependent pulse velocity. Unlike the shock wave formation in a
nonlinear medium with normal dispersion, here, the self-steepening of the pulse
trailing edge takes place due to the fact that the more intense parts of the
pulse travel slower. The predicted effect can be easily observed in the well
known schemes employed for studying of nonlinear magneto-optical rotation. The
upper bound of sample length is found from the criterion that the pulse
self-steepening and group-advance time are observable without pulse distortion
caused by the group-velocity dispersion.Comment: 16 pages, 7 figure
Negative group delay for Dirac particles traveling through a potential well
The properties of group delay for Dirac particles traveling through a
potential well are investigated. A necessary condition is put forward for the
group delay to be negative. It is shown that this negative group delay is
closely related to its anomalous dependence on the width of the potential well.
In order to demonstrate the validity of stationary-phase approach, numerical
simulations are made for Gaussian-shaped temporal wave packets. A restriction
to the potential-well's width is obtained that is necessary for the wave packet
to remain distortionless in the travelling. Numerical comparison shows that the
relativistic group delay is larger than its corresponding non-relativistic one.Comment: 10 pages, 5 figure
Transparent Anomalous Dispersion and Superluminal Light Pulse Propagation at a Negative Group Velocity
Anomalous dispersion cannot occur in a transparent passive medium where
electromagnetic radiation is being absorbed at all frequencies, as pointed out
by Landau and Lifshitz. Here we show, both theoretically and experimentally,
that transparent linear anomalous dispersion can occur when a gain doublet is
present. Therefore, a superluminal light pulse propagation can be observed even
at a negative group velocity through a transparent medium with almost no pulse
distortion. Consequently, a {\it negative transit time} is experimentally
observed resulting in the peak of the incident light pulse to exit the medium
even before entering it. This counterintuitive effect is a direct result of the
{\it rephasing} process owing to the wave nature of light and is not at odds
with either causality or Einstein's theory of special relativity.Comment: 12 journal pages, 9 figure
Small Corrections to the Tunneling Phase Time Formulation
After reexamining the above barrier diffusion problem where we notice that
the wave packet collision implies the existence of {\em multiple} reflected and
transmitted wave packets, we analyze the way of obtaining phase times for
tunneling/reflecting particles in a particular colliding configuration where
the idea of multiple peak decomposition is recovered. To partially overcome the
analytical incongruities which frequently rise up when the stationary phase
method is adopted for computing the (tunneling) phase time expressions, we
present a theoretical exercise involving a symmetrical collision between two
identical wave packets and a unidimensional squared potential barrier where the
scattered wave packets can be recomposed by summing the amplitudes of
simultaneously reflected and transmitted wave components so that the conditions
for applying the stationary phase principle are totally recovered. Lessons
concerning the use of the stationary phase method are drawn.Comment: 14 pages, 3 figure
The Exact Correspondence between Phase Times and Dwell Times in a Symmetrical Quantum Tunneling Configuration
The general and explicit relation between the phase time and the dwell time
for quantum tunneling or scattering is investigated. Considering a symmetrical
collision of two identical wave packets with an one-dimensional barrier, here
we demonstrate that these two distinct transit time definitions give connected
results where, however, the phase time (group delay) accurately describes the
exact position of the scattered particles. The analytical difficulties that
arise when the stationary phase method is employed for obtaining phase
(traversal) times are all overcome. Multiple wave packet decomposition allows
us to recover the exact position of the reflected and transmitted waves in
terms of the phase time, which, in addition to the exact relation between the
phase time and the dwell time, leads to right interpretation for both of them.Comment: 11 pages, 2 figure
The Human Proteins MBD5 and MBD6 Associate with Heterochromatin but They Do Not Bind Methylated DNA
BACKGROUND: MBD5 and MBD6 are two uncharacterized mammalian proteins that contain a putative Methyl-Binding Domain (MBD). In the proteins MBD1, MBD2, MBD4, and MeCP2, this domain allows the specific recognition of DNA containing methylated cytosine; as a consequence, the proteins serve as interpreters of DNA methylation, an essential epigenetic mark. It is unknown whether MBD5 or MBD6 also bind methylated DNA; this question has interest for basic research, but also practical consequences for human health, as MBD5 deletions are the likely cause of certain cases of mental retardation. PRINCIPAL FINDINGS: Here we report the first functional characterization of MBD5 and MBD6. We have observed that the proteins colocalize with heterochromatin in cultured cells, and that this localization requires the integrity of their MBD. However, heterochromatic localization is maintained in cells with severely decreased levels of DNA methylation. In vitro, neither MBD5 nor MBD6 binds any of the methylated sequences DNA that were tested. CONCLUSIONS: Our data suggest that MBD5 and MBD6 are unlikely to be methyl-binding proteins, yet they may contribute to the formation or function of heterochromatin. One isoform of MBD5 is highly expressed in oocytes, which suggests a possible role in epigenetic reprogramming after fertilization
Immune Response to Lactobacillus plantarum Expressing Borrelia burgdorferi OspA Is Modulated by the Lipid Modification of the Antigen
Over the past decade there has been increasing interest in the use of lactic acid bacteria as mucosal delivery vehicles for vaccine antigens, microbicides and therapeutics. We investigated the mechanism by which a mucosal vaccine based in recombinant lactic acid bacteria breaks the immunological tolerance of the gut in order to elicit a protective immune response.We analyzed how the lipid modification of OspA affects the localization of the antigen in our delivery vehicle using a number of biochemistry techniques. Furthermore, we examined how OspA-expressing L. plantarum breaks the oral tolerance of the gut by stimulating human intestinal epithelial cells, peripheral blood mononuclear cells and monocyte derived dendritic cells and measuring cytokine production. We show that the leader peptide of OspA targets the protein to the cell envelope of L. plantarum, and it is responsible for protein export across the membrane. Mutation of the lipidation site in OspA redirects protein localization within the cell envelope. Further, we show that lipidated-OspA-expressing L. plantarum does not induce secretion of the pro-inflammatory cytokine IL-8 by intestinal epithelial cells. In addition, it breaks oral tolerance of the gut via Th1/Th2 cell mediated immunity, as shown by the production of pro- and anti-inflammatory cytokines by human dendritic cells, and by the production of IgG2a and IgG1 antibodies, respectively.Lipid modification of OspA expressed in L. plantarum modulates the immune response to this antigen through a Th1/Th2 immune response
Induction of effective and antigen-specific antitumour immunity by a liposomal ErbB2/HER2 peptide-based vaccination construct
Efficient delivery of tumour-associated antigens to appropriate cellular compartments of antigen-presenting cells is of prime importance for the induction of potent, cell-mediated antitumour immune responses. We have designed novel multivalent liposomal constructs that co-deliver the p63β71 cytotoxic T Lymphocyte epitope derived from human ErbB2 (HER2), and HA307β319, a T-helper (Th) epitope derived from influenza haemagglutinin. Both peptides were conjugated to the surface of liposomes via a Pam3CSS anchor, a synthetic lipopeptide with potent adjuvant activity. In a murine model system, vaccination with these constructs completely protected BALB/c mice from subsequent s.c. challenge with ErbB2-expressing, but not ErbB2-negative, murine renal carcinoma (Renca) cells, indicating the induction of potent, antigen-specific immune responses. I.v. re-challenge of tumour-free animals 2 months after the first tumour cell inoculation did not result in the formation of lung tumour nodules, suggesting that long-lasting, systemic immunity had been induced. While still protecting the majority of vaccinated mice, a liposomal construct lacking the Th epitope was less effective than the diepitope construct, also correlating with a lower number of CD8+ IFN-Ξ³+ T-cells identified upon ex vivo peptide restimulation of splenocytes from vaccinated animals. Importantly, in a therapeutic setting treatment with the liposomal vaccines resulted in cures in the majority of tumour-bearing mice and delayed tumour growth in the remaining ones. Our results demonstrate that liposomal constructs which combine Tc and Th peptide antigens and lipopeptide adjuvants can induce efficient, antigen-specific antitumour immunity, and represent promising synthetic delivery systems for the design of specific antitumour vaccines
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