Toward determining amyloid fibril structures using experimental constraints from Raman spectroscopy.

We present structural models for three different amyloid fibril polymorphs prepared from amylin20-29 (sequence SNNFGAILSS) and amyloid-ß25-35 (Aß25-35) (sequence GSNKGAIIGLM) peptides. These models are based on the amide C=O bond and Ramachandran ψ-dihedral angle data from Raman spectroscopy, which were used as structural constraints to guide molecular dynamics (MD) simulations. The resulting structural models indicate that the basic structural motif of amylin20-29 and Aß25-35 fibrils is extended ß-strands. Our data indicate that amylin20-29 forms both antiparallel and parallel ß-sheet fibril polymorphs, while Aß25-35 forms a parallel ß-sheet fibril structure. Overall, our work lays the foundation for using Raman spectroscopy in conjunction with MD simulations to determine detailed molecular-level structural models of amyloid fibrils in a manner that complements gold-standard techniques, such as solid-state nuclear magnetic resonance and cryogenic electron microscopy.

Spinodal de-wetting of light liquids on graphene.

We demonstrate theoretically the possibility of spinodal de-wetting in heterostructures made of light-atom liquids (hydrogen, helium, and nitrogen) deposited on suspended graphene. Extending our theory of film growth on two-dimensional (2D) materials to include analysis of surface instabilities via the hydrodynamic Cahn-Hilliard-type equation, we characterize in detail the spatial and temporal scales of the resulting spinodal de-wetting patterns. Both linear stability analysis and direct numerical simulations of the surface hydrodynamics show micron-sized (generally material dependent) patterns of 'dry' regions. The physical reason for the development of such instabilities on graphene can be traced back to the inherently weak van der Waals interactions between atomically thin materials and atoms in the liquid. Thus 2D materials could represent a new theoretical and technological platform for studies of spinodal de-wetting.

All-optical regenerator of multi-channel signals.

One of the main reasons why nonlinear-optical signal processing (regeneration, logic, etc.) has not yet become a practical alternative to electronic processing is that the all-optical elements with nonlinear input-output relationship have remained inherently single-channel devices (just like their electronic counterparts) and, hence, cannot fully utilise the parallel processing potential of optical fibres and amplifiers. The nonlinear input-output transfer function requires strong optical nonlinearity, e.g. self-phase modulation, which, for fundamental reasons, is always accompanied by cross-phase modulation and four-wave mixing. In processing multiple wavelength-division-multiplexing channels, large cross-phase modulation and four-wave mixing crosstalks among the channels destroy signal quality. Here we describe a solution to this problem: an optical signal processor employing a group-delay-managed nonlinear medium where strong self-phase modulation is achieved without such nonlinear crosstalk. We demonstrate, for the first time to our knowledge, simultaneous all-optical regeneration of up to 16 wavelength-division-multiplexing channels by one device. This multi-channel concept can be extended to other nonlinear-optical processing schemes.Nonlinear optical processing devices are not yet fully practical as they are single channel. Here the authors demonstrate all-optical regeneration of up to 16 channels by one device, employing a group-delay-managed nonlinear medium where strong self-phase modulation is achieved without nonlinear inter-channel crosstalk.

NALM-based, phase-preserving 2R regenerator of high-duty-cycle pulses.

We explore the potential of the nonlinear amplifying loop mirror (NALM)-based phase-preserving 2R (reamplification and reshaping) regenerator for simultaneous regeneration of multiple wavelength-division-multiplexed (WDM) channels. While not considering nonlinear multi-channel propagation, we address two issues of the phase-preserving NALM that appear to us as the major obstacles in adopting it for realistic WDM applications: a high operating power and a detrimental effect of non-small (33% - 50%) pulse duty cycles. After thorough optimization, we find a new operating regime of this regenerator with the non-small duty-cycle capability and approximately an order of magnitude reduction of the required operating power. In addition, we show that the plateau in the input-output power transfer curve does not automatically lead to the reduction of the amplitude jitter, which is particularly noticeable for the non-small duty-cycle pulses.

A comparative study of noisy signal evolution in 2R all-optical regenerators with normal and anomalous average dispersions using an accelerated Multicanonical Monte Carlo method.

In [Opt. Express 15, 10061 (2007)] we proposed a new regime of multichannel all-optical regeneration that required anomalous average dispersion. This regime is superior to the previously studied normal-dispersion regime when signal distortions are deterministic in their temporal shape. However, there was a concern that the regenerator with anomalous average dispersion may be prone to noise amplification via modulational instability. Here, we show that this, in general, is not the case. Moreover, in the range of input powers that is of interest for multichannel regeneration, the device with anomalous average dispersion may even provide less noise amplification than the one with normal dispersion. These results are obtained with an improved version of the parallelized modification of the Multicanonical Monte Carlo method proposed in [IEEE J. Sel. Topics Quantum Electron. 14, 599 (2008)].

A new robust regime for a dispersion-managed multichannel 2R regenerator.

We study the performance of a multichannel version [M. Vasilyev and T.I. Lakoba, Opt. Lett. 30, 1458 (2005)] of the all-optical Mamyshev regenerator in a practically important situation where one of its key components - a periodic-group-delay device - has a realistic amplitude characteristic of a bandpass filter. We show that in this case, the regenerator can no longer operate in the regime reported in our original paper. Instead, we have found a new regime in which the regenerator's performance is robust not only to such filtering, but also to considerable variations of regenerator parameters. In this regime, the average dispersion of the regenerator must be (relatively) large and anomalous, in constrast to what was considered in all earlier studies of such (single-channel) regenerators based on spectral broadening followed by off-center filtering. In addition, hardware implementation of a regenerator in the new regime is somewhat simpler than that in the original regime.

All-optical multichannel 2R regeneration in a fiber-based device.

We propose the design of an all-optical 2R regenerator capable of handling multiple wavelength-division-multiplexed channels simultaneously. It extends the known concept of off-center filtering of self-phase-modulation-broadened signal spectra. The novel feature of the proposed device is a dispersion map that strongly suppresses interchannel impairments. The map employs several sections of nonlinear fiber with high normal dispersion, separated by dispersion compensators with spectrally periodic group delay. The results of our numerical simulations indicate the feasibility of such a multichannel regenerator.