Layer-by-layer assembly and tribological property of multilayer ultrathin films constructed by modified graphene sheets and polyethyleneimine

A series of graphene sheets (GS)-based multilayer films was constructed in virtue of layer-by-layer electrostatic self-assembly technique based on the negatively charged poly(sodium 4-styrenesulfonate) (PSS) mediated GS (PSS-GS) and the positively charged polyethyleneimine (PEI). High-resolution transmission electron microscope, atomic force microscope and micro-Raman spectrum characterizations demonstrated that the PSS-GS has been synthesized and could be assembled on the single-crystal silicon substrate. Ellipsometric thickness measurement and ultraviolet-visible absorption spectroscope confirmed the successive assemblies of GS. Finally, the macrotribological behaviors of different multilayer films were evaluated on a ball-on-plate macrotribometer and the results indicated that the prepared three- and five-layer films had high load affording ability and long anti-wear life, which could be highly dependent upon the high coverage and excellent self-lubricant properties that the GS owns intrinsically.     

Quantum field theoretical analysis on unstable behavior of Bose-Einstein condensates in optical lattices

We study the dynamics of Bose-Einstein condensates flowing in optical lattices on the basis of quantum field theory. For such a system, a Bose-Einstein condensate shows an unstable behavior which is called the dynamical instability. The unstable system is characterized by the appearance of modes with complex eigenvalues. Expanding the field operator in terms of excitation modes including complex ones, we attempt to diagonalize the unperturbative Hamiltonian and to find its eigenstates. It turns out that although the unperturbed Hamiltonian is not diagonalizable in the conventional bosonic representation the appropriate choice of physical states leads to a consistent formulation. Then we analyze the dynamics of the system in the regime of the linear response theory. Its numerical results are consistent with those given by the discrete nonlinear Schrödinger equation.     

Coupled-resonator optical waveguide: a proposal and analysis

We propose a new type of optical waveguide that consists of a sequence of coupled high-Q resonators. Unlike other types of optical waveguide, waveguiding in the coupled-resonator optical waveguide (CROW) is achieved through weak coupling between otherwise localized high-Q optical cavities. Employing a formalism similar to the tight-binding method in solid-state physics, we obtain the relations for the dispersion and the group velocity of the photonic band of the CROW's and find that they are solely characterized by coupling factor k(1) . We also demonstrate the possibility of highly efficient nonlinear optical frequency conversion and perfect transmission through bends in CROW's.     

A nonadiabatic semi-classical method for dynamics of atoms in optical lattices

We develop a semi-classical method to simulate the motion of atoms in a dissipative optical lattice. Our method treats the internal states of the atom quantum mechanically, including all nonadiabatic couplings, while position and momentum are treated as classical variables. We test our method in the one-dimensional case. Excellent agreement with fully quantum mechanical simulations is found. Our results are much more accurate than those of earlier semi-classical methods based on the adiabatic approximation.     

Photonic nanojets as three-dimensional optical atom traps: A theoretical study

We show that an efficient three-dimensional optical atom trap can be achieved by light scattered off a dielectric microsphere. Namely, under suitable conditions, a plane wave incident on a polymer sphere produces a focal point in the forward scattering direction known as photonic nanojet. The photonic nanojet is formed at a distance of a few micrometers away from the surface of the sphere wherein the Casimir–Polder interaction felt by an atom is negligible compared to the optical and gravitational potentials. When many polymer spheres are brought together so as to form a linear chain, a one-dimensional periodic optical lattice filled with cold atoms is possible since interference between the incident and scattered beams is minimal when the spheres are not too close.     

Oscillating superfluidity of bosons in optical lattices

Following a suggestion by Orzel et al. [Science 291, 2386 (2001)]], we analyze bosons in an optical lattice undergoing a sudden parameter change from the Mott to superfluid phase. We introduce a modified coherent states path integral to describe both phases. The saddle point theory yields collective oscillations of the uniform superfluid order parameter. We calculate its damping rate by phason pair emission. In two dimensions the overdamped region largely overlaps with the quantum critical region. Measurements of critical dynamics on the Mott side are proposed.     

Implementation of spin Hamiltonians in optical lattices

We propose an optical lattice setup to investigate spin chains and ladders. Electric and magnetic fields allow us to vary at will the coupling constants, producing a variety of quantum phases including the Haldane phase, critical phases, quantum dimers, etc. Numerical simulations are presented showing how ground states can be prepared adiabatically. We also propose ways to measure a number of observables, like energy gap, staggered magnetization, end-chain spins effects, spin correlations, and the string-order parameter.     

Observation of photon-assisted tunneling in optical lattices

We have observed tunneling suppression and photon-assisted tunneling of Bose-Einstein condensates in an optical lattice subjected to a constant force plus a sinusoidal shaking. For a sufficiently large constant force, the ground energy levels of the lattice are shifted out of resonance and tunneling is suppressed; when the shaking is switched on, the levels are coupled by low-frequency photons and tunneling resumes. Our results agree well with theoretical predictions and demonstrate the usefulness of optical lattices for studying solid-state phenomena.     

Delocalized entanglement of atoms in optical lattices

We show how to detect and quantify entanglement of atoms in optical lattices in terms of correlation functions of the momentum distribution. These distributions can be measured directly in the experiments. We introduce two kinds of entanglement measures related to the position and the spin of the atoms.     

Transport of cold atoms in optical lattices

The work discusses transport of cold atoms in optical lattices. Two related but different problems are considered: interacting Bose atoms subject to a static field (i.e., the atoms in a tilted lattice); and non-interacting atoms in a tilted lattice in the presence of a buffer gas. For these two systems we found, respectively: periodic, quasiperiodic, or decaying Bloch oscillations, as it depends on the strength of atom-atom interactions and the magnitude of the static field; diffusive directed current of atoms, similar to the electron current in ordinary conductors.     

A novel optical multilayer hydrophone with a triangular pyramid substrate

A novel concept for an optical multilayer ultrasonic hydrophone with the sensing film deposited on a triangular pyramid glass substrate is proposed.Using the calculation model for the spectral coefficients’ derivatives of a dielectric multilayer optical coating,the acousto-optic sensitivity characteristic of the hydrophone is analyzed with different measurement laser polarizations and incident angles.We present a reasonable method and adjusting strategy for the optimum working point selection of the ultrasound measurement.Analytic results show that the novel hydrophone possesses all the other merits of a plate glass substrate optical multilayer hydrophone but with improved detection sensitivity.A longer measurement time without distortion decreases the difficulty of high frequency signal circuits.Spatial split of the ultrasound signal caused by the substrate’s triangular pyramid roof simplifies the spatial spot area correction,which contributes to the accurate calibration of the hydrophone’s wideband frequency response.     

Layer-by-layer assembly of a magnetic nanoparticle shell on a thermoresponsive microgel core

We describe the surface modification of magnetic nanoparticles (MNPs), the coverage of poly(N-isopropylacrylamide) (PNiPAM) microgel with the MNPs and the inductive heating of these carriers. PNiPAM surface itself was modified using the layer-by-layer (LbL) assembly of polyelectrolytes to facilitate the deposition of surface-modified MNPs. One advantage of this concept is it allows the tuning of the magnetic and thermoresponsive properties of individual components (nanoparticles and microgels) separately before assembling them. Characterisations of the hybrid core–shell are discussed. In particular, it is shown that (i) each layer is successfully deposited and, more importantly, (ii) the coated microgel retains its thermoresponsive and magnetic behaviour.     

Localization of Bose-Einstein condensates in optical lattices

The dynamics of repulsive bosons condensed in an optical lattice is effectively described by the Bose-Hubbard model. The classical limit of this model, reproduces the dynamics of Bose-Einstein condensates, in a periodic potential, and in the superfluid regime. Such dynamics is governed by a discrete nonlinear Schrödinger equation. Several papers, addressing the study of the discrete nonlinear Schrödinger dynamics, have predicted the spontaneous generation of (classical) breathers in coupled condensates. In the present contribute, we shall focus on localized solutions (quantum breathers) of the full Bose-Hubbard model. We will show that solutions exponentially localized in space and periodic in time exist also in absence of randomness. Thus, this kind of states, reproduce a novel quantum localization phenomenon due to the interplay between bounded energy spectrum and non-linearity.     

Cooling in reduced period optical lattices: Non-zero Raman detuning

In a previous paper [Phys. Rev. A 72 (2005) 033415], it was shown that sub-Doppler cooling occurs in a standing-wave Raman scheme (SWRS) that can lead to reduced period optical lattices. These calculations are extended to allow for non-zero detuning of the Raman transitions. New physical phenomena are encountered, including cooling to non-zero velocities, combinations of Sisyphus and “corkscrew” polarization cooling, and somewhat unusual origins of the friction force. The calculations are carried out in a semi-classical approximation and a dressed state picture is introduced to aid in the interpretation of the results.     

Enhancement of optical switching parameter and third-order optical nonlinearities in embedded Si nanocrystals: A theoretical assessment

Third-order bound-charge electronic nonlinearities of Si nanocrystals (NCs) embedded in a wide band-gap matrix representing silica are theoretically studied using an atomistic pseudopotential approach. Nonlinear refractive index, two-photon absorption and optical switching parameter are examined from small clusters to NCs up to a size of 3 nm. Compared to bulk values, Si NCs show higher third-order optical nonlinearities and much wider two-photon absorption-free energy gap which gives rise to enhancement in the optical switching parameter.     

Stabilization of optical solitons in chirped PT-symmetric lattices

We investigate the stability properties of optical solitons in a chirped PT-symmetric lattice whose frequency changes in the transverse direction. Linear-stability analysis together with the direct propagation simulations demonstrates that the chirped lattice can improve the stability of optical solitons dramatically. The instability of fundamental solitons can be completely suppressed if the chirp rate exceeds a critical value. A broad stability area of dipole solitons appears if the lattice is appropriately chirped. Thus, we propose an effective way to suppress the instability of solitons in PT-symmetric potentials.     

Mixtures of strongly interacting bosons in optical lattices

We investigate the properties of strongly interacting heteronuclear boson-boson mixtures loaded in realistic optical lattices, with particular emphasis on the physics of interfaces. In particular, we numerically reproduce the recent experimental observation that the addition of a small fraction of 41K induces a significant loss of coherence in 87Rb, providing a simple explanation. We then investigate the robustness against the inhomogeneity typical of realistic experimental realizations of the glassy quantum emulsions recently predicted to occur in strongly interacting boson-boson mixtures on ideal homogeneous lattices.