On the efficiency of small-angle Čerenkov second harmonic generation in optical waveguides

It is well known that in the non-depleted pump approximation, the efficiency of a second harmonic generation (SHG) of a guided mode in a non-linear optical waveguide increases quadratically with the interaction length (P ₂ L ²), and linearly (P ₂ L) in the erenkov regime. The efficiency of the erenkov SHG in the waveguide with a non-linear substrate and linear guiding layer is known to be strongly peaked at a particular pump wavelength and a particular waveguide thickness, with the erenkov angle approaching zero. The known theory predicts an infinite efficiency value at the peak, however. In this contribution, a simple integral expression for the SHG efficiency in the erenkov regime is derived. For large erenkov angles and interaction lengths it yields the expected P ₂ L dependence, while in the limit of small erenkov angles the dependence is found to have the form of P L ^3/2, possessing also a finite value at the efficiency peak. The condition determining the accurate position of the efficiency peak in the waveguide thickness–pump wavelength plane is given, too.     

Controlling SERS intensity by tuning the size and height of a silver nanoparticle array

It is demonstrated that the surface-enhanced Raman scattering (SERS) intensity of R6G molecules adsorbed on a Ag nanoparticle array can be controlled by tuning the size and height of the nanoparticles. A firm Ag nanoparticle array was fabricated on glass substrate by using nanosphere lithography (NSL) combined with reactive ion etching (RIE). Different sizes of Ag nanoparticles were fabricated with seed polystyrene nanospheres ranging from 430 nm to 820 nm in diameter. By depositing different thicknesses of Ag film and lifting off nanospheres from the surface of the substrate, the height of the Ag nanoparticles can be tuned. It is observed that the SERS enhancement factor will increase when the size of the Ag nanoparticles decreases and the deposition thickness of the Ag film increases. An enhancement factor as high as 2×10⁶ can be achieved when the size of the polystyrene nanospheres is 430 nm in diameter and the height of the Ag nanoparticles is 96 nm. By using a confocal Raman mapping technique, we also demonstrate that the intensity of Raman scattering is enhanced due to the local surface plasmon resonance (LSPR) occurring in the Ag nanoparticle array.     

Nonlinear coefficient and temperature dependence of the refractive index of lithium niobate crystals in the terahertz regime

The effective nonlinear coefficient d _eff of lithium niobate is determined to be 94 pm/V for a process that converts infrared light to 1.35 THz radiation. This value is inferred from the performance of a continuous-wave, singly-resonant optical parametric oscillator, in which the cavity-enhanced signal wave of a primary parametric process acts as a pump wave for a cascaded process, generating terahertz waves. To quantify the nonlinear coefficient, the coupled wave equations including absorption are evaluated. Furthermore, from our measurements we also determine the temperature dependence of the refractive index to be dn _THz/dT=0.0013/K around 1.4 THz.     

New simplified coupling scheme for the delivery of 20 MW Nd:YAG laser pulses by large core optical fibers

The feasibility of transmitting 20 MW, 5 ns laser pulses from a frequency doubled Nd:YAG laser through a standard 1500 μm multi mode optical fiber is demonstrated. A new coupling scheme employing an optical homogenizer prevents breakdown in air without the use of a vacuum chamber. At the same time a very homogeneous flat top beam profile on the fiber surface is achieved. The new scheme therefore clearly simplifies fiber coupling of high power laser pulses. Experiments on the delivery of more than 20000 pulses with 110 mJ mean energy without fiber damage have been performed. Received: 2 August 2000 / Revised version: 18 August 2000 / Published online: 22 November 2000     

Subpicosecond pulse compression in nonlinear photonic crystal waveguides based on the formation of high-order optical solitons

We investigate by numerical simulation the compression of subpicosecond pulses in two-dimensional nonlinear photonic crystal (PC) waveguides. The compression originates from the generation of high-order optical solitons through the interplay of the huge group-velocity dispersion and the enhanced self-phase modulation in nonlinear PC waveguides.Both the formation of Bragg grating solitons and gap solitons can lead to efficient pulse compression. The compression factors under different excitation power densities and the optimum length for subpicosecond pulse compression have been determined. As a compressor, the total length of the nonlinear PC waveguide is only ten micrometres and therefore can be easily incorporated into PC integrated circuits.     

Birefringent modifications induced by femtosecond filaments in optical glass

We present a study of the refractive index modifications spontaneously induced by infrared femtosecond filaments propagating in the bulk of transparent solids. It was found that extended modified refractive index channels up to several mm in length can be formed under loose focusing conditions in fused silica. The observed birefringence zone at the beginning of these channels is attributed to the anisotropic stress induced via nonlinear losses in the high intensity region of the filament.     

Frequency correlation and group time delay of ambient noise and ship radiated-noise in the sea

I.IntroductionTheambientnoiseintheseaisthenoisebackgroundforsonarsignaldetection.Ontheotherhand,theambientnoiseistheuscfu1signa1forextractingtheoceanenvironmenta1parametersbecausethereisagreatdealofinformationonoceanenvironmcntinambientnoise.Theshipradiated-noiscofothershipsistheusefulsignalforpassivesonardetectionwhereastheshipselfnoiseisthenoisebackground.Variouskindsofsignalprocessingmeth-odsessentia1lyutilizethediffcrencesofcharactCristicsbctweensignalandnoisebackgroundtoextractuscfulinfo…     

Anisotropic optical conductivity and electron–hole asymmetry in doped monolayer graphene in the presence of the Rashba coupling

In this study, the optical conductivity of substitutionary doped graphene is investigated in the presence of the Rashba spin orbit coupling (RSOC). Calculations have been performed within the coherent potential approximation (CPA) beyond the Dirac cone approximation. Results of the current study demonstrate that the optical conductivity is increased by increasing the RSOC strength. Meanwhile it was observed that the anisotropy of the band energy results in a considerable anisotropic optical conductivity (AOC) in monolayer graphene. The sign and magnitude of this anisotropic conductivity was shown to be controlled by the external field frequency. It was also shown that the Rashba interaction results in electron–hole asymmetry in monolayer graphene.     

Validity of the generalized second law of thermodynamics of the universe bounded by the event horizon in brane scenario

In this paper, we examine the validity of the generalized second law of thermodynamics of the universe bounded by the event horizon in brane-world gravity. Here we consider a homogeneous and isotropic model of the universe in the one case where it is filled with a perfect fluid and in another case where a holographic dark energy model of the universe has been considered.     

Fully analytical evaluation of optical gain coefficient in bulk semiconductors in quasi-equilibrium free-carrier approximation

In this letter, a new analytical method is presented to calculate of the semiconductor optical gain coefficient. This method is particularly suitable for theoretical analyses to determine the dependence of semiconductor gain on the total carrier density and temperature in the semiconductor lasers. Also, the optical gain functions for semiconductor optical gain coefficient are presented analytically. The analytical evaluation is verified with numerical methods, which illustrates the accuracy of these obtained analytical expressions.     

A scalable quantum computer with ultranarrow optical transition of ultracold neutral atoms in an optical lattice

We propose a new quantum-computing scheme using ultracold neutral ytterbium atoms in an optical lattice, especially in a monolayer of three-dimensional optical lattice. The nuclear Zeeman sublevels define a qubit. This choice avoids the natural phase evolution due to the magnetic dipole interaction between qubits. The Zeeman sublevels with large magnetic moments in the long-lived metastable state are also exploited to address individual atoms and to construct a controlled-multiqubit gate. Estimated parameters required for this scheme show that this proposal is scalable and experimentally feasible.     

Tunable ground-state solitons in spin–orbit coupling Bose–Einstein condensates in the presence of optical lattices

Properties of the ground-state solitons, which exist in the spin–orbit coupling(SOC) Bose–Einstein condensates(BEC) in the presence of optical lattices, are presented. Results show that several system parameters, such as SOC strength,lattice depth, and lattice frequency, have important influences on properties of ground state solitons in SOC BEC. By controlling these parameters, structure and spin polarization of the ground-state solitons can be effectively tuned, so manipulation of atoms may be realized.     

Improving the fidelity of continuous-variable quantum teleportation by tuning displacement gain

The fidelity of teleportation of continuous quantum variables can be improved by tuning the local displacement gain. We investigate the optimization of the fidelity for the teleportation of Schrodinger cat states, and of coherent states. It is found that the gain corresponding to the maximum fidelity is not equal to one for the two input states in the case of the small squeezing degree of the entanglement resource, while unity displacement gain is the best choice for teleporting arbitrary quantum states in the case of large squeezing.     

Recent trends in the tuning of polymersomes’ membrane properties

“Polymersomes” are vesicular structures made from the self-assembly of block copolymers. Such structures present outstanding interest for different applications such as micro- or nano-reactor, drug release or can simply be used as tool for understanding basic biological mechanisms. The use of polymersomes in such applications is strongly related to the way their membrane properties are controlled and tuned either by a precise molecular design of the constituting block or by addition of specific components inside the membrane (formulation approaches). Typical membrane properties of polymersomes obtained from the self-assembly of “coil coil” block copolymer since the end of the nineties will be first briefly reviewed and compared to those of their lipidic analogues, named liposomes. Therefore the different approaches able to modulate their permeability, mechanical properties or ability to release loaded drugs, using macromolecular engineering or formulations, are detailed. To conclude, the most recent advances to modulate the polymersomes’ properties and systems that appear very promising especially for biomedical application or for the development of complex and bio-mimetic structures are presented.     

Improved gate-control in InAs nanowire structures by the use of GdScO3 as a gate dielectric

We investigated the properties of gadolinium scandate (GdScO₃) as a gate dielectric for top-gate electrodes on undoped InAs nanowires. It is demonstrated that due to the high dielectric constant of GdScO₃ (k=22), a better control of the conductance of the nanowire is achieved compared to a reference SiO₂-isolated back-gate electrode. We analyzed the output and transfer characteristics of top-gate-controlled InAs wires at room temperature and at temperatures down to 4 K. Owing to the good coverage of the InAs nanowire by the 50-nm-thick GdScO₃ layer, which was deposited by pulsed-laser deposition, the gate leakage current is sufficiently suppressed.     

Nonlinear optical absorption properties of porphyrins confined in Nafion membrane

The nonlinear absorption (NLA) properties of five different types of porphyrins were studied using the Z-scan technique. The porphyrins under investigation were confined into Nafion column matrix membrane in order to protect them from possible degradation. The results of the experiments have indicated that all the porphyrins tested exhibited interesting NLA properties. The nonlinear absorption coefficients (β’s) were determined at different porphyrin concentrations by comparing the Z-scan data with the theoretical functions.     

Generation of an ultrashort optical pulse by four-wave Raman mixing in deuterium cooled at 77 K

When a 100-fs Ti:sapphire laser is focused into molecular deuterium cooled at 77 K, rotational stimulated Raman emission is generated based on four-wave Raman mixing. By phase locking the emission lines, the laser pulsewidth was reduced to 40 fs, when a nearly transform-limited pulse was utilized as a fundamental laser. The pulsewidth was further reduced to 20 fs, when a positively chirped pulse was employed. Computer simulation suggests that the Raman emission is strongly chirped and a train of pulses with lower intensities are formed when a chirp pulse is used as a fundamental laser. PACS 42.65.Dr; 42.55.Ye; 42.50-p     

Laser nano-fabrication of large-area plasmonic structures and surface plasmon resonance tuning by thermal effect

A simple and flexible technique aimed to generate large-area periodic nano-dot array features on metal thin films by laser interference lithography (LIL) has been demonstrated. In this paper, gold nano-dot arrays with a period of ∼450 nm and a dot diameter of ∼100 nm on quartz substrates coated with a gold film of 50 nm thick were fabricated. Multiple enhanced transmission peaks were observed in this patterned film. In addition to the characteristic peak of the gold surface plasmon resonance around 500 nm, multiple shoulder peaks that range from 550 to 700 nm were also observed in the nano-chain array structures. These shoulder peaks disappeared after thermal annealing. It was found that the nano-dots became smaller and well-separated nano-balls under the high temperature annealing process. These nano-structures have potential applications in solar cell, nano-lithography and biosensing.     

Observation of the Higgs Boson of strong interaction via Compton scattering by the nucleon

It is shown that the Quark-Level Linear σ Model (QLLσM) leads to a prediction for the diamagnetic term of the polarizabilities of the nucleon which is in excellent agreement with experimental data. The bare mass of the σ meson is predicted to be m _σ =666 MeV and the two-photon width Γ(σ→γ γ)=(2.6±0.3) keV. It is argued that the mass predicted by the QLLσM corresponds to the $\gamma\gamma\to\sigma\to N\bar{N}$ reaction, i.e. to a t-channel pole of the γ N→N γ reaction. Large-angle Compton scattering experiments revealing effects of the σ meson in the differential cross section are discussed. Arguments are presented that these findings may be understood as an observation of the Higgs boson of the strong interaction while being a part of the constituent quark.