Compact efficient intracavity optical parametric oscillator with a passively Q-switched Nd : YVO<Subscript>4</Subscript>/Cr<Superscript>4+</Superscript> : YAG laser in a hemispherical cavity

A compact eye-safe optical parametric oscillator (OPO) using a noncritically phase-matched KTP crystal intracavity pumped by a passively Q-switched Nd:YVO₄ laser is experimentally demonstrated. To enhance the performance of passive Q-switching, a Cr⁴⁺:YAG saturable absorber crystal is coated as an OPO output coupler in a nearly hemispherical cavity. With an incident pump power of 2.5 W, the compact intracavity OPO cavity, operating at 62.5 kHz, produces average powers at 1573 nm up to 255 mW and peak powers higher than 1 kW. PACS 42.60.Gd; 42.65.Yj; 42.55.Xi     

Generalized Transvectants-Rankin–Cohen Brackets

We introduce o(p+1q+1)-invariant bilinear differential operators on the space of tensor densities on Rⁿ generalizing the well-known bilinear sl₂-invariant differential operators in the one-dimensional case, called Transvectants or Rankin–Cohen brackets. We also consider already known linear o(p+1q+1)-invariant differential operators given by powers of the Laplacian.     

Noncommutative Ricci Curvature and Dirac Operator on C q [SL2] at Roots of Unity

We find a unique torsion free Riemannian spin connection for the natural Killing metric on the quantum group C _q [ SL₂], using a recent frame bundle formulation. We find that its covariant Ricci curvature is essentially proportional to the metric (i.e. an Einstein space). We compute the Dirac operator and find for q an odd rth root of unity that its eigenvalues are given by q-integers [m] _q for m=0,1...,r–1 offset by the constant background curvature. We fully solve the Dirac equation for r=3.     

Spectral Gaps for Periodic Schrödinger Operators with Strong Magnetic Fields

We consider Schrödinger operators H^h=(ihd+A)*(ihd+A) with the periodic magnetic field B=dA on covering spaces of compact manifolds. Using methods of a paper by Kordyukov, Mathai and Shubin [14], we prove that, under some assumptions on B, there are in arbitrarily large number of gaps in the spectrum of these operators in the semiclassical limit of the strong magnetic field h0.Acknowledgement I am very thankful to Bernard Helffer for bringing these problems to my attention and useful discussions and to Mikhail Shubin for his comments.     

The early oxynitridation stages of hydrogen-terminated (100) silicon after exposure to N2:N2O. II. Silicon and oxygen bonding states

The early oxidation stages of hydrogen-terminated single-crystalline (100) silicon have been studied by X-ray photoemission spectroscopy, following the evolution of the Si2p and O1s signals after exposure to N₂:N₂O ambient at 850 °C for different durations. Evidence is given that the usual analysis of the film in terms of the Si2p peak leads to inconsistencies, related to the presence in the film of non-siloxanic bridges, as oxygen-rich defects (like hydroxyl terminations or peroxo bridges) or oxygen-deficient defects (like amino bridges). Information on the film structure is obtained by combining the analysis of the Si2p peak with that of the O1s peak. PACS 82.65.+r; 68.35.Fx; 68.35.Dv; 79.60.Jv     

Silver Nanoparticles Synthesized by Vapor Deposition onto an Ice Matrix

As a strategy for synthesizing metal nanoparticles, thermally evaporated Ag was deposited onto a thin (1.2nm) crystalline ice layer on hafnia (HfO₂) at 100K. The Ag atoms penetrate into the ice matrix but do not reach the underlying HfO₂ substrate. After controlled thermal desorption of water by heating to 300K, atomic force microscopy reveals Ag particle formation. Their lateral dimensions are between 5 and 20nm and, in many cases, their heights exceed the thickness of the original water layer. Fewer, higher and more regular Ag particles are formed in the presence, as compared to the absence, of ice. This is discussed in terms of two factors – Ag atoms reaching HfO₂ are thermally colder when they arrive from the ice matrix and desorption of water involves formation of liquid droplets, a process that concentrates Ag into the volumes occupied by the water droplets.     

Universal Properties of the Four-Boson System in Two Dimensions

We consider the nonrelativistic four-boson system in two dimensions interacting via a short-range attractive potential. For a weakly attractive potential with one shallow two-body bound state with binding energy B₂, the binding energies B_N of shallow N-body bound states are universal and thus do not depend on the details of the interaction potential. We compute the four-body binding energies in an effective quantum mechanics approach. There are exactly two bound states: the ground state with B₄⁽⁰⁾=197.3(1)B₂ and one excited state with B₄⁽¹⁾=25.5(1)B₂. We compare our results to recent predictions for N-body bound states with large N1.On leave from FZ Jülich, Institut für Kernphysik (Theorie), D-52425 Jülich and HISKP (Theorie), Universität Bonn, Nußallee 14–16, D-53115 Bonn, Germany     

Optimizing non-resonant frequency conversion in periodically poled media

Non-resonant frequency conversion into the blue, green, orange, and red spectral regions is reported. Fundamental light sources were continuous-wave non-planar monolithic single-mode ring Nd:YAG lasers as well as a standing-wave multi-mode Nd:YAG laser. Periodically poled KTiOPO₄ was employed as the nonlinear medium, but the considerations could also be applied to other periodically poled materials. A multi-pass scheme resulted in a normalized conversion efficiency as high as 27.2 %W^-1 for frequency doubling in the small-signal regime at 1064 nm. PACS 42.65.Ky; 42.79.Nv; 42.70.Mp     

Luminescent CdS Nanoparticles Embedded in Polyethylene Glycol (PEG 300) Matrix Thin Film

A simple method for synthesis of well dispersed cadmium sulphide (CdS) nanoparticles embedded in a polyethylene glycol matrix (PEG 300) in thin film form is presented. The molar ratio of PEG and CdS was varied within the range 70:30 to as high as 50:50. Films with controllable sizes (r 3 –8nm) of nanocrystals could be obtained by adjusting post-deposition annealing temperature (T) and time (t). The obtained films showed systematic variation in optical properties with decreasing crystal size due to quantum confinement. Transmission electron micrographs (TEM) indicated well resolved nanoparticles for films annealed at lower temperature (T = 373K) and time (t < 45 min). Photoluminescence (PL) studies indicated blue shift with decreasing particle size. The films did not degrade with aging in a humid atmosphere (relative humidity 40%) for several weeks.     

Diode-pumped passively Q-switched Nd : YAG laser at 1123 nm

A diode-pumped Nd:YAG laser at 1123 nm is passively Q-switched by using a low doping concentration Cr⁴⁺:YAG crystal as a saturable absorber. When pumped by a 1.5-W laser diode, the laser produces pulses of 50-ns duration with a pulse energy of as much as 15 J and a peak power of 300 W at a pulse-repetition rate of 10 kHz. PACS 42.60.Gd; 42.55.Rz; 42.55.Xi     

Reaction-Induced Transformations in Pt–Sn/SiO2 Catalysts: In Situ 119Sn Mössbauer Study

Reaction-induced separation of tin-rich surface layers and tin-depleted inner region was observed in metallic particles of Pt–Sn/SiO₂ catalysts in two reactions: (i) dechlorination of 1,2-dichloroethane at 473 K (modeling catalytic removal of chlorine from hazardous chlorocarbons) and (ii) oxidation of carbon monoxide at room temperature. In the former, a Pt:Sn catalyst (1:2 atomic ratio, 1 wt% metal content), prepared via co-impregnation, showed high selectivity (>80%) toward ethylene (at the expense of ethane), but only after a prolonged (ca. 24 h) period. In situ Mössbauer studies revealed stabilization of a homogeneous Pt–Sn alloy and SnCl₂ after activation in hydrogen; whereas tin-depleted and tin-rich components were separated after a 24-h period. Hence, inhibition of the hydrogenation activity of Pt, by surface tin enrichment and Cl deposition favors high ethylene selectivity. For the oxidation of CO at room temperature, a catalyst with a Pt:Sn atomic ratio of 3:2 (3 wt% Pt) was prepared by an organometallic (CSR) method using ¹¹⁹Sn(CH₃)₄. Platinum-rich PtSn(1) and tin-rich PtSn(2) components were separated in the Mössbauer spectra of catalyst activated at 570 K. The PtSn(2) component is primarily involved in surface reactions. Both in CO oxidation and the subsequent re-activation in hydrogen at room temperature a reversible PtSn(2) Sn⁴⁺ interconversion occurred. d ln(A ₇₇/A ₃₀₀)/dT data indicate the surface location of the involved components.     

Reversible hydrogen-storage functions for mixtures of Li3N and Mg3N2

The hydriding and dehydriding reactions occurring in mixtures of lithium and magnesium nitrides (Li₃N and Mg₃N₂) were investigated for the first time in fixed composition ratios. A mixture of Li₃N–20 at.%Mg₃N₂ after heat treatment at 833 K was hydrogenated at 523 K in 35 MPa according to the following overall reaction: 4Li₃N+Mg₃N₂+12H₂ 12LiH+3Mg(NH₂)₂. This reaction is one of the candidates for reversible hydrogen-storage functions in which 9.1 mass% of hydrogen can be stored in the solid state. PACS 81.05.Zx; 71.20.Ps; 82.30.-b     

Model for the hydrogen adsorption on carbon nanostructures

The hydrogen sorption capacity of carbon nanostructures was for several years a very controversial subject. Theoretical models have been published demonstrating a great potential for a large hydrogen sorption capacity of carbon nanostructures. Here we present a simple empirical model where condensation of hydrogen as a monolayer at the surface of nanotubes as well as bulk condensation in the cavity of the tube is assumed. The maximum potential amount of hydrogen absorbed according to the model was calculated to be 2.28×10^-3 mass%S[m²g^-1]=3.0 mass% for the adsorption of a monolayer hydrogen at the surface. The condensation of hydrogen in the cavity of the tube leads to a potential absorption for single wall nanotubes starting at 1.5 mass% and increasing with the diameter of the tubes. The experimentally measured hydrogen capacity of the nanotube samples correlates with the B.E.T. specific surface area. The slope of the linear relationship is 1.5×10^-3 mass%/m²g^-1. Therefore, the extrapolated maximum discharge capacity of a carbon sample is 2 mass%. Furthermore, it can be concluded, that the hydrogen sorption mechanism is related to the surface of the sample, i.e. a surface adsorption process. PACS 81.05.Uw; 81.07.De; 82.33.Pt     

Correlations between eigenvalues of a random matrix

Exact analytical expressions are found for the joint probability distribution functions ofn eigenvalues belonging to a random Hermitian matrix of orderN, wheren is any integer andN. The distribution functions, like those obtained earlier forn=2, involve only trigonometrical functions of the eigenvalue differences.     

Q-switching of a diode cladding-pumped Erbium Fibrelaser at 2.7 μm

Reliable Q-switched operation of a cladding-pumped 2.7m ZBLAN fibre laser in the range of the power limit of self-destruction is shown for the first time. The laser is pumped by a multimode diode. Q-switching is achieved by two different techniques: a rotating mirror and an acousto-optical modulator. Pulse widths of 500ns with the rotating mirror and 300ns (FWHM) with the acousto-optical modulator and pulse energies in the J range are achieved.     

On-axis calcium magneto-optical trap loaded with a focused decelerating laser

We report on a simple scheme to efficiently load an on-axis magneto-optical trap (MOT) from a decelerated atomic beam, which avoids perturbation by radiation pressure from the decelerating laser. This has been tightly focused near the MOT center, with a waist size much smaller than the atomic cloud. For comparison, and in order to test the efficiency of this non-optimum deceleration geometry we have employed a second, independent decelerating laser, with a profile mode matched to the atomic beam. Using a Calcium MOT, good performance has been achieved and for an oven temperature of 580 °C we loaded 1.2(2)×10⁷ atoms in 16(1)ms. The technique described here has been essential for the sensitive detection of cold collisions, which represent minor losses in MOTs of alkaline-earth metal elements (R.L. Cavasso-Filho, A. Scalabrin, D. Pereira, F.C. Cruz: Phys. Rev. A, 67, 021402(R) (2003)). PACS 32.80.Pj, 39.25.+k, 39.10.+j     

KrF excimer laser processing of thick diamond-like carbon films

Ablation characteristics of a commercially available diamond-like carbon (DLC) film (the EVERSCAN^TM bar code scanner window; Diamonex Division of Morgan Advanced Ceramics Inc.) in the 0–7 Jcm^-2 fluence window are reported. Glass pieces covered with 6 m thick DLC coating are ablated by 22 ns pulses of a KrF excimer laser in medium vacuum (10^-1 Pa), and in Ar and O₂ atmospheres of 10⁵ Pa. The ablation depth increases strictly linearly with an increasing number of pulses, independently of the atmosphere and the applied fluence. The threshold lies at 0.13 Jcm^-2 and is independent of the atmosphere. While above 1 Jcm^-2 the ablation rate vs. fluence plots recorded in different atmospheres coincide within experimental error, processing under vacuum between 0.5 and 1 Jcm^-2 results in significantly higher ablation rate values. Ablation rates exceeding 100 nm/pulse above 1 Jcm^-2 ensure extremely efficient machining over extended areas even at relatively low fluences. Identical characteristics in Ar and O₂ atmospheres suggest that solely physical (ablative) processes contribute to material removal. PACS 42.62.-b; 61.80.Ba; 68.35.Rh; 81.05.Uw     

Publisher's Note

When the article 'Modification of the finite difference scheme for efficient analysis of thin lossy metal layers in optical devices' by O. Conradi, S. Helfert, and R. Pregla was first published in the Optical Waveguide special issue of Optical and Quantum Electronics (volume 30, nos. 5–6, pages 369–373) the authors' affiliation given was incorrect. The article is printed again in full with the original pagination and credit lines. A misprint in the Results section has been corrected as well: the refractive index of both silver layers is 0.14–j11.0 at a wavelength of = 1523 nm, and not 0.41–j11.0.     

A Particle in a Magnetic Field of an Infinite Rectilinear Current

We consider the Schrödinger operator H=(i+A)² in the space L ₂(R ³) with a magnetic potential A created by an infinite rectilinear current. We show that the operator H is absolutely continuous, its spectrum has infinite multiplicity and coincides with the positive half-axis. Then we find the large-time behavior of solutions exp(–i H t)f of the time dependent Schrödinger equation. Our main observation is that a quantum particle has always a preferable (depending on its charge) direction of propagation along the current. Similar result is true in classical mechanics.     

Superintegrable chiral Potts model: Thermodynamic properties,an “inverse” model,and a simple associated Hamiltonian

The partition function of theN-state Superintegrable chiral Potts model is obtained exactly and explicitly (if not completely rigorously) for a finite lattice with particular boundary conditions. This yields the bulk and surface free energies, and horizontal and vertical correlation lengths and interfacial tensions. The critical exponents are =1–2/N, _hor=_hor=2/N, and _vert=_vert=1, and the finite-size corrections are obtained at criticality. The eigenvalue spectrum of the column-to-column transfer matrix is that of a direct product ofN byN matrices. Inverting this matrix gives a related solvable model which is a generalization of the free-fermion model. The associated Hamiltobian has a very simple form, suggesting there may be a more direct algebraic method (perhaps a generalized Clifford algebra) for obtaining its eigenvalues.