Athermal polarisation-independent arrayed- waveguide grating (AWG) multiplexer using an all-polymer approach

An athermal arrayed-waveguide grating (AWG) multiplexer relying on an all-polymer approach is reported. By properly adjusting the positive coefficient of thermal expansion of the polymer substrate and the negative thermo-optic coefficient of the polymer waveguide, athermal and polarisation-independent AWG devices are demonstrated. Received: 21 May 2001 / Revised version: 26 July 2001 / Published online: 2 November 2001     

Determination of the mass of polymer films using an AFM cantilever

There are no established methods that have a sensitivity during the determination of an adsorbed polymer film mass that is not worse than 1 ng that do not require additional calibration and the usage of reference measures. A highly sensitive method for measuring an adsorbed polymer is proposed in this work. The added mass was determined by the change of the resonance frequency of a cantilever used in atomic-force microscopy (AFM) as a probe. A modification of the cantilever surface is proposed that allows one to avoid the affect of the adsorbed polymer on the cantilever force constant. The mass of poly(diallyldimethylammonium) (PDDA) chloride adsorbed on the cantilever surface was obtained with a sensitivity of 0/01 ng using an AFM cantilever.     

A model for the interaction of near-infrared laser pulses with metal powders in selective laser sintering

A thermal model of the interaction of pulsed near-infrared laser radiation from a Nd:YAG laser was made, taking the measured powder properties such as reflectance, optical penetration depth and thermal conductivity into account. It allows an estimation of the evolution of two different temperatures: the average temperature of the powder (taken over the grains in a volume given by the laser beam diameter and the optical penetration depth) and the temperature distinction within a single grain. It showed that in pulsed mode consolidation can be achieved at much lower average power as the surface of the powder particles are molten but their cores remain at nearly room temperature. This leads to a much lower average temperature and therefore a dramatic decrease in residual thermal stresses in the finished piece. The results of the model were experimentally tested and confirmed. Received: 26 July 2001 / Accepted: 23 November 2001 / Published online: 23 January 2002     

Photoacoustic cavitation and heat transfer effects in the laser-induced temperature jump in water

The finite element method is employed to analyze photoacoustic cavitation and heat transfer occurring when modest temperature jumps (T-jumps) are induced by a laser in D₂O solution, which may contain a small concentration of a protein or peptide sample. Cavitation can be initiated through a photoacoustic mechanism at intensities well below optical breakdown thresholds. Cavitation probability is related to test medium properties, initial temperature, T-jump magnitude and test region geometry. Parameters affecting thermal conduction losses are also examined because such losses limit the useful duration of the T-jump induced in protein folding experiments. From this study, guidelines are offered for reducing the occurrence of cavitation and extending the useful duration of the T-jump. Received: 2 April 2001 / Revised version: 17 October 2001 / Published online: 29 November 2001     

Fast imaging method combining cantilever and feedback signals in contact-mode atomic force microscopy

A fast imaging method in a contact-mode atomic force microscope (AFM) is examined for its principle and performance, where the image is acquired by combining a cantilever signal and a feedback signal applied to a piezotube. The frequency component of the feedback signal is restricted in the lower frequency region to keep the linear relationship between the feedback signal and the displacement of the piezotube. It is shown that the image is basically independent of the feedback details since a wide detection bandwidth is certified by the cantilever response much faster than by the feedback response, allowing a fast scanning. The fast scanning, however, enhances the distortion in the transient region where surface height changes abruptly. This influence can be reduced by choosing the scan line direction for the data acquisition. The combination procedure also reduces the low-frequency noise in the feedback signal. A 512×512-pixel image was obtained in 90 s without sacrificing the resolution. Received: 24 November 1998 / Accepted: 31 March 1999 / Published online: 2 June 1999     

Kerr-lens, mode-locked lasers as transfer oscillators for optical frequency measurements

We introduce a novel concept for optical frequency measurement and division which employs a Kerr-lens, mode-locked laser as a transfer oscillator whose noise properties do not enter the measurement process. We experimentally demonstrate that this method opens up the route to phase-link signals with arbitrary frequencies in the optical or microwave range while their frequency stability is preserved. Received: 23 July 2001 / Published online: 29 November 2001     

Anderson Transitions for a Family of Almost Periodic Schrödinger Equations in the Adiabatic Case

This work is devoted to the study of a family of almost periodic one-dimensional Schr?dinger equations. Using results on the asymptotic behavior of a corresponding monodromy matrix in the adiabatic limit, we prove the existence of an asymptotically sharp Anderson transition in the low energy region. More explicitly, we prove the existence of energy intervals containing only singular spectrum, and of other energy intervals containing absolutely continuous spectrum; the zones containing singular spectrum and those containing absolutely continuous are separated by asymptotically sharp transitions. The analysis may be viewed as utilizing a complex WKB method for adiabatic perturbations of periodic Schr?dinger equations. The transition energies are interpreted in terms of phase space tunneling. Received: 2 July 2001 / Accepted: 13 November 2001     

Sampling high-frequency signals using a poled-polymer asymmetric Fabry–Perot microcavity as an electro-optic probe tip

The sampling of high-frequency signals using poled electro-optic polymer films as electro-optic probe tips has been demonstrated for the first time. The electro-optic polymer, which was spin coated onto a high-reflectivity glass substrate, was corona poled; thus an asymmetric Fabry–Perot microcavity was formed, based on the difference between the polymer reflectivity and that of high-reflectivity glass. This converts phase modulation to amplitude modulation, so only one laser beam is needed in this system. The sampling technique has been analyzed by multiple reflection and index ellipsoid methods. A 1.2 GHz microwave signal propagating on a coplanar waveguide transmission line has been sampled, and a voltage sensitivity of about 0.5 mV/ was obtained. Received: 16 August 2001 / Revised version: 20 November 2001 / Published online: 14 March 2002     

Flexible ceramic–polymer composite films with temperature-insensitive and tunable dielectric permittivity

BaTiO₃–polymer composite layers have been produced by the spin-on technique (thickness 3–10 μm). The dielectric permittivity of the layers at room temperature can be tuned from 2.8 to approximately 33 by varying the ceramic filling from 0 to 60% by volume. The dielectric properties of the films are almost insensitive to temperature variations in the range 20–180 °C. Free-standing composite layers with ceramic content ≤50% are flexible without noticeable change of permittivity after repeated mechanical bending. Received: 22 November 2001 / Accepted: 24 November 2001 / Published online: 23 January 2002     

Renormalization group analysis of a confined, interacting Bose gas

The renormalization group is not only a powerful method for describing universal properties of phase transitions, but it is also useful for evaluating non-universal thermodynamic properties beyond mean-field theory. In this contribution we concentrate on these latter aspects of the renormalization group approach. We introduce its main underlying ideas in the familiar context of the ideal Bose gas and then apply them to the case of an interacting, confined Bose gas within the framework of the random phase approximation. We model confinement by periodic boundary conditions and demonstrate how confinement modifies the flow equations of the renormalization group, thus changing the thermodynamic properties of the gas. Received: 20 July 2001 / Revised version: 20 August 2001 / Published online: 23 November 2001     

Lattice dynamics of RbC60, RbC70 and phase transitions in A1C60: A neutron scattering study

1 C₆₀ (A = Rb, Cs, K) and RbC₇₀ have been investigated by neutron scattering. The studies include measurements in the high temperature rotator-, the polymer-, the dimer- and the low temperature isolator-phase and include investigations of the particular phase transitions. It is found that each of the phases can be characterized by specific features of its lattice dynamics which can be used for detailed studies of the dynamics of the rotator-polymer and the dimer-polymer transitions. Particular insight in the lattice dynamics is obtained from model calculations for the RbC₆₀ polymer and dimer phase. Received: 13 September 1996/Accepted: 11 November 1996     

Spin-dependent internal photo-electron emission over metal–semiconductor junctions – a study with circularly polarised infrared radiation

The spin asymmetry of internal photo-electron emission over Schottky barriers has been investigated for Fe/GaAs (001) having close-to-ideal current–voltage characteristics. Using a low-power circularly polarised YAG laser operated at its fundamental frequency (λ=1064 nm) as well as a visible diode laser (λ=670 nm) we demonstrate that, by eliminating the photo-current due to inter-band transitions in GaAs with the infrared source, a significant enhancement to the magnetic asymmetry could be achieved. The bias dependence of the asymmetry was also measured. It was found that the values were considerably different for the photo-electrons traversing the barriers in opposite directions. Received: 15 November 2001 / Revised version: 23 March 2002 / Published online: 2 May 2002     

Estimating thermal transport in deep X-ray lithography with an inversion method

This study proposes a general methodology for estimating the depth profile of the heat source of the thermal transport system during deep X-ray lithography. The exposure process in a lithography system is considered as an inverse heat conduction problem with an unknown heat source. The conjugate gradient method is used to solve the inverse problem. Numerical results confirm that the method proposed herein can accurately estimate the heat source even involving the inevitable measurement errors. Furthermore, this methodology can also be applied to estimate the local distribution of temperatures when using scanning thermal microscopy (SThM) to microthermally machine materials and will contribute to increase the quality of microthermally machined products. In addition, a thermomechanical data-storage system, which utilizes a resistively heated atomic-force-microscopy (AFM) cantilever tip to read and write data bits, can also adopt this inverse methodology to control the temperature of a polymer substrate.     

Photoluminescence and capacitance transients in highly Mg-doped GaN

Behaviors of the photoluminescence blue-band and near-bandgap peak and the relevant thermal ionization energies of the shallow and deep Mg-related acceptors have been studied, respectively. The 2.989 eV blue-band is attributed to the deep donor–acceptor-pair transitions involving a deep Mg-related acceptor at E_v+0.427 eV. The blueshift with increasing excitation power is explained by variation in the contribution of close and distant donor–acceptor-pairs to the luminescence. The redshift with increasing temperature results from thermal release of carriers from close donor–acceptor-pairs. The 3.26 eV near-bandgap peak is attributed to the shallow donor–acceptor-pair transitions involving a shallow Mg-related acceptor at E_v+0.223 eV. The relevant thermal ionization energies of the shallow and deep Mg-related acceptors, being about E_v+0.16 and E_v+0.50 eV, are determined from deep-level transient Fourier spectroscopy measurements. Received: 11 July 2001 / Accepted: 9 August 2001 / Published online: 2 May 2002     

Hybrid polymer/silica thermo-optic vertical coupler switches

Thermo-optic 1×2 vertical coupler switches (VCSs) using a hybrid polymer/silica integration technology were designed using a finite element method and a coupled mode method for different refractive index contrasts. The multilayer structures were optimized by thermal analysis. Based on these designs and simulations, hybrid polymer/silica thermo-optic 1×2 VCSs exhibiting low insertion loss, low crosstalk, low switching power and polarisation independence were demonstrated. Based on this building block, a 1×8 VCS has been fabricated. Received: 21 May 2001 / Published online: 23 October 2001     

Nuclear spin relaxation induced by a mechanical resonator

We report on measurements of the spin lifetime of nuclear spins strongly coupled to a micromechanical cantilever as used in magnetic resonance force microscopy. We find that the rotating-frame correlation time of the statistical nuclear polarization is set by the magnetomechanical noise originating from the thermal motion of the cantilever. Evidence is based on the effect of three parameters: (1) the magnetic field gradient (the coupling strength), (2) the Rabi frequency of the spins (the transition energy), and (3) the temperature of the low-frequency mechanical modes. Experimental results are compared to relaxation rates calculated from the spectral density of the magnetomechanical noise.     

Holographic recording mechanisms of gratings in indium oxide films using 325 nm helium–cadmium laser irradiation

UV (325 nm) holographic recording of gratings in indium oxide films fabricated by reactive pulsed laser deposition has been investigated as a function of growth temperature, oxygen pressure and angle of incidence of the plasma plume on the substrate. The influence of the ambient environment (air or vacuum) and the film temperature during recording has also been studied. Large steady state refractive index changes up to 6×10^-3 were observed in layers grown at an oblique angle of 75°. About 77% of the magnitude of these changes residues after thermal annealing and is attributed to UV-induced permanent structural rearrangements. In contrast, refractive index changes in films grown at normal incidence were smaller in magnitude and completely reversible. Received: 30 July 2001 / Accepted: 20 November 2001 / Published online: 23 January 2002     

Surface modification of a polymer film by cryogenic laser ablation of organosilicon compounds

An organosilicon compound, hexamethylcyclotrisilazane (HMCTS), was photolyzed in a solid film at 84 K with an ArF excimer laser. A polymeric film was prepared as a result of polymerization and/or crosslinking using a reactive species such as 1-(methylsilyl)methanimine, generated by photolysis. At room temperature in air, the film changed into a polymer having siloxane units. However, fragments generated by laser irradiation of HMCTS in the frozen film made a similar polymer film having a siloxane structure on a PVA film in air. The polymeric film with siloxane units prepared by the cryogenic laser ablation method displayed hydrophobic properties. Received: 23 May 2001 / Accepted: 30 May 2001 / Published online: 30 August 2001     

Optical nonlinearity enhancement for an anisotropic semiconductor–insulator composite

We investigate the effect of geometric anisotropy on the optical nonlinearity enhancement for a periodic composite with a rectangular array of elliptic semiconducting cylinders in an insulating host. By using a series expression of the space-dependent electric field obtained by a simple Fourier method in a periodic composite, we calculate the frequency dependence of the effective third-order nonlinear susceptibility as a function of anisotropy. The results show that the height of the nonlinearity enhancement peak may be increased by several orders of magnitude as the aspect ratio of the ellises is decreased or the lattice edge length ratio is increased. At resonance frequency, there exists a strong anomalous dispersion. We also investigate the effect of the volume fraction of the semiconductor phase for composites with a square array of circular semiconducting cylinders. Received: 23 November 2000 / Accepted: 2 August 2001 / Published online: 17 October 2001     

Time-resolved thermal lens study on the heat dissipation effects in solid polymeric matrices used as laser dyes

A dual-beam pulsed-laser time-resolved thermal lens (TRTL) has been used to study the heat dissipation effects in solid polymers employed as laser dyes. The laser-dye samples studied are polymer solutions of rhodamine 6G (Rh6G), namely homopolymers of methyl methacrylate (MMA) and copolymers of methyl methacrylate and 2-hydroxyethyl methacrylate (HEMA) with MMA:HEMA compositions 1:1 and 7:3 vol./vol., where the dye is either dissolved or modified and copolymerized with methacrylate monomers. The long-term stability of the laser material is independent of the thermal diffusivity of the samples, as determined by the TRTL technique. This suggests that the rate of heat dissipation does not play a major role in the photostability of the samples. In contrast, the TRTL experiments have revealed permanent changes in the optical properties of the polymers when subjected to a large number of laser pulses. This is explained in terms of permanent plastic deformation of the matrices. Received: 19 June 2002 / Revised version: 1 August 2002 / Published online: 15 November 2002 RID="*" ID="*"Dedicated to Prof. S.E. Braslavsky on the occasion of her 60th birthday. This work was presented at the V Congreso de Fotoquímica, Torremolinos, Spain, 2001 RID="**" ID="**"Corresponding author. Fax: +34-93/205-6266, E-mail: s.nonell@iqs.es