Elasticity,fracture and thermoreversible gelation of highly filled physical gels<Superscript>⋆</Superscript>

We describe a physically associating triblock copolymer-based gel that exhibits a reversible transition between solid and liquid states at a temperature of approximately 55^°C. The thermal transition of the gel enables us to compare the properties of liquid suspensions and elastic composites with identical particle loadings, with particle volume fractions as large as 0.55. The suspension viscosity and the composite elasticity scale in a similar manner with the overall particle volume fraction, a result that is rationalized in terms of an effective strain amplification factor that depends only on the particle loading. Measured values of the strain amplification factor are in good agreement with the expected form for well-dispersed spheres. We also find that the elastic composites are exceptionally strong, with fracture strengths that exceed the modulus of the base gel by a factor of 100 or more. Deviations from purely elastic behavior became important for high particle volume fractions, and were probed by stress relaxation experiments.     

Self-starting five optical cycle pulse generation in Cr<Superscript>4+</Superscript>:YAG laser

We report stable self-starting near-transform-limited 26-fs pulses at 250-mW output power from a Kerr-lens mode-locked (KLM) Cr⁴⁺:YAG laser using chirped mirrors in combination with prisms. The highest output power achieved in KLM regime was 600 mW at 55-fs pulse duration. The experimental results agree well with the results of theoretical analysis with respect to KLM self-starting ability and stability against continuous-wave and multi-pulse operation. Parameter ranges for stable transform-limited KLM pulses as well as the shortest achievable pulse durations are established. Using an InGaAs-InP semiconductor saturable absorber mirror we could obtain self-starting 57-fs pulses at the average output power of 200 mW. Two-photon absorption was found to be the main mechanism favoring the multiple-pulse operation. Received: 2 May 2002 / Revised version: 16 July 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +43-1/58801-38799, E-mail: sorokina@tuwien.ac.at     

Control of amphiphilic block copolymer morphologies using solution conditions

It is well known that the morphology of block copolymer aggregates depends on polymer properties such as the molecular weight, the relative block length, and the chemical nature of the repeat unit. Recently, we have shown that if aggregates are allowed to self-assemble in solution, then in addition to the above factors, a high degree of control over the aggregate architecture can be achieved by adjusting the solution conditions. Factors such as the water content in the solvent mixture, the solvent nature and composition, the presence of additives (ions, surfactants, and homopolymer) and the polymer concentration were successfully employed to control the aggregate shape and size. In this paper, we review a series of studies performed in our group to show how solution properties can control the architecture of aggregates prepared from a given copolymer. The control mechanism is explained in terms of the effect of each property on the forces that govern the formation of any given morphology, namely the core-chain stretching, corona-chain repulsion and interfacial tension. Received 30 April 2002 and Received in final form 3 September 2002 Published online: 21 January 2003     

2.0-MeV Er+ implanted in silicon: depth distribution, damage profile and annealing behaviour

Si crystals were implanted with 2.0- MeV Er⁺ at the doses of 5×10¹² ions/cm², 1×10¹⁴ ions/cm², 5×10¹⁴ions/cm², 1×10¹⁵ ions/cm² and 2.5×10¹⁵ ions/cm². Conventional furnace thermal annealing was carried out in the temperature range from 600 °C to 1150 °C. The depth distribution of Er, associated damage profiles and annealing behavioar were investigated using the Rutherford backscattering spectrometry and channelling (RBS/C) technique. A proper convolution program was used to extract the distribution of Er from the experimental RBS spectrum. The obtained distribution parameters, projected range R_p, projected range straggling ΔR_p and skewness SK were compared with those of TRIM96 calculation.The experimental R_p and SK values agree well with the simulated values, while the experimental ΔR_p is larger than TRIM 96 simulated value by a factor of 18%. The damage profile of silicon crystal induced by 2.0-MeV Er⁺ at a dose of 1×10¹⁴ ions/cm² was extracted using the multiple-scattering dechannelling model based on Feldman’s method, which is in a good agreement with the TRIM96 calculation. For the samples with dose of 5×10¹⁴ ions/cm² and more, an abnormal annealing behavioar was found and a qualitative explaination has been given. Received: 11 October 1999 / Accepted: 28 March 2000 / Published online: 5 July 2000     

Tailoring the nonlinear refractive index of fluoride-phosphate glasses for laser applications

In this paper we report on measurements of the nonlinear refractive index n₂ at 1047 nm of various fluoride-phosphate laser glasses relative to fused silica using degenerate four-wave mixing. We find good agreement with empirical estimates obtained from the d-line linear refractive index and the Abbe number for pure fluoride-phosphate glasses. Significant increase of n₂ is observed when adding sulfate and niobium oxide to the glass composition offering tailorable nonlinear properties for glasses employed in short pulse laser oscillators. Received: 27 December 1999 / Published online: 24 March 2000     

Optical spectra and luminescence dynamics of Pr3+ and Nd3+ in SrLaAlO4 single crystals

Single crystals of SrLaAlO₄: Pr³⁺ and SrLaAlO₄: Nd³⁺ have been grown by the Czochralski method and their optical properties have been studied for different activator concentrations. The absorption and emission spectra of the activators are inhomogeneously broadened because the Sr²⁺ and La³⁺ ions are distributed randomly on the sites ofC ₄ symmetry, however, the symmetry selection rules are still obeyed. Strong self-quenching of the praseodymium luminescence occurs by cross relaxation which is supposed to be assisted by phonon emission in the case of the³ P ₀ level. Self-quenching of Nd³⁺ luminescence disappears at low temperature indicating that the condition of resonance in the cross-relaxation process is fulfilled only for higher components of the ground state.Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wroclaw, Poland     

Spectroscopic properties and exicted-state relaxation dynamics of Er3+ in LiNbO3

Nonequivalent Er³⁺ sites in LiNbO₃ crystals have been evidenced by low-temperature optical absorption measurements. The radiative transition rates for the ⁴ S _3/2, ⁴ F _9/2, ⁴ I _9/2, ⁴ I _11/2, and ⁴ I _13/2 levels have been evaluated by the Judd-Ofelt method, and the contribution of multiphonon relaxation to the decay of excited states has been estimated. The quantum efficiency of the ⁴ I _11/2 level has been estimated to be 6% only, thus the excitation of levels lower than ⁴ S _3/2 is expected to populate efficiently the ⁴ I _13/2 level.     

Vacuum ultraviolet luminescence from a micro-pulling-down method grown Nd:(La0.9,Ba0.1)F2.9

Vacuum ultraviolet (VUV) luminescence from a Nd³⁺:(La_1−x,Ba_x)F_3−x (x=0.1) and a Nd³⁺:LaF₃ single crystal grown by the micro-pulling-down method modified for fluoride crystal growth is discussed. Emission resulting from excitation with 157 nm pulses of a F₂ laser and by 290 nm femtosecond pulses of a Ti:sapphire laser show that the luminescence spectral and temporal characteristics are similar for both excitation cases and that they have good prospects as a VUV laser material.     

Investigation of 2.0 μm emission in Tm and Ho co-doped oxyfluoride tellurite glass

This paper reports 2.0 μm emission properties of Tm³⁺/Ho³⁺ co-doped oxyfluoride tellurite glass exited by 808 nm laser diode (LD). Mid-infrared transmittance property of glass was investigated by Fourier transform infrared (FTIR) spectrometer. The real chemical composition of investigated glass was identified by X-ray photoelectric spectroscopy (XPS). Thermal stability of the glass was determined by differential thermal analysis (DTA) measurement. The Judd-Ofelt parameters, spontaneous radiative transition probabilities, branching ratios and radiative lifetime of Ho³⁺ were calculated based on the absorption spectra by using Judd-Ofelt theory. Results indicate that the maximum 2.0 μm emission intensity attributed to the ⁵I₇→⁵I₈ transition of Ho³⁺ was achieved at 1.5 mol% Tm₂O₃ and 1 mol% Ho₂O₃ concentrations in oxyfluoride tellurite glass. OH⁻ absorption at 3000 cm⁻¹ was greatly depressed by introduction of 10 mol% F⁻. The maximum absorption and stimulated emission cross-section of Ho³⁺ near 2.0 μm are 7.0×10⁻²¹ cm² at 1950 nm and 8.8×10⁻²¹ cm² at 2048 nm, respectively. The calculated radiative lifetime of 4.4 ms for ⁵I₇→⁵I₈ transition and large stimulated emission cross-section of the Tm³⁺/Ho³⁺ co-doped oxyfluoride tellurite glass indicate that the glass has a potential application in efficient 2.0 μm laser.     

Photo-induced synthesis of DNA-templated metallic nanowires and their integration into micro-fabricated contact arrays

We report the synthesis of metallic nanowires accomplished by site-specific integration of single DNA duplexes into micro-fabricated contact arrays and their subsequent selective metallization. DNA interconnects between metallic contacts are formed by tethering the ends of DNA molecules, stretched in hydrodynamic flow into a linear conformation, at different gold contact pads via thiol functional groups. To transform the DNA interconnects into metallic cluster chains or nanowires, we use an electroless metal deposition technique where platinum ions bound along the DNA molecules from a salt solution are reduced to metallic clusters of less than 10 nm in diameter by applying UV light.