Actively mode-locked fiber laser using a deformable micromirror

We present what we believe to be the first fiber laser system that is actively mode-locked by a deformable micromirror. The micromirror device is placed within the laser cavity and performs a dual function of modulator and end-cavity mirror. The mode-locked laser provides ~1-ns-long pulses with 20 nJ/pulse energy at 5 MHz repetition rates.     

Silicon Carbide Nanoparticles Produced by CO2 Laser Pyrolysis of SiH4/C2H2 Gas Mixtures in a Flow Reactor

Pulsed CO₂-laser-induced decomposition of different mixtures of SiH₄ and C₂H₂ in a flow reactor has been employed to produce silicon carbide clusters and nanoparticles with varying content of carbon. The as-synthesized species were extracted from the reaction zone by a conical nozzle and expanded into the source chamber of a cluster beam apparatus where, after having traversed a differential chamber, they were analyzed with a time-of-flight mass spectrometer. Thin films of silicon carbide nanoclusters were produced by depositing the clusters at low energy on potassium bromide and sapphire windows mounted into the differential chamber. At the same time, Si and SiC nanoparticles were collected in a filter placed into the exhaust line of the flow reactor. Both beam and powder samples were characterized by FTIR spectroscopy. The close resemblance of the spectra suggests that the composition of the beam and powder particles obtained during the same run is nearly identical. XRD spectroscopy could only be employed for the investigation of the powders. It was found that CO₂ laser pyrolysis is ideally suited to produce silicon carbide nanoparticles with a high degree of crystallinity. Nanopowders produced from the pyrolysis of a stoichiometric (2:1) mixture of SiH₄/C₂H₂ were found to contain particles or domains of pure silicon. The characteristic silicon features in the FTIR and XRD spectra, however, disappeared when C₂H₂ was applied in excess.     

Ti:sapphire rib channel waveguide fabricated by reactive ion etching of a planar waveguide

We report, to our knowledge, the first active channel waveguide in Ti:sapphire. We have created ∼1.4-μm high ribs in a ∼10-μm thick Ti:sapphire planar waveguide by reactive ion etching. Following excitation by an Ar-ion laser, the rib structure showed channel-waveguide fluorescence emission. The mode profiles and the beam-parameter values (M²) were measured. The coupling efficiency of fluorescence emission into a single-mode fiber was an order of magnitude higher than for fluorescence from unstructured planar regions of the waveguide. Such devices are of interest as low-threshold tunable lasers and as broadband light sources in low-coherence interferometry. Received: 22 December 2002 / Revised version: 30 March 2002 / Published online: 8 August 2002     

Three-dimensional structuring of sapphire by sequential He+ ion-beam implantation and wet chemical etching

We present a method for the selective two- and three-dimensional patterning of sapphire using light ion-beam implantation to generate severe lattice damage to depths exceeding 1 μm and subsequent selective wet chemical etching of the damaged regions by hot H₃PO₄. C-cut sapphire crystals were implanted through contact masks using ion fluences of 1×10¹⁶ to 5×10¹⁷ He⁺/cm² and energies up to 400 keV. The etching process is characterized by a high selectivity and a rate of approximately 19 nm/min. Whereas an implantation that produces a continuously damaged pathway results in complete etching from the surface, sole in-depth implantation using only high-energy ions leads to under-etching of the crystalline surface layer. By a combination of these processes we have fabricated three-dimensional structures such as channels and bridges in sapphire. Received: 14 October 2002 / Accepted: 15 October 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax +41-21/693-3701, E-mail: aurelian.crunteanustanescu@epfl.ch     

Filamentary iron nanostructures from laser-induced pyrolysis of iron pentacarbonyl and ethylene mixtures

In this work, we present results of the synthesis and characterization of iron and iron oxide nanoparticles aggregated in filamentary, spider-web-like structures. The particles were produced in a flow reactor by CO₂ laser pyrolysis of gaseous mixtures of iron pentacarbonyl and ethylene. Low- and high-resolution electron microscopy reveals chain-like structures of particles, most of them being composed of an α-iron core and an iron oxide shell, identified as magnetite and, to a lesser extent, hematite. These results are in good agreement with a M?ssbauer analysis carried out for the same samples. The role of the reaction temperature on the synthesis of filamentary iron nanostructures by infrared laser pyrolysis of Fe(CO)₅/C₂H₄ mixtures is discussed. Received: 31 May 2000 / Accepted: 6 June 2000 / Published online: 2 August 2000     

Magnetic Properties of Iron/Graphite Core-shell Structured Nanoparticles Prepared by Annealing of Fe-C-N Nanocomposite

We are reporting the core-shell structured iron/graphite nanoparticles formed during annealing of a nanopowder prepared by laser pyrolysis of gas phase reactants. The originally synthesized Fe-C-N nanocomposite powder has been characterized by TEM, XRD and magnetic measurements. Nanopowder was heated up to 800 °C at ～ 1 Pa vacuum. Presence of iron nanoparticles with mean diameter 40 nm in the annealed state of nanopowder was proved by XRD and TEM analyses. Mössbauer spectroscopy was used for characterization of synthesized/annealed nanopowder to confirm the qualitative change in phase composition.     

Preparation of carbon nitride fine powder by laser induced gas-phase reactions

3 N₄ structure, with an α-C₃N₄-like form being prevalent. The powders were found to be slightly contaminated by SF₆ sensitizer products. Our results suggest that by improving the experimental parameters this contamination might be reduced and that the laser pyrolysis method offers possibilities for production of C_xN_y materials, with controlled composition. Received: 4 February 1997/Accepted: 7 April 1997