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Opt Express ; 25(19): 23035-23044, 2017 Sep 18.
Artigo em Inglês | MEDLINE | ID: mdl-29041607


We demonstrate low-loss GaN/AlGaN planar waveguides grown by molecular beam epitaxy on sapphire substrates. By using a proper AlGaN cladding layer and reducing surface roughness we reach <1dB/cm propagation losses at 633nm. These low propagation losses allow an efficient second harmonic generation using modal phase matching between a TM0 pump at 1260nm and a TM2 second harmonic at 630nm. A maximal power conversion of 2% is realized with an efficiency of 0.15%·W-1cm-2. We provide a modelling that demonstrates broadband features of GaN/AlGaN platform by showing second harmonic wavelengths tunability from the visible up to the near-infrared spectral region. We discuss drawbacks of modal phase matching and propose a novel solution which allows a drastic improvement of modal overlaps with the help of a planar polarity inversion. This new approach is compatible with low propagation losses and may allow as high as 100%·W-1cm-2 conversion efficiencies in the future.

Inorg Chem ; 51(15): 8202-11, 2012 Aug 06.
Artigo em Inglês | MEDLINE | ID: mdl-22780452


The reaction of the trivacant Dawson polyoxometalate α-[P(2)W(15)O(56)](12-) and the divalent cations Co(2+) is known to form a symmetrically derived sandwich complex of formula ßß-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](16-) [symbolized as ßß-Co(4)(P(2)W(15))(2)] at low pH (ca. pH 3). We have shown previously that, by a slight modification of the reaction conditions, trinuclear αß-[(NaOH(2))Co(3)(H(2)O)(P(2)W(15)O(56))(2)](17-) and dinuclear [(NaOH(2))(2)Co(2)(P(2)W(15)O(56))(2)](18-) complexes [symbolized as αß-NaCo(3)(P(2)W(15))(2) and Na(2)Co(2)(P(2)W(15))(2), respectively] can be synthesized as aqueous-soluble sodium salts. αß-NaCo(3)(P(2)W(15))(2) is a "lacunary" sandwich complex that can add a Co(2+) cation to form nearly quantitatively an unsymmetrical Dawson tetracobalt sandwich polyoxometalate, αß-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](16-) [symbolized as αß-Co(4)(P(2)W(15))(2)]. Thus, for Co(4)(P(2)W(15))(2), the junctions between the trivacant {P(2)W(15)} subunits and the central tetrameric unit can be either both ß type or ß and α types. The interconversion between αß-Co(4)(P(2)W(15))(2) and ßß-Co(4)(P(2)W(15))(2) and the decomplexation process at low pH, leading to the formation of αß-NaCo(3)(P(2)W(15))(2) and/or Na(2)Co(2)(P(2)W(15))(2), have been followed in aqueous solution at various pH values by electrochemistry, UV-visible absorption spectroscopy, and (31)P NMR spectroscopy.

Ânions/química , Cobalto/química , Complexos de Coordenação/química , Compostos de Tungstênio/química , Cátions , Técnicas Eletroquímicas , Concentração de Íons de Hidrogênio , Espectroscopia de Ressonância Magnética , Modelos Moleculares , Conformação Molecular , Sais , Sódio , Espectrofotometria Ultravioleta , Água