[show abstract][hide abstract] ABSTRACT: We report the formation of two-dimensional disordered arrays of poly(methyl)methacrylate (PMMA) microcolumns with embedded single size distribution of Lu0.990Er0.520Yb0.490 nanocrystals, (Er,Yb):Lu2O3, using a disordered porous silicon template. The cubic (Er,Yb):Lu2O3 nanocrystals, which crystallize into the cubic system with Ia3 space group, were synthesized using the modified Pechini method. Electronic microscopic techniques were used to study the distribution of the nanocrystals in the PMMA columns. Cathodoluminescence was used to observe the visible luminescence of the particles. Red emission attributed to 4 F9/2 [rightwards arrow] 4I15/2 erbium transition is predominant in these new composites.
Nanoscale Research Letters 09/2013; 8(1):385. · 2.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: Porous GaN layers are grown on silicon from gold or platinum catalyst seed layers, and self-catalyzed on epitaxial GaN films on sapphire. Using a Mg-based precursor, we demonstrate p-type doping of the porous GaN. Electrical measurements for p-type GaN on Si show Ohmic and Schottky behavior from gold and platinum seeded GaN, respectively. Ohmicity is attributed to the formation of a Ga2Au intermetallic. Porous p-type GaN was also achieved on epitaxial n-GaN on sapphire, and transport measurements confirm a p-n junction commensurate with a doping density of ∼1018 cm−3. Photoluminescence and cathodoluminescence confirm emission from Mg-acceptors in porous p-type GaN.
[show abstract][hide abstract] ABSTRACT: Investigations of the specific heat of the potassium gadolinium double tungstate KGd(WO4)2 have been performed over the temperature range from 0.05 K up to 4 K in zero magnetic field. The λ -type anomaly observed at T = 0.42 K was interpreted as an indication of a second order magnetic phase transition within the Gadolinium sublattice. The critical exponent α was determined from experimental data.
Central European Journal of Physics 08/2013; 11(3). · 0.91 Impact Factor
[show abstract][hide abstract] ABSTRACT: Simultaneous continuous-wave laser oscillation at two wavelengths has been observed and studied in a diode-pumped monoclinic N
p-cut Tm:KLu(WO4)2 for different transmission of the output coupler. The maximum output power reached 1.15 W with a slope efficiency of 20.4 % with respect to the absorbed power for polarization parallel to the N
m optical axis. In an analogous N
g-cut crystal, the dual-wavelength laser operation is accompanied by polarization switching with increasing pump power and the switching point depends on the output coupling. The thresholds are slightly higher, and the slope efficiency reached a maximum of 25.5 % for polarization parallel to N
m at low pump levels, but at high pump levels, the oscillating polarization is parallel to N
p, reaching maximum output power of 3.09 W. Simple modelling with rate equations taking into account reabsorption losses explains qualitatively the complex behavior observed in the continuous-wave laser experiments with this anisotropic biaxial laser crystal.
[show abstract][hide abstract] ABSTRACT: Porous GaN crystals have been successfully grown and electrically contacted simultaneously on Pt-and Au-coated silicon substrates as porous crystals and as porous layers. By the direct reaction of metallic Ga and NH3 gas through chemical vapor deposition, intermetallic metal-Ga alloys form at the GaN-metal interface, allowing vapour-solid-solid seeding and subsequent growth of porous GaN. Current-voltage and capacitance-voltage measurements confirm that the intermetallic seed layers prevent interface oxidation and give a high-quality reduced workfunction contact that allows exceptionally low contact resistivity. Additionally, the simultaneous formation of a lower workfunction intermetallic permits ohmic electron transport to n-type GaN grown using high workfunction metals that best catalyze the formation of porous GaN layers and may be employed to seed and ohmically contact a range of III-N compounds and alloys for broadband absorption and emission. Additionally, we show how a porous GaN rectifying diode can be formed by oxidatively crystallizing Mg typically employed for p-doping GaN, as a layer formed under porous structure resulting in a high-k polycrystalline MgO dielectric.
[show abstract][hide abstract] ABSTRACT: We present the fabrication of single-mode rib waveguides in (Yb,Nb) : RbTiOPO4 ((Yb,Nb) : RTP) by reactive ion etching (RIE) with a combination of SF6 and Ar gases. The influence of the gas pressure, RF power and gas ratio on the etch rate and surface quality was studied to optimize the etching of RTP. Optimized parameters were used to fabricate rib waveguides in a (Yb,Nb) : RTP film, grown by liquid phase epitaxy, with an etch rate of 10 nm min−1. Channel waveguide propagation was demonstrated for the first time in an RIE etched (Yb,Nb) : RTP rib structure.
Journal of Physics D Applied Physics 03/2013; 46(14):145108. · 2.53 Impact Factor
[show abstract][hide abstract] ABSTRACT: CW mirrorless laser operation at 1840 nm at room temperature was observed in buried rib waveguides of
KY0.58Gd0.22Lu0.17Tm0.03(WO4)2 fabricated by structuring KY(WO4) substrates by Ar-ion milling and subsequent liquid phase epitaxial growth of the active layer on these substrates. Laser efficiency and laser
threshold seems to change with the width of the channels fabricated.
[show abstract][hide abstract] ABSTRACT: We studied up-conversion emission of triply doped
(Ho,Tm,Yb):KLu(WO4)2 (KLuW) nanocrystals at the
range of temperature 296-673 K at different excitation wavelengths. The
intensity ratio between two emission lines was used for monitoring the
temperature. Pumping Yb3+ at 980 nm provides a good response
at relatively high temperatures, while pumping Tm3+ at 802 nm
provides an excellent sensitivity in the biological range of
temperatures., which make the material also attractive for biological
[show abstract][hide abstract] ABSTRACT: In this Letter we report Cs<sup>+</sup> ion exchange channel waveguides on RbTiOPO<sub>4</sub> (RTP) for what we believe is the first time. A Ti channel mask was fabricated on an RTP substrate by conventional photolithography. The ion exchange process was carried out in a CsNO<sub>3</sub> melt, and the channels produced ranged from 6 to 11 μm in width. The near-field pattern of the modes was recorded, and type II second harmonic generation in waveguide regime was obtained, producing 512.5 nm green light. The optical characterization shows optical losses of 3.8 dB/cm.
[show abstract][hide abstract] ABSTRACT: We review our recent results on the fabrication and characterization of Tm3+-based waveguide lasers on monoclinic potassium double tungstates. We demonstrated laser oscillation in the CW regime, at ∼1.84 μm, in slab and channel waveguides. In addition, using such guiding layers, we have demonstrated passive Q-switching in a slab waveguide laser.
Journal of Luminescence 01/2013; 133:262-267. · 2.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: Several lanthanum hydroxides (28–146 m2/g) were prepared by different procedures involving precipitation and hydrothermal methods by conventional heating or with microwaves. The use of ultrasounds during precipitation led to the formation of additional crystalline phases whereas the aging treatment with microwaves decreased the temperature needed to form the lanthanum oxide phase when compared with the samples aged by conventional heating. After calcination, La2O3 samples showed similar BET surface areas (3–5 m2/g) but different particle sizes ranging from 150 to 600 nm depending on the La(OH)3 preparation method, as observed by TEM. La2O3 samples were completely rehydroxylated after 80 h of exposure to atmospheric air at controlled humidity conditions recovering only partially the surface areas of the La(OH)3 precursors (14–18 m2/g). The progress of rehydroxylation with time occurred in several steps at different rates. Rehydroxylation rate mainly depended on the particle size and surface area of the lanthanum oxide sample. Therefore, the method used to prepare the initial lanthanum hydroxide affects the surface and rehydroxylation properties of the subsequent lanthanum oxide sample.
Journal of Nanoparticle Research 01/2013; 15:1479. · 2.18 Impact Factor
[show abstract][hide abstract] ABSTRACT: Using the overlapping integral method, a rib waveguide laser of monoclinic potassium double tungstate, KRE
, co-doped with Erbium and Ytterbium has been modelled. The laser operation at 1.5
m is based on an efficient pump scheme via the energy transfer from Yb
ions. The numerical simulation requires spectroscopic parameters of the ions involved and the waveguide geometry and index profiles. This model allows determining the laser power as a function of controllable parameters such as ions doping level, pump power, cavity length or reflectance of the input/output mirrors. It has been found that, for the standard doping level used in this matrix, the optimum cavity length is only few millimeters. Overall, using simulation tools is possible to optimize fabrication parameters, and thus saving effort in the development of experimental prototypes.
Optical and Quantum Electronics 01/2013; 45:349-356. · 0.99 Impact Factor
[show abstract][hide abstract] ABSTRACT: Europium single doped potassium lutetium tungstate Eu3+:KLu(WO4)2 single crystals have been grown with an active ion doping concentration from 0.5 to 5 at% using the top seeded solution growth slow cooling method. The experimental parameters for growing these single crystals have been optimized. Optical absorption and emission cross section were subjected to a detailed spectroscopic study. 58 sublevels were determined. Red visible emission was observed and attributed to the pure electronic transition under UV laser pumping. The CIE coordinates were (0.59,0.35) with 604 nm as the dominant wavelength and 84% of purity.
Journal of Luminescence 01/2013; 138:77–82. · 2.14 Impact Factor
[show abstract][hide abstract] ABSTRACT: Porous GaN crystals have been successfully grown and electrically contacted simultaneously on Pt- and Au- coated silicon substrates as porous crystals and as porous layers. By the direct reaction of metallic Ga and NH3 gas through chemical vapor deposition, intermetallic metal-Ga alloys form at the GaN-metal interface allowing vapor-solid-solid seeding and subsequent growth of porous GaN. Current-voltage and capacitance-voltage measurements confirm that the intermetallic seed layers prevent interface oxidation, and give a high quality reduced workfunction contact that allows exceptionally low contact resistivities. Additionally, the simultaneous formation of a lower workfunction intermetallic permits ohmic electron transport to n-type GaN grown using high workfunction metals that best catalyze the formation of porous GaN layers and may be employed to seed and ohmically contact a range of III-N compounds and alloys for broadband absorption and emission.
[show abstract][hide abstract] ABSTRACT: Large-area monolayer graphene, synthesized by chemical vapor deposition, was transferred to a 1-in. quartz substrate. The high-quality monolayer graphene has been subject to characterization of the nonlinear properties near 1 μm and was successfully applied as saturable absorber for passive mode-locking of a femtosecond Yb:KLuW laser. The diode-pumped mode-locked Yb:KLuW laser was tunable around 1.04 μm and delivered pulses as short as 160 fs. The maximum output power of 160 mW was demonstrated for 203 fs pulse duration. The mode-locked laser results are comparable to those demonstrated with the same laser gain medium using single-walled carbon nanotubes as saturable absorbers.
[show abstract][hide abstract] ABSTRACT: We present a technique for the direct deposition of nanoporous GaN particles on Si substrates without requiring any post-growth treatment. The internal morphology of the nanoporous GaN particles deposited on Si substrates by using a simple chemical vapor deposition approach was investigated, and straight nanopores with diameters ranging between 50 and 100 nm were observed. Cathodoluminescence characterization revealed a sharp and well-defined near band-edge emission at ∼365 nm. This approach simplifies other methods used for this purpose, such as etching and corrosion techniques that can damage the semiconductor structure and modify its properties.
Microscopy and Microanalysis 07/2012; 18(4):905-11. · 2.50 Impact Factor
[show abstract][hide abstract] ABSTRACT: Europium-doped lanthanum oxide (5 mol % Eu(3+):La(2)O(3)) was prepared by calcining europium-doped lanthanum hydroxide (5 mol % Eu(3+):La(OH)(3)) previously synthesized by a simple hydrothermal method. Interestingly, we observed different emission Eu(3+) signatures depending on the phase of the host (lanthanum oxide or hydroxide) by cathodoluminescence. Taking into account that lanthanum oxide easily rehydroxylates in air, for the first time, we report the use of cathodoluminiscence as a novel characterization technique to follow the lanthanum oxide rehydroxylation reaction versus time according to different annealing procedures. Additionally, differential thermal-thermogravimetric analysis, infrared spectroscopy, and X-ray diffraction techniques were used to identify the phases formed from the Eu(3+):La(OH)(3) depending on temperature and to study the evolution of La(2)O(3) to La(OH)(3) versus time. The results showed that the higher the temperature and the longer the annealing time, the higher the resistance to rehydroxylation of the Eu(3+):La(2)O(3) sample.
[show abstract][hide abstract] ABSTRACT: We report on laser operation in a (6 at. % Tm, 5 at. % Yb):KLu(WO4)2 codoped crystal. The vibrational frequencies of KLu(WO4)2 are coupled to the electronic transitions of Tm3+ at 1946 nm, creating virtual final laser levels at higher energy than the ground level 3H6 of Tm3+. The longest recorded laser wavelength was 2039 nm, which is longer than permitted by a pure electronic transition in Tm3+ ions in KLu(WO4)2. We show that every laser wavelength can be explained with the electron-phonon coupling effect, where the vibration frequencies were determined through Raman spectroscopy.