Laser induced damage threshold at 355 and 1064 nm of Ta2O5 films of different phases

Ta2O5 films are deposited on fused silica substrates by conventional electron beam evaporation method. By annealing at different temperatures, Ta2O5 films of amorphous, hexagonal and orthorhombic phases are obtained and confirmed by x-ray diffractometer ( XRD) results. X-ray photoelectron spectroscopy ( XPS) analysis shows that chemical composition of all the films is stoichiometry. It is found that the amorphous Ta2O5 film achieves the highest laser induced damage threshold ( LIDT) either at 355 or 1064 nm, followed by hexagonal phase and finally orthorhombic phase. The damage morphologies at 355 and 1064 nm are different as the former shows a uniform fused area while the latter is centred on one or more defect points, which is induced by different damage mechanisms. The decrease of the LIDT at 1064nm is attributed to the increasing structural defect, while at 355nm is due to the combination effect of the increasing structural defect and decreasing band gap energy.

Investigation of annealing effects on the laser-induced damage threshold of amorphous Ta2O5 films

Ta2O5 films were deposited using the conventional electron beam evaporation method and then annealed at temperatures in the range 373-673 K. Chemical composition, scattering and absorption were examined by X-ray photoelectron spectroscopy (XPS), total integrated scattering (TIS) measurement and the surface thermal lensing (m) technique, respectively. The laser-induced damage threshold (LIDT) was assessed using the output from an Nd:YAG laser with a pulse length of 12 ns. The results showed that the improvement of the LIDT after annealing was due to the reduced substoichiometric and structural defects present in the film. The LIDT increased slightly below 573K and then increased significantly with increase in annealing temperature, which could be attributed to different dominant defects. Moreover, the root mean square (RMS) roughness and scattering had little effect on the LIDT, while the absorption and the LIDT were in accord with a general relation. (c) 2008 Elsevier Ltd. All rights reserved.

Single mode vertical-cavity surface emitting laser with surface plasmon nanostructure

The vertical-cavity surface-emitting laser(VCSEL) has proved to be a low cost light source with attractive properties such as surface emission, circular and low divergence output beam, and simple integration in two-dimensional array. Many new applications such as in spectroscopy, optical storage, short distance fiber optic interconnects, and in longer distance communication, are continuously arising. Many of these applications require stable and single-mode high output power. Several methods that affect the transverse guiding and/or introduce mode selective loss or gain have been developed. In this study, a method for improving the single mode output power by using metal surface plasmons nanostructure is proposed. Theoretical calculation shows that the outpout power is improved about 50% compared to the result of standard VCSELs.

Crystallization of amorphous Si films by pulsed laser annealing and their structural characteristics

Nanocrystalline silicon (nc-Si) films were prepared by pulsed laser annealed crystallization of amorphous silicon (alpha-Si) films on SiO2-coated quartz or glass substrates. The effect of laser energy density on structural characteristics of nc-Si films was investigated. The Ni-induced crystallization of the a-Si films was also discussed. The surface morphology and microstructure of these films were characterized by scanning electron microscopy, high-resolution electron microscopy, atomic force microscopy and Raman scattering spectroscopy. The results show that not only can the alpha-Si films be crystallized by the laser annealing technique, but also the size of Si nanocrystallites can be controlled by varying the laser energy density. Their average size is about 4-6 nm. We present a surface tension and interface strain model used for describing the laser annealed crystallization of the alpha-Si films. The doping of Ni atoms may effectively reduce the threshold value of laser energy density to crystallize the alpha-Si films, and the flocculent-like Si nanostructures could be formed by Ni-induced crystallization of the alpha-Si films.

Study on the thermal stability of InN by in-situ laser reflectance system

The thermal stability of InN in the growth environment in metalorganic chemical vapor deposition was systematically investigated in situ by laser reflectance system and ex situ by morphology characterization, X-ray diffraction and X-ray photoelectron spectroscopy. It was found that InN can withstand isothermal annealing at temperature as high as 600 degrees C in NH3 ambient. While in N-2 atmosphere, it will decompose quickly to form In-droplets at least at the temperature around 500 degrees C, and the activation energy of InN decomposition was estimated to be 2.1 +/- 0.1 eV. Thermal stability of InN when annealing in NH3 ambient during temperature altering would be very sensitive to ramping rate and NH3 flow rate, and InN would sustain annealing process at small ramping rate and sufficient supply of reactive nitrogen radicals. Whereas In-droplets formation was found to be the most frequently encountered phenomenon concerning InN decomposition, annealing window for conditions free of In-droplets was worked out and possible reasons related are discussed. In addition, InN will decompose in a uniform way in the annealing window, and the decomposition rate was found to be in the range of 50 and 100 nm/h. Hall measurement shows that annealing treatment in such window will improve the electrical properties of InN. (c) 2005 Elsevier B.V. All rights reserved.

Design and characteristics of quantum cascade laser-based CO detection system

National High Technology Research and Development Program of China 2007AA03Z112；Program of Ministry of Education of China 20060183030；Program of Jilin Provincial Science and Technology Department of China 20070709；Program of Bureau of Science and Technology of Changchun City 2007107

Impact of diode facet reflectivity on the fiber grating external cavity semiconductor laser

The scattering matrix method is used to analyze the multiple reflection effect between the laser diode facet and the fiber grating facet by considering the fiber grating external cavity laser diode (FGECL) as a four-mirror cavity laser. When neglecting other important parameters such as butt-coupling distance between the diode and the fiber facets, coupling efficiency, external cavity length, it is shown that low reflectivity is not a crucial factor for the laser characteristics such as SMSR. Experimentally high SMSR fiber grating external cavity laser is fabricated with a relatively large residual facet reflectivity (about 1%), which is coincident with our simulation results.

Near-Infrared Femtosecond Laser for Studying the Strain in Si1-xGex Alloy Films via Second-Harmonic Generation

The second-harmonic generation (SHG) from Si1-xGex alloy films has been investigated by near-infrared femtosecond laser. Recognized by s-out polarized SHG intensity versus rotational angle of sample, the crystal symmetry of the fully strained Si0.83Ge0.17 alloy is found changed from the O-h to the C-2 point group due to the inhomogeneity of the strain. Calibrated by double crystal X-ray diffraction, the strain-induced chi((2)) is estimated at 5.7 x 10(-7) esu. According to the analysis on p-in/s-out SHG, the strain-relaxed Si0.10Ge0.90 alloy film is confirmed to be not fully relaxed, and the remaining strain is quantitatively determined to be around 0.1%.

Deep centers in AlGaAs/GaAs GRIN-SCH SQW laser structures grown by MBE and MOCVD

The deep centers in AlGaAs/GaAs graded index-separate confinement heterostructure single quantum well (GRIN-SCHSQW) laser structures grown by MBE and MOCVD have been investigated using deep level transient spectroscopy (DLTS) technique, The majority and minority carrier DLTS spectra show that the deep (hole and electron) traps (Hi and E3), having large capture cross sections and concentrations, are observed in the graded n-AlxGa1-xAs layer of laser structures in addition to the well-known DX centers. For laser structures grown by MBE, the deep hole trap H1 and the deep electron trap E3 may be spatially localized in the interface regions of discontinuous variation Al mole fraction of the n-AlxGa1-xAs layer with x = 0.20-0.43. For laser structures grown by MOCVD, the deep electron trap E3 may be spatially localized in the n-AlxGa1-xAs layer with x = 0.18-0.30, and the DX center may be spatially localized in the interface regions of discontinuous variation Al mole fraction of the AlxGa1-xAs layer with x = 0.22-0.30.

Microphotoluminescence investigation of InAs quantum dot active region in 1.3 mu m vertical cavity surface emitting laser structure

Microphotoluminescence (mu-PL) investigation has been performed at room temperature on InAs quantum dot (QD) vertical cavity surface emitting laser (VCSEL) structure in order to characterize the QD epitaxial structure which was designed for 1.3 mu m wave band emission. Actual and precise QD emission spectra including distinct ground state (GS) and excited state (ES) transition peaks are obtained by an edge-excitation and edge-emission (EEEE) mu-PL configuration. Conventional photoluminescence methods for QD-VCSELs structure analysis are compared and discussed, which indicate the EEEE mu-PL is a useful tool to determine the optical features of the QD active region in an as-grown VCSEL structure. Some experimental results have been compared with simulation results obtained with the aid of the plane-wave admittance method. After adjustment of epitaxial growth according to EEEE mu-PL measurement results, QD-VCSEL structure wafer with QD GS transition wavelength of 1300 nm and lasing wavelength of 1301 nm was obtained.

Selecting ionization path by dynamic stark shift with strong laser pulse

Multiphoton ionization of NO via intermediate Rydberg states with ultra-short laser pulses is investigated with time-resolved photoelectron spectroscopy in combination with fermosecond pump-probe technology. The Rydberg states of NO, which are characterized by obvious ac-Stark shift in ultra-strong laser field, can be tuned in resonance to ionize NO molecule at one's will with identical laser pulses, i.e., one can 'select' resonance path to ionization. The results shown in this Letter demonstrate that the states holding notable dynamic Stark shift provide us another dimension to chemical control with strong laser field. (C) 2003 Elsevier Science B.V. All rights reserved.

Identifying the isolated transition metal ions/oxides in molecular sieves and on oxide supports by UV resonance Raman spectroscopy

Isolated transition metal ions/oxides in molecular sieves and on surfaces are a class of active sites for selective oxidation of hydrocarbons. Identifying the active sites and their coordination structure is vital to understanding their essential role played in catalysis and designing and synthesizing more active and selective catalysts. The isolated transition metal ions in the framework of molecular sieves (e.g., TS-1, Fe-ZSM-5, and V-MCM-41) or on the surface of oxides (e.g., MoO3/Al2O3 and TiO2/SiO2) were successfully identified by UV resonance Raman spectroscopy. The charge transfer transitions between the transition metal ions and the oxygen anions are excited by a UV laser and consequently the UV resonance Raman effect greatly enhances the Raman signals of the isolated transition metal ions. The local coordination of these ions in the rigid framework of molecular sieves or in the relatively flexible structure on the surface can also be differentiated by the shifts of the resonance Raman bands. The relative concentration of the isolated transition metal ion/oxides could be estimated by the intensity ratio of Raman bands. This study demonstrates that the UV resonance Raman spectroscopy is a general technique that can be widely applied to the in-situ characterization of catalyst synthesis and catalytic reactions. (C) 2003 Elsevier Science (USA). All rights reserved.

ZnO clusters encapsulated inside micropores of zeolites studied by UV Raman and laser-induced luminescence spectroscopies

Using microporous zeolites as host, sub-nanometric ZnO clusters were prepared in the micropores of the host by the incipient wetness impregnation method. A small amount of sub-nanometric ZnO clusters were introduced into the channels of HZSM-5 zeolite, whereas a large quantity of sub-nanometric ZnO clusters can be accommodated in the supercages of HY zeolite and no macrocrystalline ZnO exists on the extra surface of the HY material. The vibrations of the zeolite framework and ZnO were characterized by UV Raman spectroscopy. The optical properties of these ZnO clusters were studied by UV-visible absorption spectroscopy and laser-induced luminescence spectroscopy. It is found that there are strong host-guest interactions between the framework oxygen atoms of zeolite and ZnO clusters influencing the motions of the framework oxygen atoms. The interaction may be the reason why ZnO clusters are stabilized in the pores of zeolites. Different from bulk ZnO materials, these sub-nanometric ZnO clusters exhibit their absorption onset below 265 nm and show a purple luminescence band (centered at 410-445 nm) that possesses high quantum efficiency and quantum size effect. This purple luminescence band most likely originates from the coordinatively unsaturated Zn sites in sub-nanometric ZnO clusters. On the other hand, the differences in the pore structure between HZSM-5 and HY zeolites cause the absorption edge and the purple luminescence band of ZnO clusters in ZnO/HZSM-5 show a red shift in comparison with those of ZnO clusters in ZnO/HY.

Measurement of the single-color three-photon resonant ionization spectrum of atomic uranium under the "equivalent wavelength effect laser" interaction

Using Nd: YAG laser (532 nm) pumped mixed-dye laser. we obtained the output of this dye enhanced at the wavelength interval equivalent to that given by the copper vapor laser pumped dye laser. This measure favored is with the measurement of single-color three-photon resonant ionization spectrum of atomic uranium in the range of 562-586 nm,which is otherwise not efficiently covered by Nd: YAG laser pumped dye laser with any single dye. Thus 140 U I energy levels were obtained and the peaks of interest 575.814 nm and 575.836 rim were well resolved and their relative intensity determined.

Degradation of polyimide induced by nitrogen laser irradiation

The degradation behavior of polyimide (PMDA-ODA) induced by nitrogen laser irradiation was studied. The changes in the surface morphology and the composition of the irradiated polyimide films were examined by scanning electron microscopy, X-ray photoelectron spectroscopy and FT-IR spectroscopy. The initial reaction was achieved by photochemical degradation of polyimide in the highly electronic excited state by the absorption of a second 337 nm photon. Atmospheric oxygen sequentially reacted with the produced radicals to form a highly oxidized layer. The formation of carbonyl group was enhanced by the heat remaining on the irradiated polyimide film surfaces. (C) 2000 Elsevier Science B.V. All rights reserved.