Analysis of a diode-pumped passively Q-switched Nd:GdVO4 self-stimulating Raman laser

This paper presents the theoretical and experimental results of a laser diode-pumped passively Q-switched Nd:GdVO₄ self-Raman laser in detail. In the experiment, by using two Cr⁴⁺:YAG crystals with different initial transmissions as the saturable absorber, using Nd:GdVO₄ as the gain medium and Raman medium simultaneously, the passively Q-switched operation of the Nd:GdVO₄ self-Raman laser at 1176 nm was investigated. The average power, pulse energy, pulse width and pulse repetition rate with respect to the incident pump power were measured. The obtained maximum output average power is 244.6 mW with respect to incident power of 5.7 W and the corresponding conversion efficiency is 4.3%. In the theoretical part, we used the rate equations to obtain the theoretical results. In the rate equations, the Gaussian distributions of the intracavity photon densities of the fundamental and Raman lasers and the initial population-inversion density were taken into account, and the ground-state population density of the saturable absorber at t = 0 is assumed to be uniform. The obtained theoretical results were in agreement with the experimental results on the whole.     

Passively Q-switched mode-locking Nd:GdVO4 laser with a chemically reduced graphene oxide saturable absorber

A diode-pumped passively Q-switched mode-locking Nd:GdVO₄ laser using a chemically reduced graphene oxide saturable absorber was demonstrated. The repetition rate and pulse width of the Q-switching envelopes were 5.7 MHz and 100 ns, respectively, while those of the mode-locking pulses that were modulated in the Q-switching envelopes were 139 MHz and 10 ps. At the maximum pump power of 7 W, the average output power was 1.38 W, corresponding to a slope efficiency of 25%.     

Efficient tri-wavelength actively Q-switched Yb:GAGG laser

An efficient acousto-optic Q-switched Yb-doped Gd₃Al_xGa_5−xO₁₂ (GAGG) (x = 0.5) laser is demonstrated. Under the absorbed pump power of 7.4 W, the maximum average output power of 1.4 W is obtained at the pulse repletion rate of 1 kHz , with the slope efficiency as high as 32%. The pulse width of 40 ns is achieved with the pulse energy and peak power of 1.4 mJ and 35 kW, respectively. What’ more, the output spectrum shows itself tri-wavelength in either CW or Q-switching mode. To our knowledge, this is the first time for realizing simultaneous tri-wavelength Yb:GAGG laser actively Q-switched operation.     

Mid-infrared ridge waveguide in MgO:LiNbO3 crystal produced by combination of swift O5+ ion irradiation and precise diamond blade dicing

We report on the fabrication of ridge waveguide operating at mid-infrared wavelength in MgO:LiNbO₃ crystal by using O⁵⁺ ion irradiation and precise diamond blade dicing. The waveguide shows good guiding properties at the wavelength of 4 μm along the TM polarization. Thermal annealing has been implemented to improve the waveguiding performances. The propagation loss of the ridge waveguide has been reduced to be 1.0 dB/cm at 4 μm after annealing at 310 °C. The micro-Raman spectra indicate that the microstructure of the MgO:LiNbO₃ crystal has no significant change along the ion track after swift O⁵⁺ ion irradiation.     

Continuous-wave lasing at 1.06 μm in femtosecond laser written Nd:KGW waveguides

We report on the buried channel waveguide laser at 1065 nm in Nd:KGW waveguides fabricated by femtosecond laser writing with dual-line approach. A relatively high scanning speed of 0.5 mm/s enables acceptable propagation loss less than 2 dB/cm. The fluorescence emission spectra of Nd³⁺ ions measured shows that the fluorescence properties were well preserved in the waveguide region. A stable continuous wave laser at 1065 nm has been obtained at room temperature in the buried channel waveguides by optical pumping at 808 nm. A maximum output power of 33 mW and a slope efficiency of 52.3% were achieved in the Nd:KGW waveguide laser system.     

Efficient upconversion-pumped continuous wave Er3+:LiLuF4 lasers

We report on detailed spectroscopic investigations and efficient visible upconversion laser operation of Er³⁺:LiLuF₄. This material allows for efficient resonant excited-state-absorption (ESA) pumping at 974 nm. Under spectroscopic conditions without external feedback, ESA at the laser wavelength of 552 nm prevails stimulated emission. Under lasing conditions in a resonant cavity, the high intracavity photon density bleaches the ESA at 552 nm, allowing for efficient cw laser operation.We obtained the highest output power of any room-temperature crystalline upconversion laser. The laser achieves a cw output power of 774 mW at a slope efficiency of 19% with respect to the incident pump power delivered by an optically-pumped semiconductor laser. The absorption efficiency of the pump radiation is estimated to be below 50%.To exploit the high confinement in waveguides for this laser, we employed femtosecond-laser pulses to inscribe a cladding of parallel tracks of modified material into Er³⁺:LiLuF₄ crystals. The core material allows for low-loss waveguiding at pump and laser wavelengths. Under Ti:sapphire pumping at 974 nm, the first crystalline upconversion waveguide laser has been realized. We obtained waveguide-laser operation with up to 10 mW of output power at 553 nm.     

Impact of high dose krypton ion irradiation on corrosion behavior of laser beam welded zircaloy-4

In order to study the effect of krypton ion irradiation on the aqueous corrosion behavior of laser beam welded zircaloy-4 (LBWZr4), the butt weld joint of zircaloy-4 was made by means of a carbon dioxide laser, subsequently the LBWZr4 samples were irradiated with Kr ions using an accelerator at an energy of 300 keV, with a dose range from 1 × 10¹⁵ to 3 × 10¹⁶ ions/cm² at about 150 °C. Three-sweep potentiodynamic polarization measurement was employed to evaluate the aqueous corrosion behavior of Kr-irradiated LBWZr4 in a 0.5 M H₂SO₄ solution. Scanning electron microscopy (SEM) was used to examine the surface topography of the Kr-irradiated LBWZr4 after the potentiodynamic polarization measurement. Transmission electron microscopy was employed to examine the change of microstructures in the irradiated surface. The polarization tests showed that compared with the passive current density of the as-received LBWZr4, the Kr-irradiated LBWZr4 is much lower; however, with the irradiation dose increasing from 1 × 10¹⁵ to 3 × 10¹⁶ ions/cm², the passive current density, closely related to the surface corrosion resistance, increased remarkably. The mechanism of the corrosion behavior transformation was due to the recrystallization of the amorphous phase induced by the lower ion irradiation.     

Fabrication of Yb3+/Er3+ codoped Bi2O3–WO3–TeO2 pedestal type waveguide for optical amplifiers

We report, for the first time to our knowledge, experimental results on pedestal waveguides produced with Yb³⁺/Er³⁺ codoped Bi₂O₃–WO₃–TeO₂ thin films deposited by RF Sputtering for photonic applications. Thin films were deposited using Ar/O₂ plasma at 5 mTorr pressure and RF power of 40 W on substrates of silicon wafers. The definition of the pedestal waveguide structure was made using conventional optical lithography followed by plasma etching. Propagation losses around 2.0 dB/cm and 2.5 dB/cm were obtained at 633 and 1050 nm, respectively, for waveguides in the 20–100 μm width range. Single-mode propagation was measured for waveguides width up to 10 μm and 12 μm, at 633 nm and 1050 nm, respectively; for larger waveguides widths multi-mode propagation was obtained. Internal gain of 5.6 dB at 1530 nm, under 980 nm excitation, was measured for 1.5 cm waveguide length (∼3.7 dB/cm). The present results show the possibility of using Yb³⁺/Er³⁺ codoped Bi₂O₃–WO₃–TeO₂ pedestal waveguide for optical amplifiers.     

Synthesis,z-scan and degenerate four wave mixing characterization of certain functionalized photosensitive polyesters containing ortho-hydroxyazo chromophores

The preparation and NLO characterization of photosensitive polyesters containing azoaromatic residues in the molecular backbone, functionalized with orthohydroxy chromophores is presented. Samples were studied for its UV–vis absorption, FT-IR and intensity dependent nonlinear absorption properties. Nonlinear characterization was carried out with z-scan using frequency doubled, Q-switched Nd:YAG laser operating at 532 nm. The closed aperture z-scan spectra reveal the self defocusing effects of the samples with negative nonlinearity coefficient (n₂) showing values as high as −1.28 × 10⁻¹⁰ (esu) for certain samples and the corresponding third order susceptibility coefficient of the order of 29.9 × 10⁻¹² (esu). Degenerate four wave mixing technique was employed to substantiate the findings. The numerical fits show that the molecules exhibit reverse saturable absorption. A study of beam fluence dependence of nonlinear absorption coefficient (β_eff) has been presented. All phenomena indicate that molecules are reverse saturable absorbers whose optical limiting property gets enhanced with increasing conjugation length.     

Electromechanical properties of [0 0 1], [0 1 1] and [1 1 1] oriented Pb(Mg1/3Nb2/3)O3–0.32PbTiO3 crystals under uniaxial stress

The electric field and the stress induced strain and polarization responses of [0 0 1], [0 1 1] and [1 1 1] oriented Pb(Mg_1/3Nb_2/3)O₃–0.32PbTiO₃ (PMN–0.32PT) relaxor ferroelectric single crystals have been systematically investigated by experimental study. The responses of [0 0 1] oriented single crystal to stress cycle and electric field are explained by polarization rotation and phase transformations mechanism. However, the responses of [0 1 1] and [1 1 1] oriented single crystal should be explained by domain switching. The differences of strain and polarization between the minimum and maximal values of electric field in [0 0 1] orientation firstly increase then decrease with enhancing of compressive stress.     

Modulations in structural and ferroelectric properties due to tensile strain in BiFeO3 films on MgAl2O4 substrates induced by thermal-expansion

The effect of tensile strain on structural and ferroelectric properties of BiFeO₃ epitaxial films was investigated. The films grown by pulsed laser deposition on MgAl₂O₄ (0 0 1) substrates revealed monoclinic structure deviated from the bulk rhombohedral structure due to a tensile strain along the in-plane direction. The strain is induced by the difference in thermal expansion coefficients between the film and the substrate. A Poisson ratio is calculated from the in-plain and out-of-plain lattice constants at different temperatures measured by reciprocal space maps of X-ray diffraction. The small Poisson ratio compared to the bulk suggests a weaker elastic response at high temperature. The ferroelectric polarization of the tensile-strained film along the (0 0 1) is also decreased from the bulk value.     

Mechanical properties and microstructure of spark plasma sintered nanostructured p-type SiGe thermoelectric alloys

SiGe based thermoelectric (TE) materials have been employed for the past four decades for power generation in radio-isotope thermoelectric generators (RTG). Recently “nanostructuring” has resulted in significantly increasing the figure-of-merit (ZT) of both n and p-type of SiGe and thus nanostructured Si₈₀Ge₂₀ alloys are evolving as a potential replacement for their conventional bulk counterparts in designing efficient RTGs. However, apart from ZT, their mechanical properties are equally important for the long term reliability of their TE modules. Thus, we report the mechanical properties of p-type nanostructured Si₈₀Ge₂₀ alloys, which were synthesized employing spark plasma sintering of mechanically alloyed nanopowders of its constituent elements with 1.2% boron doping. Nanostructured p-type Si₈₀Ge₂₀ alloys exhibited a hardness of ~ 9 ± 0.1 GPa, an elastic modulus of ~ 135 ± 1.9 GPa, a compressive strength of 108 ± 0.2 MPa, and fracture toughness of ~ 1.66 ± 0.04 MPa√m with a thermal shock resistance value of 391 ± 21 Wm^− 1. This combination of good mechanical properties coupled with higher reported ZT of nanostructured p-type Si₈₀Ge₂₀ alloys are rendered to be a potential material for power generation applications, compared to its bulk counterpart.     

Microstructures and mechanical property of laser butt welding of titanium alloy to stainless steel

Laser butt welding of titanium alloy to stainless steel was performed. The effect of laser-beam offsetting on microstructural characteristics and fracture behavior of the joint was investigated. It was found that when the laser beam is offset toward the stainless steel side, it results in a more durable joint. The intermetallic compounds have a uniform thickness along the interface and can be divided into two layers. One consists of FeTi + α-Ti, and other consists of FeTi + Fe₂Ti + Ti₅Fe₁₇Cr₅. When laser beam is offset by 0 mm and 0.3 mm toward the titanium alloy side, the joints fracture spontaneously after welding. Durable joining is achieved only when the laser beam is offset by 0.6 mm toward the titanium alloy. From the top to the bottom of the joint, the thickness of intermetallic compounds continuously decreases and the following interfacial structures are found: FeAl + α-Ti/Fe₂Ti + Ti₅Fe₁₇Cr₅, FeAl + α-Ti/FeTi + Fe₂Ti + Ti₅Fe₁₇Cr₅ and FeAl + α-Ti, in that order. The tensile strength of the joint is higher when the laser beam is offset toward the stainless steel than toward the titanium alloy, the highest observed value being 150 MPa. The fracture of the joint occurs along the interface between two adjacent intermetallic layers.     

Alkaline aluminum phosphate glasses for thermal ion-exchanged optical waveguide

Alkaline aluminum phosphate glasses (NMAP) with excellent chemical durability for thermal ion-exchanged optical waveguide have been designed and investigated. The transition temperature Tg (470 °C) is higher than the ion-exchange temperature (390 °C), which is favorable to sustain the stability of the glass structure for planar waveguide fabrication. The effective diffusion coefficient D_e of K⁺–Na⁺ ion exchange in NMAP glasses is 0.110 μm²/min, indicating that ion exchange can be achieved efficiently in the optical glasses. Single-mode channel waveguide has been fabricated on Er³⁺/Yb³⁺ doped NMAP glass substrate by standard micro-fabrication and K⁺–Na⁺ ion exchange. The mode field diameter is 9.6 μm in the horizontal direction and 6.0 μm in the vertical direction, respectively, indicating an excellent overlap with a standard single-mode fiber. Judd–Ofelt intensity parameter Ω₂ is 5.47 × 10⁻²⁰ cm², implying a strong asymmetrical and covalent environment around Er³⁺ in the optical glasses. The full width at half maximum and maximum stimulated emission cross section of the ⁴I_13/2 → ⁴I_15/2 are 30 nm and 6.80 × 10⁻²¹ cm², respectively, demonstrating that the phosphate glasses are potential glass candidates in developing compact optoelectronic devices. Pr³⁺, Tm³⁺ and Ho³⁺ doped NMAP glasses are promising candidates to fabricate waveguide amplifiers and lasers operating at special telecommunication windows.     

Crystal growth and frequency conversion of BaMgF4 single crystal by temperature gradient technique

The BaMgF₄ single crystal was grown by the temperature gradient technique. Highly pure MgF₂ and BaF₂ powders were used as raw materials and CaF₂ single crystal was chosen as a seed to induce BaMgF₄ single crystal grow along 〈0 0 1〉 orientation. The short cut-off wavelength of BaMgF₄ single crystal was determined to be 130 nm which is consistent with the previously reported result. We realized second harmonic generation from 1064 nm to 532 nm laser, and 800 nm to 400 nm femtosecond laser in single domain BaMgF₄ by using birefringent phase matching. 355-nm-wavelength laser was produced by Sum-Frequency Generation from 1064-nm- and 532-nm-wavelength laser, which is the shortest coherent radiation obtained in the UV in the BaMgF₄ crystal.     

Ab initio study of electronic structures of InAs and GaSb nanowires along various crystallographic orientations

InAs and GaSb nanowires oriented along different crystallographic axes—the [0 0 1], [1 0 1] and [1 1 1] directions of zinc-blende structure—have been studied utilizing a first-principles derived nonlocal screened atomic pseudopotential theory, to investigate the band structure, polarization ratio and effective masses of these semiconductor nanowires and their dependences on the wire lateral size and axis orientation. The band energy dispersion over entire Brillouin zone and orbital energy are determined and found to exhibit different characteristics for three types of wires. There is an explicit dispersion hump in the conduction bands of [0 0 1] nanowires with two larger diameters and [1 0 1] nanowires with the smallest diameter considered. Moreover, the [1 1 1] nanowires are shown to exhibit very different orbital energy for the maximum valence state at the zone-boundary point, compared with [0 0 1] and [1 0 1] nanowires. These differences present significant and detailed insight for experimental determination of the band structure in InAs and GaSb nanowires. Furthermore, we study the polarization ratio of these nanowires for different orientations. Our calculation results indicate that, for the same lateral size, the [1 1 1] nanowires give extraordinarily higher polarization ratio compared to nanowires along the other two directions, and at the same time have larger band-edge photoluminescence transition intensity. Consequently, the [1 1 1] nanowires are predicted to be better suitable for optoelectronic applications. We also significantly find that polarization ratio and transition intensity displays different varying trend of dependence on lateral size of nanowires. Specially, the calculated polarization ratio is shown to increase with the decreasing size, which is opposite to the behavior displayed by the optical transition intensity. The predicted polarization ratios of [1 0 1] and [1 1 1] nanowires for 10.6 Å diameter approach the limit of 100%. In addition, the electron and hole masses for InAs and GaSb nanowires with different crystallographic axes have been calculated. For the [1 0 1] and [1 1 1] oriented nanowires, the hole masses are predicted to be around 0.1–0.2 m₀, which are notably smaller than the values (∼0.5 m₀) along the same direction for their bulk counterparts. Thus, we demonstrates an inspired possibility of obtaining a high hole mobility in nanowires that is not available in bulk. The small hole mobility is interpreted as to be associated with the strong electronic band mixing in nanowires.     

Efficient laser operation at 1.06 μm in co-doped Lu3+, Nd3+:GdCa4O(BO3)3 single crystal

Efficient laser emission at 1.06 μm was obtained from a diode-laser quasi-continuous wave pumped Nd_0.04Gd_0.86Lu_0.10Ca₄O(BO₃)₃ (Nd:GdLuCOB) single crystal. An uncoated, 6.0-mm long, ZX-cut Nd:GdLuCOB medium yielded laser pulses with 1.76 W peak power for absorbed pump pulses of 5.49 W peak power, corresponding to an overall optical-to-optical efficiency η_oa = 0.32; the slope efficiency was η_sa = 0.44. Comparison is made with an uncoated XY-cut Nd:GdCOB medium (4.0-at.% Nd doping and 6.8-mm length) from which laser pulses with 1.74 W peak power (at optical efficiency η_oa = 0.25) and 0.31 slope efficiency were obtained. The improvements in laser emission of Nd:GdLuCOB at the fundamental wavelength are important for future self-frequency doubling in ZX principal plane of this crystal.     

Planar waveguide in beta barium borate formed by proton implantation and optical properties in visible and near-infrared band

β-BaB₂O₄ is nonlinear crystal that provides attractive features for various nonlinear optical application. We report on z-cut β-BaB₂O₄ planar waveguide produced by multi-energy proton implantation in total of 3 × 10¹⁶ ion/cm² at room temperature. The refractive index profile is a barrier-confined distribution, which is reconstructed according to the dark mode spectroscopy at the wavelength of 633 nm. The near-field light intensity profiles in the visible (633 nm) and near-infrared band are measured by end-face coupling method. The absorption spectra recorded over the wavelength range 185–2500 nm shows there is hardly change after proton implantation. The investigation results of the Raman spectra demonstrate that the positions and widths of all the peaks are very similar in the β-BaB₂O₄ crystal and waveguide layer.     

Enhanced sintering ability of biphasic calcium phosphate by polymers used for bone scaffold fabrication

Biphasic calcium phosphate (BCP), which is composed of hydroxyapatite [HAP, Ca₁₀(PO₄)₆(OH)₂] and β-tricalcium phosphate [β-TCP, β-Ca₃(PO₄)₂], is usually difficult to densify into a solid state with selective laser sintering (SLS) due to the short sintering time. In this study, the sintering ability of BCP ceramics was significantly improved by adding a small amount of polymers, by which a liquid phase was introduced during the sintering process. The effects of the polymer content, laser power and HAP/β-TCP ratios on the microstructure, chemical composition and mechanical properties of the BCP scaffolds were investigated. The results showed that the BCP scaffolds became increasingly more compact with the increase of the poly(l-lactic acid) (PLLA) content (0–1 wt.%) and laser power (6–10 W). The fracture toughness and micro-hardness of the sintered scaffolds were also improved. Moreover, PLLA could be gradually decomposed in the late sintering stages and eliminated from the final BCP scaffolds if the PLLA content was below a certain value (approximately 1 wt.% in this case). The added PLLA could not be completely eliminated when its content was further increased to 1.5 wt.% or higher because an unexpected carbon phase was detected in the sintered scaffolds. Furthermore, many pores were observed due to the removal of PLLA. Micro-cracks and micro-pores occurred when the laser power was too high (12 W). These defects resulted in a deterioration of the mechanical properties. The hardness and fracture toughness reached maximum values of 490.3 ± 10 HV and 1.72 ± 0.10 MPa m^1/2, respectively, with a PLLA content of approximately 1 wt.% and laser power of approximately 10 W. Poly(l-lactic-co-glycolic acid) (PLGA) showed similar effects on the sintering process of BCP ceramics. Rectangular, porous BCP scaffolds were fabricated based on the optimum values of the polymer content and laser power. This work may provide an experimental basis for improving the mechanical properties of BCP bone scaffolds fabricated with SLS.