Pump Power Density Research Articles

Gd3+ ion induced UV upconversion emission and temperature sensing in Tm3+/Yb3+:Y2O3 phosphor

Cubic phase Tm3+/Yb3+:Y2O3 and Tm3+/Yb3+/Gd3+:Y2O3 phosphors were prepared by low temperature combustion technique for upconversion emission in UV–visible range. The 980 nm excitation has generated UV emission at 314 nm in tridoped phosphor due to the energy transfer from Tm3+ to Gd3+ ion. Characteristic emission bands from Tm3+ are also observed in both the phosphors. Thermally coupled Stark sublevels 1G4(a) (476 nm) and 1G4(b) (488 nm) of Tm3+ ion were utilised for optical thermometry using fluorescent intensity ratio (FIR) method. The result shows that maximum absolute sensitivity in tridoped phosphor is observed to be 1.33 × 10−3 K−1 at 298 K. Moreover, temperature rise of phosphor at various pump power densities was also measured and it is estimated to achieve 407 K at the pump power density of 38.46 W/cm2.

Optical net gain measurement on Al0.07Ga0.93N/GaN multi-quantum wells.

We report net gain measurements at room temperature in Al0.07Ga0.93N/GaN 10-period multi-quantum well layers emitting at 367 nm, using the variable stripe length method. The separate confinement heterostructure was designed targeting electron-beam pumped lasing at 10 kV. The highest net gain value was 131 cm-1, obtained at the maximum pumping power density of the experimental setup (743 kW/cm2). The net gain threshold was attained at 218 kW/cm2 using 193 nm optical pumping. From these experiments, we predict an electron-beam-pumped lasing threshold of 370 kW/cm2 at room temperature, which is compatible with the use of compact cathodes (e.g. carbon nanotubes). In some areas of the sample, we observed an anomalous amplification of the photoluminescence intensity that occurs for long stripe lengths (superior to 400 µm) and high pumping power (superior to 550 kW/cm2), leading to an overestimation of the net gain value. We attribute such a phenomenon to the optical feedback provided by the reflection from cracks, which were created during the epitaxial growth due to the strong lattice mismatch between different layers.

Observation of distributed feedback lasing in silicon nanocrystals under electrical pumping

Electrically pumped distributed feedback (DFB) lasing in silicon nanocrystals (SiNCs) has been observed. The DFB SiNC laser was made based on a SiNC light-emitting diode (LED) by incorporating a second-order DFB grating. The light emission covered the visible regime. Optical gains in the order of 102 cm−1 were found in the light emitting regime. The DFB grating was so designed and fabricated that lasing wavelength in the red regime was selected. Signals emitting from the surface of the DFB SiNC laser were recorded. The laser was pumped by a pulsed direct-current power supply at room temperature. Four typical criteria of lasing were identified, i.e., 1) threshold behavior of the electroluminescence (EL) intensity versus pumping power density, 2) evident spectral narrowing of the EL spectrum, 3) well-recognized degree of polarization, and 4) small spread of light emitting angle. The latter three were for the pumping power density beyond the threshold one.

Fluorescence intensity ratio technique and its reliability

The present article reports the optical absorption and upconversion (UC) studies of 1.0 mol% Er3+/2.0 mol% Yb3+ doped/codoped glasses prepared by melt-quenching technique. The elements present and the composition of the prepared glass have been confirmed from XPS and XRF analysis respectively. Judd-Ofelt intensity parameters have been calculated using the absorption spectrum which is further utilized to predict the nature of Er_O bond, the transition probabilities, branching ratios and radiative lifetimes. The CIE study shows non-colour tunable and highly pure green emission (94.2%). The temperature-dependent UC emission spectra of the 2.0 mol% Yb3+ sensitized glass have been recorded at three different pump power densities to establish a reliable FIR based temperature scale. Furthermore, the Arrhenius fitting of the temperature-dependent spectra reveals low thermal quenching of green luminescence in the codoped glass.

SPECTRAL EFFICIENCY OF THE ACTIVE MEDIUM Ti:SAPPHIRE PUMPING BY AlInGaN LED EMISSION

The electroluminescent characteristics of powerful AlInGaN LEDs in the regime of high-pulsed current are investigated. The current dependencies of power and emission spectra of blue-green LEDs are established in their relationship with the efficiency of active medium Ti:Sapphire pumping. The reached values of the optical pumping power density using LEDs are estimated.

Analysis on the thermal effect of Pr:YLF crystal for power scaling

In a high-power Pr:YLF solid-state laser, the thermal effect of gain medium is one of the prime limiting factors, and its thermal damage has become the major concern. The thermal effect of Pr:YLF crystal was analyzed theoretically, and the distribution of temperature, thermal stress, thermal focal length, and pump polarization effects of the Pr:YLF crystal were simulated. The thermal effect investigation indicates that under reasonable pumping power density, crystal length, and beam waist size and location, the temperature rise and nonuniformity of thermal distortion are not intensified under high-power operation. Additionally, the relationship between Gauss or Super-Gaussian pump mode and thermal focal length of Pr:YLF crystal was simulated. To the best of our knowledge, this analysis is the first to examine the thermal effect of Pr:YLF crystal for power scaling, and this thermal effect investigation of Pr:YLF crystal provides first-hand data for a high-power, visible, solid-state laser that could be helpful for high-power Pr:YLF solid-state laser design.

Opto-Thermally Tunable Axicon-Like Device Based on PDMS-Au Bilayers

A simple method to build up a reflective axicon-like structure is proposed based on the localized swelling of a PDMS-gold bilayer heated by a tightly focused pump laser source. When the distorted reflective surface is illuminated by a probe beam, the theoretical modeling and the experimental tests put in evidence the formation of virtual foci with tens <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> diameter with a depth of focus to 103 mm. Other than being tunable in a flexible and simple way, the reflecting device is wavelength independent and completely reconfigurable with up to 207.3 W/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> pump power densities. As the width of the apical region is triggered by the dimension of the pump focal spot, the device is taylorable for miniaturized systems.

Importance of channel dimension for flow-electrode flowing in flow-electrode capacitive mixing (F-CapMix): Evaluation of net power density under high-pressure-drop conditions

Flow-electrode capacitive mixing (F-CapMix) utilizes ion adsorption/desorption on active materials in the flow-electrode to harvest salinity gradient power. In order to facilitate flow-electrode flow in the F-CapMix unit cell, the current collectors should include a meandering flow channel for the cell assembly and should facilitate efficient electron transport between them and the flow-electrode active materials. This study evaluates the F-CapMix performance variation and pumping power of the flow-electrode and feed water depending on flow channel depth. It was found that both parameters increased as the cross-sectional area of the flow channel narrowed. This result was due to the increasing linear velocity of the flow-electrode, which itself was associated with the narrowing cross-sectional area. The maximum gross power densities of the F-CapMix increased in a parabolic shape with respect to the flow-electrode linear velocity increase. (0.68 → 0.83 → 0.86 → 1.03 → 1.14 W/m2) On the contrary, pumping power densities of the flow-electrode increased linearly (2.22 → 2.98 → 3.51 → 5.93 → 19.67 W/m2) with linear velocity increment of the flow-electrode. Further, the F-CapMix net power density was negatively affected by the increasing linear velocity of the flow-electrode (net power density variation from −2.06 to −19.10 W/m2), likely owing to the higher pumping power density required due to the narrower flow channel. This study verifies the importance of securing an appropriate cross-sectional area of the meandering flow channel to maintain positive net power of the F-CapMix.

Four Novel Embedded Z-Source DC–DC Converters

This article develops a series of improved Z-source dc–dc converters to realize additional voltage pumping and high power density through two propositions. Proposition 1 develops two improved positive- and negative-connected Z-source dc–dc converters (IPZSC and INZSC), using the same number of components as in the existing voltage pumping Z-source converter. Based on Proposition 1, Proposition 2 proposes four novel embedded Z-source dc–dc converters (EZSCs) by placing the source in a specifically designed position, which realizes lower voltage stresses across capacitors. Various technical aspects of the proposed EZSCs, including operations, power losses, and small signal stability analyses are detailed in this paper. Besides, comparisons between the proposed series of improved Z-source dc–dc converters and other existing Z-source dc–dc converters are thoroughly made to demonstrate the better performance of the EZSCs. Finally, experiments are conducted to well validate the effectiveness and superiority of the proposed converter circuits.

Стимулированная эмиссия в сильно легированных Al-=SUB=-0.68-=/SUB=-Ga-=SUB=-0.32-=/SUB=-N : Si-структурах с поперечной оптической накачкой при комнатной температуре

The broadband stimulated emission in the spectral range λ = 380−700 nm with the inhomogeneous broadening has been experimentally obtined in the heavily doped Al0.68Ga0.32N : Si structures grown by molecular beam epitaxy. The behavior of the intensities and spectra of stimulated emission from the edge of the active element with transverse pulsed pumping by radiation with λ = 266 nm, measured at room temperature, demonstrate the threshold behavior and optical gain. For stimulated emission with a maximum at λ = 500 nm, the minimum threshold pump power density was 6.5 kW/cm2 for excited region length of 1.5 mm. The parameters and contributions of the two main processes e − A and D − A of radiative recombination in the excited structures for stimulated emission and optical gain are studied.

Stimulated emission in the heavily doped Al-=SUB=-0.68-=/SUB=-Ga-=SUB=-0.32-=/SUB=-N : Si structures with transverse optical pumping at room temperature

The broadband stimulated emission in the spectral range λ=380-700 nm with the inhomogeneous broadening has been experimentally obtined in the heavily doped Al0.68Ga0.32N : Si structures grown by molecular beam epitaxy. The behavior of the intensities and spectra of stimulated emission from the edge of the active element with transverse pulsed pumping by radiation with λ=266 nm, measured at room temperature, demonstrate the threshold behavior and optical gain. For stimulated emission with a maximum at λ=500 nm, the minimum threshold pump power density was 6.5 kW/cm2 for excited region length of 1.5 mm. The parameters and contributions of the two main processes e-A and D-A of radiative recombination in the excited structures for stimulated emission and optical gain are studied. Keywords: stimulated emission, heavily doped AlxGa1-xN structures, luminescence, optical gain, donor-acceptor recombination.

High-power Yb:YAG thin-disk laser with 80% efficiency pumped at the zero-phonon line.

We report a 3-kW thin-disk laser with 80% optical efficiency employing zero-phonon line pumping at 970 nm. A detailed comparison to conventional pumping at 940 nm is provided, which shows almost twice the pump power density handling capability.

Low-threshold wavelength-tunable ultraviolet vertical-cavity surface-emitting lasers from 376 to 409 nm

Optically pumped wavelength-tunable vertical-cavity surface-emitting lasers (VCSELs) operating in the ultraviolet A (UVA) spectrum were demonstrated. The VCSELs feature double dielectric distributed brag reflectors and a wedge-shaped cavity fabricated using the substrate transfer technique and laser lift off, resulting in a graded cavity length in one device. A resonant period gain structure is used in the InGaN/GaN multi-quantum well active region to enhance the coupling between the cavity mode field and the active layers. The optical field inside the cavity is modulated by the cavity length; thus, tunable lasing at different wavelengths is realized at different points of a single VCSEL chip. The lasing wavelength extends from 376 to 409 nm, covering most of the UVA band below the band gap of GaN. The threshold pumping power density of the UVA VCSELs at different wavelengths ranges from 383 to 466 kW/cm2, which is among the lowest values for ultraviolet (UV) VCSELs. This study is promising for the development of small-footprint, power-efficient UV light sources.

Laser Diode Pumped Polymer Lasers with Tunable Emission Based on Microfluidic Channels.

Tunable whispering-gallery-mode (WGM) lasers have been paid lots of attention for their potential applications in the photonic field. Here, a tunable polymer WGM laser based on laser diode pumping is realized with a threshold of 0.43 MW/cm2 per pulse. The WGM laser is realized by a microfluidic microcavity, which consists of a quartz capillary and gain materials. The laser performance keeps stable for a long time (3.5 h), pumped by a 50-ns 50 Hz laser diode with a pumping peak power density of 1.08 MW/cm2 per pulse. The lasing wavelength can be tuned over 15 nm by changing the gain material concentration from 3.5 mg/mL to 12.5 mg/mL in the microfluidic channel. Moreover, the lasing mode can be switched between transverse magnetic (TM) and transverse electric (TE) modes by adjusting the pump polarization. These results provide the basis for designing nanophotonic devices with laser diode pumping.

Upconversion luminescence and energy transfer in the fluorophosphate glasses with low phosphate content doped with Tm3+ and Yb3+ions

We report an experimental observation of the upconversion in fluorophosphate glasses doped with different concentrations of Tm 3+ ions (from 10 20 to 10 15 ion/cm 3 ) and a constant content of Yb 3+ ions (10 21 ion/cm 3 ) when pumped by a diode laser with a wavelength of 975 nm with a change in the pumping power density from 2.0 to 14.5 kW/cm 2 . Possible mechanisms of population of the upper 1 D 2 , 2 G 4 and 3 F 2,3 states of Tm 3+ ions were discussed depending on their concentration. The role of cross-relaxation processes in the population of the upper excited levels is considered. A change in the values of n -parameter of the luminescence at 475 nm ( 1 G 4 ) from 3 to 2 and high upconversion at low Tm 3+ ions concentrations (about 10 18 -10 15 ion/cm 3 ) were attributed to the cooperative energy transfer from Yb-Yb* dimers. It was found that Tm 3+ /Yb 3+ co-doped fluorophosphate glasses could be promising as up-converting materials. • The concentration dependence of the UC luminescence of Tm 3+ /Yb 3+ co-doped fluorophosphate glasses was studied. • Mechanisms of population of the state 2 D 1, 3 F 2,3 , 1 G 4 and 1 P 0 of Tm 3 + ions were discussed depending on their concentration. • The cooperative energy transfer from Yb-Yb* dimmers led to the high upconversion at low Tm 3+ ions concentrations.

Flow field designs developed by comprehensive CFD model decrease system costs of vanadium redox-flow batteries

Different flow field designs are known for vanadium redox-flow batteries (VFB). The best possible design to fulfil a variety of target parameters depends on the boundary conditions. Starting from an exemplary interdigitated flow field design, its channel and land dimensions are varied to investigate the impact on pressure drop, channel volume, flow uniformity and limiting current density. To find a desirable compromise between these several partly contrary requirements, the total costs of the VFB system are evaluated in dependence of the flow field’s dimensions. The total costs are composed of the electrolyte, production and component costs. For those, the production technique (injection moulding or milling), the pump and nominal power density as well as depth of discharge are determined. Finally, flow field designs are achieved, which lead to significantly reduced costs. The presented method is applicable for the design process of other flow fields and types of flow batteries.Graphical abstract

Effect of pump-induced loss on the spectral dependence of Rhodamine 6G laser efficiency under microsecond coherent pumping

We examine the effect of the pump-induced loss in the gain medium of a Rhodamine 6G laser on the spectral dependence of the laser efficiency under microsecond coherent pumping. It is shown theoretically and experimentally that the pump-induced loss is mainly due to nonlinear refraction and thermo-optic inhomogeneities of the gain medium, which produce a negative thermal lens, whose defocusing effect reduces the laser efficiency. The induced absorption by excited-state singlet and triplet levels of dye molecules is shown to make no significant contribution to changes in laser efficiency. The pump-induced thermal loss at the peak absorption wavelength of Rhodamine 6G plays a more important role well above the threshold pump power density (γ ⩾ 5). We demonstrate that the maximum dye laser efficiency under microsecond coherent pumping can only be achieved at an optimal pump wavelength.

High-efficiency generation of harmonics in polythiophene

High-efficiency harmonic generation is observed in a semiconducting polymer RR-P3HT [regioregular poly(3-hexylthiophene)], which exhibits a high nonlinear susceptibility [χ (2) > 10−6 m V−1] under pumping by a pulsed fibre laser. The harmonic generation efficiency in RR-P3HT is comparable or higher than that observed for liquid crystals, which are used in reference experiments to estimate the susceptibility [the quadratic susceptibility of a nematic liquid crystal NLC 1289 is χ (2) ≈ 2 × 10−6 m V−1]. No generation is observed in polythiophene with a random structure, RRa-P3HT [regiorandom poly(3-hexylthiophene)], at the same pump power. It is experimentally demonstrated that a necessary condition for generating the second and third harmonics in a polymer, along with a high pump power density, is the presence of a large power density gradient (exceeding 1013 W m−3). Based on a preliminary theoretical analysis, we can suggest that the quadrupole mechanism, which is a consequence of the regularity of RR-P3HT structure in the thin near-wall layer, may contribute significantly to the nonlinear radiation conversion in semiconducting polymers.

Stimulated Raman scattering in CH4 gas using single cylindrical lens focusing

In this work, single cylindrical lens, an extreme of the astigmatic case, is used to study Stimulated Raman scattering (SRS) in CH4 gas. In comparison with the conventional spherical optics case, single cylindrical lens focusing has three obvious advantages: (1) an increase of the dimension of the focus will decrease pump power density, and the laser-induced breakdown is suppressed. (2) It will efficiently disperse the heat generated in the SRS process, and benefit for the heat management for SRS. Consequently, SRS with cylindrical lens focusing configuration shows stronger resistance to thermal defocusing effect. (3) The forward high-order Stokes and backward SRS are suppressed. Therefore, the utilization of cylindrical lens has improved forward first Stokes in CH4, acquiring the maximum photon conversion efficiency of 67.1% at 188.5 mJ pump energy and 1Hz, and 44.9% at 109.4 mJ pump energy and 10 Hz, for the condition of 3 Mpa pressure. This effect has great potential for the gaseous SRS with high repetition rates and high pressure.

Visible whispering gallery mode lasing via Li+ ion doped ZnO microspheres

Here we have fabricated ZnO based microcavity as optical resonator to achieve whispering gallery mode (WGM) luminescence through visible excitations. We have observed defect related WGM emissions by individual ZnO microsphere through visible laser excitations of 532 nm wavelength. Further, in order to get visible stimulated emissions, we have considered the doping of 2 mol.% of monovalent (Li+) ion to increase the number of defects in ZnO. Nevertheless, here the ZnO and 2 mol.% Li doped ZnO (Li-ZnO) microcavities (microspheres) are synthesized by using hydrothermal synthesis technique. Here we have considered four distinguished 2 mol.% Li-ZnO microspheres to check the consistency of the stimulated emission behavior. We have observed that the stimulated emissions are achieved through all the four Li-ZnO microspheres at threshold pumping power density of 48 mW/cm2 of the laser source. Therefore, we have proposed these doped microresonators for their promising applications to design the WGM based visible laser cavities.