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Ultrathin Pt/Co/Pt trilayers with initial in-plane magnetization were irradiated with femtosecond laser pulses. In this way, an irreversible structural modification was introduced, which resulted in the creation of numerous pulse fluence-dependent magnetic phases. This was particularly true with the out-of-plane magnetization state, which exhibited a submicrometer domain structure. This effect was studied in a broad range of pulse fluences up to the point of ablation of the metallic films. In addition to this single-pulse experiment, multiple exposure spots were also investigated, which exhibited an extended area of out-of-plane magnetization phases and a decreased damage threshold. Using a double exposure with partially overlapped spots, a two-dimensional diagram of the magnetic phases as a function of the two energy densities was built, which showed a strong inequality between the first and second incoming pulses. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a selection scheme of lasing direction by imposing a loss imbalance structure into the single mode square ring cavity. The control of the traveling direction is realized by introducing a taper-step section in one of the straight waveguides of the square ring cavity. It was shown by semi-analytic calculation that the taper-step section in the cavity provides effective loss imbalance between two travelling directions as the round trip repeats. Various kinds of square cavities were fabricated using InGaAsP/InGaAs multiple quantum well semiconductor materials in order to test the direction selectivity while maintaining the single mode. We also measured the pump power dependent lasing spectra to investigate the maintenance property of the lasing direction. The experimental results demonstrated that the proposed scheme is an efficient means for a unidirectional lasing in a single mode laser. [ABSTRACT FROM AUTHOR]

Experiments of laser-driven hypervelocity flyers have been conducted at the SG-III prototype laser facility. Using the continuum phase plate technique, four laser beams each with a 3-ns quadratic profile are configured to produce relatively uniform irradiated spots of diameter size either 500 im or 2000 im. With the former, specifically designed multi-layered flyers (polyimide/copper) were accelerated by shock impedance and reverberation techniques via direct laser ablation to a super-high averaged velocity of 55 km/s, much faster than recently reported results. Light-emission signals of shock breakout and flyer impact on flat or stepped windows were obtained that indicated good planarity and integrity for the flyer. In the latter, single-layered aluminum flyers were gradually accelerated to a terminal velocity of 11 km/s, as measured by optical velocimetry, without melting and vaporization. The results suggest that the SG-III prototype laser facility has the capability to launch high-speed flyers to create extreme conditions for investigating the science of shock compression and its equation of state. [ABSTRACT FROM AUTHOR]

Experimental investigations on the electrical explosion of aluminum wire using negative polarity current in vacuum are presented. Current pulses with rise rates of 40 A/ns, 80 A/ns, and 120 A/ns are generated for investigating the influence of current rise rate on energy deposition. Experimental results show a significant increase of energy deposition into the wire before the voltage breakdown with the increase of current rise rate. The influence of wire dimension on energy deposition is investigated as well. Decreasing the wire length allows more energy to be deposited into the wire. The energy deposition of a 0.5 cm-long wire explosion is ∼2.5 times higher than the energy deposition of a 2 cm-long wire explosion. The dependence of the energy deposition on wire diameter demonstrates a maximum energy deposition of 2.7 eV/atom with a diameter of ∼18 μm. Substantial increase in energy deposition is observed in the electrical explosion of aluminum wire with polyimide coating. A laser probe is applied to construct the shadowgraphy, schlieren, and interferometry diagnostics. The morphology and expansion trajectory of exploding products are analyzed based on the shadowgram. The interference phase shift is reconstructed from the interferogram. Parallel dual wires are exploded to estimate the expansion velocity of the plasma shell. [ABSTRACT FROM AUTHOR]

We have investigated excitation power and temperature dependent PL spectra to systematically study the influences of the interfaces in the both InAs/Ga0.71In0.29Sb and InAs/AlSb on the optical properties of AlSb/Ga0.71In0.29Sb/InAs quantum wells (QWs). The localized states as well as the activation energy were analyzed to discuss the possible thermal quenching and non-radiative recombination mechanisms. We found two non-radiative recombination processes were involved in the thermal quenching of radiative emission for the QW structures. The GaAs-like interface in InAs/Ga0.71In0.29Sb with higher activation energy (62.7 meV) in high temperature region (70 K-300 K) supplies a deeper hole confinement and less roughness than the InSb-like one, which suppress non-radiative recombination process and promote the optical qualities of the quantum wells. The peak energy of the InSb-like sample exhibited "step-curve" behavior with increase temperature. Neither InSb-like nor AlAs-like interface in InAs/AlSb favored the radiative emission efficiency. [ABSTRACT FROM AUTHOR]

Controlled photo-induced oxidation and amorphization of elemental trigonal tellurium are achieved by laser irradiation at optical wavelengths. These processes are monitored in situ by time-resolved Raman scattering and ex situ by electron microscopies. Ultrathin TeO2 films form on Te surfaces, as a result of irradiation, with an interface layer of amorphous Te intervening between them. It is shown that irradiation, apart from enabling the controllable transformation of bulk Te to onedimensional nanostructures, such as Te nanotubes and hybrid core-Te/sheath-TeO2 nanowires, causes also a series of light-driven (athermal) phase transitions involving the crystallization of the amorphous TeO2 layers and its transformation to α multiplicity of crystalline phases including the γ-, β-, and α-TeO2 crystalline phases. The kinetics of the above photo-induced processes is investigated by Raman scattering at various laser fluences revealing exponential and non-exponential kinetics at low and high fluence, respectively. In addition, the formation of ultrathin (less than 10 nm) layers of amorphous TeO2 offers the possibility to explore structural transitions in 2D glasses by observing changes in the short- and medium-range structural order induced by spatial confinement. [ABSTRACT FROM AUTHOR]

Temperature- (T-) and laser power-dependent photoluminescence (PL) measurements have been made for the tensile-strained, undoped GeSn (0.03% Sn) film grown on Si substrate. The PL results show not only clear strain-split direct bandgap transitions to the light-hole (LH) and heavy-hole (HH) bands at energies of 0.827 and 0.851 eV at 10 K, respectively, but also clearly show both strong direct and indirect bandgap related PL emissions at almost all temperatures, which are rarely observed. This split of PL emissions can be directly observed only at low T and moderate laser power, and the two PL peaks merge into one broad PL peak at room temperature, which is mainly due to the HH PL emission rather than LH transition. The evolution of T-dependent PL results also clearly show the competitive nature between the direct and indirect bandgap related PL transitions as T changes. The PL analysis also indicates that the energy gap reduction in Γ valley could be larger, whereas the bandgap reduction in L valley could be smaller than the theory predicted. As a result, the separation energy between Γ and L valleys (~86meV at 300 K) is smaller than theory predicted (125 meV) for this Ge-like sample, which is mainly due to the tensile strain. This finding strongly suggests that the indirect-to-direct bandgap transition of Ge1-ySny could be achieved at much lower Sn concentration than originally anticipated if one utilizes the tensile strain properly. Thus, Ge1-ySny alloys could be attractive materials for the fabrication of direct bandgap Si-based light emitting devices. [ABSTRACT FROM AUTHOR]

The influence of an intense laser field on one-electron states and intraband optical absorption coefficients is investigated in two-dimensional GaAs/Ga0.7Al0.3As quantum rings. An analytical expression of the effective lateral confining potential induced by the laser field is obtained. The one-electron energy spectrum and wave functions are found using the effective mass approximation and exact diagonalization technique. We have shown that changes in the incident light polarization lead to blue- or redshifts in the intraband optical absorption spectrum. Moreover, we found that only blueshift is obtained with increasing outer radius of the quantum ring. [ABSTRACT FROM AUTHOR]

We demonstrate random lasing action in flexible bacterial cellulose (BC) membrane containing a laser-dye and either dielectric or metallic nanoparticles (NPs). The novel random laser system consists of BC nanofibers attached with Rhodamine 6G molecules and having incorporated either silica or silver NPs. The laser action was obtained by excitation of the samples with a 6 ns pulsed laser at 532 nm. Minimum laser threshold of ≈0.7 mJ/pulse was measured for the samples with silica NPs, whereas a laser threshold of 2.5mJ/pulse for a system based on silver NPs was obtained. In both cases a linewidth narrowing from ≈50 to ≈ 4 nm was observed. Potential applications in biophotonics and life sciences are discussed for this proof-of-concept device. [ABSTRACT FROM AUTHOR]

We consider the concept of injection terahertz lasers based on double-graphene-layer (double-GL) structures with metal surface-plasmon waveguide and study the conditions of their operation. The laser under consideration exploits the resonant radiative transitions between GLs. This enables the double-GL laser room temperature operation and the possibility of voltage tuning of the emission spectrum. We compare the characteristics of the double-GL lasers with the metal surface-plasmon waveguides with those of such laser with the metal-metal waveguides. [ABSTRACT FROM AUTHOR]

When a shock wave propagating in a solid sample reflects from a free surface, geometrical effects predominantly governed by the roughness and defects of that surface may lead to the ejection of tiny jets that may breakup into high velocity, approximately micrometer-size fragments. This process referred to as microjetting is a major safety issue for engineering applications such as pyrotechnics or armour design. Thus, it has been widely studied both experimentally, under explosive and impact loading, and theoretically. In this paper, microjetting is investigated in the specific loading conditions associated to laser shocks: very short duration of pressure application, very high strain rates, small spatial scales. Material ejection from triangular grooves in the free surface of various metallic samples is studied by combining transverse optical shadowgraphy and time-resolved velocity measurements. The influences of the main parameters (groove angle, shock pressure, nature of the metal) on jet formation and ejection velocity are quantified, and the results are compared to theoretical estimates. [ABSTRACT FROM AUTHOR]

We studied the random laser (RL) bichromatic emission (BCE) from a powder consisting of silica particles infiltrated with Rhodamine 640 (Rh640) molecules. The BCE is attributed to Rh640 monomers and dimers. Because of the efficient monomer-dimer energy transfer, we observed RL wavelength switching from ≈ 620 nm to ≈ 650 nm and the control of the emitted wavelength was made by changing only the excitation laser intensity. None of external parameters such as excitation laser spot size or radiation detector position was changed as in previous experiments. Two laser thresholds associated either to monomers or dimers were clearly observed. Moreover, an effect analog to frequency-pulling among two coupled oscillators was identified measuring the RL spectra as a function of the excitation laser intensity. A wavelength shift, Δλ, was measured between the monomer and dimer resonance wavelengths, changing only the excitation laser intensity. The maximum value of Δλ ≈ 16 cm-1 was obtained for laser pulses of 7 ns with 30 µJ. [ABSTRACT FROM AUTHOR]

Band structure properties of the type-II W-design AlSb/InAs/GaIn(As)Sb/InAs/AlSb quantum wells have been investigated theoretically in a systematic manner and with respect to their use in the active region of interband cascade laser for a broad range of emission in mid infrared between below 3 to beyond 10 μm. Eight-band k·p approach has been utilized to calculate the electronic subbands. The fundamental optical transition energy and the corresponding oscillator strength have been determined in function of the thickness of InAs and GaIn(As)Sb layers and the composition of the latter. There have been considered active structures on two types of relevant substrates, GaSb and InAs, introducing slightly modified strain conditions. Additionally, the effect of external electric field has been taken into account to simulate the conditions occurring in the operational devices. The results show that introducing arsenic as fourth element into the valence band well of the type-II W-design system, and then altering its composition, can efficiently enhance the transition oscillator strength and allow additionally increasing the emission wavelength, which makes this solution prospective for improved performance and long wavelength interband cascade lasers. [ABSTRACT FROM AUTHOR]

We investigate the evolution of the species from both the target and the air, and the plasma parameter distribution of the nanosecond laser-produced plasmas in atmospheric air. The technique used is spatially resolved optical emission spectroscopy. It is argued that the N II from the air, which is distributed over a wider region than the target species in the early stages of the discharge, is primarily formed by the shock wave. The ionized species have a larger expansion velocity than the excited atoms in the first ∼100 ns, providing direct evidence for space-charge effects. The electron density decreases with the distance from the target surface in the early stages of the discharge, and both the electron density and the excited temperature variation in the axial direction are found to become insignificant at later stages. [ABSTRACT FROM AUTHOR]

Although the laser shockwave cleaning process offers a promising alternative to conventional dry-cleaning processes for nanoscale particle removal, its difficulty in removing organic particles has been an unexplained problem. This work elucidates the physics underlying the ineffectiveness of removing organic particles using laser shock cleaning utilizing polystyrene latex particles on silicon substrates. It is found that the shockwave pressure is high enough to deform the particles, increasing the contact radius and consequently the particle adhesion force. The particle deformation has been verified by high-angle scanning electron microscopy. The Maugis-Pollock theory has been applied to predict the contact radius, showing good agreement with the experiment. [ABSTRACT FROM AUTHOR]

An ultrathin metal film deposited on dielectric plate supports low loss THz surface plasma wave (SPW). The SPW field falls off away from the metal film both inside the dielectric as well as in free space. Two lasers normally incident, from free space on the planar structure, exert a difference frequency ponderomotive force on the free electrons of the film and resonantly excite THz SPW. The ratio of SPW amplitude to lasers is 10-3 at laser intensity of 3 × 1012 W/cm2 at 1 μ m wavelength. The growth rate falls monotonically because at higher frequency the coupling of SPW is weak. [ABSTRACT FROM AUTHOR]

We present an approach to characterize the relative saturation current density (Joe) and sheet resistance (RSH) of laser doped regions on silicon wafers based on rapid photoluminescence (PL) imaging. In the absence of surface passivation layers, the RSH of laser doped regions using a wide range of laser parameters is found to be inversely proportional to the PL intensity (IPL). We explain the underlying mechanism for this correlation, which reveals that, in principle, IPL is inversely proportional to Joe at any injection level. The validity of this relationship under a wide range of typical experimental conditions is confirmed by numerical simulations. This method allows the optimal laser parameters for achieving low RSH and Joe to be determined from a simple PL image. [ABSTRACT FROM AUTHOR]

Focussed ion beam milling can be used to introduce aperiodic distributed feedback (ADFB) gratings into fully packaged, operational terahertz (THZ) quantum cascade lasers to achieve electronically controlled, discretely tunable laser emission. These aperiodic gratings-designed using computer-generated hologram techniques-consist of multiple slits in the surface plasmon waveguide, distributed along the length of the laser cavity. Tuning behaviour and output power in ADFB lasers operating around 2.9 THz are investigated with a variety of slit dimensions and grating scales. Mode selectivity and grating losses are found to be strongly dependent on milling depth into the upper waveguide layers, dramatically increasing as the metallic layers are penetrated, then rising more slowly with deeper milling into the laser active region. Grating scale and placement along the laser cavity length are also shown to influence mode selection. [ABSTRACT FROM AUTHOR]

We propose a device structure to lower the lasing threshold of a uniformly lying helix cholesteric liquid crystal (ChLC) laser. We place a π-conjugated polymer active layer beneath the ChLC layer to provide auxiliary gain, and demonstrate an improvement in the lasing threshold by a factor of 2.3. We also perform finite difference time domain calculations coupled with rate equations for a four-level system, and clarify the effect of the additional active layer on both the photonic density of states and the inversion population density. Although the addition of an extra layer lowers the photonic density of states, the gain provided by the auxiliary layer is sufficient to overcome the losses and decrease the lasing threshold. Our concept is useful for obtaining high-performance ChLC lasers. [ABSTRACT FROM AUTHOR]

Using 800 nm, 25-fs pulses from a mode locked Ti:Al2O3 laser, we have measured the ultrafast optical reflectivity of MBE-grown, single-layer In0.4Ga0.6As/GaAs quantum-dot (QD) samples. The QDs are formed via two-stage Stranski-Krastanov growth: following initial InGaAs deposition at a relatively low temperature, self assembly of the QDs occurs during a subsequent higher temperature anneal. The capture times for free carriers excited in the surrounding GaAs (barrier layer) are as short as 140 fs, indicating capture efficiencies for the InGaAs quantum layer approaching 1. The capture rates are positively correlated with initial InGaAs thickness and annealing temperature. With increasing excited carrier density, the capture rate decreases; this slowing of the dynamics is attributed to Pauli state blocking within the InGaAs quantum layer. [ABSTRACT FROM AUTHOR]

Dye-doped chiral-nematic liquid crystal lasers have great potential as small size, low-cost, widely tunable lasers. We present a numerical model for stimulated emission and lasing in liquid crystal films based on thin film optics. The gain threshold is modelled and the results are confirmed experimentally. The effect of the orientation of dye molecules and the matching of the photonic bandgap to the dye spectrum on the threshold for lasing is discussed. [ABSTRACT FROM AUTHOR]

In this work, we analyze a hybrid laser/liquid jet micromachining process for several metals and ceramics based on the optical breakdown of a microdroplet. In the process, materials are removed by the combined effects of a laser pulse and a high-speed pulsed microjet ejected from the microdroplet. The opto-hydrodynamic phenomena occurring during this process and the interaction of the laser/liquid jet with various materials, including copper, aluminum, stainless steel, alumina, and boron nitride, are investigated experimentally. The results show that the laser/liquid jet can remove the materials with substantially increased removal rates and reduced thermal side effects compared with the conventional pulsed laser ablation process. Visualization of the process reveals that the materials are partially ablated and melted by the laser pulse during the early stage of the process and that the molten material is subsequently eliminated by the hydrodynamic impact of the liquid jet. [ABSTRACT FROM AUTHOR]

The temporal evolution of the ablation plume of cerium was investigated by absorption spectroscopy. Cerium oxide pellets were ablated in a helium atmosphere by second-harmonic radiation (532 nm) from a Nd:YAG laser at a fluence of 0.5 J/cm2. The lateral velocity (expansion velocity horizontal to the sample surface) of the plume was determined from the magnitude of the Doppler splitting of the absorption spectra measured close to the sample surface. The lateral velocities of neutral and singly ionized atoms were systematically investigated by varying several parameters, such as ambient gas pressure, ablation laser fluence, observation timing, and observation height. In addition, temporal profiles of the absorption signal were measured by detuning the probe laser frequency from the atomic resonant frequency in order to obtain the temporal variation of the velocity. On the basis of the drag force model, the slowing coefficients for atomic and ionic species in a helium atmosphere were evaluated along with lateral velocity in a vacuum. This study may help in understanding the plume dynamics effect on deposited film properties as well as optimizing experimental conditions for ablation-based spectroscopic analysis. [ABSTRACT FROM AUTHOR]

LiCaAlF6 (LiCAF) is a promising inorganic neutron scintillator material. Here, we report the electronic structure as well as properties of the small hole polaron and F center in LiCAF based on first-principles calculations. The results suggest that lowering the Fermi level by acceptor doping will stabilize F+ against F- and reduce the reabsorption of Ce emission by F centers in LiCAF:Ce3+, thereby increasing light yield and radiation resistance. [ABSTRACT FROM AUTHOR]

A one-dimensional mathematical model based on the front tracking method was developed to predict the melt depth as a function of internal and external parameters of laser spot remelting process in conduction mode. Power density, pulse duration, and thermophysical properties of material including thermal diffusivity, melting point, latent heat, and absorption coefficient have been taken into account in the model of this article. By comparing the theoretical results and experimental welding data of commercial pure nickel and titanium plates, the validity of the developed model was examined. Comparison shows a reasonably good agreement between the theory and experiment. For the sake of simplicity, a graphical technique was presented to obtain the melt depth of various materials at any arbitrary amount of power density and pulse duration. In the graphical technique, two dimensionless constants including the Stefan number (Ste) and an introduced constant named laser power factor (LPF) are used. Indeed, all of the internal and external parameters have been gathered in LPF. The effect of power density and pulse duration on the variation of melt depth for different materials such as aluminum, copper, and stainless steel were investigated. Additionally, appropriate expressions were extracted to describe the minimum power density and time to reach melting point in terms of process parameters. A simple expression is also extracted to estimate the thickness of mushy zone for alloys. [ABSTRACT FROM AUTHOR]

We investigated the effects of laser excitation wavelength on water-window emission lines of laser-produced boron-nitride plasmas. Plasmas are produced by focusing 1064 nm and harmonically generated 532 and 266 nm radiation from a Nd:YAG laser on BN target in vacuum. Soft x-ray emission lines in the water-window region are recorded using a grazing-incidence spectrograph. Filtered photodiodes are used to obtain complementary data for water-window emission intensity and angular dependence. Spectral emission intensity changes in nitrogen Ly-α and He-α are used to show how laser wavelength affects emission. Our results show that the relative intensity of spectral lines is laser wavelength dependent, with the ratio of Ly-α to He-α emission intensity decreasing as laser wavelength is shortened. Filtered photodiode measurements of angular dependence showed that 266 and 532 nm laser wavelengths produce uniform emission. [ABSTRACT FROM AUTHOR]

We report a simulation of quantum cascade lasers based on the integration of a number of optoelectronic models on both microscopic and macroscopic scales. On the microscopic scale, quantum mechanical computation was performed to find the quantization states and a rate equation approach was used to compute the optical gain. On the macroscopic scale, we solved the drift-diffusion equations with modification of current density to account for long-range carrier transport, including quantum tunneling, mini-band tunneling, and hot carrier transport. Multiple lateral optical modes were computed by solving a scalar wave equation as an eigenvalue problem. Finally, multiple lateral mode laser cavity photon rate equations were solved with the drift-diffusion equations in a self-consistent manner to predict the lasing characteristics of a quantum cascade laser. The simulation compared the integrated models with experimental data from a number of AlInGaAs/InP systems with variable quantum wells and at different temperatures. Reasonable agreements with experiments have been obtained for both electrical and lasing characteristics. [ABSTRACT FROM AUTHOR]

Laser-produced plasmas (LPP) from Sn targets are seriously considered to be the light source for extreme ultraviolet (EUV) next generation lithography, and optimization of such a source will lead to improved efficiency and reduced cost of ownership of the entire lithography system. We investigated the role of reheating a prepulsed plasma and its effect on EUV conversion efficiency (CE). A 6 ns, 1.06 μm Nd:yttrium aluminum garnet laser was used to generate the initial plasma that was then reheated by a 40 ns, 10.6 μm CO2 laser to generate enhanced EUV emission from a planar Sn target. The effects of prepulsed laser intensity and delay timings between the prepulsed and the pumping pulse were investigated to find the optimal pre-plasma conditions before the pumping pulse. The initial optimization of these parameters resulted in 25% increase in CE from the tin LPP. The cause of increased EUV emission was identified from EUV emission spectra and ion signal data. [ABSTRACT FROM AUTHOR]

A setup with Q-switched Nd:YAG laser inducing acoustic wave at air-solid interface and air-coupled optical deflection sensor was developed to research the influences of low air pressure on laser inducing leaky Lamb and Scholte waves at air-solid interface. The solid plate is settled in an airtight vessel abounded with standard air and its pressure can be adjusted by a vacuum pump. By experiments, the waveforms of laser induced interface acoustic waves, leaky Lamb and Scholte waves, under air pressures from 0.02 to 0.08 MPa with 0.01 MPa interval were first measured comprehensively. From the waveforms, we find that with increasing air pressure, the leaky energy of Lamb waves increase linearly and the amplitude of Scholte waves increase exponentially, but the velocity of Scholte waves decreases obviously. [ABSTRACT FROM AUTHOR]

Laser induced breakdown at copper-water interface is employed to synthesize copper oxide nanoparticles. Copper forms two stable oxides: monoclinic CuO and cubic Cu2O. The characteristic traits of laser induced plasma at copper-water interface are altered to analyze the size induced structural modifications in these oxides. The properties of laser produced plasma were varied by changing the focusing conditions of the source laser. Tightly focused condition led to formation of CuO of size ≤ 200 nm whereas laser defocusing condition produces nanocolloids of Cu2O of size less than 10 nm. These findings were attributed to high pressure (60 GPa) accompanied by high temperature at tightly focused condition which results in growth of covalent CuO whereas lower pressure (1.3 GPa) and low temperature at defocused condition probably forms symmetric Cu2O. [ABSTRACT FROM AUTHOR]

We present a study of heterogeneous terahertz quantum cascade lasers using micro-probe photoluminescence. Simulations and experiments are first presented on a homogeneous terahertz quantum cascade laser; these indicate that the population of the upper laser level and the energy of laser transition can be tracked by this technique. We then focus on heterogeneous terahertz quantum cascade lasers and demonstrate the utility of micro-photoluminescence for these devices by measuring the state populations and energy separations for each sub-stack independently. [ABSTRACT FROM AUTHOR]

2.7 μm emission properties and energy transfer mechanism of Nd3+/Er3+ co-doped sodium tellurite glasses are investigated in present paper. Absorption and emission spectra were tested to characterize the 2.7 μm emission properties of Nd3+/Er3+ co-doped sodium tellurite glasses and a reasonable energy transfer mechanism of 2.7 μm emission between Er3+ and Nd3+ ions was proposed. Intense 2.7 μm emission was obtained from Nd3+/Er3+ co-doped sodium tellurite glasses due to the efficient energy transfer from Nd3+ to Er3+ ions under 808 nm LD pumping. Meanwhile, strongly decreased 545 nm up-conversion and 1.5 μm emissions were observed. Additionally, to obtain efficient 2.7 μm emission, the optimized concentration ratio of Nd3+ to Er3+ was found to be 0.5:1 in present glass system. Our results suggest that the present Nd3+/Er3+ co-doped sodium tellurite glass might have potential application in mid-infrared lasers. [ABSTRACT FROM AUTHOR]

A midinfrared quantum cascade laser with high wall-plug efficiency is analyzed by means of an ensemble Monte Carlo method. Both the carrier transport and the cavity field dynamics are included in the simulation, offering a self-consistent approach for analyzing and optimizing the laser operation. It is shown that at low temperatures, photon emission and absorption can govern the carrier transport in such devices. Furthermore, we find that photon-induced scattering can strongly affect the kinetic electron distributions within the subbands. Our results are validated against available experimental data. [ABSTRACT FROM AUTHOR]

High power lasers such as NIF in the USA or LMJ in France are being developed for inertial confinement fusion applications. However, the performance of the optics is limited by laser-induced damage (LID), which occurs, for instance, in the potassium dihydrogen phosphate (KH2PO4 or KDP) crystals utilized for frequency conversion. An accurate equation of state (EOS) is required to explain the LID process and to predict damage size. For the design of such EOS, a pulsed electron beam was used to generate a quasi-plane stress wave of 0.7 GPa in KDP. The sample response was deduced from photonic Doppler velocimetry. Equations of state and deviatoric stress components are designed and compared to experimental data. They are used in laser-induced bulk damage simulations, showing that strength may play a significant role. [ABSTRACT FROM AUTHOR]

Microscale laser dynamic forming (μLDF) shows great potential in fabricating robust and high-aspect-ratio metallic microcomponents. Experiments revealed that strain rate and sample size play important roles in determining the dynamic plasticity and final results of μLDF. To further understand these effects, a multiscale modeling methodology is adopted to characterize the microscale dynamic plasticity considering the evolutions of nano-to-submicron dislocations avalanches under shock loading. In this methodology, 3D discrete dislocation dynamics simulations are implemented to derive the yield strength and the initial strain hardening dependence on size and strain rate. It is observed that there exist three dynamic stages during deformation process. The initial strain hardening rate in Stage II increases with strain rate. The mechanical threshold stress model, intrinsically equipped with strain-rate-dependent flow stress and initial hardening, is chosen and modified to incorporate size effect quantitatively. This scale-dependent model, implemented in abaqus/explicit, provides deformation depths and thickness variations in good agreement with experimental results in μLDF. [ABSTRACT FROM AUTHOR]

We analyze a microscopically based rate equation model for quantum dot lasers. The model separately treats the dynamics of electrons and holes, and the carrier-carrier scattering rates depend nonlinearly on the wetting layer carrier densities. Our objective is to determine analytical expressions for the relaxation oscillation frequency and damping rate. To this end, we consider the Class B limit of the five rate equations and apply asymptotic techniques. We consider two cases corresponding to either equivalent or drastically different decay rates for the electrons and holes. We show how they contribute to increase the relaxation oscillation damping rate compared to the damping rate of the conventional laser and that there exist optimal conditions on the control parameters in order to observe maximum damping. [ABSTRACT FROM AUTHOR]

Near-infrared solid laser-induced blast waves were investigated using a Q-switched Nd:glass laser oscillating at 1053 nm wavelength, up to 2.0 J/pulse laser energy Ei, and 33 ns pulse duration (full width at half maximum). Half-shadowgraph half-self-emission visualization elucidated laser detonation waves and blast waves. Laser output and transmission through the waves were measured using pairs of a photodetectors and energy meters. Results show volumetric absorption in this near-infrared region, which is mainly attributable to low inverse-Bremsstrahlung absorption coefficients at short wavelengths. Some input laser energy was converted into blast wave energy: 59% at Ei=1.0 J/pulse, which approximates that of CO2 laser-induced blast waves. [ABSTRACT FROM AUTHOR]

The relationship between facet stress and reliability of AlGaInAs edge-emitting lasers is unclear despite it being an important issue. We prepared two 1.3 μm AlGaInAs Fabry–Pérot buried-heterostructure (BH) lasers that were identical except that they had tensile and compressive stress at the facet. The magnitude of the facet stress was controlled to be approximately 200 MPa in both lasers. We performed three reliability tests. In forward-biased electrostatic discharge tests, which can evaluate the resistance to optical damage, the cumulative degradation ratio of the compressive stressed laser was 33% lower than that of the tensile stressed laser. This result indicates a reduction in the optical absorption due to enlargement of the energy band gap at the facet. In the long-term aging of the light output power of 8 mW at 85 °C over 5 000 h, no dependence of the facet stress on the lifetime was observed. Since a major limitation of InP-based BH lasers is the BH interface, the aging results are reasonable. In the accelerated aging of a large current of 200 mA at 85 °C over 800 h, degradation (defined as an increase in the threshold current of over 10%) was observed only in the tensile stressed laser. This degradation differed completely from that caused by optical absorption; dislocation loops covered the entire active layer at the facet. To the best of our knowledge, this is the first time such degradation has been reported. We found the degradation depends on the facet stress and that compressive stress can suppress the degradation. [ABSTRACT FROM AUTHOR]

Terahertz (THz) lasers on optically pumped multiple-graphene-layer (MGL) structures as their active region are proposed and evaluated. The developed device model accounts for the interband and intraband transitions in the degenerate electron-hole plasma generated by optical radiation in the MGL structure and the losses in the slot or dielectric waveguide. The THz laser gain and the conditions of THz lasing are found. It is shown that the lasers under consideration can operate at frequencies >=1 THz at room temperatures. [ABSTRACT FROM AUTHOR]

We propose an electrically pumped intersubband laser capable of operating at short infrared wavelengths ∼1.5–2.5 μm. Short-wavelength operation is achieved via resonant intracavity second harmonic generation in quantum cascade (QC) lasers based on high band offset heterostructures. This approach overcomes the fundamental problem of intervalley scattering that prohibits direct lasing in QC lasers in this spectral range. The proposed devices can be modulated at a rate exceeding 100 GHz, which may be interesting for a variety of applications. We present detailed calculations of bandstructure design, phase-matched waveguide, and current-output power dependence for the devices based on GaInAs/AlAsSb/InP heterostructures. Accurate position of highly excited subbands is determined by calculating the conduction band (CB) dispersion with the energy-dependent 14-band effective mass, in which matrix elements are adjusted to fit the CB structure obtained with a 30-band k·p method. We also discuss the controversial location of lateral valleys, which imposes the limitation on the fundamental laser transition energy. [ABSTRACT FROM AUTHOR]

Laser field with superintensity beyond 1029 W/cm2 can be generated by coherent superposition of multiple 100 fs laser pulses in circular geometry setup upon retroreflection by a ring mirror. We have found the criteria for attaining such intensities using broadband ring mirror within the practical damage threshold and paraxial focusing regime. Simple expressions for the intensity enhancement factor are obtained, providing insight for achieving unlimited laser intensity. Higher intensities can be achieved by using few-cycle laser pulses. [ABSTRACT FROM AUTHOR]

Stimulated Brillouin scattering of two collinear lasers in a plasma is investigated. Lasers exert a longitudinal ponderomotive force on electrons, imparting them oscillatory axial velocity at the beat frequency. This velocity acts as a driver for parametric excitation of an ion acoustic wave (ω,kvector ) and a noncollinear sideband electromagnetic wave (ω′,kvector ′). The driver velocity vvector 0- couples to the sideband wave to exert a ponderomotive force at (ω,kvector ) on the electrons, driving the ion acoustic wave. The density perturbation of ion acoustic wave beats with vvector 0- to produce a nonlinear current at (ω′,kvector ′), driving the sideband. In the case of finite spot size Gaussian laser beams, the beat wave has a Gaussian profile and excites an ion acoustic wave (ω,kvector ) and a backscattered TM mode (ω′,kvector z′). The growth rate scales as the product of amplitudes of the lasers and maximizes at optimum values of scattering angles. The parametric instability of difference frequency driver is stronger than the sum frequency driver. [ABSTRACT FROM AUTHOR]

Purpose: In order to investigate the possibility of using the Er:YAG laser for apicoectomy and root-end sealing, the procedure time, dye penetration at the site of root-end sealing, and surrounding morphological differences were investigated. Materials and Methods: In 48 extracted human mandibular second premolars, root canal therapy was performed in the conventional manner. The Er:YAG laser was used for apicoectomy at either 3, 5, or 7 W. After preparing a root-end cavity about 5 mm deep, root-end sealing was performed using amalgam or light-cured resin. For control, an engine-driven fissure bur was used. Next, the root apex was polished, and dye penetration was assessed at depths of 1, 2, 3, and 4 mm. Morphological observations were carried out by x-ray imaging, stereoscopic microscopy, and scanning electron microscope (SEM). Results: When compared to the conventional rotary cutting device, the time required for 5- or 7-W laser apicoectomy was significantly shorter. The cut dentin surface in the laser application group was smooth, and carbonization, fissures, and marked evaporation were not seen. Dye leakage was only slight, with no significant difference between the laser and control groups (p < 0.01). Conclusions: The time for apicoectomy using the Er:YAG laser was shorter when compared to apicoectomy using the engine-driven rotary cutting device, and the level of dye leakage was comparable to the conventional method. The cut surface for root-end sealing had no residual smear layer and was clean. Favorable results were obtained with 3-W laser application in terms of cut surface condition, therapy time, and sealant gap. [ABSTRACT FROM AUTHOR]

In this work, we report a systematic investigation of upconversion losses and their effects on fluorescence quantum efficiency and fractional thermal loading in Nd3+-doped fluoride glasses. The energy transfer upconversion (γup) parameter, which describes upconversion losses, was experimentally determined using different methods: thermal lens (TL) technique and steady state luminescence (SSL) measurements. Additionally, the upconversion parameter was also obtained from energy transfer models and excited state absorption measurements. The results reveal that the microscopic treatment provided by the energy transfer models is similar to the macroscopic ones achieved from the TL and SSL measurements because similar γup parameters were obtained. Besides, the achieved results also point out the migration-assisted energy transfer according to diffusion-limited regime rather than hopping regime as responsible for the upconversion losses in Nd-doped glasses. [ABSTRACT FROM AUTHOR]

The cylindrical confinement of laser-induced plasmas in round pipes has been investigated by optical emission spectra and fast imaging. An obvious enhancement in the emission intensity of Al atomic lines was observed when a round pipe was placed to confine the laser-induced Al plasmas. The enhancement factor for the emission intensities of the Al atomic lines was measured to be around 9 at a time delay of 12 μs when the pipe diameter is 10.8 mm. Assuming local thermodynamic equilibrium conditions, the plasma temperatures are estimated to be in the range from 4000 to 5800 K. It shows that the plasma temperature increased by around 1000 K when the cylindrical confinement was applied. Images of the laser-induced Al plasmas show that the plasmas were compressed into a smaller volume with a pipe presented. The spatial-confinement effects are attributed to the reflection and compression of the shock wave. [ABSTRACT FROM AUTHOR]

The physical mechanisms and molecular-level picture of laser-induced material ejection from frozen solutions of polymer molecules in a volatile matrix are investigated in a series of coarse-grained molecular dynamics simulations. The simulations are performed for polymer concentrations up to 6 wt % and laser fluences covering the range from the regime where molecular ejection is limited to matrix evaporation from the surface up to more than twice the threshold fluence for the onset of the collective molecular ejection or ablation. The results of the simulations are related to experimental observations obtained in matrix-assisted pulsed laser evaporation (MAPLE) thin film depositions and are used to address unresolved research questions that are of direct relevance to MAPLE performance. Contrary to the original picture of the ejection and transport of individual polymer molecules in MAPLE, the simulations indicate that polymer molecules are only ejected in the ablation regime and are always incorporated into polymer-matrix clusters/droplets generated in the process of the explosive disintegration of the overheated matrix. The entanglement of the polymer molecules facilitates the formation of intricate elongated viscous droplets that can be related to the complex morphologies observed in polymer films deposited by MAPLE. Analysis of the state of the irradiated target reveals a substantial increase of the polymer concentration and complex surface morphology generated in the new surface region by the ablation process. The ramifications of the computational predictions for interpretation of experimental data and the directions for future experimental exploration are discussed based on the physical picture of molecular ejection and transport in MAPLE emerging from the simulations. [ABSTRACT FROM AUTHOR]

We review the technique named heterodyne speckle velocimetry and present two applications for testing the method with a fluid moving under the conditions of Poiseuille flow. The fluid was seeded with small tracking particles (diameter 300 nm) and fluxed between the two parallel planes of a cell with constant or variable cross section. In the first case the velocity distribution was constant along the direction parallel to the planes and was in excellent agreement with the expected Poiseuille profile along the orthogonal direction. In the second case, where velocity gradients were present also along the planes, the technique was able to reconstruct both the orthogonal Poiseuille profile and the in-plane two dimensional mapping of the velocity vectors, with the possibility of measuring the fluid flux within an accuracy of 1%. [ABSTRACT FROM AUTHOR]

Ytterbium doped strontium fluoroapatite Yb3+:Sr5(PO4)3F (Yb:S-FAP) crystals have been used in high average power laser systems as gain medium. Growth induced defects associated with the crystal often affect their performance. In order to improve the crystal quality and its optical applications, it is imperative to understand the nature of these defects. In this study, we utilize micro-Raman spectroscopy to characterize two common growth-induced defects: bubble core and cloudiness. We find the bubble core consist of voids and microcrystals of Yb:S-FAP. These microcrystals have very different orientation from that of the pure crystal outside the bubble core. In contrast to a previous report, neither Sr3(PO4)2 nor Yb2O3 are observed in the bubble core regions. On the other hand, the cloudy regions are made up of the host materials blended with a structural deformation along with impurities which include CaCO3, YbPO4, SrHPO4, and Sr2P2O7 . The impurities are randomly distributed in the cloudy regions. This analysis is necessary for understanding and eliminating these growth defects in Yb:S-FAP crystals. [ABSTRACT FROM AUTHOR]

Unlike at any single TEM (n, m) mode laser, there is a subluminous phase velocity region located along the central region of a TEM (1,0)+TEM (0,1) mode laser. In conjunction with the high longitudinal electric field in this region, it forms another acceleration channel, which also locates inside the transverse ponderomotive potential trap. Through simulation, it is found that relativistic electrons injected into this acceleration channel can stand at the acceleration phase for a long time and be synchronously accelerated to high energies. Also, the accelerated electrons can be well confined inside the trap avoiding the transverse scattering problem. [ABSTRACT FROM AUTHOR]

We investigate the effects of the intense terahertz laser field and the spin-orbit coupling on single spin in a quantum dot. The laser field and the spin-orbit coupling can strongly affect the electron density of states and can excite a magnetic moment. The direction of the magnetic moment depends on the symmetries of the system, and its amplitude can be tuned by the strength and frequency of the laser field as well as the spin-orbit coupling. [ABSTRACT FROM AUTHOR]