Threshold Power Density Research Articles

The sensitization of laser-induced heat of Er3+ doped Y2O3 nanocrystals by codoped with Mn2+

Low threshold power density cw laser-induced heat has been observed in [Formula: see text] and [Formula: see text] codoped [Formula: see text] nanocrystals under excitation by a 980 nm IR laser. Codoped [Formula: see text] remarkably reduces the power density threshold of laser-induced heat compared with [Formula: see text] doped [Formula: see text] nanocrystals. When the excitation power density exceed [Formula: see text], [Formula: see text] codoped [Formula: see text] nanocrystals emit strong blackbody radiation. The thermal emission of [Formula: see text] should originate from the multiphonon relaxation between neighboring energy levels. One additional UC-PL enhancement is observed. The UC-PL intensity can be enhanced by an order of magnitude through high temperature calcination caused by light into heat.

On-axis radio frequency magnetron sputtering of stoichiometric BaTiO3 target: Localized re-sputtering and substrate etching during thin film growth

BaTiO3 thin films were prepared on Nb–SrTiO3 (100) and Pt/Al2O3/SiO2/Si substrates by radio frequency (rf) magnetron sputtering using a stoichiometric BaTiO3 ceramic target. This on-axis BaTiO3 thin film growth encountered severe re-sputtering and substrate etching, above a threshold power density (4W/cm2), due to negative ion formation at the target surface and subsequent acceleration towards the substrate. However, the film deposition with reduced or negligible re-sputtering was possible below 4W/cm2 of rf-power. The rf-voltage vs. power curve showed two distinct linear regimes with high and low slopes; the change in the slope coincides with substrate etching. Optical emission spectroscopy was employed to establish the link between the onset of excessive re-sputtering and could be used as a control tool. Since, negative oxygen ions (O−) are responsible for the re-sputtering, additional processing parameters like the oxygen partial pressure [Po=(O2∕O2+Ar) %] and total pressure were also adjusted to realize target stoichiometry on the grown films. Finally, through optimization steps, as revealed by the X-ray photoelectron spectroscopy, stoichiometric BaTiO3 films were obtained, at a pressure ≥2.7Pa, power density of 2W/cm2 and Po around 50%.

Spine growth and seismogenic faulting at Mt. Unzen, Japan

AbstractThe concluding episode of activity during the recent eruption of Mt. Unzen (October 1994 to February 1995) was characterized by incremental spine extrusion, accompanied by seismicity. Analysis of the seismic record reveals the occurrence of two dominant long‐period event families associated with a repeating, nondestructive source mechanism, which we attribute to magma failure and fault‐controlled ascent. We obtain constraints on the slip rate and distance of faulting events within these families. That analysis is complemented by an experimental thermomechanical investigation of fault friction in Mt. Unzen dacitic dome rock using a rotary‐shear apparatus at variable slip rates and normal stresses. A power density threshold is found at 0.3 MW m−2, above which frictional melt forms and controls the shear resistance to slip, inducing a deviation from Byerlee's frictional law. Homogenized experimentally generated pseudotachylytes have a similar final chemistry, thickness, and crystal content, facilitating the construction of a rheological model for particle suspensions. This is compared to the viscosity constrained from the experimental data, to assess the viscous control on fault dynamics. The onset of frictional melt formation during spine growth is constrained to depths below 300 m for an average slip event. This combination of experimental data, viscosity modeling, and seismic analysis offers a new description of material response during conduit plug flow and spine growth, showing that volcanic pseudotachylyte may commonly form and modify fault friction during faulting of dome rock. This model furthers our understanding of faulting and seismicity during lava dome formation and is applicable to other eruption modes.

GaSb-based mid infrared photonic crystal surface emitting lasers

We demonstrated for the first time above room temperature (RT) GaSb-based mid-infrared photonic crystal surface emitting lasers (PCSELs). The lasers, under optical pumping, emitted at λ(lasing)~2.3μm, had a temperature insensitive line width of 0.3nm, and a threshold power density (P(th)) ~0.3KW/cm2 at RT. Type-I InGaAsSb quantum wells were used as the active region, and the photonic crystal, a square lattice, was fabricated on the surface to provide optical feedback for laser operation and light coupling for surface emission. The PCSELs were operated at temperatures up to 350K with a small wavelength shift rate of 0.21 nm/K. The PCSELs with different air hole depth were studied. The effect of the etched depth on the laser performance was also investigated using numerical simulation based on the coupled-wave theory. Both the laser wavelength and the threshold power decrease as the depth of the PC becomes larger. The calculated results agree well with the experimental findings.

Optically pumped GaN vertical cavity surface emitting laser with high index-contrast nanoporous distributed Bragg reflector.

Laser operation of a GaN vertical cavity surface emitting laser (VCSEL) is demonstrated under optical pumping with a nanoporous distributed Bragg reflector (DBR). High reflectivity, approaching 100%, is obtained due to the high index-contrast of the nanoporous DBR. The VCSEL system exhibits low threshold power density due to the formation of high Q-factor cavity, which shows the potential of nanoporous medium for optical devices.

Ultraviolet lasing action in aligned ZnO nanowall

A vertically aligned ZnO nanowall arrays have been synthesized on a patterned ZnO buffer layer using an interference laser beam. The average height and thickness of the ZnO nanowall were 4 μm and 200 nm, respectively. Room-temperature ultraviolet (UV) lasing was obtained from single piece of ZnO nanowall detached from the substrate. The threshold power density for lasing was estimated to be about 80 kW/cm2. In addition, UV lasing from vertically aligned ZnO nanowall was also observed owing to the presence of holes. The lasing spectra had no regularity in the mode spacing and many different spatial modes, suggesting that the action was supported by the random lasing mechanism. The threshold power density for lasing was about 300 kW/cm2, which was much higher than that of the single piece of ZnO nanowall because of the leakage of the light into buffer layer.

Molecular-beam epitaxy of heterostructures of wide-gap II–VI compounds for low-threshold lasers with optical and electron pumping

The paper presents basic approaches in designing and growing by molecular beam epitaxy of (Zn,Mg)(S,Se)-based laser heterostructures with multiple CdSe quantum dot (QD) sheets or ZnCdSe quantum wells (QW). The method of calculation of compensating short-period ZnSSe/ZnSe superlattices (SLs) in both active and waveguide regions of laser heterostructures possessing the different waveguide thickness and different number of active regions is presented. The method allowing reduction of the density of nonequilibrium point defects in the active region of the II–VI laser structures has been proposed. It utilizes the migration enhanced epitaxy mode in growing the ZnSe QW confining the CdSe QD sheet. The threshold power density as low as P thr ∼ 0.8 kW/cm2 at T = 300 K has been demonstrated for laser heterostructure with single CdSe QD sheet and asymmetric graded-index waveguide with strain-compensating SLs.

Kinetics of Ultrasonic Drug Delivery from Targeted Micelles.

To minimize the adverse side effects of conventional chemotherapy, a targeted micellar drug carrier was investigated that retains hydrophobic drugs in its core and then releases the drug via ultrasonic activation. This paper compares the percent drug release from folated versus non-folated micelles by insonation at 70 kHz and different acoustic power densities. The encapsulated drug is Doxoru- bicin (Dox). A physical model of zero-order release with first-order re-encapsulation was used to fit the experimental kinetic data. Additionally, the acoustic activation power density and Gibbs free energy were introduced and calculated for folated and non-targeted micelles. The data suggests an important role of inertial cavitation in drug release and the presence of a power density threshold for inertial cavitation.

A smart load-sensing system using standardized mechano-luminescence measurement.

The mechano-luminescence (ML) of phosphors has stirred a great deal of interest for its potential application in inexpensive, non-destructive load sensors. However, the most serious drawback of ML phosphors has been responses that differ according to the loading conditions. This has led to a lack of standardization in realizing smart ML sensor applications. We improved the applicability of ML phosphors to that of a smart, standardized load sensor by detecting ML based on the UV excitation above the threshold power density during the entire loading process. The ML behavior under these conditions was completely different from that of conventional ML behavior with UV excitation turned off. The ML output was clearly represented as a simple linear function of the applied load under conditions that could be either static or dynamic. In addition, neither a ML loss angle nor hysteresis behavior was observed under these ML measurement conditions.

Lasing Characteristics of a Metal-Coated GaN Shallow Grating Structure at Room Temperature

This study demonstrated a metal-coated GaN laser from a defect cavity with shallow grating structure. With a structure optimization for surface plasmon waves, the shallow grating can support a high quality factor (Q) resonant mode, which leads to a lasing action in the metal-coated GaN cavity. The defect lasing mode was observed at a wavelength of approximately 365 nm at room temperature. The defect grating structure was 840 nm in period, 220 nm in width, and 200 nm in height, and the cavity length of the defect region was 300 nm. The quality factor was 480 and the threshold power density was 4.6 W/cm 2 . Lasing modes were also characterized with FEM simulation. Furthermore, the optical image and far-field angle of the metal-coated defect mode GaN grating structure were analyzed, and the lasing behavior of the defect grating structure was attributed to the defect mode.

Laser influence to biosystems

In this paper a continous (cw) lasers in visible region were applied in order to study the influence of quantum generator to certain plants. The aim of such projects is to analyse biostimulation processes of living organizms which are linked to defined laser power density thresholds (exposition doses). The results of irradiation of corn and wheat seeds using He-Ne laser in the cw regime of 632.8nm, 50mW are presented and compared to results for other laser types. The dry and wet plant seeds were irradiated in defined time intervals and the germination period plant was monitored by days. Morphological data (stalk thickness, height, cob lenght) for chosen plants were monitored. From the recorded data, for the whole vegetative period, we performed appropriate statistical data processing. One part of experiments were the measurements of coefficient of reflection in visible range. Correlation estimations were calculated and discussed for our results. Main conclusion was that there were a significant increments in plant's height and also a cob lenght elongation for corn.

Energy Transfer Modes in Pulsed Laser Seam Welding

A systematic parametric study has been made in pulsed Nd:YAG laser welding to understand the energy transfer modes. Four different energy transfer zones, namely conduction, transition, penetration, and keyhole, have been identified. The traditional classification of energy transfer modes based on the power density value of 106 W/cm2 is not strictly applicable as the transfer mode varies with pulse duration. The threshold power density to form keyhole is not constant, but the threshold energy density has been found to be invariant around 2.4 kJ/cm2. The pulse duration has been optimized to be of about 8 ms to achieve welds of higher aspect ratio.

Optically-pumped 285 nm edge stimulated emission from AlGaN-based LED structures grown by MOCVD on sapphire substrates

285 nm stimulated emission (SE) is demonstrated at room temperature from optically-pumped AlGaN multiple quantum wells (MQWs) deposited between a Si-doped AlGaN current spreading layer and a 25 nm Mg-doped p-type AlGaN clad layer. The epitaxial structures were grown by low pressure metalorganic chemical vapor deposition (MOCVD) on high-quality AlN/sapphire templates and a pulsed ArF excimer laser was used as the excitation source for lasing experiments. The threshold power density (Pth) was measured to be 970 kW/cm2 and the SE was strongly polarized in transverse electric (TE) mode. The minimum emission linewidth was ∼2 nm. The high value of Pth is primarily attributed to optical losses due to pump laser absorption in the top p-AlGaN. Our results show the viability of the MOCVD growth technology in conjunction with c-plane sapphire substrates to develop electrically-pumped deep-UV laser diodes.

A denoising method based on multiple-stage acousto-optic tunable filter in the green laser measurement system

The charge handling capacity of CCD camera is limited, so image distortion is caused by bright light generated by the hot workpiece. The green laser stripe captured by CCD camera is fuzzy in the green laser measurement system. In order to make the image of the stripe clearer, the filter system is designed based on multiple-stage acousto-optic tunable filter. In the filter system, a computational model is built by combining the visible radiation character of hot workpiece and the threshold power density of CCD saturation. The curve of the filtering rate of bright light can be obtained using the computational model. In order to realize the goal of intelligent filter, each stage of the acousto-optic tunable filter is adjusted according to the curve of the filtering rate. Finally, the image of green laser stripe on hot workpiece is clearer. The filter system designed in the paper is viable according to the experiment results. And the accuracy of the green laser measurement system is improved.

Plasma-assisted molecular beam epitaxy of Al(Ga)N layers and quantum well structures for optically pumped mid-UV lasers on c-Al2O3

This paper reports on novel approaches developed for plasma-assisted molecular beam epitaxy of Al-rich AlGaN epilayers and quantum well heterostructures on c-sapphire, which allowed us to fabricate low-threshold optically-pumped separate confinement heterostructure lasers emitting in the mid-UV spectral range (258–290 nm) with the threshold power density below 600 kW cm−2. The optimum buffer structure has been developed which provides lowering the near-surface threading dislocation density down to 1.5 × 108 and 3 × 109 cm−2 for screw and edge types, respectively, and improving the surface morphology (rms < 0.7 nm at the area of 3 × 3 μm−2). It comprises the high-temperature (780 °C) migration enhanced epitaxy growth of a (30–70) nm thick AlN nucleation layer on c-Al2O3, followed by a 2 μm thick AlN buffer grown under the metal-rich conditions in the Al-flux modulation mode and containing several (up to 6) ultra-thin (∼3 nm) GaN interlayers grown at N-rich conditions. Proper strain engineering in AlGaN single quantum well heterostructure grown atop of the AlN buffer layer enables one to preserve dominant TE polarization of both spontaneous and stimulated emission even at shortest obtained wavelength (258 nm). The threshold power density of stimulated emission as low as 150 kW cm−2 at 289 nm for a single quantum well laser structure has been demonstrated.

Propagation mechanism of polymer optical fiber fuse.

A fiber fuse phenomenon in polymer optical fibers (POFs) has recently been observed, and its unique properties such as slow propagation, low threshold power density, and the formation of a black oscillatory damage curve, have been reported. However, its characterization is still insufficient to well understand the mechanism and to avoid the destruction of POFs. Here, we present detailed experimental and theoretical analyses of the POF fuse propagation. First, we clarify that the bright spot is not a plasma but an optical discharge, the temperature of which is ~3600 K. We then elucidate the reasons for the oscillation of the damage curve along with the formation of newly-observed gas bubbles as well as for the low threshold power density. We also present the idea that the POF fuse can potentially be exploited to offer a long photoelectric interaction length.

The Change of the Surface Morphology of Organic Films by Laser Pulses

In this work by the means of atomic force microscopy (AFM) was investigated the morphology of thin films of organic molecules based on perylene and MPc, irradiated by laser radiation. It is known that the action of nanosecond laser pulse leads to a fragmentation of organic films of perylene, and MnPc derivatives. In this connection, the task was to study the surface topography of the films of perylene and MPc by the means of AFM after irradiation of them by pulsed laser radiation of nanosecond duration. The radiation source used laser with photon energy of 2.34 eV and a pulse width of 10 ns. The films were deposited on the surface of the GaAs (100) using a Knudsen cell in a vacuum 10-5 Pa. The study of the surface topography of the irradiated films was carried out under the forvacuum (10-1 Pa) using an atomic force microscope. It was discovered that modification of organic films, which manifests itself as a change in the morphology of the surface, takes place. In particular, for the first time found that the effect of laser radiation leads to an irreversible change in the topography of the surface, which is not observed with optical instruments. The comparative analysis of topography measurements of the films parts of the surface before and after laser irradiation shows, that from a certain threshold power density of 2.4 mJ/cm2, the removing of the layer from the surface of the organic film takes place, whereby the film thickness is reduced by approximately 20 nm.

Room-temperature mid-infrared “M”-type GaAsSb/InGaAs quantum well lasers on InP substrate

We have demonstrated experimentally the InP-based “M”-type GaAsSb/InGaAs quantum-well (QW) laser lasing at 2.41 μm at room temperature by optical pumping. The threshold power density per QW and extracted internal loss were about 234 W/cm2 and 20.5 cm−1, respectively. The temperature-dependent photoluminescence (PL) and lasing spectra revealed interesting characteristics for this type of lasers. Two distinct regions in the temperature dependent threshold behavior were observed and the transition temperature was found to coincide with the cross over point of the PL and lasing emission peaks. The current-voltage characteristic of “M”-type QW laser was superior to the inverse “W”-type one due to its thinner barrier for holes. Further improvement of the “M”-type QW structure could lead to a cost-effective mid-infrared light source.

Observation of polymer optical fiber fuse

Although high-transmission-capacity optical fibers are in demand, the problem of the fiber fuse phenomenon needs to be resolved to prevent the destruction of fibers. As polymer optical fibers become more prevalent, clarifying their fuse properties has become important. Here, we experimentally demonstrate a fuse propagation velocity of 21.9 mm/s, which is 1–2 orders of magnitude slower than that in standard silica fibers. The achieved threshold power density and proportionality constant between the propagation velocity and the power density are 1/180 of and 17 times the values for silica fibers, respectively. An oscillatory continuous curve instead of periodic voids is formed after the passage of the fuse. An easy fuse termination method is also presented.

Optically pumped AlGaN quantum‐well lasers at sub‐250 nm grown by MOCVD on AlN substrates

AbstractIn this study, we employed bulk (0001) AlN substrates for the metalorganic chemical vapor deposition growth of AlGaN multi‐quantum‐well heterostructures in an Aixtron 6 × 2″ close‐coupled showerhead reactor. The wafers were fabricated into cleaved bars with a cavity length of ∼1 mm. Two different layer structure designs are presented in this work. Both laser bars were optically pumped by a pulsed 193 nm ArF excimer laser at room temperature. The lasing wavelengths are 243.5 nm and 245.3 nm with threshold power density 427 kW/cm2 and 297 kW/cm2, respectively. Both laser bars showed transverse electric‐polarization‐dominated optical emission when operating above threshold. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)