Search

Thermal stress effects on continuous-wave second-harmonic generation in periodically poled LiTaO

The optical properties of pure liquid copper were investigated using density functional theory with the Quantum ESPRESSO package. The effects of structural changes were investigated by comparing the electron density of states and imaginary part of the dielectric function between the crystalline and liquid states with densities near the melting point. The results indicated that the effect of interband transitions remains in the structural changes near the melting point.

Bead-on-plate welding of pure copper sheet with a thickness of 200 μm were carried out with a 200 W blue diode laser. Output power was varied from 150 to 200 W. The spot diameter was set the 50,100 μm constant. The maximum laser intensity was 1×107 W/cm2. The threshold of the output power required for full penetration welding to 200 μm thick pure copper was obtained. Next, at each laser output power, the welding speed was changed to control the input energy, and the welding speed required for full penetration welding was determined. The maximum welding speed 35 mm/s was obtained with a laser intensity of 1×107 W/cm2. To investigate welding quality the bead appearance and crosssectional observation were performed.

Visual systems play a vital role in guiding the behaviour of animals. Understanding the visual information animals are able to acquire is therefore key to understanding their visually-mediated decision making. Compound eyes, the dominant eye type in arthropods, are inherently low-resolution structures. Their ability to resolve spatial detail depends on sampling resolution (interommatidial angle) and the quality of ommatidial optics. Current techniques for estimating interommatidial angles are difficult, and generally require in vivo measurements. Here, we present a new method for estimating interommatidial angles based on the detailed analysis of 3D Micro-CT images of fixed samples. Using custom-made MATLAB software we determine the optical axes of individual ommatidia and project these axes into the three-dimensional space around the animal. The combined viewing directions of all ommatidia, estimated from geometrical optics, allow us to estimate interommatidial angles and map the animal's sampling resolution across its entire visual field. The resulting topographic representations of visual acuity match very closely the previously published data obtained from both fiddler and grapsid crabs. However, the new method provides additional detail that was not previously detectable and reveals that fiddler crabs, rather than having a single horizontal visual streak as is common in flat world inhabitants, likely have two parallel streaks located just above and below the visual horizon. A key advantage of our approach is that it can be used on appropriately preserved specimens allowing the technique to be applied to animals such as deep-sea crustaceans that are inaccessible or unsuitable for in vivo approaches.

To reduce the minimum pulse energy required for laser ignition (MPE), thereby reducing the cost of the laser device and realizing multipoint ignition with the same laser energy, the ignition mechanism flammable gas using laser ablation was investigated by varying the distance between the target and the focusing lens, together with schlieren and direct color emission observation, which reveals the behavior of the ablative target material and initial flame kernel. It was found that the MPE in the ablative ignition was consistent with the ablation threshold energy of laser fluence on the target, except when the laser focal point was near the target surface. In this latter case, vortex flow induced by laser breakdown and successive collision of the initial flame kernel with the target was observed. The increase in MPE, in this case, was explained by heat loss in the initial flame kernel.

To understand the aggregation mechanism of serum protein dispersed in the solution of distilled water induced by a low temperature atmospheric pressure plasma, we compared the relationship between the amount of aggregation and experimental condition dependencies such as gas species, gas flow rate, and the distance between the plasma device and the solution. In this experiment, pure argon, helium, and/or a mixture of helium and argon gases were used. From statistical analyses of various experimental conditions, it was found that a monotonic supply of total absolute charge in one period was important for protein aggregation. When the coefficient of variation (standard deviations/averaged total absolute charges during one period) is small, the amount of protein aggregation becomes larger. Excess or scarcity of averaged total absolute charges does not appear to relate to protein aggregation phenomena; however, the coefficient of variation strongly relates to the mechanism of protein aggregation. Therefore, a possible serum protein aggregation process by low-temperature plasma treatment was proposed in which the serum protein, which had negative charges in the solution, was locally aggregated through the trigger of charge neutralization induced by a stable charge supply, and/or positively charged aggregated protein on the solution surface attracted serum protein in the solution to the surface.

Dynamics of albumin agglomeration and dissolution induced by a helium plasma jet using the Schlieren visualization technique were demonstrated. The Schlieren technique enabled us to visualize the plasma-induced agglomeration of albumin on the albumin solution and dissolution from the agglomerate. Through the dissolution, a high-density albumin region moved downward, forming downward flows. During the plasma treatment, the downward flows were observed with a flow velocity of around 0.2 mm/s, while the flow velocity was 0.7 mm/s–0.9 mm/s without the plasma treatment. It was found that the plasma treatment decelerates the downward flows.

It is well known that pulsed laser can easily generate breakdown plasmas when the beam is focused. The laser-based gas breakdown phenomenon is caused by multiphoton ionization (MPI) and cascade ionization (CI) in a high-intensity electric field near the focal region. However, the phenomenon is complicated because it is affected by the purity of the gas and the focusing quality of the laser and optical systems. For a comprehensive understanding of the laser-induced breakdown phenomenon, the influence of an electric field on the threshold intensity of the laser-induced breakdown was investigated. The applied field disperses electrons generated in the focal area by the intense laser field, thereby increasing the laser-induced breakdown threshold intensity. The observed phenomenon is explained by a simple electron balance model.

Nonlinear propagation equations and thermal conduction equation are numerically solved to investigate the laser-induced thermal effects on the laser propagation. The thermal performance of modules is evaluated by comparing the numerical and experimental results.

Striation phenomena in a plasma flare produced by an atmospheric pressure plasma jet had been observed. In this study, we measured the striation structure in a neon plasma flare by optical emission spectroscopy. As a result, it shows that the emission from the plasma flare is synchronized with the plasma current. We discuss the temperature estimated from the Boltzmann plot using the two emission lines of Ne I at 692.9 and 724.5 nm whose upper levels are Ne [3p (2p6)] and Ne [3p (2p10)], respectively. This temperature has peaks with a spatial interval of about 0.5 mm, and the peak positions correspond to the spatial variation of the emission intensity. We also discuss the mechanism of the striation phenomena with the estimated temperature, and collisional relaxation between excited and neutral atoms.

We investigate light-induced heating by nonlinear absorption in LiNbO 3 -type crystals under continuous-wave (CW) laser irradiation. The heat source is one-photon absorption by long-lived excited states created by two-photon absorption. The accumulated effect of excited states, which act as light absorbers, is significant for a CW laser. Light-induced heating causes catastrophic CW laser breakdown in LiNbO 3 -type crystals in which the band-gap energy is less than twice the photon energy and in which long-lived excited states, like polarons, can exist. For LiNbO 3 , the threshold intensity for catastrophic breakdown is estimated to be in the order of MW/cm 2 .

LiNbO 3 and LiTaO 3 are frequently used in second harmonic conversion of continuous-wave light from the infrared to the visible regions. Optical damage of LiNbO 3 -type crystals is a crucial issue in the high-average-power laser systems. The optical damage by a light-induced heating is investigated. We have proposed a light-induced heating by the accumulated long-lived states like polarons or self-trapped excitons. In addition to the long-lived states, point defects as color centers are created by a radiation. A new model includes these states. As a results, it is shown that a creating rate of the color center becomes a important parameter on the determination of critical power.

The MU radar was built in 1984 as the first large-scale MST (Mesosphere, Stratosphere, and Troposphere) / IS (Incoherent Scatter) radar with a two-dimensional active phased array antenna system by Kyoto University with the collaboration of Mitsubishi Electric Corporation. IEEE Milestone was dedicated to the MU radar in 2015. This article describes the history of development of the MU radar, its historical and technological significances, influence to the subsequent system, continuous research and development towards the future, and the milestone dedication.

Beam propagation equations, rate equations of the excited states, and thermal conduction equation are numerically solved to investigate the laser-induced thermal effects on the laser propagation in nonlinear optical materials. The beam output power and profile of second harmonic waves are evaluated in quasi-phase-matched second-harmonic generation.

The authors investigate the formation of color centers (CC) using a femtosecond laser, which causes the darkening of TiO2 film surfaces and reduction of their electrical resistance under visible-light illumination. They propose a kinetic model to explain the CC formation from femtosecond laser irradiation. The kinetic model treats three components, which are conduction-band electrons, self-trapped excitons or polarons, and color centers. The CC formation coefficient is assumed to be expressed by the Arrhenius equation. The laser fluence dependence on the decrease of the electrical resistance is well explained by the kinetic model.The authors investigate the formation of color centers (CC) using a femtosecond laser, which causes the darkening of TiO2 film surfaces and reduction of their electrical resistance under visible-light illumination. They propose a kinetic model to explain the CC formation from femtosecond laser irradiation. The kinetic model treats three components, which are conduction-band electrons, self-trapped excitons or polarons, and color centers. The CC formation coefficient is assumed to be expressed by the Arrhenius equation. The laser fluence dependence on the decrease of the electrical resistance is well explained by the kinetic model.

In a low-temperature atmospheric pressure plasma jet using helium gas, emission-propagation phenomena, such as streamers and striations were measured using a high-speed intensified charge-coupled device camera. A particular focus was placed on the study of the dependence of the phenomena on the distance between the nozzle of the plasma device and a target plate. When the distance decreased, a transition from the positive streamer to a spatially continuous emission resulted. A further distance reduction resulted in a new propagation mode in which the positive and negative streamers appeared alternately with different current waveforms over two cycles of applied voltage. This phenomenon may be related to residual charges of the preceding cycle when streamer propagation begins. Striation structures were observed in the tail of the positive streamer head and in the successive spatially continuous-emission region. These structures can be measured only within a shorter period than one voltage cycle.