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Transient analysis of TMSR-SF0, a molten salt thermal-hydraulic experimental system of molten salt reactor, is investigated using RELAP5/MOD4.0 code. Simulation model consists of two coolant circui...

International audience; Barium germano-gallate glasses are attractive glass hosts for photonic applications in the mid-infrared region up to 6 µm. In this work, we investigate the photosensitivity of such glasses under femtosecond laser, with an emphasize on the formation of refractive index changes. Six glasses with varying compositions (including addition of K, Na, Y, and La) were studied. We observed several transformation regimes in the pulse energyrepetition rate landscape: Type I (isotropic refractive index change) and a spatial broadening regime with a phase shift Δφ > 2π rad at 550 nm. This translates into refractive index changes Δn > 10-2 and is comparable to values obtained in most chalcogenide glasses. The effect of glass composition on Δφ appears correlated to the number of non-bridging oxygen presented in the glass and is brought to evidence by monitoring the Cations/GaO3/2 ratio. This provides a way to design a range of germano-gallate glasses suitable to imprinting high refractive index contrast.

The period ( $$\wedge $$ ) of nanograting in sapphire varied from 320 to 398 nm with increasing the laser fluence, which is similar to the change trend of period of the near-subwavelength ripples previously observed on the material surface ( $$0.4

In this paper, a non-reciprocal writing effect is proposed on the silicon by a femtosecond laser with tailor of temporal chirp. The induced structures which belong to the written lines with two opposite scans show obvious difference in the ablated regions. The grating compressor of a laser system is controlled to change the temporal chirp so as to flip the direction of the pulse front tilt, which results in 2 types of induced structures exchanging between two opposite scans. We assume, during the laser scans along the pulse front tilt, there may have a snowplow effect on the excited free electrons, leading to an asymmetrical electrical field distribution around the focal area. This process could further boost an ionization efficiency in the next pulse irradiation as well as make the induced structures grow faster. Current result possibly offers a new route to control the density of free electrons excited when femtosecond laser interacts with silicon wafer.

In this paper, without circular scanning of laser focus, annular ablation region was reported to be induced by femtosecond laser static irradiation of ZnO surface immersed in oil, and these annular ablation regions were all covered by laser-induced periodic surface structures (LIPSS) upon irradiation of either linearly-polarized light or circularly-polarized light. This novel phenomenon was found to be related to the bubbles in-situ generated and attached at laser focus inside oil during irradiation process. With increasing irradiation pulse number, the shallow annular ablation region transited to a deep annular groove, and the unablated region surrounded by the annular ablation region was finally ablated to a LIPSS-covered spherical-crown-shaped bump. Finite-difference-time-domain (FDTD) simulation shows that the bubble at the laser focus may act as a beam shaping device to partly transform a Gaussian-intensity-profile beam to an annular-intensity-profile beam and the increased bubble size induces a decrease of the radius of the annular beam, which can roughly explain the experimentally observed morphology transformation phenomenon. Further analysis of light field transformation based on matrix optics further demonstrates that the annular intensity profile is originated from hollow beams generated after incident light passing through the bubble-type radial polarizer.

Self-organized fiber nanogratings (FNGs) induced by femtosecond pulse laser direct writing are demonstrated in optical silica fibers for the first time. Ge doping in the silica fiber core plays an important role in the formation of nanogratings. Negative changes in refractive index with sawtooth fluctuation and strong birefringence of FNGs were investigated by an optical fiber refractive index analyzer and a crossed polarization microscope, respectively. The polarization features of FNGs were characterized by the Mueller matrix analytical method and in-line polarizers based on FNGs were fabricated. FNGs are promising integrated components because of their prominent properties of polarization and salient compatibility with most fiber-optic systems.

A Polarizing beam splitter (PBS) based on pendulum effect in volume holographic grating (VBG) is proposed. Using the modified coupled-wave equations of Kogelnik, expressions of transmitted and diffracted intensities are derived when a polarized femtosecond pulse incidents on a VHG. Simulation results show that the intensities of the transmitted and diffracted pulses are periodic oscillation and exchange along the propagation depth, and this property is pendulum effect. Period of the pendulum effect is sensitive to polarization state of the incident pulse. By suitably using this property, it is possible to realize a PBS where the incident TE polarization outputs in transmitted direction of the VHG and TM component outputs in diffracted direction.

Based on the advanced high temperature reactor (AHTR), which is an advanced concept combining attractive attributes by adopting liquid salt and coated particle fuel, more and more reactor designs that use fuel pebbles are developed. The bed structure in liquid salt cooled pebble bed reactor is important for reactor design. To investigate the bed structure in the reactor, the Pebble Recirculation Experiment Device (PRED) and the Pebble Bed Dense Experiment facility (PBDE), which were mock-ups for TMSR-SF (Thorium-based Molten Salt Reactor with Solid Fuel) were developed. Packing experiments were performed under different conditions. The results show that, the packing factor of TMSR-SF is supposed to be about 0.574, which is much less than 0.60 in gaseous environment; the loading rate of pebbles and the velocity of inlet flow has great effect on the bottom shape of the pebble bed; the structure of the pebble bed remains stable during normal operation and changes under accident conditions; loss of the coolant in the core may cause rearrangement of pebble bed; increase in packing factor that caused by strong vibration may induce reactivity in reactor core. The pebble bed behavior under different conditions that investigated in this paper provides the basis phenomenological analysis for reactor design. Moreover, the running of PRED demonstrates the feasibility of the methods for pebble loading and defueling in liquid salted cooled pebble bed reactors.

In this paper, a femtosecond laser line-scanning irradiation was used to induce the periodic surface microstructure on HgCdTe crystal. Low spatial frequency laser induced periodic surface structures of 650–770 nm and high spatial frequency laser induced periodic surface structures of 152–246 nm were respectively found with different scanning speeds. The evolution process from low spatial frequency laser induced periodic surface structures to high spatial frequency laser induced periodic surface structures is characterized by scanning electron microscope. Their spatial periods deduced by using a two-dimensional Fourier transformation partly agree with the predictions of the Sipe-Drude theory. Confocal micro-Raman spectral show that the atomic arrangement of induced low spatial frequency laser-induced structures are basically consistent with the crystal in the central area of laser-scanning line, however a new peak at 164 cm −1 for the CdTe-like mode becomes evident due to the Hg vaporization when strong laser ablation happens. The obtained surface periodic ripples may have applications in fabricating advanced infrared detector.

We report the space-selective precipitation of Cr3+-doped ZnAl2O4 crystals in aluminosilicate glass by femtosecond (fs) laser irradiation. The precipitation of ZnAl2O4 crystal was confirmed by x-ray diffraction (XRD), Raman spectra and transmission electron microscopy (TEM), and the incorporation of octahedron Cr3+ in ZnAl2O4 crystal was testified by electron paramagnetic resonance (EPR) and luminescence spectra. The laser irradiated area shows greatly enhanced luminescence due to Cr3+ transitions. This method is promising for waveguide laser and three dimensional (3D) optical storage.

Small modular thorium-based graphite-moderated molten salt reactors (smTMSRs), which combine the advantages of small modular reactors and molten salt reactors, are regarded as a wise development path to speed deployment time. In a smTMSR, low enriched uranium and thorium fuels are used in once-through mode, which makes a marked difference in their neutronic properties compared with the case when a conventional molten salt breeder reactor is used. This study investigated the temperature reactivity coefficient (TRC) in a smTMSR, which is mainly affected by the molten salt volume fraction (VF) and the heavy nuclei concentration in the fuel salt (HN). The four-factor formula method and the reaction rate method were used to indicate the reasons for the TRC change, including the fuel density effect, the fuel Doppler effect, and the graphite thermal scattering effect. The results indicate that only the fuel density has a positive effect on the TRC in the undermoderated region. Thermal scattering from both salt and graphite has a significant negative influence on the TRC in the overmoderated region. The maximal effective multiplication factor, which shows the highest fuel utilization, is located at 10% VF and 12 mol% HN and is still located in the negative TRC region. In addition, on increasing the heavy nuclei amount from 2 mol% HN to 12 mol% HN (VF = 10%), the total TRC undergoes an obvious change from − 11 to − 3 pcm/K, which implies that the change in the HN caused by the fuel feed online should be small to avoid potential trouble in the reactivity control scheme.

The solid fuel thorium molten salt reactor (TMSR-SF1) is a 10-MWth fluoride-cooled pebble bed reactor. As a new reactor concept, one of the major limiting factors to reactor lifetime is radiation-induced material damage. The fast neutron flux (E > 0.1 MeV) can be used to assess possible radiation damage. Hence, a method for calculating high-resolution fast neutron flux distribution of the full-scale TMSR-SF1 reactor is required. In this study, a two-step subsection approach based on MCNP5 involving a global variance reduction method, referred to as forward-weighted consistent adjoint-driven importance sampling, was implemented to provide fast neutron flux distribution throughout the TMSR-SF1 facility. In addition, instead of using the general source specification cards, the user-provided SOURCE subroutine in MCNP5 source code was employed to implement a source biasing technique specialized for TMSR-SF1. In contrast to the one-step analog approach, the two-step subsection approach eliminates zero-scored mesh tally cells and obtains tally results with extremely uniform and low relative uncertainties. Furthermore, the maximum fast neutron fluxes of the main components in TMSR-SF1 are provided, which can be used for radiation damage assessment of the structural materials.

In this paper, based on Kogelnik’s coupled-wave theory and transfer matrix method, we discussed the temporal diffraction properties of multilayer volume holographic gratings under femtosecond pulses readout. Results show that grating layers generate diffracted pulses, with each grating layer diffracting out one sub-pulse, whose waveform is determined only by parameters of the corresponding grating layer. The buffer layers placed on the right side of the grating layer translates the former diffracted sub-pulse along the negative time axis, and the translation is proportional to the total thickness of buffer layers placed on the right side of it, whereas the buffer layers placed on the left side of the grating layer has no effect on the diffracted sub-pulse. We also give explanations of these phenomena based on the diffraction efficiency of the grating layers and the phase shift of the buffer layers.

In this paper, we report the birefringence effect of surface self-assembly periodic nanostructures induced on 6H-SiC by femtosecond laser irradiation. Birefringence characteristic (e.g. cross-polarized image), measured by cross polarized microscopy, was found to be controlled by both single pulse energy and scanning velocity. Comparing birefringence measurement results of nanostructures and morphology characterization by Scanning electron microscopy, it is shown that ∼200 nm-period deep-subwavelength periodic ripples (DSWR) plays a dominating role in the birefringence effect. Raman spectra show that the change of retardance with pulse energy and scanning velocity is most possibly caused by the thickness variation of DSWR. Finally, a light attenuator based on a single layer of DSWR structure on 6H-SiC surface was constructed and tested by light source of 800 nm to have a tunable attenuating ratio of 69–100%.

BACKGROUND: To identify the main determinants of intraocular lens (IOL) tilt and decentration after cataract surgery using a novel anterior segment optical coherence tomography (AS-OCT) method. METHODS: Fifty-six patients who underwent phacoemulsification with IOL implantation in one eye were continuously enrolled in this cohort study. Axial length (AL) was measured with IOL Master 700. The tilt and decentration of patients’ preoperative crystalline lenses and postoperative IOLs, as well as crystalline lens thickness (LT), were measured using AS-OCT before surgery and 1 week after surgery. RESULTS: The mean tilt and decentration of the patients’ preoperative crystalline lenses were 4.90°±1.81° and 0.21±0.02 mm, and the mean tilt and decentration of IOLs were 4.75°±1.66° and 0.21±0.02 mm, respectively. There were no significant differences in magnitude, direction of tilt, or decentration between crystalline lenses and IOLs. The strongest determinant of IOL tilt was preoperative crystalline lens tilt (R(2)=0.512, P

Producing medical radionuclide 99Mo in molten salt reactors is a very attractive choice to solve its global shortage. In this study, we evaluated the yield of 99Mo in the small modular thorium based molten salt reactor (SM-MSR). Firstly, the fuel burn-up analysis of SM-MSR was carried out by an in-house developed code (MOBAT), which takes the unique characteristics of molten salt reactor into account, and the variation of fission yield of 99Mo with burn-up time was obtained. The minimum value of the fission yield of 99Mo is 1.13 × 10−3 (6-day TBq/MW/s) at approximately 600 equivalent full power day. Then, based on the behavior of noble metals in the fuel salt and the experimental results of MSRE gas sample composition measurements, 99Mo migration probability from the primary loop to the off-gas module with the burn-up time was calculated, and the equilibrium value of the migration probability is obtained as 18.4%. When the load factor of SM-MSR is 0.75 (300 MWth), the annual 99Mo amount of migration to the off-gas module would be 1.96 × 106 (6-day TBq) under the most conservative calculation. Finally, a filter system was added in the off gas module, and 99Mo would be extracted from the off gas. As long as the utilization percentage of 99Mo in the off gas module can reach 0.94%, the global demand under current data could be met, which implies a huge additional economic value for the SM-MSR.

A molten salt reactor (MSR) is one of the six advanced reactor concepts selected by the generation IV international forum because of its advantages of inherent safety, and the promising capabilities of Th-U breeding and transuranics transmutation. A dynamics model for the channel-type MSR is developed in this work based on a three-dimensional thermal–hydraulic model (3DTH) and a point reactor model. The 3DTH couples a three-dimensional heat conduction model and a one-dimensional single-phase flow model that can accurately consider the heat conduction between different assemblies. The 3DTH is validated by the RELAP5 code in terms of the temperature and mass flow distribution calculation. A point reactor model considering the drift of delayed neutron precursors is adopted in the dynamics model. To verify the dynamics model, three experiments from the molten salt reactor experiment are simulated. The agreement of the experimental data and simulation results was excellent. With the aid of this model, the unprotected step reactivity addition and unprotected loss of flow of the 2 MWt experimental MSR are modeled, and the reactor power and temperature evolution are analyzed.

The formation process of self-organized nanograting in fused silica glass with femtosecond laser (1 kHz, 120 fs, 800 nm) double-pulse sequences irradiation was investigated by combined control of polarization state of individual pulse, interpulse temporal delay (from -330 to 350 ps) and energy ratio in each sequence. It is found that the as-formed nanograting induced by the first pulse could be erased by the second pulse in repeated sequences irradiation, and moreover a new one would be rewritten in situ if the two pulses in a sequence were temporally well separated. The rewritten nanograting shows a new orientation perpendicular to the polarization direction of the second pulse. Additionally, the erasing and rewriting of nanograting can become more efficient as long as the second pulse energy increases to be higher than the first one regardless of the interpulse delay. Our result shows that the energy ratio in double-pulse sequences plays a decisive role on the erasing and rewriting, because formation of nanograting mainly depends on the pulse energy deposition process. This study provides a useful reference for application of high-density data storage and amendment of nanograting.

Thorium blanket fuel is investigated in Pebble Bed Fluoride salt-cooled High temperature Reactor (PB-FHR) with 19.9 at% U-235 seed fuel to improve the utilization of uranium fuel. Uranium fuel utilization is optimized with lots of parameters, such as graphite-to-thorium atom ratio (C/Th), graphite-to-uranium atom ratio (C/U), discharge burnup of thorium and uranium, and the dimension of the seed/blanket region. It is found that the equivalent discharge burnup, defined as total released energy over the mass of uranium, could be improved to around 265 MWd/kgU, which is 20% higher than discharge burnup using pure uranium fuel. In equilibrium state, the temperature reactivity coefficients of fuel and coolant are both negative. Other properties such as radial power peak factor, life of reflector, in-pile residence time of thorium pebble and radioactive waste are analyzed. Finally, baseline design parameters are recommended for further thermal-hydraulic analysis and TRISO fuel performance. (C) 2015 Elsevier Ltd. All rights reserved.

Integrating multiple functionalities into individual nanoscale hybrids with strong nonlinear optical (NLO) response to ultrafast laser pulses is of tremendous importance. Here, a series of ultra-small and well-dispersed nanoparticles (NPs) supported on the undoped and doped reduced graphene oxide (rGO) are obtained via a general and versatile photochemical technique. Extremely fast heterogeneous nucleation rate and cooling rate having negligible thermal effect induced by a femtosecond laser are proposed to play a determined role in the formation of ultra-small NPs. No surfactants, reduction reagents, or toxic materials are needed. The prepared rGO hybrids exhibit significantly enhanced ultrafast NLO response with very low optical limiting threshold, which originates from the fast and efficient electron and/or energy transfer from ultra-small NPs to rGO. This study may represent an important universal step toward the generation of graphene hybrid nanostructures and even complex 3D functional systems consisting of multiple functional ultra-small subunits with new horizons for numerous applications, especially in ultrafast nonlinear optics.

Self-organized nanogratings induced by femtosecond laser pulse direct writing are demonstrated in optical fibers. The results reveal strong anisotropic birefringence of nanogratings. Depolarization and retardation of nanograting are investigated through analyzing Mueller matrices method.

With the progress of China’s nuclear power industry, nuclear power plant valve products is in a period of rapid development, high performance, low cost, short cycle of design requirements for nuclear power valve is proposed, so there is an urgent need for advanced digital design method and integrated design platform to provide technical support. Especially in the background of the nuclear power plant leakage in Japan, it is more practical to improve the design capability and product performance of the nuclear power valve. The finite element numerical analysis is a common and effective method for the development of nuclear power valves. Nuclear power valve has high safety, complexity of valve chamber and nonlinearity of seal joint surface. Therefore, it is urgent to establish accurate prediction models for earthquake prediction and seal failure to meet engineering accuracy and calculation conditions. In this paper, a general method of finite element modeling for nuclear power valve assembly and key components is presented, aiming at revealing the characteristics and rules of finite element modeling of nuclear power valves, and putting forward aprecision control strategy for finite element models for nuclear power valve characteristics analysis.With the progress of China’s nuclear power industry, nuclear power plant valve products is in a period of rapid development, high performance, low cost, short cycle of design requirements for nuclear power valve is proposed, so there is an urgent need for advanced digital design method and integrated design platform to provide technical support. Especially in the background of the nuclear power plant leakage in Japan, it is more practical to improve the design capability and product performance of the nuclear power valve. The finite element numerical analysis is a common and effective method for the development of nuclear power valves. Nuclear power valve has high safety, complexity of valve chamber and nonlinearity of seal joint surface. Therefore, it is urgent to establish accurate prediction models for earthquake prediction and seal failure to meet engineering accuracy and calculation conditions. In this paper, a general method of finite element modeling for nuclear power valve assembly and key compone...

The 10 MWth solid-fueled thorium molten salt reactor (TMSR-SF1) is a FLiBe salt-cooled pebble bed reactor to be deployed in 5–10 years, designed by the TMSR group. Due to a large amount of beryllium in the core, the photoneutrons are produced via (γ, n) reactions. Some of them are generated a long time after the fission event and therefore are considered as delayed neutrons. In this paper, we redefine the effective delayed neutrons into two fractions: the delayed fission neutron fraction and the delayed photoneutron fraction. With some reasonable assumptions, the inner product method and the k-ratio method are adopted for studying the effective delayed photoneutron fraction. In the k-ratio method, the Monte Carlo code MCNP6 is used to evaluate the effective photoneutron fraction as the ratio between the multiplication factors with and without contribution of the delayed neutrons and photoneutrons. In the inner product method, with the Monte Carlo and deterministic codes together, we use the adjoint neutron flux as a weighting function for the neutrons and photoneutrons generated in the core. Results of the two methods agree well with each other, but the k-ratio method requires much more computing time for the same precision.

Directly controlling the growth process of clusters and nanoparticles is an effective way to tune their specific properties, which has been considered as a significant issue lying at the heart of nanotechnology. For technological applications, great strides have been made in the assembly of clusters and nanoparticles. However, controllable synthesis of clusters and nanoparticles inside a bulk solid-state medium remains a tremendous challenge, which is important for integrated devices. Here we report a strategy for space-selective control of elemental tellurium (Te) precipitation as clusters or nanoparticles in glass by femtosecond (fs) laser irradiation. After irradiation by a 1 kHz fs laser at 800 nm, Te2 clusters, which emit near-infrared (NIR) light, are observed at the focal point of the laser beam inside the glass sample. By shifting the repetition rate to 250 kHz, a temperature field forms around the focal area that facilitates transformation of Te clusters into nanoparticles. Raman mapping shows that the clusters are localized in the center of the laser-induced microstructure, while the nanoparticles exhibit an annular distribution. The possible mechanisms of generation and distribution of different species are discussed. We have also demonstrated optical data storage and embedded micro-grating by using this technique.

In this paper, we report a non-reciprocal writing process for inducing asymmetric microstructure using a femtosecond laser with tilted pulse fronts in fused silica. The shape of the induced microstructure at the focus closely depends on the laser scan direction. An elongated end is observed as a kind of structural difference between the written lines with two reverse scans along +x and –x, which further leads to a birefringence intensity difference. We also find a bifurcation in the head region of the induced microstructure between the written lines along x and y. That process results from the focal intensity distortion caused by the pulse front tilt by comparing the simulated intensity distribution with the experimental results. The current results demonstrate that the pulse front tilt not only affects the free electron excitation at the focus but also further distorts the shape of the induced microstructure during a high-energy femtosecond laser irradiation. These results offer a route to fabricate optical elements by changing the spatiotemporal characteristics of ultrashort pulses.

In this paper, the effect of the irradiation pulse number on the morphology of the near-wavelength ripples perpendicular to the laser polarization was investigated. The results show that the ripple spacing first fast decreases with the increased irradiation pulse number and finally reaches a stable value. The change of ripple morphology with different incidence angles was also studied. The ripples are found keeping always perpendicular to the laser polarization and have respective ripple spacings of 650, 480 and 390 nm upon the laser irradiation with pulse number N = 16 and fluence of 1.33 J/cm2 at the incidence angles of 0°, 30° and 60°. The change of the ripple spacing with different excitation levels was also studied. The theory for interference of the incident wave with the surface plasmon polariton wave (SPP) and the Sipe–Drude theory are both employed to analyze the experimental results. Detailed calculations show that the Sipe–Drude theory works better for explaining the main features of the ripples on silicon wafer surface. We suggest that the interaction of the incident wave with some form of surface electromagnetic wave except SPP is most possibly responsible for the ripple formation on silicon surface.

We report on the formation of Bi2ZnB2O7 crystal structures with designated patterns in ZnO–Bi2O3–B2O3 glass by femtosecond laser direct writing. The crystallization mechanism in glass is investigated by crystallization kinetics analysis and simulation of the three-dimensional temperature field distribution. The crystallized regions show larger third-order optical nonlinearity than the unirradiated region in glass by Z-scan technique. This finding is of great potential in application of nonlinear optical integrated devices and development of new nonlinear materials.

Based on the coupled wave theory of Kogelnik and Fourier optics, the time-domain diffraction characteristics of tramsmitted multilayer volume holographic grating (MVHG) under an ultrashort pulse readout are investigated. It is shown that the temporal diffraction characteristics depend not only on the numbers of the grating layers, but also on the thicknesses of the grating layers and buffer layers, grating period and the refractive index modulation. Furthermore, using group velocity dispersion we give explanation on the time-delay of diffraction pulse with respect to the center of the readout pulse. Results of our discussion may find applications in optical communications, pulse shaping and processing.

Planar structure consisting of line array with embedded nanogratings, which can function as a polarization-dependent light attenuator, was fabricated inside fused silica using a tightly focused 1 kHz fs laser scanning. Microstructure in the laser-modified region was characterized with Raman spectroscopy and scanning electron microscopy. Wavelength response and thermal stability of the structure were studied. The polarization-dependent attenuation efficiency decreases with increasing wavelength. The structure can resist high temperature up to 900 °C, and proper heat treatment can improve its attenuation efficiency. The results provide some new insights into the characteristics of fs laser-induced self-organized nanogratings and an alternative way to produce a polarization-dependent light attenuator that can be used in harsh environment.

We investigate the morphology change of Au film on sapphire substrate by irradiating with a 1 kHz femtosecond pulse laser. Under observation of a scanning electron microscope, a textured nanostructure was formed in the exposed area on Au film due to laser ablation and subsequent stress relaxation. This process was strongly determined by the laser intensity profile and the dynamics of molten liquid. With the increasing of laser pulses number, the Au film was broken down and then a few polarization-dependent nanoripples arranged in the same direction appeared on the sapphire surface, which may result from a spatial modulation of energy due to the interference between the incident light and the excited surface plasmon polaritons. In addition, we used an energy dispersive spectrometer to analyze the chemical composition of nanoripples on the surface and in the ablated crater, respectively. The changes of O and Al elements implied that a complicated chemical transformation participated in the nanoripples formation process. We believe that present results are very useful for the analysis of the interaction between femtosecond laser and solids, especially the film material.

We report the observation of an anomalous polarization dependent process in an isotropic glass induced by long time stationary irradiation of a high repetition rate near-infrared femtosecond laser. Two distinctive types of polarization dependent microstructures were induced at different irradiation stages. At early stage (a few seconds), a dumbbell-shaped structure elongated perpendicularly to the laser polarization formed at the top of the modified region, which was later erased by further irradiation. At later stage (above 30 s), bubbles filled with Osub2/subformed by the irradiation, which were distributed along the laser polarization at a distance far beyond the radius of the laser beam. Based on a simple modeling of light reflection on boundaries, a thermal accumulation process was proposed to explain the formation and evolution of the dumbbell-shaped microstructure. The possible factors responsible for polarization dependent distribution of bubbles are discussed, which needs further systematic investigations. The results may be helpful in the development of femtosecond laser microprocessing for various applications.

In this paper, a line-scanning irradiation of 6H-SiC single crystal by a femtosecond laser focus was implemented for laser fluence of 2.2 J/cm2 at different scanning velocities from 1000 down to 100 μm/s. The morphology of the obtained ablation lines characterized by a scanning electron microscope shows that there progressively appear three-level ripples with average period of about 222, 600 nm and that between them. Possible formation mechanisms for these three-level ripples were analyzed by numerical simulation employing finite-difference time-domain method (FDTD). Analysis shows that as Obara proposed, the interference of incident wave with the wave scattered by scratches possibly left by fine polishing is the most possible reason for inducing 222 nm-period and 600 nm-period ripples. Main discussions were focused on the orientation characteristics of ripples. Both isolated scratches and crossed scratches with different orientation angles were considered for evaluating their role in field redistribution. The detailed simulations indicate that for scratches oriented 90° with respect to laser polarization generate a pair of strongest ripple-like optical field enhancement compared to scratches with other orientation angles. It is these advantages of 90°-oriented scratch in field enhancement that make the final ripple perpendicular to laser polarization most competitive and finally left on the surface. This complements the physical picture of Obara et al. in terms of their negligence in addressing the orientation characteristics of ripples. Further FDTD simulation of the interaction of incident wave with as-formed 600 nm-period ripples shows that the optical field enhancement located between any two adjacent 600 nm-period ripples is the driving force for splitting 600 nm-period ripples into the third ripples with period between 222 and 600 nm.

In this paper, a time-delay-adjustable double-pulse train with 800-nm wavelength, 200-fs pulse duration and a repetition rate of 1 kHz, produced by a collinear two-beam optical system like a Mach–Zehnder interferometer, was employed for irradiation of 6H-SiC crystal. The dependence of the induced structures on time delay of double-pulse train for parallel-polarization configuration was studied. The results show that as the time delay of collinear parallel-polarization dual-pulse train increased, the induced near-subwavelength ripples (NSWRs) turn from irregular rippled pattern to regularly periodic pattern and have their grooves much deepened. The characteristics timescale for this transition is about 6.24 ps. Besides, the areas of NSWR were found to decay exponentially for time delay from 0 to 1.24 ps and then slowly increase for time delay from 1.24 to 14.24 ps. Analysis shows that multiphoton ionization effect, grating-assisted surface plasmon coupling effect, and timely intervene of second pulse in a certain physical stage experienced by 6H-SiC excited upon first pulse irradiation may contribute to the transition of morphology details.

In this paper, a modified spectral holography structure is demonstrated. Combining the direct space-to-time pulse shaping theory with the modified structure, we can convert a spatial domain x–y image into a y–t image, where one spatial dimension is now transformed into the time domain. Thus we realize the space-to-time or parallel-to-serial conversion. As an example, we generate the temporal equivalent of letter “A”, where each pixel of the image is now represented by a short optical pulse. As a possible application of our scheme, we demonstrate the generation of trains of a femtosecond pulse sequence by our structure. The results of the paper can be applied in ultrashort pulse shaping, ultrafast communication and other relevant areas.

In this paper, we report an evolution of surface morphology of silver film irradiated by a 1 kHz femtosecond laser. By SEM observations, it is noted that different nanostructures with respective surface features depend highly on the number of pulses and the laser fluence. Especially when the laser fluence is below the threshold fluence of film breakdown, a textured nanostructure including many nanobumps and nanocavities will appear on the surface of silver film. In order to determine an optimal regime for nanostructuring silver film and to further study the underlying mechanism, we perform a quantitative analysis of laser fluence and pulse number. The results show that this nanostructure formation should be due to a sequential process of laser melting, vapor bubbles bursting, heat stress confinement, and subsequent material redistribution. As a potential application, we find this nanostructured silver film can be used as the active substrate for surface enhanced Raman scattering effect.

We report on the precipitation of Si nanocrystals inside a borosilicate glass by using an 800 nm, 250 kHz femtosecond laser irradiation, which was confirmed with X-ray diffraction, Raman spectra and transmission electron microscopy analyses. Refractive index profile reveals that the refractive index of the Si nanocrystals precipitated region increased up to 8.7% in comparison with that of the unirradiated area, leading to a large diffraction efficiency of the fabricated dot structure. Furthermore, the third-order optical nonlinearity of the Si nanocrystals precipitated glass is greatly enhanced based on the Z -scan measurement. These results may find applications for the fabrication diffractive optical devices and optical switches.

Spatio-temporal coupling characteristics of ultrafast laser pulses are quantitatively tailored. An asymmetric microstructure is induced in the focal volume when the laser scans perpendicularly to the direction of the spatial chirp in fused silica. The tilted direction reverses when adding a Dove prism into the light path. The sign of the pulse front tilt can be turned from positive to negative by changing the group delay dispersion by steps. We reveal that the tilted direction of a microstructure depends on spatial chirp, and the interplay between spatio-temporal chirp leads to the change of tilted angles.

Recent research progress of terahertz (THz) radiation generation based on the ultrafast spin dynamics is reviewed. The transient spin-charge conversion based on the inverse spin Hall effect and the Rashba-Edelstein effect is introduced, and it is pointed out that the ferromagnetic/non-magnetic heterostructure has been used to design a low-cost and high-efficiency THz radiation source. The efficiency and bandwidth of a spintronics-based THz emitter can be improved by optimizing layer thickness, growth conditions, substrates, and construction. The applications of the THz emission spectrum are outlined in the study of the ultrafast formation dynamics of the spin Seebeck effect. © 2019, Chinese Lasers Press. All right reserved.

We report the formation of β′-Gd2(MoO4)3 (GMO) crystal on the surface of the 21.25Gd2O3–63.75MoO3–15B2O3 glass, induced by 250 kHz, 800 nm femtosecond laser irradiation. The morphology of the modified region in the glass was clearly examined by scanning electron microscopy (SEM). By micro-Raman spectra, the laser-induced crystals were confirmed to be GMO phases and it is found that these crystals have a strong dependence on the number and power of the femtosecond laser pulses. When the irradiation laser power was 900 mW, not only the Raman peaks of GMO crystals but also some new peaks at 214 cm−1, 240 cm−1, 466 cm−1, 664 cm−1 and 994 cm−1which belong to the MoO3 crystals were observed. The possible mechanisms are proposed to explain these phenomena.

A 250 kHz femtosecond laser was used to induce a one-step precipitation of Ag nanoparticles and the simultaneous element redistribution in Ag ion doped glass. After femtosecond laser exposure, a ring-shape region was formed. Besides, as a result of an optical spherical aberration effect, a nonuniform laser intensity distribution along the incident direction caused most of the formed Ag nanoparticles to precipitate in the lower part of cross section of this structure. By an EPMA analysis, the relative concentration of the Ag element was both high in the center and in the boundary of the ring-shape region. These Ag nanoparticles could potentially increase the refractive index in their precipitation regions. We proposed that the induced ion redistribution and the precipitation of Ag nanoparticles should be due to the heat accumulation effect by the high repetition rate femtosecond laser irradiation.

Based on anisotropic diffraction of a volume phase grating in a photorefractive crystal, we theoretically discuss an optical multi- and demultiplexing scheme implemented by one single grating in photorefractive LiNbO3 crystal. It shows that our scheme can simultaneously demultiplex 93 channels in the telecommunication wavelength around 1550 nm. Using only one grating to realize WDM can avoid the multiple exposure problems encountered by multiple hologram scheme. Moreover, in our scheme, the transmitted and diffracted beams are orthogonal to each other, thus we don't need to use a normal recording and readout structure. Our theoretical results can be used in the design of a WDM device.

We report on the formation and structural evolution of embedded self-organized, polarization-dependent nanogratings in sodium germanate glasses induced by an 800 nm, 1 kHz femtosecond laser. Optical birefringence dependent on the femtosecond laser polarization as well as the sodium oxide content is observed when the sample surface is perpendicular to the laser propagation direction. Scanning electron microscopy images of the written lines reveal the formation of periodic platelet or nanovoid arrays, which are aligned perpendicularly to the laser polarization direction after mechanical polishing. The influences of sodium oxide content on the morphology and period of the nanogratings are discussed.

Recording, erasing, and rewriting of ripples are achieved by applying femtosecond laser pulses on tungsten surfaces. Ripples oriented perpendicular to the polarization direction of the writing beam can be recorded on a metal surface by exposing the sample to a series of linearly polarized pulses. When applying the second series of pulses with varied polarization direction on the same place, the original ripples can be erased, and new ripples are rewritten with the orientation perpendicular to the polarization of the second group of pulses. The simulation shows that when original ripples exist, laser intensity is focused above the grooves with polarization parallel to original ripples, which can erase the ripples. However, when the polarization is perpendicular to the existing ripples, laser intensity is almost confined in the grooves, which accelerates the formation of ripples.

A 1 kHz femtosecond double-pulse sequence irradiation is used to study the temporal evolution of nanograting in fused silica by controlling the delay times and polarization combinations of two independent beams from a Mach–Zehnder interferometer. A lateral laser-scan experiment with speed at 5 µm s−1 and each pulse energy of 1 µJ is firstly performed with the delay time from sub-picosecond to 10 ps, and then the written nanostructures are systematically studied under a cross-polarized microscope because the intensity of birefringence signal nearly corresponds to optical retardance and development level of the induced nanograting. The trend shows that the induced nanogratings can continue developing with a decrease of delay time in the case of the linear polarization pulse arriving before. In another vertical laser-scan experiment at the same speed and pulse energy, the morphologies of nanogratings embedded in the lines are characterized by scanning electron microscope after mechanical polishing and chemical etching. The self-organized patterns have a commonly spatial period of 200–300 nm and the orientation is always perpendicular to the polarization of the first laser pulse, and the second pulse in each sequence seems to promote the as-formed nanograting developing further even if the polarized direction is different from the previous pulse. These new findings verify again that a localized memory effect can make positive feedback to reinforce the patterned nanostripes. In that process, the impact ionization from the seed electrons left by the first pulse excitation and the photoionization of self-trapped excitons with lower ionization threshold results in an increase of the re-excited carriers during the second pulse irradiation and the subsequent development of the as-formed nanograting. Our result provides further proofs for understanding the physical mechanism of nanograting strongly connection with the interplay on multiple ionization channels.

In this article, we report the effects of the thickness of metal and oxide layers of the Al/WO3/Au interconnecting structure on the electrical and optical characteristics of the and bottom units of the two-unit stacked organic-light-emitting-devices (OLEDs). It is found that light emission performance of the upper unit is sensitive to the transmittance of semitransparent Al/WO3/Au structure, which can be improved by changing the thickness of each layer of the Al/WO3/Au structure. It is important to note that the introduction WO3 between Al and Au significantly enhances the current efficiency of both the upper and bottom units with respect to that of the corresponding Al/Au structure without WO3. In addition, the emission spectra of both the upper and bottom units are narrower than that of the control device due to microcavity effect. Our results indicate that the All WO3/Au interconnecting structure is a good candidate for fabricating independently controllable high efficiency stacked OLEDs.