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In this paper, we propose a demodulation method for refractive index (RI) sensing with tilted fiber Bragg gratings (TFBGs). It operates by monitoring the TFBG cladding mode resonance “cut-off wavelengths.” The idea of a “cut-off wavelength” and its determination method are introduced. The RI sensitivities of TFBGs are significantly enhanced in certain RI ranges by using our demodulation method. The temperature-induced cross sensitivity is eliminated. We also demonstrate a parallel-double-angle TFBG (PDTFBG), in which two individual TFBGs are inscribed in the fiber core in parallel using a femtosecond laser and a phase mask. The RI sensing range of the PDTFBG is significantly broader than that of a conventional single-angle TFBG. In addition, its RI sensitivity can reach 1023.1 nm/refractive index unit in the 1.4401–1.4570 RI range when our proposed demodulation method is used. [ABSTRACT FROM AUTHOR]

We studied the photoluminescence (PL) from black silicon that was fabricated using an 800 nm, 250 kHz femtosecond laser in air. By changing the scan velocity and the fluence of the femtosecond laser, the formation of the PL band between the orange (600 nm) and red bands (near 680 nm) could be controlled. The red band PL from the photoinduced microstructures on the black silicon was observed even without annealing due to the thermal accumulation of high-repetition rate femtosecond laser pulses. The orange band PL was easily quenched under 532 nm cw laser irradiation, whereas the red band PL was more stable; this can be attributed to 'defect luminescence' and 'quantum confinement', respectively. [ABSTRACT FROM AUTHOR]

The nonlinear optical response of gold nanoparticles/reduced graphene oxide (AuNPs/rGO) nanocomposites prepared by laser ablation method was studied using a femtosecond laser open-aperture (OA) Z-scan method, and an enhanced nonlinear absorption effect was observed. Using the femtosecond time-resolved pump-probe measurements, the nonlinear response mechanism in the hybrid was studied. When graphene was excited by an ultrashort laser pulse, the bleaching of valence band (VB) and the filling of conduction band (CB) took place, causing a saturable absorption (SA) effect. When decorated with gold nanoparticles, the excited carriers from the CB of graphene could transfer to the sp band of the metal before returning to the VB of graphene. This process would take place in a much longer time scale (∼50 ps) than the direct relaxation in pure graphene, causing the enhancement of the SA effect. The excited carrier dynamics were further demonstrated using femtosecond time-resolved transient absorption spectroscopy, and the interband relaxation of the excited electrons from the sp band of AuNPs to the CB of graphene was observed, resulting in the further filling of the CB of graphene and enhancement of the SA effect. [ABSTRACT FROM AUTHOR]

Single‐crystal halide perovskites are among the most promising semiconductor candidates for highly efficient optoelectronic devices due to their higher stability against moisture and light radiation and lower trap density than their polycrystalline counterparts. In this work, single‐crystal MAPbBr3 microcavities with controllable geometries and sizes are fabricated using a template‐limited method combined with an anti‐solvent diffusion growth method. The single‐crystal microcavities emit lasers around 550 nm under femtosecond laser excitation, with a lasing threshold of ≈35 µJ cm−2 and a quality factor of 1135, as well as excellent long‐term operation stability. The temporal‐spatial evolution of the lasing process in an individual microcavity is achieved by the microscopic optical Kerr‐gating technique. The temporal behaviors including pulse duration and build‐up time, emission spectra shift, and lasing mode evolution as a function of pump fluence are unveiled, and the complex relationship between the light‐induced carrier dynamics and lasing properties is clarified. Spatial heterogeneous lasing dynamics are also observed in an individual microdisk, which is attributed to the non‐uniform optical losses and carrier densities driven by structural imperfections. This study can provide a fabrication strategy for perovskite on‐chip optoelectronic devices, as well as a deeper understanding of the carrier‐density‐driven lasing dynamics in microcavities. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) periodic structures were fabricated on silicon surfaces by femtosecond laser irradiation in air and water, with the assistance of a microlens array (MLA) placed in the beam's path. By scanning the laser beam along the silicon surface, multiple grooves were simultaneously fabricated in parallel along with smaller laser-induced ripples. The 2D periodic structures contained long-periodic grooves and perpendicular short-periodic laser-induced ripples, which had periods of several microns and several hundred nanometers, respectively. We investigated the influence of laser power and scanning velocity on the morphological evolution of the 2D periodic structures in air and water. Large-area grid-like structures with ripples were fabricated by successively scanning once along each direction of the silicon's surface, which showed enhanced optical absorption. Hydrofluoric acid was then used to remove any oxygen and laser-induced defects for all-silicon structures. [ABSTRACT FROM AUTHOR]

A femtosecond pulse laser was used to fabricate carbon nanodots (CDs), of which the particle size and photoluminescence (PL) properties could be effectively controlled by adjusting ambient pressure. By increasing the reaction pressure, the particle size of CDs gradually decreased and finally reached less than 1 nm at 4 MPa. Simultaneously, the fluorescence intensity of the CDs first increased and then decreased by further increasing the pressure. By examining the PL dynamics and the chemical structure of the CDs, we found that the PL change of products was attributed to the quantity change of functional groups attached to the CDs due to the surface area change of the carbonic core. [ABSTRACT FROM AUTHOR]

We demonstrate an environmentally stable femtosecond laser for industrial application based on all polarization‐maintaining Yb‐doped laser and experimentally explore the effect of the net cavity dispersion (NCD) on output characteristics. The oscillator emits pulses with 5.1 ps and a 23 nm spectral bandwidth, then the pulse is compressed to 88 fs. The instabilities over 120 h gives 0.32% relative root mean square noise, indicating high stability of the pulse laser. With effective regulation of NCD, we obtained the maximum spectral bandwidth of 27 nm. Such broadband oscillator with compact all‐fiber configurations provide extremely high stability should be very attractive for long‐term industrial application. [ABSTRACT FROM AUTHOR]

We demonstrated a linear-cavity fiber laser using a fiber Bragg grating (FBG) fabricated by femtosecond laser and a Sagnac loop as cavity mirrors. The temperature sensing response of the fiber laser was characterized by placing the FBG in a high-temperature environment. The stability of the fiber laser at high temperature was improved after the FBG was annealed at 1100 °C. The fiber laser can work stably as a temperature sensor at 1000 °C and its temperature sensing sensitivity is approximately 15.9 pm/°C from 300 °C to 1000 °C. • A fiber laser based on an FBG fabricated by femtosecond laser and a Sagnac loop as cavity mirrors is demonstrated. • The fiber laser can work stably as a temperature sensor at 1000 °C after the FBG was annealed at 1100 °C. • The wavelength of the fiber laser had a quadratic dependence on temperature from room temperature to 1000 °C. • The temperature–wavelength had a good linear relationship from 300 °C to 1000 °C. Its sensing sensitivity was about 15.9 pm/°C. [ABSTRACT FROM AUTHOR]

Gold nanorod (GNR) and reduced graphene oxide (rGO) hybrids are synthesized using photochemical method based on femtosecond laser ablation without the usage of strong chemical reducing agents. The products are characterized by absorption spectra, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The formed GNR are well located on the surface of rGO sheets. By adjusting the laser ablation time, the aspect ratio and optical properties of GNR can be well controlled. The produced nanocomposites exhibited an excellent near-infrared (NIR) photothermal performance with a photothermal conversion efficiency of 77.8%, suggesting that the hybrids have large potential as agents in photothermal therapy. • Gold nanorod and graphene hybrids are synthesized using femtosecond laser. • The aspect ratio of gold nanorod can be well controlled by adjusting laser ablation time. • The produced hybrids exhibited an excellent photothermal performance at 808 nm. [ABSTRACT FROM AUTHOR]

Zinc oxide (ZnO) nanoparticles (NPs) are synthesized by one-step femtosecond laser ablation of zinc powders in n-methyl pyrrolidone (NMP) at room temperature. The as-prepared ZnO NPs are fairly stable, water-soluble and have abundant surface functional groups resulting in a strong fluorescence in the visible region. By further annealing the ZnO NPs in the reacting solvent up to 120∘C, the photoluminescence (PL) intensity of material can be significantly enhanced. Besides, the PL of ZnO NPs can change from blue to green by controlling the annealing temperature. The products exhibit excellent temperature sensing with high temperature sensitivity and a 1.3% change per ∘C response over a linear temperature sensing range (6–84 ∘ C) in aqueous buffer, which matches well with the physiologically relevant temperatures. Hence, the ZnO nanoparticle system can serve as a promising candidate for practical fluorescent temperatures nanosensors which can be used for accurate temperature monitoring within biological systems. Water-soluble and photoluminescent ZnO NPs are synthesized by femtosecond laser ablation of zinc powders at room temperature. By further annealing, the photoluminescence intensity of the products can be significantly enhanced. The ZnO NPs exhibit excellent temperature sensing with a high sensitivity of 1.3% change per ∘C over a linear range of 6–84∘C in aqueous buffer, which can serve as a promising candidate for fluorescent temperature nanosensors to be used for accurate temperature monitoring within biological systems. [ABSTRACT FROM AUTHOR]

The influence of the size parameter of the scatterers on ultrafast optical Kerr gate (OKG) imaging is investigated in highly scattering poly-disperse turbid media. The results show that in a poly-disperse turbid medium, which in our case, is a suspension of two different sized mono-disperse microspheres, the temporal and spatial behaviors of the light pulses transmitted through it are dominated by the smaller microspheres. The contrasts of the OKG images for the poly-disperse microsphere sample are closer to the contrasts of the OKG images for the smaller sized mono-disperse microsphere sample. [ABSTRACT FROM PUBLISHER]

Highlights: [•] We proposed a simple method of fabricating through holes in 6H–SiC using femtosecond laser and chemical selective etching. First, laser affected zones (LAZ) were induced on the SiC pattern. Then the through holes were produced as the LAZ were removed by chemical selective etching with mixed solution of HF and HNO3. [•] The formation mechanism of the holes was attributed to the chemical reaction of the LAZ with mixed solution of HF and HNO3. [•] The method has several prominent advantages, such as: fast removal rate, being independent of etch mask, being simple technological process and high precision and flexibility of controlling the depth of the hole. [Copyright &y& Elsevier]

Abstract: A simple method using 800-nm femtosecond laser irradiation and chemical selective etching has been proposed for fabrication of high-aspect-ratio all-silicon grooves. Grooves with the maximum aspect ratio of 44 were produced. A scanning electronic microscopy equipped with an energy dispersive X-ray spectroscopy was employed to characterize the morphology and chemical composition of the grooves respectively. The formation mechanism of the grooves was attributed to the chemical reaction of the laser induced refractive index change microstructures and hydrofluoric acid solution. The dependences of the aspect ratio of the grooves on the laser irradiation parameters, such as: the numerical aperture of the microscope objective lens, the laser average power and the laser scanning velocity, are discussed. [Copyright &y& Elsevier]

We studied the structural and elemental evolutions during the femtosecond laser percussion drilling of high-aspect-ratio diamond microholes. Microholes 225-μm-deep having an aspect ratio of 15 were drilled with an exposure time of 100 s and a laser power of 60 mW. It is found that when the specimen was machined by femtosecond laser at low power and short exposure time, the laser-affected zone (LAZ) may be still solid in the interior rather being void, even it reached the bottom of the diamond. A clear crack appeared between the solid portion and pristine diamond. Elemental analysis revealed that oxygen was incorporated into the solid portion of the LAZs, and its atomic percentage reached 6.5% for a laser power of 10 mW at initial position and decreased as the depth increased in the solid portion. The wall of the void contained nearly no oxygen. Furthermore, nanoripples were observed on the sidewall surface of the hole. [ABSTRACT FROM AUTHOR]

A compact 51.6-W, 26- μ J all-fiber integrated Yb-doped femtosecond laser source with pulse durations of 692 fs despite ∼ 18 π nonlinear phase shift accumulation in the main amplifier is demonstrated by using fiber quasi-rectangular pulse pre-shaping. The numerical results are in good agreement with the experiment. Due to the advantages of an all-fiber spliced structure and a minimal pulse stretching ratio (from 26.3 ps to ∼ 70 ps), just a small size of gratings and a short separation distance between the two gratings in the compressor is required. Therefore, the laser source is exceedingly compact, robust, cost-effective, and easy to assemble. This technique is anticipated to accelerate the use of fiber femtosecond lasers in industrial applications. • All-fiber spliced structure with ∼ 18 π nonlinear phase shift in the main amplifier. • High peak power and density as much as 0.4 MW and 80 GW/cm 2 respectively in the main amplifier • Compact structure with small compressor makes the laser robust, reliable, and cost-effective. • Near-diffraction-limit beam quality suitable for micro-machining and related applications. [ABSTRACT FROM AUTHOR]

Abstract: An alternative approach for femtosecond laser induced black silicon in ambient air is proposed, in which, black silicon is fabricated on a tellurium coated silicon substrate via femtosecond laser irradiation in ambient air, and selectively etching with hydrofluoric acid is employed to remove the incorporated oxygen. Results of energy dispersive X-ray spectroscopy analysis and absorption measurement show that oxygen is effectively eliminated via etching, and the optical absorption of the black silicon is enhanced. [Copyright &y& Elsevier]

Abstract: We performed the femtosecond nondegenerate optical Kerr shutter (OKS) measurements in a 10-mm-thick optical glass. The time-resolved OKS signals showed an obvious asymmetry and a large broadening with respect to that for thin samples. The time-resolved OKS signals in samples with different thicknesses showed that, the temporal broadening of the OKS signals was attributed to the group velocity mismatch between pump and probe beams of different wavelengths. By imaging the filament induced inside the glass, we attributed the asymmetry of the time-resolved OKS signals to the focusing position change of pump beam caused by self-focusing and re-focusing effect. [Copyright &y& Elsevier]

Abstract: Time resolved two-photon absorption induced electron-hole plasma (EHP) luminescence of Ga-doped ZnO thin film was measured by an ultrafast optical Kerr gate (OKG) in femtosecond time regime. Experimental results showed that the buildup time of the EHP luminescence was strongly dependent on the excitation fluence. The dependence of the buildup time of EHP on excitation fluence probably arose mainly from the relaxation of the hot carriers due to the carrier–carrier interaction, which increased with the increase of excitation fluence. [Copyright &y& Elsevier]

We demonstrate the control of femtosecond single-filament formation via feedback-based wavefront shaping. It is observed that the optimal phase profile forms a single-filament with closed-loop genetic algorithm. Using this method, the position stability of the filament can be significantly improved, and the position of the single filament can be flexibly adjusted. Additionally, a two-step approach combining the premodulation of pinhole with wavefront shaping is presented for forming a bright single filament rapidly. • Succeeded in getting a stable and bright single filament. • Optimization processes from multiple filaments to single filament are displayed. • A two-step strategy is an effective technique to fast control single filament. [ABSTRACT FROM AUTHOR]

We have developed an ultrafast time-resolved imaging technique for the propagation dynamics of ultrashort laser pulses in transparent media. This method utilizes the optical polarigraphy technique and a chirped supercontinuum as the probe light. The supercontinuum senses the instantaneous birefringence induced by the propagation of an intense pulse, and a polarigraphy image with different color distributions could be obtained. Using this method, we performed a space-time characterization of the filament induced by a femtosecond laser pulse in CS2, indicating that this technique could be used for the single-shot imaging of pulse propagation dynamics. [ABSTRACT FROM AUTHOR]

Abstract: Microchannels are fabricated inside silicon along the direction of laser beam by a femtosecond laser at 800nm, which is located at the absorption region of silicon. Formation mechanism of the microchannels is attributed to spherical aberration. The element change in surrounding of the microchannel is characterized, and results show that oxygen is incorporated into silicon after irradiation. The origin of the oxygen incorporation and the laser intensity dependence of the amount of oxygen are discussed, respectively. [Copyright &y& Elsevier]

High-aspect-ratio grooves have been fabricated in silicon by a single pass of femtosecond laser pulses in water and ambient air. Scanning electron microscopy and energy dispersive x-ray spectroscopy were employed to image for the morphology of the photoinduced grooves and analyze the chemical composition in the surrounding of the grooves. It was observed that the sidewall of the grooves fabricated in water was much smoother than that in ambient air, and there were homogeneous nano-scale protrusions on the sidewall of the grooves fabricated in water. Meanwhile, oxygen species, which was incorporated into the grooves fabricated in air, was not observed in those in water. [ABSTRACT FROM AUTHOR]

In order to investigate the decay of nonadiabatic molecular unidirectional rotation (UDR) for nitrogen molecules, we measured the time-resolved rotational Doppler spectroscopy in first four revival periods after instantaneous excitation by a polarization shaped femtosecond laser. The results show that the rotational Doppler shifts decrease with increasing order of revival periods for full and half rotational revivals. Moreover, the maximum alignment of the molecular UDR at half rotational revivals was demonstrated to delay with revival periods increasing. This work is helpful to understand the kinetic processes of collisional decay for nonadiabatic molecular UDR, especially in the several revival periods after excitation. • Early collisional decay in nonadiabatic molecular UDR was observed. • The RDS at rotational revivals decrease with revival periods increasing. • Maximum alignment at half revivals delays with revival periods increasing. [ABSTRACT FROM AUTHOR]

Morphologies of high-aspect-ratio silicon grooves fabricated by using femtosecond laser irradiation and selective chemical etching of hydrofluoric acid (HF) were studied. Oxygen was deeply doped into silicon under femtosecond laser irradiation in air, and then the oxygen-doped regions were removed by HF etching to form high-aspect-ratio grooves. After HF etching, periodic nano-ripples which were induced in silicon by femtosecond laser were observed on the groove sidewalls. The ripple orientation was perpendicular or parallel to the laser propagation direction ( z direction), which depended on the relative direction between the laser polarization direction and the scanning direction. The formation of nano-ripples with orientations perpendicular to z direction could be attributed to the standing wave generated by the interference of the incident light and the reflected light in z direction. The formation of nano-ripples with orientations parallel to z direction could be attributed to the formation of self-organized periodic nanoplanes (bulk nanogratings) induced by femtosecond laser inside silicon. Materials in the tail portion of laser-induced oxygen doping (LIOD) regions were difficult to be etched by HF solution due to low oxygen concentration. The specimen was etched further in KOH solution to remove remaining materials in LIOD regions and all-silicon grooves were fabricated. [ABSTRACT FROM AUTHOR]

Gradient-index modulation prefabrication in the cladding of standard telecom fibers was proposed for femtosecond laser processing of Type II-IR fiber Bragg gratings. The refractive index of the prefabricated region in the cladding had a large gradient across the laser beam, which could act as a cylindrical lens to enhance the focusing of laser beam when writing Type II-IR gratings. With the help of prefabrication, the threshold pulse energy for processing Type II-IR FBGs was lowered from 750 μJ to 520 μJ. The fabricated FBGs showed good thermal stability at temperatures over 900 °C. [ABSTRACT FROM AUTHOR]

In nature, a frog can easily rest on a lotus leaf even though the frog's weight is several times the weight of the lotus leaf. Inspired by the lotus leaf, we fabricated a planar superhydrophobic microboat (SMB) with a superhydrophobic upper surface on a PDMS sheet which was irradiated by a focused femtosecond laser. The SMB can not only float effortlessly over the water surface but can also hold up some heavy objects, exhibiting an excellent loading capacity. The water surface is curved near the edge of the upper surface and the SMB's upper edge is below the water level, greatly enhancing the displacement. Experimental results and theoretical analysis demonstrate that the superhydrophobicity on the edge of the upper surface is responsible for the SMB's large loading capacity. Here, we call it the ‘superhydrophobic edge effect’. [ABSTRACT FROM AUTHOR]

Abstract: We investigated the ultrafast optical Kerr effect of the fluorotellurite glass (Ft glass), the nonlinear response time of which was measured to be less than 230fs, while the nonlinear refractive index was estimated to be 9.62×10−16 cm2/W. Using optical Kerr gate (OKG) technique with Ft glass as Kerr medium, we obtained narrow-bandwidth and symmetric gated spectra from a supercontinuum (SC) generated by a femtosecond laser. Experimental results indicated that Ft glass was a good candidate for an OKG medium due to its fast response time, large nonlinearity, and high transparency. [Copyright &y& Elsevier]

A high-fidelity numerical model for investigations of the ultrafast heating is highly desirable for simulating the pulsed laser damage and the ultrafast electron emission characteristics. However, realization of accurate predictions of thermal dynamics and thermionic electron emission remains challenging due to the high non-equilibrium state, in which the equilibrium heating parameters are invalid. Here, we report an axisymmetric two-dimensional (2-D) high-fidelity numerical model for predictions of the thermionic emission with respect to the temperature-dependent dynamics parameters. The spatio-temporal temperature evolution dynamics and the thermionic emission rate characteristics on Au film target are demonstrated, whose credibility is approved by the Au film ablation threshold experiments. [ABSTRACT FROM AUTHOR]

The pump power dependence of the spatial gating properties of femtosecond optical Kerr gate (OKG) was investigated using coaxial two-color optical Kerr measurements in CS. As the pump power increased, the spatial pattern of the optical Kerr signals changed from a Gaussian spot to a ring form, and then a spot surrounded by a concentric ring, successively. By comparing the experimental data with the calculation results and measuring the pump power dependence of the OKG signal intensity, we demonstrated that the spatial variation of OKG transmittance could be attributed to the non-uniform spatially distributed phase change of the probe beam, due to the transient birefringence effect induced by pump beam with transverse mode of a Gaussian distribution. [ABSTRACT FROM AUTHOR]

Abstract: Materials with curvilinear surface microstructures are highly desirable for micro-optical and biomedical devices. However, realization of such devices efficiently remains technically challenging. This paper demonstrates a facile and flexible method to fabricate curvilinear microstructures with controllable shapes and dimensions. The method composes of femtosecond laser exposures and chemical etching process with the hydrofluoric acid solutions. By fixed-point and step-in laser irradiations followed by the chemical treatments, concave microstructures with different profiles such as spherical, conical, bell-like and parabola were fabricated on silica glasses. The convex structures were replicated on polymers by the casting replication process. In this work, we used this technique to fabricate high-quality microlens arrays and high-aspect-ratio microwells which can be used in 3D cell culture. This approach offers several advantages such as high-efficient, scalable shape-controllable and easy manipulations. [Copyright &y& Elsevier]

Inspired by the lotus leaf, a new superhydrophobic surface with hierarchical mesh-porous structure is fabricated by femtosecond laser irradiation on silicon. The fabricated surface shows a superhydrophobic character with water contact angle being found to reach up to 158±1 and sliding angle of 4±0.5. The superhydrophobicity is stable even if the PH of solution changes from 1 to 14. And the surface also exhibits excellent self-cleaning effect and bouncing behavior, implying that the adhesion of the surface is extremely low. This work will enhance further understanding of the wettability of a solid surface with special surface morphology. [ABSTRACT FROM AUTHOR]

Abstract: Compared with bismuth glasses and chalcogenide glasses, tellurite glasses have a wider transparency window. Especially, these glasses with excellent transparency in visible region could offer a larger dynamic range for the time-resolved simultaneous three-dimensional imaging based on the optical Kerr gate (OKG). We investigated the ultrafast nonlinear optical properties of a TeO2–ZnO–Na2O glass using a femtosecond optical Kerr gate at wavelength of 800nm. The nonlinear refraction n 2 and the ultrafast nonlinear response time of the tellurite glass were estimated to be 4.56×10–15 cm2/W and less than 200fs, respectively. We also demonstrated the time-resolved simultaneous three-dimensional imaging using the tellurite glass as the Kerr medium, which indicated that tellurite glasses may also be good candidates as the Kerr media for the time-resolved imaging based on the OKG. [Copyright &y& Elsevier]

Abstract: Netlike or porous microstructures are highly desirable in metal implants and biomedical monitoring applications. However, realization of such microstructures remains technically challenging. Here, we report a facile and environmentally friendly method to prepare netlike microstructures on a stainless steel by taking the full advantage of the liquid-mediated femtosecond laser ablation. An unordered netlike structure and a quasi-ordered array of holes can be fabricated on the surface of stainless steel via an ethanol-mediated femtosecond laser line-scan method. SEM analysis of the surface morphology indicates that the porous netlike structure is in the micrometer scale and the diameter of the quasi-ordered holes ranges from 280nm to 320nm. Besides, we find that the obtained structures are tunable by altering the laser processing parameters especially scanning speed. [Copyright &y& Elsevier]

The ultrafast third-order nonlinear optical properties of C60+ vinyltriethoxysilane H2C=CHSi(OC2H5)3 (C60+VTES) sol-gel were investigated by the femtosecond optical Kerr gate (OKG) technique at 800 nm. The third-order optical susceptibility was measured to be 5.42 × 10−14 esu for C60+VTES at a weight concentration of 0.06 wt.%. Using C60+VTES as the OKG material, we acquired a series of narrow bandwidth and symmetric gated spectra continuously from the chirped white light continuum generated in water with femtosecond laser pulses. The gated spectra obtained using the C60+VTES OKG have distinct superiorities compared with CS2. [ABSTRACT FROM AUTHOR]

Abstract: We theoretically investigated different thermal relaxation participating in the ultrafast thermionic emission processes on gold film surface with a femtosecond pulse excitation. The thermionic emission regimes under the two temperature relaxation and the thermal diffusion relaxation were demonstrated. The simulations showed that the thermionic emission properties can be defined in the regime under two temperature relaxation by reducing the laser fluence, or widening the pulse duration or increasing the laser wavelength. It was also found that there exists a transition between the two distinct thermionic emission regimes under peculiar laser parameters of laser fluence, pulse duration and laser wavelength. The results were explained as significant intervene of laser irradiation parameters into gold film thermal relaxation processes. [Copyright &y& Elsevier]

We demonstrate that quasi-periodic void structure can be self-formed in transparent materials by single femtosecond laser pulse. Compared to the multiple-pulse induced structures, the single-pulse induced void structures are very short and may contain absent voids. The formation mechanisms have been discussed comparatively in detail. Based on this, a technique for high-speed and large-area fabrication of micro-void arrays in transparent materials has been presented. The experimental results show that 3D micro-void structures which contain over several hundred thousand voids in micrometer scales are produced in areas of square millimeters within a few minutes, and the periods of micro-void structures can be easily varied by processing parameters. This work has potential applications in 3D optical storage, photonic crystal and integrated optics, and provides novel insight into the interaction between the single femtosecond pulse and the transparent materials. [ABSTRACT FROM AUTHOR]

Abstract: By femtosecond laser line-by-line scanning irradiating, large-scale microstructures were formed on the surface of silicon with dimensions of 1×1mm2. Scanning electron microscope investigations exhibited that homogeneous surface microstructures, such as directional-arranged bacilliform mesoporous structures, have been successfully prepared. The dependence of the surface morphology on laser pulse energy was analyzed, and the results indicated that the bacilliform mesoporous structures only can be textured within a certain energy range. The optical reflective spectrum measurement revealed that the presence of bacilliform mesoporous structures can significantly enhance the absorptivity of silicon at visible light range. This work would help to control the formation of surface micro/nanostructures on silicon and other materials, which has potential applications in solar energy, photoelectronics, biology and material science. [ABSTRACT FROM AUTHOR]

Abstract: Liquid-assisted ablation of solids by femtosecond laser pulses has proved to be an efficient tool for highly precise microfabrication, which evokes numerous research interests in recent years. In this paper, we systematically investigate the interaction of femtosecond laser pulses with silicon wafer in water, alcohol, and as a comparison, in air. After producing a series of multiple-shot craters on a silicon wafer in the three types of environments, surface morphologies and femtosecond laser-induced periodic surface structures are comparatively studied via the scanning electron microscope investigations. Meanwhile, the influence of liquid mediums on ablation threshold fluence and ablation depth is also numerically analyzed. The experimental results indicate that the ablation threshold fluences of silicon are reduced by the presence of liquids (water/alcohol) and ablation depths of craters are deepened in ambient water. Furthermore, smoother surfaces tend to be obtained in alcohol-mediated ablation at smaller shot numbers. Finally, the evolution of the femtosecond laser-induced periodic surface structures in air, water and alcohol is also discussed. [Copyright &y& Elsevier]

Abstract: Periodic surface nanostructures induced by femtosecond laser pulses on polycrystalline ZnO are presented. By translating the sample line-by-line under appropriate irradiation conditions, grating-like nanostructures with an average period of 160nm are fabricated. The dependence of surface morphologies on the processing parameters, such as laser fluence, pulse number and laser polarization, are studied by scanning electronic microscope (SEM). In addition, photoluminescence (PL) analysis at room-temperature indicates that the PL intensity of the irradiated area increases significantly compared with the un-irradiated area. Using femtosecond laser pulses irradiation to fabricate periodic surface nanostructures on polycrystalline ZnO is efficient, simple and low cost, which shows great potential applications in ZnO-based optoelectronic devices. [Copyright &y& Elsevier]

Abstract: Ultrafast temperature relaxation processes in Au film including two temperature relaxation and thermal diffusion relaxation with femtosecond laser pulse excitation were investigated numerically by Finite Element Method (FEM). With the temperature dependent thermal parameters, the full 2D temperature field evolution in picosecond and nanosecond domains were obtained. It is proposed that the heat transfer depth can be alternatively localized or enhanced by the distinct temperature relaxation mechanisms. Moreover, the effect of laser parameters and Au film thickness and surface reflectivity on the two temperature relaxation time were analysed. [Copyright &y& Elsevier]

Abstract: We fabricated spherical microlenses on optical glasses by femtosecond laser direct writing (FLDW) in ambient air. To achieve good appearances of the microlenses, a meridian-arcs scanning method was used after a selective multilayer removal process with spiral scanning paths. A positive spherical microlens with diameter of 48μm and height of 13.2μm was fabricated on the surface of the glass substrate. The optical performances of the microlens were also tested. Compared to the conventional laser direct writing (LDW) technique, this work could provide an effective method for precise shape-controlled fabrication of three-dimensional (3D) microstructures with curved surfaces on difficult-to-cut materials for practical applications. [Copyright &y& Elsevier]

Abstract: We encoded surface relief micro-gratings on Au–Cr thin films using two-beam interference of femtosecond laser pulses with the durations from 25fs to 70fs. The dependence of the fabrication quality on the pulse duration has been investigated both numerically and experimentally. The results revealed that the shorter pulses were preferable to prepare periodical microstructures with minimal ablation fringe width and satisfied fabrication quality. This work has potential applications on periodic functional microstructures fabrication for ultra-fine processing and modification on various materials, especially for intractable materials. [Copyright &y& Elsevier]

• We apply spectroscopy to monitor the real-time full evolution of molecular UDR for N 2 molecules under ambient conditions. • The time-dependent alignment degree of the molecular UDR and 1-D alignment have different features. • The rotational Doppler shift was observed to change according to these features at various rotational revivals. Molecular unidirectional rotation (UDR) induced by femtosecond lasers has attracted significant attention in recent years. We monitored the molecular UDR for nitrogen molecules under ambient conditions in real time by observing time-resolved spectroscopic probes, based on the rotational Doppler effect. The time-dependent alignment degree of the molecular UDR was experimentally observed to differ from that of the 1-D alignment. Moreover, the rotational Doppler shift exhibited different features at various rotational revivals. Finally, we used spectroscopy to demonstrate the manifestation of the quantum nature of the molecular UDR. [ABSTRACT FROM AUTHOR]