Surface birefringence in FTO thin film fabricated by ultrafast laser
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Ultrafast laser-induced periodic surface structures (LIPSSs) have been studied intensely in the last two decades, which have become a useful method for surface nanostructures and are widely used to tune surface properties. This paper reported the fabrication of high-spatial-frequency LIPSSs (HSFLs) on FTO film induced by 1,030[Formula: see text]nm femtosecond laser. The morphology, duty cycle and birefringence effects of HSFLs were studied in detail by changing the laser fluence and scanning speed. Clear and uniform HSFLs were formed over the entire ablated area when the laser fluence and scanning speed were 89[Formula: see text]mJ/cm 2 and 0.01[Formula: see text]mm/s, respectively. The duty cycle of the HSFL was measured to be as high as 0.36, and the thickness of the HSFL layer was found to be in the range of 409–546[Formula: see text]nm. The phase retardation of the FTO film with HSFLs could reach up to 96[Formula: see text]nm and could be used as an optical attenuator with a tunable range of 71–100% for 532[Formula: see text]nm linear polarized light.Keywords:
Duty cycle
Summary form only given. In recent years, the use of focused femtosecond laser pulses to direct-write three-dimensional arrays of waveguides, couplers and other devices within the bulk of glass has become increasingly prevalent. The study of the physical damage created by such lasers has also begun to be investigated with properties such as anomalous anisotropic light scattering, and uniaxial birefringence being observed in irradiated samples. At present, the microscopic processes underlying such anisotropies remain unclear. We have identified an additional property present in fused silica which is apparent after being irradiated by an amplified Ti:sapphire femtosecond laser system-strong reflection from the damaged region occurs only along the direction of polarization of the writing laser.
Reflection
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The paper is devoted to new trends in dispersive control techniques for ultrafast light emission.Acoustooptical dispersive delay lines for controlling the spectral components and phase composition of ultrashort laser pulses are considered.The method of super high frequency modulation of chirped femtosecond laser pulses is proposed.Theoretical approach to dispersive pulse shaping is supported by the experiments.
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Structured optical elements that control the spatial and temporal characteristics of femtosecond light pulses are analyzed and synthesized. We show that unique spatiotemporal effects can be attained based on the diffraction, refraction, and dispersive effects that appear in the femtosecond regime. We argue that the design requirements for ultrafast optics are beyond the achromatization considerations that are usually applied to incoherent illumination because of the need to consider coherent effects. Despite fundamental limitations in the space-time control of ultrashort pulses, we show the potential of this technique to improve simultaneously the spatial and the temporal resolution of a lens and to generate ultrafast pulse sequences.
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Pulse duration
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Periodic structures self-formed on the surface of several metals by femtosecond laser pulses are investigated. The laser fluence dependence can be explained by the induction of a surface plasma wave through parametric decay of laser.
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The control of layer thickness and phase structure in two-dimensional transition metal dichalcogenides (2D TMDCs) like MoTe2 has recently gained much attention due to their broad applications in nanoelectronics and nanophotonics. Continuous-wave laser-based thermal treatment has been demonstrated to realize layer thinning and phase engineering in MoTe2, but requires long heating time and is largely influenced by the thermal dissipation of the substrate. The ultrafast laser produces a different response but is yet to be explored. In this work, we report the nonlinear optical interactions between MoTe2 crystals and femtosecond (fs) laser, where we have realized the nonlinear optical characterization, precise layer thinning, and phase transition in MoTe2 using a single fs laser platform. By using the fs laser with a low fluence as an excitation light source, we observe the strong nonlinear optical signals of second-harmonic generation and four-wave mixing in MoTe2, which can be used to identify the odd–even layers and layer numbers, respectively. With increasing the laser fluence to the ablation threshold (Fth), we achieve layer-by-layer removal of MoTe2, while 2H-to-1T′ phase transition occurs with a higher laser fluence (2Fth to 3Fth). Moreover, we obtain highly ordered subwavelength nanoripples on both the thick and few-layer MoTe2 with a controlled fluence, which can be attributed to the fs laser-induced reorganization of the molten plasma. Our study provides a simple and efficient ultrafast laser-based approach capable of characterizing the structures and modifying the physical properties of 2D TMDCs.
Thinning
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Multimicrogratings are one-off written on silicate glass by two interfered femtosecond pulsed laser beams with the aid of a mask. The period and depth of the multimicrogratings are revealed by optical microscopy and atomic force microscopy. The depth is dependent on both the colliding angle between the two interfered laser beams and the laser pulse energy, but the period relies on the colliding angle only. We also observe a series of grooves formed at the middle of each bulge of the multimicrogratings and attribute it to the higher-order modulation arising from second-harmonic generation of the femtosecond laser pulse during the one-off writing processes.
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The interaction between femtoseocnd laser and transparent materials has been studied intensively in recent years. When the femtosecond laser was focused onto the surface of the transparent materials, if the laser fluence applied to the sample exceeds the material’s fluence threshold, ablation occurs. In this paper, we study the surface ablation of lithium niobate by femtosecond laser. We produced a two-dimensional array of voids in the sample surface by varying the number of shots and laser energy, and analyze of the damage depth with the relation to the pulse energy and the number of the pulse. It has important reference on the microfabrication in such materials by femtosecond laser.
Laser Ablation
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Following femtosecond (fs) laser pulse irradiation, we produce a type of periodic surface structure with a period tens of times greater than the laser wavelength and densely covered by an iterating pattern that consists of stripes of nanostructures and microscale cellular structures. The morphology of this large scale wave pattern crucially depends on laser fluence and the number of laser pulses, but not on the laser wavelength. Our study suggests that this large scale wave is initiated by fs laser induced surface unevenness followed by periodically distributed nonuniform surface heating from fs pulse irradiation.
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