Nonlinear optical properties of a series of azobenzene liquid-crystalline materials

The nonlinear optical properties of a series of azobenzene liquid-crystalline materials, which have different side-chain lengths in their molecular structure from one to another, were investigated using Z-scan method under picosecond pulse laser at 532 nm, 1064 nm and CW 488 nm excitation. The mechanism accounting for the process of nonlinear refraction was discussed under different laser excitations. The polymer films possess very large nonlinear refraction at all the three different laser excitations. Especially, the nonlinear refractive index becomes larger as the length of side-chain, where azobenzene group is contained, increases under pulse excitation at 1064 nm.     

Diffraction patterns and nonlinear optical properties of gold nanoparticles

Stable gold nanoparticles have been prepared by using soluble starch as both the reducing and stabilizing agents; this reaction was carried out at 40 °C for 5 h. The obtained gold nanoparticles were characterized by UV–Vis absorption spectroscopy, transmission electron microscopy (TEM) and z-scan technique. The size of these nanoparticles was found to be in the range of 12–22 nm as analyzed using transmission electron micrographs. The optical properties of gold nanoparticles have been measured showing the surface plasmon resonance. The second-order nonlinear optical (NLO) properties were investigated by using a continuous-wave (CW) He–Ne laser beam with a wavelength of 632.8 nm at three different incident intensities by means of single beam techniques. The nonlinear refractive indices of gold nanoparticles were obtained from close aperture z-scan in order of 10^?7 cm²/W. Then, they were compared with diffraction patterns observed in far-field. The nonlinear absorption of these nanoparticles was obtained from open aperture z-scan technique. The values of nonlinear absorption coefficient are obtained in order of 10^?1 cm/W.     

Characterization and nonlinear optical properties of PVP/TiO2 nano-fibers doping with Ag colloid nano-particles

We synthesized PVP/TiO₂ nano-fibers doping with Ag colloid nano-particles by electro-spinning method. These nano-fibers were characterized by UV/visible/NIR spectroscopy, SEM and XRD. The image of SEM showed that the synthesized nano-fibers were monotonous and without knot and had a diameter about 150 nm. We also measured the nonlinear refractive and absorption indexes of the sample in three different intensities using the single beam Z-scan method by a continuous wave (CW) He–Ne laser at 632.8 nm wavelength. The nonlinear refraction indexes of these nano-fibers were measured in order of 10⁻⁷ (cm²/W) with negative sign and the nonlinear absorption coefficient was obtained in order of 10⁻³ (cm/W).     

Two photon absorption characteristics of bulk GaTe crystal

We have investigated the structural and optical properties of bulk GaTe crystal grown by vertical Bridgman method. Two photon absorption (TPA) properties of GaTe crystal have been investigated by the open aperture Z-scan technique under 1064 nm wavelength with 4 ns or 65 ps pulse durations. The TPA coefficients are greater in ns regime than that of ps regime. Upon increasing intensity of incident light from 5.02×10⁷ W/cm² to 1.07×10⁸ W/cm², the TPA coefficients increased from 3.47×10^?6 cm/W to 8.53×10^?6 cm/W for nanosecond excitation. Similarly, when intensity of incident light was increased from 6.81×10⁸ W/cm² to 9.94×10⁸ W/cm² the TPA coefficients increased from 3.53×10^?7 cm/W to 6.83×10^?7 cm/W for picosecond excitation. Measured TPA coefficient of GaTe crystal is larger than that of GaSe and GaS layered crystals.     

Pulsed laser induced damage in SOI material

Laser-induced damage in silicon-on-insulator (SOI) material is investigated with 1064 nm laser pulses. As the laser pulse duration is increased from 190 ps to 1.14 s, the damage threshold of SOI material decreases from 1.3×10¹⁰ to 7.7×10³ W/cm² in laser flux. It is found that the damage threshold varies inversely as the pulse duration for a short irradiation time, and is independent of pulse duration for a long irradiation time. The time dependence is in good agreement with a thermal model which well describes the thermal-induced damage in a semi-finite material irradiated by a Gaussian laser beam. The values of absorption coefficient and thermal conductivity under laser irradiation are calculated as 1.1×10³ cm^?1 and 0.18 Wcm^?1 K^?1, respectively, by fitting the model to the experimental results. These results on material damage can be used to predict the damage thresholds of SOI-based devices.     

Nonlinear refraction and photoinduced birefringence in chlorophosphonazo I doped polymer thin films

The nonlinear refraction and photoinduced birefringence of chlorophosphonazo I (CPA I ) doped PVA thin films were investigated. The single-beam Z-scan measurement showed that CPA I doped PVA thin film possessed a large value of nonlinear refractive index (n₂=1.82×10⁻¹² cm²/W) under a pulse 532 nm excitation, and the mechanism accounting for the process of nonlinear refraction was discussed in term of resonant electronic effect. Moreover, fast and stable molecular reorientation was observed when investigating the photoinduced birefringence of CPA I doped PVA thin film with a CW 532 nm laser as pump light and a CW 650 nm laser as probe light.     

Third order nonlinear optical properties of ethyltriphenylphosphonium bis(2-thioxo-1,3-dithiole-4,5-dithiolato)aurate (III)

A novel organometallic compound, ethyltriphenylphosphonium bis(2-thioxo-1,3-dithiole-4,5-dithiolato)aurate (III), abbreviated as TPEPADT, was synthesized. The TPEPADT doped poly(methyl methacrylate) (PMMA) thin film with a mass fraction of 1% (1 wt.%) was prepared by using a spin-coating method. The third-order nonlinear optical properties of TPEPADT in acetonitrile solution and TPEPADT-doped PMMA thin film were investigated by using the laser Z-scan technique at the wavelength 1064 nm with laser duration of 20 ps. The linear refractive index of the polymer thin film was also studied. The Z-scan curves revealed that both TPEPADT in acetonitrile solution and the polymer thin film possessed negative nonlinear refraction, exhibiting a self-defocusing effect and nonlinear absorption was negligible under the experimental conditions used. The nonlinear refractive index was calculated to be ?1.9 × 10^?18 m²/W for TPEPADT in acetonitrile solution and ?8.9 × 10^?15 m²/W for the polymer thin film. These results suggest that TPEPADT have potential for the application of all-optical switching devices.     

Diode-pumped actively Q-switched Nd:GGG laser operating at 938 nm

The stimulated emission cross-section of Nd:GGG crystal in 938 nm transition was measured by the amplifier approach. It is 2.3×10^?20 cm². A quasi-continuous-wave diode pumped, actively Q-switched Nd:GGG laser operating at 938 nm was demonstrated. Pumped by laser diodes with 900 W peak power and 300 μs pulse duration, it generated 168 mJ energy in long pulse mode. The slope efficiency was 36%. Q-switched by a KD?P Pockels cell, 41 mJ output pulse energy was obtained. The pulse duration and peak power were 120 ns and 340 kW, respectively. The optical to optical efficiency was 7%.     

Laser-ablation-induced refractive index fields studied using pulsed digital holographic interferometry

Pulsed digital holographic interferometry has been used to investigate the plume and the shock wave generated in the ablation process of a Q-switched Nd-YAG (λ=1064 nm and pulse duration=12 ns) laser pulse on a polycrystalline boron nitride (PCBN) target under atmospheric air pressure. A special setup based on two synchronised wavelengths from the same laser for simultaneous processing and measurement has been used. Digital holograms were recorded for different time delays using collimated laser light (λ=532 nm) passed through the volume along the target. Numerical data of the integrated refractive index field were calculated and presented as phase maps showing the propagation of the shock wave and the plume generated by the process. Radon inversion has been used to estimate the 3D refractive index fields measured from the projections assuming rotational symmetry. The shock wave density has been calculated using the point explosion model and the shock wave condition equation and its behaviour with time at different power densities ranging from 1.4 to 9.1 GW/cm² is presented. Shock front densities have been calculated from the reconstructed refractive index fields using the Gladstone–Dale equation. A comparison of the shock front density calculated from the reconstructed data and that calculated using the point explosion model at different time delays has been done. The comparison shows quite good agreement between the model and the experimental data. Finally the reconstructed refractive index field has been used to estimate the electron number density distribution within the laser-induced plasma. The electron number density behaviour with distance from the target at different power densities and its behaviour with time are shown. The electron number densities are found to be in the order of 10¹⁸ cm^?3 and decay at a rate of 3×10¹⁵ electrons/cm³ ns.     

Optical temperature sensor through infrared excited blue upconversion emission in Tm3 +/Yb3 + codoped Y2O3

An analysis of the intense blue upconversion emission at 476 and 488 nm in Tm^3 +/Yb^3 + codoped Y₂O₃ under excitation power density of 86.7 W/cm² available from a diode laser emitting at 976 nm, has been undertaken. Fluorescence intensity ratio (FIR) variation of temperature-sensitive blue upconversion emission at 476 and 488 nm in this material was recorded in the temperature range from 303 to 753 K. The maximum sensitivity derived from the FIR technique of the blue upconversion emission is approximately 0.0035 K^? 1. The results imply that Tm^3 +/Yb^3 + codoped Y₂O₃ is a potential candidate for the optical temperature sensor.     

Nonlinear refractive index of some anthraquinone dyes in 1294-1b liquid crystal

Third order nonlinear refractive index of three anthraquinone dyes, i.e., Solvent Blue 59, Solvent Blue 35 and Solvent Green 3 doped in 1294-1b nematic liquid crystal (NLC) were studied by the single beam Z-scan technique using a continuous-wave He–Ne laser at 632.8 nm. The negative nonlinear refractive index (n₂) in the order of 10^? 5 cm²/w for all samples was obtained. We believe that, this large nonlinearity is owing to Janossy effect and the difference in the nonlinear refractive index of our dyes can be described by the structures of dyes and the interactions between dyes and 1294-1b molecules. So as to understand the effect of dye structure on nonlinearity enhancement, the dichroic ratio of these dyes in 1294-1b was measured using polarized spectroscopy.     

Variety of nanostructures induced by femtosecond laser pulses on surface of Au/Cr film stack

Several nanostructures were obtained after irradiation with femtosecond laser pulse (130 fs, 800 nm, 1 kHz pulse repetition frequency) on Au/Cr film stack. The influence of laser parameters such as fluence (0.5 J/cm², 1.5 J/cm², 3 J/cm²) and the number of pulse were investigated. With single pulse irradiation, the nanoline and nonoparticle were obtained for the pulse fluence of 0.5 J/cm² and 3 J/cm², respectively. The formation mechanism of those nanostructures was discussed. The results of this experiment demonstrate that different kinds of nanostructures could be formed by varying the laser parameters such as fluence and the number of pulse.     

Cr4+:YAG passively Q-switched c-cut Nd:YVO4 self-Raman laser at 1168.6 nm

We demonstrate the first Cr⁴⁺:YAG passively Q-switched c-cut Nd:YVO₄ self-Raman laser at 1168.6 nm based on the Stokes shift of 816 cm⁻¹. At the pump power of 4.7 W, the maximum output power of the Stokes line at 1168.6 nm is 270.5 mW, corresponding to an optical conversion efficiency of 5.8%. The pulse width, pulse repetition rate, pulse energy and peak power are 8.8 ns, 35.8 kHz, 7.6 μJ and 0.86 kW, respectively. At the pump of 5.0 W, the Stokes line at 1097.2 nm based on Raman shift of 259 cm⁻¹ also appears.     

Third-order optical nonlinearities of an organometallic complex: Bis(tetra-n-propylammonium)bis(2-thioxo-1,3-dithiole-4,5-dithiolato)cuprate(II) doped in solid PMMA films

An organometallic complex, bis(tetra-n-propylammonium)bis(2-thioxo-1,3-dithiole-4,5-dithiolato)cuprate(II), [(C₃H₇)₄N]₂[Cu(dmit)₂] (dmit^2?=4,5-dithiolate-1,3-dithiole-2-thione), abbreviated as PrCu, was synthesized. The films of PrCu were prepared using spin coating method. The third-order nonlinear optical properties of PrCu in acetone solution and PMMA films were investigated by Z-scan technique at 1064 nm with laser duration of 20 ps. The Z-scan spectra reveal that the composite films exhibit large negative nonlinear refractive indices of the order of 10^?15 m²/W, which are three orders larger than that in acetone solution. The nonlinear absorption coefficients were calculated to be 9.416×10^?10 m/W. For the composite films, the figure of merit, W and T, meet the requirement of all-optical switching devices. The experimental results show that the PrCu-doped PMMA films have potential applications for nonlinear optical devices.     

Cascaded continuous-wave singly resonant optical parametric oscillator pumped by a single-frequency fiber laser

We present a cascaded continuous-wave singly resonant optical parametric oscillator (SRO) delivering idler output in mid-IR and terahertz frequency range. The SRO was pumped by an ytterbium-doped fiber laser with 27 W linear polarization pump powers, and based on periodically poled MgO:LiNbO₃ crystal (PPMgLN) in two-mirror linear cavity. The PPMgLN is 50 mm long with 29.5 μm period. The idler power output at 3811 nm was obtained 2.6 W. The additional spectral components that have been attributed to cascaded optical parametric processes are described at increasing pump levels. Besides the initial signal component at about 1476.8 nm, further generated wavelengths with frequency shifts about 47 cm^?1, 94 cm^?1 and 104 cm^?1 were observed. It was speculated that the idler waves lie in the terahertz (THz) domain from the observed results.     

Growth and characterization of Nd:Bi12SiO20 single crystal

A Nd:Bi₁₂SiO₂₀ crystal was grown by the Czochralski method. The thermal properties of the crystal were systematically studied. The thermal expansion coefficient was measured to be α=11.42×10^?6 K^?1 over the temperature range of 295–775 K, and the specific heat and thermal diffusion coefficient were measured to be 0.243 Jg^?1 k^?1 and 0.584 mm²/s, respectively at 302 K. The density was measured to be 9.361 g/cm³ by the buoyancy method. The thermal conductivity of Nd:Bi₁₂SiO₂₀ was calculated to be 1.328 Wm^?1 K^?1 at room temperature (302 K). The refractive index of Nd:Bi₁₂SiO₂₀ was measured at room temperature at eight different wavelengths. The absorption and emission spectra were also measured at room temperature. Continuous-wave (CW) laser output of a Nd:Bi₁₂SiO₂₀ crystal pumped by a laser diode (LD) at 1071.5 nm was achieved with an output power of 65 mW. To our knowledge, this is the first time LD pumped laser output in this crystal has been obtained. These results show that Nd:Bi₁₂SiO₂₀ can serve as a laser crystal.     

Behaviour of scintillators under XUV free electron laser radiation

Free electron lasers (FEL) are new generation accelerator-based short wavelength light sources providing high pulse intensity and femtosecond pulse duration, which enable investigation of interaction of elementary excitations in solids under extreme conditions. Using the FLASH facility of HASYLAB at DESY (Hamburg, Germany), we investigated the response of different materials with scintillating properties based on intrinsic emissions to the 25.6 and 13.8 nm FEL radiation by means of time-resolved luminescence spectroscopy. FLASH delivered single pulses of 25 fs duration having energy per pulse up to 30 μJ resulting in power densities of ～10¹² W/cm² on crystals. As a function of excitation density we observed the shortening of lifetime and non-exponential behaviour of emission decays in CaWO₄, while the emission spectra recorded are comparable to those obtained at conventional excitation sources.     

Effect of Ag addition on the third-order nonlinearity and physicochemical property of chalcohalide glass

In this work, the 70GeS₂–20In₂S₃–10CsI glass introduced with 0–10 mol% Ag₂S were prepared by the vacuumed melt-quenching technique. The physicochemical properties, such as glass transition temperature, density, refractive index, transmittance, hardness as well as third-order nonlinearity are investigated with the increasing Ag₂S contents. It was found that the refractive index (@632.8 nm), density, and hardness of glasses increase distinctly from 2.204 to 2.270, from 3.520 to 3.675 g cm⁻³ and from 180.9 to 227.9 kg mm⁻², respectively. Meanwhile, the nonlinear refractive index increases from 3.2 × 10⁻¹⁸ to 4.6 × 10⁻¹⁸ m²/W due to the increased refractive index. Finally, the Raman spectra are performed to structurally illustrate the role of Ag addition on the changes of the physicochemical properties. With the Ag₂S contents increasing, the vibration intensity of the [InS₄] and [InS₃I] tetrahedrons increases and the heavy Ag atoms result in the increased density and refractive index, as well as the nonlinear refractive index. The Ag-containing glass, which exhibited good thermal stability, excellent infrared transparency and ultrafast nonlinear optical properties, can be find applications for the IR window material or ultrafast infrared optics.     

Terahertz emission dependence on the intensity ratio of 400–800 nm in generating terahertz waves from two-color laser-induced gas plasma

A transient photocurrent model is used to explain terahertz emission from gas plasma irradiated by a laser pulse and the second harmonic. By introducing the second harmonic, 400 nm, the corresponding terahertz emission is greatly enhanced. The exact dependence of terahertz emission on the intensity ratio of 400–800 nm is studied for the case with total intensity of 5.00 × 10¹⁴ W/cm². Results show the emission reaches the maximum at about the case for energy distribution of I_ω = 4.00 × 10¹⁴ W/cm², I_2ω = 1.00 × 10¹⁴ W/cm².     

Generation of tunable and narrow linewidth continuous-wave yellow laser by sum–frequency mixing of diode-pumped solid-state Nd:YAG ring lasers

We report a tunable, narrow linewidth and high beam quality continuous-wave (CW) yellow laser system at 589 nm. The system is an all solid-state design employing single-pass sum–frequency generation in a KTP crystal by mixing the 1064 nm with 1319 nm lines of two side-pumped Nd:YAG enforcing unidirectional ring lasers. With this method, a CW yellow laser at 589.159 nm with an output power of 0.8 W, a linewidth less than 1.5 GHz and a beam quality M² = 1.29 is obtained. The wavelength of the laser also can be precisely tuned from 589.112 to 589.181 nm in step-length of about 0.22 pm.