Large optical nonlinearity in CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> thin films

CaCu₃Ti₄O₁₂ (CCTO) thin films were successfully prepared on LaAlO₃ substrates by pulsed laser deposition technique. We measured the nonlinear optical susceptibility of the thin films using Z-scan method at a wavelength of 532 nm with pulse durations of 25 ps and 7 ns. The large values of the third-order nonlinear optical susceptibility, χ ⁽³⁾, of the CCTO film were obtained to be 2.79×10⁻⁸ esu and 3.30×10⁻⁶ esu in picosecond and nanosecond time regimes, respectively, which are among the best results of some representative nonlinear optical materials. The origin of optical nonlinearity of CCTO films was discussed. The results indicate that the CCTO films on LaAlO₃ substrates are promising candidate materials for applications in nonlinear optical devices.     

Phase-coherent comparison of two optical frequency standards over 146 km using a telecommunication fiber link

We have explored the performance of two “dark fibers” of a commercial telecommunication fiber link for a remote comparison of optical clocks. The two fibers, linking the Leibniz University of Hanover (LUH) with the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, are connected in Hanover to form a total fiber length of 146 km. At PTB the performance of an optical frequency standard operating at 456 THz was imprinted to a cw transfer laser at 194 THz, and its frequency was transmitted over the fiber. In order to detect and compensate phase noise related to the optical fiber link we have built a low-noise optical fiber interferometer and investigated noise sources that affect the overall performance of the optical link. The frequency stability at the remote end has been measured using the clock laser of PTB’s Yb⁺ frequency standard operating at 344 THz. We show that the frequency of a frequency-stabilized fiber laser can be transmitted over a total fiber length of 146 km with a relative frequency uncertainty below 1×10⁻¹⁹, and short term frequency instability given by the fractional Allan deviation of σ _y (τ)=3.3×10⁻¹⁵/(τ/s).     

A high-stability semiconductor laser system for a <Superscript>88</Superscript>Sr-based optical lattice clock

We describe a frequency-stabilized diode laser at 698 nm used for high-resolution spectroscopy of the ¹S₀–³P₀ strontium clock transition. For the laser stabilization we use state-of-the-art symmetrically suspended optical cavities optimized for very low thermal noise at room temperature. Two-stage frequency stabilization to high-finesse optical cavities results in measured laser frequency noise about a factor of three above the cavity thermal noise between 2 Hz and 11 Hz. With this system, we demonstrate high-resolution remote spectroscopy on the ⁸⁸Sr clock transition by transferring the laser output over a phase noise-compensated 200-m-long fiber link between two separated laboratories. Our dedicated fiber link ensures a transfer of the optical carrier with frequency stability of 7×10⁻¹⁸ after 100 s integration time, which could enable the observation of the strontium clock transition with an atomic Q of 10¹⁴. Furthermore, with an eye toward the development of transportable optical clocks, we investigate how the complete laser system (laser+optics+cavity) can be influenced by environmental disturbances in terms of both short- and long-term frequency stability.     

Nonlinear optical properties of Cr<Superscript>3+</Superscript>-doped laser crystals

The nonlinear optical properties in six Cr³⁺-doped laser crystals LiCaAlF₆, LiSrGaAlF₆, Gd₃Ga₅O₁₂, Gd₃Sc₂Ga₃O₁₂, LaMgAl₁₁O₁₉ and Alexandrite are investigated with the help of the Z-Scan technique at λ = 532 nm in the CW regime. The data reported here include particularly the excited state absorption cross section and the third-order nonlinear susceptibilities. It is found that the three first systems only exhibit both refractive and absorptive nonlinear effects, whereas the three others have only absorptive effects. Gd₃Ga₅O₁₂ shows the best nonlinear potentialities. The excited state absorption cross section corresponding mainly to the ⁴T₂ →   ⁴T₁ transition is found to be ranging between 8.9 × 10⁻²² cm² in LiSGaF and 3.1 × 10⁻²⁰ cm² in LaMgAl₁₁O₁₉. The calculated ratio of the third order nonlinear susceptibility to the ground state absorption coefficient is found to be largest in GGG with a value of 146 × 10⁻⁶ esu.cm and smallest in Alexandrite (0.6 × 10⁻⁶ esu.cm).     

Near IR laser light visualizators using nonlinear GaSe and other layered crystallites

Two methods of preparation of the devices for visualization of pulsed and continuous near-IR (near infrared) are described and the results of conversion of pulsed and continuous IR (800–1360 nm) laser radiation into the visible range of spectra (400–680 nm) by using a transparent substrate covered with the particles (including nanoparticles) of effective nonlinear materials of GaSe _x S_{1 − x} (0.2 ≤ x ≤ 0.8) are presented. Converted light can be detected in transmission or reflection geometry as a visible spot corresponding to the real size of the incident laser beam. Developed device structures can be used for checking if the laser is working or not, for optical adjustment, for visualization of distribution of laser radiation over the cross of the beam and for investigation of the content of the laser radiation. Low energy (power density) limit for visualization of the IR laser pulses with 2–3 ps duration for these device structures are: between 4.6–2.1 μJ (3 × 10⁻⁴−1 × 10⁻⁴ W/cm²) at 1200 nm; between 8.4–2.6 μJ (4.7 × 10⁻⁴−1.5 × 10⁻⁴ W/cm²) at 1300 nm; between 14.4–8.1 μJ (8.2 × 10⁻⁴–4.6 × 10⁻⁴ W/cm²) at 1360 nm. Threshold damage density is more than 10 MW/cm² at λ = 1060 nm, pulse duration τ = 35 ps. The results are compared with commercially existing laser light visualizators.     

Large third-order nonlinear optical susceptibility of Au-Al2O3 composite films near the resonant frequency

2 O₃ composite films with high Au concentrations (30%–60% in volume fraction) were prepared by reactive co-sputtering and post rapidly thermal annealing. The structure of the films and the size distributions of the Au nanoclusters were examined by TEM, and the third-order nonlinear optical susceptibility χ⁽³⁾ was measured by degenerated four-wave mixing using a 70-ps pulse laser at 532 nm. The maximum value of the χ⁽³⁾ was about 1.2×10^-6 esu in the annealed films and occurred at around 45% Au concentration. The figure of merit, χ⁽³⁾/α (α is the absorption coefficient), has a value of 7×10^-12 esu cm over a wide range of Au concentrations. Received: 23 July 1997     

Nonlinear optical properties of nanometer-size silver composite azobenzene containing polydiacetylene film

Diacetylene monomer containing p-nitrophenyl azobenzene moiety (NADA) was synthesized. Silver nanoparticles with different concentrations were adulterated in the above polymerized NADA (PNADA) films and the third-order nonlinear optical properties were investigated in detail. UV–vis spectra and transmission electron microscopy were used to confirm the formation of PNADA/Ag nanocomposite films. The silver nanoparticles (average size of 10 nm) were well dispersed in the polymer films. The value of the nonlinear refractive index n ₂ for PNADA films (8.48×10⁻¹⁵ cm²/W) was much higher than that of pure polydiacetylene films. Further, the introduction of silver nanoparticles into the PNADA polymer films led to the further enhancement of nonlinear optical properties. The maximum value of n ₂ for PNADA/Ag nanocomposite films could be 11.6×10⁻¹⁵ cm²/W. This enhancement should be ascribed to the surface plasmon resonance of silver nanoparticles.     

Nonlinear optical characterization of LaEr(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> thin films using the <Emphasis Type="BoldItalic">Z</Emphasis> -scan technique

The nonlinear optical properties of thin films of LaEr(MoO₄)₃ were studied using a ∼30 ps Nd:YAG laser at 532 nm with a repetition rate of 250 Hz. Closed aperture Z-scan measurement revealed a negative nonlinearity in the LaEr(MoO₄)₃. The nonlinear refractive index γ=1.38×10^-10 cm²/W and nonlinear absorption coefficient β=16.8×10^-6 cm/W were calculated from the Z-scan data. The fluorescent upconversion spectra were recorded with 980 nm excitation. An optical switching mechanism based on nonlinear absorption is also presented experimentally. PACS 81.15.Fg; 77.84.Bw; 33.50.Dq; 42.70.Mp     

Femtosecond fiber laser based methane optical clock

An optical clock based on an Er³⁺ fiber femtosecond laser and a two-mode He–Ne/CH₄ optical frequency standard (λ=3.39 μm) is realized. Difference-frequency generation is used to down convert the 1.5-μm frequency comb of the Er³⁺ femtosecond laser to the 3.4-μm range. The generated infrared comb overlaps with the He–Ne/CH₄ laser wavelength and does not depend on the carrier–envelope offset frequency of the 1.5-μm comb. In this way a direct phase-coherent connection between the optical frequency of the He–Ne/CH₄ standard and the radio frequency pulse repetition rate of the fiber laser is established. The stability of the optical clock is measured against a commercial hydrogen maser. The measured relative instability is 1×10⁻¹² at 1 s and for averaging times less than 50 s it is determined by the microwave standard, while for longer times a drift of the He–Ne/CH₄ optical standard is dominant.     

Nonlinear optical absorption and refraction in Ru, Pd, and Au nanoparticle suspensions

We present the results of studies of the nonlinear optical properties of Pd, Ru, and Au nanoparticles. We studied the nonlinear refraction and nonlinear absorption of suspensions of these nanoparticles at 1064-nm wavelength. A relatively strong nonlinear absorption of the Pd nanoparticles was observed in the case of 1064-nm, 50-ps pulses (β=2×10⁻⁹ m W⁻¹). The Ru and Pd nanoparticles showed weak negative nonlinear refraction (γ∼−(6–8)×10⁻¹⁶ m² W⁻¹) in this spectral range. In the case of the Au nanoparticles, a saturated absorption at 532 nm dominated over other nonlinear optical processes.     

Nonlinear optical absorption and refraction of epitaxial Ba0.6Sr0.4TiO3 thin films on (001) MgO substrates

Highly epitaxial Ba_0.6Sr_0.4TiO₃ (BST) ferroelectric thin films were fabricated on (001) MgOsubstrates by pulsed laser deposition. The nonlinear optical absorption coefficients (β) and refraction indices (γ) of the BST thin films on (001) MgO substrates were investigated using the single beam Z-scan technique with femtosecond laser pulses at the wavelengths of 790 nm and 395 nm, respectively, at room temperature. The nonlinear absorption coefficients of BST thin films were measured to be ∼0.087 cm/GW and ∼0.77 cm/GW at 790 nm and 395 nm, respectively. The nonlinear refraction indices of BST thin films exhibit a strong dispersion from a positive value of 6.1×10^-5 cm²/GW at 790 nm to a negative value of -4.0×10^-5 cm²/GW at 395 nm near band gap. The dispersion of γ is roughly consistent with Sheik-Bahae’s theory for the bound electronic nonlinear refraction resulting from the two-photon resonance. These results show that the BST film is a promising material as a candidate for nonlinear optical applications. PACS 42.70.Mp; 78.20.-e; 81.05.-t     

The nonlinear optical property and photoinduced anisotropy of?a?novel azobenzene-containing fluorinated polyimide

A novel azobenzene-containing fluorinated polyimide was synthesized. The nonlinear optical property and photoinduced birefringence of a polyimide thin film were investigated. Large third-order nonlinear refraction (n ₂=−4.49×10⁻¹¹ cm²/W) was observed in the polyimide thin film by carrying out Z-scan measurement. The polyimide thin film exhibited larger nonlinear refraction than that of a mono-azo dye doped PMMA thin film (n ₂=−1.63×10⁻¹² cm²/W). The photoinduced birefringence of the polyimide thin film (▵ n∼10⁻²) under different pump intensities was investigated; it was much larger than that of the mono-azo dye doped PMMA thin film (▵ n∼10⁻³). Moreover, the time constants for birefringence growth and relaxation processes were determined.     

Nonlinear refraction of silver nanowires from nanosecond to femtosecond laser excitation

In order to investigate the effect of pulse width and solvent on the nonlinear properties of metal nanostructures, silver nanowires were fabricated in a direct current electric field (DCEF) using a solid-state ionic method and characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The nonlinear refractive index (γ) of silver nanowires suspended in ethanol was measured using the Z-scan technique and laser radiation of various (femto-, pico-, and nanosecond) pulse durations. Experimental results indicated that silver nanowires have obvious positive refractive nonlinearities and γ (the Kerr-induced self-focusing) increases as the pulse duration increases from 7.4×10⁻⁸ cm²/GW at 110 fs to 1.6×10⁻⁴ cm²/GW at 8 ns, due to the additional influence of the atomic reorientational Kerr effect in the case of longer pulses. Due to the solvent dependence of the nonlinear behavior of the silver nanowires, the nonlinear absorption and refraction of silver nanowires suspended in de-ionized water are smaller than those of silver samples suspended in ethanol. The thermal nonlinearities are insignificant in our experimental conditions.     

Nonlinear refraction of undoped and Fe-doped KTiOAsO4 crystals in the femtosecond regime

The third-order optical nonlinearities in undoped and Fe-doped KTA crystals have been measured using the Z-scan technique with femtosecond pulses at 780-nm wavelength. The nonlinear refractive index is determined to be 1.7×10⁻¹⁵ cm²/W and 0,9×10⁻¹⁵ cm²/W for undoped and Fe-doped KTA, respectively. No two-photon absorption occurs in these crystals. It is shown that doping with Fe₂O₃ could weaken refractive nonlinearity of KTA, suggesting that Fe:KTA will improve the performance of KTA in high-intensity femtosecond laser applications. In addition, the measured nonlinear index of refraction in KTA crystals is about five times lower than that predicted by the two-band theory. One of the reasons for the discrepancy is given as the applicable limit of the simple theory in which a two-parabolic band model has been assumed in the analysis. Received: 3 June 1999 / Published online: 20 October 1999     

Cilostazol determination by the enhancement of the green emission of Tb3+ optical sensor

The efficiency of excited-state interaction between Tb³⁺ and the industrial product Cilostazol (CIL) has been studied in different solvents. High luminescence intensity peak at 545 nm of terbium complex in acetonitrile was obtained. The photophysical properties of the green emissive Tb³⁺ complex have been elucidated, the terbium was used as optical sensor for the assessment of CIL in the pharmaceutical tablets and body fluids at pH 3.1 and λ_ex = 320 nm with a concentration range 1.0 × 10⁻⁹–1.0 × 10⁻⁶ mol L⁻¹ of CIL, correlation coefficient of 0.998 and detection limit of 7.5 × 10⁻¹⁰ mol L⁻¹.     

Generation of nanosecond terahertz pulses by the optical rectification method

The possibility of the generation of quasi-cw terahertz radiation by the optical rectification method for broad-band Fourier unlimited nanosecond laser pulses has been experimentally demonstrated. The broadband radiation of a LiF dye-center laser is used as a pump source of a nonlinear optical oscillator. The energy efficiency of terahertz optical frequency conversion in a periodically polarized lithium niobate crystal is 4 × 10⁻⁹ at a pump power density of 7 MW/cm².     

三-α-(2,4-二甲基-3-戊氧基)溴硼亚酞菁的三阶非线性光学性质研究

应用Z扫描技术，在波长为532nm、脉宽为8ns条件下，对一种新型亚酞菁配合物三-α-(2,4-二甲基-3-戊氧基)溴硼亚酞菁的三阶非线性光学特性进行了研究，结果发现，该新化合物有较强的反饱和吸收(β=3.7×10^-10 m/W)效应和非线性自聚焦折射效应(n₂=7.2×10^-11esu)，其三阶非线性极化率χ⁽³⁾=1.1×10^-11esu.分析了溴硼亚酞菁配合物特殊的锥形分子结构和重原子溴的轴向配位等因素对其三阶非线性光学性质的重要影响. 关键词： 亚酞菁 Z扫描 三阶光学非线性     

A narrow linewidth and frequency-stable probe laser source for the <Superscript>88</Superscript>Sr<Superscript>+</Superscript> single ion optical frequency standard

In this paper, we describe in detail a narrow linewidth and frequency-stable laser source used to probe the 5s ² S _1/2–4d ² D _5/2 clock transition of the ⁸⁸Sr⁺ optical frequency standard. The performance of the laser system is investigated with studies of its frequency drift rates and with high resolution spectra of the ⁸⁸Sr⁺ clock transition. The observed short-term drift rates are typically in the range of 10 to 23 mHz/s, and the current long-term drift rate is 13.9(3) mHz/s. The laser stability, after subtraction of linear drifts, reaches 5×10⁻¹⁶ at an averaging time of 3000 s. This high level of stability is attributed for the most part to stabilization of the reference cavity at the temperature where the coefficient of linear thermal expansion crosses zero. An upper bound for the laser linewidth is given by the observation of a Fourier-transform limited resonance of 4.3 Hz (Δν/ν=1×10⁻¹⁴) on the ⁸⁸Sr⁺ clock transition. The effective averaging time during the linewidth measurements was about 100 s.     

Thermal conductivity and thermal diffusivity of SiO<Subscript>2</Subscript> nanopowder

A 3ω approach for the simultaneous determination of the effective thermal conductivity and thermal diffusivity of nanopowder materials was developed. A 3ω experimental system was established, and the thermal properties of water and alcohol were measured to validate and estimate the accuracy of the current experimental system. The effective thermal conductivity and thermal diffusivity of the SiO₂ nanopowder with 375, 475, and 575 nm diameters were measured at 290–490 K and at different densities. At room temperature, the effective thermal conductivity and thermal diffusivity of the SiO₂ nanopowder increased with temperature; however, both values decreased as the particle diameter was reduced. An optimum SiO₂ powder density that decreased with decreasing diameter was also observed within the measurement range. The minimum effective thermal conductivity and maximum effective thermal diffusivity were obtained at 85 × 10⁻³ kg/L, when the particle diameter was 575 nm. The optimum densities of the particles with 375 and 475 nm diameters were less than 50.23 × 10⁻³ and 64.82 × 10⁻³ kg/L, respectively.     

SnO2 thin films grown by pulsed Nd:YAG laser deposition

SnO₂ thin films have been deposited on glass substrates by pulsed Nd:YAG laser at different oxygen pressures, and the effects of oxygen pressure on the physical properties of SnO₂ films have been investigated. The films were deposited at substrate temperature of 500°C in oxygen partial pressure between 5.0 and 125 mTorr. The thin films deposited between 5.0 to 50 mTorr showed evidence of diffraction peaks, but increasing the oxygen pressure up to 100 mTorr, three diffraction peaks (110), (101) and (211) were observed containing the SnO₂ tetragonal structure. The electrical resistivity was very sensitive to the oxygen pressure. At 100 mTorr the films showed electrical resistivity of 4×10⁻² Ω cm, free carrier density of 1.03×10¹⁹ cm⁻³, mobility of 10.26 cm² V⁻¹ s⁻¹ with average visible transmittance of ∼87%, and optical band gap of 3.6 eV.