[en] Second harmonic generation from a frequency stable Nd : YAG laser using an external cavity is studied. Noncritically phase matched KNbO₃ crystal is used for efficient generation of 532 nm light. Maximum 65% conversion of the infrared to green light is achieved.

[en] Second harmonic generation(SHG)is widely used process in nonlinear optical spectroscopy. If the input light field is Gaussian beam, the dynamics of SHG is determined by the power of the input beam, the spatial shape of the input beam, its confocal parameter, the Poynting vector walk off length, the phase matching condition and so on. Our goal in this study is to investigate the spatial filtering process by aperture, the spatial shape of the focused input fundamental beam and its confocal parameter. Particularly we study the power characteristic of cw SHG for the periodically poled lithium niobate

[en] The fluorescence upconversion setup for the detection of photoluminescence (PL) decay lifetime with subpicosecond time resolution was constructed, and the photoluminescence phenomena of several hemicyanine dyes with alkyl chains of different chain lengths tethered to the N atom of the pyridine moiety (HC-n, n=6, 15, 22) in methanol were investigated. The average decay lifetimes of the solutions determined from the measured data by multi-order exponential decay curve fitting were ∼27 ps at the PL peak wavelength. It was found that the PL decay properties did not depend on the alkyl chain length in the molecule, implying that the twist of the alkylpyridinium ring of the molecule is not possible as a nonfluorescing relaxation pathway. The time-dependent PL spectra constructed from the PL lifetime data showed the dynamic Stokes shift of ∼1000 cm^-1

[en] We report a simple method for determining photovoltaic constant and conductivity of any photorefractive crystal which have no inversion symmetry by utilizing the electric field dependence of non-phase-matched second harmonic generation. The photovoltaic constant, dark conductivity and photoconductivity are easily deduced from an analysis of the observed relaxation behavior of space-charge field at two different light intensities for photorefraction. We demonstrate this method for LiNbO₃ doped with 0.1 mole% Fe at 514.5 nm. We derived the theoretical expression for the DC electric field dependence of he intensity of second harmonic wave generated under phase miss-matched condition

[en] Third-order optical nonlinearities of poly (3-dodecylthiophene) thin films were investigated using pulsed and continuous-wave (cw) lasers. The nonlinearity measured by the z-scan method at 532 nm (second harmonic output of the Nd:YAG laser) with nanosecond laser pulses yielded the value χ⁽³⁾∼10^-7 esu, similar to the previous experimental results. The value of the off-diagonal element χ⁽³⁾ determined from the nanosecond degenerate four-wave mixing method indicated that the nonlinearity with the pulsed laser input mainly electronic in origin. On the other hand, the nonlinearity measured using the z-scan method with a cw He-Ne laser at 633 nm was more than 10^-4 esu, too large to be accounted for by an electronic or saturation effect. Local heating in the polymer film by light absorption is thought to be a cause of the large nonlinearity. This temperature increase at the beam center by the He-Ne laser beam was estimated from measurements of the UV-visible absorption spectra and the refractive indices of the polymer film at various temperatures

[en] We demonstrate an experimental method for measuring sub-second temporal evolution of the internal field after polarization reversal in ferroelectrics, lithium niobate and lithium tantalate for quasi-phase matching(QPM) applications. In each crystal the internal field exhibited two distinct relaxations, suggesting that they originate from two different types of defects. The fast one decays in sub-second range while the slow one lasts over days at room temperature. The sub-second relaxation could be fit to a stretched exponential. This method can probe microscopic electrical properties related to the defects, and can be used to assess ferroelectric crystals for QPM applications in terms of intrinsic/extrinsic defects

[en] The optical parametric oscillation (OPO) characteristics of LiNbO₃ normally pumped by 1.064 μ m laser output from the nanosecond Nd:YAG laser were studied. The 5 cm long LiNbO₃ crystal was cut at θ = 47^o and the OPO cavity was formed by two plain mirrors. The output coupler reflectivity in the 1.40-1.60 μ m range was 99.5 percent and back mirror in the 1.45-1.55 μ m range was 80 percent. At the cavity length of 7 cm and the pump pulse energy of 156 mJ, the optical parametric oscillator generates the nanosecond pulses (signal 1.6 μ m, idler 3.18 μ m) up to 16 mJ. It was found that the output energy was changed as the cavity length was varied from 7 cm to 16 cm at the fixed pump pulse energy of 145 mJ. (author)

[en] For the periodic poling of Lithium Niobate, domain walls typically want to align with the x-planes and therefore the determination of X and Y axes is very important. However, this is not so trivial, normally requiring the x-ray diffraction measurements.We propose a simple method for determining these axes of the nonlinear uniaxial crystals such as Lithium Niobate by suing the second-harmonic generation. For several uniaxial crystals (with 3 m, 32, 62 m and 42 m point groups), the intensity change of the second harmonic output is calculated as the crystal is rotated azimuthally with respect to Z-axis. The appropriateness of our method is confirmed through the second harmonic transmission experiment on Lithium Niobate (3 m point group) and the second harmonic reflection experiment on quartz (32 point group) crystals.

[en] Polyimide materials for NLO applications have attracted attentions because of their high glass transition temperature (T_g), which can be utilized to stabilize the dipole orientation of the NLO chromophore at high temperatures. Another attractive merit of polyimides is their high thermal stability that enables them to endure at elevated temperature during operation in electro-optic devices. There are two types of NLO polyimides, which have been used either as polymer hosts for composite materials or as polymer backbones for side-chain NLO chromophores. Side-chain polymer systems have the advantages such as homogeneity and high level of NLO chromophore relative to the host-guest systems, which suffer from poor homogeneity of chromophores and diffusion of the chromophores at high temperatures. The number average molecular weight (Mn) determined by gel permeation chromatography (GPC) was 23100 (M_w/M_n = 1.64) for polymer 4. The polyimide 4 was soluble in common solvents such as acetone, DMF, and DMSO, but was not soluble in methanol and diethyl ether

[en] The optical parametric oscillation (OPO) characteristics of LiNbO₃, which is normally pumped by using the 1.064-μm laser output from a nanosecond Nd:YAG laser, were investigated. A 5-cm-long LiNbO₃ crystal was cut at θ=47 .deg. , and the OPO cavity was formed by using two plain mirrors. The output coupler reflectivity in the 1.40 - 1.60 μm range was 80.0%, and the back mirror had a reflectivity of 99.5% in the 1.45 - 1.55 μm range. At a cavity length of 7 cm and a pump pulse energy of 156 mJ (1.7 times above threshold), the optical parametric oscillator generated nanosecond pulses (signal 1.6 μm and idler 3.18 μm) up to 16mJ. The change in the output energy was studied as the cavity length was varied from 7 cm to 16 cm at a fixed pump pulse energy of 145 mJ. We also measured the OPO output energy as a function of the reflectivity of the output coupler