Characterization of second and third order optical nonlinearities of ZnO sputtered films

We measured the second and third order optical nonlinearity of zinc oxide, grown on glass substrates by the ion beam sputtering technique. Second and third harmonic generation measurements were performed by means of the rotational Maker fringes technique for different polarization configurations, thus allowing the determination of all non-zero components of the second order susceptibility at three different fundamental beam wavelengths, i.e., 1064 nm, 1542 nm and 1907 nm. The dispersion of the nonlinear optical coefficients has been evaluated, while the nonlinear optical coefficients were found to range between 0.9 pm/V and 0.16 pm/V for d₃₃, 0.53 pm/V and 0.08 pm/V for |d₁₅|, 0.31 and 0.08 pm/V for |d₃₁|, with increasing wavelength. Finally, one third order susceptibility, χ_ijkl ⁽³⁾, has been determined by third harmonic generation measurements at a fundamental wavelength λ=1907 nm and a value for χ₃₃₃₃ ⁽³⁾ of 185×10^-20 m²/V² has been found. PACS 42.65.An; 42.65.Ky; 42.70.Nq     

Method for measurements of second-order nonlinear optical coefficient based on Z-scan

The method for measuring second-order nonlinear optical coefficients based on well-known Z-scan is presented. The influence of linear absorption coefficients on normalized transmittance is discussed. Using this method, we obtained the second-order nonlinear coefficient d₃₁(5%MgO:LiNbO₃) = 4.5 × 10⁻¹² m/v at 1064 nm, which agrees well with theoretical calculations and previous well-known values.     

Thermally Stable and Resonance Enhanced Large Nonlinear Optical Properties In Poled Silica Films Doped with Thiazole Azo Dye

Sol-gel processed poled silica films doped with novel nonlinear organic materials, thiazole azo dye (TA), are prepared in high concentration. Resonance enhanced second-order nonlinear optical coefficient of d₃₃ = 154 pm/V is obtained from TA doped silica film (40 wt%) for the fundamental wavelength of 1064 nm. The value d₃₃ is unvaried for at least 100 h under a high temperature condition (100°C) without any relaxation. Wavelength dependent second-order nonlinearity is also measured.     

Measurement of Second-Order Optical Nonlinear Coefficient from the Absolute Radiant Power of Parametric Fluorescence in LiI03

This work presents a new technique for the measurement of second-order optical nonlinear coefficients from the absolute radiant power of the parametric fluorescence emission. It is based on calibrated detectors regarded as secondary radiometric standards. This technique makes it possible to skip a number of problems usually encountered with standard techniques based on calibrated neutral density filters and reference sources of light at much higher power levels. We infer a value for the d₃₁ nonlinear coefficient of LiIO₃ from spectral measurements of the fluorescence power integrated over all solid angles, by denning the wavelength bandwidths with interference filters. Moreover, by performing the measurements on the ultraviolet and visible emission lines of an argon ion laser in the range from 351 to 488 nm, the wavelength dependence of d₃₁ is compared with the expected one obtained by Miller’s dispersion law.     

Studies of the second-order nonlinear optical properties of cubic boron nitride

Cubic boron nitride (cBN) is a kind of artificial wide-energy-gap semiconductor crystal, which has zinc blende structure with Td symmetry. The second-order nonlinear optical properties of cBN single crystal were investigated for the first time. Using a Q-switching Nd:YAG laser, the optical rectification and the second-harmonic generation at 532 nm from cBN single crystal were observed. In order to determine the nonlinear optical coefficient of a minute-size cBN sample, a simple method based on modified transverse linear electro-optic modulation was also introduced. This approach is convenient because it is unnecessary to know the absolute intensity of a probing beam for measuring the half-wave voltage of a cBN sample. Finally, cBN’s linear electro-optic coefficient γ₄₁=1.07×10^-14 m/V and nonlinear optical coefficient d₁₄(0,ω,-ω)=1.07×10^-13 m/V were obtained. PACS 42.65.An; 42.65.Ky; 78.20.Jq     

Enhancement of optical nonlinearity in a guest-host system by nematic ordering

Second-harmonic generation (SHG) has been studied for understanding the enhancement mechanism for the second-order optical nonlinearity by the nematic (or axial) ordering in a liquid crystal doped with one-dimensional nonlinear optical (NLO) organic molecules. An extended version of the Maier-Saupe mean-field theory for nematic liquid crystals was developed to obtain analytical expressions for the second-order NLO coefficients in terms of the axial order, the polar order and the effective nematic potential. From the SHG data in a guest-host system composed ofN,N'-dimethylaminonitrostilbene molecules (0.5% by weight) and a liquid crystal, the enhancement of the second-order NLO coefficient,d ₃₃, by nematic ordering becomes almost 3, which agrees well with our theoretical predictions.     

Preparation and optical properties of HgWO<Subscript>4</Subscript> nanorods by hydrothermal method coupled with ultrasonic technique

For the first time, a newly luminescent nanomaterial, monoclinic wolframite-type HgWO₄ nanorods (diameter: ∼200 nm; length: ~2000 nm) are prepared by hydrothermal method together with ultrasonic technique. Fluorescent (FL) and UV–Vis results both show that for HgWO₄, ultrasonic irradiation procedure will change its optical behaviors greatly. When the crystals become into nanorods, the fluorescent emitting peaks (365 and 495 nm) shift to central region, and finally form a wider one at 435 nm. Similar results of UV–visible absorption peaks are observed for these two products. FTIR spectra further characterize their structure. All above unique optical performances might result from both small sizes caused by ultrasonic irradiation procedure and involvement of incompact d¹⁰ electrons. Moreover, possible synthesis mechanisms of HgWO₄ nanorods are also investigated.     

Optical characterization of poly (ethylene oxide)/zinc oxide thin films

The optical characterization of poly (ethylene oxide)/zinc oxide thin films has been done by analyzing the absorption spectra in the spectral wavelength region 380–800 nm using a ultraviolet-spectrophotometer at room temperature. Thin film polymer composites made of poly (ethylene oxide) (PEO) containing zinc oxide (ZnO) filler concentrations (0%, 2%, 6%, 10%, and 14%) by weight were used in this study. The optical results obtained were analyzed in terms of the absorption formula for non-crystalline materials. The optical energy gap and other basic optical constants such as dielectric constants and optical conductivity were investigated and showed a clear dependence on the ZnO filler concentration. It was found that the optical energy gap for the composite films is less than that for the neat PEO, and that it decreases as the ZnO concentration increases. Enhancement of the optical conductivity was observed with increase in the ZnO concentration. Dispersion of refractive index was analyzed using the Wemple–DiDomenico single oscillator model. The refractive index (n), extinction coefficient (k), and dispersion parameters (E_o, E_d) were calculated for the investigated films.     

Nonlinear optical and optical limiting properties of individual single-walled carbon nanotubes

A large number of individual single-walled carbon nanotubes (SWNTs) were obtained by dilution of nanotube dispersions in N-methyl-2-pyrrolidone (NMP). Up to 70% individual SWNTs are contained in the NMP dispersions with concentrations of less than 4.0×10^-3 mg/mL. The nonlinear optical and optical limiting properties of SWNT dispersions were studied by using the Z-scan technique at 532 nm. As the concentration of SWNTs is increased, the nonlinear extinction (NLE) and optical limiting effects improve significantly, while the limiting thresholds decrease gradually. The individual SWNTs show similar NLE effect to zinc phthalocyanine nanoparticles, while also exhibiting larger NLE coefficients than Mo₆S_4.5I_4.5 nanowires.     

Fabrication of ZnO nanorods for optoelectronic device applications

Hydroxyl free zinc oxide nanorods have been synthesized by a catalyst free surfactant based one-step solid state reaction process. The powder X-ray diffraction studies reveal well defined wurtzite peaks due to crystalline ZnO, while optical absorption spectra represent prominent exciton absorption and remarkable blueshift in the onset of absorption. As predicted by transmission electron microscopy, the ZnO nanorods are ∼100 nm long and of ∼20 nm dia. Further, luminescence aspects of such nanorods are studied for possible deployment in optoelectronics devices.      

A study on morphology control and optical properties of ZnO nanorods synthesized by microwave heating

In this study, we present morphology control investigations on zinc oxide (ZnO) nanorods synthesized by microwave heating of a mixture of zinc nitrate hexahydrate and hexamethylenetetramine (HMTA) precursors in deionized water (DI water). To study the morphology and structural variations of the obtained ZnO nanorods in different molar ratio of zinc nitrate hexahydrate to HMTA, X-ray diffraction (XRD), scanning electron microscopy (SEM) images, Raman scattering, and photoluminescence (PL) spectroscopy were measured. XRD and SEM images are utilized to examine the crystalline quality as well as the morphological properties of the ZnO nanorods. It is found that morphology control can be achieved by simply adjusting the reactant concentrations and the molar ratio of zinc nitrate hexahydrate to HMTA. Raman scattering and PL spectroscopy measurements were demonstrated to study the size- and shape-dependent optical response of the ZnO nanorods. The Raman scattering result shows that the intensity of LO mode at around 576 cm^?1 decreases with the increase in the molar ratio of zinc nitrate hexahydrate to HMTA, indicating the reduction of defect concentrations in the synthesized ZnO nanorods. Room temperature PL spectrum of the synthesized ZnO nanorods reveals an ultraviolet (UV) emission peak and a broad visible emission. An enhancement of UV emission appears in the PL spectra as the molar ratio of zinc nitrate hexahydrate to HMTA increases, indicating that the defect concentration of the synthesized ZnO nanorods can be reduced by increasing the molar ratio.     

Linear and Nonlinear Optical Properties Of Racemic (±)2-(α-Methylbenzylamino)-5-Nitropyridine Single Crystals

Linear and nonlinear optical properties of racemic (±)2-(α-methylbenzylamino)-5-nitropyridine ((±)MBANP) single crystals have been comprehensively investigated and compared with those of the enantiomorph (–)2-(α-methylbenzylamino)-5-nitropyridine ((–)MBANP) crystals. (±)MBANP crystal exhibits very high chemical and physical stability, but relatively small nonlinear optical coefficients (d₃₁ = 6.8 pm/V, d₃₂ = 4.7 pm/V, d₃₃ = 0.84 pm/V). A comparison between the nonlinear optical coefficients of (±)MBANP and (–)MBANP demonstrates the validity of the oriented-gas model in molecular crystals that neglects all the contributions from intermolecular interaction.     

Manifestation of nano-sized effects in photoinduced nonlinear optics of CdI2–Cu layered single crystals

The infrared (IR) induced second harmonic generation in the thin CdI₂–Cu crystalline layers was discovered. With decreasing thickness of the CdI₂–Cu crystals up to several nanometers value of second-order nonlinear optical susceptibility d₁₄ (at wavelength λ=2.76 μm) for the output IR-induced SHG significantly increases. For the Cu content about 0.4% the output SHG signal achieves its maximum at the sample thickness below 2 nm. It is important that for the samples with larger film thickness the corresponding changes have substantially different properties. Limitation of the d₁₄ at the larger concentration is probably caused by formation of the Cu clusters limiting the enhancement of the hyperpolarizability for particular cluster as well as total nonlinear dielectric susceptibility.     

Room temperature synthesis of needle-shaped ZnO nanorods via sonochemical method

Single crystalline needle-shaped zinc oxide nanorods were synthesized via sonochemical methods using zinc acetate dihydrate and sodium hydroxide at room temperature. Morphological investigation revealed that the nanoneedles are of hexagonal surfaces along the length. The typical diameter and length vary from 120 to 160 nm and 3 to 5 μm, respectively. Sonication time appears to be a critical parameter for the shape determination. Detailed structural characterization confirmed that the nanorods are single crystalline with wurtzite hexagonal phase. A standard peak of zinc oxide was observed at 520 cm⁻¹ from the Fourier transform infrared spectroscopy. The ultra-violet visible and room temperature photoluminescence (PL) spectroscopic results demonstrate that the synthesized material has good optical properties.     

Two-photon absorption of high-power picosecond pulses in PbWO<Subscript>4</Subscript>, ZnWO<Subscript>4</Subscript>, PbMoO<Subscript>4</Subscript>, and CaMoO<Subscript>4</Subscript> crystals

The nonlinear process of two-photon interband absorption is studied in tungstate and molybdate oxide crystals excited by a sequence of high-power picosecond pulses with a wavelength of 523.5 nm. The transmission of the crystals is measured for the excitation pulse intensity up to 100 GW/cm². The pulse intensity in the crystals initially transparent at a wavelength of 523.5 nm is strongly limited due to two-photon absorption (TPA), and the reciprocal transmission in PbWO₄ and ZnWO₄ crystals reaches 50–60. In all crystals, TPA induces long-lived one-photon absorption, which affects the nonlinear process dynamics and leads to a hysteresis in the dependence of the transmission on the laser excitation intensity. Absorption dichroism manifests itself in a significant difference in the transmission intensities when the principal orthogonal optical axes of the crystals are excited. The TPA coefficients are determined during the excitation of two optical axes of the crystals. TPA coefficients β for the crystals vary over a wide range, namely, from β = 2.4 cm/GW for PbMoO₄ to β = 0.14 cm/GW for CaMoO₄, and the values of β can differ almost threefold when different optical axes of a crystal are excited. Good agreement is achieved between the measured intensities limited by TPA and the estimates calculated from the measured nonlinear coefficients. Stimulated Raman scattering (SRS) upon excitation at a wavelength of 523.5 nm is only detected in two of the four crystals under study. The experimental results make it possible to explain the suppression of SRS by its competition with TPA, and the measured nonlinear coefficients are used to estimate this suppression.     

Effect of surface preparation on the morphology of ZnO nanorods

The effects of Si substrate orientation and surface treatment on the morphology and density of Zinc oxide (ZnO) nanorods were investigated. The size and density of ZnO nanorods were influenced by Si substrate orientation and surface preparation. ZnO nanorods synthesized on the ideally H-terminated Si(1 1 1) prepared with an NH₄F solution resulted in the biggest size and the lowest density. It is suggested that the smoother surface of the Si substrate and lattice shape match with a larger atomic distance result in the increase of the ZnO seedlayer's grain size, which in turn enhances the size of ZnO nanorods grown on it. The optical properties of the ZnO nanorods were affected by their size and crystallinity. The smallest ZnO nanorods with a preferential c-axis orientation synthesized on the HF-treated Si(1 1 1) surface showed the highest intensity ratio of UV to visible emission, and the biggest ZnO nanorods synthesized on the N₂-sparged NH₄F-treated Si(1 1 1) surface showed the lowest intensity ratio of UV to visible emission. Therefore, it can be concluded that Si substrate orientation and surface preparation significantly affect the optical properties of ZnO nanorods.     

Studies on structural and magnetic properties of Co-doped pyramidal ZnO nanorods synthesized by solution growth technique

Large-area arrays of highly oriented Co-doped ZnO nanorods with pyramidal hexagonal structure are grown on silica substrates by wet chemical decomposition of zinc–amino complex in an aqueous medium. In case of undoped ZnO with an equi-molar ratio of Zn²⁺/hexamethylenetetramine (HMT), highly crystalline nanorods were obtained, whereas for Co-doped ZnO, good quality nanorods were formed at a higher Zn²⁺/HMT molar ratio of 4:1. Scanning electron microscope (SEM) studies show the growth of hexagonal-shaped nanorods in a direction nearly perpendicular to the substrate surface with a tip size of ～50 nm and aspect ratio around 10. The XRD studies show the formation of hexagonal phase pure ZnO with c-axis preferred orientation. The doping of Co ions in ZnO nanorods was confirmed by observation of absorption bands at 658, 617 and 566 nm in the UV–vis spectra of the samples. The optical studies also suggest Co ions to be present both in +2 and +3 oxidation states. From the photoluminescence studies, a defect-related emission is observed in an undoped sample of ZnO at 567 nm. This emission is significantly quenched in Co-doped ZnO samples. Further, the Co-doped nanorods have been found to show ferromagnetic behavior at room temperature from vibrating sample magnetometer (VSM) studies.     

Synthesis and photoluminescent and nonlinear optical properties of manganese doped ZnS nanoparticles

In this work we synthesized ZnS:Mn²⁺ nanoparticles by chemical method using PVP (polyvinylpyrrolidone) as a capping agent in aqueous solution. The structure and optical properties of the resultant product were characterized using UV-vis optical spectroscopy, X-ray diffraction (XRD), photoluminescence (PL) and z-scan techniques. UV-vis spectra for all samples showed an excitonic peak at around 292 nm, indicating that concentration of Mn²⁺ ions does not alter the band gap of nanoparticles. XRD patterns showed that the ZnS:Mn²⁺ nanoparticles have zinc blende structure with the average crystalline sizes of about 2 nm. The room temperature photoluminescence (PL) spectrum of ZnS:Mn²⁺ exhibited an orange-red emission at 594 nm due to the ⁴T₁-⁶A₁ transition in Mn²⁺. The PL intensity increased with increase in the Mn²⁺ ion concentration. The second-order nonlinear optical properties of nanoparticles were studied using a continuous-wave (CW) He-Ne laser by z-scan technique. The nonlinear refractive indices of nanoparticles were in the order of 10⁻⁸ cm²/W with negative sign and the nonlinear absorption indices of these nanoparticles were obtained to be about 10⁻³ cm/W with positive sign.     

Photoelectric properties of ZnO: In nanorods/SiO2/Si heterostructure assembled in aqueous solution

In-doped zinc oxide (ZnO:In) nanorods were grown onto SiO₂/n-Si substrate without catalyst in aqueous solution. The ZnO:In nanorods/SiO₂/n-Si heterostructure photovoltaic device was prepared. The structural and photoelectric properties of the as-grown ZnO:In nanorods were analyzed. ZnO:In nanorods had a strong and broad UV surface photovoltage response in the range of 300–400 nm, and the bands were identified. The photoelectric conversion properties of ZnO:In nanorods/SiO₂/n-Si heterostructure were investigated. ZnO:In/SiO₂/n-Si heterostructure showed a wide range photocurrent spectral response with high intensity in the UV and visible region. The rectifying behavior of this heterostructure was observed. Moreover, the device had a low turn-on voltage and a high breakdown voltage. Current–voltage characteristic was studied for the heterostructure, and the open-circuit voltage and short-circuit current were obtained. PACS 73.40.Lq; 85.35.Be; 81.16.Dn     

Effect of laser annealing using high repetition rate pulsed laser on optical properties of phosphorus-ion-implanted ZnO nanorods

The effect of high repetition rate pulsed laser annealing with a KrF excimer laser on the optical properties of phosphorus-ion-implanted zinc oxide nanorods has been investigated. The recovery levels of phosphorus-ion-implanted zinc oxide nanorods have been measured by photoluminescence spectra and cathode luminescence images. Cathode luminescence disappeared over 300 nm below the surface due to the damage caused by ion implantation with an acceleration voltage of 25 kV. When the annealing was performed at a low repetition rate of the KrF excimer laser, cathode luminescence was recovered only in a shallow area below the surface. The depth of the annealed area was increased along with the repetition rate of the annealing laser. By optimizing the annealing conditions such as the repetition rate, the irradiation fluence and so on, we have succeeded in annealing the whole damaged area of over 300 nm in depth and in observing cathode luminescence. Thus, the effectiveness of high repetition rate pulsed laser annealing on phosphorus-ion-implanted zinc oxide nanorods was demonstrated.