Enhanced optical limiting performance in phthalocyanine-quantum dot nanocomposites by free-carrier absorption mechanism

Enhanced nonlinear optical properties (in dimethyl sulphoxide) is observed for 2(3),9(10),16(17),23(24)-tetrakis-(4-aminophenoxy)phthalocyaninato indium(III) chloride (InPc) when covalently linked to CdSe/ZnS or CdSe quantum dots (QDs). The experimental nonlinear optical parameters were obtained from Z-Scan measurements. Contributions from two-photon absorption (2PA) due to the InPc, and free-carrier absorption (FCA) by QDS have been identified as the main factors responsible for the enhanced optical limiting. The effective nonlinear absorption coefficient for InPc-CdSe/ZnS was found to be 700.0 cm/GW. The FCA cross-sections for InPc-CdSe/ZnS and InPc-CdSe composites were found to be 1.52 × 10⁻¹⁹ and 6.00 × 10⁻²⁰ cm² respectively. A much lower limiting threshold of 92 mJ cm⁻² was observed for InPc-CdSe/ZnS nanocomposite, hence, making it suitable for use as optical limiting material. Density Functional Theory (DFT) calculations on similar phthalocyanine-quantum dots system was modeled in order to explain the enhancement in the observed nonlinear optical properties of the Pc in the presence of the QDs. The experimentally determined nonlinear optical properties are well within the range of the DFT calculated properties.     

Experimental and theoretical characterization of semiorganic nonlinear optical material l-leucine hydrobromide

Semiorganic nonlinear optical material l-leucine hydrobromide (LEHBr) has been synthesized and single crystals were grown by solvent evaporation method at a constant temperature of 35 °C. CHN test, FTIR and XRD analysis were performed for identification of the material. Thermal and mechanical stability of the grown crystal were investigated by DTA-TGA analysis and microhardness measurement respectively. Suitability of LEHBr for NLO application was studied by optical transmission study and second harmonic generation (SHG) efficiency measurement. SHG efficiency was found to be 4 times of KDP. First hyperpolarizability, dipole moment and polarizability of LEHBr were calculated at the framework of Hartree–Fock (HF) with 6-311G(d) basis set. First order hyperpolarizability was found to be 3.233 × 10⁻³³ esu which is 4.9 times of urea.     

Synthesis and nonlinear optical properties of some donor–acceptor oxadiazoles

Heterocyclic 1,3,4-oxadiazole moiety has been used as a new π-conjugation relay to prepare a number of donor–acceptor molecules carrying a π-electron-rich aromatic ring as a donor and either a p-nitrophenyl or a pyridine/pyridinium ring as an acceptor. Several of these oxadiazoles display significant second-order molecular nonlinearity, β (14–124 × 10⁻³⁰ esu) and provide the basis for future design of efficient nonlinear optical materials having the oxadiazole core.     

Synthesis,z-scan and degenerate four wave mixing characterization of certain functionalized photosensitive polyesters containing ortho-hydroxyazo chromophores

The preparation and NLO characterization of photosensitive polyesters containing azoaromatic residues in the molecular backbone, functionalized with orthohydroxy chromophores is presented. Samples were studied for its UV–vis absorption, FT-IR and intensity dependent nonlinear absorption properties. Nonlinear characterization was carried out with z-scan using frequency doubled, Q-switched Nd:YAG laser operating at 532 nm. The closed aperture z-scan spectra reveal the self defocusing effects of the samples with negative nonlinearity coefficient (n₂) showing values as high as −1.28 × 10⁻¹⁰ (esu) for certain samples and the corresponding third order susceptibility coefficient of the order of 29.9 × 10⁻¹² (esu). Degenerate four wave mixing technique was employed to substantiate the findings. The numerical fits show that the molecules exhibit reverse saturable absorption. A study of beam fluence dependence of nonlinear absorption coefficient (β_eff) has been presented. All phenomena indicate that molecules are reverse saturable absorbers whose optical limiting property gets enhanced with increasing conjugation length.     

Nonlinear optical properties of the PbS nanorods synthesized via surfactant-assisted hydrolysis

PbS nanorods were synthesized by surfactant-assisted homogenous hydrolysis. The products were characterized by UV–vis spectrophotometer, X-ray powder diffraction (XRD) and transmission electron microscope (TEM). PbS nanorods were measured by the Z-scan technique to investigate the third-order nonlinear optical (NLO) properties. The result of the NLO measurements shows that the PbS nanorods have the third-order nonlinear optical properties of both NLO absorption and NLO refraction with self-focusing effects. The nonlinear absorption coefficient and refractive index of the PbS nanorods are 2.16 × 10^− 9 m/W and 3.52 × 10^− 16 m²/W respectively.     

Investigation on third order nonlinear optical,electrical and surface properties of organic stilbazolium crystal of 4-N,N-dimethylamino-N′-methylstilbazolium p-methoxybenzenesulfonate

The third order nonlinear optical, electric and dielectric properties of an organic stilbazolium derivative of 4-N,N-dimethylamino-N′-methylstilbazolium p-methoxybenzenesulfonate (DSMOS) crystal are reported. The nonlinear refractive index (n₂), two photon absorption coefficient (β) and third order optical susceptibility χ⁽³⁾ have been measured by Z-scan technique using Gaussian beam from the Nd:YAG laser at 1064 nm. The results show a large negative nonlinear refractive index (n₂ = −1.122 × 10−9 cm²/W) with a molecular two photon absorption coefficient β value of 3.625 × 10⁻⁶ cm/W. The low dielectric constant observed in the high frequency region indicates the suitability of the sample for electro-optic applications. The surface features are also investigated by atomic force microscopy (AFM).     

Low temperature phase barium borate: A new optical limiter in continuous wave and nano pulsed regime

Low temperature phase barium borate was synthesized by hydrothermal method. XRD analysis confirms the formation of γ-BBO or hydrated barium polyborate (Ba₃B₆O₉(OH)₆) which crystallizes in monoclinic system in the P2/c space group. The molecular structure analysis shows the presence of dominant BO₄ unit and the hydrated nature of material. γ-BBO exhibits sharp absorption edge at 202 nm and highly transparency in the UV–Visible–NIR region. The peak at 347 nm in the emission spectrum is due to the presence of self-trapped exciton. The third order nonlinear optical properties and limiting behavior of low temperature barium borate in both pulsed and continuous wave regime were studied. The effective 2PA absorption coefficient of γ-BBO under ns pulse excitation is estimated to be 0.38 × 10⁻¹⁰ m/W. The nonlinear absorption coefficient, refractive index and optical susceptibility of the material in cw regime were found to be in the order of 10⁻⁵ m W⁻¹, 10⁻¹² m² W⁻¹, 10⁻⁶ esu respectively. In both regimes, low temperature phase barium borate exhibits better optical limiting properties than high temperature phase β-BBO.     

Reverse saturable absorption studies in polymerized indole – Effect of polymerization in the phenomenal enhancement of third order optical nonlinearity

We report our results on the identification of large order enhancement in nonlinear optical coefficients of polymerized indole and its comparative study with reference to its monomer counterpart. Indole monomer shows virtually little third order effects whereas its polymerized version exhibits phenomenal increase in its third order nonlinear optical parameters such as nonlinear refractive index and nonlinear absorption. Open aperture Z-scan trace of polyindole done with Q-switched Nd:YAG laser source (532 nm, 7 ns), shows β value as high as 89 cm/GW at a beam energy of 0.83 GW/cm². Closed aperture Z-scan done at identical energies reveals nonlinear refractive index of the order of −3.55 × 10⁻¹⁷ m²/W. Band gap measurement of polyindole was done with UV–Vis absorption spectra and compared with that of Indole. FTIR spectra of the monomer and polymerized versions were recorded and relevant bond formations were confirmed from the characteristic peaks. Photo luminescent spectra were investigated to know the emission features of both molecules. Beam energy (I₀) versus nonlinear absorption coefficient (β) plot indicates reverse saturable type of absorption behaviour in polyindole molecules. Degenerate Four Wave Mixing (DFWM) plot of polyindole reveals quite a cubic dependence between probe and phase conjugate signal and the resulting χ⁽³⁾ is comparable with Z-scan results. Optical limiting efficiency of polyindole is comparable with certain derivatives of porphyrins, phthalocyanines and graphene oxides.     

Structure and optical properties of Li2Ga2GeS6 nonlinear crystal

Structure and optical properties of new nonlinear crystal – Li₂Ga₂GeS₆ single crystal of optical quality, grown by the Bridgman technique were studied. The data on transmission, Raman scattering, luminescence emission, excitation and thermal quenching as well as thermostimulated luminescence are presented. Fundamental absorption edge is determined by the direct allowed electronic transitions: The values of optical band gap are estimated. Absorption band at 8.0 μm is due to S–S vibrations. Features in photoluminescence spectra are associated with excitons: both free (narrow line at 371 nm) and self-trapped ones (broad bands at 596, 730 and 906 nm). Spontaneous emission in the 80–170 K range, both at crystal heating and cooling, is typical of pyroelectrics: This confirms the absence of symmetry center in Li₂Ga₂GeS₆ and an opportunity of laser frequency nonlinear conversion.     

Nonlinear optical response of silica doped with copper nanoclusters under 1064 nm laser excitation

Metal nanocluster composite glass was formed by Cu ion implantation into silica using metal vapor vacuum arc (MEVVA) ion source. The microstructural properties of the nanoclusters were analyzed by optical absorption spectra and transmission electron microscopy (TEM). Third-order nonlinear optical properties of the nanoclusters were measured at 1064 nm excitations using Z-scan technique. The nonlinear refraction index, nonlinear absorption coefficient, and the real and imaginary parts of the third-order nonlinear susceptibility were deduced. The mechanisms responsible for the nonlinear response were discussed. Third-order nonlinear susceptibility χ⁽³⁾ of this kind of sample was determined to be (4.2 ± 1.0) × 10⁻⁸ esu.     

Picosecond measurement of the nonlinear refractive index of new salts of carboxylate anions with chiral ammonium cations

The third order nonlinear optical properties of novel carboxylate anions with chiral amines and ammonium cations have been studied by a new alternative of the classical z-scan technique with picosecond laser pulses (pulse duration of 30 ps at 1064 nm wavelength). The materials do not present linear and nonlinear (two-photon) absorptions at the wavelength and the intensities used for the experiments. The non-resonant molecular second order hyperpolarizability γ obtained can reach 4.4 × 10⁻⁴⁸ SI. These values are two and three orders of magnitude larger than those CS₂ and of C₆H₆ respectively.     

Third-order nonlinear optical properties of bis(tetrabutylammonium)bis(4,5-dithiolato-1,3-dithiole-2-thione)copper

Bis(tetrabutylammonium)bis(4,5-dithiolato-1,3-dithiole-2-thione)copper (BCDT) was synthesized and its third-order optical nonlinearity was characterized using picosecond Z-scan at 1064 nm. The Z-scan spectra reveal a large negative nonlinear refraction coefficient n₂ as high as 4.0 × 10⁻¹⁸ m²/W and a slight two-photon absorption β, which is determined to be 3.4 × 10⁻¹² m/W. The molecular second-order hyperpolarizability γ was calculated to be 6.5 × 10⁻³² esu. All these results suggest that this material has potential for the application of all-optical switching.     

A pulsed optical absorption spectroscopy study of wide band-gap optical materials

The paper presents the results of a study on the formation and evolution of short-lived radiation-induced defects in wide band-gap optical materials with the mobile cations. The spectra and decay kinetics of transient optical absorption (TOA) of radiation defects in crystals of potassium and ammonium dihydro phosphates (KH₂PO₄ and NH₄H₂PO₄) were studied by means of the method of pulsed optical absorption spectroscopy with the nanosecond time resolution under excitation with an electron-beam (250 keV, 10 ns). A model of electron tunneling between the electron and hole centers under conditions of the thermally stimulated mobility of one of the recombination process partners was developed. The model describes all the features of the induced optical density relaxation kinetics observed in nonlinear optical crystals KH₂PO₄ and NH₄H₂PO₄ in a broad decay-time range of 10 ns–10 s after the pulse of radiation exposure. The paper discusses the origin of radiation defects that determine the TOA, as well as the dependence of the decay kinetics of the TOA on the temperature, excitation power and other experimental conditions.     

Off-resonant third-order nonlinear optical properties of multiply-bonded multinuclear organometallic complexes measured with femtosecond pulses

Third-order nonlinear optical properties of multinuclear organometallic complexes consisting of metal–metal multiple bonds previously studied by picosecond degenerate four-wave mixing (DFWM) are reexamined by femtosecond optical heterodyned optical Kerr effect (OHD-OKE) and femtosecond Z-scan techniques. The real part of off-resonant second hyperpolarizabilities are obtained at a wavelength of 1280 nm to be +3.0 ± 10⁻³³ esu for a Pt–Mo–Mo–Pt tetranuclear complex and less than 10⁻³³ esu for a Mo–Mo dinuclear complex. The present results clarify that the previously reported values are not off-resonant values and the off-resonant property of the tetranuclear complex is superior to that of the dinuclear one.     

Growth and dielectric,mechanical, thermal and etching studies of an organic nonlinear optical l-arginine trifluoroacetate (LATF) single crystal

l-arginine trifluoroacetate, an organic nonlinear optical material, has been synthesized from aqueous solution. Bulk single crystal of dimension 57 mm × 5 mm × 3 mm has been grown by temperature lowering technique. Powder X-ray diffraction studies confirmed the monoclinic structure of the grown l-arginine trifluoroacetate crystal. Linear optical property of the grown crystal has been studied by UV–vis spectrum. Dielectric response of the l-arginine trifluoroacetate crystal was analysed for different frequencies and temperatures in detail. Microhardness study on the sample reveals that the crystal possesses relatively higher hardness compared to many organic crystals. Thermal analyses confirmed that the l-arginine trifluoroacetate material is thermally stable upto 212 °C. The etching studies have been performed to assess the perfection of the l-arginine trifluoroacetate crystal. Kurtz powder second harmonic generation test confirms the nonlinear optical properties of the as-grown l-arginine trifluoroacetate crystal.     

Transparent amorphous zinc oxide thin films for NLO applications

This review focuses on the growth and optical properties of amorphous zinc oxide (ZnO) thin films. A high quality ZnO films fabricated by dip-coating (sol–gel) method were grown on quartz and glass substrates at temperature equal to 350 K. The amorphous nature of the films was verified by X-ray diffraction. Atomic Force Microscopy was used to evaluate the surface morphology of the films. The optical characteristics of amorphous thin films have been investigated in the spectral range 190–1100 nm. Measurement of the polarized optical properties was shows a high transmissivity (80–99%) and low absorptivity (<5%) in the visible and near infrared regions at different angles of incidence. Linear optical properties were investigated by classic and Time-Resolved Photoluminescence (TRPL) measurements. Photoluminescence spectrum exhibits a strong ultraviolet emission while the visible emission is very weak. An innovative TRPL technique has enabled the measurement of the photoluminescence decay time as a function of temperature. TRPL measurements reveal a multiexponential decay behavior typical for amorphous thin films. Second and third harmonic generation measurements were performed by means of the rotational Maker fringe technique using Nd:YAG laser at 1064 nm in picosecond regime for investigations of the nonlinear optical properties. The obtained values of second and third order nonlinear susceptibilities were found to be high enough for the potential applications in the optical switching devices based on refractive index changes. Presented spectra confirm high structural and optical quality of the investigated zinc oxide thin films.     

Optical properties of petal-like aggregated nanocrystalline zinc oxide synthesized by laser ablation

The results of the investigations carried out on the third-order nonlinearity in zinc oxide (ZnO) nanocrystals (NCs) by Z-scan technique are included in this paper. ZnO NCs show negative nonlinearity and good nonlinear absorption behavior at 532 nm. The third-order optical susceptibility χ(3) increases with enlargement of NCs due to the size dependent enhancement of exciton oscillator strength.The synthesis of ZnO NCs was performed by laser ablation from a high-purity metallic target of Zn in distilled water medium. For the ablation process, a high frequency pulsed Nd:YAG laser was employed operating at 532 nm with 100 ns pulse duration. UV–vis absorption spectroscopy illustrated the enhancement of the size of ZnO NCs upon increasing the laser pulse energy applied in ablation process. Accordingly the corresponding optical band gap (E_g) decrease by increasing the size of NCs. X-ray diffraction (XRD) associated with transmission electron microscopy (TEM) was utilized to characterize the crystalline phase and also for determining the ZnO NCs morphology.     

Studies on optical and mechanical properties of new organic NLO crystal: Guanidinium 4-aminobenzoate (GuAB)

Organic nonlinear optical crystal of guanidinium 4-aminobenzoate (GuAB) has been successfully grown by slow evaporation solution growth technique. The grown crystal is up to the dimension of 36 × 8 × 6 mm³. The crystal is characterized by single crystal XRD for obtaining unit cell parameters. Optical properties of the grown crystal have been studied by means of UV–Vis–NIR transmission and absorption spectra in the range of 200 to 1000 nm. The refractive index and band gap energy of the GuAB crystal are obtained as 1.68 and 3.73 eV respectively. Mechanical hardness has been carried out on the grown crystal.     

Study on optical weak absorption of borate crystals

Borate crystal is an important type of nonlinear optical crystals used in frequency conversion in all-solid-state lasers. Especially, LiB₃O₅ (LBO), CsB₃O₅ (CBO) and CsLiB₆O₁₀ (CLBO) are the most advanced. Although these borate crystals are all constructed by the same anionic group-(B₃O₇)⁵⁻, they show different nonlinear optical properties. In this study, bulk weak absorption values of three borate crystals have been studied at 1064 nm by a photothermal common-path interferometer. The bulk weak absorption values of them along [1 0 0], [0 1 0] and [0 0 1] directions were obtained, respectively, to be approximately 17.5 ppm cm⁻¹, 15 ppm cm⁻¹ and 20 ppm cm⁻¹ (LBO); 80 ppm cm⁻¹, 100 ppm cm⁻¹ and 40 ppm cm⁻¹ (CBO); 600 ppm cm⁻¹, 600 ppm cm⁻¹ and 150 ppm cm⁻¹ (CLBO) at 1064 nm. The results showed an obvious discrepancy of the values of these crystals along three axis directions. A correlation between the bulk weak absorption property and crystal intrinsic structure was then discussed. It is found that the bulk weak absorption values strongly depend on the interstitial area surrounded by the B–O frames. The interstitial area is larger, the bulk weak absorption value is higher.     

Planar waveguide in beta barium borate formed by proton implantation and optical properties in visible and near-infrared band

β-BaB₂O₄ is nonlinear crystal that provides attractive features for various nonlinear optical application. We report on z-cut β-BaB₂O₄ planar waveguide produced by multi-energy proton implantation in total of 3 × 10¹⁶ ion/cm² at room temperature. The refractive index profile is a barrier-confined distribution, which is reconstructed according to the dark mode spectroscopy at the wavelength of 633 nm. The near-field light intensity profiles in the visible (633 nm) and near-infrared band are measured by end-face coupling method. The absorption spectra recorded over the wavelength range 185–2500 nm shows there is hardly change after proton implantation. The investigation results of the Raman spectra demonstrate that the positions and widths of all the peaks are very similar in the β-BaB₂O₄ crystal and waveguide layer.