Picosecond Z-scan Technique Research Articles

Modulating the nonlinear absorption response of SnOx thin films via phase engineering.

Phase (composition) is known to play a key role in determining the electronic and optical properties of amorphous oxide semiconductors. In this work, modulating the ultrafast nonlinear optical (NLO) response of SnO2 and SnO thin films by tuning oxygen partial pressure during film sputtering is explored. Femtosecond Z-scan results demonstrate that intermediate phases have no profound impact on the two-photon absorption (TPA) response of SnO2 and SnO films. Interestingly, the magnitude of the effective nonlinear absorption coefficient (βeff) of both intermediate SnO2-x and SnOx are enhanced after the change of Sn2+/Sn4+ composition ratio, as measured by picosecond Z-scan technique. Femtosecond degenerate pump-probe measurements show that intermediate phases accelerate the carrier trapping and improve the defect-related carrier absorption in SnOx (SnO-rich) film, while intermediate phase suppress the TPA response of SnO2-x (SnO2-rich) films, therefore carrier-induced absorption dominates the NLO behavior of SnO2-x film on picosecond regime. Our results indicate a simple and effective way to modulate the NLO response of transparent conductive oxide SnO2 and SnO.

Copper Nanoparticles Embedded in Natural Plagioclase Mineral Crystals: In Situ Formation and Third-Order Nonlinearity

Copper nanoparticles are low-cost plasmonic materials with the ability of manipulating light at the nanometer scale. The exploration of copper nanoparticles toward photonic applications suffers instability due to the easy prone oxidation. In this paper, we developed the in situ formation of copper nanoparticles with a diameter between 40 and 50 nm in a natural plagioclase mineral crystal matrix by controlling the Cu+–Na+ ion exchange process in the temperature range of 1050–1200 °C. By combining quantitative element distribution characterization and spectroscopic measurement, we illustrate the in situ formation process and mechanism of Cu nanoparticles in natural plagioclase mineral crystals. Moreover, we determined the nonlinear optical properties of the materials by using the picosecond Z-scan technique. The third-order nonlinear optical susceptibility χ(3) at 1064 and 532 nm was estimated to be (1.046 ± 0.213) × 10–10 esu and (3.236 ± 0.198) × 10–10 esu, respectively. The natural plagioclase mineral crystals embedded with copper nanoparticles are believed to hold great potential to serve as plasmonic materials for a variety of practical applications, especially in nonlinear optics.

Nonlinear optical enhancement induced by synergistic effect of graphene nanosheets and CdS nanocrystals

CdS nanocrystals are attached on graphene nanosheets and their nonlinear optical properties are investigated by picosecond Z-scan technique at 532 nm. We found that synergistic effect between the graphene and CdS makes a major enhancement on the nonlinear optical absorption of graphene/CdS nanohybrid in comparison with cooperative effect, and the synergistic improvement is restricted by nonradiative defects in hybrid. The synergistic mechanism involving the local field theory and charge transfer evolution is proposed.

Structural, photoluminescence and picosecond nonlinear optical effect of In-doped ZnO nanowires

In-doped ZnO (IZO) nanowires were grown using the chemical vapour deposition method. The IZO nanowires have been characterized by scanning electronic microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and UV–Visible spectroscopy. The PL results demonstrated a larger band-gap for the IZO nanowires in comparison to the undoped ZnO. Two major emission peaks were observed for the IZO nanowires, one originated from the free exciton recombination (ultraviolet emission) and another possibly related to the deep-level emission (visible emission). Furthermore, the nonlinear optical characteristic of the nanowires was studied using picosecond Z-scan technique. The experimental results show that the two and three-photon absorption coefficient of samples were able to be observed. These studies make the promising potential applications of the samples in the development of multifunctional all-optical devices.

Plasmon assisted enhanced nonlinear refraction of monodispersed silver nanoparticles and their tunability.

Nonlinear optical characterizations were performed on monodispersed silver (Ag) nanoparticles (NPs) of various sizes using a picosecond Z-scan technique with excitation wavelengths of 532 nm and 1064 nm. The Ag NPs were fabricated using a heterogeneous condensation technique in a gas medium. The nonlinear refraction values were higher for the monodispersed Ag NPs whose surface plasmon resonance (SPR) peak is closer to the excitation wavelength. The higher nonlinear optical response is explained in terms of an electric field enhancement near the SPR. Moreover, the fabrication method allows the tailoring of the nonlinear refraction index of the Ag NPs by tuning the SPR peak of the sample. A comparison of the nonlinear refraction index of the monodispersed and polydispersed Ag NPs showed that the nonlinear refractive index of the monodispersed Ag NPs is higher.

Photophysical and nonlinear optical properties of an azobenzene substituted zinc phthalocyanine

Photophysical property and third-order optical nonlinearity of an azobenzene substituted zinc phthalocyanine (azo-ZnPc) in chloroform solution were studied by UV–Vis spectra method and a picosecond Z-scan technique at 532nm with pulse duration of 25ps, respectively. It was found that the azo-ZnPc shows large positive nonlinear refraction and positive nonlinear absorption, exhibiting the defocusing effect and reverse saturable absorption, respectively. The molecular second hyperpolarizability of the azo-ZnPc dyad was measured to be 3.9×10−30esu. All the results suggest that the studied azo-ZnPc dyad may have potential applications in the field of nonlinear optics.

Thiazole based novel functional colorants: Synthesis, characterization and nonlinear optical studies using picosecond Z-scan technique

Organic push–pull chromophoric system containing thiazole Schiff bases were synthesized from aminothiazole intermediate. These novel chromophores possessed excellent thermal and optical properties with complete transparency in the visible spectral region. Thermogravimetric analysis revealed that all dyes were thermally stable at temperatures above 300°C. The third-order nonlinear optical studies, performed using the Z-scan technique with ∼2ps pulses, revealed excellent three photon absorption coefficients/cross-sections combined with reasonably high values of nonlinear refractive index (n2). The solute n2 was of opposite sign compared to that of solvents. To the best of our knowledge, this is the first report of three-photon absorption studies in Schiff base compounds.

Ultrafast Excited-State Dynamics and Dispersion Studies of Third-Order Optical Nonlinearities in Novel Corroles

Ultrafast nonlinear optical (NLO) properties of four novel Corroles in the visible spectral range (660–800 nm) were evaluated using picosecond Z-scan technique. Ultrafast excited state dynamics have also been appraised with picosecond (ps) and femtosecond (fs) degenerate pump–probe techniques using excitation wavelengths of 800 and 600 nm, respectively. The excitation by 800 nm photons resulted in two-photon absorption at adequately high peak intensities, thereby facilitating the access to higher excited states (Sn). The nonradiative relaxation mechanisms from these states, reflected in the pump–probe data, consisted of double-exponential decay with a slow component in the range of 54–277 ps and faster component in the range of 2.0 to 2.5 ps. When excited with 600 nm photons (unfocused), photoinduced absorption was observed with the first excited state S1 being populated, and as a consequence single decay was observed in the data of all molecules studied. These retrieved lifetimes were analogous to those obtained with picosecond pump–probe data. The long lifetime is attributed to nonradiative decay from the S1 state with possible contribution from triplet states, whereas the shorter lifetime is attributed to the internal conversion (S2 to S1*), followed by vibrational relaxation (S1* to S1) processes. Time-resolved fluorescence lifetime measurements revealed the magnitude of radiative lifetimes to be in the nanosecond regime. NLO coefficients were evaluated from the Z-scan data at wavelengths of 660, 680, 700, 740, and 800 nm. Large two-photon absorption coefficients (β)/cross-sections (σ2) at 740 nm/680 nm were recorded for these molecules, making them apposite for applications such as two-photon induced photodynamic therapy and lithography. Figure of merit, T, was <1 for all molecules at 740 and 800 nm, suggesting that these molecules find use in photonic device applications.

Third-Order Optical Nonlinearities of Squarylium Dyes with Benzothiazole Donor Groups Measured Using the Picosecond Z-Scan Technique

Third-order optical nonlinearities of two squarylium dyes with benzothiazole donor groups (BSQ1 and BSQ2) in chloroform solution are measured by a picosecond Z-scan technique at 532 nm. It is found that the two compounds show the saturation absorption and nonlinear self-focus refraction effect. The molecular second hyperpolarizabilities are calculated to be 7.46 × 10−31 esu and 5.01 × 10−30 esu for BSQ1 and BSQ2, respectively. The large optical nonlinearities of squarylium dyes can be attributed to their rigid and intramolecular charge transfer structure. The difference in γ values is attributed to the chloro group of benzene rings of BSQ2 and the one-photon resonance effect. It is found that the third-order nonlinear susceptibilities of two squarylium dyes are mainly determined by the real parts of χ(3), and the large optical nonlinearities of studied squarylium dyes can be attributed to the nonlinear refraction.

Third-order nonlinear optical properties of the phenothiazinium chlorides at 532 nm

A series of phenothiazinium chlorides, which have good solubility in polar solvents and good thermal stability, were evaluated in dimethyl sulfoxide solution for third-order nonlinear optical properties at 532 nm using a nanosecond and picosecond Z-scan technique. The title compounds exhibit strong reverse saturable absorption and nonlinear refraction. The third-order nonlinear optical properties were obtained under irradiation with either a nanosecond or picosecond laser. The nonlinear mechanism is revealed as excited-state nonlinearity by a picosecond pump-probe response experiment for one of the compounds. The absorption and emission maxima, Stokes shift and quantum yield are also reported.

Nonlinear Optical Properties of Novel Polymeric Rare Earth Phthalocyanine Studied Using Picosecond Z-Scan Technique

Three novel tri-dimensional phthalocyanine polymers, with lanthanum (LaPPc), gadolinium (GdPPc) and ytterbium (YbPPc) as centric atoms, have been synthesized from a tetranuclear phthalonitrile. Third-order optical nonlinearities of these compounds in DMF solution are measured by a picosecond Z-sacn technique at 532 nm. It is found that all the compounds show reverse saturation absorption and nonlinear self-focus refraction effect. The second-order molecular hyperpolarizabilities are calculated to be 1.82 × 10−29, 1.48 × 10−29 and 1.45 × 10−29 esu for LaPPc, GdPPc and YbPPc, respectively. The differences among their nonlinear optical properties are attributed to the special tri-dimensional structure and the variation in rare earth atoms.

The optical and nonlinear optical properties of Au:TiO2 and Au:Al2O3 composite films

On the basis of measurement and theoretical analysis，optical and nonlinear optical properties of the Au：TiO2 and Au：Al2O3 composite films with different Au nanoparticle size and different substrates were investigated. The relationship between the Au nanoparticle size and substrates and the surface plasmon resonance (SPR) of Au composite films were characterized by UV-VIS-NIR spectroscopy，and the dependence of third-order nonlinear susceptibility of the composite films on Au nanoparticle size and substrates was investigated by picosecond Z-scan technique with different exciting wavelengths (532nm and 1064nm). The analysis of optical properties of these films was carried out using the theories of surface plasmon resonance and localized field enhancement，respectively. The experimental and theoretical results revealed the dependence of third-order nonlinear susceptibilities χ(3) on Au nanoparticle size and substrates. For Au nanoparticle of size around 35nm embedded in TiO2 substrate with larger permittivity and the exciting wavelength at 532nm close to the surface plasmon resonance (618nm—632nm)，the third-order nonlinear susceptibility χ(3) showed a maximum value of 25×10-9esu.

Linear and nonlinear optical properties of CdSe quantumn dots

On the basis of measurement and theoretical analysis, the linear and nonlinear optical properties of the CdSe quantumn dots with different nanoparticle sizes were investigated. The relation between the nanoparticle size and the absorption peak of CdSe quantumn dots was characterized by UV-VIS-NIR spectroscopy, and the dependence of third-order nonlinear susceptibility of quantumn dots on nanoparticle size was investigated by picosecond Z-scan technique with different excitation wavelengths (532nm and 1064nm,resp.). The analysis of optical properties of the films was carried out using the theory of localized field enhancement. The experimental and theoretical results showed the dependence of third-order nonlinear susceptibilities χ(3) on the CdSe nanoparticle size. With CdSe nanoparticle size around 4.3nm and the excitation wavelength of 532nm, which was close to the resonance, the third-order nonlinear susceptibility χ(3) showed a maximum value of 2.0×10-11 esu.

Linear and nonlinear optical properties of Ag:Bi2O3 composite films

On the basis of measurements and theoretical analysis, linear and nonlinear optical properties on the Ag:Bi2O3 composite films with different Ag concentrations were investigated. The relationship between the Ag concentrations and the surface plasmon resonance (SPR) of Ag:Bi2O3 composite films were characterized by UV-VIS-NIR spectroscopy, and the dependence of third-order nonlinear susceptibility of composite films on Ag concentrations was investigated by picosecond Z-scan technique with different excitation wavelength (532nm and 1064nm). The analysis of optical properties of these films was carried out using the theories of surface plasmon resonance and localized field enhancement respectively. From the experimental and theoretical results, the dependence of third-order nonlinear susceptibilities χ(3) on Ag concentration was shown. With Ag concentration around 35％ and the excitation wavelength at 532nm near the surface plasmon resonance (560nm—622nm), the third-order nonlinear susceptibility χ(3) showed a maximum value of 2.4×10-9 esu.

Dispersion of the nonlinear refractive index in sapphire.

The nonlinear refractive index, n2, of sapphire was experimentally measured in the 550-1550-nm wavelength range by use of a picosecond Z-scan technique. It was found that in this spectral region the value of n2 decreases monotonically from approximately 3.3 x 10(-16) to approximately 2.8 x 10(-16) cm2/W. An empirical expression for the wavelength dependence of the nonlinear refractive index in the 270-1550-nm range was obtained.

High third-order nonlinear optical susceptibility in new fluorinated poly(p-phenylenevinylene) copolymers measured with the Z-scan technique

The third-order nonlinear optical properties of a series of copoly(2, 3, 5, 6-tetrafluoro-1, 4-phenylenevinylene-2, 5-dioctyloxy-1, 4-phenylenevinylene) that contain variable ratios of two differently substituted monomers have been studied in chloroform solutions at l=1064 nm by the picosecond Z-scan technique. Nonlinear refractive index n(2) of the samples investigated has been found to be negative, and a strong dependence of its magnitude on the copolymer's composition has been observed. The highest third-order nonlinear optical susceptibility, |x((3))|=(6 +/- 2)x 10(-10) esu, was measured for a copolymer obtained by reaction of equimolar quantitites of the parent monomers.

Substituent-dependence of the optical nonlinearities in poly(2,5-dialkoxy- p-phenylenevinylene) polymers investigated by the Z-scan technique

We investigated the relationship existing between the third-order nonlinear optical (NLO) properties and the chemical structure of some poly(2,5-dialkoxy-1,4-phenylenevinylene) polymers. The latter have similar degrees of polymerization and polydispersity indexes and differ only for the structure of the side groups in the 2,5 positions of the phenylene rings. The two-photon absorption coefficient and the nonlinear refractive index have been measured using the picosecond Z-scan technique. Experimental results show that all the investigated organic molecules exhibit a small Im χ (3) and a large, negative Re χ (3) at λ=1064 nm. A considerable dependence of the third-order NLO properties on the steric effect of the substituents has been observed.

Competitive Optical Nonlinearities and Photoinduced Transformation of Epoxy Polymer Based on Diglicidyl Ether of Bisphenol A

We have studied competitive optical nonlinearities - excited state absorption (ESA), absorption saturation (SA) and two-photon absorption (TPA) - of epoxy polymer based on diglicidyl ether of bisphenol A with picosecond Z-scan technique at 532 nm. The complexity of the analysis was dealt with problem of the selection of the part of Z-scan curve which corresponds to the dominant contribution of the proper nonlinear mechanism on the background of competitive others. The TPA coefficient is β=4.0±0.5 cm/GW for the polymer. At high laser intensities (>7 GW/cm2) photoinduced transformation into some new configuration is taken place that is more absorbent at 532 nm (β=16.7±3 cm/GW for the photoproduct). Up to a pumping irradiance of 20 GW/cm2 no optical damage of the polymer was observed.

Z-scan measurements of nonlinear refraction and Kerr-lens mode-locking with Yb3+:KY(WO4)2

The nonlinear refractive index n2 of Yb3+:KY(WO4)2 crystal has been measured using picosecond Z-scan technique. The magnitude of n2 was found to be 8.7 × 10−16 cm2/W at wavelength of 1.08 μm. The numerical modeling based on fluctuation model showed a great potential of this crystal as active medium for Kerr-lens mode-locking.