A quantitative comparison of third-order nonlinear optical properties of colloidal gold nanoshells (NSs) and gold nanorods (NRs) in water solutions has been carried out using open- and closed-aperture Z-scan measurements, performed with femtosecond laser pulses over a broad range of wavelengths. Absorption saturation was found to be a dominant effect for all the studied nanoparticles; however, two-photon absorption (2PA) properties were also detected, and were clearly resolved especially at the shortest wavelengths used. The value of the merit factor σ2/M (2PA cross section scaled by the molecular weight) for the NRs (10×35 nm) at 530 nm is 7.5 (GM·mol/g), while for the NSs it is 1.9 (GM·mol/g) at the same wavelength.
A detailed comparison of third-order nonlinear optical properties of colloidal gold nanoshells (NSs) and gold nanorods (NRs) in water solutions has been carried out with the open- and closed-aperture Z-scan measurements, performed with femtosecond laser pulses over a broad range of wavelengths. Absorption saturation was found to be a dominant effect for all the studied nanoparticles, however two-photon absorption properties are also detected, especially at the shortest wavelengths studied. The value of the merit factor σ2/M (two-photon absorption cross section scaled by the molecular weight) for the NRs (10nm × 35 nm) at 530 nm is 7.5 (GM·mol/g), while for the NSs is 1.9 (GM·mol/g) at the same wavelength.
Single nanoparticle imaging is a powerful method to characterize nanoobjects and gain better understanding of their structural and optical properties. In our research we focus on plasmonic nanoparticles and particularly on anisotropic gold nanorods, which present interesting, polarization-dependent optical properties strictly correlated with their surface plasmon resonances. Here we discuss our results on two-photon excited luminescence imaging of a single gold nanorod. We analyze the dependence of the two-photon luminescence of a nanorod on the excitation wavelength, incident laser power and polarization, and contrast them with the data available in the literature.
We synthesized a mixture composed of gold nanoparticles of various shapes using the wet chemistry method. The final
solution contained long nanorods, balls, disks and different spherical nanoparticles. To separate particles of individual
shapes from the reaction mixture, the solution was centrifuged in a glucose density gradient. A distribution of
nanoparticles based on their diameters was observed and each section was collected independently and each type of
nanoobjects was characterised separately. Finally, the difference in nanoparticle shapes depending on the presence of
Ag+ ions in the growth solution is reported and its influence on the separation is discussed.
Both nonlinear absorption and nonlinear refraction are effects that are potentially useful for a plethora of applications in
photonics, nanophotonics and biophotonics. Despite substantial attention given to these phenomena by researchers
studying the merits of disparate systems such as organic materials, hybrid materials, metal-containing molecules and
nanostructures, it is virtually impossible to compare the results obtained on different materials when varying parameters
of the light beams and different techniques are employed. We have attempted to address the problem by studying the
properties of various systems in a systematic way, within a wide range of wavelengths, and including the regions of onephoton,
two-photon and three-photon absorption.
The objects of our studies have been typical nonlinear chromophores, such as π-conjugated molecules, oligomers and
polymers, organometallics and coordination complexes containing transition metals, organometallic dendrimers, small
metal-containing clusters, and nanoparticles of various kinds, including semiconductor quantum dots, plasmonic
particles and rare-earth doped nanocrystals. We discuss herein procedures to quantify the nonlinear response of all of
these systems, by defining and comparing the merit factors relevant for various applications.
There is much interest in nonlinear absorbing chromophores for applications in photonics, nanophotonics and biophotonics. We have performed studies of dispersion of the nonlinear absorption cross sections and the refractive nonlinearities of organic and organometallic nonlinear chromophores using the technique of Z-scan, with a tunable amplified femtosecond laser system. Z-scan is less sensitive than the popular technique of two-photon induced fluorescence but has advantages of being suitable for non-fluorescent substances and providing information on both absorptive and refractive nonlinearities. We have analysed the experimental results in terms of simple models and using the Kramers-Kronig transformation as shown in this paper for Coumarine 307 and an organometallic dendrimer. The dispersion curves are often dominated by two-photon resonances but inclusion of other nonlinear mechanisms seems to be necessary for better understanding of their features.
DNA is a polyelectrolyte capable of forming thin films with interesting optical properties. We investigated refractive
indices and optical anisotropy of films of the native, sodium ion-based DNA (Na-DNA) and DNA bearing the
cetyltrimethylammonium ion (DNA-CTMA) using a prism coupler technique. The light polarization direction was
either parallel (nTE) or perpendicular to the surface plane of the films (nTM). The index values and the birefringence of
DNA films vary considerably depending on the type of the counter-ion, the film fabrication method and the relative
humidity (RH) of the environment. A high negative birefringence in films of Na-DNA, nTE-nTM = -0.03 at an RH ~ 55
%, was measured in solution-cast films, indicating that the optically anisotropic DNA molecules are aligned in the plane
parallel to the film surface. Refractive indices of DNA-CTMA thin films were smaller and more isotropic than those for
films of Na-DNA polymer. The prism coupler reflectance curves showed a hysteresis of the index values when the RH
of a DNA-CTMA film environment varied. Polarization microscopy studies showed liquid-crystalline textures at the
edges of Na-DNA and DNA-CTMA films.
Third-order nonlinear optical properties were investigated for the system incorporating the Disperse Red 1 dye and the deoxyribonucleic acid - cetyltrimethylammonium complex (DNA-CTMA). The interaction of the dye with the DNA chains does not lead to major changes of the nonlinear optical effects due to the DR1 dye. Polarization dependences of the nonlinear absorption in this system reveal, however, some indications that the interactions may lead to ordering of the dye molecules against the DNA chains.
DNA is emerging as a novel exciting photonic polymer material due to its unique double-helix structure and the ability to
act as a host capable to be aligned itself and capable of inducing orientation of nonlinear optical (NLO) chromophores.
Physical and optical properties of DNA are remarkably modified with the alteration of the nucleic acid counter-ions. We
determined optical properties of salmon-derived DNA and DNA complexed with cetyltrimethyl-ammonium (CTMA)
surfactant in solutions and films. Absorption coefficients derived for an average nucleotide formula weight indicated
DNA of high purity. Prism coupling measurements showed a large birefringence in refractive indices in the direction
parallel and perpendicular to the surface plane of the films indicating anisotropic alignment of DNA molecules. Almost
isotropic refractive indices were measured in DNA-CTMA films indicating disorder in orientation of DNA-CTMA
molecules in the films. Doping with about 5 wt% Disperse Red 1 (DR1) essentially did not change this very weak
birefringence in the DNA-CTMA films. Optical properties of DNA films were sensitive to environmental humidity while
the DNA-CTMA films were less susceptible to it. The Z-scan technique using femtosecond pulsed laser system was
employed to determine the NLO properties of DNA in solutions in the 530-1300 nm wavelength range.
The non-equilibrium transformations induced by sub-picosecond lasers on space scale of nanometers and time
scale of less than picosecond are considered in this presentation. We demonstrate that the fast (during the pulse
time) change in the inter-atomic potential due to the electrons excitation is responsible for the swift coherent
atomic displacement. We calculate the coherent displacement of atoms in non-equilibrium and compare it to
that following from the familiar Lindemann approach to the melting in thermodynamic equilibrium.
We compare our analysis with the experiments on non-equilibrium phase transformation of Gallium by 150 fs
pulses at intensity well below the ablation threshold. The presented analysis and direct measurements indicate
that the melting in its conventional sense either is not completed, or that, most probably, some transient state of
matter has been created during the interaction even when the deposited energy exceeds three times the equilibrium
enthalpy of melting.
In conclusion we address some unresolved problems in understanding of ultra-fast phase transformations induced
by ultra-short laser pulses in non-equilibrium conditions.
We investigated linear optical and second-order nonlinear optical (NLO) properties of films of urethane-urea copolymer (UU2) functionalised with a high concentration of an azobenzene chromophore. The polymer films on ITO-coated substrate were corona poled to induce a noncentrosymmetric organization of chromophore dipoles and data on the second harmonic generated with the laser beam (the fundamental wavelength 1053 nm, 6 ps/pulse, 20 Hz repetition rate) was acquired as a function of time and temperature. Second harmonic generation (SHG) was used to monitor in situ the polar alignment and relaxation of orientation of the side-chain Disperse Red-like chromophore molecules in the films poled at room temperature and high above the glass transition temperature (Tg 140-150oC). The deff coefficient was determined from the Maker-fringe method and corrected for absorption. A strong second harmonic effect with a fast relaxation was observed in "cold" (room temperature) poling experiments. A large second-order resonantly enhanced optical nonlinearity (d33 of the order of 200 pm/V) was obtained in high temperature poling. A strong and stable nonlinearity has persisted for years after the films were high-temperature poled.
We have been studying third-order NLO properties of ruthenium-containing organometallic dendrimers. These molecules offer large hyperpolarizabilities and the possibility to control both the refractive and absorptive parts of the nonlinear response by electrochemical switching of the oxidation state of the ruthenium centres. Time-resolved studies indicate that it is possible to switch the molecules between the form in which they are two-photon absorbers and the state where they are fast (~ 1 picosecond time scale) saturable absorbers. Measurements of the dispersion of third-order nonlinearity have been made by the Z-scan technique on a nitro decorated dendrimer. They indicate that competition between two-photon absorption and absorption saturation is present in some wavelength ranges. The dispersion of both the real and imaginary parts of the cubic polarizability could be modelled by considering leading terms of the dispersion relation.
The reflectivity of Gallium films excited by femtosecond laser can be raised from ~55% to up to ~85% on a picosecond time-scale. Temporal behavior of the reflectivity exhibits three clearly distinguished stages: an initial 2 - 4 ps sharp rise, a relatively slow increase to a maximum value in a few 100 ps, and afterwards a long slope in ~ (0.1 - 1) μs to the original value. In this paper we present reflectivity measurements in a pump-probe scheme with one pump and two identical simultaneous femtosecond probes set at two different angles, which completely determines the real and imaginary parts of the dielectric function with time resolution ~ 200 fs. The analysis of the experimental data uncovered a number of new phenomena: (1) the energy density threshold to initiate phase transition is several times lower than the equilibrium enthalpy of melting; (2) the initial 2 - 4 ps rise of reflectivity relates to the transformation to a new phase in the absence of energy loss due to cooling. The second, slower stage (~100 ps) relates to a heat conduction dominated process; (3) the rate of the reflectivity change strongly increases with the increase of the pump laser intensity; (4) the volume fraction of the new phase reaches only 60% even with the deposited energy exceeds more than two times the equilibrium enthalpy of melting; (5) the electron-to-lattice coupling rate is a transient non-linear function of temperature that is drastically different from the equilibrium conditions. The results suggest a mechanism to control of the reflectivity switching, and thus the duty cycle of the reversible phase transition (crystal-metal-crystal), through an optimal combination of the laser parameters, target and substrate material. As a result, new all-optical switching devices with ps-range switching time could be designed utilizing the nonlinear dielectric properties of the non-equilibrium solid-state plasma.
Transition metal carbonyl clusters incorporating group 6 (molybdenum, tungsten) and iridium atoms in a tetrahedral or butterfly-shaped four-atom cluster core, together with carbonyl, cyclopentadienyl and alkyne ligands, have been synthesized and incorporated into oligourethanes, and their optical limiting properties assessed by open-aperture Z-scan (ns pulses, 523 nm) and time-resolved pump-probe studies (ps pulses, 527 nm). The Z-scan studies reveal that the tetrahedral [M2Ir2] cluster cores (M = Mo, W) displayed a greater effective nonlinear absorption coefficent β2 than the [MoIr3] cores; the tungsten example, W2Ir2(CO)10(η-C5H5)2, exhibited the highest response. Substitution at the cyclopentadienyl group (including incorporation into a polymer backbone) had little effect on the response measured. A time-resolved investigation of the alkyne-adduct Mo2Ir2(μ4-η2-MeC2Ph)(CO)8(η-C5H4Me)2 using picosecond pulses at 527 nm reveals optical-power-limiting behaviour that results from electronic processes [specifically, a fast nonlinear absorption process followed by reverse saturable absorption involving long-lived (>1000 ps) metastable excited states].
The nonlinear optical properties of multiwalled carbon nanotubes (MWNT) was investigated using femtosecond, picosecond and nanosecond laser pulses by the Z-scan and Degenerate four wave mixing techniques. Measurements show a significant third order nonlinear response in the both visible and near-infrared wavelengths regions, with Χ(3) values as high as 10-10 esu obtained on nanosecond and picosecond excitation and somewhat diminished Χ(3) values of the order of 10-12 esu obtained on femtosecond excitation. The temporal response at both picosecond and femtosecond excitation show a significant fast component indicating that electronic processes contribute to the third order nonlinear optical response of MWNT. This electronic role is highlighted by the observation of Van Hove singularities (VHS) in the density of states of MWNT. Unexpected visible luminescence from MWNT, observed on photo excitation at 1064 nm, is believed to arise from radiative transitions between energy states in the VHS. Our results shows that the presence of VHS enable efficient optical transitions in MWNT and furthermore the enhancement of the third order nonlinear optical response.
Linear and nonlinear optical properties of a new soluble polymer bearing distyrylbenzene chromophore, the alkoxy-sulphono-substituted p-phenylenevinylene oligomer (MTPV-ORSO) attached as a side-chain to the methyl methacrylate backbone are reported. This chromophore was developed for introduction into single-mode nonlinear optical polymer fibres. The molecular second-order and third-order nonlinearities were estimated with quantum chemical calculations (MOPAC). The side-chain polymer shows an optical absorption maximum at 326 nm. The polymer was incorporated into the core of the fibre preform with a procedure which led to a step index profile, as measured with a preform profiler, suitable for a single-mode optical fibre. Second harmonic of the fundamental 1200 nm wave was measured in the corona poled polymer films giving the second-order susceptibility d33 = 0.8 pm/V for the polymer containing about 1.3 wt% of the side-chain chromophore and 0.1 pm/V in a guest-host system containing 0.24 wt% of the chromophore in the fibre preform. The coherence length of the side-chain polymer, equal to 28 μm at 1200 nm, was evaluated from the dispersion of the linear refractive indices. Degenerate four-wave mixing (DFWM) with amplified femtosecond pulses was also applied to measure the nonlinear refractive index of the co-polymer at 800 nm.
We have investigated a large number of organometallic structures possessing high second and third order optical nonlinearities. Most third-order NLO experiments were performed with 100 femtosecond light pulses at 800 nm. While investigating structure-property relationships we note an enhancement of the real and imaginary parts of the cubic hyperpolarizability in structures of trigonal symmetries, and, in particular, of dendritic geometries with multipolar charge distribution, due to the presence of charge acceptor groups. Two-photon absorption cross sections achievable in these structures are comparable with the best values reported in the literature. We also show that, for some of the ruthenium alkynyl complexes investigated in this study, it is possible to perform electrochemical switching between two forms of the compound. The nonlinear properties of the two forms were investigated by the technique of Z-scan carried out in situ in an electrochemical cell. The neutral form is essentially nonabsorbing in the infrared but shows two-photon absorption at 800 nm. Upon oxidation the complex becomes infrared absorbing and the oxidized form exhibits an absorption saturation effect at 800 nm. This electrochromic switching of both the linear absorption and the sign of the imaginary part of the third-order susceptibility is found to be reversible and is, therefore, of potential application interest.
Time-resolved degenerate four-wave mixing (DFWM) experiments performed on films of triphenylamino-phenylene vinylene (TPA-PPV) copolymer show the modulus of the nonlinear refractive index to be (2.1#0.4) x10-13 cm2/W at 800 nm. The polymer was synthesized in the Hoerner-type polycondensation reaction. The films were characterized by optical absorption spectra, molecular weight and glass transition temperature measurements. The linear refractive index measurements performed with a prism coupler indicate that the annealed polymer films are isotropic. The films showed waveguiding of light. The DFWM experiments were performed in the forward BOXCARS geometry with simultaneous monitoring of the phase-matched and the non-phase-matched signals. This allowed measuring the nonlinearity of sub-micrometer thick films even in the presence of signals from a thick glass substrate. A cubic power dependence of the diffracted signal vs. pump intensity was observed as expected for the Kerr-type electronic nonlinearity. The signals showed a strong instantaneous response followed by a slow decay with the time constant 9#2 ps. Z-scan measurements showed the presence of two-photon absorption in the polymer.
Third-order optically nonlinear polymeric material built of rigid-rod molecules are often insoluble and transmit visible light poorly. We report on optical and waveguiding properties of a new soluble derivative of PPV: a π- conjugated polymer DFP-PDPV. We found that solvents used for processing of the DFP-PDPV polymer may influence the film optical properties. Changes in absorption spectra, birefringence, optical attenuation and the nonlinear refractive index were observed in the films made by spin coating and solution cast using different solvents.
The films are birefringent and dispersive. Refractive indices vary from 1.76 to 1.63 for the TE polarisation of the incident
light and from 1.70 to 1.60 for the TM polarisation for wavelengths from 476.5 nm to 1.55μ, respectively. The birefringence is an order ofmagnitude lower than that in films ofunsubstituted PPV.
DFP-PDPV is suitable for fabrication of optical waveguides. Low loss waveguiding layers could be prepared. Propagation losses were measured at 632.8nm, 8lOnm and 1064nm. The losses decrease at longer wavelengths reaching the level of 1 dB/cm at 1.06μ. The waveguiding properties of DFP-PDPV films depend on the solvents used in processing of the polymer. We investigated thin films of DFP-PDPV for their potential for nonlinear waveguide applications. The modulus of nonlinear refractive index |n2| in the range (0.9-1.5) x10-14 cm2/W was measured in DFP-PDPV films at 8OOnm using a femtosecond degenerate four wave mixing (DFWM) technique. These studies supplement the results of nonlinear optical studies of this polymer in solution using the Z-scan technique described in Proceedings of SPIE 3473, 79-90 (1998).
The techniques of closed and open aperture Z-scan as well as that of phase-matched and non-phase-matched degenerate four- wave mixing (DFWM) were used to investigate nonlinear optical properties of two poly(p-phenylenevinylenes) substituted with the phenoxyphenyl and fluorophenyl groups at the vinylene position. The polymers studied were poly[1,4-phenylene-1,2- di(4-phenoxyphenyl)vinylene] (DPOP-PPV) and poly[4,4'-biphenyl-diyl-1,2-di(4- fluorophenyl)vinylene] (DFP-PDPV). Spectroscopic studies showed a blue shift of absorption spectra of both polymers in comparison to that of poly(p-phenylenevinylene) (PPV). Z-scan experiments for solutions of the polymers in 1,4-dioxane and DFWM experiments for thin films of the polymers were carried out at 800 nm with amplified 100 femtosecond pulses. The results of both techniques are in good agreement, indicating that the nonlinear refractive index n2 is on the order of 10-14 cm2/W for the substituted polymers. The sign of the real and imaginary parts of n2 is positive. The polymers possess relatively low values of two-photon absorption merit factors at 800 nm. The nonlinear properties of the substituted polymers were compared to the results obtained on unsubstituted PPV films.
We show that the chemical and physical environment used to synthesise and process poly(p-phenylenevinylene) (PPV) by the soluble precursor route in the matrix of poly(N-vinylpyrrolidone) (PVP) has a remarkable effect on the effective conjugation length of the resulting polymer, which directly affects the third order nonlinearity. A peculiar bathochromic shift of the position of the PPV absorption maximum, an increase of absorption coefficients and the anisotropy of refractive indices upon conversion have been observed. A large third-order optical nonlinearity (In2I>lO cm2/W) at 800 nm can be achieved in the composites. The nonlinearity of the composites depends on the PPV content and the conversion conditions.
The PPV-PVP system is interesting from the point of view of its good waveguiding properties in the thin film planar waveguide structures.
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