Plasmon-enhanced multi-photon excited photoluminescence of Au, Ag, and Pt nanoclusters.

In this work, we have studied the multi-photon excited photoluminescence from metal nanoclusters (NCs) of Au, Ag and Pt embedded in Al2O3matrix by ion implantation. The thermal annealing process allows to obtain a system composed of larger plasmonic metal nanoparticles (NPs) surrounded by photoluminescent ultra-small metal NCs. By exciting at 1064 nm, visible emission, ranging from 450 to 800 nm, was detected. The second and fourth-order nature of the multiphoton process was verified in a power-dependent study measured for each sample below the damage threshold. Experiments show that Au and Ag NCs exhibit a four-fold enhanced multiphoton excited photoluminescence with respect to that observed for Pt NCs, which can be explained as a result of a plasmon-mediated near-field process that is of less intensity for Pt NPs. These findings provide new opportunities to combine plasmonic nanoparticles and photoluminescent nanoclusters inside a robust inorganic matrix to improve their optical properties. Plasmon-enhanced multiphoton excited photoluminescence from metal nanoclusters may find potential application as ultrasmall fluorophores in multiphoton sensing, and in the development of solar cells with highly efficient energy conversion modules.

Enhanced ultrafast optomagnetic effects in room-temperature ferromagnetic Pt nanoclusters embedded in silica by ion implantation.

Ferromagnetic-like behavior at room temperature (300 K) was observed in Pt particles embedded in ion-implanted silica matrices. Results in samples integrated by ultra-small photoluminescent Pt clusters (<2 nm) were compared with samples containing exclusively larger plasmonic Pt nanoparticles (>3 nm). The ferromagnetic behavior coexists simultaneously with a diamagnetic response. Enhanced diamagnetic response of one order of magnitude was observed compared to typical diamagnetism in pure silica, and it is increased with the mean diameter of the Pt particles. Besides, a larger sensitivity to an external field was observed in the ferromagnetic response of the nanostructures with a characteristic saturation at 20 kOe. This ferromagnetic behavior was only observed in the samples with nucleated Pt particles. The magnitude of the saturation magnetization shows up to a fivefold increase in the samples with smaller particle size and larger particle density. Saturation magnetization was observed between 3-15 × 10-4 emu g-1, with remanent magnetization of 0.2-0.6 × 10-4emu g-1, measured at 300 K. Coercitive fields also decrease in samples with smaller size and particles density, with values of 114 and 300 Oe. At lower temperatures (5 K) the saturation magnetization increases, as it would be expected from a ferromagnetic state. Optomagnetic response was studied by inverse Faraday effects and induced photomagnetization with circular polarized picosecond pulsed light at 1064 nm wavelength. Results showed that samples with a stronger ferromagnetic response exhibit larger Faraday rotation up to 5.3 × 103deg cm-1 by light excitations with irradiances between 50 and 180 GW cm-2. These findings have immediate applications in multifunctional solid-state magneto-optical devices such as optical isolators, high-data storage devices and ultrafast all-optical switching of magnetization.

Plasmon coupling interactions and inhibition of nonlinear absorption in a complex system with Ag and Pt nanoparticles in silica.

The optical response exhibited by a complex hybrid system integrated by Pt ultrasmall fluorescent particles and plasmonic Ag nanoparticles is reported. The system was synthesized by coimplantation of Ag and Pt ions into a silica matrix followed by a proper thermal annealing. The energies and fluences were chosen in order to overlap the spatial regions of the Ag and Pt ion distributions below the silica surface. Optical absorption and emission spectroscopies show that the complex nanostructures exhibit an important plasmonic response, together with photoluminescence excited at 355 nm, which is enhanced when compared to the reference sample with only Pt particles. Off-resonance nonlinear transmission and Z-scan measurements were undertaken using ultrafast pulses. High-irradiance excitation at 1064 nm with picosecond pulses shows that the Pt or Ag nanoparticles exhibit a two-photon absorption effect, while the complex system shows the absence of any nonlinear absorption. Similar observations were made using femtosecond pulses at 800 nm wavelength. This inhibition of the two-photon absorption effect and enhancement in the emission of the complex hybrid samples by the synergic participation of Ag and Pt particles can be explained as a result of a plasmon coupling via the near-field interaction between plasmonic and emitting sources.

Superlinear Photoluminescence by Ultrafast Laser Pulses in Dielectric Matrices with Metal Nanoclusters.

An intense photoluminescence emission was observed from noble metal nanoclusters (Pt, Ag or Au) embedded in sapphire plates, nucleated by MeV ion-implantation and assisted by an annealing process. In particular, the spectral photoluminescence characteristics, such as range and peak emission, were compared to the behavior observed from Pt nanoclusters embedded in a silica matrix and excited by UV irradiation. Correlation between emission energy, nanoclusters size and metal composition were analyzed by using the scaling energy relation EFermi/N1/3 from the spherical Jellium model. The metal nanocluster luminescent spectra were numerically simulated and correctly fitted using the bulk Fermi energy for each metal and a Gaussian nanoclusters size distribution for the samples. Our results suggest protoplasmonics photoluminescence from metal nanoclusters free of surface state or strain effects at the nanoclusters-matrix interface that can influence over their optical properties. These metal nanoclusters present very promising optical features such as bright visible photoluminescence and photostability under strong picosecond laser excitations. Besides superlinear photoluminescence from metal nanoclusters were also observed under UV high power excitation showing a quadratic dependence on the pump power fluence.

Laser-induced electrical signal filtering by multilayer reduced graphene oxide decorated with Au nanoparticles.

Nanoscale plasmonic particles represent a crucial transformation on optical and electronic properties exhibited by advanced materials. Herein are reported remarkable interferometric optical effects with dependence on polarization for filtering or modulating electronic signals in multilayer nanostructures. Metallic nanoparticles were incorporated in randomly distributed networks of reduced graphene oxide by an in-situ vapor-phase deposition method. The polarization-selectable nonlinear optical absorption contribution on the photoconductivity of reduced graphene oxide decorated with gold nanoparticles was analyzed. Nanosecond pulses at 532 nm wavelength were employed in a two-wave mixing experiment to study photoconduction and nonlinear optical absorption in this nanohybrid material. The ablation threshold of the sample was measured in 0.4 J/cm2. Electrochemical impedance spectroscopy measurements revealed a capacitive response that can be enhanced by gold decoration in carbon nanostructures. A strong two-photon absorption process characterized by 5 × 10-7 m/W was identified as a physical mechanism responsible for the nonlinear photoconductive behavior of the nanostructures. Experimental shift of 1 MHz for the cutoff frequency associated with an electrical filter function performed by the sample in film form was demonstrated. Moreover, amplitude modulation of electronic signals controlled by the polarization of a two-wave mixing experiment was proposed. All-optical and optoelectronic nanosystems controlled by multi-photonic interactions in carbon-based materials were discussed. The key role of the vectorial nature of light in two-wave mixing experiments is a fascinating tool for the exploration of low-dimensional systems.

Removal of aqueous chromium and environmental CO2 by using photocatalytic TiO2 doped with tungsten.

Removal of hexavalent chromium was accomplished by using photocatalyst materials of TiO2 doped with tungsten oxide, environmental air as oxygen supply and white light as irradiation source. Dichromate anions in concentration ranges of 50 to 1000 µg/L were removed by means of aqueous dispersions of TiO2 doped with tungsten. The aqueous chromium analyses were performed by Differential Pulse Voltammetry technique. Additionally, mineralization of CO2 gas was promoted by the photocatalysis process, as was clearly shown by Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS) analyses obtained from the TiO2 samples recovered after photocatalytic experiments. Results of sample analyses by Scanning Electron Microscopy (SEM) and High Resolution Transmission Electron Microscopy (HRTEM) are presented and discussed.

Mechano-optical effects in multiwall carbon nanotubes ethanol based nanofluids.

A highly sensitive technique for analyzing surface tension and dynamic viscosity of nanofluids was reported. Multiwall carbon nanotubes suspended in ethanol were evaluated. The assistance of a Fabry-Perot interferometer integrated by a small sample volume fluid allowed us to explore the stability and mechanical properties exhibited by the nanostructures. The surface tension and dynamic viscosity of the colloid was examined by using interferometric optical signals reflected from a remnant drop pending at the end of an optical fiber. Nanosecond pulses provided by a Nd:YAG laser source with 9.5 MW/mm2 at 532 nm wavelength were used to induce mechano-optical effects in the liquid drop. The mechanical parameters were approximated, taking into account single optical pulses interacting with an inelastic mass-spring-damper system.

Magneto-conductivity and magnetically-controlled nonlinear optical transmittance in multi-wall carbon nanotubes.

The impact of vectorial magnetic field effects on electrical conductivity and nonlinear optical transmittance exhibited by multi-wall carbon nanotubes was studied. The samples were synthetized by an aerosol pyrolysis processing route in a thin film form. Optical signals in a two-wave mixing configuration allowed us to identify two orthogonal directions of propagation for a magnetic field travelling through the nanomaterials studied. A selective modification in optical absorption was considered to be induced by magnetic perturbations in the sample. Standard optical Kerr gate measurements were carried out for exploring the third order nonlinear optical behavior of the film. A capacitive effect influenced by optical and magnetic excitations was distinguished to be characteristic of the sample. Magneto-quantum conductivity sensitive to the direction of an external magnetic field interacting with the tubes was analyzed. Magnetically-induced changes in electronic band parameters seem to be the main responsible for the optical and electrical modulation observed in the nanostructures. Immediate applications for developing magneto-optical and magneto-electrical functions can be contemplated.

Low-dimensional multiplexing: the magneto-optical Kerr effect in an individual FeCoCu nanowire.

Due to their selective and fascinating effects, metallic nanoparticles have become a very significant topic for science. A modification in morphology and structure of low-dimensional materials can result in extraordinary ultrafast physical phenomena. New findings related to electronic, optical and magnetic processes have emerged from surface plasmon resonance excitations in nanoparticles. Moreover, multi-functional systems can be obtained from the integration of different elements in a nanostructured configuration. Recently, Palmero et al (2015 Nanotechnology 26 461001) have reported magneto-optical Kerr effect explorations in individual FeCuCo nanowires; the influence of tailored morphologies exhibited by particular samples was analyzed. An important magnetization reversal action was revealed and it was concluded that the demagnetization may be responsible for vortex domain wall propagation. The report can provide a solid base for future research and immediate applications in modern spintronics or magnetic data storage can be contemplated.

Exclusive-OR Encryption by Photoconduction and Two-Photon Absorption in Carbon Nanotubes.

A two-wave photoconductive system dependent on the nonlinear optical absorption in carbon nano-tubes is presented. Optical irradiation at 532 nm wavelength and 1 nanosecond pulse duration was employed for performing the experiments. A vectorial two-wave mixing configuration was used in order to measure the absorptive and refractive nonlinearities. A single-beam transmittance technique was carried out to evaluate the photoconductivity and also it allows us to confirm the participation of the nonlinear optical absorption displayed by the samples. A two-photon absorption effect was identified as the main physical mechanism associated to the third order absorptive nonlinearity. The exclusive disjunctive logic function was achieved by the optoelectronic response of an interferometric configuration. An ultrasonic spray pyrolysis processing route was utilized for the preparation of the samples. The morphology of the nanotubes was estimated by using scanning electronic microscopy. By combining the photoconductive response of two different carbon nanotubes thin film samples, a straightforward XOR encryption was performed.

Collective optical Kerr effect exhibited by an integrated configuration of silicon quantum dots and gold nanoparticles embedded in ion-implanted silica.

The study of the third-order optical nonlinear response exhibited by a composite containing gold nanoparticles and silicon quantum dots nucleated by ion implantation in a high-purity silica matrix is presented. The nanocomposites were explored as an integrated configuration containing two different ion-implanted distributions. The time-resolved optical Kerr gate and z-scan techniques were conducted using 80 fs pulses at a 825 nm wavelength; while the nanosecond response was investigated by a vectorial two-wave mixing method at 532 nm with 1 ns pulses. An ultrafast purely electronic nonlinearity was associated to the optical Kerr effect for the femtosecond experiments, while a thermal effect was identified as the main mechanism responsible for the nonlinear optical refraction induced by nanosecond pulses. Comparative experimental tests for examining the contribution of the Au and Si distributions to the total third-order optical response were carried out. We consider that the additional defects generated by consecutive ion irradiations in the preparation of ion-implanted samples do not notably modify the off-resonance electronic optical nonlinearities; but they do result in an important change for near-resonant nanosecond third-order optical phenomena exhibited by the closely spaced nanoparticle distributions.

Optical Kerr phase shift in a nanostructured nickel-doped zinc oxide thin solid film.

The optical Kerr effect exhibited by a nickel doped zinc oxide thin solid film was explored with femto- and pico-second pulses using the z-scan method. The samples were prepared by the ultrasonic spray pyrolysis technique. Opposite signs for the value of the nonlinear refractive index were observed in the two experiments. Self-defocusing together with a two-photon absorption process was observed with 120 ps pulses at 1064 nm, while a dominantly self-focusing effect accompanied by saturated absorption was found for 80 fs pulses at 825 nm. Regarding the nanostructured morphology of the resulting film, we attribute the difference in the two ultrafast optical responses to the different physical mechanism responsible of energy transfer generated by multiphoton processes under electronic and thermal effects.

Photoconductive logic gate based on platinum decorated carbon nanotubes.

Electrical and nonlinear optical experiments were performed on multiwall carbon nanotubes (CNTs) prepared by a chemical vapor deposition method. We report that the incorporation of platinum particles on the CNTs surface originates an enhancement in the photoconductive properties with noticeable capabilities to modulate optical and electrical signals. The photoconductive logic gate function OR was experimentally demonstrated using a simple photoconductive platform based on our samples. A two-photon absorption effect was identified as the main mechanism of third-order optical nonlinearity under a nonresonant nanosecond excitation. Multiphotonic interactions were described in order to explain the observed behavior.

Optoelectronic modulation by multi-wall carbon nanotubes.

We report the observation of photoconduction and a strong nonlinear optical absorptive response exhibited by multi-wall carbon nanotubes. An aerosol pyrolysis method was employed for the preparation of the samples. Measurements of the optical transmittance with 7 ns pulses at 1064 nm wavelength allowed us to identify a two-photon absorption effect as the main mechanism of third-order nonlinearity. Photoconductive experiments at 445 nm wavelength seem to confirm the possibility for generating non-resonant multi-photonic absorption processes in the multi-wall carbon nanotubes. By the optical control of the conductivity in the nanotubes, we implement an optoelectronic amplitude modulator device with potential applications for sharp selective functionalities.

Modulation of the propagation speed of mechanical waves in silicon quantum dots embedded in a silicon-nitride film.

Using a vectorial picosecond self-diffraction method, we evaluate the modification of the speed of the sound in a silicon-nitride film containing silicon quantum dots prepared by remote plasma-enhanced chemical vapor deposition. Our non-contact technique is based on the stimulation of the electrostriction contribution to the nonlinearity of index exhibited by the sample in a multiwave mixing laser experiment. We identified the electronic birefringence using two of the incident beams to generate a self-diffraction signal, then, we modified the third order nonlinear response by means of the optical Kerr effect given by a phase-mismatched third beam which induced electrostriction. Our results indicated that the speed of the sound in a silicon-nitride film can be simultaneously tailored by an electronic nonlinear refractive index, and by an electrostriction effect, both resulting from silicon quantum dots doping.

Ultrafast nonlinear optical response of photoconductive ZnO films with fluorine nanoparticles.

The absorptive and refractive third order nonlinear optical properties exhibited by a ZnO thin solid film with fluorine nanoparticles were studied with picosecond and femtosecond pulses using different techniques. We were able to evaluate the photoconductivity of the material and the quenching of the induced birefringence observed in the presence of two-photon absorption. The samples were prepared by a chemical spray deposition technique. In order to investigate the different contributions of the third order nonlinearities of the film, we analyzed the vectorial self-diffraction effect and the optical Kerr transmittance observed in the sample. A dominantly absorptive nonlinearity was measured at a 532 nm wavelength with 50 ps pulses, while nonlinear refraction was found to be negligible in this regime. On the other side, a pure electronic refractive third order nonlinearity without the contribution of nonlinear absorption was detected at 830 nm with 80 fs pulse duration. A quasi-instantaneous optical response and a strong enhancement in the ultrafast nonlinear refraction with the inhibition of the picosecond two-photon absorption mechanism were measured for the case of the femtosecond excitation.

Ultrafast optical phase modulation with metallic nanoparticles in ion-implanted bilayer silica.

The nonlinear optical response of metallic-nanoparticle-containing composites was studied with picosecond and femtosecond pulses. Two different types of nanocomposites were prepared by an ion-implantation process, one containing Au nanoparticles (NPs) and the other Ag NPs. In order to measure the optical nonlinearities, we used a picosecond self-diffraction experiment and the femtosecond time-resolved optical Kerr gate technique. In both cases, electronic polarization and saturated absorption were identified as the physical mechanisms responsible for the picosecond third-order nonlinear response for a near-resonant 532 nm excitation. In contrast, a purely electronic nonlinearity was detected at 830 nm with non-resonant 80 fs pulses. Regarding the nonlinear optical refractive behavior, the Au nanocomposite presented a self-defocusing effect, while the Ag one presented the opposite, that is, a self-focusing response. But, when evaluating the simultaneous contributions when the samples are tested as a multilayer sample (silica-Au NPs-silica-Ag NPs-silica), we were able to obtain optical phase modulation of ultra-short laser pulses, as a result of a significant optical Kerr effect present in these nanocomposites. This allowed us to implement an ultrafast all-optical phase modulator device by using a combination of two different metallic ion-implanted silica samples. This control of the optical phase is a consequence of the separate excitation of the nonlinear refracting phenomena exhibited by the separate Au and Ag nanocomposites.

Inhibition of the two-photon absorption response exhibited by a bilayer TiO2 film with embedded Au nanoparticles.

We use two different synthesis approaches for the preparation of TiO(2) films in order to study their resulting third order optical nonlinearity, and its modification by the inclusion of Au nanoparticles in one of the samples. An ultrasonic spray pyrolysis method was used for preparing a TiO(2) film in which we found two-photon absorption as a dominant nonlinear effect for 532 nm and 26 ps pulses; and a purely electronic nonlinearity at 830 nm for 80 fs pulses. A strong optical Kerr effect and the inhibition of the nonlinear optical absorption in 532 nm can be obtained for the first sample if Au nanoparticles embedded in a second TiO(2) film prepared by a sol-gel technique are added to it. We used an optical Kerr gate, z-scan, a multi-wave mixing experiment and an input-output transmittance experiment for measuring the optical nonlinearities.

Modification of the nonlinear optical absorption and optical Kerr response exhibited by nc-Si embedded in a silicon-nitride film.

We studied the absorptive and refractive nonlinearities at 532 nm and 26 ps pulses for silicon-nitride films containing silicon nanoclusters (nc-Si) prepared by remote plasma-enhanced chemical vapor deposition (RPECVD). Using a self-diffraction technique, we measured for the as-grown sample beta=7.7x10(-9)m/W, n(2)=1.8x10(-16)m(2)/W, and /chi(3)1111/ = 4.6x10(-10)esu; meanwhile, when the sample was exposed to an annealing process at 1000 degrees C during one hour in a nitrogen atmosphere, we obtained beta=-5x10(-10)m/W, n2=9x10(-17)m(2)/W, and /chi(3)1111/=1.1x10(-10)esu. A pure electronic nonlinear refraction was identified and a large threshold ablation of 41 J/cm(-2) was found for our films. By fitting nonlinear optical transmittance measurements, we were able to estimate that the annealed sample exhibits a response time close to 1 fs. We report an enhancement in the photoluminescence (PL) signal after the annealing process, as well as a red-shift due to an increment in size of the nc-Si during the thermal process.

Thermo-optic effect and optical third order nonlinearity in nc-Si embedded in a silicon-nitride film.

Using a self-diffraction experiment with 7ns pulses at 532nm we studied a silicon nitride film containing silicon nanoclusters (nc-Si) of 3.1+/-0.37 nm mean size. The sample was prepared by remote plasma-enhanced chemical vapor deposition (RPECVD), and we found that its nonlinearity consists of a combination of electronic and thermal contributions. By varying the repetition rate of the laser, we discriminated the responsible mechanisms for the nonlinear response. Using this procedure we determined a total /chi((3))1111/ = 3.3x10(-10)esu, n2 = 2.7x10(-16) m(2)/W, beta = 1x10(-9) m/W and dn/dT =1x10(-4) degrees C(-1) for our sample. We also show results for the optical Kerr effect using 80 fs pulses at 820 nm. The purely electronic nonlinearity measured is characterized by /chi((3))1111/=9.5 x10(-11) esu.