Magnetooptical Kerr effect measurements of ultrafast spin dynamics in cobalt nanodots

We present our magnetooptical Kerr effect (MOKE) studies on picosecond spin dynamics in closely spaced rectangular Co dots with sizes ranging from 2/spl times/6 /spl mu/m/sup 2/ down to 100 /spl times/ 300 nm/sup 2/ under in-plane, picosecond, magnetic pulse excitation. A low-temperature-grown GaAs photoconductive switch, excited by femtosecond laser pulses, was used to produce /spl sim/25-ps-long magnetic transients along the surface of the coplanar-waveguide centerline, which contained arrays of patterned Co dots. The resulting spin dynamics in the dots was detected by a time-synchronized train of femtosecond optical probe pulses employing the time-resolved MOKE. Our experimental technique allowed us to measure the initial picosecond dynamics of spins in Co nanodots, followed by damped oscillations. We ascribe the observed results to the small-angle coherent spin precession and show that it depends on the size of magnetic dots.     

Glass formation and third-order optical nonlinear characteristics of bismuthate glasses within Bi2O3–GeO2–TiO2 pseudo-ternary system

Glass-forming region of Bi₂O₃–GeO₂–TiO₂ (BGT) pseudo-ternary system was determined by using melt-quench method. A series of high transparent glass samples were selected and their structural characteristics were investigated by FT-IR and Raman spectra. By employing Z-scan and optical Kerr shutter techniques with femtosecond laser pulses as excitation source, third-order optical nonlinearities (TON) of the BGT glasses as well as the TON response time were investigated at wavelength of 800 nm. The ultrafast nonlinear response and high figure of merit suggest great potentials of BGT glasses in applications of all-optical switching or related optical devices.     

Realization of wavelength conversion with hyperbolic secant femtosecond pulse in normal dispersion regime

Realization of wavelength conversion based on second-order femtosecond dark solitons with hyperbolic secant pulse is presented. This is achieved by introducing localized dispersion perturbation along the optical fiber. We demonstrate that an initial 30 fs second-order pulse decays to similar sub-pulses by applying perturbation using a step increment of β₂ from 6.3 to 15.75 ps² km^?1. This shows that the realization of a 1 × 2 channel wavelength converter for femtosecond pulses is possible. Recent research shows the possibility of realizing wavelength conversion generated from picosecond solitons neglecting nonlinear effects. However, employing the same method for femtosecond pulses fails due to the manifestation of nonlinear effects. In this paper, pulse deformation under different levels of perturbation was tested, and appropriate perturbation leading to similar sub-pulses is achieved.     

Surface plasmon-mediated energy transfer in heterogap Au-Ag nanowires

We report the observation of energy transfer from a gold (Au) nanodisk pair to a silver (Ag) nanowire across a 120 nm gap via surface plasmon resonance (SPR) excitation. The enhanced electromagnetic (EM) fields generated by Au SPR excitation induce oscillation of the conduction electrons in the Ag segment, transferring energy to it even though the Ag segment has only weak resonant interactions with the incident EM radiation. The induced Ag SPR produces strong EM fields at the position of the Ag segment, leading to a Raman signal approximately 15 times greater than when the Ag segment is alone (not adjacent to the Au nanodisk pair). The Raman intensity is found to depend nonlinearly on the incident laser intensity for laser power densities of 10 kW/cm(2), which is consistent with the results of EM theory calculations which are not able to account for the factor of 15 enhancement based on a linear mechanism. This suggests that energy transfer from the Au disk pair to the Ag segment involves an enhanced nonlinear polarization mechanism such as can be produced by the electronic Kerr effect or stimulated Raman scattering.     

Necklace-shaped Au-Ag nanoalloys: laser-assisted synthesis and nonlinear optical properties

Here in this paper, necklace-shaped Au-Ag nanoalloys (NAs) have been synthesized by a laser-based approach. A chain of Ag nanoparticles (NPs), which were joined together with Au junctions, was formed upon copper vapor laser (CVL) irradiation of a colloidal mixture of Ag and Au NPs; while the corresponding NPs were separately provided by laser ablation of gold and silver targets in deionized water by a 1064 nm Q-switched Nd:YAG laser. Dependence of the NAs development process on the CVL irradiation time in three distinct stages of as-mixed, nucleation and complete formation has been systematically studied by UV-vis optical absorption spectroscopy analysis as well as by transmission electron microscopy (TEM), which was exploited to visually confirm the NAs evolution through the process. Furthermore, the x-ray photoelectron spectroscopy (XPS) technique was accurately employed to determine the synthesized alloy content. On the other hand, using the open-and closed-aperture Z-scan technique, the nonlinear absorption (NLA) as well as nonlinear refraction (NLR) changes in Au-Ag NAs were investigated through their formation. The deduced results from the nonlinear optical properties of the colloidal NAs in the mentioned stages were interpreted considering the spectroscopic and microscopic observations. The total change of individual Au and Ag NPs saturable absorption (SA) into the reverse saturable absorption (RSA) behavior was concluded through the evolution into Au-Ag NAs.     

Linear and nonlinear optical refractions of CR39 composite with CdSe nanocrystals

Hybrid composites of CdSe nanocrystals embedded in allyl diglycol carbonate (CR39) matrices have been prepared and characterized. The measurements show that the linear refractive index of the composite decreases as the CdSe nanocrystal’s weight-percentage concentration increases at the laser wavelengths of 632.8 nm and 532 nm. The room temperature nonlinear optical properties of the hybrid composites were investigated using a single-beam Z-scan technique with femtosecond laser pulses at the wavelengths of 794 nm and 397 nm. The experimental data reveals that the Kerr nonlinear refractive index n₂ of the composite increase at these wavelengths when the CdSe nanocrystal’s weight-percentage concentration increases. Also, the nonlinear refractive index n₂ of the CdSe/CR39 hybrid composites exhibit dispersion from a positive value at 794 nm (below the band gap) to a positive value at 395 nm (above the band gap). The measured dispersion of n₂ is roughly consistent with the Sheik-Bahae’s theory for the bound electronic nonlinear refraction resulting from the two-photon resonance.     

Investigation of nonlinear dynamics in CdS by time-resolved nondegenerate pump–probe system with phase object

The time-resolved nondegenerate pump–probe system with phase object is employed for investigation of nonlinear absorption and refraction dynamics in CdS. The 532?nm laser beam with 21?ps duration is used as the excitation and the laser beams of 600 and 680?nm with 10?ps duration from optical parametric generation are used for probing. The experimental results at both probe wavelengths show free-carrier absorption and large free-carrier refraction along with two-photon absorption and bound electronic optical Kerr effect. By numerically fitting the experimental data based on the nondegenerate pump–probe theory, the nondegenerate two-photon absorption coefficient, the nondegenerate Kerr coefficient, the free-carrier decay time, the free-carrier absorptive cross-section and free-carrier refractive coefficient at different wavelengths are all determined.     

Limitations arising from two-photon absorption of solvent in pulsed-laser thermal lens detection: determination of the two-photon absorption coefficient of ethanol at 266 nm

Two-photon absorption of the solvent under pulsed-laser excitation at 266 nm produces a high background thermal lens signal interfering with the analyte signal. Discrimination of both solvent and analyte signals along with calibration of the photothermal response has allowed the determination of the two-photon absorption coefficient of ethanol. The obtained value, 3.0x10(-10) cm W-1, is close to the literature values obtained from transmittance measurements using picosecond or femtosecond laser pulses.     

Localized-surface-plasmon enhanced emission from porous silicon by gold nanoparticles

The porous silicon (PS) samples, decorated by Au nanoparticles (NPs) possessing localized-surface-plasmon (LSP) resonance, are prepared by the conventional anodization method. Photoluminescence (PL) is studied systematically, in particular, its dependence on the excitation power. It is found that undecorated PS samples exhibit a saturation behavior in PL intensity with increasing the pumping laser power, while the luminescence of Au-decorated PS hybrid samples have a purely linear dependence on the excitation power. In the linear response region of PS samples, addition of metal NPs layer moderately suppresses the emission while, in the saturation region, the net emission is enhanced by approximately up to 4-fold. Several possible mechanisms are discussed. We believe that the observed PL enhancement in saturation region is dominantly due to the resonant coupling between the LSP of Au NPs and the electronic excitation of PS, which inhibits the nonradiative Auger recombination process at high excitation power. These results indicate that the plasmon effect could be useful for designing even more efficient optoelectronic devices such as super bright light emitting devices and solar cells with high efficiencies. Despite many challenges, Au NPs can potentially be applied to introduce LSP resonance for the future silicon-based optoelectronics or photonics.     

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.     

A Solution‐Processed Ultrafast Optical Switch Based on a Nanostructured Epsilon‐Near‐Zero Medium

All the optical properties of materials are derived from dielectric function. In spectral region where the dielectric permittivity approaches zero, known as epsilon‐near‐zero (ENZ) region, the propagating light within the material attains a very high phase velocity, and meanwhile the material exhibits strong optical nonlinearity. The interplay between the linear and nonlinear optical response in these materials thus offers unprecedented pathways for all‐optical control and device design. Here the authors demonstrate ultrafast all‐optical modulation based on a typical ENZ material of indium tin oxide (ITO) nanocrystals (NCs), accessed by a wet‐chemistry route. In the ENZ region, the authors find that the optical response in these ITO NCs is associated with a strong nonlinear character, exhibiting sub‐picosecond response time (corresponding to frequencies over 2 THz) and modulation depth up to ≈160%. This large optical nonlinearity benefits from the highly confined geometry in addition to the ENZ enhancement effect of the ITO NCs. Based on these ENZ NCs, the authors successfully demonstrate a fiber optical switch that allows switching of continuous laser wave into femtosecond laser pulses. Combined with facile processibility and tunable optical properties, these solution‐processed ENZ NCs may offer a scalable and printable material solution for dynamic photonic and optoelectronic devices.     

Laser-induced damage of hafnia coatings as a function of pulse duration in the femtosecond to nanosecond range

Laser-damage thresholds and morphologies of hafnia single layers exposed under femtosecond, picosecond, and nanosecond single pulses (1030/1064 nm) are reported. The samples were made with different deposition parameters in order to study how the damage behavior of the samples evolves with the pulse duration and how it is linked to the deposition process. In the femtosecond to picosecond regime, the scaling law of the laser-induced damage threshold as a function of pulse duration is in good agreement with the models of photo and avalanche ionization based on the rate equation for free electron generation. However, differences in the damage morphologies between samples are shown. No correlation between the nanosecond and femtosecond/picosecond laser-damage resistance of hafnia coatings could be established. We also report evidence of the transition in damage mechanisms for hafnia, from an ablation process linked to intrinsic properties of the material to a defect-induced process, that exists between a few picoseconds and a few tens of picoseconds.     

Optical nonlinearities of Au/TiO2 films excited by high-repetition-rate femtosecond laser

The Au/TiO₂ composite films have been fabricated using the pulsed laser deposition technique. The optical transmission spectra of the films were investigated in the range from 300 to 900 nm and the surface plasmon resonance peaks were observed varying from 550 nm to 650 nm. The optical nonlinearities of the as-deposited and annealed samples were investigated by the Z-scan method using a high-repetition-rate femtosecond laser. It was found that, for the as-deposited samples, the sign of nonlinear absorption coefficients (β) changed from negative to positive with increasing Au concentration. However, for all the annealed samples, the sign of β was found to be negative. The nonlinear responses induced by the cumulative thermal effects were suggested to contribute to this phenomenon.     

Energy dependent saturable and reverse saturable absorption in cube-like polyaniline/polymethyl methacrylate film

Solid films of cube-like polyaniline synthesized by inverse microemulsion polymerization method have been fabricated in a transparent PMMA host by an in situ free radical polymerization technique, and are characterized by spectroscopic and microscopic techniques. The nonlinear optical properties are studied by open aperture Z-scan technique employing 5 ns (532 nm) and 100 fs (800 nm) laser pulses. At the relatively lower laser pulse energy of 5 μJ, the film shows saturable absorption both in the nanosecond and femtosecond excitation domains. An interesting switchover from saturable absorption to reverse saturable absorption is observed at 532 nm when the energy of the nanosecond laser pulses is increased. The nonlinear absorption coefficient increases with increase in polyaniline concentration, with low optical limiting threshold, as required for a good optical limiter.     

Fine-pitched microgratings encoded by interference of UV femtosecond laser pulses

Fine-pitched microgratings are encoded on fused silica surfaces by a two-beam laser interference technique employing UV femtosecond pulses from the third harmonics of a Ti:sapphire laser. A pump and prove method utilizing a laser-induced optical Kerr effect or transient optical absorption change has been developed to achieve the time coincidence of the two pulses. Use of the UV pulses makes it possible to narrow the grating pitches to an opening as small as 290 nm, and the groove width of the gratings is of nanoscale size. The present technique provides a novel opportunity for the fabrication of periodic nanoscale structures in various materials.     

Encryption of nonlinear optical signals in ZnO:Al thin films by ultrashort laser pulses

Described herein is the effect of optical annealing on the third-order non-linear optical properties exhibited by nanostructured Al-doped ZnO thin films. The samples were synthetized by an ultrasonic spray pyrolysis method. The optical annealing process was carried out by laser pulses at 532, 835 and 1064 nm wavelengths with, ps, fs and ps pulse duration, respectively. The optical non-linearity of the films was measured by the z-scan method with three different irradiations of excitation: 100 fs at 835 nm, 120 ps at 532 nm, and 150 ps at 1064 nm. The as-grown samples showed a saturable optical absorption that evolves into two-photon absorption transitions by a picosecond optical annealing phenomenon induced at 532 nm wavelength. Potential applications for developing optical encryption functions were considered.     

Carbon Nanotubes as an Ultrafast Emitter with a Narrow Energy Spread at Optical Frequency

Ultrafast electron pulses, combined with laser‐pump and electron‐probe technologies, allow ultrafast dynamics to be characterized in materials. However, the pursuit of simultaneous ultimate spatial and temporal resolution of microscopy and spectroscopy is largely subdued by the low monochromaticity of the electron pulses and their poor phase synchronization to the optical excitation pulses. Field‐driven photoemission from metal tips provides high light‐phase synchronization, but suffers large electron energy spreads (3–100 eV) as driven by a long wavelength laser (>800 nm). Here, ultrafast electron emission from carbon nanotubes (≈1 nm radius) excited by a 410 nm femtosecond laser is realized in the field‐driven regime. In addition, the emitted electrons have great monochromaticity with energy spread as low as 0.25 eV. This great performance benefits from the extraordinarily high field enhancement and great stability of carbon nanotubes, superior to metal tips. The new nanotube‐based ultrafast electron source opens exciting prospects for extending current characterization to sub‐femtosecond temporal resolution as well as sub‐nanometer spatial resolution.     

Multiwavelength picosecond and single wavelength femtosecond pulses emission in a passively mode-locked fiber laser using a semiconductor saturable absorber mirror and a contrast ratio tunable comb filter

We propose and demonstrate a highly flexible fiber laser capable of generating stable multiwavelength picosecond and single wavelength femtosecond pulses by using a semiconductor saturable absorber mirror and a contrast ratio tunable comb filter. In the multiwavelength lasing regime, up to 11-wavelength stable mode-locked pulses in 3 dB bandwidth with a channel spacing of 0.8 nm were obtained. While in the single wavelength with broadband spectrum lasing regime, the fiber laser emitted 576 fs soliton pulse. Through changing the contrast ratio of the comb filter, the conversion between the multiwavelength picosecond and single wavelength femtosecond pulsed operations could be efficiently achieved.     

Ultrafast near-field spectroscopy of single semiconductor quantum dots

Excitonic and spin excitations of single semiconductor quantum dots (QDs) currently attract attention as possible candidates for solid-state-based implementations of quantum logic devices. Due to their rather short decoherence times in the picosecond to nanosecond range, such implementations rely on using ultrafast optical pulses to probe and control coherent polarizations. We combine ultrafast spectroscopy and near-field microscopy to probe the nonlinear optical response of a single QD on a femtosecond time-scale. Transient reflectivity spectra show pronounced oscillations around the QD exciton line. These oscillations reflect phase-disturbing Coulomb interactions between the excitonic QD polarization and continuum excitations. The results show that although semiconductor QDs resemble in many respects atomic systems, Coulomb many-body interactions can contribute significantly to their optical nonlinearities on ultrashort time-scales.