Order Nonlinear Response Research Articles

Efficient second harmonic generation by para-nitroaniline embedded in electro-spun polymeric nanofibres

Intense well polarized second harmonic light was generated by poly(methyl methacrylate) nanofibres with embedded para-nitroaniline nanocrystals. Subwavelength diameter fibres were electro-spun using a 1:2 weight ratio of chromophore to polymer. Analysis of the generated second harmonic light indicates that the para-nitroaniline molecules, which nominally crystalize in the centrosymmetric space group, were organized into noncentrosymmetric structures leading to a second order susceptibility dominated by a single tensor element. Under the best deposition conditions, the nanofibrers display an effective nonlinear optical susceptibility approximately two orders of magnitude greater than that of potassium dihydrogen phosphate. Generalizing this approach to a broad range of organic molecules with strong individual molecular second order nonlinear responses, but which nominally form centrosymmetric organic crystals, could open a new pathway for the fabrication of efficient sub-micron sized second harmonic light generators.

Media polarity and concentration roles on the third order nonlinear behaviors of thiazine dyes

Nonlinear optical materials play important roles in optics and photonics. In order to improve the performance of these materials, in this paper, we tried to find effective methods for controlling the nonlinear responses of thiazine dyes. Our experimental results indicate that molecular linear properties, media polarity, concentration and excitation beam properties with different contributions play significant roles on the saturable and reverse saturable absorption characteristics of thiazine dyes. In real, competition between these effective parameters leads to appearance of various third order nonlinear responses. Although by excitation wavelengths near the absorption peaks and at high powers, dye molecules tend to indicate reverse saturable absorption characteristics in polar protic solvents by increasing the concentration, saturable absorption characteristics were observed in polar aprotic solvents. Furthermore, the samples displayed the same behaviors in different solvent media at low powers and for excitation wavelengths far from absorption peaks.Therefore, by controlling the nonlinear responses of thiazine dyes, these materials can indicate both saturable and reverse saturable absorption characteristics that increase their applications in optics and photonics devices.

Spectral dependence of nonlinear absorption in ordered silver metallic nanoprism arrays

Ordered metallic nanoprism arrays have been proposed as novel and versatile systems for the observation of nonlinear effects such as nonlinear absorption. The study of the effect of the local field reinforcement on the fast optical third order nonlinear response around the Surface Plasmon Resonance is of great interest for many plasmonic applications. In this work, silver nanoprism arrays have been synthesized by the nanosphere lithography method. A low repetition rate tunable picosecond laser source was used to study the irradiance and wavelength dependence of the nonlinear absorption properties around the dipolar and quadrupolar resonances of the nanoarray with the use of the z-scan technique. The irradiance dependence of the on-resonance nonlinearity was studied, and a spectral region where nonlinear absorption is negligible was identified. This is important for the possible application of these materials in optical information processing devices.

Relaxation of Microwave Nonlinearity in a Cuprate Superconducting Resonator

The 2nd and 3rd order nonlinear microwave response of a superconducting YBa2Cu3O7 thin film resonator was synchronously measured using three input tones. This technique permits the local measurement, and hence mapping, of intermodulation distortion (IMD) inside the resonator. 2nd and 3rd order IMD measured with a fixed probe relaxed in remarkably different ways after the removal of a static magnetic field. The 2nd order IMD relaxed by two different magnetic processes, a fast process that appears related to bulk remanent magnetization, and a slow process that fits the description of Bean and Livingston. The 3rd order IMD relaxes by only one process which is distinct from the two processes controlling 2nd order relaxation.

Environment effects on the nonlinear absorption properties of Methylene blue under different power of excitation beam

Abstract In this paper, the nonlinear optical absorption properties of Methylene blue solutions were studied by open aperture Z-scan technique under a continuous-wave laser irradiation with various input powers. The experimental results show that the mechanism of nonlinear absorption and the calculated coefficients depend highly on the molecular surrounding media properties and the nature of intermolecular interactions. From the experimental results, the contributions of various intermolecular interactions were determined as well. Exploiting these results, it is possible to determine the effective media polarity parameters on the nonlinear responses of optical materials. According to the experimental results, by increasing of excitation beam power, the strong hydrogen bonds with high contributions increase the third order nonlinear responses of Methylene blue molecules.

Nonlinear coupling in graphene-coated nanowires.

We propose and analyze nonlinear coupler based on a pair of single mode graphene-coated nanowires. Nonlinear wave interactions in such structure are analyzed by the coupled mode equations derived from the unconjugated Lorentz reciprocity theorem. We show that the routing of plasmons in the proposed structure can be controlled by the input power due to the third order nonlinear response of graphene layer. Our findings show that graphene nonlinearity can be exploited in tunable nanoplasmonic circuits based on low-loss, edgeless cylindrical graphene waveguides.

Third order nonlinear responses of Basic Blue 55 molecules in polar solvents

Molecular surrounding environment is one of the important factors that can modify nonlinear properties of materials. For precise investigation, in this paper, single beam Z-scan technique was used for measuring the third order nonlinear responses of Basic Blue 55 molecules in different solvent media. Reverse saturable absorption and self-defocusing characteristics were found to be highly dependent on molecular surrounding media properties.Moreover, from these experimental results the contribution of various intermolecular interactions on the third order nonlinear responses of Basic blue 55 molecules was determined. Our results show that strong hydrogen bonds between dye molecules and their surrounding medium have dominant role in increasing the nonlinear coefficients. So, by a quantitative studying, the precise investigation of nonlinear responses of dye solutions will be facilitated.

Measuring the nonlinear refractive index of graphene using the optical Kerr effect method.

By means of the ultrafast optical Kerr effect method coupled to optical heterodyne detection (OHD-OKE), we characterize the third-order nonlinear response of graphene and compare it to experimental values obtained by the Z-scan method on the same samples. From these measurements, we estimate a negative nonlinear refractive index for monolayer graphene, n2=-1.1×10-13 m2/W. This is in contradiction to previously reported values, which leads us to compare our experimental measurements obtained by the OHD-OKE and the Z-scan method with theoretical and experimental values found in the literature and to discuss the discrepancies, taking into account parameters such as doping.

Enhanced Third Harmonic Generation in Single Germanium Nanodisks Excited at the Anapole Mode.

We present an all-dielectric germanium nanosystem exhibiting a strong third order nonlinear response and efficient third harmonic generation in the optical regime. A thin germanium nanodisk shows a pronounced valley in its scattering cross section at the dark anapole mode, while the electric field energy inside the disk is maximized due to high confinement within the dielectric. We investigate the dependence of the third harmonic signal on disk size and pump wavelength to reveal the nature of the anapole mode. Each germanium nanodisk generates a high effective third order susceptibility of χ((3)) = 4.3 × 10(-9) esu, corresponding to an associated third harmonic conversion efficiency of 0.0001% at an excitation wavelength of 1650 nm, which is 4 orders of magnitude greater than the case of an unstructured germanium reference film. Furthermore, the nonlinear conversion via the anapole mode outperforms that via the radiative dipolar resonances by about 1 order of magnitude, which is consistent with our numerical simulations. These findings open new possibilities for the optimization of upconversion processes on the nanoscale through the appropriate engineering of suitable dielectric materials.

Third order non-linear response of II-VI semiconductor polymer nanocomposites with different polymers

Effect of two different host polymer matrices: Polyvinyl pyrrolidone (PVP) and Poly(vinylcarbazole) (PVK) on third order nonlinear optical properties of Ag:CdSe hybrid polymer nanocomposites has been investigated and discussed. Higher nonlinear behaviour of the PVK based nanocomposites as compared to PVP based nanocomposites is attributed to large electron delocalization in PVK matrix due to the two additional benzene rings. Nanocomposites show self focussing behaviour with two photon absorption processes. Large nonlinearity of CdSe hybrid polymer nanocomposites is the collective outcome of dielectric & Quantum confinement and surface Plasmon-exciton interaction effects. Third order nonlinear refraction parameters of these nanocomposites have been found to be improved with metal doping upto specific doping concentration due to the increased electron density.

Optomechanical Enhancement of Doubly Resonant 2D Optical Nonlinearity.

Emerging two-dimensional semiconductor materials possess a giant second order nonlinear response due to excitonic effects while the monolayer thickness of such active materials limits their use in practical nonlinear devices. Here, we report 3300 times optomechanical enhancement of second harmonic generation from a MoS2 monolayer in a doubly resonant on-chip optical cavity. We achieve this by engineering the nonlinear light-matter interaction in a microelectro-mechanical system enabled optical frequency doubling device based on an electrostatically tunable Fabry-Perot microresonator. Our versatile optomechanical approach will pave the way for next generation efficient on-chip tunable light sources, sensors, and systems based on molecularly thin materials.

Electronic resonances in broadband stimulated Raman spectroscopy.

Spontaneous Raman spectroscopy is a formidable tool to probe molecular vibrations. Under electronic resonance conditions, the cross section can be selectively enhanced enabling structural sensitivity to specific chromophores and reaction centers. The addition of an ultrashort, broadband femtosecond pulse to the excitation field allows for coherent stimulation of diverse molecular vibrations. Within such a scheme, vibrational spectra are engraved onto a highly directional field, and can be heterodyne detected overwhelming fluorescence and other incoherent signals. At variance with spontaneous resonance Raman, however, interpreting the spectral information is not straightforward, due to the manifold of field interactions concurring to the third order nonlinear response. Taking as an example vibrational spectra of heme proteins excited in the Soret band, we introduce a general approach to extract the stimulated Raman excitation profiles from complex spectral lineshapes. Specifically, by a quantum treatment of the matter through density matrix description of the third order nonlinear polarization, we identify the contributions which generate the Raman bands, by taking into account for the cross section of each process.

Numerical study of the optical nonlinearity of doped and gapped graphene: From weak to strong field excitation

Numerically solving the semiconductor Bloch equations within a phenomenological relaxation time approximation, we extract both the linear and nonlinear optical conductivities of doped graphene and gapped graphene under excitation by a laser pulse. We discuss in detail the dependence of second harmonic generation, third harmonic generation, and the Kerr effects on the doping level, the gap, and the electric field amplitude. The numerical results for weak electric fields agree with those calculated from available analytic perturbation formulas. For strong electric fields when saturation effects are important, all the effective third order nonlinear response coefficients show a strong field dependence.

Nonlinear Imaging Method Using Second Order Phase Symmetry Analysis and Inverse Filtering

This paper presents a nonlinear imaging method based on nonlinear elastic guided waves, for the damage detection and localisation in a composite laminate. The proposed technique relies on the study of the structural nonlinear responses by means of a combination of second order phase symmetry analysis (PSA) with chirp excitation and inverse filtering (IF) method. PSA was used to exploit the invariant properties of the propagating elastic waves with the phase angle of the pulse compressed chirp signals, in order to characterise the second order nonlinear behaviour of the medium. Then, the IF approach was applied to a library of second order nonlinear responses to obtain a two-dimensional image of the damage. The experimental tests carried out on an impact damage composite sample were compared to standard C-scan. The results showed that the present technique allowed achieving the optimal focalisation of the nonlinear source in the spatial and time domain, by taking advantage of multiple scattering and a small number of receiver sensors.

A nanolens-type enhancement in the linear and second harmonic response of a metallic dimer

In this paper we explore the linear and second-order nonlinear response of gold nanoparticle pairs (dimers). Despite that even-order nonlinear processes are forbidden in bulk centrosymmetric media like metals, second order nonlinear response exhibits a high degree of sensitivity for spherical nanoparticles where inversion symmetry is broken at the surface. Recent experiments demonstrate significant dependence of linear response and second-harmonic surface nonlinear response arising from the local fundamental field distribution in a dimer configuration. Our calculations are carried out taking into account high order multipolar interactions between metal nanoparticles, and demonstrate that linear and nonlinear optical responses of the dimer exhibit periodic behavior dependent on the separation distance between nanoparticles. This response increases for dimers with a large difference between particle sizes.

Linear and nonlinear density response functions for a simple atomic fluid

We use molecular dynamics simulations to investigate the linear and nonlinear density response functions for simple fluids under the influence of spatially periodic external fields. Using a direct Fourier space decomposition of the instantaneous microscopic density for the perturbed fluid we can clearly identify the distinct order of response. Using a single component sinusoidal longitudinal force for a set of wavelengths and amplitudes we show that in the linear response regime the proportionality between the external field amplitude and the density perturbation can be used to determine the linear density response function, and hence the pair correlation function, static liquid structure factor, and lowest order direct correlation function. We show also that for large external field amplitudes a single component external field can be used to determine the form for lowest order and second lowest order nonlinear response functions for restricted regions of the total response function spaces.

Analytical Volterra-based models for nonlinear low order flight dynamics approximation systems

AbstractAnalytical methodology is presented to conduct dynamical assembly of simple low order nonlinear responses for system synthesis and prediction using Volterra theory. The procedure is set forth generically and then applied to several atmospheric flight examples. A two-term truncated Volterra series, which is enough to capture the quadratic and bilinear nonlinearities, is developed for first and second order generalised nonlinear single degree of freedom systems. The resultant models are given in the form of first and second kernels. A parametric study of the influence of each linear and nonlinear term on kernel structures is investigated. A step input is then employed to quantify and qualify the nonlinear response characteristics. Uniaxial surge and pitch motions are presented as examples of the low order flight dynamic systems. These examples show the ability of the proposed analytical Volterra-based models to predict, understand, and analyse the nonlinear aircraft behaviour beyond that attainable by linear-based models. The proposed analytical Volterra-based model offers an efficient nonlinear preliminary design tool in qualifying the aircraft responses before computer simulation is available or invoked.

Parametric projection operator technique for second order non-linear response

We demonstrate the application of the recently introduced parametric projector operator technique to a calculation of the second order non-linear optical response of a multilevel molecular system. We derive a parametric quantum master equation (QME) for the time evolution of the excited state of an excitonic system after excitation by the first two pulses in the usual spectroscopic four-wave-mixing scheme. This master equation differs from the usual QME by a correction term which depends on the delay τ between the pulses. In the presence of environmental degrees of freedom with finite bath correlation time and in the presence of intramolecular vibrations we find distinct dynamics of both the excited state populations and the electronic coherence for different delays τ.

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.

Optical and dielectric properties of nitrophenolate based nonlinear optical crystal

A semi-organic nonlinear optical (NLO) material, lithium-p-nitrophenolate trihydrate (LPNP) was synthesized. Single crystals of dimensions 20 × 7 × 3 mm3 were harvested following the solvent evaporation technique. The functional groups present in the compound were identified from FT-IR and FT-Raman spectral analyses, and its molecular structure was confirmed. Identification of the compound was accomplished by X-ray diffraction technique (powder and single crystal XRD). The unit-cell dimensions and the morphology of the grown crystals were identified from single crystal XRD measurements. The thermal transport properties, thermal effusivity (e), thermal diffusivity (α), thermal conductivity (k) and heat capacity (C p) were measured by the photopyroelectric technique at room temperature. Dielectric constant and dielectric loss were also measured as a function of frequency between 42 Hz and 5 MHz, and temperature between 32 and 100 °C. From optical transmittance measurements, the direct optical band gap of the LPNP crystal was estimated to be 2.47 eV. Laser damage threshold is 60.91 GW cm−2. Powder second harmonic generation (SHG) measurement was carried out using a modified Kurtz–Perry technique. Third order nonlinear response was studied using Z-scan technique with a He–Ne laser (632.8 nm, 35 mW). The magnitude and the sign of the nonlinear absorption and nonlinear refraction are derived from a transmittance curve. The NLO parameters Intensity dependent refractive index n 2, nonlinear absorption coefficient β and third order susceptibility χ(3) were estimated.