Nonlinear plasmonic response in atomically thin metal films.

We fabricated Au nanoparticles on the surface of TiO2 by photo-reduction method. The structural property of film is investigated using transmission electron microscopy technique. The surface plasmon resonance absorption peaks of TiO2 and Au-TiO2 were recorded using Shimadzu (UV-2450) UV-visible spectrophotometer. The work function of films was measured by scanning Kelvin probe microscopy (SKP5050) from KP technology, United Kingdom. The nonlinear optical refractive index and nonlinear optical absorption coefficient of Au-TiO2 nanocomposites were simultaneously measured using z-scan technique. The nonlinear optical response of Au-TiO2 nanocomposites is due to pure electronic transfer effects in Au nanoparticles. The observed nonlinearity is due to the dielectric constant of Au, which is due to the surface plasmon resonance and surface polarization between Au nanoparticles and TiO2. The dielectric constant of Au shows maximum value at 532 nm wavelength (2.33 eV). Surface plasmon resonance effect of Au nanoparticles is directly related to the metal dielectric constant, therefore it is increased. The increase in surface plasmon rsonance of Au-TiO2 nanocomposites can be observed from UV-visible absorption spectra. The optical nonlinearity depends strongly on the dielectric constant of Au. Therefore, the optical nonlinearity increases with the increase in surface plasmon resonance peak. The absorption peak of surface plasmon resonance at 544 nm is inversely proportional to the work function. Therefore, we can say that the Au dopant decreases the work function that effectively increases the surface plasmon resonance absorption peak at 544 nm, so that the nonlinear response is enhanced. The systematic change in the work function with Au concentration plays a major role in optical nonlinearity. The estimated optical nonlinearity was found to increase from 3.80×10-6 to 9.69×10-6 esu with increase in Au concentrations from 0 to 1.0x10-2 mole. This observed increment in nonlinearity is due to the enhancement of local electric field created by excitation of surface plasmon resonance that affects the work function. Therefore, the surface plasmon resonance and work function help in tuning the optical nonlinearity. The tunable nonlinear optical response of the Au-TiO2 nanocomposites may find applications in nonlinear optics at wavelength 532 nm and the development of materials for generating the higher order harmonics.

Chapter 11 - Nonlinear optics and photonics applications of two-dimensional materials

The size, shape and composition of nanoparticles in a composite become design parameters that can be manipulated to enhance the third-order nonlinear response of the composite. In this work we report the study of the nonlinear response of a composite material consisting of elongated silver nanoparticles that are aligned in a preferential direction. The absorptive and refractive contributions to the nonlinear response of the sample were studied using the Z-scan technique with 80 fs pulses at 825 nm. The nonlinear response was studied as a function of the intensity, and the polarization angle of the light. A large anisotropy was observed in both the refractive and absorptive contributions to the nonlinearity. The fast electronic, and slow thermal contributions to the nonlinear response were resolved by conducting experiments using a chopper to change the thermal load to the sample, while maintaining a constant peak irradiance. The results were compared with the nonlinear optical response of spherical silver nanoparticles.

Majority of acoustic instruments widely used in industry and technology for non-destructive evaluation (NDE) make use of a linear elastic response of materials. The nonlinear approach to ultrasonic NDE is concerned with nonlinear material response, which is inherently related to the frequency changes of the input signal, and is a new technology for monitoring of deterioration in material properties and diagnostics of damage. The application field now includes both the nonlinear wave and nonlinear vibration modes. The former is based on the assumption and is applicable to the case studies of the distributed material nonlinearity. It profits from accumulation of the wave nonlinear response along the propagation distance and relies on the higher harmonic signals. A strong nonlinear response of non-bonded interfaces in planar defects introduces the nonlinearity localized in the defect area where the vibration nonlinearity steps up. The concept of local defect resonance (LDR) combined with its nonlinearity identifies a nonlinear inclusion as a nonlinear oscillator and brings about different dynamic and frequency scenarios in vibration nonlinear phenomena. The LDR-induced trapping of the nonlinearity generates a defect-selective nonlinearity and conditions for efficient and even noncontact nonlinear diagnostic imaging of damage.

A review of nonlinear oscillatory shear tests: Analysis and application of large amplitude oscillatory shear (LAOS)

In most practical scenarios, optical susceptibilities can be treated as a local property of a medium. For example, in the context of nonlinear optics we can typically treat the Kerr and Raman response as local, such that optical fields at one location do not produce a nonlinear response at distinct locations in space. This is because the electronic and vibrational disturbances produced within the material are confined to a region that is smaller than an optical wavelength. By comparison, Brillouin interactions, mediated by traveling-wave acoustic phonons, can result in a highly nonlocal nonlinear response as the elastic waves generated in the process can occupy a region in space much larger than an optical wavelength. The unique properties of these interactions can be exploited to engineer new types of processes, where highly delocalized phonon modes serve as an engineerable channel that mediates scattering processes between light waves propagating in distinct optical waveguides. These types of nonlocal optomechanical responses have recently been demonstrated as the basis for information transduction, however the nontrivial dynamics of such systems has yet to be explored. In this work, we show that the third-order nonlinear process resulting from spatially extended Brillouin-active phonon modes involves mixing products from spatially separated, optically decoupled waveguides, yielding a nonlocal susceptibility. Building on these concepts, we illustrate how nontrivial multi-mode acoustic interference can produce a nonlocal susceptibility with a multi-pole frequency response, as the basis for new optical and microwave signal processing schemes within traveling wave systems.

Progress in the development of new tools for modem apphed mathematics resulted in a better scientific under­ standing of nonlinear waves in various fields of nonlinear optics, hydrodynamics, and plasmas. The universahty of the equations describing wave processes is one of the most important components of this success. Nonlinear Schrodinger equation (NLS) and Korteweg de Vries equation are among best illustrations of successful application of apphed mathematics as a tool to analyze various nonlinear phenomena ranging from optical communications and Bose con­ densation to ocean waves. Solutions of nonlinear equations usually result in the formation of singularities , coherent structures or solitary waves. Examples of the corresponding phenomena can be observed in filamentation of laser beams in nonlinear media, wave breaking in hydrodynamics, collapse, and Langmuir waves in plasmas. NLS in dimension one (ID) is a basis for the description of modem optical fiber communication systems. A key technological tool for development of ultrafast high-bitrate optical communication lines is a dispersion management. A dispersion-managed optical fiber is described by NLS with periodic variation of dispersion along an optical line which dramatically reduces pulse broadening. Solitary wave solutions in such systems are important information carriers in optical lines. NLS in 2D describes a stationary self-focusing of light in optical media with Kerr nonlinearity, Bose-Einstein condensate in plane geometry as well as a self-focusing in plasma. Due to self-focusing the amplitude of an NLS solution tends to infinity after finite distance of propagation, representing a blow up phenomenon. The blow up is accompanied by a dramatic contraction of the spatial size of solution, which is called collapse. Near the collapse point NLS loses its applicability and there is usually a quahtative change in the underlying nonlinear phenomena, the new mechanisms become important such as dissipation, inelastic two- and three-body collisions, which can cause a loss of atoms from the Bose-Einstein condensate, breakdown of slow envelope approximation in nonlinear optical media and plasma density depletion. If optical power is very large in optics or if a number of particles in the BoseEinstein condensate significantly exceeds the critical number, then collapse turbulence occurs with random positions of collapses in space and time. A study of collapse regularization is important in describing collapse turbulence and to be able to go beyond an individual collapse event. 3D NLS is a model for non-stationary self-focusing of hght in optical media with Kerr nonlinearity, Bose-Einstein condensate in 3D geometry and laser-plasma interactions. Self-focusing results in collapse, which is of a different type as compare with 2D, and it is called weak collapse. NLS in 3D is a model for ultrashort pulse (optical bullet) that is a key for current developments in microfabrication of photonics devices by means of intense femtosecond laser pulses. Regularization of collapse for such pulses requires an extension of the model beyond NLS and a study of the full set of MaxwellSs equations. Maxwell's equations are also necessary to study ultrashort hght propagation through thin films and guided-wave metamaterials. New challenges for nonlinear optics and apphed mathematics include description of nonlinear optical phenomena in negative-refractive-index metamaterials, which are artificial nano-composites materials that show great promise for opening the new avenues in manipulating light. Light propagating through them could exhibit extraordinary properties and brings a wealth of novel phenomena to nonlinear optics. Electrodynamical properties of metamaterials result from many factors: plasmonic oscillations in metallic nanostructures, multi-scale interaction of near field electromagnetic resonances with optical far field dynamics, surface roughness of nanostructures, high values of local fields that cause nonlinear response and etc. It has recently been shown that mathematical modeling of hght interaction with thin film metamaterials requires the use of classical hydrodynamics models. Therefore, metamaterials represent a new research

Two dimensional materials with nonlinear optical response are of interest for bioimaging and optical information processing. However, achieving a measurable second order nonlinear signal in thin films and two dimensional materials has relied on using pulsed lasers and intense optical focusing, which limits potential applications require nonlinear response by using low laser power or from nanoscale materials. Here we achieved second harmonic generation from nanoflakes of two dimensional materials with lateral size smaller than the diffraction limit by using a double nanohole plasmonic optical tweezer with a low-power continuous-wave laser. The plasmonic double nanohole aperture enhances the local field intensity and allows for single nanoflake trapping and significant second harmonic generation at the nanoscale. The two dimensional property of nanoflakes allows for positioning in an area with high local field intensity and achieving higher nonlinear response than bulk nonlinear nanoparticlesWe observed an increase in second harmonic generation power two orders of magnitude higher than. other bulk materials such as lithium niobate nanoparticles. This allows for having strong nonlinear generation at the nanoscale for applications such as subwavelength nonlinear imaging or information processing.

Light propagation effects on nonlinear optical response of a size-controlled GaAs thin film at exciton resonance are investigated by transient gradient using picosecond pulses (1.5 ps). The signals are enhanced at the exciton resonance and the temporal profiles strongly depend on the excitation pulse energies. The result indicates that the nonlinear response is induced by excitonic polaritons and its decay is determined by the round-trip of the polariton in the film.

Periodical concentration of surface plasmon polaritons by wave interference in metallic film with nanocavity array

Hierarchy in nonlinear optical responses induced by metal cation tailoring effect in the In-containing chalcogenide compounds

Compared to standard nonlinear ultrasonic techniques based on harmonic generation or resonance tests that provide an average nonlinearity level, Dynamic Acousto-Elastic Testing (DAET) is a pump-probe approach that allows one to track the nonlinear elastic response at each phase of the dynamic cycle, and to quantify additional features such as transient softening/weakening and hysteresis. These so-called nonlinear signatures can help us unravel the origins of the nonlinear response at the micro/mesoscopic scale. In this talk, we first review some of the DAET results obtained by our group and others in a wide range of materials from cracked metals to rocks and unconsolidated granular media. Next, we present some of our most recent results where we investigate the effect of relative humidity (RH) on the nonlinear elastic response of granular media. We use DAET on samples of glass beads under dry (∼10%), ambient (∼60%) and humid (∼100%) conditions. We find that all extracted nonlinear parameters in humid samples increases by an order of magnitude compared to those in dry samples. This is consistent with the idea that water adsorption on the grains makes the contact junctions weaker. This has great relevance when trying to quantify the nonlinearity of porous materials.

Femtosecond-laser induced two-photon absorption of GaN and AlxGa1-xN thin films: Tuning the nonlinear optical response by alloying and doping

Nonlinear optics is generally first presented as an extension of conventional optics. Typically the subject is introduced with reference to a classical oscillatory electric polarization, accommodating correction terms that become significant at high intensities. The material parameters that quantify the extent of the nonlinear response are cast as coefficients in a power series—nonlinear optical susceptibilities signifying a propensity to generate optical harmonics, for example. Taking the subject to a deeper level requires a more detailed knowledge of the structure and properties of each nonlinear susceptibility tensor, the latter differing in form according to the process under investigation. Typically, the derivations involve intricate development based on time-dependent perturbation theory, assisted by recourse to a set of Feynman diagrams. This paper presents a more direct route to the required results, based on photonic rather than semiclassical principles, and offers a significantly clearer perspective on the photophysics underlying nonlinear optical response. The method, here illustrated by specific application to harmonic generation and down-conversion processes, is simple, intuitive and readily amenable for processes of arbitrary photonic order.

The nonlinear optical (NLO) response of photonic materials plays an important role in the understanding of light-matter interaction as well as pointing out a diversity of photonic and optoelectronic applications. Among the recently studied materials, 2D-LTMDs (bi-dimensional layered transition metal dichalcogenides) have appeared as a beyond-graphene nanomaterial with semiconducting and metallic optical properties. In this article, we review most of our work in studies of the NLO response of a series of 2D-LTMDs nanomaterials in suspension, using six different NLO techniques, namely hyper Rayleigh scattering, Z-scan, photoacoustic Z-scan, optical Kerr gate, and spatial self-phase modulation, besides the Fourier transform nonlinear optics technique, to infer the nonlinear optical response of semiconducting MoS2, MoSe2, MoTe2, WS2, semimetallic WTe2, ZrTe2, and metallic NbS2 and NbSe2. The nonlinear optical response from a thermal to non-thermal origin was studied, and the nonlinear refraction index and nonlinear absorption coefficient, where present, were measured. Theoretical support was given to explain the origin of the nonlinear responses, which is very dependent on the spectro-temporal regime of the optical source employed in the studies.