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The predominant methods currently used to determine nonlinear optical constants like the nonlinear refractive index n2 or the third order susceptibility χ(3) rely mostly on experimental, open and closed z-scan techniques and beam deflection methods. While these methods work well when the linear absorption is relatively small or negligible, the retrieval process is more complicated for a strongly scattering, dispersive or absorbing medium. The study of optics at the nanoscale in the picosecond or femtosecond laser pulsed regimes demands the development of new theoretical tools, and diverse experimental approaches, to extract and verify both linear and nonlinear optical dispersions exhibited by matter, especially when material constituents are fashioned into nanostructures of arbitrary shape. We present a practical, combined experimental and theoretical approach based on the hydrodynamic model that uses experimental results of harmonic generation conversion efficiencies to retrieve complex, nonlinear dispersion curves, not necessarily only for third order processes. We provide examples for materials that are of special interest to nanophotonics, for example, silicon, gold, and indium tin oxide (ITO), which displays nonlocal effects and a zero-crossing of the real part of the dielectric constant. The results for silicon and gold compare well with analytical predictions of nonlinear dispersion based on the nonlinear oscillator model. Based on our assessment of third harmonic generation conversion efficiencies in silicon, we predict χω(3) and χ3ω(3) are of order 10−17 (m/V)2 in the visible and near IR ranges, with respective peaks of 10−14 (m/V)2 and 10−16 (m/V)2 in the UV range. Similarly, gold’s χω(3) and χ3ω(3) are of order 10−17–10−16 (m/V)2, and predict χω(3)∼10−17(m/V)2 and χ3ω(3)∼10−18(m/V)2 for ITO. These results clearly suggest that judicious exploitation of the nonlinear dispersion of ordinary semiconductors has the potential to transform device physics in spectral regions that extend well into the UV range.

Methods currently used to determine nonlinear optical constants like n2 or chi3 rely on open and closed z-scan techniques. The study of optics at the nanoscale in the femtosecond regime requires new tools and approaches to extract linear and nonlinear dispersions exhibited by matter. We present a practical approach that amounts to numerical ellipsometry that utilizes experimental harmonic generation conversion efficiencies to retrieve complex, nonlinear dispersion curves. We provide examples of retrieved linear and nonlinear dispersions for a variety of materials, and show that for Silicon the numerical retrieval method yields chi3~10^(-16) (m/V)^2 and chi33w~10^(-17) (m/V)^2 , and visible and near IR ranges. Similarly, we predict chi3~10^(-17) (m/V)^2 and chi33w~10^(-19) (m/V)^2 for ITO as it exhibits linear and nonlinear anisotropic responses due to nonlocal effects.

Nanostructures made of semiconductors or metals are nowadays routinely integrated in photonic devices. At this scale light-matter interaction displays interesting new phenomena. We report a collection of experimental results of nonlinear harmonic generation in different nanolayers: semiconductors, conductive oxides and metals. The comparison of these experimental results with numerical predictions of our theoretical model identifies, distinguishes and explains the different nonlinear contributions to the harmonics generated by these materials at nanoscale. Our model accounts for surface, magnetic and bulk nonlinearities arising from free and bound charges, preserving linear and nonlinear dispersion, nonlocal effects due to pressure and viscosity.

Currently, nanostructures are routinely fabricated and integrated in different photonic devices for a variety of purposes and applications. At this scale light-matter interaction displays completely new phenomena. Traditionally, second and third harmonic generation (SHG and THG) have been studied in highly nonlinear transparent materials and under phase matching conditions ensuring large conversion efficiencies. In this case, the dominant nonlinear polarization term is given by the bulk electric dipole contribution through the nonlinear susceptibilities. However, at nanoscale the scenario is totally different: the electric dipole contribution no longer predominates and other terms, usually neglected, become important. Understanding how light interacts at the nanoscale with metals, semiconductors or dielectrics is pivotal to engineer and implement nano-antennas, filters, etc. In this work, we present a collection of experimental results regarding the SHG and THG in a semiconductor (GaAs) a conductive oxide (Indium Tin Oxide (ITO) nanolayer) at epsilon-near-zero (ENZ) condition and a metal (Au nanolayer). We explain the different nonlinear contribution in each case comparing our measurements with numerical results predicted by our theoretical model which accounts for surface, magnetic and bulk nonlinearities arising from free and bound charges, preserving linear and nonlinear dispersion, nonlocal effects due to pressure and viscosity, and changes in the effective mass of free electrons triggered by absorption [1] .

Understanding how light interacts with matter at the nanoscale is pivotal if one is to properly engineer nano-antennas, filters and other devices whose geometrical features approach atomic size. We report experimental results on second and third harmonic generation from 20 nm- and 70 nm-thick gold layers, for TE- and TM-polarized incident light pulses. We discuss the relative roles that bound electrons and an intensity dependent free electron density (hot electrons) play in third harmonic generation. While planar structures are generally the simplest to fabricate, metal layers that are only a few nanometers thick and partially transparent are almost never studied. Yet, transmission offers an additional reference point to compare experimental measurements with theoretical models. Our experimental results are explained well within the context of the microscopic hydrodynamic model that we employ to simulate second and third harmonic conversion efficiencies. Using our experimental observations we estimate ∣ χ 1064 n m ( 3 ) ∣≈ 10 − 18 ( m/V ) 2 , triggered mostly by hot electrons.

The use of semiconductors, metals and conductive oxides in the process of fabrication of actual nano devices is at the front edge of nowadays technology, exploiting the properties of light propagation and localization at nanometric scale in new and surprising ways. At these scales the usual theory describing the nonlinear (NL) effects of electromagnetic fields should be revisited and analyzed. We report a collection of experimental results of nonlinear harmonic generation in different nanolayers: semiconductors, conductive oxides and metals. The comparison of these experimental results with numerical predictions of our theoretical model identifies, distinguishes and explains the different nonlinear contributions to the harmonics generated by these materials at nanoscale. Our model accounts for surface, magnetic and bulk nonlinearities arising from free and bound charges, preserving linear and nonlinear dispersion, nonlocal effects due to pressure and viscosity.

Currently, nanostructures are routinely fabricated and integrated in different photonic devices for a variety of purposes and applications. For instance, in order to engineer properly nano-antennas or filters, it is important to understand accurately how light interacts with metals, semiconductors, or ordinary dielectrics at the nanoscale. When the nanoscale is reached, light-matter interactions displays new phenomena and conventional approximations may not always be applicable. Thus, new strategies must be sought in order to study and understand light-matter interactions at the nanoscale. In this work, we present experimental results of second and third harmonic generation from gold nanolayers, raveling novel behavior at nanoscale. These measurements are compared with numerical simulations based on a microscopic hydrodynamic model which accounts for surface, magnetic and bulk nonlinearities arising from both free and bound charges, preserving linear and nonlinear dispersion, nonlocal effects due to pressure and viscosity, and an intensity dependent free electron density, to which we refer as hot electrons contribution.

In this work, we report an experimental and numerical study of the intracavity spatial filtering in edge-emitting lasers using a chirped photonic crystal (PhC) as the filtering element in the near-field domain. We provide a comprehensive analysis of the near-field PhC filtering scheme and compare it to conventional spatial filtering using a variable width slit in the far-field domain. Using a two-dimensional chirped PhC as a spatial filter, we experimentally demonstrate a brightness enhancement by a factor of 1.3, considering an edge-emitting laser with a 1.5 mm cavity length, consistent with a numerical prediction of brightness enhanced by a factor of 1.7. The experimental results are theoretically confirmed by numerical integration of a spatio-temporal model of the edge-emitting laser. Furthermore, numerical results show that brightness can be further increased over a factor of 2, applying optimized spatial-filtering elements at both the front and rear facets of the lasers.

The use of semiconductors such as GaAs, GaP or Si in the process of fabrication of actual nano devices is at the front edge of nowadays technology, exploiting the properties of light propagation and localization at nanometric scale in new and surprising ways. At these scales the usual theory describing the nonlinear effects of electromagnetic fields are pushed to the limit of usual approximations and should be revisited and analyzed. Recently, we have studied in detail the generation of the second and third harmonic the opaque region of GaAs and Si, going beyond the previous studies and deeply analyzing the nonlinear process in order to infer which are the different mechanisms leading to the second and third harmonic generation at the surface of these materials. We demonstrate that the bulk nonlinearity is not the only one active term and that we have strong contributions coming from the surface and magnetic Lorentz terms, which usually are either hidden by the bulk contributions or assumed to be negligible. Experimental and theoretical simulations are contrasted, using a hydrodynamic model [1,2] that accounts for all salient aspects of the dynamics, including surface and bulk generated harmonic components. [3] The study, made in detail for GaAs is extended here to other semiconductors as Si and GaP. We also consider resonant structures as gratings and nanowires capable to strongly enhance the nonlinear efficiencies. Although the harmonic generation in this regime and materials still has low efficiency, these findings have significant repercussions and are consequential in nanoscale systems, which are usually investigated using only dispersion less bulk nonlinearities, with near-complete disregard of surface and magnetic contributions and their microscopic origins.

We present an experimental study of second harmonic generation from a 20nm-thick indium tin oxide layer in the proximity of its epsilon-near-zero condition. We report measurements of second harmonic efficiencies as a function of the input wavelength and angle of incidence close to the epsilon-near-zero crossing point. We perform theoretical simulations using a hydrodynamical model that is able to capture all major physical mechanisms driving the nonlinear electrodynamic behaviour of conductive oxides. The experimental and theoretical results are compared and a very good quantitative and qualitative agreement is found.

We demonstrate a novel dispersion-scan (d-scan) scheme for single-shot temporal characterization of ultrashort laser pulses. The novelty of this method relies on the use of a highly dispersive crystal featuring antiparallel nonlinear domains with a random distribution and size. This crystal, capable of generating a transverse second-harmonic signal, acts simultaneously as the dispersive element and the nonlinear medium of the d-scan device. The resulting in-line architecture makes the technique very simple and robust, allowing the acquisition of single-shot d-scan traces in real time. In addition, the technique can be further simplified by avoiding the need of dispersion pre-compensation. The retrieved pulses are in very good agreement with independent FROG measurements. We also apply the new single-shot d-scan to a TW-class laser equipped with a programmable pulse shaper, obtaining an excellent agreement between the applied and the d-scan retrieved dispersions., 7 pages, 5 figures

In this work, we numerically investigate the diffraction management of longitudinal elastic waves propagating in a two-dimensional metallic phononic crystal. We demonstrate that this structure acts as an &ldquo, ultrasonic lens&rdquo, providing self-collimation or focusing effect at a certain distance from the crystal output. We implement this directional propagation in the design of a coupling device capable to control the directivity or focusing of ultrasonic waves propagation inside a target object. These effects are robust over a broad frequency band and are preserved in the propagation through a coupling gel between the &ldquo, and the solid target. These results may find interesting industrial and medical applications, where the localization of the ultrasonic waves may be required at certain positions embedded in the object under study. An application example for non-destructive testing with improved results, after using the ultrasonic lens, is discussed as a proof of concept for the novelty and applicability of our numerical simulation study.

In high power regimes, broad area semiconductor lasers usually suffer from poor beam quality due to their asymmetric beam divergence, large beam quality factor (M2) and from the absence of any intrinsic filtering mechanism that can be integrated inside the cavity. In this work, we present a compact photonic crystal spatial filter, fabricated by periodically modulating refractive index media on a glass substrate using tightly focused femtoseconds laser. This filter work by deflecting the higher angular field components in a given frequency range. We demonstrate the spatial filtering effect when placed in an extended cavity configuration for a single BAS emitter, with transverse width of 400-µm and cavity length 1500-µm. We report a decrease of the laser M2 value along the slow axis with the introduction of the photonic crystal inside the cavity, together with a brightness enhancement by a factor of 1.5 compared to that of an unfiltered case. These results were compared with those obtained in the far field domain, with a conventional spatial filter consisting of an intra-cavity slit.

[EN] This work envisages a detailed study of two-dimensional defect localization and reconstruction, using laser generated ultrasound and its application as a remotely controlled non-destructive testing method. As an alternative to full ultrasonic or full optical approaches, we propose a hybrid configuration where ultrasound is generated by impact of laser pulses, while the detection is done with conventional transducers. We implement this approach for defect reconstruction in metallic elements and show that it combines advantages of both photonic and ultrasonic devices, reducing the drawbacks of both methods. We combine our experimental results with a high-resolution signal processing procedure based on the synthetic aperture focusing technique for the benefit of the final two-dimensional visualization of the defects., The work was supported by Spanish Ministry of Economy and Innovation (MINECO) and European Union FEDER through project FIS2015-65998-C2-1 and FIS2015-65998-C2-2 and by project AICO/2016/060 by Conselleria de Educacion, Investigacion, Cultura y Deporte de la Generalitat Valenciana.

We study electromagnetic pulse propagation in an indium tin oxide nanolayer in the linear and nonlinear regimes. We use the constitutive relations to reconstruct the effective dielectric constant of the medium, and show that nonlocal effects induce additional absorption resonances and anisotropic dielectric response: longitudinal and transverse effective dielectric functions are modulated differently along the propagation direction, and display different epsilon-near-zero crossing points with a discrepancy that increases with increasing intensity. We predict that hot carriers induce a dynamic redshift of the plasma frequency and a corresponding translation of the effective nonlinear dispersion curves that can be used to predict and quantify nonlinear refractive index changes as a function of incident laser peak power density. Our results suggest that large, nonlinear refractive index changes can occur without the need for epsilon-near-zero modes to couple with plasmonic resonators. At sufficiently large laser-pulse intensities, we predict the onset of optical bistability, while the presence of additional pump absorption resonances that arise from longitudinal oscillations of the free electron gas give way to corresponding resonances in the second and third harmonic spectra. A realistic propagation model is key to unraveling the basic physical mechanisms that play a fundamental role in the dynamics.

We report a comparative experimental and theoretical study of second harmonic generation from a 20nm-thick indium tin oxide nanolayer in the proximity of the epsilon-near-zero condition. We record the efficiency of the second harmonic signal both as a function of wavelength as well as of the angle of incidence around the epsilon-near-zero crossing point. We compare our experimental results with numerical simulations based on a hydrodynamical model able to capture all major physical mechanisms driving the electrodynamic behavior of conductive oxide layers, with unique aspects of the different nonlinear sources. We found a very good quantitative and qualitative agreement between experiment and theory.

Broad aperture semiconductor lasers usually suffer from low spatial quality of the emitted beams. Due to the highly compact character of such lasers, the use of conventional intracavity spatial filters is problematic. We demonstrate that extremely compact photonic crystal spatial filters, incorporated into a laser resonator, can improve the beam spatial quality and, correspondingly, increase the brightness of the emitted radiation. We report the decrease in the M2 value from 47 down to 28 due to photonic crystal spatial intracavity filtering and the increase in the brightness by a factor of 1.5, giving a proof of principle of intracavity photonic crystal spatial filtering in broad area semiconductor lasers.

Phase-locked second and third harmonic generation in the opaque region of a GaAs wafer is experimentally observed and analyzed both in transmission and reflection. These harmonic components, which are generated close to the surface, can propagate through an opaque material as long as the pump is tuned to a region of transparency or semitransparency and correspond to the inhomogeneous solutions of Maxwell’s equations with nonlinear polarization sources. We show that measurement of the angular and polarization dependence of the observed harmonic components allows one to infer the different nonlinear mechanisms that trigger these processes, including not only the bulk nonlinearity but also the surface and magnetic Lorentz contributions, which usually are either hidden by the bulk contributions or assumed to be negligible. The experimental results are compared with a detailed numerical model that takes into account these different effects, including for the first time combined linear and nonlinear material dispersions in a nonlinear Lorentz oscillator model of the bulk nonlinearities. Our results suggest that the intensity of the second harmonic signal generated by the surface can be more intense than the signal generated by the bulk. These findings have significant repercussions and are consequential in nanoscale systems, which are usually investigated using only dispersionless bulk nonlinearities, with near-complete disregard of surface and magnetic contributions and their microscopic origins.

When an intense laser beam is tuned in the transparency region of a generic nonlinear crystal, three components of second (SH) and third (TH) harmonic beams are generated: one backward, in reflection and two forward, in transmission. The first transmitted SH component comes from the homogeneous solution of the nonlinear equations and travels with the group velocity and the absorption coefficient according to material dispersion at the harmonic wavelength. The second component has its origin in the inhomogeneous solution of the nonlinear equation and it propagates into the material with the same phase velocity and absorption coefficient as the fundamental beam, regardless of the dispersion at the harmonic frequency. It is trapped and dragged along in the same direction as the pump and it is known as "phase locked" (PL) harmonic component. These two different components of the SH can only be distinguished when they are induced with short pulses and far from the usual phase-matching condition [1]. These theoretical predictions were experimentally validated first by the measurement of the two forward SH components in transparent materials [2]. Later, it was demonstrated that phase and group velocity locking leads to the inhibition of linear absorption of the harmonics, when the pump is tuned to a transparency region and the harmonics well below the absorption edge of the nonlinear material. As a result, the inhomogeneous SH and TH components propagate inside the material under conditions of strong absorption, in the opaque region of the spectrum. [3,4].

Broad Area Semiconductor (BAS) lasers usually show poor beam quality, especially in a high power regime. The emitting region of this lasers has typical dimensions of 1 μm in the vertical direction (fast axis) and hundreds of μm in the horizontal direction (slow axis). Along the fast axis, the beam is tightly confined to single mode emission and can be collimated using a cylindrical micro-lens. The collimation problem arises in the slow axis, where the aperture size can range between 50–400 μm (Fig 1.a) and the light distribution over many spatial modes makes it difficult to collimate. The beam quality of these lasers is worse in terms of focusability and increased M2.

Laser-generated ultrasound represents an interesting nondestructive testing technique that is being investigated in the last years as performative alternative to classical ultrasonic-based approaches. The greatest difficulty in analyzing the acoustic emission response is that an in-depth knowledge of how acoustic waves propagate through the tested composite is required. In this regard, different signal processing approaches are being applied in order to assess the significance of features extracted from the resulting analysis. In this study, the detection capabili-ties of internal defects in a metallic sample are proposed to be studied by means of the time-frequency analysis of the ultrasonic waves resulting from laser-induced thermal mechanism. In the proposed study, the use of the wavelet transform considering different wavelet variants is considered due to its multi-resolution time-frequency characteristics. Also, a significant time-frequency technique widely applied in other fields of research is applied, the synchrosqueezed transform.

The use of semiconductors, metals, or ordinary dielectrics in the process of fabrication of nanodevices is at the front edge of nowadays technology, exploiting the properties of light propagation and localization at nanometric scale in new and surprising ways. Understanding accurately how light interacts with these materials at the nanoscale is crucial if one is to properly engineer nano-devices. When the nanoscale is reached, light-matter interactions display new phenomena where conventional approximations may not always be applicable and they should be either revised or voided. In this work, we measure the efficiency of second and third harmonic generation from gold nanolayers. The experimental results are compared with numerical simulations based on a detailed microscopic hydrodynamic model that considers different effects playing a role in the nonlinear response, not usually considered by more generic models. The agreement between experimental and theoretical results proves the importance of all these contributions.

We report comparative experimental and theoretical studies of second and third harmonic generation from a 20nm-thick indium tin oxide layer in proximity of the epsilon-near-zero condition. Using a tunable OPA laser we record both spectral and angular dependence of the generated harmonic signals close to this particular point. In addition to the enhancement of the second harmonic efficiency close to the epsilon-near-zero wavelength, at oblique incidence third harmonic generation displays unusual behavior, predicted but not observed before. We implement a comprehensive, first-principles hydrodynamic approach able to simulate our experimental conditions. The model is unique, flexible, and able to capture all major physical mechanisms that drive the electrodynamic behavior of conductive oxide layers: nonlocal effects, which blueshift the epsilon-near-zero resonance by tens of nanometers; plasma frequency redshift due to variations of the effective mass of hot carriers; charge density distribution inside the layer, which determines nonlinear surface and magnetic interactions; and the nonlinearity of the background medium triggered by bound electrons. We show that by taking these contributions into account our theoretical predictions are in very good qualitative and quantitative agreement with our experimental results. We show that by taking these contributions into account our theoretical predictions are in very good qualitative and quantitative agreement with our experimental results. We expect that our results can be extended to other geometries where ENZ nonlinearity plays an important role.

We discuss a dynamical model of harmonic generation that arises from surfaces that demarcate two different metals or conductors, or the electron cloud that spills outside a simple metal surface and the interior bulk, having different electron densities, for example a noble metal such as gold, and indium tin oxide (ITO). While in general two adjacent materials may contain free and bound charges that determine their respective dielectric constants, the transition region may be characterized by a large discontinuity in the free electron density, , epsilon-near-zero conditions, or multiple, nested plasmonic resonances. For example, , while the free-electron cloud that spills outside a noble metal surface decreases as a function of position from the hard ionic surface. These discrepancies lead to the prediction that the angular dependence of second harmonic generation (SHG) from a simple planar structure is direction-dependent, and highlights the sensitivity of both SHG and third harmonic generation to the makeup of the surface and what surrounds it. Our calculations also suggest that the nonlinear optical analysis of more complicated, hybrid structures, such as metal/oxide nanoantennas or metasurfaces, should always be performed by including effects that are generally overlooked, such as nonlocal effects (viscosity and pressure); the presence of linear and nonlinear quantum tunneling currents in the nano- and sub nano-gaps between the nanoantenna and embedded nanoparticles; linear and nonlinear contributions of bound (inner-core) electrons to the dielectric constant.

We present an indirect and non-destructive optical method for domain statistic characterization in disordered nonlinear crystals, having a spatially random distribution of ferroelectric domains with homogeneous refractive index. This method, based on a combination of numerical simulations and experimental measurements, analyses the wavelengthdependent second harmonic spatial distribution. We apply this technique to the characterization of two different random media, with drastically different statistical distributions of ferroelectric domains.

© 2018 [Optical Society of America]. One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modifications of the content of this paper are prohibited. We have investigated multiple scattering of light in a disordered system based on liquid crystals for a temperature-controllable random laser. Coherent backscattering measurements at several temperatures have been well fitted by the theoretical model deduced for a random collection of spherical point scatters based on a diffusion approximation. The transport mean free path exclusively depends on the diffusivity of the liquid crystalline phase of the hybrid scattering system. It is shown how the laser threshold excitation intensity is strongly correlated with the transport mean free path.

© 2018 [Optical Society of America]. Users may use, reuse, and build upon the article, or use the article for text or data mining, so long as such uses are for non-commercial purposes and appropriate attribution is maintained. All other rights are reserved. We present an indirect, non-destructive optical method for domain statistic characterization in disordered nonlinear crystals having homogeneous refractive index and spatially random distribution of ferroelectric domains. This method relies on the analysis of the wave-dependent spatial distribution of the second harmonic, in the plane perpendicular to the optical axis in combination with numerical simulations. We apply this technique to the characterization of two different media, Calcium Barium Niobate and Strontium Barium Niobate, with drastically different statistical distributions of ferroelectric domains.

Aligned with current research efforts and industrial applications on nondestructive testing, in this work, a hybrid system combining remotely induced laser ultrasonics with conventional transducer detection is studied for defects detection in metallic parts. The processing of the induced acoustic emission waves is proposed to be approached by means of a high-resolution volumetric signal processing procedure based on the synthetic aperture focusing technique for the benefit of the final 2D visualization of the defects. The advantages of the hybrid non-destructive testing approach and the performance of the processing technique are experimentally validated.

We explore the outcomes of detailed microscopic models by calculating second- and third-harmonic generation from thin film surfaces with discontinuous free-electron densities. These circumstances can occur in structures consisting of a simple metal mirror, or arrangements composed of either different metals or a metal and a free electron system like a conducting oxide. Using a hydrodynamic approach we highlight the case of a gold mirror, and that of a two-layer system containing indium tin oxide (ITO) and gold. We assume the gold mirror surface is characterized by a free-electron cloud of varying density that spills into the vacuum, which as a result of material dispersion exhibits epsilon-near-zero conditions and local field enhancement at the surface. For a bylayer consisting of a thin ITO and gold films, if the wave is incident from the ITO side the electromagnetic field is presented with a free-electron discontinuity at the ITO/gold interface, and wavelength-dependent, epsilon-near-zero conditions that enhance local fields and conversion efficiencies, and determine the surface's emission properties. We evaluate the relative significance of additional nonlinear sources that arise when a free-electron discontinuity is present, and show that harmonic generation can be sensitive to the density of the screening free-electron cloud, and not its thickness. Our findings also suggest the possibility to control surface harmonic generation through surface charge engineering., Comment: 19 pages, 6 figures, 49 references

We demonstrate an indirect non-destructive optical method for the characterization of ferroelectric domain size and distribution based on the analysis of the generated second harmonic spatial distribution. This method is based on a combination of experimental measurements and numerical simulations and allows obtaining information of domain statistics in disordered structures via the analysis of wavelength dependence of the angular second harmonic diffraction pattern.

Flat focusing mirrors without any optical axis have been investigated by two-dimensional chirped photonic crystals. The principle of flat focusing mirror is similar to Veselago flat lens, which allows the beam focusing in the near field. The key idea of 2D chirped PhCs is to provide anomalous diffraction that can restrain a beam spreading and even focus a divergent beam upon reflection. Different focusing phenomena of the three 2D PhCs are shown in this paper and importantly, all of them have transversal invariance (lack of an optical axis).

Nondestructive testing of metallic objects that may contain embedded defects of different sizes is an important application in many industrial branches for quality control. Most of these techniques allow defect detection and its approximate localization, but few methods give enough information for its 3D reconstruction. Here we present a hybrid laser&ndash, transducer system that combines remote, laser-generated ultrasound excitation and noncontact ultrasonic transducer detection. This fully noncontact method allows access to scan areas on different object&rsquo, s faces and defect details from different angles/perspectives. This hybrid system can analyze the object&rsquo, s volume data and allows a 3D reconstruction image of the embedded defects. As a novelty for signal processing improvement, we use a 2D apodization window filtering technique, applied along with the synthetic aperture focusing algorithm, to remove the undesired effects due to side lobes and wide-angle reflections of propagating ultrasound waves, thus enhancing the resulting 3D image of the defect. Finally, we provide both qualitative and quantitative volumetric results that yield valuable information about defect location and size.

Laser-generated ultrasound is a modern non-destructive testing technique. It has been investigated over recent years as an alternative to classical ultrasonic methods, mainly in industrial maintenance and quality control procedures. In this study, the detection and reconstruction of internal defects in a metallic sample is performed by means of a time-frequency analysis of ultrasonic waves generated by a laser-induced thermal mechanism. In the proposed methodology, we used wavelet transform due to its multi-resolution time frequency characteristics. In order to isolate and estimate the corresponding time of flight of eventual ultrasonic echoes related to internal defects, a density-based spatial clustering was applied to the resulting time frequency maps. Using the laser scan beam&rsquo, s position, the ultrasonic transducer&rsquo, s location and the echoes&rsquo, arrival times were determined, the estimation of the defect&rsquo, s position was carried out afterwards. Finally, clustering algorithms were applied to the resulting geometric solutions from the set of the laser scan points which was proposed to obtain a two-dimensional projection of the defect outline over the scan plane. The study demonstrates that the proposed method of wavelet transform ultrasonic imaging can be effectively applied to detect and size internal defects without any reference information, which represents a valuable outcome for various applications in the industry.

We present a novel single-shot cross-correlation technique based on the analysis of the transversally emitted second harmonic generation in crystals with random distribution and size of anti-parallel nonlinear domains. We implement it to the measurement of ultrashort laser pulses with unknown temporal duration and shape. We optimize the error of the pulse measurement by controlling the incident angle and beam width. As novelty and unlike the other well-known cross correlation schemes, this technique can be implemented for the temporal characterization of pulses over a very wide dynamic range (30 fs–1ps) and wavelengths (800–2200 nm), using the same crystal and without critical angular or temperature alignment.

Copyright 2015 AIP Publishing. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. We demonstrate infrared femtosecond laser-induced inversion of ferroelectric domains. This process can be realised solely by using tightly focused laser pulses without application of any electric field prior to, in conjunction with, or subsequent to the laser irradiation. As most ferroelectric crystals like LiNbO3, LiTaO3, and KTiOPO4 are transparent in the infrared, this optical poling method allows one to form ferroelectric domain patterns much deeper inside a ferroelectric crystal than by using ultraviolet light and hence can be used to fabricate practical devices. We also propose in situ diagnostics of the ferroelectric domain inversion process by monitoring the Cerenkov second harmonic signal, which is sensitive to the appearance of ferroelectric domain walls.

We study second and third harmonic generation of ultrashort pulses in a cavity environment and focus on the role of a phase locking phenomenon that causes the pump to trap and drag the generated harmonic fields. The results reveal strong field localisation and resonance conditions for the phase locked components, which are tuned in a spectral region of huge nominal material absorption. This unique behavior suggests the possibility of new optical phenomena and new applications for opaque materials (i.e. semiconductors) in the visible and UV ranges, such as new laser sources with relatively high conversion efficiencies.

We study, both experimentally and theoretically, the process of second-harmonic generation by two noncollinear beams in ferroelectric crystals with a disordered distribution of ferroelectric domains. We show that this parametric process results in generation of second-harmonic wave in the direction transverse to the propagation of the fundamental beam. We demonstrate how this effect can be used for the femtosecond pulse characterization enabling the estimation of both width and chirp of the pulse.