Backward Phase-Matched Second-Harmonic Generation from Stacked Metasurfaces.

We demonstrate phase-matched second-harmonic generation (SHG) from three-dimensional metamaterials consisting of stacked metasurfaces. To achieve phase matching, we utilize a novel mechanism based on phase engineering of the metasurfaces at the interacting wavelengths, facilitating phase-matched SHG in the unconventional backward direction. Stacking up to five metasurfaces,we obtain a phase-matched SHG signal, which scales superlinearly with the number of layers. Our results motivate further investigations to achieve higher conversion efficiencies also with more complex wave fronts.

Spatiospectral features of a soliton-assisted random laser in liquid crystals.

We report on novel features of random lasers assisted by near-infrared spatial solitons in nematic liquid crystals. Specifically, we study the role of light-induced reorientational waveguides (nematicons) on the spatial and spectral distributions of the laser modes. We show that the spatially spiky character of the laser emission propagating backwards with respect to the pump tends to disappear in the forward direction, due to the soliton confinement of the generated light. Moreover, the spectral features associated with various random laser resonances appear to merge upon guided-wave propagation along the nematicon, making the nematicon-aided random laser a bidirectional device with distinct emission properties at the two opposite outputs.

Tailoring Second-Harmonic Emission from (111)-GaAs Nanoantennas.

Second-harmonic generation (SHG) in resonant dielectric Mie-scattering nanoparticles has been hailed as a powerful platform for nonlinear light sources. While bulk-SHG is suppressed in elemental semiconductors, for example, silicon and germanium due to their centrosymmetry, the group of zincblende III-V compound semiconductors, especially (100)-grown AlGaAs and GaAs, have recently been presented as promising alternatives. However, major obstacles to push the technology toward practical applications are the limited control over directionality of the SH emission and especially zero forward/backward radiation, resulting from the peculiar nature of the second-order nonlinear susceptibility of this otherwise highly promising group of semiconductors. Furthermore, the generated SH signal for (100)-GaAs nanoparticles depends strongly on the polarization of the pump. In this work, we provide both theoretically and experimentally a solution to these problems by presenting the first SHG nanoantennas made from (111)-GaAs embedded in a low index material. These nanoantennas show superior forward directionality compared to their (100)-counterparts. Most importantly, based on the special symmetry of the crystalline structure, it is possible to manipulate the SHG radiation pattern of the nanoantennas by changing the pump polarization without affecting the linear properties and the total nonlinear conversion efficiency, hence paving the way for efficient and flexible nonlinear beam-shaping devices.

Plasmonic mode conversion in individual tilted 3D nanostructures.

We investigate mode conversion in 3D asymmetric nanocones using angle-dependent linear optical spectroscopy and second-harmonic generation microscopy supported by corresponding simulations. The results prove the efficient excitation of the plasmonic out-of-plane mode that enhances the electric near-field at the sharp tip. Furthermore, we introduce two advanced fabrication processes including either etch mask transfer by tilted etching into a metallic layer or tilted electron-beam lithography followed by tilted evaporation and lift-off. These processes enable the fabrication of tilted nanostructures which can be optimized for a given purpose. The combination of the optical properties and the introduced fabrication processes enables a new design of plasmonic nanostructures for ultra-compact sensors or photon sources.

Resonant harmonic generation in AlGaAs nanoantennas probed by cylindrical vector beams.

We investigate second- and third-harmonic generation from individual AlGaAs nanoantennas using far-field mapping with radially- and azimuthally-polarized cylindrical vector beams. Due to the unique polarization structure of these beams, we are able to determine the crystal orientation of the nanoantenna in a single scanning map. Our method thus provides a novel and versatile optical tool to study the crystal properties of semiconductor nanoantennas. We also demonstrate the influence of cylindrical vector beam excitation on the resonant enhancement of second- and third-harmonic generation driven by electric and magnetic anapole-like modes, despite falling in the strong absorption regime of AlGaAs. In particular, we observe a greater nonlinear conversion efficiency from a single nanoantenna excited with a radially-polarized beam as compared to an azimuthally polarized cylindrical vector beam. The fundamental field of the radially-polarized beam strongly couples to the multipoles increasing the near-field enhancement of the nanoantenna. Our work introduces new ways to study individual nanostructures and to tailor the efficiencies of nonlinear phenomena at the nanoscale using non-conventional optical techniques.

Demonstration of longitudinally polarized optical needles.

Longitudinally polarized optical needles are beams that exhibit ultra-long depth of field, subwavelength transverse confinement, and polarization oriented along the longitudinal direction. Although several techniques have been proposed to generate such needles, their scarce experimental observations have been indirect and incomplete. Here, we demonstrate the creation and full three-dimensional verification of a longitudinally polarized optical needle. This needle is produced by generating a radially polarized Bessel-Gauss beam at the focus of a high numerical aperture microscope objective. Using three-dimensional spatial mapping of second-harmonic generation from a single vertically aligned GaAs nanowire, we directly verify such a longitudinally polarized optical needle's properties, which are formed at the focus. The needle exhibits a dominant polarization, which is oriented along the longitudinal direction, an ultra-long depth of field (30 λ), and high spatial homogeneity. These are in agreement with corresponding focal field calculations that use vector diffraction theory. Our findings open new opportunities for manipulation and utilization of longitudinally polarized optical needles.

Less Is More: Enhancement of Second-Harmonic Generation from Metasurfaces by Reduced Nanoparticle Density.

We investigate optical second-harmonic generation (SHG) from metasurfaces where noncentrosymmetric V-shaped gold nanoparticles are ordered into regular array configurations. In contrast to expectations, a substantial enhancement of the SHG signal is observed when the number density of the particles in the array is reduced. More specifically, by halving the number density, we obtain over 5-fold enhancement in SHG intensity. This striking result is attributed to favorable interparticle interactions mediated by the lattice, where surface-lattice resonances lead to spectral narrowing of the plasmon resonances. Importantly, however, the results cannot be explained by the improved quality of the plasmon resonance alone. Instead, the lattice interactions also lead to further enhancement of the local fields at the particles. The experimental observations agree very well with results obtained from numerical simulations including lattice interactions.

On the molecular optical nonlinearity of halogen-bond-forming azobenzenes.

We study hyper-Rayleigh scattering and computed molecular hyperpolarizability in a series of azobenzene chromophores in chloroform and dimethylformamide as solvents. The chromophores form halogen or hydrogen bonds of varying strength with dimethylformamide molecules, differently from what is expected for chloroform. We show that hyperpolarizability is unaffected or sligthly lower with the azobenzene forming the strongest halogen bond. Solid supramolecular polymers with the same chromophores have previously demonstrated clearly higher second-order nonlinear responses when a halogen-bond-accepting polymer is used, the larger increase being associated with the stronger halogen bond. The present study proves that the higher optical nonlinearity in polymers lies in the better ordering of the chromophores instead of changes in molecular hyperpolarizability, highlighting the unique properties of halogen bonding in supramolecular chemistry.

Beaming random lasers with soliton control.

Random lasers are resonator-less light sources where feedback stems from recurrent scattering at the expense of spatial profile and directionality. Suitably-doped nematic liquid crystals can random lase when optically pumped near resonance(s); moreover, through molecular reorientation within the transparency region, they support self-guided optical spatial solitons, i.e., light-induced waveguides. Here, we synergistically combine solitons and collinear pumping in weakly scattering dye-doped nematic liquid crystals, whereby random lasing and self-confinement concur to beaming the emission, with several improved features: all-optical switching driven by a low-power input, laser directionality and smooth output profile with high-conversion efficiency, externally controlled angular steering. Such effects make soliton-assisted random lasers an outstanding route towards application-oriented random lasers.

Nonresonant Local Fields Enhance Second-Harmonic Generation from Metal Nanoislands with Dielectric Cover.

We study second-harmonic generation from gold nanoislands covered with amorphous titanium oxide (TiO_{2}) films. As the TiO_{2} thickness increases, the plasmon resonance of the nanoislands shifts away from the second-harmonic wavelength of 532 nm, diminishing the resonant enhancement of the process at this wavelength. Nevertheless, the second-harmonic signal is enhanced by up to a factor of 45 with increasing TiO_{2} thickness. This unexpected effect arises from the scaling of local fields at the fundamental wavelength of 1064 nm-which is at the far tail of the resonance-due to a change in the dielectric environment of the nanoislands.

Collective Effects in Second-Harmonic Generation from Plasmonic Oligomers.

We investigate collective effects in plasmonic oligomers of different symmetries using second-harmonic generation (SHG) microscopy with cylindrical vector beams (CVBs). The oligomers consist of gold nanorods that have a longitudinal plasmon resonance close to the fundamental wavelength that is used for SHG excitation and whose long axes are arranged locally such that they follow the distribution of the transverse component of the electric field of radially or azimuthally polarized CVBs in the focal plane. We observe that SHG from such rotationally symmetric oligomers is strongly modified by the interplay between the polarization properties of the CVB and interparticle coupling. We find that the oligomers with radially oriented nanorods exhibit small coupling effects. In contrast, we find that the oligomers with azimuthally oriented nanorods exhibit large coupling effects that lead to silencing of SHG from the whole structure. Our experimental results are in very good agreement with numerical calculations based on the boundary element method. The work describes a new route for studying coupling effects in complex arrangements of nano-objects and thereby for tailoring the efficiency of nonlinear optical effects in such structures.

Nonlinear microscopy using cylindrical vector beams: Applications to three-dimensional imaging of nanostructures.

The three-dimensional (3D) optical fields that arise from the focusing of cylindrical vector beams (CVB) with radial and azimuthal polarizations provide new sources of contrast for optical microscopy of nano-objects. So far, these demonstrations have been restricted to two-dimensional transversal scanning, i.e., along the focal plane of interest, or use of point-like objects, i.e., single molecules and nanoparticles. Here, we demonstrate the first application of CVBs for 3D imaging of 3D nano-objects. This technique is done by acquiring 3D image scans of the second-harmonic generation signal from vertically-aligned semiconductor nanowires, whose second-order response is primarily driven by the longitudinal electric field, i.e., the field component along the nanowire axis. Our technique provides a new way to study individual nano-objects in three dimensions through the unique combination of nonlinear microscopy and CVBs.

Nonlinear imaging of nanostructures using beams with binary phase modulation.

We demonstrate nonlinear microscopy of oriented nanowires using excitation beams with binary phase modulation. A simple and intuitive optical scheme comprising a spatial light modulator gives us the possibility to control the phase across an incident Hermite-Gaussian beam of order (1,0) (HG10 mode). This technique allows us to gradually vary the spatial distribution of the longitudinal electric fields in the focal volume, as demonstrated by second-harmonic generation from vertically-aligned GaAs nanowires. These results open new opportunities for the full control of polarization in the focal volume to enhance light interaction with nanostructured materials.

All-optical guided-wave random laser in nematic liquid crystals.

Spatial solitons can affect and enhance random lasing in optically-pumped dyedoped nematic liquid crystals. Upon launching two collinear beams in the sample, the first to pump the fluorescent guest molecules and the second to induce a reorientational soliton, strikingly the second beam not only guides the emitted photons in the soliton waveguide, but also enhances the lasing efficiency and modulates its spectral width. By altering the scattering paths of the emitted photons, the soliton also contributes to the selection of the lasing modes, as further confirmed by the observed kinks in the input/output characteristics. These experimental results demonstrate that random lasing can be efficiently controlled by a light beam which does not interact with the gain molecules, opening a route towards light-controlled random lasers.

Bulk second-harmonic generation from thermally evaporated indium selenide thin films.

We investigate bulk second-order nonlinear optical properties of amorphous indium selenide thin films fabricated by thermal evaporation. Such films are shown to exhibit strong and photostable second-harmonic generation (SHG). We report strong thickness dependence of the second-harmonic signals as characterized by the Maker-fringe method. The absolute value of the nonlinear susceptibility tensor of the film is addressed by analyzing the interference of SHG signals from the film and the glass substrate. The value of the joint non-diagonal component of the susceptibility is found to be 4 pm/V, which is comparable to that of widely used second-order nonlinear materials.

On the determination of χ(2) in thin films: a comparison of one-beam second-harmonic generation measurement methodologies.

The determination of the second-order susceptibility (χ(2)) of thin film samples can be a delicate matter since well-established χ(2) measurement methodologies such as the Maker fringe technique are best suited for nonlinear materials with large thicknesses typically ranging from tens of microns to several millimeters. Here we compare two different second-harmonic generation setups and the corresponding measurement methodologies that are especially advantageous for thin film χ(2) characterization. This exercise allows for cross-checking the χ(2) obtained for identical samples and identifying the main sources of error for the respective techniques. The development of photonic integrated circuits makes nonlinear thin films of particular interest, since they can be processed into long waveguides to create efficient nonlinear devices. The investigated samples are ABC-type nanolaminates, which were reported recently by two different research groups. However, the subsequent analysis can be useful for all researchers active in the field of thin film χ(2) characterization.

Fabrication of Ion-Shaped Anisotropic Nanoparticles and their Orientational Imaging by Second-Harmonic Generation Microscopy.

Ion beam shaping is a novel and powerful tool to engineer nanocomposites with effective three-dimensional (3D) architectures. In particular, this technique offers the possibility to precisely control the size, shape and 3D orientation of metallic nanoparticles at the nanometer scale while keeping the particle volume constant. Here, we use swift heavy ions of xenon for irradiation in order to successfully fabricate nanocomposites consisting of anisotropic gold nanoparticle that are oriented in 3D and embedded in silica matrix. Furthermore, we investigate individual nanorods using a nonlinear optical microscope based on second-harmonic generation (SHG). A tightly focused linearly or radially-polarized laser beam is used to excite nanorods with different orientations. We demonstrate high sensitivity of the SHG response for these polarizations to the orientation of the nanorods. The SHG measurements are in excellent agreement with the results of numerical modeling based on the boundary element method.

Surface-Enhanced Impulsive Coherent Vibrational Spectroscopy.

Surface-enhanced Raman spectroscopy (SERS) has attracted a lot of attention in molecular sensing because of the remarkable ability of plasmonic metal nanostructures to enhance the weak Raman scattering process. On the other hand, coherent vibrational spectroscopy triggered by impulsive excitation using ultrafast laser pulses provides complete information about the temporal evolution of molecular vibrations, allowing dynamical processes in molecular systems to be followed in "real time". Here, we combine these two concepts and demonstrate surface-enhanced impulsive vibrational spectroscopy. The vibrational modes of the ground and excited states of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), spin-coated on a substrate covered with monodisperse silver nanoparticles, are impulsively excited with a sub-10 fs pump pulse and characterized with a delayed broad-band probe pulse. The maximum enhancement in the spectrally and temporally resolved vibrational signatures averaged over the whole sample is about 4.6, while the real-time information about the instantaneous vibrational amplitude together with the initial vibrational phase is preserved. The phase is essential to determine the vibrational contributions from the ground and excited states.

Surface lattice resonances in second-harmonic generation from metasurfaces.

We investigate the role of surface-lattice resonances (SLRs) in second-harmonic generation (SHG) from arrays of metal nanoparticles. The SLRs affect the generated signal when the sample is rotated away from normal incidence. The adjustment of the incident angle tunes the SLRs to the fundamental wavelength for SHG and improves the quality of the resonance for better resonance enhancement of SHG. Compared to normal incidence, an enhancement by a factor of 10 is observed. However, at certain incident angles, the enhancement is interrupted by diffraction anomalies, which redirect light into the substrate, increasing radiation damping and compromising the quality of the resonance.

Third-harmonic generation imaging of three-dimensional microstructures fabricated by photopolymerization.

We demonstrate the capability of polarized third-harmonic generation (THG) for high contrast imaging of three-dimensional microstructures fabricated by photopolymerization. Using circular polarization of fundamental light, background-free optically-sectioned THG images were obtained from laser-written photopolymerized microstructures. The technique has great potential for simple and noninvasive characterization of photopolymerized devices, which typically show poor contrast in conventional light microscopy.