Enhanced Second Harmonic Generation from Ferroelectric HfO2-Based Hybrid Metasurfaces.

The nonlinear process of second harmonic generation (SHG) in monolayer (1L) transition metal dichalcogenides (TMD), like WS2, strongly depends on the polarization state of the excitation light. By combination of plasmonic nanostructures with 1L-WS2 by transferring it onto a plasmonic nanoantenna array, a hybrid metasurface is realized impacting the polarization dependency of its SHG. Here, we investigate how plasmonic dipole resonances affect the process of SHG in plasmonic-TMD hybrid metasurfaces by nonlinear spectroscopy. We show that the polarization dependency is affected by the lattice structure of plasmonic nanoantenna arrays as well as by the relative orientation between the 1L-WS2 and the individual plasmonic nanoantennas. In addition, such hybrid metasurfaces show SHG in polarization states, where SHG is usually forbidden for either 1L-WS2 or plasmonic nanoantennas. By comparing the SHG in these channels with the SHG generated by the hybrid metasurface components, we detect an enhancement of the SHG signal by a factor of more than 40. Meanwhile, an attenuation of the SHG signal in usually allowed polarization states is observed. Our study provides valuable insight into hybrid systems where symmetries strongly affect the SHG and enable tailored SHG in 1L-WS2 for future applications.

Wide bandgap semiconductors such as gallium nitride (GaN) are essential constituents of future optical circuits, as their optical response can accommodate a broad wavelength range while suppressing two-photon absorption and free-carrier absorption effects that are encountered with silicon (Si) structures. Their direct wide bandgap is also favourable for the incorporation of active elements. Furthermore, nonlinear optical processes can be harnessed by exploiting the higher-order susceptibility tensors of the crystal structure to achieve advanced light control modalities, enabling all-optical processing and the generation of entangled photon states. We will report on nonlinear frequency conversion from the telecom range via second harmonic generation (SHG) and third harmonic generation (THG) in suspended gallium nitride slab photonic crystal (PhC) cavities on silicon, under continuous-wave resonant excitation. Genetic optimization is applied to sweep parameter space for the highest cavity quality factors, and simultaneously accounting for power in-coupling. While there is a clear trade-off theoretically between coupling efficiency and Q-factor for a given cavity design, the upper limit on the Q-factor that is imposed by loss channels, given the disorder figure of current fabrication technology, makes room for introducing improved far-field coupling to enhance nonlinear processes without sacrificing the experimentally achievable light confinement. Far-field coupling is addressed through various PhC cavity designs, which enable the excitation of the fundamental mode with a Gaussian beam. Optimized two-dimensional PhC cavities with increased far-field coupling have been characterized with quality factors as high as 44′000, approaching the computed theoretical values. The strong enhancement in light confinement has enabled second harmonic generation (SHG) under continuous-wave excitation, with up-conversion from both 1300 nm and 1550 nm wavelength bands, confirmed by spectral and power dependence measurements. At 1550 nm, normalized SHG conversion efficiency as large as 2.4×10−3 W−1 are measured as well as simultaneous THG. SHG emission power of up to 0.74 nW has been detected without saturation

UV irradiation of ultrathin ${\mathrm{SiO}}_{2}$ on Si can lead to injection of both holes and electrons into the oxide. 4.9-eV photons lead to electron injection while 6.7-eV photons open up the hole injection channel. The holes and electrons can trap, generating electric fields of opposite signs. Second harmonic generation (SHG) is sensitive to both the sign and magnitude of the electric field at the buried interface, revealed by opposing electric field induced second harmonic (EFISH) generation contributions. The SHG response of UV-irradiated ${\mathrm{S}\mathrm{i}\ensuremath{-}\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ slowly evolves to the SHG pattern characteristic for unirradiated ${\mathrm{S}\mathrm{i}\ensuremath{-}\mathrm{S}\mathrm{i}\mathrm{O}}_{2}.$ We attribute the phenomenon to combined dynamics of electrons and holes at the ${\mathrm{S}\mathrm{i}\ensuremath{-}\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ interface.

The surface freezing and wetting behaviour at the liquid/vapour interface of Ga-rich alloys of Ga and Pb has been studied as a function of temperature at selected compositions both in the homogeneous liquid and the fluid two phase demixing regime. The interfacial phase transitions have been probed by second harmonic generation (SHG) in combination with plasma generation (PG) measurements at different cooling and heating cycles. In the composition range between the eutectic and monotectic points the occurrence of surface freezing is clearly indicated by distinct changes of the SHG and PG intensities, respectively. It is not observed below the eutectic composition for solution very dilute in Pb. Qualitatively, these results agree with the recent observations made in Ga–Bi alloys. A new wetting characteristic is found crossing the miscibility gap at constant composition with varying temperature. A Pb-rich macroscopic wetting film forms which must be concluded from the SHG intensities. This interfacial phase transition causes a clearly detectable drop of the sample temperature which is explained by the corresponding discontinuous change of the optical emissivity at the interface. These interfacial phenomena related with the miscibility gap have not been reported before.

Using in vivo second harmonic generation (SHG) and third harmonic generation (THG) microscopies, we tracked the course of collagen remodeling over time in the same melanoma microenvironment within an individual mouse. The corresponding structural and morphological changes were quantitatively analyzed without labeling using an orientation index (OI), the gray level co-occurrence matrix (GLCM) method, and the intensity ratio of THG to SHG (RTHG/SHG). In the early stage of melanoma development, we found that collagen fibers adjacent to a melanoma have increased OI values and SHG intensities. In the late stages, these collagen networks have more directionality and less homogeneity. The corresponding GLCM traces showed oscillation features and the sum of squared fluctuation VarGLCM increased with the tumor sizes. In addition, the THG intensities of the extracellular matrices increased, indicating an enhanced optical inhomogeneity. Multiplying OI, VarGLCM, and RTHG/SHG together, the combinational collagen remodeling (CR) index at 4 weeks post melanoma implantation showed a 400-times higher value than normal ones. These results validate that our quantitative indices of SHG and THG microscopies are sensitive enough to diagnose the collagen remodeling in vivo. We believe these indices have the potential to help the diagnosis of skin cancers in clinical practice.

Strategic improvement of second and third harmonic generation in multifunctional Cu-Sn-S3 ternary semiconducting thin films

Forthcoming low-energy consumption oxide electronics rely on the deterministic control of ferroelectric and multiferroic domain states at the nanoscale. In this review, we address the recent progress in the field of investigation of ferroic order in thin films and heterostructures, with a focus on non-invasive optical second harmonic generation (SHG). For more than 50 years, SHG has served as an established technique for probing ferroic order in bulk materials. Here, we will survey the specific new aspects introduced to SHG investigation of ferroelectrics and multiferroics by working with thin film structures. We show how SHG can probe complex ferroic domain patterns non-invasively and even if the lateral domain size is below the optical resolution limit or buried beneath an otherwise impenetrable cap layer. We emphasize the potential of SHG to distinguish contributions from individual (multi-) ferroic films or interfaces buried in a device or multilayer architecture. Special attention is given to monitoring switching events in buried ferroic domain- and domain-wall distributions by SHG, thus opening new avenues towards the determination of the domain dynamics. Another aspect studied by SHG is the role of strain. We will finally show that by integrating SHG into the ongoing thin film deposition process, we can monitor the emergence of ferroic order and properties in situ, while they emerge during growth. Our review closes with an outlook, emphasizing the present underrepresentation of ferroic switching dynamics in the study of ferroic oxide heterostructures.

For RbTiOPO4 (RTP) crystal, the second harmonic generation (SHG) properties of 1064nm are discussed, including phase matching (PM) direction, effective nonlinear optical coefficient (deff), walk-off angle, and angular acceptance. For the frequency doubling of nano-second and pico-second Nd:YAG laser, the conversion efficiency of RTP crystal can reach 51 % and 57 %, respectively. The laser damage threshold of RTP crystal is measured.

Electrical tuning of second-order nonlinearity in optical materials is attractive to strengthen and expand the functionalities of nonlinear optical technologies, though its implementation remains elusive. Here, we report the electrically tunable second-order nonlinearity in atomically thin ReS2 flakes benefiting from their distorted 1T crystal structure and interlayer charge transfer. Enabled by the efficient electrostatic control of the few-atomic-layer ReS2, we show that second harmonic generation (SHG) can be induced in odd-number-layered ReS2 flakes which are centrosymmetric and thus without intrinsic SHG. Moreover, the SHG can be precisely modulated by the electric field, reversibly switching from almost zero to an amplitude more than 1 order of magnitude stronger than that of the monolayer MoS2. For the even-number-layered ReS2 flakes with the intrinsic SHG, the external electric field could be leveraged to enhance the SHG. We further perform the first-principles calculations which suggest that the modification of in-plane second-order hyperpolarizability by the redistributed interlayer-transferring charges in the distorted 1T crystal structure underlies the electrically tunable SHG in ReS2. With its active SHG tunability while using the facile electrostatic control, our work may further expand the nonlinear optoelectronic functions of two-dimensional materials for developing electrically controllable nonlinear optoelectronic devices.

For high power industrial, intracavity frequency doubled, Nd:YAG laser systems, KTiOPO4 (KTP) is a very difficult nonlinear optical material to use because of the low damage threshold, smaller acceptance angle and higher birefringence (larger walk-off angle). In orderto achieve high power (more than 80 watts) of 532 nm laser light from a single Nd:YAG cavity, we have studied the Second Harmonic Generation (SHG) of Lithium Triborate (LiB3O5 or LBO), a relatively new nonlinear optical crystal discovered and developed in 1989. Both theoretical and experimental results are discussed in terms of the damage threshold, SHG conversion efficiency, acceptance and walk-off angles, as well as the techniques for both critical and non-critical phase-matching.For high power industrial, intracavity frequency doubled, Nd:YAG laser systems, KTiOPO4 (KTP) is a very difficult nonlinear optical material to use because of the low damage threshold, smaller acceptance angle and higher birefringence (larger walk-off angle). In orderto achieve high power (more than 80 watts) of 532 nm laser light from a single Nd:YAG cavity, we have studied the Second Harmonic Generation (SHG) of Lithium Triborate (LiB3O5 or LBO), a relatively new nonlinear optical crystal discovered and developed in 1989. Both theoretical and experimental results are discussed in terms of the damage threshold, SHG conversion efficiency, acceptance and walk-off angles, as well as the techniques for both critical and non-critical phase-matching.

Although tightly focused input beams for efficient second harmonic (SH) generation (SHG) are usually avoided because of material damage, this is exactly the condition needed for the generation of spatial solitons (QSS) based on quadratic (χ(2)) nonlinearities.[1,2] These QSS’es are mutually self-trapped fundamental (FW) and harmonic (SH) optical beams in which the mutual beam narrowing due to photon exchange incipient in up and down conversion balances diffraction. The generation of quadratic solitons significantly affects the SHG process, for example by placing an upper limit on the amount of fundamental that can be converted to SH, by increasing the bandwidth of the process etc.[3] Of particular interest are periodically poled ferroelectric media which are an important limit of non-critical phase-matching. In fact, periodically poled LiNbO3 (PPLN) has been shown to have the lowest soliton threshold of 1.5 GW/cm2 reported to date [4]. That pioneering work established the existence of quadratic solitons in periodically poled media but neither this work, or any other work on QSSes has addressed the impact of QSS on the SHG tuning curves.

\Ve present a new heterodyne technique for detecting weak signals beyond the standard shot-noise limit by employingamplitude-squeezed light as the local oscillator (LO) field. The amplitude-squeezed LO field at 428.8 nm is generated from ahighly efficient (up to 60% conversion efficiency) single-pass second-harmonic generation in a nonlinear crystal (KNbO3)pumped by femtosecond (130 fs) pulses at 857.7 nm. The weak signal field is introduced from the entrance port ofthe crystalto combine with the generated squeezed LO field. An enhancement of 0.7 dB (1.4 dB inferred) in signal-to-noise ratiobeyond the shot-noise limit is directly observed for this new heterodyne detection scheme. Practical applications of proposedtechnique in laser radar systems are discussed.Keywords: second harmonic generation, squeezed light 1. INTRODUCTION Second harmonic generation (SHG) is one of the nonlinear frequency conversion methods used as a means to extend coherentlight sources to shorter wavelengths. Having at the input of the nonlinear medium a fundamental field at a given frequency(no, one can obtain a field at doubled frequency 2wo with a quite high conversion efficiency, especially in case of pulse lasersbeing used as the fundamental field for pumping. The efficiency of the SHG conversion process is dependent on suchparameters as the input pump power, traveled distance inside the nonlinear medium, and the values of the medium's nonlinearcoefficients.i When one considers SHG from ultrashort pulses (such as pulse widths below half of a picosecond) in certainnonlinear media (such as KNbO3) processes like group velocity dispersion have to be taken into account.2One of the attractive features of SHG is the resulting suppression of the amplitude noise of the generated light. Amplitude-squeezed states of light have less quantum fluctuations in photon numbers of the light field than those of the coherent field(such as lasers).3 Unlike squeezed vacuum states of light, amplitude squeezed states of light can have intense optical power4,which allows direct replacement of coherent lasers in precision optical measurements where quantum shot-noise associatedwith the laser field needs to be suppressed.It has been observed that the degree of squeezing is proportional to the efficiency of SHG process,56 where the efficiency isdefined as the ratio of the generated second-harmonic light power to the input fundamental power. Therefore, configurationsof highly efficient SHG processes are very desirable for the generation of squeezed light. Using short pulse laser to pumpSHG process has a main advantage, which is the gain of its high peak power. Since the input power is one of the parametersthat the efficiency of SHG depends on, the high peak intensities that can be reached in short laser pulses contribute to theachievement of high conversion efficiencies in the nonlinear crystal. Recently, single-pass traveling-wave (TW) SHG hasbeen proposed and experimentally demonstrated in a bulk crystal7 and in quasi-phase-matched nonlinear waveguide8.Conversion efficiencies up to 60% were obtained in the experiments with nonlinear waveguides, and O.35-dB squeezing inthe generated second harmonic light was observed. We have shown that conversion efficiencies higher than 60% andamplitude squeezing of more than 1.0 dB could be obtained in a conceptually simple system of type I TW SHG in a bulkcrystal .

Linear and Nonlinear Polarizability: a Primer Second-Order Nonlinear Optical Processes in Molecules and Solids Third-Order Nonlinear Optical Effects in Molecular and Polymeric Materials Nonlinear Optical Properties of Molecules and Materials Electronic Hyperpolarizability and Chemical Structure Electrooptic Polymer Waveguide Devices: Status and Applications Waveguiding and Waveguide Applications of Nonlinear Organic Materials Nonlinear Optical Materials: The Great and Near Great Donor- and Acceptor-Substituted Organic and Organometallic Compounds: Second-Order Nonlinear Optical Properties Use of a Sulfonyl Group in Materials for Nonlinear Optical Materials: A Bifunctional Electron Acceptor Organic and Organometallic Compounds: Second-Order Molecular and Macroscopic Optical Nonlinearities Chemistry of Anomalous-Dispersion Phase-Matched Second Harmonic Generation Applications of Organic Second-Order Nonlinear Optical Materials Chromophore-Polymer Assemblies for Nonlinear Optical Materials: Routes to New Thin-Film Frequency-Doubling Materials Novel Covalently Functionalized Amorphous *y^2 Nonlinear Optical Polymer: Synthesis and Characterization Second-Order Nonlinear Optical Polyphosphazanes Molecular Design for Enhanced Electric Field Orientation of Second-Order Nonlinear Optical Chromophores Nonlinear Optical Chromophores in Photocrosslinked Matrices: Synthesis, Poling, and Second-Harmonic Generation Thermal Effects on Dopant Orientation in Poled, Doped Polymers: Use of Second Harmonic Generation Organic Polymers as Guided Wave Materials Observing High Second Harmonic Generation and Control of Molecular Alignment in One Dimension: Cyclobutenediones as a Promising New Acceptor for Nonlinear Optical Materials Strategy and Tactics in the Search for New Harmonic-Generating Crystals Development of New Nonlinear Optical Crystals in the Borate Series What is Materials Chemistry? Defect Properties and the Photorefractive Effect in Barium Titanate Defect Chemistry of Nonlinear Optical Oxide Crystals From Molecular to Supramolecular Nonlinear Optical Properties Control of Symmetry and Asymmetry in Hydrogen-Bonded Nitroaniline Materials Molecular Orbital Modeling of Monomeric Aggregates in Materials with Potentially Nonlinear Optical Properties Strategies for Design of Solids with Polar Arrangement Ferroelectric Liquid Crystals Designed For Electronic Nonlinear Optical Applications Model Polymers with Distyrylbenzene Segments for Third-Order Nonlinear Optical Properties Composites: Novel Materials for Second Harmonic Generation Clathrasils: New Materials for Nonlinear Optical Applications Inorganic Sol-Gel Glasses as Matrices for Nonlinear Optical Materials Intrazeolite Semiconductor Quantum Dots and Quantum Supralattices: New Materials for Nonlinear Optical Applications Small Semiconductor Particles: Preparation and Characterization Synthetic Approaches to Polymeric Nonlinear Optical Materials Based on Ferrocene Systems Transition Metal Acetylides for Nonlinear Optical Properties Third-Order Near-Resonance Nonlinearities in Dithiolenes and Rare Earth Metallocenes Nonlinear Optical Properties of Substituted Phthalocyanines Nonlinear Optical Properties of Substituted Polysilanes and Polygermanes Design of New Nonlinear Optic-Active Polymers: Use of Delocalized Polaronic or Bipolaronic Charge States New Polymeric Materials with Cubic Optical Nonlinearities: From Ring-Opening Metathesis Polymerization Polymers and an Unusual Molecular Crystal with Nonlinear Optical Properties Quadratic Electrooptic Effect in Small Molecules Third-Order Nonlinear Optical Properties of Organic Materials

Lead‐free halide perovskites is a novel class of environment‐friendly nonlinear optical materials, that already demonstrate high potential for second harmonics generation (SHG). Here a synthesis protocol is optimized to create single‐crystal CsGeI3 nanoparticles (NPs) supporting optical Mie resonances that efficiently convert infrared light to visible both via lasing and SHG mechanisms. Such high‐quality resonant NPs allow us to achieve up‐conversion lasing in the broadband excitation wavelength (1200–1520 nm), which is accompanied by efficient spectrally tunable SHG with intensity comparable with that for lasing depending on temperature. Experimental and theoretical study of linear and nonlinear optical properties of CsGeI3 material in the form of high‐quality thin film and NPs of different sizes reveal that coupling incident light with a magnetic dipole resonance leads to a strong enhancement of SHG by one order of magnitude. As a result, the study provides a novel strategy where individual NPs can support both up‐conversion lasing and SHG in a broad range of excitation wavelengths.

We theoretically investigated the electronic and nonlinear optical properties of 3D quantum dot-double quantum ring structures of GaAs/Al0.3Ga0.7As under electric field. We studied four representative geometries with variable dot height and inner ring width, performing full 3D calculations. The nonlinear optical rectification (NOR) is strongly influenced by the structural parameters and increases with the degree of asymmetry induced by the electric field. We found enhanced NOR values especially for the structure with small dot height. Large second harmonic generation (SHG) at the double resonance condition was obtained in all cases at specific values of the electric field. As for NOR, the small dot height structure exhibits the biggest SHG signal at resonance. Significant third harmonic generation (THG) coefficients are obtained only for medium dot height geometry. The stronger THG signal is emitted by the structure with larger inner ring.