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This paper presents second-order broadband optical generation based on random domain size orientation in a birefringent nonlinear crystal. A higher degree of structural disorder seems to enhance the efficiency in response to an increasing number of domains, while the higher level of disorder growth is characterized by a wider dispersion. This observation holds true across various domain sizes, rendering the system robust to structural fluctuations. The simulation results for broadband second-harmonic generation in birefringent, transparent media with arbitrary domain configurations are explored in this paper. Additionally, the idea of expanding the mid-infrared second-harmonic generation’s bandwidth is described analytically for the proposed configuration. The analysis includes the disordered assembly of magnesium-oxide-doped lithium niobate and investigates the impact of birefringence on the random quasi-phase matching process in both monodispersed and polydispersed assemblies, considering the random rotation of different domains. The efficiency of 3.49% and 2.01% with 3-dB bandwidth of 135nm and 210nm, centered at 1170nm and 1135nm for monodispersed and polydispersed cases, respectively, have been obtained in phase matching case.

We have observed fairly strong second-harmonic generation (SHG) in vacuum-deposited thin films of phthalocyanines (Pc) having central metal atoms; Co, Cu, Zn, VO, TiO, Pb, Sn and a metal-free Pc. Based on the remarkable spectral dependences, the SHG was concluded to be due to higher-order mechanisms: in all the films, a resonance peak due to an electric quadrupole transition was observed at the SH wavelength corresponding to the higher-energy side of the Q band. The spectral structures in the molecules with a similar shape are similar to each other, indicating that the electronic states responsible for the spectra are ascribed to Pc macrocycles. The χ⁽²⁾ values corresponding to in C_∞v symmetry and the conventional electric dipole mechanism are in the range of 1.4 ∼ 10 × 10⁻⁹ esu for these Pc films.

The nonlinear second-harmonic generation (SHG) and third-harmonic generation (THG) in “12–6” tuned GaAs/GaAlAs double quantum wells (QWs) are calculated in the presence of applied electric field under the effective mass approximation. The influence of the structural parameter of the double QW is also considered, which is the main means to adjust its nonlinear optical properties. The eigenvalues and corresponding eigenfunctions of the double QWs system are obtained via the finite difference technique. The nonlinear SHG and THG are presented as a function of the photon energy, structural parameter, and applied electric field. It is shown that the SHG and THG are sensitive to both the applied electric field and the structure parameters, which can tune the nonlinear optical properties efficiently by adjusting the symmetry and confined potential of the double QWs.

Primary shear horizontal (SH) mode propagation and its second harmonic are generated and detected using wedge transducers on the surface of an elastic plate. In the far field region, the clear second-harmonic response can be observed at the frequency where the phase velocity of the primary SH mode equals that of the symmetric double-frequency Lamb mode. Based on the measured results, the ratio of the second-harmonic amplitude to the square of the primary SH mode amplitude is numerically evaluated for different propagation distances. It is experimentally verified that under some conditions the second harmonic of the primary SH mode propagation grows in amplitude with propagation distance.

Several parameters that affect the optical second-harmonic generation (SHG) coefficients in the two-dimensional Coulomb system have been discussed. Due to the changes of the Coulomb potential coefficients and geometric factor, it is shown that the optical SHG coefficients' changes are effectively controlled. Moreover, the SHG coefficients depend strongly on the system size and relaxation time.

This letter proposes a procedure for generation and control of chaotic beats in a dynamical system that is initially in the periodic state. The dynamical system describes a simple nonlinear optical process — second-harmonic generation of light. The periodic states of the system are found to be in analytical forms. We also investigate some aspects of synchronization of chaotic beats in two systems, detuned in the pump fields.

The optical properties of nanoscopic arrays of metal particles are dominated by plasmon resonances and electromagnetic interaction between the particles. We use electron-beam lithography to prepare arrays of noncentrosymmetric gold particles and study their linear and second-order nonlinear optical properties. By varying the orientation of the particles in a fixed lattice, we observe shifts in the polarized linear extinction spectra. The second-harmonic generation efficiencies of the two types of samples differ by up to 60%. The results show that the properties of the samples are sensitive to the smallest details of their structure.

We present two new techniques with increased precision to determine the second-harmonic susceptibility tensor of thin films. The tensor is associated with the macroscopic structure of such samples but cannot be measured directly. Instead, it must be extracted from experimental results using a theoretical model. The first technique is based on regression analysis of a large number of independent measurements. The second technique relies on the use of two, instead of one, input beams at the fundamental frequency. Both techniques allow for addressing the quality of the theoretical models used to describe the experiment.

Although nonlinear optical (NLO) polymers often exhibit very high optical nonlinearities, they have barely been applied to tentative NLO devices. The main reason lies in their high absorption losses, and this fundamental trade-off between optical nonlinearity and absorption, stems further advancement. Our theoretical analyses inform us that absorption effect is much less critical in cascaded wavelength conversion (CWC) than in second-harmonic generation (SHG), especially when absorption loss is very high at a given second-harmonic wavelength. Based on this result, we propose CWC as a more practical application of NLO polymers than SHG.

Nonlinear optical (NLO) polymer is emerging as a new alternative material for a wavelength converter in optical fiber communication systems. We designed and synthesized a polyetherimide-based side-chain polymer with a 4-(dialkylamino)-4′(-(styrylsulfonyl)-stilbene chromophore as a new NLO polymer. We designed and fabricated periodically-poled NLO polymer waveguides for the wavelength converter. Difference-frequency generation (DFG) process with a quasi-phase-matching (QPM) scheme was used. For the first time, we demonstrated a wavelength converter with a DFG conversion efficiency of -47 dB in the periodically-poled NLO polymer waveguides, operating at ~ 1.55 μm.

We describe our research involving arrays of gold nanoparticles fabricated with electron beam lithography. Small defects in the particles introduced during the fabrication process result in broken design symmetry. The broken symmetry renders the arrays chiral, and signatures of chirality can be observed in both the linear and nonlinear optical responses through precise polarization measurements. The defects enhance the local electric field, enabling more efficient nonlinear optical generation and therefore making nonlinear optical measurements more sensitive to their presence. We also discuss future objectives for clarifying the microscopic processes underlying the optical responses in gold nanoparticle arrays.

In this paper, we present the experimental proof that even for highly nonlinear (NL), such as 2-methyl-4-nitroaniline (MNA), wave mixing has negligible effect on the second-harmonic generation (SHG) using 250 fs fundamental pulses. We explain our experimental results with a simple formalism based on monochromatic plane wave (MPW).

The distribution details of strongly-localized fields in metal nanostructures are important for strong interaction with the locally-varying non-linearity. Using gold nanodimers with nanogaps, we demonstrate that field localization alone is insufficient to drive a strong second-order response. Instead, polarization conversion of the local fundamental field combines with its asymmetric distribution along the dimer perimeter to yield strong responses. Strongly-localized fields may also favor higher-multipole (magnetic-dipole and electric-quadrupole) contributions to their nonlinear response. We show that the polarization-dependent interference of multipole sources in second-harmonic generation from gold nanoparticles allows the separation of their relative contributions.

This paper analyzes a highly efficient broadband second-harmonic generator in the mid infrared region using an isotropic reverse tapered semiconductor slab configuration, using fractional total internal reflection quasi-phase matching technique. A computer aided simulation has been performed using ZnTe as the nonlinear material which provides an extremely high conversion efficiency of 22.94% with a 3dB bandwidth of 200nm. Effect of varying the slab dimensions on the performance parameters has also been analyzed. Finally, the catastrophic effect caused by destructive interference due to nonlinear law of reflection has also been incorporated in the analysis thereby giving more accurate results.

Vector beams with the desired polarization structure interacting with matter result in many novel nonlinear optical effects. Herein, we review the tight focusing and weak focusing properties of three types of light beams, namely, linearly polarized beams, cylindrical vector beams, and hybridly polarized beams. In particular, we revisit the second- and third-order nonlinear optical effects in nonlinear optical media under the excitation of several types of vector beams with the desired polarization distribution. The prospects of their applications in nonlinear optical microscopy and materials characterization are also briefly discussed.

A three-dimensional (3D) Cd-CP [Cd(H₂L)]_n was synthesized under hydrothermal conditions [${\mathrm{\text{H}}}_4\mathrm{\text{L}}=1\mathrm{\text{H}},1^{\prime }\mathrm{\text{H}}$-[2,4${}^{\prime }$-biimidazole]-4,5-dicarboxylicacid]. The structure of [Cd(H₂L)]_n was characterized by single-crystal X-ray diffraction and the result shows that it exhibits a non-centrosymmetric lig(LiGe) network constructed from interconnecting C₂-symmetric metallacalix[4]arenes Cd₄(H₂L)₄ MBBs. This complex exhibits light blue fluorescence emission originated from intraligand ${\pi }^{\ast }\to \pi$ electron transition, indicating that the d${}^{10}$ metal ions can hardly absorb energy and thus have no interference with the fluorescence of the ligand. The UV–Vis was used to investigate the optical property of the sample, which is essentially not absorbed in the visible light area and the band gap of the sample is about 2.2eV. Moreover, nonlinear optical (NLO) determination shows that the sample exhibits an obvious second-harmonic generation (SHG) response of $\sim 1.2\times \mathrm{\text{urea}}$.

The work is devoted to the study of the second-harmonic generation (SHG) process as a result of random quasi-phase-matching of thulium laser radiation in ZnSe polycrystals. A linear dependence of the SHG efficiency on the nonlinear medium length has been observed in the experiment. The efficiencies of SHG in ZnSe polycrystals with average grain sizes from 35$\mu$m to 380$\mu$m were compared. The obtained results are in good agreement with the theory in the case of a Gaussian grain size distribution.

Epithelial cancer comprises more than 85% of human cancers. The detection and treatment at the early stage has been demonstrated to apparently improve patient survival. In this review, we summarize our recent research works on the diagnostic application of epithelial tissue based on multiphoton microscopy (MPM), including identification of the layered structures of esophagus, oral cavity, skin and bronchus tissues, establishment of the diagnostic features for distinguishing gastric normal tissue from cancerous tissue, linking collagen alteration and ectocervical epithelial tumor progression for evaluating epithelial tumor progression, and differentiating normal, inflammatory, and dysplastic ectocervical epithelial tissues. These results provide the groundwork for developing MPM into clinical multiphoton endoscopy.

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors arising in the digest tract. It brings a challenge to diagnosis because it is asymptomatic clinically. It is well known that tumor development is often accompanied by the changes in the morphology of collagen fibers. Nowadays, an emerging optical imaging technique, second-harmonic generation (SHG), can directly identify collagen fibers without staining due to its noncentrosymmetric properties. Therefore, in this study, we attempt to assess the feasibility of SHG imaging for detecting GISTs by monitoring the morphological changes of collagen fibers in tumor microenvironment. We found that collagen alterations occurred obviously in the GISTs by comparing with normal tissues, and furthermore, two morphological features from SHG images were extracted to quantitatively assess the morphological difference of collagen fibers between normal muscular layer and GISTs by means of automated image analysis. Quantitative analyses show a significant difference in the two collagen features. This study demonstrates the potential of SHG imaging as an adjunctive diagnostic tool for label-free identification of GISTs.

A novel non-centrosymmetric (NCS) orthorhombic complex Zr(dmpu)₂Cl₄ (1) has been solvothermally prepared in situ from starting materials of Zirconium(IV) acetylacetonate (Zr(acac)₄) and N,N-dimethylpropyleneurea (dmpu), in which dmpu can act not only as solvent but also as organic ligand of monodentate oxygen donor. The second-harmonic generation (SHG) measurements reveal that complex 1 has a strong response of 1.4 times that of potassium dihydrogen phosphate (KDP) at 532 nm based on the Kurtz–Perry method. Furthermore, optical transmission study indicates that complex 1 has a wide transparency range in the UV region (200–850 nm) with a cutoff at 291 nm. According to first-principles calculations, the enhanced macroscopic SHG response of 1 can be attributed to the organic ligand dmpu. Therefore, 1 could be a promising candidate for non-linear optical material.