Significant Second Harmonic Generation Research Articles

Anisotropic Strain-Tailoring Nonlinear Optical Response in van der Waals NbOI2.

Two-dimensional (2D) NbOI2 demonstrates significant second-harmonic generation (SHG) with a high conversion efficiency. To unlock its full potential in practical applications, it is desirable to modulate the SHG behavior while utilizing the intrinsic lattice anisotropy. Here, we demonstrate direction-specific modulation of the SHG response in NbOI2 by applying anisotropic strain with respect to the intrinsic lattice orientations, where more than 2-fold enhancement in the SHG intensity is achieved under strain along the polar axis. The strain-driven SHG evolution is attributed to the strengthened built-in piezoelectric field (polar axis) and the enlarged Peierls distortions (nonpolar axis). Moreover, we provide quantifications of the correlation between strain and SHG intensity in terms of the susceptibility tensor. Our results demonstrate the effective coupling of orientation-specific strain to the anisotropic SHG response through the intrinsic polar order in 2D nonlinear optical crystals, opening a new paradigm toward the development of functional devices.

Hierarchical van der Waals Assemblies to Boost Deep‐Ultraviolet Nonlinear‐Optical Capabilities in Boron Phosphate

AbstractCristobalite‐type boron phosphate BPO4, an important nonlinear‐optical (NLO) crystal, exhibits nearly the largest bandgap (Eg ≈ 9.3 eV) and a significant second‐harmonic‐generation (SHG) effect (d36 ≈ 0.7 pm V−1) in the deep‐ultraviolet (DUV, < 200 nm) spectral region. However, its conventional tetrahedral framework structure tends to have low structural anisotropy, resulting in a severe deficiency of birefringence (Δn < 0.01), which fails to achieve DUV birefringent phase matching. In this work, the local anisotropy of tetrahedral structures are proposed to enlarge by constructing van der Waals (vdW) anisotropic motifs under synthetic chemical conditions, which can effectively convert unattainable extreme external forces into internal stresses in chemical bond structures. Depending on unique hierarchical vdW assemblies, enormous tetrahedral distortions can be achieved by stable in‐plane or in‐line bond matching between rigid tetrahedral primitives. First‐principles calculations demonstrate that the birefringence of 2D layered and 1D chained BPO4 achieved by vdW engineering are scaled from Δn < 0.01 to 0.07–0.13, with the shortest birefringent phase‐matched wavelengths down to DUV 189–154 nm. The finding is of seminal importance for the design of vdW NLO crystals in structural chemistry and would provide opportunities to explore unique vdW DUV NLO materials under extreme synthetic conditions.

Free Halogen Substitution of Chiral Hybrid Metal Halides for Activating the Linear and Nonlinear Chiroptical Properties.

Halogen substitution has been proven as an effective approach to the band gap engineering and optoelectronic modulation of organic-inorganic hybrid metal halide (OIHMH) materials. Various high-performance mixed halide OIHMH film materials have been primarily obtained through the substitution of coordinated halogens in their inorganic octahedra. Herein, we propose a new strategy of substitution of free halogen outside the inorganic octahedra for constructing mixed halide OIHMH single crystals with chiral structures, resulting in a boost of their linear and nonlinear chiroptical properties. The substitution from DMA4[InCl6]Cl (DMA = dimethylammonium) to DMA4[InCl6]Br crystals through a facile antisolvent vaporization method produces centimeter-scale single crystals with high thermal stability along with high quantum yield photoluminescence, conspicuous circularly polarized luminescence, and greatly enhanced second harmonic generation (SHG). In particular, the obtained DMA4[InCl6]Br single crystal features an intrinsic chiral structure, exhibiting a significant SHG circular dichroism (SHG-CD) response with a highest reported anisotropy factor (gSHG-CD) of 1.56 among chiral OIHMH materials. The enhancements in both linear and nonlinear chiroptical properties are directly attributed to the modulation of octahedral distortion. The mixed halide OIHMH single crystals obtained by free halogen substitution confine the introduced halogens within free halogen sites of the lattice, thereby ensuring the stability of compositions and properties. The successful employment of such a free halogen substitution approach may broaden the horizon of the regulation of structures and the optoelectronic properties of the OIHMH materials.

Quadruple‐Bidentate Nitrate‐Ligated A2Hg(NO3)4 (A=K, Rb): Strong Second‐Harmonic Generation and Sufficient Birefringence

AbstractThe design of efficient nonlinear optical (NLO) crystals continues to pose significant challenges due to the difficulty of assembling polar NLO‐active modules in an optimal additive fashion. We report herein the first NLO‐active mercuric nitrates A2Hg(NO3)4 (A=(KHNO), Rb (RHNO)), for which assembly is induced by ionic polarization of the d10 cations. The two new crystalline compounds are isostructural, featuring interesting pseudo‐diamond‐like structures with parallel [Hg(NO3)4] modules, and leading to strong powder second‐harmonic generation (SHG) responses of 9.2 (KHNO) and 8.8 (RHNO) times that of KH2PO4. In combination with the simple solution preparation of centimeter‐scale crystals, sufficient birefringence, and short ultraviolet (UV) cutoff edges, these attributes make KHNO and RHNO promising candidates for UV NLO materials. Theoretical calculations and single‐crystal structure analysis reveal that the newly‐developed highly condensed and distorted [Hg(NO3)4] module, with an Hg2+ cation that is quadruply bidentate nitrate‐ligated, is crucial for the significant SHG responses. This work highlights the potential importance of modules with multiple bidentate ligands for the development of high‐performing next‐generation NLO materials.

Quadruple-Bidentate Nitrate-Ligated A2 Hg(NO3 )4 (A=K, Rb): Strong Second-Harmonic Generation and Sufficient Birefringence.

The design of efficient nonlinear optical (NLO) crystals continues to pose significant challenges due to the difficulty of assembling polar NLO-active modules in an optimal additive fashion. We report herein the first NLO-active mercuric nitrates A2 Hg(NO3 )4 (A=(KHNO), Rb (RHNO)), for which assembly is induced by ionic polarization of the d10 cations. The two new crystalline compounds are isostructural, featuring interesting pseudo-diamond-like structures with parallel [Hg(NO3 )4 ] modules, and leading to strong powder second-harmonic generation (SHG) responses of 9.2 (KHNO) and 8.8 (RHNO) times that of KH2 PO4 . In combination with the simple solution preparation of centimeter-scale crystals, sufficient birefringence, and short ultraviolet (UV) cutoff edges, these attributes make KHNO and RHNO promising candidates for UV NLO materials. Theoretical calculations and single-crystal structure analysis reveal that the newly-developed highly condensed and distorted [Hg(NO3 )4 ] module, with an Hg2+ cation that is quadruply bidentate nitrate-ligated, is crucial for the significant SHG responses. This work highlights the potential importance of modules with multiple bidentate ligands for the development of high-performing next-generation NLO materials.

Intermolecular interactions enhanced second harmony generation of iridium complex for bio-labeling

Second harmonic generation (SHG) microscopy avoids the inherent drawbacks of traditional fluorescent probes and brings a major breakthrough in live cell imaging. However, not all ordered non-centrosymmetric matter has the ability to generate SHG signals due to the different modes of intermolecular interaction. Here, we designed a non-centrosymmetric metal iridium complex (Ir-SHG) and obtained single crystals with two different intermolecular interaction modes in red (Ir-R) and yellow (Ir-Y), in which Ir-Y was shown to acquire a significant SHG effect. Interestingly, the SHG imaging of Ir-SHG exhibits high imaging signal-to-ratio (SNR) and brightness, as well as can be used in combination with a large number of fluorophores without signal cross-talk. We believe this work brings new insights into the development of SHG materials for multimodal nonlinear optical imaging.

Anomalous wavefront control via nonlinear acoustic metasurface through second-harmonic tailoring and demultiplexing

We propose a nonlinear acoustic metasurface concept by exploiting the nonlinearity of locally resonant unit cells formed by curved beams. The analytical model is established to explore the nonlinear phenomenon, specifically the second-harmonic generation (SHG) of the nonlinear unit cell, and validated through numerical and experimental studies. By tailoring the phase gradient of the unit cells, nonlinear acoustic metasurfaces are developed to demultiplex different frequency components and achieve anomalous wavefront control of SHG in the transmitted region. To this end, we numerically demonstrate wave steering, wave focusing, and self-bending propagation. Our results show that the proposed nonlinear metasurface provides an effective and efficient platform to achieve significant SHG and separate different harmonic components for wavefront control of individual harmonics. Overall, this study offers an outlook to harness nonlinear effects for acoustic wavefront tailoring and develops potential toward advanced technologies to manipulate acoustic waves.

Improved Polarization in the Sr6 Cd2 Sb6 O7 Se10 Oxyselenide through Design of Lateral Sublattices for Efficient Photoelectric Conversion.

Highly-polarizable materials are favorable for photoelectric conversion due to their efficient charge separation, while precise design of them is still a big challenge. Herein a novel polar oxyselenide, Sr6 Cd2 Sb6 O7 Se10 , is rationally designed. It contains lateral sublattices of polarizable [Sb2 OSe4 ]4- chains and highly-orientated [CdSe3 ]4- chains. The intense polarization was evaluated by significant second-harmonic generation (SHG) signal (maximum: 12.6×AgGaS2 ) in broad spectrum range. The polarization was found to mainly improve the carrier separation with a much longer recombination lifetime (76.5 μs) than that of the nonpolar compound Sr2 Sb2 O2 Se3 (18.0 μs), resulting in better photoelectric performance. The single-crystal photoelectric device exhibited excellent response covering broad spectrum in 500-1000 nm with stable reproducibility. This work provides some new insights into the structure design of highly-polarizable heteroanionic materials for photoelectric conversion.

Improved Polarization in the Sr6Cd2Sb6O7Se10 Oxyselenide through Design of Lateral Sublattices for Efficient Photoelectric Conversion

AbstractHighly‐polarizable materials are favorable for photoelectric conversion due to their efficient charge separation, while precise design of them is still a big challenge. Herein a novel polar oxyselenide, Sr6Cd2Sb6O7Se10, is rationally designed. It contains lateral sublattices of polarizable [Sb2OSe4]4− chains and highly‐orientated [CdSe3]4− chains. The intense polarization was evaluated by significant second‐harmonic generation (SHG) signal (maximum: 12.6×AgGaS2) in broad spectrum range. The polarization was found to mainly improve the carrier separation with a much longer recombination lifetime (76.5 μs) than that of the nonpolar compound Sr2Sb2O2Se3 (18.0 μs), resulting in better photoelectric performance. The single‐crystal photoelectric device exhibited excellent response covering broad spectrum in 500–1000 nm with stable reproducibility. This work provides some new insights into the structure design of highly‐polarizable heteroanionic materials for photoelectric conversion.

Homovalent cation substitutions leading to new lithium-containing La3CuSiS7 family members having chiral, uniaxial structures and exhibiting second- and third-harmonic generation

Eighteen isotypic, lithium-containing rare-earth sulfides, crystallizing in the chiral, polar, and noncentrosymmetric space group P63 with the La3CuSiS7-structure type have been prepared via direct combination of the elements or binary sulfides; nine of these compounds are reported for the first time. The structures of these compounds, with the formulae Ln3LiTS7 (Ln = La, Ce, Pr, Nd, Sm, Gd, and Dy for T = Si or Ge; Ln = La, Ce, Pr, and Nd for T = Sn), were determined by single crystal X-ray diffraction. According to X-ray powder diffraction data, the Ln3LiTS7 compounds are the major phase of the reaction products. All of the compounds are semiconductors with optical bandgaps spanning nearly the entire visible region. The Si- and Ge-containing analogs show high thermal stability,> 1000 °C, while the Sn-containing compounds melt in the vicinity of ~740 °C. The compounds show potentially broad regions of optical transparency in the infrared regime. Calculated bond valence sums (BVSs) and global instability index (G) values confirm the Ln3+ oxidation state and stable crystal structures with reasonable strain, respectively. Ce3LiGeS7 and Ce3LiSnS7 display non-phase-matching but significant second-harmonic generation responses with χ2 values of 21 ± 1 and 28 ± 1 pm/V, respectively at λ = 1.8 µm. The laser-induced damage threshold values for Ce3LiGeS7 and Ce3LiSnS7 are> 3 × commercial AgGaSe2, for picosecond pulses at λ = 1.064 µm. These two compounds also exhibit moderate third-harmonic generation responses. Additionally, crystal-chemical correlations are discussed.

Electric-field-induced second harmonic generation in silicon dioxide.

Electric-field-induced second harmonic generation (EFISH) as a third order nonlinear process is of high practical interest for the realization of functional nonlinear structures. EFISH in materials with vanishing χ(2) and non-zero χ(3) offers huge potential, e.g., for background-free nonlinear electro-optical sampling. In this work, we have investigated SiO2 as a potential EFISH material for such applications using DC-electric fields. We were able to observe significant second harmonic generation (SHG) in comparison to the background SHG signal. The fundamental excitation at 800 nm results in a SHG signal at 400 nm for high applied DC electric fields, which is a clear indication for EFISH. Additionally, we were are able to precisely model the EFISH signal using time-domain simulations. This numerical approach will be of great importance for efficiency enhancement and prove as a valuable tool for future device design.

Second harmonic generation in glass-based metasurfaces using tailored surface lattice resonances

Abstract Dielectric metasurfaces have shown prominent applications in nonlinear optics due to strong field enhancement and low dissipation losses at the nanoscale. Chalcogenide glasses are one of the promising materials for the observation of nonlinear effects thanks to their high intrinsic nonlinearities. Here, we demonstrate, experimentally and theoretically, that significant second harmonic generation (SHG) can be obtained within amorphous Selenium (Se)-based chalcogenide metasurfaces by exploiting the coupling between lattice and particle resonances. We further show that the high-quality factor resonance at the origin of the SHG can be tuned over a wide wavelength range using a simple and versatile fabrication approach. The measured second harmonic intensity is orders of magnitude higher than that from a dewetted Se film consisting of random Se nanoparticles. The achieved conversion efficiency in the resonance region is of the order of 10−6 which is comparable with direct bandgap materials and at least two orders of magnitude higher than that of conventional plasmonics- and Si-based structures. Fabricated via a simple and scalable technique, these all-dielectric architectures are ideal candidates for the design of flat nonlinear optical components on flexible substrates.

Significant second-harmonic generation and bulk photovoltaic effect in trigonal selenium and tellurium chains.

One-dimensional (1D) selenium and tellurium crystalize in helical chainlike structures and thus exhibit fascinating properties. By performing first-principles calculations, we have researched the linear and nonlinear optical (NLO) properties of 1D Se and Te, and find that both systems exhibit pronounced NLO responses. In particular, 1D Se is found to possess a large second-harmonic generation coefficient with the χ value being up to 7 times larger than that of GaN, and is even several times larger than that of the bulk counterpart. On the other hand, 1D Te also produces significant NLO susceptibility χ which exceeds that of bulk GaN by 5 times. Furthermore, 1D Te is shown to possess a prominent linear electro-optic coefficient rxxx(0). In particular, the Te chain exhibits a large shift current response and the maximum is twice as large as the maximal photovoltaic responses obtained from BaTiO3. Therefore, 1D Se and Te may find potential applications in solar energy conversion, electro-optical switches, and so on. Finally, the much stronger NLO effects of 1D Se and Te are attributed to their one-dimensional structures with high anisotropy, strong covalent bonding and lone-pair electrons. These findings will contribute further to experimental studies and the search for excellent materials with large NLO effects.

Large second-harmonic generation and linear electro-optic effect in trigonal selenium and tellurium

Trigonal selenium and tellurium crystalize in helical chain-like structures and thus possess interesting properties such as current-induced spin polarization, gyrotropic effects and nonlinear optical responses. By performing systematic ab initio calculations based on density functional theory in the generalized gradient approximation plus scissors correction, we study their linear and nonlinear optical (NLO) properties. We find that the two materials exhibit significant second-harmonic generation (SHG) and linear electro-optic (LEO) effect. In particular, the SHG coefficients of tellurium are huge in the photon energy range of 0 ~ 3 eV, with the magnitudes of SHG(xxx) being as large as 3640 pm/V, which are about sixteen times larger than that of GaN, a widely used NLO material. On the other hand, trigonal selenium is found to possesses large LEO coefficients r(xxx,0) which are more than six times larger than that of GaN.Therefore, trigonal tellurium and selenium may find valuable applications in NLO and LEO devices such as frequency conversion, electro-optical switch and light signal modulator.The energy positions and shapes of the principal features in the calculated optical dielectric functions of both materials agree rather well with the available experimental ones, and are also analyzed in terms of the calculated band structures especially symmetries of the involved band states and dipole transition selection rules.

Integrated silicon nitride electro-optic modulators with atomic layer deposited overlays.

Silicon nitride (SiN) is currently the most prominent CMOS-compatible platform for photonics at wavelengths <1 μm. However, realizing fast electro-optic (EO) modulators, the key components of any integrated optics platform, remains challenging in SiN. Modulators based on the plasma dispersion effect, as in silicon, are not available. Despite the fact that significant second-harmonic generation has been reported for silicon-rich SiN, no efficient Pockels effect-based modulators have been demonstrated. Here we report the back-end CMOS-compatible atomic layer deposition (ALD) of conventional second-order nonlinear crystals, zinc oxide, and zinc sulfide, on existing SiN waveguide circuits. Using these ALD overlays, we demonstrate EO modulation in ring resonators.

Abstract P5-02-02: Second harmonic generation in combination with nuclear morphometry in the evaluation of DCIS

Abstract Purpose/Objective: Collagen is a major extracellular matrix (ECM) constituent in normal breast and is extensively remodeled in breast carcinoma. Therefore, features of remodeled collagen in the stroma adjacent to ductal carcinoma in situ (DCIS) could indicate cancer progression. The major objective of this study is to identify potential tumor-associated collagen signatures unique to DCIS that will allow us to predict progression based on the collagen texture and nuclear morphology. In this present study, we develop two image analysis pipelines (SHG Texture Extraction and H&amp;E Nuclear Morphology Extractor) to quantify 1) stromal changes, 2) collagen signatures and 3) nuclear morphology from normal breast to DCIS in order to predict local breast cancer recurrence. Method: We used second harmonic generation (SHG) images and H&amp;E to analyze collagen features and to study nuclear morphology using a data set of 336 patients (from which 310 normal and 327 DCIS regions were imaged). The 336 patients were a subset of patients with pure DCIS taken from a case-control study. Clinical-pathologic factors were associated with risk of subsequent ipsilateral cancer (DCIS or invasive). The SHG framework consisted of collagen segmentation using 1) adaptive thresholding and 2) morphological operations. The H&amp;E framework consisted of nuclear segmentation using adaptive thresholding and a maker-controlled watershed algorithm; and nuclear feature extractions including intensity, texture and morphology. Overall, the SHG framework segments collagen regions and computes textural features specifically at collagen regions. Furthermore, the H&amp;E framework segments nuclei and computes nuclei morphology and textural features. These features were used in L1-regularized logistic regression to construct classification models to discriminate normal vs DCIS regions; and to distinguish regions from DCIS patients with vs. without local recurrences. Results: In first experiment, we performed L1-regularized logistic regression to construct a classification model to discriminate normal vs DCIS regions. Our results suggest that using only SHG collagen features, this logistic model selected 19 significant features to build a classification model that achieved area under curve (AUC) 90% and accuracy 83% using 5-Fold cross validation. When H&amp;E nuclei features are used, the logistic model selected 88 significant features and achieved AUC 91% and accuracy 86%. By combined both SHG and H&amp;E features, the model achieved classification AUC 93% and accuracy 88%. By using L1-regularized logistic model with combined significant SHG and H&amp;E features, we achieved AUC 59% with an accuracy of 61% for DCIS and recurrent DCIS regions. Conclusions: Our study suggests that SHG and nuclear morphology features extracted from H&amp;E can improve the classification of normal and DCIS regions. Overall, these results suggest that second harmonic generation and H&amp;E nuclear morphology analysis could aid in the assessment of prognosis and risk of progression to invasive breast cancer. Citation Format: Park CC, Irshad H, Ziaee S, Martin-Tuite P, Habel L, Weaver VM, Schnitt SJ, Beck AH. Second harmonic generation in combination with nuclear morphometry in the evaluation of DCIS [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P5-02-02.

Synthesis and properties of novel nonlinear optical polyester containing carbomethoxycyanovinylnitrocatecholic group

ABSTRACTA new X-type polyester 5 containing 4-(2′-carbomethoxy-2′-cyano)vinyl-5-nitrocatecholic groups as nonlinear optical (NLO) chromophores, which are parts of the polymer backbone, was prepared and studied its properties. Polyester 5 is soluble in common organic solvents such as acetone and dimethylsulfoxide. Polymer 5 shows a thermal stability up to 280°C from thermogravimetric analysis with glass-transition temperature (Tg) obtained from differential scanning calorimetry of near 110°C. The second harmonic generation (SHG) coefficient (d33) of poled polymer 5 film at the 1064 nm fundamental wavelength is 6.13 × 10−9 esu. The dipole alignment of polymer 5 exhibits a thermal stability up to Tg, and no significant SHG decay is observed below 110°C due to the partial main-chain character of polymer structure, which is acceptable for NLO device applications.

Synthesis and nonlinear optical properties of novel polyester containing dicyanovinylnitrocatecholic group

ABSTRACTA novel X-type polyester containing 4-(2′,2′-dicyanovinyl)-5-nitrocatecholic groups as nonlinear optical (NLO) chromophores, which constitute part of the polymer backbone, was prepared and characterized. The polyester is soluble in common organic solvents such as N,N-dimethylformamide and acetone. It shows a thermal stability up to 300°C from thermogravimetric analysis with glass-transition temperature obtained from differential scanning calorimetry of near 105°C. The second harmonic generation (SHG) coefficient (d33) of a poled polymer film at the 1064 nm fundamental wavelength is 5.31 × 10−9 esu. The dipole alignment exhibits a thermal stability even at 5°C higher than glass-transition temperature (Tg), and no significant SHG decay is observed below 110°C due to the partial main-chain character of polymer structure, which is acceptable for NLO device applications.

Quantitative Analysis of Collagen Produced by Rabbit Keratocytes using Second Harmonic Generation Microscopy

ABSTRACTPurpose: To quantitatively analyze the collagen produced by rabbit keratocytes treated with mitomycin C (MMC) and/or transforming growth factor-beta (TGF-β1), and to investigate the collagen arrangement in situ, using second harmonic generation (SHG) microscopy.Methods: Keratocytes were divided into four groups: The keraotocytes in group rabbit keratocytes were treated with serum free medium containing 1% methylcellulose as controls. The cells in group TGF were received with TGF-β1 (at 2 ng/ml) and the same medium as the control. After exposure to MMC at 0.02 mg/ml for 5 minutes, the cells in group MMC were received with the same treatment as controls. In group TGF-MMC, the cells were exposed to MMC at 0.02 mg/ml for 5 minutes first, and then treated as group TGF. The signals of collagen were detected by SHG at 2 and 4 weeks post-treatment. The collagen content, as well as the keratocyte numbers, was calculated by MetaMorph Offline.Results: The collagen arrangement could be detected clearly by SHG. The cell number of group TGF was higher than other groups at 2 and 4 weeks post-treatment. The collagen content in group MMC was the least among all the groups at 2 weeks, while the collagen content in group TGF was higher than in group rabbit keratocytes and MMC at 4 weeks. No significant SHG side effects were found in cell proliferation and collagen deposition.Conclusions: SHG can provide a highly sensitive method to quantitatively measure collagen content, as well as to analyze its arrangement in situ.

Far-field radiation patterns of second harmonic generation from gold nanoparticles under tightly focused illumination.

We study far-field angular radiation patterns of second harmonic generation (SHG) from gold nanosphere, nanocube, nanorod, and nanocup illuminated by tightly focused linearly and radially polarized beams, respectively. It is found that under linearly polarized illumination, far-field forward-scattering SHG (FSHG) dominates second harmonic (SH) responses generated by those gold particles. On the contrary, it is amazing that significant backward-scattering SHG (BSHG) can be observed when those gold nanoparticles are excited by a focused radially polarized beam. For the case of gold nanosphere, the effective point dipole systems are developed to reasonably elucidate this interesting difference. Our investigations suggest that for SHG microscopy with backward detection scheme, tightly focused radially polarized beam could be a promising excitation field to improve the backward SH signal.