Second Harmonic Generation Intensity Research Articles

Hexagonal In2Se3: A Defect Wurtzite-Type Infrared Nonlinear Optical Material with Moderate Birefringence Contributed by Unique InSe5 Unit.

The balance between second harmonic generation (SHG) intensity and laser-induced damage threshold (LIDT), together with phase-matchable behavior, is the key point for exploration of novel nonlinear optical (NLO) materials. In this work, the NLO property of defect wurtzite-type hexagonal-In2Se3 (γ) is extensively explored first. It exhibits a strong SHG intensity of 2.6 × AgGaS2 (AGS) at 2.1 μm, and a high powder LIDT of 7.3 × AGS. From wurtzite to γ-In2Se3, the birefringence changes from 0.003 to 0.075, resulting in the phase-matchable phenomenon of γ-In2Se3. This is well ascribed to the contribution of the unique InSe5 unit in γ-In2Se3 from the result of birefringence calculation and analysis.

Modulation of Second-Harmonic Generation in Bulk MoS2 via Excitation Wavelength and Metal Film Thickness

We numerically investigated the impact of the excitation wavelength and metal film thickness on second-harmonic generation (SHG) in bulk MoS2, which originates from the change in optical-path difference and phase difference among the multistage reflected lights during the thin-film interference process. The enhancement factor of SHG for a MoS2 layer on metal-dielectric substrates was calculated using the finite-difference time-domain technique. It was shown that the intensity of SHG was strongly affected by the excitation wavelength and the thickness of the metal film. We also obtained the dependence of the thickness of the MoS2 layer with the strongest SHG on the excitation wavelength. The reflection, transmission, and absorption of MoS2 layers with different thicknesses were studied by changing the metal film thickness of substrates, and the MoS2 thickness with the strongest SHG was found at different excitation wavelengths. We found that changing the metal film thickness has little effect on the color of the MoS2 layer, indicating that the thickness of the MoS2 layer can still be roughly estimated from the color. Therefore, it is inferred that MoS2 layers with the strongest SHG have the potential to be used for optical information storage or in the design of optical devices provided that the excitation wavelength or the thickness of the metal film is selected appropriately.

Magneto-Stark and Zeeman effect as origin of second harmonic generation of excitons in Cu2O

We report on the experimental and theoretical investigation of magnetic-field-induced second harmonic generation (SHG) and two-photon absorption of excited exciton states ($n\ensuremath{\geqslant}3$) of the yellow series in the cuprous oxide ${\mathrm{Cu}}_{2}\mathrm{O}$. In this centrosymmetric material, SHG can occur due to constructive interplay of electric dipole and electric quadrupole/magnetic dipole transitions for light propagating along the low-symmetry directions [111] or [112]. By application of a magnetic field in Voigt configuration, SHG gets also allowed for excitation along the [110] axis and even the high-symmetry cubic direction [001]. Combining a symmetry analysis and a microscopic theory, we uncover the two key contributions to the magnetic-field-induced SHG: the Zeeman effect and the magneto-Stark effect. We demonstrate systematic dependencies of the SHG intensity on the linear polarization angles of the ingoing fundamental laser and the outgoing SHG beam, complementary to the paper by Rommel et al. [Phys. Rev. B 101, 115202 (2020)]. In general, the resulting contour plots in combination with a symmetry analysis allow one to determine uniquely the character of involved transitions. Moreover, we can separate in magnetic field the Zeeman and the magneto-Stark effect through appropriate choice of the experimental geometry and polarization configuration. We present a microscopic theory of the second harmonic generation of excitons in a centrosymmetric cubic semiconductor taking into account the symmetry and the band structure of cuprous oxide. Based on the developed microscopic theory, we identify the main contributions to the second-order nonlinear susceptibility of $S, P$, and $D$ excitons. We analyze the redistribution of SHG intensities between the excitonic states both in the absence and presence of the magnetic field and show good agreement with the experimental data. With increasing exciton principal quantum number, the magneto-Stark effect overpowers the influence of the Zeeman effect.

Designing Silicates as Deep-UV Nonlinear Optical (NLO) Materials using Edge-Sharing Tetrahedra.

Discovering new deep-ultraviolet (DUV) nonlinear optical (NLO) materials is currently a great challenge. The reported DUV NLO materials are almost exclusively borates or phosphates. Silicates-the largest constituent of the earth's crust-are excluded owing to their weak second harmonic generation (SHG) response. We report a silicate, Li2 BaSiO4 , with edge-sharing LiO4 -SiO4 tetrahedra that achieves the balance between a short UV absorption edge, below 190 nm, and a large SHG response, 2.8×KDP. The SHG intensity is the largest for silicates without second-order Jahn-Teller cations, and exceeds that of non-isomorphic Li2 SrSiO4 by more than an order of magnitude. As such Li2 BaSiO4 may be seen as a promising DUV-UV NLO material. This research indicates that edge-sharing tetrahedra is a new design parameter for discovering new DUV NLO materials.

Designing Silicates as Deep‐UV Nonlinear Optical (NLO) Materials using Edge‐Sharing Tetrahedra

AbstractDiscovering new deep‐ultraviolet (DUV) nonlinear optical (NLO) materials is currently a great challenge. The reported DUV NLO materials are almost exclusively borates or phosphates. Silicates—the largest constituent of the earth's crust—are excluded owing to their weak second harmonic generation (SHG) response. We report a silicate, Li2BaSiO4, with edge‐sharing LiO4–SiO4 tetrahedra that achieves the balance between a short UV absorption edge, below 190 nm, and a large SHG response, 2.8×KDP. The SHG intensity is the largest for silicates without second‐order Jahn–Teller cations, and exceeds that of non‐isomorphic Li2SrSiO4 by more than an order of magnitude. As such Li2BaSiO4 may be seen as a promising DUV‐UV NLO material. This research indicates that edge‐sharing tetrahedra is a new design parameter for discovering new DUV NLO materials.

Second-harmonic generation spectroscopy in gold nanorod-based epsilon-near-zero metamaterials.

The interest in hyperbolic metamaterials is fueled by fascinating optical properties exhibited by this class of artificial media. Their optical features originate from hyperbolic dispersion emerging due to the shape anisotropy of the metal-dielectric composite. In this work, we study experimentally and numerically the second-harmonic generation (SHG) in ordered arrays of Au nanorods embedded in porous aluminum oxide. Strong increase of the SHG intensity in the vicinity of the epsilon-near-zero (ENZ) spectral point accompanied by dramatic phase modulation of the SHG wave is revealed. These effects are attributed to resonant enhancement of the electric field of the light wave and transition from the elliptical to hyperbolic dispersion law in hyperbolic metamaterials near the ENZ point.

Solid-State Nonlinear Optical Switch with the Widest Switching Temperature Range Owing to Its Continuously Tunable Tc.

Interest on the nonlinear optical (NLO) switches that turn on/off the second-harmonic generation (SHG) triggered by the external stimulus (such as heat) have continuously grown, especially on the solid-state NLO switches showing superior stability, reversibility, and reproducibility. Herein, we discover (NH4)2PO3F, as an entirely new solid-state NLO switch showing outstanding switch contrast and reversibility as well as strong SHG intensity (1.1 × KH2PO4 (KDP)) and high laser-induced damage threshold (2.0 × KDP), undergoes a unique first-order phase transition that originates from a reversible hydrogen-bond rearrangement and needs to overcome an energy barrier. Accordingly, we put forward a strategy to continuously modify such an energy barrier by reducing the number of hydrogen bonds per unit cell via an isoelectronic replacement of NH4+ by K+ with a similar size yet incapability of providing any hydrogen bond. Consequently, Kx(NH4)2-xPO3F (x = 0-0.3) exhibiting excellent switching performance are obtained. Remarkably, Kx(NH4)2-xPO3F not only realizes a continuously tunable Tc spanning from 270 to 150 K, representing the widest NLO switching temperature range ever known but also indicates the first solid-state NLO switch example with continuous Tc. Intrinsically, such a Tc decline depends on the weakening degree of the hydrogen-bonding interactions in the unit cell. These new insights will shed useful light on the future material design and open new application possibilities.

Polarimetric second-harmonic generation microscopy of the hierarchical structure of collagen in stage I-III non-small cell lung carcinoma.

Polarimetric second-harmonic generation (P-SHG) microscopy is used to quantify the structural alteration of collagen in stage-I,-II and -III non-small cell lung carcinoma (NSCLC) ex vivo tissue. The achiral and chiral molecular second-order susceptibility tensor components ratios (R and C, respectively), the degree of linear polarization (DLP) and the in-plane collagen fiber orientation (δ) were extracted. Further, texture analysis was performed on the SHG intensity, R, C, DLP and δ. The distributions of R, C, DLP and δ as well as the textural features of entropy, correlation and contrast show significant differences between normal and tumor tissues.

New scandium borates RxLayScz(BO3)4 (x+y+z=4, R=Sm, Tb): Synthesis, growth, structure and optical properties

Herein we report the growth of new RxLayScz(BO3)4 (x + y+z = 4, R = Sm, Tb) crystals with large second-harmonic generation (SHG) responses. The obtained compounds, grown from eutectic LiBO2-LiF flux by the TSSG method, crystallize in the space group P321 with the cell parameters a = 9.800(15) Å and c = 7.9565(7) Å for Sm0.34La0.68Sc2.98(BO3)4 and a = 9.7976(7) Å and c = 7.9567(12) Å for Tb0.23La0.77Sc3(BO3)4. These crystalline materials also exhibit luminescent properties related to the optical transitions of Tb3+ and Sm3+; specifically, Tb0.23La0.77Sc3(BO3)4 shows a strong green emission that is dominated by the 5D4 to 7F5 transition. The studied crystals’ SHG intensities were higher than those of KDP crystals, revealing their potential uses in various nonlinear optics applications.

Synthesis and Characterization of Two New Second Harmonic Generation Active Iodates: K3Sc(IO3)6 and KSc(IO3)3Cl.

Transparent single crystals of two new iodates K3Sc(IO3)6 and KSc(IO3)3Cl have been synthesized hydrothermally. Single-crystal X-ray diffraction was used to determine their crystal structures. Both compounds crystallize in non-centrosymmetric space groups. The compound K3Sc(IO3)6 crystallizes in the orthorhombic space group Fdd2. The crystal structure is made up of [ScO6] octahedra, [IO3] trigonal pyramids, and [KO8] distorted cubes. The compound KSc(IO3)3Cl crystallizes in the trigonal space group R3. The building blocks are [ScO6] octahedra, [KO12] polyhedra, and [IO3] trigonal pyramids. The Cl– ions act as counter ions and reside in tunnels in the crystal structure. The second harmonic generation (SHG) measurements at room temperature, using 1064 nm radiation, on polycrystalline samples show that the SHG intensities of K3Sc(IO3)6 and KSc(IO3)3Cl are around 2.8 and 2.5 times that of KH2PO4 (KDP), respectively. In addition, K3Sc(IO3)6 and KSc(IO3)3Cl are phase-matchable at the fundamental wavelength of 1064 nm. The large anharmonicity in the optical response of both compounds is further supported by an anomalous temperature dependence of optical phonon frequencies as well as their enlarged intensities in Raman scattering. The latter corresponds to a very large electronic polarizability.

Mixed-valent selenium compounds: Noncentrosymmetric Cd3(SeO3)2(SeO4) and Hg3(SeO3)2(SeO4) and centrosymmetric Pb2(SeO3) (SeO4)

Three mixed-valent selenium compounds: Cd3(SeO3)2(SeO4), Hg3(SeO3)2(SeO4) and Pb2(SeO3) (SeO4) were successfully synthesized by hydrothermal methods. Among these three compounds, Cd3(SeO3)2(SeO4) and Hg3(SeO3)2(SeO4) are isostructural and crystalize in the noncentrosymmetric (NCS) polar space group P21. Powder second-harmonic generation (SHG) measurements using 1064 ​nm radiation indicate that Cd3(SeO3)2(SeO4) and Hg3(SeO3)2(SeO4) exhibit SHG intensities of approximately 0.6 and 0.8 times of KH2PO4 (KDP), respectively. Pb2(SeO3) (SeO4) is centrosymmetric (CS) and crystalizes in the space group Pnma. Pb2(SeO3) (SeO4) exhibits a 3D framework composed of chains of edge-sharing [PbO6] octahedra linked to [SeO3] and [SeO4] groups. M3(SeO3)2(SeO4) (M ​= ​Cd, Hg) also exhibits a 3D structure consisting of layers of [MO8], [MO6] and [SeO3] polyhedra connected by [SeO4] groups. For all of the reported compounds, infrared spectroscopy, UV–vis diffuse reflectance spectroscopy and thermogravimetric measurements were performed.

Multipolar analysis of second-harmonic generation in (111) Gallium Arsenide nanoparticles

We perform multipolar analysis of second-harmonic generation (SHG) from (111)-grown gallium arsenide (GaAs) nanoantennas and discuss its specifics. It was experimentally demonstrated that the conversion efficiency in axially-symmetric (111) GaAs nanoparticles remains constant under the polarization rotation of normally incident radiation in a wide range of particle sizes, while the SHG radiation pattern changes. We apply the analytical method based on the Lorentz lemma to explain this behaviour. The induced nonlinear current is decomposed into two rotating contributions, which are shown to generate multipoles of different parities. Thus, the total SHG intensity in the far-field is proved to be independent of the in-plane rotation of the pump polarization. Nevertheless, due to the threefold symmetry of the crystal with regard to the (111) direction, the SHG radiation pattern rotates around the polar axis repeating its shape every 60°.

2(C3H7N6)+·2Cl-·H2O: an ultraviolet nonlinear optical crystal with large birefrigence and strong second-harmonic generation.

Large birefringence (Δn = 0.277 at 546 nm) and strong second-harmonic generation (SHG) intensity (4.3 × KDP) achieved a remarkable balance in a potential ultraviolet nonlinear optical crystal: 2(C3H7N6)+·2Cl-·H2O. Theoretical calculations showed that planar benzene-like π-conjugated [C3N6] groups had important optical properties.

The impact of age, mineralization, and collagen orientation on the mechanics of individual osteons from human femurs

Osteons are the end product of cortical bone remodeling. Changes in their mechanical properties may be an indicator of overall bone health. Aszenci and Bonucci first evaluated the compressive properties of individual osteons in 1968. However, these results have never been independently validated and technological advances allow for further investigation of osteon mechanics. In the present study, an experimental protocol was established such that osteons ~250 µm in diameter were successfully extracted from human cortical bone (males, ages 93 and 64) and loaded in compression. Both micro-computed tomography and second harmonic generation (SHG) microscopy imaging methods were incorporated into the protocol to improve the structural and compositional assessment of each osteon. Further, a high-resolution videography system was used to monitor the osteons during mechanical testing, permitting insight into deformation mechanisms in situ. The results show that the older donor osteons (male, age 93 (n = 13)) were stiffer (p = 0.04) and more highly mineralized (p = 0.03), while the younger donor osteons (male, 64 (n = 14)) exhibited more heterogeneity in the measured mechanical properties with the presence of both stiff and more compliant osteons. SHG intensity indicated predominantly longitudinal fiber alignment for all osteons while the videography data revealed three distinct failure modes and confirmed that whitening observed during yielding of macroscale bone specimens represents failure at the local level. The stiffness and strength of the younger donor osteons were heavily dependent on the gross structure of the osteon, while these properties were dominated by collagen orientation in the older donor osteons.

Single-scan interferometric second harmonic generation microscopy using a kHz phase-scanner.

In conventional laser-scanning microscopy, images are formed by acquiring the signal from pixel to pixel. Here, we report more than one order of magnitude reduction in acquisition time of Interferometric Second Harmonic Generation (I-SHG) by scanning the phase within each pixel, to characterize the relative polarity of various samples. Using an electro-optic phase-scanner, we show that the phase-shift patterns required for interferometry can be applied at each pixel during the scanning of the sample, allowing single-scan I-SHG (1S-ISHG) measurements. Requiring exposure times comparable to standard SHG intensity images, the additional phase information of the signal can thus be retrieved in parallel to its amplitude at the time-scale of seconds. Moreover, slower modulations can be used to enhance the precision of the phase measurement, without any spatial or temporal shift between interferograms, in contrast to conventional frame phase-shifting I-SHG (standard I-SHG). This continues to extend I-SHG to dynamical processes, and opens it to large-scale studies, as well as to imaging samples where the signal-to-noise ratio is an issue.

An intrinsic, label-free signal for identifying stem cell-derived cardiomyocyte subtype.

Human-induced pluripotent stem cell (hiPSC)-derived cardiomyocytes have many promising applications, including the regeneration of injured heart muscles, cardiovascular disease modeling, and drug cardiotoxicity screening. Current differentiation protocols yield a heterogeneous cell population that includes pluripotent stem cells and different cardiac subtypes (pacemaking and contractile cells). The ability to purify these cells and obtain well-defined, controlled cell compositions is important for many downstream applications; however, there is currently no established and reliable method to identify hiPSC-derived cardiomyocytes and their subtypes. Here, we demonstrate that second harmonic generation (SHG) signals generated directly from the myosin rod bundles can be a label-free, intrinsic optical marker for identifying hiPSC-derived cardiomyocytes. A direct correlation between SHG signal intensity and cardiac subtype is observed, with pacemaker-like cells typically exhibiting ~70% less signal strength than atrial- and ventricular-like cardiomyocytes. These findings suggest that pacemaker-like cells can be separated from the heterogeneous population by choosing an SHG intensity threshold criteria. This work lays the foundation for developing an SHG-based high-throughput flow sorter for purifying hiPSC-derived cardiomyocytes and their subtypes.

An Investigation of Field-Effect Passivation Layer Characteristics Using Second Harmonic Generation Measurement

Second harmonic generation (SHG) characteristics of single and multilayer field-effect passivation (FEP) structures were investigated for CMOS image sensor application. By compensating the internal multiple reflection effect, the SHG characteristics of single and multilayer FEP structures were compared under equivalent conditions. Analysis of the time-dependent SHG intensity confirmed that the multilayer FEP structure has a 2.7 times higher SHG intensity than the single-layer structure, and the FEP layer is negatively charged from the initial state. In particular, when atomic layer deposition conditions were varied to control the Al2O3 composition ratio, the SHG demonstrated greater detecting sensitivity than conventional material analysis methods such as RBS and XPS, even though it cannot distinguish the elements. Moreover, SIMS composition analysis verified that the initial state SHG intensity increase under high oxygen ambient Al2O3 growth condition can be attributed to the change of impurities rather than the effect of oxygen areal density increase.

Enhanced piezoresponse and nonlinear optical properties of fluorinated self-assembled peptide nanotubes

Self-assembled L,L-diphenylalanine (FF) nanostructures offer an attractive platform for photonics and nonlinear optics. The nonlinear optical (NLO) coefficients of FF nanotubes depend on the diameter of the tube [S. Khanra et al. Phys. Chem. Chem. Phys. 19(4), 3084–3093 (2017)]. To further enhance the NLO properties of FF, we search for structural modifications. Here, we report on the synthesis of fluorinated FF dipeptides by replacing one ortho-hydrogen atom in each of the phenyl groups of FF by a fluorine atom. Density-functional theoretical calculations yield insights into minimum energy conformers of fluorinated FF (Fl-FF). Fl-FF self-assembles akin to FF into micron-length tubes. The effects of fluorination are evaluated on the piezoelectric response and nonlinear optical properties. The piezoelectric d15 coefficient of Fl-FF is found to be more than 10 times higher than that of FF nanotubes, and the intensity of second harmonic generation (SHG) polarimetry from individual Fl-FF nanotubes is more than 20 times that of individual FF nanotubes. Furthermore, we obtain SHG images to compare the intensities of FF and Fl-FF tubes. This work demonstrates the potential of fluorine substitution in other self-assembled biomimetic peptides for enhancing nonlinear optical response and piezoelectricity.

Twist Angle mapping in layered WS2 by Polarization-Resolved Second Harmonic Generation

Stacked atomically thin transition metal dichalcogenides (TMDs) exhibit fundamentally new physical properties compared to those of the individual layers. The twist angle between the layers plays a crucial role in tuning these properties. Having a tool that provides high-resolution, large area mapping of the twist angle, would be of great importance in the characterization of such 2D structures. Here we use polarization-resolved second harmonic generation (P-SHG) imaging microscopy to rapidly map the twist angle in large areas of overlapping WS2 stacked layers. The robustness of our methodology lies in the combination of both intensity and polarization measurements of SHG in the overlapping region. This allows the accurate measurement and consequent pixel-by-pixel mapping of the twist angle in this area. For the specific case of 30° twist angle, P-SHG enables imaging of individual layers.

Broadband mid-infrared second harmonic generation using epitaxial polydomain barium titanate thin films

The mid-infrared (mid-IR) second-order optical nonlinearity of the barium titanate (BTO) thin films was characterized by second harmonic generation (SHG). The epitaxial BTO thin films were grown on strontium titanate substrates by pulsed-laser deposition. From the azimuthal-dependent polarized SHG measurements, the tensorial optical nonlinear coefficients, dij, and ferroelectric domain fraction ratio, δAY/δAz, were resolved. Strong SHG signals were obtained at the pumping laser wavelength λ between 3.0 and 3.6 μm. The SHG intensity was linearly dependent upon the square of the pumping laser power. The broadband mid-IR optical nonlinearity enables BTO thin films for applications in chip-scale quantum optics and nonlinear integrated photonic circuits.