Second Harmonic Generation Technique Research Articles

Inversion Symmetry-Broken Tetralayer Graphene Probed by Second-Harmonic Generation.

Stacking orders provide a unique way to tune the properties of two-dimensional materials. Recently, ABCB-stacked tetralayer graphene has been predicted to possess atypical elemental ferroelectricity arising from its symmetry breaking but has been experimentally explored very little. Here, we observe pronounced nonlinear optical second-harmonic generation (SHG) in ABCB-stacked tetralayer graphene while absent in both ABAB- and ABCA-stacked allotropes. Our results provide direct evidence of symmetry breaking in ABCB-stacked tetralayer graphene. The remarkable contrast in the SHG spectra of tetralayer graphene allows straightforward identification of ABCB domains from the other two kinds of stacking order and facilitates the characterization of their crystalline orientation. The employed SHG technique serves as a convenient tool for exploring the intriguing physics and novel nonlinear optics in ABCB-stacked graphene, where spontaneous polarization and intrinsically gapped flat bands coexist. Our results establish ABCB-stacked graphene as a unique platform for studying the rare ferroelectricity in noncentrosymmetric elemental structures.

Switchable diode effect in 2D van der Waals ferroelectric CuCrP2S6

Two-dimensional (2D) ferroelectrics has emerged as a promising building block for nonvolatile memory devices. In this work, we demonstrate the out-of-plane ferroelectricity of 2D CuCrP2S6 (CCPS) at the room temperature and the switchable diode effect in 2D CCPS-based ferroelectric nanodevices. The spontaneous out-of-plane ferroelectric polarization switching and hysteresis loops are directly evidenced by the piezoresponse force microscopy. The intrinsic ferroelectricity originates from the non-centrosymmetric structure of 2D CCPS, which is confirmed by optical second-harmonic generation technique. A ferroelectric tunnel junction was built up by using 2D CCPS as a function layer. The observed diode-like forward rectification effect of CCPS diode is opposite to the direction of remnant polarization, which is attributed to the ferroelectric polarization modulation of Schottky barrier. Our work highlights the great potential of 2D CCPS in ultrathin ferroelectric memory device and motivates the development of multifunctional nanodevices.

Optical Second Harmonic Generation on LaAlO3/SrTiO3 Interfaces: A Review.

As we approach the limits of semiconductor technology, the development of new materials and technologies for the new era in electronics is compelling. Among others, perovskite oxide hetero-structures are anticipated to be the best candidates. As in the case of semiconductors, the interface between two given materials can have, and often has, very different properties, compared to the corresponding bulk compounds. Perovskite oxides show spectacular interfacial properties due to the the rearrangement of charges, spins, orbitals and the lattice structure itself, at the interface. Lanthanum aluminate and Strontium titanate hetero-structures (LaAlO3/SrTiO3) can be regarded as a prototype of this wider class of interfaces. Both bulk compounds are plain and (relatively) simple wide-bandgap insulators. Despite this, a conductive two-dimensional electron gas (2DEG) is formed right at the interface when a LaAlO3 thickness of n≥4 unit cells is deposited on a SrTiO3 substrate. The 2DEG is quite thin, being confined in only one or at least very few mono-layers at the interface, on the SrTiO3 side. A very intense and long-lasting study was triggered by this surprising discovery. Many questions regarding the origin and characteristics of the two-dimensional electron gas have been (partially) addressed, others are still open. In particular, this includes the interfacial electronic band structure, the transverse plane spatial homogeneity of the samples and the ultrafast dynamics of the confined carriers. Among a very long list of experimental techniques which have been exploited to study these types of interfaces (ARPES, XPS, AFM, PFM, …and many others), optical Second Harmonic Generation (SHG) was found to be suitable for investigating these types of buried interfaces, thanks to its extreme and selective interface-only sensitivity. The SHG technique has made its contribution to the research in this field in a variety of different and important aspects. In this work we will give a bird's eye view of the currently available research on this topic and try to sketch out its future perspectives.

Investigation of Physiochemical Impact of Organic Molecule L-Lysine on Ammonium Dihydrogen Phosphate Single Crystal for Optoelectronics Applications

Ammonium dihydrogen phosphate (ADP) single crystals along with the incorporated 0.5 and 1% L-lysine, an organic molecule which possesses a good nonlinear response, were grown with the vision to meet the requirements of the optoelectronic industry. The inclusion of the L-lysine molecule in the crystal was confirmed by the XRD and EDX. The experiment not only confirms the inclusion level of the impurity but also the capability of the amino acid molecule to bond hydrogen within the crystal facet. A minor decrease in lattice parameters was reported for all ADP: L-lysine crystals compared with pure ADP. The structures of the grown crystals were identified as tetragonal with the space group I42d by the single-crystal XRD analysis. Vibrational signatures and functional groups were confirmed using FTIR spectroscopy. The thermal stability and decomposition temperatures of 0.5 and 1% L-lysine-added crystals were measured by TG/DTA and found to be 203 °C and 207 °C, respectively. The UV–visible transmission spectra prove a higher transparency for doped crystals as compared to pure crystals; therefore, these doped crystals can be considered the best option for the frequency doubling process in a broad range of visible and near-IR spectra. The improved hardness of the doped crystals was confirmed by the Vickers hardness data. The nonlinear optical (NLO) behaviour investigated using a second-harmonic generation (SHG) technique, indicating an efficient quadratic nonlinear coefficient of ADP: Lysine crystals at a 1064 nm initial wavelength, shows about 1.5-fold higher efficiency compared with undoped ADP.

Nonlinear Optical Imaging of Carrier Transport at the Semiconductor-Insulator Interface in Organic Field-Effect Transistors

Organic field-effect transistors (FETs) are witnessing a revolution in their performance, in part, due to a reduction in contact resistance, and through controlled morphology of the organic semiconducting film. Another major consideration is the role of the dielectric layer in dictating transport properties. The transient electric-field-induced second-harmonic generation (EFISHG) technique allows visualization of carrier dynamics in the channel region of a FET by capturing the movement of the induced dielectric polarization. A microscopic imaging system using a broadband femtosecond laser and a pulse compensation arrangement is constructed for imaging transient EFISHG from organic FETs by synchronized laser and voltage pulses. By varying the dielectric layer in organic FETs comprising pentacene and a donor-acceptor conjugated polymer as semiconducting layers, we show that EFISHG images provide an accurate estimate of the carrier mobility along with providing insights into channel formation and the electric field distribution within the channel region. We further correlate the interface charging effects observed in EFISHG to the interface trap density obtained by capacitance-voltage and conductance-voltage from metal-insulator-semiconductor diodes.

Cover Picture: Materials and Corrosion. 4/2023

Cover: Second-harmonic generation (SHG) was used to monitor in-situ compositional changes to carbon steel surfaces immersed in aqueous solutions and during electrochemical experiments. For passive film formed on carbon steel samples, the response in SHG intensity showed a peak in presence of Fe3O4, while it shows a decrease in presence of Fe2O3. Upon application of cathodic protection for 30 minutes, SHG response revealed the formation and growth of a Fe3O4 film on the carbon steel surface. In this work, the results obtained with SHG were compared with in-situ active reflectance spectroscopy measurements. More detailed information can be found in: Federico Martinelli-Orlando, Eric Dénervaud, Rachel Grange, Ueli Angst, Second-harmonic generation technique for in-situ study of passive film formation on carbon steel surfaces in aqueous solutions, Materials and Corrosion 2023, 74, 508.

Crystallographic parameters of spherulitic structures in ferroelectric film

The symmetry and structure of ferroelectric films determine most of their properties. Nonlinear optical technique of second harmonic generation (SHG) is widely accepted for remote and fast qualitative characterization of ferroelectric films. In this article, we develop a method for quantitative characterization of the local symmetry and structure of PZT thin films. Depending on the annealing time of pyrochlore phase two types of structure have been obtained: continuous perovskite polycrystalline film consisting of truncated spherulites and isolated spherulites embedded into pyrochlore matrix. The method is based on the SHG azimuthal dependences measurements followed by fitting procedure and gives the ratio of monoclinic to tetragonal phases as well as the fraction of differently oriented crystallographic faces of both of these phases.

Single crystal growth of 4-aminobenzophenone (ABP) by micro-capillary Czochralski melt technique for second harmonic generation (SHG) applications

Single crystal of 4-aminobenzophenone (ABP) was grown by micro-capillary Czochralski (mc-CzT) pulling technique from melt using a laboratory designed mini-Czochralski crystal puller set-up with its seed-rod attached with micro-capillary tube. The capillary action of the melt at the tip of the micro tube was carefully controlled in such a way that a single crystal nucleation was induced at the melt interface. Optimization of the melt temperature along with successive control of the rotation and upward pulling rates of the micro-capillary tube led to a successful single crystal growth of ABP. Crystal structure of the grown ABP was confirmed by single crystal X-ray diffraction (SCXRD) structural refinement and the molecular packing stability in the unit cell was studied through intermolecular hydrogen bonding interactions via theoretical means. Further, powder X-ray diffraction (PXRD), UV–visible-near IR (UV–vis-NIR), Fourier-transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC) analyses were carried out to confirm the crystalline purity, optical transparency, modes of molecular vibrations and thermal stability of the grown crystal. Kurtz and Perry technique was used to study second harmonic generation (SHG) ability of the grown ABP crystal and found that the SHG efficiency was about 15 times that of potassium dihydrogen phosphate (KDP) and 5 times that of urea.

Single Crystal Growth of 4‐Aminobenzophenone (ABP) by Solution and Seeded Czochralski Pulling Techniques for Second Harmonic Generation Applications

AbstractHighly efficient 4‐aminobenzophenone (ABP) single crystals are grown from melt by employing the seeded Czochralski pulling technique for second harmonic generation (SHG) applications. The as‐grown seed crystals from solution with dominant (100) and (001) facets are utilized to trigger the directional solidification. For successive growth, initial measurements of the furnace temperature distribution, heat flow structure, and solidification temperature of ABP in correlation with differential scanning calorimetry (DSC) analysis are made. The controlled diffusion is ensured by monitoring the interfacial layer between the melt and the growing crystal and is maintained by optimizing the melt temperature and pulling and rotation rates. Color change of the melt due to thermal‐oxidation is prevented by appropriate control of the growth environment. A slow and steady growth rate affirms the good quality of the ABP single crystal. The anisotropic growth behavior of ABP from solution and melt is reported. Internal molecular structure, crystalline purity, molecular vibrational modes, optical transmission, and thermal behavior are studied using single crystal and powder X‐ray diffraction, FTIR, UV–vis–NIR and DSC analyses. The obtained higher SHG efficiency of the grown ABP in comparison with that of the organic and inorganic standards urea and KDP reveals its suitability for SHG applications.

Second-Harmonic-Generation Effect and Giant Optical Birefringence in the Weyl Material CaAgAs.

Nonlinear-optical (NLO) materials suitable for the "8-14 μm" atmospheric transparent window are in urgent need. Arsenide is one of the most promising material systems for such an application. However, the second-harmonic-generation (SHG) effect of arsenide is difficult to characterize using the conventional powder SHG technique with a 2 μm fundamental laser. To overcome this problem, we focused on a novel arsenide Weyl material, CaAgAs, with a zero band gap and proposed a modified powder SHG measurement method for narrow-band-gap materials. We successfully observed the SHG signal of CaAgAs, which was approximately 0.7 times that of CdGeAs2. Moreover, on the basis of first-principles calculations, the largest SHG coefficient for CaAgAs is equal to 236 pm/V, and the ionic bonds in the [Ca3As13] motif play an indispensable role for the SHG effect because of the distorted Kagome lattice pattern. CaAgAs is predicted to have strong optical anisotropy with a giant optical birefringence of 0.52.

Surface symmetry determination by a deep learning application of optical second-harmonic generation results

Optical second-harmonic generation (SHG) technique is widely used to characterize the structural symmetry of condensed matters not only for the bulk but also for the surface states. Since experimental results of the SHG anisotropy patterns often contain multiple contributions from bulk and surface states, a conventional regression process may not provide a unique solution about the symmetry of each part. With a given symmetry for the bulk state, we here develop a discriminator for the surface symmetry by exploiting neural architectures based on 1D convolution layers where simulation results of SHG anisotropy patterns are taken as learning sequence data. Since SHG experimental results have limited information for determining a unique symmetry out of several symmetry candidates, our discriminator consists of tens of neural architectures optimized with different hyperparameters. As a final output, the surface symmetry discriminator gives a weighted sum of results suggested from all the individual neural architectures. We demonstrate that the discrimination results of the deep learning approach agree well with those of the conventional regression method for GaAs and Bi2Se3.

Early-stage assessment of drying shrinkage in Portland limestone cement concrete using nonlinear ultrasound

Second harmonic generation (SHG), a nonlinear ultrasonic technique, holds great promise for reliable structural health management owing to its nondestructive detection of early-stage damage evolution in cement-based materials. Utilizing the SHG technique, this study focuses on elucidating the contribution of limestone to dimensional stability under drying shrinkage in cement-based materials. Specifically, the acoustic nonlinearity parameter β obtained from the SHG is used, together with uniaxial strain and mass reduction, to characterize the microstructural condition near the surface (∼50 mm) of Portland limestone cement (PLC, Type IL) concrete, which is compared to ordinary Portland cement (OPC, Type I/II) concrete. In both PLC and OPC specimens, β increases remarkably compared with other measured parameters (i.e., change in mass, strain). This demonstrates the sensitivity of this approach to the microstructural modification caused by drying shrinkage, allowing for the discrimination in performance of PLC and OPC.

Monitoring of stress variation of strands in prestressed concrete by second harmonic generation measurements based on piezoelectric sensors

The stress loss of prestressed concrete may cause the ultimate failure of structures and the monitoring of stress variation of prestressed structures is therefore important to ensure the service safety. This paper developed a nondestructive evaluation method of nonlinear ultrasonic second harmonic generation (SHG) based on piezoelectric (PZT) sensors and applied for the distinction of different stress level of post-tensioned steel strands in concrete. The nonlinear ultrasonic behavior of both free strand and embedded strand having different length is studied with the developed SHG technique and a defined nonlinear parameter is introduced in SHG experiments to correlate with the tension stress in the strand. It is found that the nonlinear parameter has a negative correlation with the increasing stress of strand in both free and embedded case, indicating the nonlinear ultrasonic behavior is getting weak corresponding to the increasing contact force among strand wires. In addition, the decreasing ratio of nonlinear parameter is very close to each other regardless of the length of steel strand, indicating the possibility of replacing the large-scale on-site testing with scaled down samples in laboratory. The experimental results of this research demonstrate that the SHG technique based on PZT sensors could be useful for the stress monitoring of prestressed reinforced concrete with consideration of the relatively high sensitivity of nonlinear parameter to the variation of contact force of strand wires.

Nonlinear ultrasonic characterization of material deterioration of asphalt mixture during cracking-healing cycles

Traditional methods to characterize material deterioration of asphalt mixture are typically destructive to the local integrity of the inspected structure, which is therefore not suitable for the large-scale assessment in an efficient fashion. In this paper, an ultrasonic nondestructive evaluation (NDE) method based on the principle of second harmonic generation (SHG) of Rayleigh surface waves is proposed to apply for the quantitation of cracking variation of asphalt specimens subjected to the fracture-healing cycles. The nonlinear parameter which is the relative amplitude of second harmonic is used to caliber the crack growing in the loading tests and crack healing under the elevated temperature. The feasibility of SHG technique in the crack characterization is justified by the excellent correlation of nonlinear parameter with the crack development corresponding to the fracture force and the crack shrinkage corresponding to the rising temperature. The sensitivity of SHG technique to the crack variation is further demonstrated through a comparison with width measurements from the crack detector. The SHG measurements present an alteration of orders of magnitude with respect to the alternative change of crack. Particularly the SHG measurement demonstrates an excellent capability to distinguish the tiny cracks after the first three healing cycles, compared to the zero reading of crack detector. The findings in this work indicate that the SHG technique is a potentially viable and robust method to characterize microstructural variation of asphalt mixture.

Ultrafast real-time tracing of surface electric field generated via hot electron transport in polar semiconductors

We trace an ultrafast real-space motion of photo-excited free carriers just near the surface of polar semiconductors GaAs and InAs by using the optical second-harmonic generation (SHG) technique. With a flat distribution of photo-carriers as an initial condition, ultrafast diffusion and drift motion followed by a rapid surface recombination develop a surface electric field which is efficiently probed by the electric-field-induced SHG. From the simulation based on the drift–diffusion equation combined with the ensemble Monte-Carlo approach, we could explain the experimental observation of the temporal evolution of the surface electric field, and extract time constants related to the hot electron cooling. Whereas the hot carrier cooling time is determined to be about 8 ps at room temperature for GaAs, it can be further extended to about 35 ps at 100 K due to the increase of the longitudinal optic phonon life time enhancing the hot phonon effect. Our work can be an important guide in designing optoelectronic nano-devices by providing a clear understanding of the ultrafast electron-hole separation near the surface of polar semiconductors in connection with the hot carrier cooling process.

Nonlinear Optical Properties of Porphyrin, Fullerene and Ferrocene Hybrid Materials.

In this research, we investigated the second-order nonlinear optical (NLO) properties of multicomponent hybrid materials formed by meso-tetraphenylporphyrin P (both as free base and ZnII complex), carrying in 2 or 2,12 β-pyrrolic position an electron donor ferrocene (Fc), and/or an electron acceptor fullerene (C60) moiety, connected to the porphyrin core via an ethynyl or an ethynylphenyl spacer. We measured the NLO response by the electric-field-induced second-harmonic generation (EFISH) technique in CH2Cl2 solution with a 1907 nm incident wavelength, recording for all the investigated compounds unexpected negative values of μβ1907. Since density functional theory (DFT) calculations evidenced for P-Fc dyads almost null ground state dipole moments and very low values for P-C60 dyads and Fc-P-C60 triads, our EFISH results suggested a significant contribution to γEFISH of the purely electronic cubic term γ(−2ω; ω, ω, 0), which prevails on the quadratic dipolar orientational one μβ(−2ω; ω, ω)/5kT, as confirmed by computational evidence.

Extreme Ultraviolet Second Harmonic Generation Spectroscopy in a Polar Metal.

The coexistence of ferroelectricity and metallicity seems paradoxical, since the itinerant electrons in metals should screen the long-range dipole interactions necessary for dipole ordering. The recent discovery of the polar metal LiOsO3 was therefore surprising [as discussed earlier in Y. Shi et al., Nat. Mater. 2013, 12, 1024]. It is thought that the coordination preferences of the Li play a key role in stabilizing the LiOsO3 polar metal phase, but an investigation from the combined viewpoints of core-state specificity and symmetry has yet to be done. Here, we apply the novel technique of extreme ultraviolet second harmonic generation (XUV-SHG) and find a sensitivity to the broken inversion symmetry in the polar metal phase of LiOsO3 with an enhanced feature above the Li K-edge that reflects the degree of Li atom displacement as corroborated by density functional theory calculations. These results pave the way for time-resolved probing of symmetry-breaking structural phase transitions on femtosecond time scales with element specificity.

Possible Persistence of Multiferroic Order down to Bilayer Limit of van der Waals Material NiI2.

Realizing a state of matter in two dimensions has repeatedly proven a novel route of discovering new physical phenomena. Van der Waals (vdW) materials have been at the center of these now extensive research activities. They offer a natural way of producing a monolayer of matter simply by mechanical exfoliation. This work demonstrates that the possible multiferroic state with coexisting antiferromagnetic and ferroelectric orders persists down to the bilayer flake of NiI2. By exploiting the optical second-harmonic generation technique, both magnitude and direction of the ferroelectric order, arising from the cycloidal spin order, are successfully traced. The possible multiferroic state's transition temperature decreases from 58 K for the bulk to about 20 K for the bilayer. Our observation will spur extensive efforts to demonstrate multifunctionality in vdW materials, which have been tried mostly by using heterostructures of singly ferroic ones until now.

Probing of Carrier Motion and Contact Electrification by Using Electric-Field-Induced Second-Harmonic Generation

Triboelectric generation has attracted significant interest for its potential as an eco-friendly way to produce power with a greatly increasing power density, thereby opening the door to various applications. Although the triboelectricity effect is one of the oldest research areas and an enormous amount of effort has been put forth in developing it for many applications, its physical origin remains a mystery. Particularly, the complexity of triboelectrification causes the motion of charge carriers, including charge generation, charge separation, and other charging processes, to be vague. A novel approach based on optical second-harmonic generation technology has shown great promise in directly monitoring the dynamic performance of the triboelectricity effect and hence visualizing the spatial distribution of triboelectric charges. In this article, several works on the direct probing of carrier motion in organic transistors and in bulk heterojunction organic solar cells and of contact electrification by using an optical second-harmonic generation technique will be briefly introduced and discussed after second-harmonic generation and electric-field-induced second-harmonic generation have been described.

Characterization of Mesenchymal Stem Cell Differentiation within Miniaturized 3D Scaffolds through Advanced Microscopy Techniques.

Three-dimensional culture systems and suitable substrates topographies demonstrated to drive stem cell fate in vitro by mechanical conditioning. For example, the Nichoid 3D scaffold remodels stem cells and shapes nuclei, thus promoting stem cell expansion and stemness maintenance. However, the mechanisms involved in force transmission and in biochemical signaling at the basis of fate determination are not yet clear. Among the available investigation systems, confocal fluorescence microscopy using fluorescent dyes enables the observation of cell function and shape at the subcellular scale in vital and fixed conditions. Contrarily, nonlinear optical microscopy techniques, which exploit multi-photon processes, allow to study cell behavior in vital and unlabeled conditions. We apply confocal fluorescence microscopy, coherent anti-Stokes Raman scattering (CARS), and second harmonic generation (SHG) microscopy to characterize the phenotypic expression of mesenchymal stem cells (MSCs) towards adipogenic and chondrogenic differentiation inside Nichoid scaffolds, in terms of nuclear morphology and specific phenotypic products, by comparing these techniques. We demonstrate that the Nichoid maintains a rounded nuclei during expansion and differentiation, promoting MSCs adipogenic differentiation while inhibiting chondrogenesis. We show that CARS and SHG techniques are suitable for specific estimation of the lipid and collagenous content, thus overcoming the limitations of using unspecific fluorescent probes.