Optical Second Harmonic Generation Technique Research Articles

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.

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.

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.

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.

Transient carrier visualization of organic-inorganic hybrid perovskite thin films by using time-resolved microscopic second-harmonic generation (TRM-SHG)

Abstract Based on the transient electric field migration directly probed by the time-resolved microscopic optical second-harmonic generation (TRM-SHG) technique, we successfully detected the transient hole transport in the perovskite for the first time. From the spectroscopic point of view, SHG signal resonantly enhanced at the fundamental wavelength of around 1560 nm through the band-transition under the voltage application. Square dependence of the SH intensity on the applied voltage at the wavelength of 1560 nm clearly indicated the electric field induced process of the SHG. We could visualize the carrier motion in the channel of perovskite field effect transistor (FET) with this wavelength. Carrier mobility was estimated as 0.02 cm2/V by analyzing the transient carrier motion. TRM-SHG technique was also employed to investigate the effect of traps on the transient carrier motion. Based on the peak of the transient electric field distribution, trap density, and dynamic carrier mobility were separately estimated.

Evolution of structural distortion in BiFeO3 thin films probed by second-harmonic generation.

BiFeO3 thin films have drawn much attention due to its potential applications for novel magnetoelectric devices and fundamental physics in magnetoelectric coupling. However, the structural evolution of BiFeO3 films with thickness remains controversial. Here we use an optical second-harmonic generation technique to explore the phase-related symmetry evolution of BiFeO3 thin films with the variation of thickness. The crystalline structures for 60 and 180-nm-thick BiFeO3 thin films were characterized by high-resolution X-ray diffractometry reciprocal space mapping and the local piezoelectric response for 60-nm-thick BiFeO3 thin films was characterized by piezoresponse force microscopy. The present results show that the symmetry of BiFeO3 thin films with a thickness below 60 nm belongs to the point group 4 mm. We conclude that the disappearance of fourfold rotational symmetry in SHG s-out pattern implies for the appearance of R-phase. The fact that the thinner the film is, the closer to 1 the tensor element ratio χ31/χ15 tends, indicates an increase of symmetry with the decrease of thickness for BiFeO3 thin films.

Optical second-harmonic generation measurement for probing organic device operation.

We give a brief overview of the electric-field induced optical second-harmonic generation (EFISHG) technique that has been used to study the complex behaviors of organic-based devices. By analyzing EFISHG images of organic field-effect transistors, the in-plane two-dimensional distribution of the electric field in the channel can be evaluated. The susceptibility tensor of the organic semiconductor layer and the polarization of the incident light are considered to determine the electric field distribution. EFISHG imaging can effectively evaluate the distribution of the vectorial electric field in organic films by selecting a light polarization. With the time-resolved technique, measurement of the electric field originating from the injected carriers allows direct probing of the carrier motion under device operation, because the transient change of the electric field distribution reflects the carrier motion. Some applications of the EFISHG technique to organic electronic devices are reviewed.

Applications of second-harmonic generation imaging microscopy in ovarian and breast cancer.

In this perspective, we discuss how the nonlinear optical technique of second-harmonic generation (SHG) microscopy has been used to greatly enhance our understanding of the tumor microenvironment (TME) of breast and ovarian cancer. Striking changes in collagen architecture are associated with these epithelial cancers, and SHG can image these changes with great sensitivity and specificity with submicrometer resolution. This information has not historically been exploited by pathologists but has the potential to enhance diagnostic and prognostic capabilities. We summarize the utility of image processing tools that analyze fiber morphology in SHG images of breast and ovarian cancer in human tissues and animal models. We also describe methods that exploit the SHG physical underpinnings that are effective in delineating normal and malignant tissues. First we describe the use of polarization-resolved SHG that yields metrics related to macromolecular and supramolecular structures. The coherence and corresponding phase-matching process of SHG results in emission directionality (forward to backward), which is related to sub-resolution fibrillar assembly. These analyses are more general and more broadly applicable than purely morphology-based analyses; however, they are more computationally intensive. Intravital imaging techniques are also emerging that incorporate all of these quantitative analyses. Now, all these techniques can be coupled with rapidly advancing miniaturization of imaging systems to afford their use in clinical situations including enhancing pathology analysis and also in assisting in real-time surgical determination of tumor margins.

A novel optical second-harmonic generation technique for analyzing carrier-dynamics in organic devices

A novel optical second-harmonic generation technique for analyzing carrier-dynamics in organic devices

Optical Second-Harmonic Generation in Hydrogenated Amorphous Silicon Single- and Double-Junction Solar Cells

Using an electric-field-induced optical second-harmonic generation (EFISHG) technique, we studied the SHG from hydrogenated amorphous silicon (a-Si:H) solar cells. The SHG signal originating from a-Si interface is independent of external bias voltage (Vex), whereas that generated from the intrinsic layer is strongly dependent on Vex. The difference in the tendency of bias-dependent EFISHG intensity between a-Si single-junction solar cells and a-Si/microcrystalline silicon (µc-Si) double-junction (tandem) solar cells indicates the presence of another origin of the EFISHG generation in the tandem cells. That is, the p–n junction formed between the p-type µc-Si:H and n-type a-Si:H. These findings indicate that the EFISHG technique is available for characterizing the local electric field in a-Si solar cells.

Stability of spontaneous polarization in ultrathin improper ferroelectrics

The competition between the substrate-induced strain and the electrostatic polarization in improper ferroelectric films (hexagonal YMnO${}_{3}$) has been studied using optical second-harmonic generation techniques. Deposited on a (111) surface of yttria-stabilized zirconia substrate, the films exhibit (1) strong strain with negligible polarization, (2) relaxation accompanied by the nonlinear buildup of the polarization, and (3) bulklike polar behavior, in the order of increasing film thickness. The role of the substrate is distinctively different from that in proper ferroelectrics.

Amorphization of Si(100) by Ar+-ion bombardment studied with spectroscopic and time-resolved second-harmonic generation

The surface and interface sensitive technique of optical second-harmonic generation (SHG) has been applied spectroscopically and time-resolved before, during, and after low energy (70–1000 eV) Ar+-ion bombardment of H-terminated Si(100). The photon energy range of the fundamental radiation was ℏω=0.76–1.14 eV. Besides physical sputtering of the silicon, ion bombardment of crystalline silicon damages and amorphizes the top layer of the sample and thereby creates a layered structure of amorphous silicon (a-Si) on crystalline silicon. The SHG radiation, which is sensitive to the Ar+-ion flux, ion energy, and the presence of reactive gas species, originates from the top surface of the sample and from the interface between a-Si and c-Si. From a comparison with the SHG results obtained at a fundamental radiation of ℏω=1.3–1.7 eV, it is concluded that the SHG radiation during and after creation of this structure dominantly originates from the tails of electronic transitions in the E0′/E1 energy region rather than from silicon dangling bonds.

Optical second-harmonic generation in thin film systems

The surface and interface sensitive nonlinear optical technique of second-harmonic generation (SHG) is a very useful diagnostic in studying surface and interface properties in thin film systems and can provide relevant information during thin film processing. An important aspect when applying SHG is the interpretation of the SHG response. In order to utilize the full potential of SHG during materials processing it is necessary to have a good understanding of both the macroscopic and the microscopic origin of the SHG response, particularly in thin film or multilayer systems where the propagation of radiation is another important aspect that should be considered carefully. A brief theoretical overview on the origin of the SHG response and a description of the propagation of radiation will be given. Furthermore, several methods will be discussed that might reveal the possible macroscopic and microscopic origins of the SHG response in thin film systems. The different approaches will be illustrated by examples of real-time and spectroscopic SHG experiments with thin film systems relevant in Si etching and deposition environments, such as (1) hydrogenated amorphous Si films deposited by hot-wire chemical vapor deposition on both Si(100) and fused silica substrates, (2) amorphous Si generated by low-energy Ar+-ion bombardment of H terminated Si(100), and (3) Al2O3 films deposited by plasma-assisted atomic layer deposition on H terminated Si(100).

Diffusionlike electric-field migration in the channel of organic field-effect transistors

Time-resolved microscopic optical second-harmonic generation (TRM-SHG) technique provides a different approach to study the dynamics of carriers in organic field-effect transistor (OFET). Different from many common methods, the TRM SHG directly probes the transient electric-field distribution in the channel of the OFET in high temporal and spatial resolutions. In this work we used this technique to quantitatively study migration of the peak of the electric field. We found that under a broad range of experimental conditions, the migration of the peak of the electric field follows a diffusionlike behavior; namely, the square of the peak position of $\overline{x}$ is proportional to time $t$. Based on a two-dimensional computational simulation and general carrier transport mechanism, we proposed that this behavior arises from a simple relation, $\ensuremath{\mu}\ensuremath{\simeq}\ensuremath{\gamma}{\overline{x}}^{2}/(t|{V}_{\text{gs}}|)$, where $\ensuremath{\mu}$ is the carrier mobility, $\ensuremath{\gamma}$ is constant about 0.5, and ${V}_{\text{gs}}$ is the gate voltage with respect to the source voltage. The experimental and simulation data well supported this relationship.

Direct imaging of carrier motion in organic transistors by optical second-harmonic generation

Interest in the dynamic behaviour of carriers in organic materials is motivated by possible applications that include organic thin-film transistors, organic electro-luminescent devices and organic photoconductors. It can also provide an insight into the modelling of carrier transport and trapping in organic semiconductors and insulators. Here, we use an advanced second-harmonic generation technique to probe and visualize real carrier motion in organic materials. This is a time-resolved microscopic optical second-harmonic generation technique that allows direct and selective probing of dynamic carrier motion in organic materials. Experiments making use of this technique and using pentacene field-effect transistors have revealed dynamic changes of second-harmonic-generation intensity profiles arising from pentacene. Carrier velocity in organic solids is thus determined from the visualized carrier motion. We anticipate that this direct visualization technique will find wide application in the illustration of space-charge field formation in organic and inorganic materials, including biomaterials and polymers.

Second harmonic generation response by gold nanoparticles at the polarized water/2-octanoneinterface: from dispersed to aggregated particles

Gold nanoparticles with a diameter of approximately 20 nm have been observed at thepolarized water/2-octanone interface by the nonlinear optical technique of second harmonicgeneration. Electric field induced adsorption of the gold particles at this liquid/liquidinterface is clearly observed and confirms that these are negatively charged. The process isquasi-reversible at high potential sweep rates, but aggregation at the interface is observedat slower sweep rates through the loss of the nonlinear optical signal. The timeevolution of the second harmonic signal is also reported during potential stepexperiments. After a rapid increase due to adsorption, a continuous decreasein the nonlinear optical signal intensity is observed due to aggregation of theparticles into large islands at the interface. Diffusion of these large islands at theinterface was observed for a longer timescale through large signal fluctuations.

Temperature effect on the transport dynamics of a small molecule through a liposome bilayer

An ion having hydrophobic parts can directly transport through the liposome bilayer without an ion channel and its transport mechanism can be explained by the free-volume theory. This was confirmed by investigating the temperature effect on the transport dynamics of organic cations through anionic liposome bilayers made of unsaturated and saturated lipids by using optical second-harmonic generation (SHG) technique. This study provides useful information to design practical temperature-controlled drug delivery systems.

Simultaneous observation of collagen and elastin based on the combined nonlinear optical imaging technique coupled with two-channel synchronized detection method

Collagen and elastin are the most important proteins of the connective tissues in higher vertebrates. In this paper, we present a combined nonlinear optical imaging technique of second-harmonic generation and two-photon excited fluorescence to simultaneously observe the collagen and elastic fiber of dermis in a freshly excised human skin and rabbit aorta using a two-channel synchronized detection method. The obtained two-channel overlay image in the backward direction can clearly distinguish the morphological structure and distribution of collagen and elastic fibers. Tissue spectrum further confirms the obtained structural information. These results suggest that the combined nonlinear optical imaging technique coupled with two-channel synchronized detection method can be an effective tool for detecting collage and elastic fibers without any invasive tissue procedure of slicing, embedding, fixation and staining when two structural proteins are simultaneously present in the biological tissue.

Controlled Reversible Adsorption of Core−Shell Metallic Nanoparticles at the Polarized Water/1,2-Dichloroethane Interface Investigated by Optical Second-Harmonic Generation

We report the observation of the reversible adsorption of core-shell gold-silver nanoparticles at the polarized water/1,2-dichloroethane interface using the nonlinear optical technique of surface second-harmonic generation. This study unambiguously demonstrates the excellent stability against aggregation of these core-shell nanoparticles, namely, gold core nanoparticles coated with silver layers of variable thickness, in the presence of an electrolyte salt like lithium chloride. Furthermore, it is also demonstrated that the adsorption of the nanoparticles is reversible by modulating the applied potential at water/1,2-dichloroethane interface. The analysis of these results is performed within the Debye-Huckel approximation of the electrostatic interactions between the nanoparticles. This approach shows that the stability of core-shell nanoparticles can be attributed to the formation of a silver oxide layer at the surface of the particles.