Optical Second Harmonic Generation Research Articles

Optimization of second harmonic generation in Au/Si core-shell nanoparticles

This study numerically investigated second harmonic generation (SHG) and the linear optical responses in core–shell nanoparticles comprising a gold core and silicon shell. The effects of the core radius and shell thickness were studied based on the resonance wavelength shifts, linear responses, and SHG efficiency in numerical simulations. By accurately setting the shell thickness, a model was designed where the resonance wavelength was almost constant relative to variations in the core radius for a specific shell thickness (i.e., the resonance wavelength did not shift). Thus, it was possible to optimize the efficiency of the linear optical responses and SHG in the core–shell nanoparticles by only changing the core radius. The desired efficiency can be achieved for this nanoparticle in specific applications by first tuning the resonance wavelength by changing the shell thickness and then by optimizing the efficiency by changing the core radius. The results obtained in this study may facilitate the development and optimization of optical nanodevices for different applications in nanophotonics and nano-optics, such as nanolasers and nanosensors.

Quantifying the coherent interaction length of second-harmonic microscopy in lithium niobate confined nanostructures

Thin-film lithium niobate (TFLN) in the form of x- or z-cut lithium-niobate-on-insulator has attracted considerable interest as a very promising and novel platform for developing integrated optoelectronic (nano)devices and exploring fundamental research. Here, we investigate the coherent interaction length lc of optical second-harmonic generation (SHG) microscopy in such samples, that are purposely prepared into a wedge shape, in order to elegantly tune the geometrical confinement from bulk thicknesses down to approximately 50 nm. SHG microscopy is a very powerful and non-invasive tool for the investigation of structural properties in the biological and solid-state sciences, especially for visualizing and analyzing ferroelectric domains and domain walls. However, unlike in bulk lithium niobate (LN), SHG microscopy in TFLN is impacted by interfacial reflections and resonant enhancement, both of which rely on film thickness and substrate material. In this paper, we show that the dominant SHG contribution measured on TFLN in backreflection is the co-propagating phase-matched SHG signal and not the counter-propagating SHG portion as is the case for bulk LN samples. Moreover, lc depends on the incident pump laser wavelength (sample dispersion) but also on the numerical aperture of the focussing objective in use. These experimental findings on x- and z-cut TFLN are excellently backed up by our advanced numerical simulations.

Narrow-band high-lying excitons\xa0with negative-mass electrons in monolayer WSe2

Monolayer transition-metal dichalcogenides (TMDCs) show a wealth of exciton physics. Here, we report the existence of a new excitonic species, the high-lying exciton (HX), in single-layer WSe2 with an energy of ~3.4 eV, almost twice the band-edge A-exciton energy, with a linewidth as narrow as 5.8 meV. The HX is populated through momentum-selective optical excitation in the K-valleys and is identified in upconverted photoluminescence (UPL) in the UV spectral region. Strong electron-phonon coupling results in a cascaded phonon progression with equidistant peaks in the luminescence spectrum, resolvable to ninth order. Ab initio GW-BSE calculations with full electron-hole correlations explain HX formation and unmask the admixture of upper conduction-band states to this complex many-body excitation. These calculations suggest that the HX is comprised of electrons of negative mass. The coincidence of such high-lying excitonic species at around twice the energy of band-edge excitons rationalizes the excitonic quantum-interference phenomenon recently discovered in optical second-harmonic generation (SHG) and explains the efficient Auger-like annihilation of band-edge excitons.

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.

Manipulating Second Harmonic Generation in Higher‐Order Topological Photonic Crystals

AbstractHigher‐order topological photonic crystals (TPCs) have attracted much attention in the past several years since they do not obey the conventional bulk‐boundary correspondence in topological insulating phases. This characteristic offers a new dimension with which to trap and manipulate the flow of light. In this work, 2D higher‐order TPCs for engineering optical second harmonic generation (SHG) are designed. In the TPCs, a topological corner‐state is achieved and it is demonstrated that the high localization of photons at the corner‐state can significantly promote optical SHG. In addition, topological edge‐states are obtained by breaking the Dirac degeneracy point near the SHG frequency. They are also topologically protected and immune to bending resistance. Therefore, the enhanced SHG signal from the corner‐state can be manipulated precisely for transmitting through the designed interface with relatively low loss due to the topological‐protection edge waves. This design strategy combines the advantages of corner‐state and edge‐state in higher‐order TPCs, demonstrating potential applications of topological photonic devices, such as optical communication and optical computing technologies.

Guest-Mediated Hierarchical Self-Assembly of Dissymmetric Organic Cages to Form Supramolecular Ferroelectrics

Guest-Mediated Hierarchical Self-Assembly of Dissymmetric Organic Cages to Form Supramolecular Ferroelectrics

Intensity-dependent nonlinear optical properties in an asymmetric Gaussian potential quantum well-modulated by external fields

In this paper, the effects of external electric, magnetic and non-resonant intense laser fields on the nonlinear optical rectification (NOR), second-harmonic (SH), and third harmonic (TH) generation in a GaAs quantum well with asymmetrical Gaussian potential are theoretically investigated. Firstly, the energy eigenvalues and eigenfunctions of a single electron confined in the structure are obtained by using the diagonalization method within the framework of the effective-mass and parabolic band approaches. Then, using these energy eigenvalues and eigenfunctions, expressions derived within the compact density matrix approximation have been employed to calculate the coefficients of the nonlinear optical response in the structure. The obtained simulation results show that the influence of the external fields leads to significant changes in the coefficients of nonlinear optical rectification, second and third harmonic generation in the system. As a result, it has been seen that the amplitude and position of the peaks of nonlinear optical rectification, second and third harmonic coefficients can be controlled by changing the applied external fields.

High-performance quantum entanglement generation via cascaded second-order nonlinear processes

In this paper, we demonstrate the generation of high-performance entangled photon-pairs in different degrees of freedom from a single piece of fiber pigtailed periodically poled LiNbO3 (PPLN) waveguide. We utilize cascaded second-order nonlinear optical processes, i.e., second-harmonic generation (SHG) and spontaneous parametric downconversion (SPDC), to generate photon-pairs. Previously, the performance of the photon-pairs is contaminated by Raman noise photons. Here by fiber-integrating the PPLN waveguide with noise-rejecting filters, we obtain a coincidence-to-accidental ratio (CAR) higher than 52,600 with photon-pair generation and detection rate of 52.36 kHz and 3.51 kHz, respectively. Energy-time, frequency-bin, and time-bin entanglement is prepared by coherently superposing correlated two-photon states in these degrees of freedom, respectively. The energy-time entangled two-photon states achieve the maximum value of CHSH-Bell inequality of S = 2.71 ± 0.02 with two-photon interference visibility of 95.74 ± 0.86%. The frequency-bin entangled two-photon states achieve fidelity of 97.56 ± 1.79% with a spatial quantum beating visibility of 96.85 ± 2.46%. The time-bin entangled two-photon states achieve the maximum value of CHSH-Bell inequality of S = 2.60 ± 0.04 and quantum tomographic fidelity of 89.07 ± 4.35%. Our results provide a potential candidate for the quantum light source in quantum photonics.

Exciton-driven linear and nonlinear optical responses in metal monoxide monolayersMO(M=Mg,Ca,Cd) from first-principles calculations

We study the linear and nonlinear [second-harmonic generation (SHG)] spectra of metal monoxide $\mathrm{MO}$ ($M$=Mg,Ca,Cd) monolayers by using a first-principles approach based on many-body theory. Our results indicate that the strongly bound excitons in $\mathrm{MO}$ with planar structure characterize the optical properties, including both linear and SHG spectra. The optical bandgap ${E}_{\text{opt}}$ is 4.63, 3.71, and 1.27 eV, while the exciton binding energy is as large as 2.49, 2.37, and 1.13 eV, for MgO, CaO, and CdO, respectively. Moreover, the largest SHG coefficient of 586 pm/V is found in CdO monolayer with the smallest bandgap among the three $\mathrm{MO}$ materials. Most importantly, we find a linear relationship between the SHG coefficient and $\mathrm{Qd}/{E}_{\text{opt}}$ ($Q$ is the transferred Bader charge and $d$ is the bond length) which can be used as a descriptor to search for new two-dimensional nonlinear materials.

Extraordinary Photostability and Davydov Splitting in BN-Sandwiched Single-Layer Tetracene Molecular Crystals.

Two-dimensional molecular crystals have been beyond the reach of systematic investigation because of the lack or instability of their well-defined forms. Here, we demonstrate drastically enhanced photostability and Davydov splitting in single and few-layer tetracene (Tc) crystals sandwiched between inorganic 2D crystals of graphene or hexagonal BN. Molecular orientation and long-range order mapped with polarized wide-field photoluminescence imaging and optical second-harmonic generation revealed high crystallinity of the 2D Tc and its distinctive orientational registry with the 2D inorganic crystals, which were also verified with first-principles calculations. The reduced dielectric screening in 2D space was manifested by enlarged Davydov splitting and attenuated vibronic sidebands in the excitonic absorption and emission of monolayer Tc crystals. Photostable 2D molecular crystals and their size effects will lead to novel photophysical principles and photonic applications.

Probing the Chiral Domains and Excitonic States in Individual WS2 Tubes by Second-Harmonic Generation.

Distinct from carbon nanotubes, transition-metal dichalcogenide (TMD) nanotubes are noncentrosymmetric and polar and can exhibit some intriguing phenomena such as nonreciprocal superconductivity, chiral shift current, bulk photovoltaic effect, and exciton-polaritons. However, basic characterizations of individual TMD nanotubes are still quite limited, and much remains unclear about their structural chirality and electronic properties. Here we report an optical second-harmonic generation (SHG) study on multiwalled WS2 nanotubes on a single-tube level. As it is highly sensitive to the crystallographic symmetry, SHG microscopy unveiled multiple structural domains within a single WS2 nanotube, which are otherwise hidden under conventional white-light optical microscopy. Moreover, the polarization-resolved SHG anisotropy patterns revealed that different domains on the same tube can be of different chirality. In addition, we observed the excitonic states of individual WS2 nanotubes via SHG excitation spectroscopy, which were otherwise difficult to acquire due to the indirect band gap of the material.

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.

Optically facet-resolved reaction anisotropy in two-dimensional transition metal dichalcogenides

Quantifying anisotropy in the chemical reactions of mesoscopic crystals has mostly resorted on the combination of electron microscopy and diffraction. In this work, we established crystal-facet-resolved kinetic measurements of oxidation reactions in two-dimensional (2D) transition metal dichalcogenides (TMDs) using optical second-harmonic generation spectroscopy and scanning probe microscopy. We show the in-plane anisotropy of their bond-breaking reactions is governed by their structure and strongly material-dependent among four TMDs. The facet-resolved analysis directly revealed that the reactions proceed fastest (slowest) for chalcogen (metal)-terminated zigzag edges with armchair edges in the middle. The degree of the anisotropy inducing trigonal oxidation patterns was much higher in MoS2 and MoSe2 than WS2 and WSe2. Kinetic Wulff construction based on edge-specific reaction rates verified the material-dependent mesoscopic reaction patterns. We also show that the reactions are initiated at substrate-mediated defects located on the bottom and top surfaces of 2D TMDs.

Inversion-Symmetry Engineering in Layered Oxide Thin Films.

Inversion-symmetry breaking is a ubiquitous concept in condensed-matter science: It is a prerequisite for technologically relevant effects such as piezoelectricity, nonlinear optical properties, and spin-transport phenomena. It also determines abstract properties, like the electronic topology in quantum materials. Therefore, the creation of materials where inversion symmetry can be turned on or off by design may be a versatile approach for controlling parity-related functionalities. Here, we engineer inversion symmetry on a sub-unit-cell level in ultrathin hexagonal manganite films. Although an odd number of half-unit-cell layers breaks inversion symmetry, an even number of such layers remains centrosymmetric. Optical second harmonic generation as an inversion-symmetry-sensitive functionality is thus activated and deactivated on demand and at the same time used for in situ tracking of the symmetry state of our films. Symmetry engineering on the sub-unit-cell level thus suggests a new platform for controlled activation and deactivation of symmetry-governed functionalities in oxide-electronic epitaxial thin films.

Topology and Symmetry of Quantum Materials via Nonlinear Optical Responses

We review recent progress in the study of photogalvanic effects and optical second-harmonic generation in topological and noncentrosymmetric metals.

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.

Effect of 1,8-Diiodooctance additive on the charge carriers behavior in the PCPDTBT:PC71BM BHJ films investigated by using electric-field-induced optical second-harmonic generation measurement

Electric-field-induced optical second harmonic generation (EFISHG) measurement was conducted to investigate the effect of 1,8-diiodooctane (DIO) additive, which is relevant to improve the efficiency of bulk heterojunction (BHJ) solar cells, on carrier behaviors in the BHJ layer, where the BHJ layer is a blend of [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) and regioregular poly [2,6-(4,4-bis-(2-Ethylhexyl)-4H-cyclopentane [2,1-b;3,4-b′] dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT). Using the laser wavelength of 1000 nm, we could selectively observe the electric field in the PCPDTBT phase of BHJ layer, which allowed us to determine the difference in charge accumulation at the interface with and without DIO. With adding DIO, a decrease in interfacial stored charge was observed, and this plays a role in improving the efficiency by about 25%. The DIO modified the internal field distribution so that the electric field promotes the electrons toward the ITO electrode and holes toward the PEDOT:PSS/Au electrode.

Optical second-harmonic generation microscopy as a tool for ferroelastic domain wall exploration

Domain walls (DWs) of four typical ferroelastics, CaTiO3 (CTO), LaAlO3 (LAO), Pb3(PO4)2 (PPO), and BiVO4 (BVO), were observed by the second harmonic generation (SHG) microscopy, and the results were compared. The DWs of the examined ferroelastics are all polar. This fact does not depend on whether the crystal is an insulator (CTO, LAO, and PPO) or a semiconductor (BVO), on whether it has ferroelectric instability (CTO) or not (LAO, PPO, and BVO), or on the DW state, namely, whether the DW is crystallographically prominent or non-prominent. The symmetry of these DWs was determined from the SHG anisotropy, and it was clarified that they are a subgroup of the high temperature phase. In the cases of CTO and PPO, stress was applied to move the DW, and its polarity and symmetry were examined. The main characteristics did not change even when stress was applied. In all samples, uniform SHG was observed for each DW. This indicates that the DWs of the examined ferroelastics seem to be in a ferroelectric single domain state.

Optical second harmonic generation from plasmonic nanoshells using the nonlocal hydrodynamic model

In this paper, we investigate the linear and nonlinear optical response of spherical plasmonic nanoshells within the nonlocal hydrodynamical model for conduction electrons. The optical absorption and surface second-harmonic generation (SHG) associated with metallic core-shell nanoparticles is studied on the basis of the hydrodynamic Drude model. The influence of nonlocality on the optical SHG is investigated. We show that nonlocal effects can be applied to boost SHG from plasmonic nanoshells. It is found that the surface SHG radiation from the surface of nanoshells can be achieved by symmetry-breaking and is sensitive to nonlocality of the electron response. It is also shown that nonlocal effects lead to enhancement and shift of the resonant wavelengths of the optical absorption and SHG in metal nanoshells. Keywords: Optical absorption, Second harmonic generation, plasmonic nanoshell, nonlocal effects.

Second-Harmonic Young's Interference in Atom-Thin Heterocrystals.

Second-harmonic generation (SHG) is a nonlinear optical process that converts two identical photons into a new one with doubled frequency. Two-dimensional semiconductors represented by transition-metal dichalcogenides are highly efficient SHG media because of their excitonic resonances. Using spectral phase interferometry, here we directly show that SHG in heterobilayers of MoS2 and WS2 is governed by optical interference between two coherent SH fields that are phase-delayed differently in each material. We also quantified the frequency-dependent phase difference between the two, which agreed with polarization-resolved data and first-principles calculations on complex susceptibility. The second-harmonic analogue of Young's double-slit interference shown in this work demonstrates the potential of custom-designed parametric generation by atom-thick nonlinear optical materials.