SH Intensity Research Articles

229 nm far-ultraviolet second harmonic generation in a vertical polarity inverted AlN bilayer channel waveguide

Far-UV light sources have attracted much attention for human-safe viral inactivation and bacterial disinfection. Due to large optical nonlinearity and transparency to this wavelength region, AlN is a promising material for compact and low-cost far-UV second harmonic generation (SHG) devices. In this study, a transverse quasi-phase-matched AlN channel waveguide with vertical polarity inversion was designed and fabricated. From wavelength spectra and a pump power dependence of an SH intensity, far-UV SHG via the largest nonlinear optical tensor component d 33 was successfully confirmed under ultrashort pulse laser excitation.

Lysozyme adsorption on carbonaceous nanoparticles probed by second harmonic light scattering.

The first hyperpolarizability (β) of two different sizes (15 and 35 nm) of carbonaceous nanoparticle (CNP) is reported for the first time using second harmonic light scattering (SHLS). The β values of the CNPs were found to be larger than those of organic molecules like pNA but lower than those of plasmonic nanoparticles like gold and silver. SHLS was further used to investigate the adsorption of a model protein Lysozyme (Lyz) on these CNPs, which is crucial for the design of safe and effective CNP-based therapeutics. The change in SH intensity from the CNPs on the addition of Lyz was recorded and fitted to the modified Langmuir adsorption model (MLM). The binding constant, free energy changes and surface coverage values show that Lyz is physisorbed on the CNPs forming less than a monolayer. The temperature dependent SH intensity measurements enabled direct determination of enthalpy and entropy changes for Lyz adsorption. The enthalpy and entropy changes reveal that Lyz adsorption is endothermic and entropically driven.

Revealing atomically sharp interfaces of two-dimensional lateral heterostructures by second harmonic generation

The interface between two different semiconductors is crucial in determining the electronic properties at the heterojunction, therefore novel techniques that can probe these regions are of particular interest. Recently it has been shown that heterojunctions of two-dimensional transition metal dichalcogenides have sharp and epitaxial interfaces that can be used to the next generation of flexible and on chip optoelectronic devices. Here, we show that second harmonic generation (SHG) can be used as an optical tool to reveal these atomically sharp interfaces in different lateral heterostructures. We observed an enhancement of the SH intensity at the heterojunctions, and showed that is due to a coherent superposition of the SH emission from each material. This constructive interference pattern reveals a phase difference arising from the distinct second-order susceptibilities of both materials at the interface. Our results demonstrate that SHG microscopy is a sensitive characterization technique to unveil nanometric features in layered materials and their heterostructures.

Enhancement of polar nature of domain boundaries in ferroelastic Pb3(PO4)2 by doping divalent-metal ions

The effect of doping metal ions in ferroelastic Pb3(PO4)2 (PPO) on the polar nature of domain boundaries (DBs) was investigated using a second harmonic generation (SHG) microscope. It has been already reported that (DBs) of non-doped PPO is SH active and polar. The present study reveals that DBs of Ca-doped and Mg-doped PPO show greatly enhanced SH activity. This indicates that doping by metal ions enhances the polar nature of the DBs of PPO. This is important for future applications of DB nanotechnology. The enhancement of SH intensity is explained by a larger displacement of Ca2+ and Mg2+ ions in DBs due to smaller ionic radii. Analyses of the SH anisotropy experiments reveal that the symmetry-adapted W-wall belongs to monoclinic m and the non-adapted W’-wall to monoclinic 2. Both point groups are classified as the polar classes, which coincides with the case of pure PPO.

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.

Thermodynamics of adsorption of lysozyme on gold nanoparticles from second harmonic light scattering.

Gold nanoparticle (GNP) interaction with hen egg white lysozyme (Lyz) has been investigated by many groups in order to understand protein mediated aggregation of GNPs and the underlying mechanism of aggregation. In this article, we have studied the interaction of citrate-capped GNPs of 16, 28, 41, and 69 nm sizes with Lyz by the non-destructive label-free second harmonic light scattering (SHLS) technique at physiological pH in phosphate buffer. The surface sensitivity of the nonlinear optical SHLS technique is very high and we have looked at the GNP-Lyz interaction at nanomolar concentrations. We have followed the increase in the SHLS intensity of GNPs as a function of the added concentration of Lyz in small aliquots. The SH intensity profile exhibits saturation behaviour and was fitted with a modified Langmuir adsorption model which yielded the binding constant (Kb), the binding stoichiometry (nsat) at saturation and the free energy change (ΔG) in the adsorption process. The free energy change was further decomposed into changes in the enthalpy (ΔH) and entropy (ΔS) of adsorption by carrying out temperature dependent SHLS measurements in a specially designed cell. The thermodynamic quantities extracted from the measurements show that the binding is exothermic (ΔH < 0) as well as spontaneous (ΔS > 0). We find that the first step in the adsorption of Lyz on the GNP surface is nanoparticle protein corona (NP-PC) formation driven predominantly by electrostatic attraction. In the second step of adsorption, the adsorbed lysozymes on the surface form a bridge between two or more GNPs leading to the latter's aggregation, which is the main reason for the enhancement of the SH scattering signal. Although the interaction between the GNPs and Lyz is driven by strong electrostatic attraction, the thermodynamic quantities reported here indicate that the protein is physisorbed on the nanoparticle surface. We have also demonstrated that SHLS provides a new tool for full thermodynamic characterization of protein adsorption on metal nanoparticles at ultralow concentrations.

Proposal for CEP measurement based on terahertz air photonics

Single-shot carrier envelope phase (CEP) measurement is a challenge in the research field of ultrafast optics. We theoretically investigate how an intense terahertz pulse modulates second harmonic emission (SH) from a gas plasma induced by a few-cycle laser pulse (FCL). Results show that the modulation quantity of SH intensity has a cosinoidal dependence on the CEP of FCL pulses, based on which we propose a low energy, all-optical method for single-shot CEP measurements via using a known intense terahertz pulse. Moreover, we propose an experimental realization.

Hydrothermal synthesis of single-crystalline one dimensional β-BaB2O4 microrods with second order nonlinear optical properties

The second harmonic generation (SHG) have been the most extensively used and well understood nonlinear optics phenomenon. It implies a generation of new output frequency, twice larger than that of the incident laser radiation. Therefore, it is of key importance to propose appropriate second harmonic generation media. To date, the one-dimensional inorganic nanostructures are expected to exhibit a strong nonlinear optical response. Herein, we fabricated single-crystalline one-dimensional β-BaB2O4 microrods via a facile hydrothermal procedure and subsequent annealing using barium chloride, sodium borate and hydrochloric acid as starting materials. The fabricated β-BaB2O4 microrods generated good spectral response within the excitation range of 800–1064 nm. The SH intensity of the microrods increased linearly with excitation intensity Iω (In, n = 2.11), which demonstrated the quasi two-photon dependence. These results reveal that the fabricated β-BaB2O4 microrods are promising for future nonlinear optical applications.

Surface Oxidation of Supported, Size-Selected Silver Clusters

We use surface second harmonic generation spectroscopy (s-SHG) to study the oxidation of supported, size-selected silver clusters under ultra-high vacuum conditions. The oxidation reaction of small silver clusters between \(\hbox {Ag}_{9}\) and \(\hbox {Ag}_{55}\) is monitored by means of their localized surface plasmon resonance. We observe a rapid decline of the SH-intensity, as soon as cluster samples are exposed to an oxygen partial pressure of \(5 \times 10^{-6}\) mbar, which is attributed to the formation of silver–oxygen-bonds. The evolution of the SH-intensity under exposure to oxygen shows a double-exponential character for all investigated cluster sizes. Since the oxidation of single crystalline silver surfaces follow single-exponential Langmuir-kinetics, the two independent pathways of SH-intensity loss are attributed to a surface- and an interface-oxidation of supported clusters, respectiveley. For small cluster sizes, a complete loss of the SH intensity is obtained, which suggests the complete oxidation of the clusters. For larger clusters a plasmonic resonance is still observed after oxidation, indicating a residual free-electron density.

Second harmony generation behavior in fully epitaxial m-plane (1 0 −1 0) non-polar ZnO thin film grown on MgO (1 0 0) substrate by radio frequency sputtering

Epitaxial m-plane non-polar ZnO (1 0 −1 0) was grown on MgO (0 0 1) substrate using radio frequency sputtering. The epitaxial relationships between film and substrate are (1 0 −1 0) ZnO||(0 0 1) MgO and (0 0 0 1) ZnO||(1 0 0) MgO. Second Harmonic Generation (SHG) signal was detected in all polarization orientations with maximum SH intensity at four angles, namely 0°, 90°, 180° and 270°. The maximum of the SH signals is derived from the contribution of 2 close c-axis tensors rotated by about ±15° and the minimum intensity from the contribution of tensors elements of 2 far c-axis rotated by about ±30°.

Plasmon assisted enhanced second-harmonic generation in single hybrid Au/ZnS nanowires

We demonstrate the enhanced second-harmonic generation (SHG) in single ZnS nanowires (NWs) attached with gold nanoparticles (Au NPs). The hybrid Au/ZnS NWs with different densities of the attached Au NPs were prepared by a simple solution impregnation method. By comparing with bare ZnS NWs, ∼1.3, ∼6.6, ∼7 and ∼2 times enhancement of SH intensity was achieved in the hybrid Au/ZnS NWs with low, moderate, high and ultrahigh densities of the attached Au NPs, respectively. The enhanced SHG in the hybrid Au/ZnS NWs is attributed to the strong local-fields from the Au cluster under the near-resonant condition, which is supported by the related dark-field scattering spectra. This hybrid Au/ZnS NWs provide a simple platform for enhancing nonlinear optical responses, which have potential applications in nano-probing and nano-sensing.

Non-linear optical properties of β-D-fructopyranose calcium chloride MOFs: an experimental and theoretical approach

The second harmonic generation (SHG) properties of two MOFs, obtained from fructose and calcium chloride, were studied using a Non-Linear Optics Multimodal microscope. The first-order hyperpolarizability and the second-order susceptibility were calculated at the DFT level of theory. Moreover, a semi-classical approach to non-linearities in the optical behaviour was used in order to determine the features responsible for the SHG. The MOFs were synthesized both in ethanol and by solid–solid interaction, with a simple, rapid and low-cost methodology with no environmental impact, and were characterized with IR and RAMAN spectroscopy and both the single-crystal and powder X-ray diffraction. Both the metal–carbohydrate-based MOFs show an interesting SHG intensity: in particular, compound 1 shows an average SH intensity more than twice that of sucrose, in agreement with the theoretical results. A favourable combination of optical properties, transparency, thermal and chemical stability makes compound 1 a potential candidate for applications in electro-optics devices.

Second Harmonic Generation Based on Strong Field Enhancement in Metallic Nanostructured Surface

Although absorption is still the primary optical property of interest, other spectroscopic techniques including optical second harmonic generation (SHG) are also used to understand anisotropic nature of nanomaterials. Benefit of SHG technique as compared with linear optical approach is that SH response is the most sensitive to surface potential changes and SH intensity mainly comes from surface/interface layers. This article reports that optical absorption spectra due to surface plasmon modes of metallic nanowires exhibit strong anisotropic absorption. Maxwell-Garnett simulation cannot fit with experimental results because of wire sizes much larger than wavelength of light, whereas finite-difference time-domain calculation can fit well with experimental findings in terms of position of photon energy of absorption peak with respect to polarization conditions. DOI: http://dx.doi.org/10.5755/j01.ms.20.4.6399

Optical second harmonic generation in ferroelectric liquid crystals under oblique incidence

The second harmonic generation (SHG) in ferroelectric liquid crystals under oblique incidence was studied by using a numerical band matrix method that uses boundary conditions directly. No approximations were used except for the nondepleted fundamental wave. By matching the edge of the full-pitch band with that of the half-pitch band, the SH intensity increased significantly. The thickness (L) dependence of the SH intensity showed L 6.93–L 7.09 dependence without the limitation of the thickness.

Precise measurement of weak strain by second-harmonic generation from silicon (111) surface

The weak strain induced by uniaxial strain device is calibrated by strain-induced second-harmonic generation (SISHG) from silicon (111) surface. Dependences of the strain-induced second-harmonic intensity on sample azimuth angle show that the strain leads to increase of SH intensity. The high consistency of the SH-measured strain and the applied strain indicates that weak strain can be accurately calibrated by SISHG. The small applied strain does not greatly affect the 3 m symmetry of silicon (111) surface, but enhances the SH intensity evidently. The bulk inversion symmetry of crystal silicon vanished under applying of uniaxial strain and this also has demonstrated by first-principles simulation. Furthermore, the theoretical relative variation of Si–Si bond length agrees exactly with the applied strain along [111] direction.

Second-Harmonic Generation of the Chalcogenide Amorphous Film by Electron Beam Irradiation

PLD(pulsed laser deposition) method was used to prepare amorphous GeS2-Ga2S3-CdS chalcogenide film. Obvious SHG(second harmonic generation) was observed in electron beam irradiated film by Maker fringe method. According to Raman spectra, we discussed the mechanism of SHG and ascribed the origination of SHG to the local electric field generation under electron beam and uneven charge distribution. With the increase of accelerating voltage and the extension of irradiation time, the SHG intensity increased and reached the maximum, which is due to the enhancement of breakage of glassy isotropy with gradually increased incident electron energy and the finite population of electric dipoles leading to the saturation of SH intensity.

Preparation and Second Harmonic Generation of CdS Doped Lead Silicate Glasses

CdS doped lead silicate glasses were prepared with the conventional fusion method. The effects of the ZnO/CdS ratio and the content of CdS on the preparation of CdS doped glasses were both studied. The crystallization process of CdS nanocrystals was investigated by X-ray diffractometry and scanning electron microscopy. Utilizing the Maker fringe method, Second harmonic generation (SHG) was observed from CdS doped glasses without any poling procedures. The SH intensity gets enhanced by increasing the heat-treatment temperature and prolonging the heat-treatment duration.

Communication: Reactions and adsorption at the surface of silver nanoparticles probed by second harmonic generation

Even though nanoparticles have dimensions much smaller than the optical wavelength and shapes commonly with inversion symmetry, we show, for the first time, direct experimental evidence that second harmonic generation (SHG) can be detected from the surface layer of metallic nanoparticles, in this case 40 nm radius Ag particles. The SH intensity detected is shown to substantially decrease upon chemical bonding of thiol molecules to the Ag particle surface. The surface generated SH intensity can be used for probing properties and processes at the nanoparticle surface.

On the resonant SHG response of ultra-thin alkali (K, Rb)-covered Si(1 1 1)-7 × 7

Room temperature adsorption of K either on Si(111)-7×7 or on dielectric SiO2-covered Si(111) followed by second-harmonic generation (SHG) observation clearly identified a characteristic resonance enhancement typical to Si(111)-7×7. The polarization selected SHG as a function of coverage obtained at different photon energies between 1.17-eV and 1.55-eV show a similar variation of the SH intensity both for K and Rb adsorption on Si(111)-7×7. For submonolayer coverage, wavelength dependent SHG peak observed around 0.4 monolayer (ML) was followed by an SHG peak around 0.9 ML that was less dependent on the wavelength. Previous proposals for an interpretation of the former peak have been questioned. Instead, the former peak has been suggested as being a resonant enhancement due to the transition between alkali atomic-derived electronic states, while the origin of the latter peak remains unidentified. Multiple SHGs of different origins are identified through a phase measurement of SH fields.

Selective measurement of the electronic states of the steps on rutile TiO2 (15 15 4) surface by optical second harmonic spectroscopy

AbstractWe have obtained optical second harmonic spectra of stepped rutile TiO2 (15 15 4) adsorbed with formic acid HCOOH. We have achieved selective measurement of the electronic states of the bunched steps on this surface in an optical configuration with nearly normal incident laser illumination. For the polarization combinations S in‐S out and P in‐S out, the SH intensity was higher near the second harmonic photon energy 3.6 eV and also above 4.45 eV. The terraces and steps were found to show remarkably different contributions to the optical second harmonic spectra at 45° incidence. Copyright © 2010 John Wiley &amp; Sons, Ltd.