Intensity Of Second Harmonic Signal Research Articles

SHG in Micron-Scale Layers of Glasses: Electron Beam Irradiation vs. Thermal Poling

In this work, we studied the introduction of second-order optical nonlinearity (SON) into subsurface layer soda-lime and alkali-free flint glasses using two different techniques: electron beam irradiation and thermal poling in an open anode configuration. The experiments showed that thermally poled soda-lime glass and e-beam poled flint glass demonstrated noticeable second-harmonic generation (SHG) with their second-harmonic (SH) signals being close in magnitude. The performed estimates showed that the coincidence in the SH signal intensity in these two glasses cannot be explained in the frames of the model that relates the second-order nonlinear susceptibility with the third-order susceptibility and the “frozen” electric field in glasses (EFISH model). This supposes another mechanism of SON in thermally poled soda-lime glass. We believe that this mechanism is the dipole orientation of entities injected into the soda-lime glass during thermal poling in an open anode configuration.

Edge effects in second-harmonic generation in nanoscale layers of transition-metal dichalcogenides

The results of studying the optical properties of nanoscale single crystals of MoS2:Cl2 and WS2:Br2 semiconductor compounds are presented. In microscopic images obtained at the wavelength of the second (400 nm), edge effects are detected, which consist in enhancement or reduction in the second-harmonic signal intensity. Unlike previously proposed interference mechanisms of edge effects, non-interference mechanisms are considered. The occurrence of edge effects is associated with either an increased Cl2 and Br2 halogen molecule concentration or with an electrically induced second harmonic caused by band bending at the edges of individual crystal layers.

Current collapse imaging of Schottky gate AlGaN/GaN high electron mobility transistors by electric field-induced optical second-harmonic generation measurement

Two-dimensional current collapse imaging of a Schottky gate AlGaN/GaN high electron mobility transistor device was achieved by optical electric field-induced second-harmonic generation (EFISHG) measurements. EFISHG measurements can detect the electric field produced by carriers trapped in the on-state of the device, which leads to current collapse. Immediately after (e.g., 1, 100, or 800 μs) the completion of drain-stress voltage (200 V) in the off-state, the second-harmonic (SH) signals appeared within 2 μm from the gate edge on the drain electrode. The SH signal intensity became weak with time, which suggests that the trapped carriers are emitted from the trap sites. The SH signal location supports the well-known virtual gate model for current collapse.

Orientational ordering of 4-pentyl-4′-cyanobiphenyl molecules evaporated on multi-layered polyimide film

Using optical second-harmonic (SH) generation and polarized absorption (PA) measurements, the orientational ordering process of 4-pentyl-4′-cyanobiphenyl (5CB) was studied during evaporation onto the alignment layer of polyimide (PI) Langmuir–Blodgett films. The intensity of SH signal increased in the beginning of the deposition, but it saturated after the first 5CB monolayer was formed. The PA measurement suggested that a layer-by-layer growth was induced after the formation of 5CB monolayer with accompanying the orientational ordering. The orientational order of the first 5CB monolayer was studied in terms of the orientational order parameters, S 1 = 〈cos θ〉 and S 2 = 〈(3cos 2 θ − 1)/2〉, using the SHG and PA measurements. It was found that S 1 decreased as the thickness of PI alignment layer increased from 0.4 to 4.4 nm, while S 2 was nearly constant. These results indicate that the Coulomb interaction between the permanent dipole of 5CB and its image dipole makes a significant contribution to the orientational ordering of deposited 5CBs.

Second harmonic generation from copper-tetratert-butyl-phthalocyanine Langmuir–Blodgett film/metal interface: Electric quadrupole or electric field induced second harmonic generation effect?

It has been found that a high electric field is created at the interface between copper-tetratert-butyl-phthalocyanine (CuttbPc) Langmuir–Blodgett (LB) film/metal due to the exchanged charges. The symmetry of the CuttbPc molecule is broken by this space charge induced electric field (SCIEF), and optical second harmonic (SH) signal is generated from the CuttbPc LB film/metal interface. A theoretical expression of the reflected SH signal intensity is derived using a four-layer model with the SCIEF being treated as a mean field. The origins of the SHG are considered as the quadrupole effect and the electric field induced SHG (EFISHG) due to the SCIEF. On the basis of experimental results and the theoretical analysis, the nonlinear susceptibilities of these two effects are calculated. It is found that at the wavelength of incident fundamental light around 1100 nm the electric quadrupole effect makes a main contribution to the SHG from the interface, while at the wavelength around 1200 nm it is attributed to the EFISHG. It is expected that the SCIEF could be determined by SHG.

Guest–host polyetherketone polymer for applications in integrated-optical devices: characterization

Thin films of polyetherketone guest–host polymer doped with 3-(1,1-dicyanothenyl)-1-phenyl-4, 5-dihydro-1H-pryazole were prepared. Linear electro-optic coefficients of poled-polymer films have been determined at λ=632.8 nm by use of a simple interferometric technique. Temporal and temperature stability of the poled-polymer film was measured by probing linear electro-optic activity and second-harmonic signal intensity, respectively. For the poled-polymer films the lower-order optical waveguide-transmission losses were measured at 632.8 nm. The measurements showed that the optical loss of the fundamental mode was less than 0.5 dB cm-1 for transverse electric-field polarization.

Ordered Films by Alternating Polyelectrolyte Deposition of Cationic Side Chain and Anionic Main Chain Chromophoric Polymers

Using the method of aqueous solution alternating polyelectrolyte deposition (APD), second-order nonlinear optical (NLO) polymer films were prepared for the first time in which both polymers were NLO-active. Films were prepared by alternately coating a solid substrate with an NLO-active side chain polycation and an NLO-active main chain polyanion. This new polyanion was comprised of α-cinnamoyl chromophores in the syndioregic configuration (an “accordion” polymer). The new polycation was derived from polyepichlorohydrin that was completely substituted with new stilbazolium side chain. The films were transparent and had no texture when observed by polarized microscopy. The increase in intensity of the second harmonic (SH) signal generated in the films was quadratic with each molecular layer to 20 layers. Beyond that, the SH signal intensity saturated as more layers were added.

Coupled second-harmonic generation, surface plasmon resonance and AC impedance studies of full and partial monolayers in (Au,Ag)-alkanethiolate electrolyte systems

Optical second-harmonic generation (SHG) has been coupled with surface plasmon resonance (SPR) and AC impedance measurements to probe Ag and Au electrolyte interfaces in the presence of full and partial alkanethiolate self-assembled monolayers (SAMs) at potentials within the double layer charging region. Both SHG and AC impedance measurements of the Ag/alkanethiolate/electrolyte systems have been performed for alkanethiols with different chain lengths (C n H 2 n+1 SH, n=8,12,16). The capacitance of the alkanethiolate SAMs is found to vary inversely with alkanethiol chain length, as expected, permitting direct determination of the dielectric constants for all films. The potential dependence of the second harmonic (SH) signal from metal/SAM/electrolyte systems is found to be much weaker than that from bare Ag or Au, a result explained in terms of a series capacitor model for the SAM-double layer combination. The coverage dependent SH signal intensity from Au/alkanethiolate/electrolyte systems has been measured. Ozonolysis is used to obtain partially covered hexedecanethiolate monolayer on Au surfaces, with the coverage being controlled by the O 3 exposure time and monitored by shifts in the surface plasmon resonance (SPR) position. A linear relationship between the film coverage and SH intensity has been observed with SH response increasing with decreasing coverage. The SH behavior for partial monolayers is dominated by the change in the DC electric field caused by SAM formation.

熱ポーリングした石英ガラスの第2高調波発生の機構

The mechanism of second-harmonic generation (SHG) was investigated for the thermal poled fused quartz and synthesized silica glasses. The current was measured for silica glasses during the thermal poling process. As a result, fused quartz with a relatively higher current generated SHG. We discussed the relationship between crystallization from glass to crystal and second-order nonlinearity, based on a continuous-heating transformation (C-H-T) diagram and the sample temperature, which was calculated from the Joule’s heat of the poling process. It was observed that grain size of the α-quartz and α-cristobalite crystal was about 20 nm at the depth at 5 μm from the anodic surface in the poled fused quartz (553 K, 4 kV, 103.56 s), by a field emission scanning electron microscope (FE-SEM) and X-ray diffraction (XRD) analysis. The distribution of intensity of the second-harmonic (SH) signal was measured in a layer of SHG near the anodic surface. As a result the mechanism of SHG was deemed due to crystallization from silica glass to the orientated α-quartz possessing second-order nonlinearity under a high electric field and Joule’s heat in the thermal poling condition. It was considered that the intensity of SH signal was higher with increasing quantity of orientated nano-crystals, but was lower with decreasing orientation of the nano-crystals, but was lower with decreasing orientation of the nano-crystals, along with decreasing electric field.