Second-harmonic Intensity Research Articles

Second-harmonic intensity and phase spectroscopy as a sensitive method to probe the space-charge field in Si(100) covered with charged dielectrics

A space-charge region (SCR) can develop in silicon due to the presence of built-in charges in dielectric thin films that are used in silicon-based device architectures. To study both the strength and polarity of the electric field in such a SCR, the authors performed second-harmonic (SH) generation spectroscopy in the vicinity of the E1 critical point (2.7–3.5 eV) of silicon. As multiple contributions add coherently to SH intensity spectra, the electric-field-induced contribution cannot always be distinguished unambiguously from the intensity data in the absence of complementary phase information. Combined SH intensity and phase measurements were therefore performed to resolve this ambiguity. Using a coherent superposition of critical-point-like resonances with excitonic line shapes, the intensity and phase spectra of several SiO2- and Al2O3-based samples were simultaneously modeled. This analysis reveals that not only the polarity of the space-charge field can be determined unambiguously but also that the sensitivity to the electric field strength is significantly enhanced.

Microwave-induced zero-resistance states and second-harmonic generation in an ultraclean two-dimensional electron gas

Microwave-induced resistance oscillations (MIRO) and "zero-resistance" states (ZRS) were discovered in ultraclean two-dimensional electron systems in 2001-2003 and have attracted great interest from researchers. A comprehensive theory of these phenomena was developed in 2011: It was shown that all experimentally observed dependencies can be naturally explained by the influence of the ponderomotive forces which arise in the near-contact regions of the two-dimensional electron gas under the action of microwaves. Now we show that the same near-contact physical processes should lead to another nonlinear electrodynamic phenomenon - the second-harmonic generation. We calculate the frequency, magnetic field, mobility, and power dependencies of the second-harmonic intensity and show that it can be as large as ~ 0.5 mW/cm2 under realistic experimental conditions. A part of this paper is devoted to a further discussion of the MIRO/ZRS phenomena: we explain how the ponderomotive-force theory explains different experimental details, including those which were not known in 2011, and critically discuss alternative theories.

Effect of the domain shape on noncollinear second-harmonic emission in disordered quadratic media

We study the role of the individual ferroelectric domain shape on the second-harmonic emission in strontium barium niobate featuring a random quadratic nonlinearity. The noncollinearly emitted second-harmonic signal is scanned in the far-field at different incident angles for different domain size distributions. This offers the possibility to retrieve the Fourier spectrum, corresponding to the spatial domain distribution and domain shape. Based on images of the domain structures retrieved by Čerenkov-type second-harmonic microscopy, domain patterns are simulated, the second-harmonic intensities are calculated, and finally compared with the measurements.

How to test and verify radiation diagnostics simulations within particle-in-cell frameworks

The particle-in-cell code PIConGPU provides the feature of calculating angular resolved radiation spectra in the far field based on Liénard–Wiechert potentials for all macroparticles of a plasma simulation. In order to verify the physics of our code we present a series of physics test scenarios, which compare numerical results to analytic solutions of nonlinear Thomson scattering at relativistic electrons. These scenarios range from single particle and electron bunch tests to full-scale laser-plasma simulations that include the collective effects of a plasma, as well as coherent and incoherent superposition of radiation of many particles. For the calculated test cases good agreement to the theoretical results with respect to absolute spectral intensities was found in all observation directions. In an electron density scan of a laser-plasma scenario, we reproduce a second-harmonic intensity scaling also observed in experiment.

Magneto- and electroinduced effects in optical second-harmonic generation from a planar Au/Co/Si nanostructure

Optical second-harmonic (SH) generation in planar Au/Co/Si(111) nanostructures under application of static magnetic and electric fields is studied experimentally. The crystallographic, magneto- and electroinduced contributions to the SH intensity reveal strong azimuthal anisotropy peculiar to the Si(111) surface. The transient times characterizing the electroinduced quadratic response are found to exceed 1 s. Relative amplitudes and phases of the magneto- and electroinduced constituents of the SH signal are estimated from the measurements data.

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.

Probing the relative orientation of molecules bound to DNA through controlled interference using second-harmonic generation

A method is described in which the interference of radiated second-harmonic electric fields generated by a pair of oriented molecules intercalated into double-stranded DNA is controlled and measured. The results show that the relative molecular orientation of the two molecules significantly changes the magnitude of the observed second-harmonic generation intensity, which is described by a simple model that accounts for the interferences of the radiated fields. The technique presented shows promise for future experiments investigating structural changes induced by the formation of a DNA-biomolecule complex.

Coherent detection of terahertz pulses via gas plasma induced by few-cycle laser pulses with fixed carrier envelope phase

The time behavior of gas plasma induced by a few-cycle laser pulse with a simultaneously incident terahertz (THz) pulse is investigated via a transient photocurrent model. The results show that the second-harmonic intensity emitted from the gas plasma can well replicate the THz waveform if the laser pulse has a fixed carrier envelope phase. Based on these we propose a approach for THz coherent detection, which could realize coherent THz detection with very low laser energies and without a bias field, and may be very useful in some fields, such as remote THz sensing.

New tartratoborates: synthesis, structure, and characterization of non-centrosymmetric ASr[C4H2O6B(OH)2]·4H2O (A = K+, Rb+)

Two new tartratoborate nonlinear optical materials, KSr[C4H2O6B(OH)2]·4H2O (KSCB) and RbSr[C4H2O6B(OH)2]·4H2O (RSCB), have been prepared through slow evaporation at room temperature. Single-crystal X-ray diffraction data reveal that KSCB crystallizes in the triclinic system, space group P1 (no. 1). It consists of KO8 polyhedra, SrO9 polyhedra, BO4 tetrahedra, tartrate molecules and H2O molecules. The isolated tartratoborate anions are connected to form a three-dimensional network structure by sharing oxygen atoms with K+ and Sr2+. The structural data show that KSCB and RSCB are isostructural and noncentrosymmetric. KSCB and RSCB produce second-harmonic generation intensities of 1.5 × KH2PO4 and 1.7 × KH2PO4, respectively, and they are both type I phase-matchable. The UV-Vis diffuse reflectance spectra of these compounds indicate that they are both transparent from 250 to 850 nm. Related infrared spectra and thermal analyses were also performed.

Simple Method of Designing the Thickness of a Nonlinear Optical Crystal of Second-Harmonic Generation under Imperfect Phase-Matching Conditions

A simple method of designing the thickness of a nonlinear optical crystal used in the second-harmonic generation corresponding to the maximum second-harmonic intensity is discussed under imperfect phase-matching conditions.

Second‐Order Nonlinear Optical Activity Induced by Ordered Dipolar Chromophores Confined in the Pores of an Anionic Metal–Organic Framework

Frequency doubling: A strategy for incorporating dipolar organic chromophores into the one-dimensional channels of an anionic metal-organic framework (MOF) has been developed to generate highly active nonlinear optical materials (see picture). The resulting MOF material shows a second-harmonic generation intensity of 18.3 versus α quartz.

Surface second harmonic generation from coumarin 343 dye-attached TiO2 nanoparticles at liquid–liquid interface

The nonlinear optical properties of coumarin 343 (C343) dye-attached TiO2 nanoparticles in the size range 5–8 nm adsorbed at the interface of water/1,2-dichloroethane have been studied by using the surface second harmonic generation technique. No second harmonic (SH) response was observed from the bare TiO2 nanoparticles adsorbed at the interface, however, a strong SH response was measured from the dye molecules attached at the surfaces of the nanoparticles. The increase in the SH intensity with the increase of TiO2 nanoparticle concentration in the aqueous solution of C343 is mainly due to the pre-alignment of the dye molecules at the surfaces of nanoparticles and is partly due to the third-order polarization contribution of the nanoparticles to the observed total SH response.

Second-harmonic confocal microscopy of layered microstructures based on porous silicon

Layered structures based on porous silicon have been studied by confocal microscopy of the second optical harmonic. A linear increase in the second-harmonic intensity with increasing porosity of the layers has been observed. This behavior may result from spatial fluctuations in the dipole quadratic response of the pore walls.

Broadband Second-Harmonic Generation in the Near-Infrared Region in a Tapered Zinc Selenide Slab Using Total Internal Reflection Quasi-Phase Matching

We analytically describe the concept of broadband second-harmonic generation in the near-IR region in an isotropic tapered semiconductor slab made of zinc selenide. A computer-aided simulation has been carried out to determine the possibility of generating broadband second-harmonic intensity when broadband fundamental laser radiation is allowed to undergo total internal reflection inside the tapered semiconductor slab. The simulated results indicate an extremely broad spectral bandwidth of 557 nm centered at 4.05 µm (3.812 to 4.369 µm) in a 10-mm-long slab with a conversion efficiency of 0.02%. The losses due to absorption and surface roughness of the semiconductor material have been considered in the analysis. The effects of variations in the temperature, length, and tapering angle of the semiconductor slab have been studied for the generated second-harmonic radiation.

Direct probing of the selective electron and hole accumulation at organic/organic interfaces in a triple-layer organic device by time-resolved optical second harmonic generation

By directly probing the electric field changes across the fullerene active layer with time-resolved optical second harmonic (SH) generation method, the selective carrier accumulation processes at organic/organic interfaces in an Au/pentacene/fullerene/polyimide/indium tin oxide triple-layer device were revealed. On the basis of the Maxwell-Wagner effect, it was shown that injected holes preferably accumulated at the pentacene/fullerene interface, resulting in a fast increase of the generated SH intensity around 10−5 s; while the injected electrons transported until to the fullerene/polyimide interface, resulting in a delayed decrease of the SH intensity after 10−3 s.

Investigation of Second-Harmonic Generation and Molecular Orientation in Electrostatically Self-Assembled Thin Films

We report the observation and measurement of second-harmonic generation in self-assembled ultra thin film nonlinear optical materials using a femtosecond high repetition rate laser system. Second-harmonic intensity, as a function of the incident angle in these films, has been measured using incident p-polarized and s-polarized optical beam components. The second-order nonlinear optical susceptibilities of the thin films have also been determined. Using a curve fitting method and a crystal reference material, we have obtained second-order susceptibilities c333 = 6.17 ± 0.18 pm/V and c311 = 0.68 ± 0.02 pm/V at a fundamental wavelength of 1,200 nm. Based on linear molecular model approximation, we have also used the fitted data to investigate the average orientation distribution of the chromophore dipoles in the self-assembled film. The result indicates that the average tilt angle of the chromophore dipoles away from the substrate normal line is 25.2° ± 0.8°.

INFLUENCE FACTORS ON SHG IN PCs CONSISTING OF CENTRO-SYMMETRIC MATERIALS

Based on the finite difference-time domain algorithm, the factors influencing second harmonic generation in photonic crystals consisting of centro-symmetric materials are analyzed. The results show that photonic band gap is the key factor inducing the generation of second harmonic (SH). The suitable angle of incidence of pumping wave, waveguide length and fill factor will be helpful for the enhancement of SH. For the structure in this paper, when the angle of incidence is 20°–30°, SH intensity will be five times to that when the angle is nearly 90.° According to the results of our computations, the critical wave-guide length is 50 μm for the maximal intensity of SHG.

Generation of second-harmonic radiations of a self-focusing laser from a plasma with density-transition

A Gaussian laser-beam resonantly generates a second-harmonic wave in a plasma in the presence of a wiggler magnetic-field of suitable period. The self-focusing of the fundamental pulse enhances the intensity of the second-harmonic pulse. An introduction of an upward plasma-density ramp strongly enhances the self-focusing of the fundamental laser pulse. The laser pulse attains a minimum spot size and propagates up to several Rayleigh lengths without divergence. Due to the strong self-focusing of the fundamental laser pulse, the second-harmonic intensity enhances significantly. A considerable enhancement of the intensity of the second-harmonic is observed from the proposed mechanism.

Growth, structure and properties of the non-centrosymmetric hydrated borate CaN 2B 8O 26H 32

Nonlinear optical crystal of CaN 2B 8O 26H 32 has been grown with sizes up to 9 × 8 × 5 mm 3 by the solvent evaporation method. The structure was determined by single-crystal X-ray diffraction and further characterized by powder X-ray diffraction and IR spectrum analysis. CaN 2B 8O 26H 32 crystallizes in the orthorhombic system, space group P2 12 12 1, with unit-cell parameters a = 11.556(7) Å, b = 12.583(8) Å, c = 16.679(8) Å, Z = 4, V = 2425.2(2) Å 3, R 1 = 0.0263, and wR 2 = 0.0655. The structure is made up of CaO 7 polyhedra, and [B 4O 5(OH) 4] 2− polyborate anions, NH 4 + cations, and H 2O molecules. UV–vis–NIR diffuse reflectance spectrum is performed on the CaN 2B 8O 26H 32, which shows an absorption edge <190 nm in the UV region. Thermal properties were investigated by TG analyses. The powder second-harmonic generation intensity measured by the Kurtz–Perry method indicates that CaN 2B 8O 26H 32 has about half times that of KH 2PO 4.

Phase effects at second-harmonic generation in metamaterials

We develop the theory of second-harmonic generation in metamaterials where fundamental radiation is considered in the constant-intensity approximation taking into account the phase changes of interacting waves. We investigate the second-harmonic wave intensity in metamaterials at different parameters of the problem under consideration. We consider a self-action effect of the light wave in metamaterials in view of the phase changes of all the interacting waves and compare this effect with an analogous effect in usual homogeneous quadratic media. We show that it is possible to change the phase velocity of the pump wave by varying such parameters as the pump intensity, nonlinear medium length, and phase mismatch between the interacting waves. The metamaterials under consideration provide a possibility to change the phase of the fundamental radiation of order of magnitude higher than those in the usual homogeneous quadratic media.