Second harmonic generation from a ferroelectric film

We derive an expression for transmittivity (T_SHG) of second harmonic generation (SHG) signals from a ferroelectric (FE) film. Intensities of up and down fields in the medium are investigated in relation to T_SHG. The derivations are made based on undepletion of input fields and nonlinear wave equation derived from the Maxwell equations. We present two cases: film without mirrors and with partial mirrors. Expressions for the newly derived nonlinear susceptibility coefficients of SHG for real crystal symmetry [J. Opt. Soc. Am. B 19 (2002) 2007] are used to get more realistic results. Variations in T_SHG with respect to film thickness are illustrated.     

Second harmonic generation using an electrically controlled asymmetric plasmonic waveguide

ABSTRACT

In this study, it was shown that field enhancement in the nonlinear metal-insulator-metal (MIM) plasmonic waveguides can result in a large enhancement of the second harmonic generation (SHG) magnitude as compared with values reported in the literature. The proposed structure has two metals at the top and two metals at the bottom of the crystal. In this structure, a voltage is applied on metals to produce a SHG electrically. Hence, the metals that define the cavity also serve as electrodes capable of generating high direct current electric fields across the nonlinear material. The frequency of a fundamental wave at 458 nm was doubled and modulated in intensity by applying a moderate external voltage to the electrodes, yielding a voltage-dependent nonlinear generation with a higher coupling efficiency. All the simulations here have been calculated by using the finite-element-based commercial COMSOL software.     

Optical second harmonic generation at platinum phthalocyanine-modified platinum electrodes

Optical second harmonic generation (SHG) has been used to observe changes within electronically conducting platinum phthalocyanine (PtPc) films deposited on polycrystalline platinum electrodes as the film undergoes electrochemical modification. PtPc-modified electrodes produced enhanced SHG responses over bare platinum surfaces. This is proposed to be due to an electric quadrupole contribution from the PtPc molecule. Examination of the polarization dependence of the SHG response reveals that electroactive PtPc molecules exist in environments both parallel and perpendicular to the electrode surface. It has been possible to observe redox processes occurring within the films by monitoring the magnitude of the SHG response with variations in potential. The decrease in SHG signal has been shown empirically to be proportional to the charge removed from the film during oxidation of the phthalocyanine (Pc) ring.     

Second harmonic generation from electrically polarized CuCl-microcrystallite-doped glass

Second harmonic generation has been observed in electrically polarized CuCl-microcrystallite-doped glass. The glass was prepared by means of melting–quenching method and poled at 200°C for 1 h under applied dc field of 4 kV. The incident angular variation of SHG intensity for the polarized glass was measured. To explain the effect, it is proposed that the second-order nonlinear polarization results from the dipolar orientation in the glass.     

Quasi-phase matching of third harmonic generation in corrugated discharge capillary

Quasi-phase matching third harmonic of Ti:sapphire laser was measured by propagation through corrugated discharge capillaries. The 1 cm long capillary with periodical varying inner radius was used with corrugation frequency of 200 μm. The longitudinal plasma density was periodically varied by a discharge current ablating the inner capillary walls, altering the refractive index accordingly. Average plasma density measured was 10¹⁸ cm⁻³ at minimum corrugation radius. Peak laser intensity was 10¹⁰ W/cm² allowing enough pondermotive energy to the electron for high harmonic generation.     

Second harmonic generation investigation on electric poling effects in fused silica

The influence of electric of poling conditions on the optical nonlinearity of fused silica has been investigated by the second harmonic generation (SHG) technique. The SHG intensity of the poled fused silica increased monotonically with increasing the poling voltage or poling time, and reached a maximum at a poling temperature of T = 250°C, but the SHG intensity decreased quickly as the thermal erasure time increased. The experimental results have been explained using an electric field induced dipole orientation model.     

Hole trapping in polydiacetylene field effect transistor studied by optical second harmonic generation

Hole trapping in polydiacetylene field effect transistor (PDA-FET) was studied by the electric field induced second harmonic generation (EFISHG). Response of SHG signal from PDA-FET with an application of external voltage was monitored. Applying positive voltage to source and drain electrodes with respect to gate electrode, SHG signal was not observed during bias application, whereas the signal was enhanced after turning off the bias. Since positive bias promotes hole injection from source and drain electrodes, electric field formed by trapped holes in PDA layer activated the SHG signal. Microscopic SHG measurement implies that the trapped holes are concentrated around source and drain electrodes.     

Second harmonic generation in self-assembled ZnO microcrystallite thin films

The second-order non-linear susceptibility components were measured using 1.064 μm incident light for ZnO thin films of various thicknesses from 24.4 to 283 nm self-assembled on sapphire substrates by laser molecular beam epitaxy. It was found that the values of the non-linear susceptibility for the films are almost the same as those of bulk material, except the samples with thicknesses ranging from 35 to 64.8 nm, which show a large enhancement effect. For the sample with a thickness of 44.4 nm, the second-order non-linear susceptibility components were found to be approximately 14.7 pm/v for d₃₁, 15.2 pm/v for d₁₅, and −83.7 pm/v, a value approximately 14 times that of the bulk material, for d₃₃. The second-order non-linear coefficient enhancement in the thin films may be resulted from the microcrystallite structures.     

Broadband second harmonic generation in an imperfect nonlinear photonic crystal with random defects

In this paper, we study broadband second harmonic generation (SHG) in an imperfect nonlinear photonic crystal in which defects are introduced with random lengths. We show that the efficient SHG output is obtained when the length of each defect varies near certain specialized values. The bandwidth of the SHG output broadens with the increasing randomness of defect length. Moreover, the SHG bandwidth is nearly unaffected only when the total length of the whole structure is long enough. The disordered structure also exhibits good tolerance to the fabrication error, which provides a way to control SHG intensity and bandwidth separately.     

Active control of highly efficient third-harmonic generation in ultrathin nonlinear metasurfaces

Active electric control of highly efficient third harmonic generation was realized in an ultrathin nonlinear metasurface by using a nanocomposite consisting of gold nanoparticles dispersed in polycrystalline strontium titanate as the electro-optic material. Owing to the nonlinearity enhancement associated with the slow light effect, quantum confinement effect, and field-reinforcement, a high conversion efficiency of 3 × 10⁻⁵ was obtained, which is two orders of magnitude larger than previously reported efficiencies at comparable pump intensities. A modulation of 12% in the intensity of the third harmonic generation and a 30-nm shift in the transparency window center were achieved by varying the applied voltage from −30 V to zero. Our results pave the way toward the realization of multi-functional integrated photonic devices and chips based on metasurfaces.     

Second harmonic generation diagnostic of layer-by-layer deposition from Disperse Red 1 – functionalized maleic anhydride copolymer

Layer-by-layer (LBL) electrostatic assembly of polyelectrolytes is proving to be an increasingly rich and versatile technique for the formation of multilayered thin films with a wide range of electrical, magnetic, and optical properties. In the present work we synthesized a new non-linear optical (NLO) maleic acid copolymer containing Disperse Red 1 moieties, built-up multilayer assemblies by alternate adsorption of poly(allylamine hydrochloride) (PAH) and maleic copolymer derivative, and carried out an investigation on their second harmonic generation (SHG) properties. The resulting multilayer assemblies exhibit SHG which arises from the non-centrosymmetric alignment of the chromophores in the copolymer. The SHG signal increases with the number of chromophore-containing polymer layers, up to five layers. Further assembly reduces the signal.     

Molecular orientation of oxacyanine dye in an LB film determined by second harmonic generation and polarized absorption techniques

The molecular orientation of an oxacyanine dye (DOOC) in an LB film has been determined based on the second harmonic generation (SHG) using a fs-pulsed Ti-sapphire laser and the polarization absorption spectrometry. Both polarization and wavelength dependence of SHG varied with surface concentration of the dye. The dye was either in a monomer, a dimer or a J-aggregate form depending on the surface concentration. The tilt angle to the surface normal was 34° for the monomer, 26–29° for the dimer and 16–23° for the J-aggregate. The J-aggregate stayed more flat on surface and showed larger SHG intensity than the monomer did.     

Theoretical modeling on resonating the second harmonic for ultraviolet laser generation

The resonance of the second harmonic was proposed to improve conversion efficiency in ultraviolet laser. A schematic to generate the fourth harmonic laser with a compound cavity was modeled based on plane-wave approximation. The simulated results show that the mode-matching condition of the multi-mode lasers depends strongly on both frequency-mode spacing and resonator loss. In the case of the same cavity lengths and losses, the tolerance on the optical cavity length should be maintained on tens or hundreds of nanometer scales. Meanwhile, the TEM₀₀ beam can be achieved due to the best mode-matching at resonance and the beam size increases arising from the cavity detune.     

An efficient method for analyzing second harmonic generation with the consideration of pump depletion

We propose a simple method to analyze second harmonic generation (SHG) in general quasi-phase-matching media with the consideration of pump depletion. It allows us to conveniently predict precise result from undepleted pump approximation solution. Based on this method, multi-wavelength generation with arbitrary target efficiency and wavelength spacing is engineered. The effect of pump depletion on the performance of constructed multi-wavelength generator is analyzed. We demonstrate that it is efficient to design nonlinear device for multi-wavelength SHG by proposed method.     

Large enhancement of second harmonic generation in highly cerium doped lead-phosphate glass

The influence of lead and cerium concentration on photoinduced second harmonic generation (SHG) in lead-phosphate glasses has been studied. It has been shown that SHG can be considerably enhanced by increasing of lead and cerium concentration. We also have found that cerium greatly extends the induced nonlinearity lifetime. Physical mechanism of the phenomenon is discussed.     

聚合物薄膜厚度和极化工艺对二次谐波的影响

使用马克条纹和实时极化方法对掺杂型聚合物( DR1/PMMA )薄膜及其极化工艺进行了深入的研究。实验表明,在一定温度下,如果聚合物薄膜预烘时间不足,则极化时二次谐波强度很弱,极化驰豫也很快;预烘时间过长对二次谐波强度变化没有明显影响。也证实薄膜厚度在相干长度下,二次谐波强度最大。     

Probing and modeling of carrier motion in organic devices by optical second harmonic generation

We here report a novel optical second harmonic generation (SHG) measurement that allows an electric field formed in organic solid to be probed. We examined the SHG intensity profile that changes depending on a space charge field caused by carrier injection. Experiments making use of time-resolved SHG technique has revealed dynamic changes of SHG intensity profiles arising from pentacene, and that carrier transport in OFET was diffusion-like. Calculations using drift-diffusion equation well accounted for the visualized carrier motion probed by time-resolved SHG. That is, interface charge propagation process dominates the carrier transport, which is regulated by applied gate-source voltage.     

Estimating the plasmonic field enhancement using high-order harmonic generation: the role of the field inhomogeneity

In strong field laser physics it is a common practice to use the high-order harmonic cutoff to estimate the laser intensity of the pulse that generates the harmonic radiation. Based on semiclassical arguments it is possible to find a direct relationship between the maximum value of the photon energy and the laser intensity. This approach is only valid if the laser electric field driving the high-order harmonic radiation is spatially homogeneous. In laser–matter processes driven by plasmonics fields, the enhanced fields present a spatial dependence that strongly modifies the electron motion and consequently all the associated laser driven phenomena. As a result, this method should be revised in order to more realistically estimate the intensity of the laser field. In this work, we demonstrate how the inhomogeneity of the enhanced plasmonic fields will affect this estimation. Furthermore, by employing both quantum mechanical and classical calculations, we show how one can obtain a better estimation for the intensity of the enhanced field in plasmonic nanostructures.     

Quantum wavepacket exploration of isolated high-order harmonic attosecond pulse generation: from two-color scheme to three-color scheme

Isolated attosecond generation under the two-color scheme and three-color scheme are investigated theoretically by use of wave-packet dynamics calculation. Optimized relative phase under the two-color field is obtained and comparison is taken between the second-harmonic control and half-harmonic control. A further exploration of the three-color scheme by adding a weak second-harmonic pulse and a half-harmonic pulse onto 8×10¹⁴?W?cm^?2 6?fs/800?nm pulse shows the potential for generating the 337?eV continuum supporting creation of an isolated 11 as pulse.     

Effect of subphase divalent ions on optical second harmonic properties of hemicyanine Langmuir-Blodgett films interleaved with compatible spacer

The surface pressure-area (π-A) isotherms of 1.10-bistearyl-4.6.13.15-tetraene-8-nitrogen-crown-6 (NC) monolayers at the air-water interface, the UV-visible absorption spectra and optical nonlinear properties of Y-type hemicyanine/NC interleaving Langmuir-Blodgett (LB) multilayers containing divalent ions in the subphase (such as Cd²⁺, Ba²⁺ and Co²⁺) were investigated in the paper. It was found that the π-A isotherms of the NC monolayer, the peak position of absorption spectra and the optical second harmonic intensity in the alternating LB multilayers strongly depend on the divalent ions in the subphase. The 2ω resonance effect of molecular hyperpolarizability β was responsible for the change of the observed second order polarizability χ⁽²⁾ of LB multilayers.