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.     

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 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.     

Nonlinear infrared plasmonic waveguide arrays

The large negative permittivity of noble metals in the infrared region prevents the possibility of highly confined plasmons in simple waveguide structures such as thin films or rods. This is a critical obstacle to applications of nonlinear plasmonics in the telecommunication wavelength region. We theoretically propose and numerically demonstrate that such limitation can be overcome by exploiting inter-element coupling effects in a plasmonic waveguide array. The supermodes of a plasmonic array span a large range of effective indices, making these structures ideal for broadband mode-multiplexed interconnects for integrated photonic devices. We show such plasmonic waveguide arrays can significantly enhance nonlinear optical interactions when operating in a high-index, tightly bound supermode. For example, a third-order nonlinear coefficient in such a waveguide can be more than three orders of magnitude larger compared to silicon waveguides of similar dimensions. These findings open new design possibilities towards the application of plasmonics in integrated optical devices in the telecommunications spectral region.

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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.     

Numerical simulation of cascaded third harmonic generation for both phase matched and mismatched schemes

Numerical solutions to the nonlinear coupling-wave equations of second harmonic (SH) and third harmonic (TH) generators are investigated for both phase matched and phase mismatched configurations. For phase mismatched TH generation, several kinds of schemes (the phase mismatch either in second harmonic generation (SHG) or sum-frequency generation (SFG) process) are considered and analyzed. The physical nature corresponding to the different ratios of the coupling coefficients is discussed.     

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.     

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 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.     

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.     

Electron trajectory control of odd and even order harmonics in high harmonic generation using an orthogonally polarised second harmonic field

We investigate quantum trajectory control in high-order harmonic generation using an additional orthogonally polarised second harmonic field. By controlling the relative phase between this field and the fundamental, we are able to suppress and enhance particular electron trajectories which results in a modulation of the harmonic emission. We observe a phase shift of the modulation between the short and long trajectories that is different for adjacent odd and even harmonics. These results show qualitative agreement with a full propagation calculation where the single atom response was obtained from the strong field approximation and the main results are explained by consideration of a single atom quantum orbit picture.     

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.     

Mid-infrared supercontinuum generation in tapered As2S3 chalcogenide planar waveguide

Abstract

We numerically demonstrate mid-infrared supercontinuum generation in a non-uniformly tapered chalcogenide planar waveguide. This planar rib waveguide of As₂S₃ glass on MgF₂ is 2 cm long with increasing etch depth longitudinally to manage the total dispersion. This waveguide has zero dispersion at two wavelengths. The dispersion profile varies along the propagation distance, leading to continuous modification of the phase-matching condition for dispersive wave emission and enhancement of energy transfer efficiency between solitons and dispersive waves. Numerical simulations are conducted for secant input pulses at a wavelength of 1.55 μm with a width of 50 fs and peak power of 2 kW. Results show this proposed scheme significantly broadens the generated continuum, extending from ~1 to ~7 μm.     

Second-order quasi-phase matched second harmonic generation in annealed proton-exchanged LiNbO3 channel waveguides

We report the first efficient blue second harmonic generation (SHG) in a second-order quasi-phase matching (QPM) LiNbO₃ waveguide. We show both theoretically and experimentally that the second-orderQPM grating is the optimum structure for waveguideSHG in LiNbO₃ waveguides. Using such a structure, we have demonstrated blue light generation with a conversion efficiency of 65%/W-cm², the highest reported so far in LiNbO₃ waveguides.     

Exploring a photonic quasi-crystal fiber for enhancing the efficiency of second harmonic generation: Modeling and analysis

In this paper, we explore the possibility of using a new type of microstructured optical fiber called photonic quasi-crystal fiber (PQF) for enhancing the efficiency of second harmonic generation (SHG). We model a six-fold, index guided, silica core PQF with a relative air hole diameter ranging from 0.1–0.5. We compute the various factors related to SHG, namely, phase mismatch factor, wave-vector mismatch, coherence length, quasi-phase matching length and overlap area. Of the various factors, we focus more on the role of wave-vector mismatch and overlap area as these parameters are highly sensitive to the variations in the geometrical arrangement of air holes in the cladding. Further, these parameters are required to be as minimum as possible for enhancing the efficiency of SHG. We find that the wave-vector mismatch factor does decrease for higher values of pitch and for lower values of relative air hole diameter. However, the overlap area increases for higher values of pitch and for lower values of relative air hole diameter. Therefore, we optimize the hitherto mentioned parameters such that the wave-vector mismatch is 0.145 μm⁻¹ and the overlap area is 3.48 μm². With these values, we demonstrate a relative efficiency of 62.96% W⁻¹ cm⁻² with the proposed PQF.     

Below-threshold harmonic generation from strong non-uniform fields

Strong-field photoemission below the ionization threshold is a rich/complex region where atomic emission and harmonic generation may coexist. We studied the mechanism of below-threshold harmonics (BTH) from spatially non-uniform local fields near the metallic nanostructures. Discrete harmonics are generated due to the broken inversion symmetry, suggesting enriched coherent emission in the vuv frequency range. Through the numerical solution of the time-dependent Schrödinger equation, we investigate wavelength and intensity dependence of BTH. Wavelength dependence identifies counter-regular resonances; individual contributions from the multi-photon emission and channel-closing effects due to quantum path interferences. In order to understand the underlying mechanism of BTH, we devised a generalized semi-classical model, including the influence of Coulomb and non-uniform field interactions. As in uniform fields, Coulomb potential in non-uniform fields is the determinant of BTH; we observed that the generation of BTH are due to returning trajectories with negative energies. Due to large distance effectiveness of the non-uniformity, only long trajectories are noticeably affected.