Efficient second-harmonic generation in nonlinear plasmonic waveguide

We theoretically studied a nonlinear optical process in a hybrid plasmonic waveguide composed of a nonlinear dielectric waveguide and a metal film with a separation of a thin air gap. Owing to the hybridization effect of guided mode and surface plasmon polariton mode, this particular waveguide is able to confine the optical-field in a deep subwavelength scale together with low propagation loss. Based on this, efficient second-harmonic generations (SHG) were revealed at the fundamental wavelength of λ=1.55?μm with good field confinement. The SHG efficiency, as well as the coupling coefficient and mode area, were analyzed and discussed in detail with respect to the structural parameters.     

Two-color nonlinear localized photonic modes

We analyze second-harmonic generation (SHG) at a thin effectively quadratic nonlinear interface between two linear optical media. We predict multistability of SHG for both plane and localized waves, and also describe two-color localized photonic modes composed of a fundamental wave and its second harmonic coupled together by parametric interaction at the interface.     

990 mW average power, 52% efficient, high-repetition-rate picosecond-pulse generation in the blue with BiB3O6

Efficient single-pass second-harmonic generation (SHG) of tunable high-repetition-rate picosecond pulses into the blue is reported in the nonlinear crystal BiB3O6. Using 2.4 ps fundamental pulses from a mode-locked Ti:sapphire laser at 76 MHz and a 10 mm crystal cut for type I (e + e --> o) phase matching in the optical yz plane, second-harmonic average powers as high as 990 mW with excellent stability have been generated at 52% conversion efficiency, and a tunable range of 370-450 nm is demonstrated. From measurements of single-pass SHG in the continuous-wave regime an effective nonlinear coefficient of 3.7 pm/V has been verified for BiB3O6, and direct comparison with beta-BaB2O4 confirms a SHG power enhancement of approximately 23% for the same crystal length. Autocorrelation measurements in a 200 microm crystal of beta-BaB2O4 result in durations of 2.8 ps for the second-harmonic blue pulses.     

Simultaneous multichannel nonlinear imaging: combined two-photon excited fluorescence and second-harmonic generation microscopy

Simultaneous two-photon excited fluorescence (TPF) and second-harmonic generation (SHG) imaging is demonstrated using a single femtosecond laser and a scanning microscope. This composite nonlinear microscopic technique was applied to imaging DNA and chromosomes, and it was shown that the two different interaction mechanisms provide complementary information on the structure and nonlinear properties of these biological materials, beyond that achievable using either TPF or SHG imaging alone. The use of separate modes of detection, in reflection and transmission respectively, and the simultaneous nature of the acquisition of the two images allows pure TPF and SHG images in precise registration to be obtained.     

Influence of Cascaded Nonlinear Phase Shifts on Second-Harmonic Generation in High-Intensity Pumped QPM Structures

Cascaded nonlinear phase shifts may be imposed on the interacting waves during second-harmonic generation （SHG） in a quasi-Phase-matched （QPM） structure, whieh are severe in high-intensity regime and may result in lower conversion efficiency. We propose a configuration of QPM structure with reduced domain-length, which may depress the nonlinear phase shifts to some extent and lead to an improvement on the conversion efficiency. The numerical analyses on the conversion efficiency as well as the relative phase angle are discussed in detail for better understanding of the SHG process.     

Single-beam and enhanced two-beam second-harmonic generation from silicon nanocrystals by use of spatially inhomogeneous femtosecond pulses

Optical second-harmonic generation (SHG) is used as a noninvasive probe of the interfaces of Si nanocrystals (NCs) embedded uniformly in an SiO2 matrix. Measurements of the generated SH mode verify that the second-harmonic polarization has a nonlocal dipole form proportional to (E x Delta inverted) E that depends on inhomogeneities in the incident field E, as proposed in recent models based on a locally noncentrosymmetric dipolar response averaged over the spherical NC interfaces. A two-beam SHG geometry is found to enhance this polarization greatly compared to single-beam SHG, yielding strong signals useful for scanning, spectroscopy, and real-time monitoring. This configuration provides a general strategy for enhancing the second-order nonlinear response of centrosymmetric samples, as demonstrated here for both Si nanocomposites and their glass substrates.     

Lissajous singularities

Coherent optical Lissajous states are easily created by nonlinear processes such as second-harmonic generation (SHG). Singular properties of such states are discussed and illustrated theoretically with non-phase-matched SHG of an ellipse field containing a C point.     

Angle-resolved second-harmonic light scattering from colloidal particles

We report angle-resolved second-harmonic generation (SHG) measurements from suspensions of centrosymmetric micron-size polystyrene spheres with surface-adsorbed dye (malachite green). The second-harmonic scattering profiles differ qualitatively from linear light scattering profiles of the same particles. We investigated these radiation patterns using several polarization configurations and particle diameters. We introduce a simple Rayleigh-Gans-Debye model to account for the SHG scattering anisotropy. The model compares favorably with our experimental data. Our measurements suggest scattering anisotropy may be used to isolate particle nonlinear optics from other bulk nonlinear optical effects in suspension.     

Spectral phase correlation of coded femtosecond pulses by second-harmonic generation in thick nonlinear crystals

We demonstrate a novel all-optical scheme for measuring the correlation of spectrally phase-coded ultrashort optical waveforms that uses second-harmonic generation (SHG) in long, periodically poled lithium niobate crystals. The SHG yield can be controlled over a range of ~30 dB, depending on the correlation of the applied phase codes. Such a spectral phase correlator has applications for ultrashort-pulse optical code-division multiple-access networking and could serve as a nonlinear optical but classical analog for certain schemes for coherent quantum control of multiphoton processes.     

Nonlinear Photoluminescence from ZnO Nanobelts

The nonlinear photoluminescence （PL） including second-harmonic generation （SHG） and the multiphoton luminescence （MPL） around 385 nm and 530 nm from ZnO nanobelts are investigated by using near-infrared excitations. The excitation wavelength dependence of MPL intensity reveals resonant energy transfer from SHG to MPL near the band gap excitation. The lifetime measurement of the MPL shows a much slower decay process of the defect emission, which results from the generation and recombination of both donors and acceptors on the disordered surface of the ZnO nanobelts.     

Second-harmonic generation in resonant waveguide gratings incorporating ionic self-assembled monolayer polymer films

Experimental results on resonantly excited second-harmonic generation (SHG) in a periodic ionically self-assembled monolayer (ISAM) film are reported. A double-layer guided-mode resonance filter (GMRF) structure is coated with 40 bilayers of pyrlium-based chi(2) ISAM thin film and excited with the fundamental of a Nd:YAG laser. Enhanced second-harmonic conversion in the ISAM film is achieved because of the local field enhancement associated with the fundamental resonating leaky mode. This method of SHG is particularly promising, as the ISAM films under investigation exhibit anomalous dispersion that may be applied for phase matching to improve nonlinear conversion efficiency.     

Application of the nonlinear crystal SrB4O7 for ultrafast diagnostics converting to wavelengths as short as 125 nm

We used non-phase-matched second-harmonic generation (SHG) in the highly nonlinear but low-birefringence crystal SrB4O7 for temporal characterization (autocorrelation) of femtosecond UV pulses to convert these pulses to wavelengths as low as 125 nm, close to the transparency limit of SrB4O7. The sensitivity of this method was approximately 1 microJ at the fundamental for a repetition rate of 1 kHz. We also used SHG at longer wavelengths to estimate the three nonlinear coefficients of SrB4O7.     

Ultrasensitive second-harmonic generation frequency-resolved optical gating by aperiodically poled LiNbO3 waveguides at 1.5 microm

We retrieve intensity and phase profiles of 280 fs, 50 MHz optical pulses with 124 aJ coupled pulse energy (960 photons) by second-harmonic generation (SHG) frequency-resolved optical gating, using aperiodically poled LiNbO3 waveguides. The strong nonlinear interaction that is due to confinement within the micrometer-sized waveguide structure and the linearly chirped poling period contribute, respectively, to high SHG efficiency and broad phase-matching bandwidth. The achieved sensitivity is 2.7 x 10(-6) mW2, improving on the previous record for self-referenced complete pulse characterization by 5 orders of magnitude.     

Corrugated-enhanced second harmonic generation in metal–insulator–metal plasmonic waveguides

Nonlinear effects such as second-harmonic generation (SHG) are important for applications such as switching and wavelength conversion. In this study, the generation of second harmonic in metal–insulator–metal (MIM) plasmonic waveguides was investigated for both symmetric and asymmetric structures. Symmetric means that the metals at the top and bottom of the dielectric layer are the same and asymmetric means that the metals at the top and bottom of the dielectric layer are different. Two different structures are considered here as plasmonic waveguide for generation of second harmonic and analyzed using finite-difference time domain method. Besides the structure has grating on both sides for more coupling between photons and plasmons. The wavelength duration of grating per length unit (number of grooves) will be optimized to reach the highest second harmonic generation. To perform this optimization, the wavelength of operation λ = 458 nm is considered. It was shown that field enhancement in symmetric MIM waveguides can result in enhancement of SHG magnitude compared to the literature values and asymmetric device results in more than two orders of magnitude enhancement in SHG compared to symmetric structure. It is also shown that the electric field of second harmonic depends on the thickness of crystal (insulator). So, its thickness is optimized to achieve the highest electric field.     

Second-harmonic generation from nonlocal interaction between terahertz waves and step quantum wells

Second-harmonic generation (SHG) excited by a p-polarized terahertz wave interacting with AlGaAs/GaAs step quantum wells (STQW’s) is investigated on the basis of the microscopic nonlocal response theory. The numerical results show that there are two resonant peaks in the energy reflection spectrum of the SHG, whose position and amplitude are tunable via the structural parameters of the STQW’s and the incident optical intensity. It is clarified that the spatial nonlocality of the linear and second-order nonlinear optical responses can lead to a blueshift of the second-harmonic spectra, while the third-order nonlinearity can induce a spectral redshift. Furthermore, an optimal STQW is presented, for obtaining the largest SHG signal and observing the SH spectra by experiment.     

Theory of enhanced second-harmonic generation in some artificial materials

We review the recent theoretical investigation on enhanced second-harmonic generation (SHG) in soft nonlinear optical materials based on ferrofluids, graded metallic films, and graded metal-dielectric films of anisotropic particles. The SHG of soft ferrofluid-based nonlinear optical materials possess magnetic-field controllabilities, i.e., magnetic-field-controllable anisotropy, red-shift and enhancement, which are caused to appear by the shift of a resonant plasmon frequency due to the formation of the chains of the coated nanoparticles. Both graded metallic films and graded metal-dielectric films of anisotropic particles can serve as a novel optical material for producing a broad structure in both the linear and SHG response and an enhancement in the SHG signal, due to the local field effects.      

三方晶系的非线性磁光克尔谱及其表面磁化强度取向的效应

本文计算了三方结构不同表面和不同磁化强度取向下的χ⁽²⁾、SHG、MSHG及非线性克尔旋转角Φ_k,a⁽²⁾.结果表明,SHG和Φ_k,a⁽²⁾值非常灵敏地依赖于薄膜的表面及磁化强度的取向;三方结构的SHG、Φ_k,a⁽²⁾与fcc、bcc和hcp结构的相比有很大的差异,这表明结构对非线性磁光效应有重要的影响;当M//X、Y轴时,MSHG信号很明显,这表明可通过A值的测量来研究磁性薄膜表面的磁畴结构以及取向.     

Phase matching limitation of high-efficiency second-harmonic generation in both phase- and group-velocity-matched structures

In this paper, we studied efficient second-harmonic generation (SHG) of femtosecond pulses in both phase- and group-velocity-matched structures. Obtained results show that phase matching becomes more critical under conditions required for high levels of conversion efficiency. And the imperfect phase mismatch caused by mismatched group-velocity dispersion (GVD) will limit conversion efficiency as well as bandwidth of generated second-harmonic (SH) pulses. The spectral characteristics of the generated SH pulses and its conversion efficiency in the strong pump regime are investigated in detail. The acceptance bandwidth of nonlinear crystal in the high-efficiency SHG is redefined in the paper, and the definition is much closer to the practical application of design.     

Study on characteristics of nonlinear optical material InS·BaSO_4

The characteristics of the new nonlinear optical material ZnS·BaSO_4 is report-ed.Its effeciency of SHG(second-harmonic generation)in powder is about one thousand timethat of KDP.The structure properties of this material has been studied by electron diffraction.     

Linear and nonlinear optical properties of yttrium formate dihydrate,Y(HCOO)<Subscript>3</Subscript>·2H<Subscript>2</Subscript>O

Yttrium formate dihydrate, Y(HCOO)₃·2H₂O (point group 222), a promising new material for Raman laser frequency converters, has been reinvestigated with respect to the linear and nonlinear optical (second-harmonic generation, SHG) properties. High-precision data of refractive indices and their dispersion in the wavelength range 365–1083 nm are given, together with the three independent components of the tensor of nonlinear optical susceptibility (SHG) [d_ijk ^SHG]. A calculation of the macroscopic nonlinear optical susceptibility from the hyperpolarizabilities of the formate groups results in good agreement with the experimental values. This signals that the nonlinear optical interaction in Y(HCOO)₃·2H₂O can mainly be attributed to the formate groups. PACS 42.65.Ky; 42.70.Mp; 78.20.Ci