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.     

Reflective-type configuration for nearly phase-matching cerenkov second-harmonic generation in a nonlinear-optical polymer waveguide

A new technique for achieving efficient Cerenkov-type second-harmonic generation (SHG) in a nonlinear-optical (NLO) polymer waveguide is presented. The configuration, which can prevent the losses of light caused by relatively long-distance propagation and the multiple reflections that appear in the conventional Cerenkov technique, exhibits ease of fabrication and compactness. We experimentally observed a conversion efficiency of 1.6% W(-1) cm(-1), which to our knowledge is the highest value reported for Cerenkov SHG in polymer, by tuning both the thickness and the refractive index of the polymer film close to phase matching between a guided fundamental wave and a guided harmonic wave. The experimental results agreed well with the theoretical prediction.     

Effect of dimension variation for second-harmonic generation in lithium niobate on insulator waveguide

We study the effect of dimension variation for second-harmonic generation[SHG]in lithium niobate on insulator[LNOI]waveguides.Non-trivial SHG profiles in both t...     

Photonic band-gap enhanced second-harmonic generation in a planar lithium niobate waveguide

Enhanced second-harmonic generation (SHG) conversion efficiency was theoretically predicted in waveguide geometry with coupling to a one-dimensional grating photonic band gap (PBG). We report a series of experiments using samples made with lithium niobate. A waveguide was fabricated near the surface by applying the proton-exchange technique. The characteristics of waveguide modes were determined by several techniques: prism coupling, diffraction, and Cherenkov radiation. The WKB method was used to analyze the results. Ultraviolet laser lithography was applied to make PBG gratings on the sample. We further investigated Cherenkov second-harmonic generation (CSHG), i.e., SHG radiated into the substrate, under the condition of a band-edge PBG resonance in the waveguides. The SHG inside planar waveguides was also experimentally investigated. We fabricated waveguides with multiple pump modes and found that the second mode was more efficient in enhancing the second harmonic signal. This result is explained by our model. Several samples were investigated in detail; the highest conversion efficiency of CSHG with a PBG was enhanced around 50 times above the CSHG signal without a PBG. A numerical model was constructed with parameters calculated from our sample characterization data to interpret the experimental results.     

Low-voltage tunable second-harmonic generation in an x-cut periodically poled lithium niobate waveguide

We report a new method of tuning the second-harmonic signal generated in a periodically poled lithium niobate (PPLN) waveguide. This technique relies on the recent progress in the fabrication of PPLNs on x-cut substrates along with periodical coplanar electrodes to reduce the tuning voltage while preserving the conversion efficiency. Our scheme exploits a type I interaction implemented in a titanium waveguide to impede cancellation of the electro-optical coefficients and to minimize the group-velocity mismatch between the first- and second-harmonic modes. The tuning range covers 58 nm with an applied voltage of +/-150 V.     

Optical second-harmonic generation in a thin-film waveguide without control of film thickness

An optical second-harmonic wave can be generated in a three-layered optical waveguide of arbitrary film thickness under phase-matching condition. Phase-matching is achieved by utilizing mode dispersion in an optical waveguide with a two-dimensional configuration. A numerical analysis of the matching condition is given and the results of the observation of the optical second-harmonic wave of a guided Nd : YAG laser in a Ti-indiffused LiNbO₃ nonlinear film are described.     

Broadening of the phase-matching bandwidth in quasi-phase-matched second-harmonic generation using GaN-based Bragg reflection waveguide

We present an analysis of a high index core symmetric Bragg reflection waveguide (BRW) design based on a GaN/AlxGa1-xN system for efficient quasi-phase-matched second-harmonic generation for broadband applications. By choosing the fundamental frequency to be a BRW mode and suitably tailoring the overall dispersion characteristics, the strong dispersion of the second-harmonic mode is partially canceled, leading to phase matching between the fundamental and second-harmonic over a broad range of wavelengths. The crucial interplay between the dispersive behavior of the fundamental and second-harmonic wave manifests as a broad acceptance bandwidth of approximately 33 nm accompanied with appreciable conversion efficiency (22.8%/W) for a 10 mm long waveguide. The impact of tailoring the dispersion characteristics on the conversion efficiency is also discussed.     

Quasi-phase-matched second-harmonic generation in a GaAs/AlAs superlattice waveguide by ion-implantation-induced intermixing

We report type I second-harmonic generation by use of first-order quasi-phase matching in a GaAs/AlAs symmetric superlattice structure with femtosecond fundamental pulses at 1.55 microm. Periodic spatial modulation of the bulklike second-order susceptibility chi(zxy)(2) was achieved with quantum-well intermixing for which the group III vacancies were created by As+-ion implantation. A narrow second-harmonic bandwidth of approximately 0.9 nm (FWHM) with an average power of approximately 1.5 microW was detected, corresponding to an internal conversion efficiency of approximately 0.06%, which was considerably limited by the spectral bandwidth of the fundamental.     

Backward second-harmonic and third-harmonic generation in a periodically poled potassium titanyl phosphate waveguide

We observed backward second-harmonic and backward third-harmonic generation in a periodically poled KTiOPO(4) waveguide using nanosecond laser pulses. The highest conversion efficiency achieved for the backward second-harmonic generation, occurring at the 25th-order grating, was ~0.6% . The backward third-harmonic generation was the result of mixing the pump beam with the forward or the backward second-harmonic beam. Conversion efficiency of ~0.4% was achieved at a pump wavelength of 1233.7 nm, where the two constituent nonlinear processes are both quasi-phase matched.     

Amplitude squeezing by means of quasi-phase-matched second-harmonic generation in a lithium niobate waveguide

We demonstrate that traveling-wave second-harmonic generation produces amplitude-squeezed light at both the fundamental and the harmonic frequencies. Quasi-phase-matched second-harmonic conversion efficiencies approaching 60% were obtained in a 26-mm-long single-mode LiNbO(3) waveguide with pulses from a mode-locked laser at 1.53 microm. The amplitude noise of the transmitted fundamental field was measured to be 0.8 dB below the shot-noise level, and the generated 0.765-microm harmonic light was measured to be amplitude squeezed by 0.35 dB. The conversion-efficiency dependence of the observed squeezing at both wavelengths agrees with theoretical predictions. Waveguide losses appear to degrade the squeezing, but the maximum observed squeezing is currently limited only by the available input power.     

Enhanced second-harmonic generation by means of high-power confinement in a photovoltaic soliton-induced waveguide

We present the first experimental demonstration of enhanced second-harmonic generation (SHG) by means of power confinement with a femtosecond laser in a photovoltaic soliton-induced waveguide. A dark spatial soliton created with a weak cw laser beam in a photovoltaic lithium niobate crystal induces an efficient waveguide for SHG, leading to a 60% enhancement of the conversion efficiency.     

Polarization-vortex-driven second-harmonic generation

Polarization vortices exist in the focus of a class of vector beams, the lowest order of which possess full vector symmetry about the axis of propagation of the beam. At high numerical apertures these beams are known to exhibit large, local, longitudinal fields in the focal region. At an interface these fields can be many times stronger than the largest available transverse component and are therefore candidates for a variety of different experiments in surface physics. The observation of vortex-driven surface second-harmonic generation at smooth metal and semiconductor surfaces and thin films is reported. By comparing the response to that of a purely transverse field, we show that the smooth surface responds primarily to the longitudinal field component.     

Effect of dimension variation for second-harmonic generation in lithium niobate on insulator waveguide[Invited]

We study the effect of dimension variation for second-harmonic generation(SHG) in lithium niobate on insulator(LNOI)waveguides. Non-trivial SHG profiles in both type-0 and type-I quasi-phase matching are observed during the wavelength tuning of the fundamental light. Theoretical modeling shows that the SHG profile and efficiency can be greatly affected by the waveguide cross-section dimension variations, especially the thickness variations. In particular, our analysis shows that a thickness variation of tens of nanometers is in good agreement with the experimental results. Such investigations could be used to evaluate fabrication performance of LNOI-based nonlinear optical devices.     

Near-surface second-harmonic generation for autocorrelation measurements in the uv

Autocorrelation measurement of ultrashort uv laser pulses using non-phasematched second-harmonic generation from the surface of nonlinear crystals is described. The technique is demonstrated for wavelengths from 378–300 nm and pulse durations of 30 ps–300 fs.     

Optimization of second-harmonic generation microscopy

We describe the principles and characteristics of second-harmonic generation imaging (SHGI) and explore various methods for optimization of the technique. Second-harmonic imaging is optimized for ultrashort laser pulses, high numerical aperture microscope objectives, a highly sensitive non-descanned large area detector, pseudo-phase-matching, and specimens with large second-order non linearity or which exhibit surface plasmon enhanced phenomena. We also compare and contrast the techniques of SHGI and two-photon excited fluorescence imaging.     

IR-poled second-harmonic generation in glass

Infrared (IR)-induced second-harmonic generation in the chalcogenide glasses is observed. A phenomenological approach of IR picosecond non-linear optical (NLO) response in glass is developed for the middle IR spectral range (5–15 μm). The observed effect is explained within the framework of fifth-order NLO susceptibilities. A model that reproduces the basic characteristics of the experimental data, in which the optical non-linearities caused by photoinduced electron–phonon anharmonic interactions, is proposed. The role of the IR-induced phase matching conditions in the observed phenomenon is discussed.     

Development of a quasi-phase-matched, second-harmonic generation periodically poled lithium niobate waveguide with an integrated electro-optical modulator

A novel type of waveguide quasi-phase-matched device with output electro-optical (EO) modulation is proposed and demonstrated. The second-harmonic generated output can be modulated with a high extinction ratio by integration of dual EO phase modulators on a periodically poled lithium niobate waveguide. Blue and green coherent light with EO modulation was demonstrated, and wavelength dispersion of r33 is discussed.     

Observation of 99% pump depletion in single-pass second-harmonic generation in a periodically poled lithium niobate waveguide

We report 99% pump depletion in single-pass second-harmonic generation. Quasi-cw pulses at 1550 nm were frequency doubled in an annealed proton-exchanged waveguide formed in periodically poled lithium niobate. Measurements of pump depletion and second-harmonic generation agree with results from numerical integration of the coupled-mode equations that describe the process.