Spiral-chirped nonlinear photonics enabling broadband orbital angular momentum second harmonic generation.

In this paper, noise characteristics of second-harmonic generation (SHG) in periodically poled lithium niobate (PPLN) using the quasi-phase-matching (QPM) technique are studied experimentally. In the experiment, a 0.78-mm second-harmonic (SH) wave was generated when a 1.55-mm fundamental wave passed through a PPLN crystal (bulk or annealed proton-exchanged waveguide). The fundamental and SH waves were then separated through a beam splitter and sent to two photodetectors, respectively. The time-domain and frequency-domain characteristics of the fundamental and SH waves were analyzed. By using the pump-probe method, the noise characteristics were studied further when 532-nm irradiation light co-propagated with the 1.55-mm fundamental light in the PPLN crystals. It is found that for the bulk and waveguides of PPLN crystals, the SH wave has a higher relative noise level than the corresponding fundamental wave. For the same fundamental wave, the SH wave has lower noise in a bulk crystal than in a waveguide, and in MgO-doped PPLN than in undoped PPLN. In addition, the photorefractive effect incurred by the irradiation light can influence the SHG noise.

The grating-assisted nonlinear directional couplers are investigated by using the singular perturbation technique with multiple space scales. One of the waveguides of the nonlinear directional couplers has second order nonlinear cladding whose surface has a grating profile. The period of the grating is determined to satisfy the quasi-phase matching condition between the fundamental wave and second harmonic (SH) wave. From the solvability condition to have nontrivial solutions for each order, the coupled-mode equations between fundamental and SH waves are derived. In the numerical examples, lithium niobate (LiNbO/sub 3/) is chosen as second-order nonlinear medium. After the coupled-mode equations are examined numerically, it is found that the SH wave separates from the fundamental wave due to the difference of the coupling length between the fundamental and SH waves.

The visibility of inverted domain structures using the second harmonic generation (SHG) microscope is discussed based upon the three-wave coupling equations. Without reference second harmonic (SH) waves (non-interference case), the SHG intensity at a domain boundary decreases steeply along the perpendicular direction to the domain wall, which is observed as a dark line. Thus the non-interference SHG microscope reveals the location of domain boundaries but not the domain polarity which the SHG interference microscope can do. In the case of periodically poled domain structures, the clear observation criterion is expressed with the wave-number offset between the fundamental and SH waves.

The transient oscillatory behavior of the nonlinear response of a two-level electron-spin system is experimentally investigated in a sample of glassy silica with ${E}_{1}^{\mathcal{'}}$ centers (S=(1/2)) at microwave frequency at T=4.2 K. The transient regime, excited by an intense step-modulated radiation tuned to double-quantum (DQ) resonance, is monitored by revealing the second-harmonic (SH) wave radiated by the spins undergoing DQ transitions. Time- and frequency-domain results show that the emitted SH wave has two components: the former, which vanishes at the DQ resonance, exhibits an overdamped transient regime, the latter consists of damped oscillations at a frequency which depends on the intensity and the polarization of the input radiation but not on its detuning from the DQ resonance condition. Using a vectorial model of the DQ resonance in an S=1/2 spin system we relate the observed oscillatory behavior to the transient nutations induced by the DQ processes; the calculated DQ Rabi frequency and its dependence on the input radiation are found in quantitative agreement with the experimental results. Moreover, we calculate that the inhomogeneous broadening of the resonance line strongly affects the time dependence and the spectral content of the emitted radiation, and in particular it makes its oscillation frequency independent of the input radiation detuning, in agreement with the experimentally observed behavior. The relationship, with recent discussions on the suitability of Bloch equations to describe coherent effects in randomly diluted solids, is also examined.

High-order harmonic generation (HHG) has been studied in various laser-produced plasma plumes using a two-color orthogonally polarized beam with a 12:1 energy ratio between the fundamental and second-harmonic (SH) components. The influence of the relative phase between the fundamental and SH waves on the HHG efficiency has been investigated. Odd and even harmonic generation in plasma plumes containing nanoparticles, fullerenes, carbon nanotubes, and other samples was optimized. The effect of the variation in the SH intensity on the HHG conversion efficiency in carbon aerogel and silver plasma plumes has also been studied. It is shown that by increasing the SH intensity, one can generate only even harmonics by suppressing the odd harmonics.

A modal method for the study of second harmonic (SH) generation in nanostructures is depicted both for plane waves and focused beam illumination. It is based on the B-spline modal method, and relies on the undepleted pump approximation. The nonlinear polarization induced by an incident plane wave or a focused beam generates a source term at the doubled frequency. The latter is divided into a finite number of sub-sources, and the SH field is subsequently computed by integration of these sub-source contributions.

Lithium niobate (LN) is a favourable material for many applications, especially in integrated optics, due to its excellent electro-optic, acousto-optic, and nonlinear optic properties. One promising application of LN is second-harmonic generation (SHG). For enhancing the efficiency of SHG, phase matching between the interacting waves is needed, which is often realized by employing quasi-phase matching (QPM) [1]. QPM is typically realized by periodic poling of LN [2,3], i.e. the periodic flipping of the crystal orientation. The needed period is determined by the wavelengths as well as by the wave vectors of the fundamental- and the second-harmonic waves (FW and SH, respectively). For the case of backward SHG with a fundamental wavelength of 1.55 μm, where the SH wave travels in the opposite direction to the FW, typically sub-micrometer periods of the poled LN are required [4].

We report on spatial mode behavior of the second harmonic (SH) generated by nonlinear interaction in the a z-cut Mg-doped periodically poled LiNbO3 (MgO:PPLN) ridge-type waveguide. Specific SH modes were selectively generated by mode mixing of the fundamental harmonic (FH) waves. In addition, high-order SH modes TM10 and TM01 were simultaneously generated under similar phase-matching conditions in a quasi-phase matching (QPM) grating structure. The interfered mode profiles of high-order SH modes demonstrated specific mode distribution to have a diagonal angle and the Laguerre–Gaussian (LG) mode at a certain phase-matching condition obtained by temperature control.

Optical poling is a very simple technique for inducing a quadratic nonlinearity in germanium-doped silica fibers [1] , [2] . By injecting into the fiber fundamental frequency (FF) pulses together with a low power second harmonic (SH) seed, a multi-photon quantum interference process naturally generates a quadratic nonlinearity whose period satisfies the quasi-phase matching (QPM) condition. The nonlinear grating writing process takes a few hours before showing a saturation of the SH conversion efficiency, which is always lower than the best values achievable by resorting to thermal poling [3] . Also, the maximum length of the poled region exhibits saturation, and this hampers the SH generation [4] .

The domain switching behaviour of undoped and Ce-doped Sr0.61Ba0.39Nb2O6 (SBN) crystals is investigated for different experimental conditions. For the investigation the photorefractive self-focusing technique is used. This method is demonstrated to be well suited for determining the polarization state of SBN crystals spatially resolved. We find that the repoling behaviour of nominally pure SBN is spatially inhomogeneous. The crystals are well repoled at the positive electrode, but are depolarized at the negative electrode. The larger the Ce doping concentration is, the smaller the depolarized crystal area. The domain switching depends on the switching velocity, while no influence of the electrode material is observed.

The combination of scanning near-field optical microscopy (SNOM) and second-harmonic generation (SHG) spectroscopy provides a superior way, sensitive on an atomic scale, to analyze surface structures in a subwavelength resolution. We apply the self-consistent integral equation formalism to investigate the optical near field of SHG for mesoscopic nonlinear surface structures excited by a total internal-reflection beam. SHG near field is shown to strongly depend on the polarization state of excitation light. However, in various polarization states, they all exhibit a much stronger lateral localization effect than fundamental wave. This contrast can be attributed to the different interference features of elementary and secondary near field for fundamental and second-harmonic waves. Such a strong three-dimensional localization effect of SHG near field is of much convenience to an experimental observation of mesoscopic surface structures with use of SNOM.

The vorticity of a free propagating second harmonic (SH) beam produced in a nonlinear quadratic crystal by a combined beam composed of two coaxial Laguerre–Gaussian vortex beams is analysed. In the near field of the SH radiation an aligned vortical structure with a number of double-charge vortices is created. The diffraction of the SH beam under free propagation is also discussed.

Vortex waves (VW) carrying orbital angular momentum (OAM) have azimuth localization property based on phase variation. A phased array (PA) with VW generating elements combine the properties of PA and the vortex array (VA) for angular localization of targets. We have also exploited VW Gaussian beam (GB) generating circular array (CA) in addition to the classical Laguerre-Gaussian (LG) type beams. A rigorous theory of OAM is outlined by the multipole expansion of the spherical vector wave functions (SVWF). Range localization using matched filtering based on SVWF azimuthal mode number (topological charge) is proposed. It is shown that this scheme can be also combined with intrapulse phase coding for a better pulse compression. Two dimensional (2D) angular localization method based on multiple topological charge transmit/receive (TX/RX) patterns is described.

The influence of a short intense (with an electric field strength up to 250 kV cm−1) terahertz (THz) pulse on the generation of second harmonic (SH) of Ti : sapphire laser radiation in crystals of zinc blende type (InAs and GaAs), characterised by nonzero bulk quadratic susceptibility, is investigated. It is experimentally shown for InAs(100) that, in the case of s-polarised first and second harmonics, an application of s-polarised THz field changes significantly the SH signal. The THz field-induced azimuthal dependence of the SH signal is in good agreement with the results of theoretical calculation within a phenomenological approach. The dependence of the SH signal on the delay time between the optical and THz pulses is investigated. This dependence for the GaAs crystal repeats the envelope of the THz pulse intensity, whereas in the case of InAs crystal there is a significant discrepancy, caused by the nonlinear dynamics of strong THz field in InAs.

Highly brilliant diode lasers at 1120nm with a high optical output power, nearly diffraction limited beam and narrow spectral line width are increasingly important for non-linear frequency conversion to 560 nm. We present experimental results about edge-emitting distributed Bragg reflector (DBR) tapered diode lasers emitting at 1120 nm. The investigated lasers show an output power of up to 8W with a conversion efficiency of about 40%, and a lifetime of more than 5000 h at 5 W. The lasers also exhibit a small vertical divergence <15° full width at half maximum (FWHM), a nearly diffraction limited beam quality, and a narrow spectral line width with FWHM smaller than 10pm. These properties allow the lasers to be used for future second harmonic (SH) generation.