Scheme For Second Harmonic Generation Research Articles

Second-Harmonic Generation of a Short-Laser Pulse From a Gas-Jet Immersed in a Magnetic Field

A scheme for second-harmonic generation (SHG) of a short-laser pulse from a gas jet immersed in a magnetic field has been investigated. A laser pulse with sufficient intensity interacting with gas produces free electrons by tunnel ionization. The electrons accelerated by the laser pulse generate nonlinear currents at the second-harmonic (SH) frequency of the fundamental laser pulse. The ambient magnetic field provides an additional resonance to enhance the SH field amplitude. Based on the plasma current model, we estimate the SH field and provide various scaling laws for optimization and controlling the SHG process. This study might provide a new way to optimize the SH field from the laser interactions with a gas jet immersed in a magnetic field.

Second harmonic generation in AB-type LaTiO3/SrTiO3 superlattices

Abstract We report the generation and control of second harmonic generation (SHG) from a series of LaTiO3/SrTiO3 superlattices. We demonstrate that the nonlinear optical susceptibility χ(2) and the SHG pattern can be tuned by tailoring the number of superlattice interfaces. Significantly, unlike the conventional ABC-type nanostructures, our AB-type nanostructures can also produce SHG signals. The χ(2) value (24.3 pm/V) is even one to two orders of magnitude greater than the conventional ones. Hence, we demonstrate that SHG is a local microscopic process—it does not rely on macroscopic broken inversion symmetry. Moreover, we show that temperature dependence or structural phase transition can further enhance the SHG intensity. Our results introduce a scheme for SHG, extending the generation and control of SHG in nano-photonics to nearly all the accessible centrosymmetric materials.

Second-harmonic generation and manipulation in lithium niobate slab waveguides by grating metasurfaces

Nonlinear optical processes in waveguides play important roles in compact integrated photonics, while efficient coupling and manipulations inside the waveguides still remain challenging. In this work, we propose a new scheme for second-harmonic generation as well as beam shaping in lithium niobate slab waveguides with the assistance of well-designed grating metasurfaces at λ = 1064 nm . By encoding the amplitude and phase into the holographic gratings, we further demonstrate strong functionalities of nonlinear beam shaping by the metasurface design, including dual focusing and Airy beam generation. Our approach would inspire new designs in the miniaturization and integration of compact multifunctional nonlinear light sources on chip.

Wavelength-scanning second-harmonic generation for determining absolute charge density at aqueous interfaces.

We develop a new, to the best of our knowledge, optical scheme based on second-harmonic generation (SHG) at multiple wavelengths for unequivocal separation of the second-order and the electric-field-induced third-order nonlinear optical contributions from aqueous interfaces. The third-order SHG originating from the field-induced reorientation order of water molecules in the electrical double layer offers an optical label-free and inherent probe to the surface charge density and surface potential in the absolute scales. We verify this wavelength-scanning SHG scheme both theoretically and experimentally, and show that the approach is applicable to water interfaces with bulk ionic strength below 500 µM and can achieve a detection sensitivity for a surface charge density of ∼10-4C/m2.

Second harmonic generation of q-Gaussian laser beam in plasma channel created by ignitor heater technique

AbstractThis paper presents a scheme for second harmonic generation (SHG) of q-Gaussian laser beam in plasma channel created by ignitor heater technique. The ignitor beam creates plasma by tunnel ionization of air. The heater beam heats the plasma electrons and establishes a parabolic density profile. The third beam (q-Gaussian beam) is guided in this plasma channel under the combined effects of density nonuniformity of the plasma channel and relativistic mass nonlinearity of the plasma electrons. The propagation of q-Gaussian laser beam through the plasma channel excites an electron plasma wave at pump frequency that interacts with the incident laser beam to produce its second harmonics. The formulation is based on finding the numerical solution of the nonlinear Schrodinger wave equation for the fields of the incident laser beams with the help of moment theory approach. Particular emphasis is put on dynamical variations of the spot size of the laser beams and conversion efficiency of the second harmonics with distance of propagation.

Thermal dephasing in an enhanced cavity based high-power second harmonic generation.

We report on the performance of an enhanced-cavity (EC) designed for obtaining high-power and efficiency second-harmonic generation (SHG). This is performed by numerical simulation of SHG coupled equations in the presence of a thermal dephasing effect that is effectively intensified through embedding the periodically poled LiNbO3 crystal by an oven-surrounded scheme. It is found that by setting the PPLN temperature at an optimum value, adjusting the mirror reflectively, and pumping power at certain values, gaining SHG efficiency of more than 90% is possible. We further realized that by an ECSHG device SHG efficiency can be improved by about 15%-50%. Moreover, compared to a single-pass SHG scheme, the EC-based SHG device is shown to be a very promising candidate to reduce and suppress the effect of thermal dephasing on the stability and efficiency of SHG radiation.

Experimental demonstration of efficient and robust second harmonic generation using the adiabatic temperature gradient method

We propose a way of achieving efficient and robust second-harmonic generation. The technique proposed is similar to the adiabatic population transfer in a two-state quantum system with crossing energies. If the phase mismatching changes slowly, e.g., due to a temperature gradient along the crystal, and makes the phase match for second-harmonic generation to occur, then the energy would be converted adiabatically to the second harmonic. As an adiabatic technique, the second-harmonic generation scheme presented is stable to variations in the crystal parameters, as well as in the input light, crystal length, input intensity, wavelength and angle of incidence.

Design considerations for quasi-phase-matching in doubly resonant lithium niobate hexagonal micro-resonators

Fabrication capabilities of high optical quality hexagonal superstructures by chemical etching of inverted ferroelectric domains in lithium niobate platform suggests a route for efficient implementation of compact hexagonal microcavities. Such nonlinear optical hexagonal micro-resonators are proposed as a platform for second harmonic generation (SHG) by the combined mechanisms of total internal reflection (TIR) and quasi-phase-matching (QPM). The proposed scheme for SHG via TIR-QPM in a hexagonal microcavity can improve the efficiency and also the compactness of SHG devices compared to traditional linear-type based devices. A simple theoretical model based on six-bounce trajectory and phase matching conditions was capable for obtaining the optimal cavity size. Furthermore numerical simulation results based on finite difference time domain beam propagation method analysis confirmed the solutions obtained by demonstrating resonant operation of the microcavity for the second harmonic wave produced by TIR-QPM. Design aspects, optimization issues and characteristics of the proposed nonlinear device are presented.

Second Harmonic Generation of Self‐Focused Cosh‐Gaussian Laser Beam in Thermal Quantum Plasma by Excitation of an Electron Plasma Wave

AbstractThis paper presents a scheme for second harmonic generation (SHG) of an intense Cosh‐Gaussian (ChG) laser beam in thermal quantum plasmas. Moment theory approach in W.K.B approximation has been adopted in deriving the differential equation governing the propagation characteristics of the laser beam with distance of propagation. The effect of relativistic increase in electron mass on propagation dynamics of laser beam has been incorporated. Due to relativistic nonlinearity in the dielectric properties of the plasma, the laser beam gets self‐focused and produces density gradients in the transverse direction. The generated density gradients excite electron plasma wave (EPW) at pump frequency that interacts with the incident laser beam to produce its second harmonics. Numerical simulations have been carried out to investigate the effects of laser parameters on selffocusing of the laser beam and hence on the conversion efficiency of its second harmonics. Simulation results predict that within a specific range of decentered parameter the ChG laser beams show smaller divergence as they propagate and, thus, lead to enhanced conversion efficiency of second harmonics. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Second harmonic generation of q-Gaussian laser beam in preformed collisional plasma channel with nonlinear absorption

This paper presents a scheme for second harmonic generation of an intense q-Gaussian laser beam in a preformed parabolic plasma channel, where collisional nonlinearity is operative with nonlinear absorption. Due to nonuniform irradiance of intensity along the wavefront of the laser beam, nonuniform Ohmic heating of plasma electrons takes place. Due to this nonuniform heating of plasma, the laser beam gets self-focused and produces strong density gradients in the transverse direction. The generated density gradients excite an electron plasma wave at pump frequency that interacts with the pump beam to produce its second harmonics. The formulation is based on a numerical solution of the nonlinear Schrodinger wave equation in WKB approximation followed by moment theory approach. A second order nonlinear differential equation governing the propagation dynamics of the laser beam with distance of propagation has been obtained and is solved numerically by Runge Kutta fourth order technique. The effect of nonlinear absorption on self-focusing of the laser beam and conversion efficiency of its second harmonics has been investigated.

Second-harmonic generation in tapered optical fibers

We present a theoretical study of second-harmonic generation (SHG) in tapered optical fibers in the continuous wave and monochromatic regimes. The mechanism is based on modal phase matching, where the taper is used to sweep the phase mismatch across the phase-matched condition during the propagation. Self- and cross-phase modulations are taken into account. We report general results about the conversion efficiency and demonstrate that this SHG scheme is not only robust with respect to phase modulation, but can even benefit from it. We also show that the conversion bandwidth is easily controlled by the taper design and can be realistically extended to tens of nanometers.

Measurements of femtosecond pulse temporal profile by means of a Michelson interferometer with a Schottky junction

We introduce a new method for femtosecond pulse shape measurement. The interference of two pulses is employed rather than the second-harmonic generation (SHG). Usually, the measurements of the femtosecond pulse is realized by an interferometer in combination with a nonlinear optical material, while the measurement that we describe is realized by means of a Michelson interferometer with a Schottky junction. Only a metal-semiconductor junction (Schottky junction) is needed, and neither the nonlinear optical material nor a photodetector is included. The two-photon absorption arises when the light is strong enough, while there is only a one-photon absorption when the light is weak. And the calculations are in good agreement with the experimental results. In principle, the new technique could be used for the measuring of pulses with any duration and with very low power. Unlike the SHG scheme, in the new method the quality of optics, mechanics, and other elements of the scheme are not essential, and the measurement is easily realized, but the results are quite precise and very sensitive to the light.

SHG in Bragg reflection waveguides for enhanced bandwidth applications

A novel scheme for second harmonic generation in asymmetric Bragg reflection waveguides is presented for enhancing the bandwidth of non-linear interaction. The strong dispersion exhibited by the Bragg reflection waveguide is exploited for countering the rapidly falling efficiency as one moves away from the phase-matching wavelength. The role of the periodic cladding on the efficiency and bandwidth of second harmonic generation in a lithium niobate based Bragg reflection waveguide is analyzed. Such an idea could be used in other parametric processes for enhancing the bandwidth and for easing fabrication tolerances.

219-nm Ultra Violet Generation Using Blue Laser Diode and External Cavity

In this paper, we describe a second-harmonic generation scheme with a blue laser diode (LD) that generates tunable coherent deep ultra violet at approximately 219 nm. Using a short fundamental wavelength with this technique, a high second-harmonic (SH) power is hardly observed. At an approximately 8 µW SH power at 219 nm and an approximately 1 nm fundamental wavelength tunability can be obtained using a single-mode blue LD chip, a nonlinear optic crystal, and an external cavity.

Quasiphase matching of second-harmonic generation in quantum cascade lasers by Stark shift of electronic resonances

We demonstrate a quasiphase matching scheme for second-harmonic generation in quantum cascade lasers with integrated resonant nonlinearity. Modulation of the nonlinear susceptibility is achieved by the periodic modulation of the bias voltage along the ridge waveguide leading to a periodic shift of electronic resonances and a change in the electron population in different energy levels. An up to tenfold enhancement of the conversion efficiency is observed. This technique is applicable to any resonant nonlinear optical process in quantum wells.

UV radiation source based on a copper vapour laser with acousto-optically controlled spectral and temporal parameters

A modified version of the optical scheme for second harmonic generation in a copper vapour laser is proposed, in which mirror focusing optics mounted at an angle of 45° to the incident light beam is used instead of lens focusing optics. A mean radiation power 1.4 W at a wavelength of 255.3 nm is achieved with a conversion efficiency of 28%. A high-speed system for controlling the spectral and temporal parameters of the UV radiation is developed and studied for the first time. The use of such a system in technological setups will considerably extend the range of their application.

Configuration to improve second-harmonic-generation conversion efficiency.

The dependence of second-harmonic-generation (SHG) conversion efficiency on the intercrystal phase shift between two nonlinear crystals is analyzed with heuristic theory, and a novel scheme for SHG with two cascaded nonlinear crystals and a polarization modulator is proposed. More than 30 W output power is obtained experimentally at 532 nm with 70% external doubling efficiency. To the best of our knowledge, this is the highest conversion efficiency obtained with LiB3O5 crystal external frequency doubling. This scheme provides a simple and effective method for improving second-harmonic conversion efficiency.

Poling-induced birefringence phase matching for 1.5-μm-band wavelength conversion in a polymeric rib waveguide

Abstract A birefringence phase-matching (BPM) scheme for second harmonic generation, difference frequency generation (DFG), and their cascading is developed in a polymer rib waveguide. Poling-induced birefringence ( n TM – n TE ) of PMMA:DANS side-chain polymer films showed quadratic increase upon applied DC poling fields, and was controlled for satisfying the BPM. The three conversion processes using the nonlinear coefficient of the d 15 were simultaneously phase matched at the poling field around 150 V/μm when the fundamental guiding modes of a pump (DFG: 0.775 μm, cascading: 1.55 μm) and a signal (∼1.55 μm) waves were used. In addition, a numerical simulation of the DFG including the waveguide losses showed high conversion efficiencies around 0 dB on the conditions: the pump power of 100 mW, the waveguide length around 2 cm, and the waveguide losses of 2 dB/cm, demonstrating that the poling-induced BPM is an effective phase-matching method in the wavelength conversion processes.

Fundamental walkoff-compensated type II second-harmonic generation in KTiOPO_4 and LiB_3O_5

To improve the frequency-doubling efficiency in type II second-harmonic generation (SHG), a fundamental walkoff-compensated SHG scheme was geometrically analyzed and its usefulness was experimentally demonstrated in a 12-mm length of KTiOPO(4) with 20% power enhancement and in a 10-mm length of LiB(3)O(5) with double enhancement.

Generation of ultrashort second-harmonic pulses by compressing the Čerenkov output

Ultrashort optical second-harmonic pulses based on a traditional phase-matched Cerenkov second-harmonic generation scheme, with an additional prism to equalize the arrival time, are proposed. Proton-exchanged KTP, LiNbO(3) waveguides, and several prism materials are considered in the evaluation. There is no constraint on crystal length, making high efficiency possible as well as preserving an ultrashort pulse width. The proposed approach may be a practical and efficient way to achieve convenient ultrashort harmonic pulses.