Nonlinear photonic structures with a modulated second-order nonlinearity are used widely for quasi-phase-matched parametric processes. Creating three-dimensional (3D) nonlinear photonic structures is promising but still challenging, since standard poling methods are limited to two-dimensional structures. Light-induced quasi-phase matching (QPM) can overcome this issue by a depletion of the second-order nonlinearity with focused femtosecond laser pulses. We report, to the best of our knowledge, the first integration of a 3D QPM structure in the core of a lithium niobate waveguide applying light-induced fabrication. Depressed-cladding waveguides and embedded QPM structures are fabricated by femtosecond laser lithography. The 3D capability is exploited by splitting the QPM gratings in the waveguide core into two or four parts, respectively. These monolithic nonlinear waveguides feature parallel multi-wavelength frequency conversion. Finally, we demonstrate a concept for second-harmonic beam shaping taking advantage of a helically twisted nonlinear structure. Our results open new avenues for creating highly efficient advanced QPM devices.
We investigate systematically the evolution of second harmonic generation in strontium barium niobate with different degrees of disorder of its χ2 nonlinearity. These different degrees of disordered domain structures are achieved through electrically switching of the polarization at room temperature. The size and distribution of the domains change during the poling process and this in turn strongly affects the spatial distribution of the second harmonic signal. The degree of disorder can be determined by analyzing the angular distribution and wavelength dependence of the second harmonic emission patterns combined with measurements of the spontaneous polarization. We demonstrate evidence of the control of the second harmonic emission pattern by creating defined states of order, and successfully reproduce the resulting patterns theoretically.
We study experimentally and numerically the second-harmonic Čerenkov emission with two different characteristic azimuthal intensity distributions in strontium barium niobate with a random structure of χ2 nonlinearity. We monitor in situ the Čerenkov emission during domain switching and show that a change of domain size and shape results in a fourfold azimuthal modulation of the Čerenkov cone.
We study experimentally and theoretically the Čerenkov-type second-harmonic generation in a one-dimensional nonlinear photonic crystal. We demonstrate that the power of emitted second-harmonic can be enhanced 270 times by varying the angle of incidence of the fundamental beam such that the reciprocal lattice vector of the crystal can be used to compensate for the phase mismatch in the transverse direction enabling interaction in the nonlinear Bragg diffraction regime.
We explore a physical approach to invert ferroelectric domains in the volume of MgO-doped lithium niobate crystals without any external electric field. Permanent defect structures are created by focused infrared femtosecond laser pulses below the material surface along the polar axis followed by a thermal treatment. This procedure leads to an inversion of ferroelectric domains beneath and above the laser-induced filaments up to the surfaces of the crystal. All domain walls are straight and up to 800 μm long. We measure the domain width in dependence on the length of the filaments and the writing energy. The smallest achieved domain width and the domain spacing is 1 μm. We propose a model taking into account the temperature dependence of the pyroelectric field and thermally activated bulk charges to explain the mechanism of domain inversion. Our findings pave the way to all-optical printing of arbitrary ferroelectric domain structures for nonlinear photonic applications.
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