Random Quasi-Phase-Matched Second Harmonic Generation in Periodically Poled Lithium Tantalate

Abstract

We observe second harmonic generation via random quasi-phase-matching in a 2.0 m periodically poled, 1-cm-long, z-cut lithium tantalate. Away from resonance, the harmonic output profiles exhibit a characteristic pattern stemming from a stochastic domain distribution and a quadratic growth with the fundamental excitation, as well as a broadband spectral response. The results are in good agreement with a simple model and numerical simulations in the undepleted regime, assuming an anisotropic spread of the random nonlinear component. © 2010 Optical Society of America OCIS codes: 190.0190, 190.2620 Parametric wavelength conversion in media with a random nonlinearity, discussed in the pioneering works by Freund [1] and by Dolino and coworkers [2], has recently attracted significant interest due to its nonresonant behavior, i.e., its low sensitivity to phase matching and its broadband response [3][4][5][6][7][8][9]; potential applications have also been investigated, such as the time-domain characterization of femtosecond pulses [10]. Moreover, in connection with quasi-periodicity, a few important aspects of nonlinear wave mixing and transverse phase matching have been unveiled [11][12][13][14]. Broadband frequency conversion can take place in both isotropic polycrystalline ferroelectrics [3] and in crystals with randomly distributed antiparallel ferroelectric micro-domains [5,6]; the former are easily accessible and support a linear trend in terms of the generated second harmonic (SH) versus the input fundamental power, while the latter exhibit the standard quadratic dependence of the generated SH versus the input fundamental power, with higher conversion efficiencies. The presence of antiparallel ferroelectric domains can intrinsically depend on the specific growth technique, as is the case for strontium barium niobate (SBN, d 33 = 12 pm/V) [4]. Here, a random domain distribution is generated during boule preparation [7,11]. In SBN and with reference to SH generation, Horowitz and coworkers studied the role of microdomains superimposed to an ordered pattern [6]. In several ferroelectric crystals with a second-order susceptibility, however, electric field poling for quasiphase matching (QPM) via domain inversion is the most used approach for engineering nonlinear gratings and realizing a variety of bulk and guidedwave structures for signal processing based on frequency conversion [15,16]. An intense electric field applied across the thickness of a ferroelectric crystal relates to the probability of domain nucleation. Domain nucleation, however, is a stochastic process with a random trend due to various defects, including vacancies, impurities, ions in wrong lattice sites, etc.[17]. The random character becomes particularly relevant when the imposed (periodic) pattern has small features, with the final structure being prone to show a disordered two-dimensional distribution of markto-space-ratio (MTSR) superimposed to the grating defined by the electrode geometry.In this Letter we report on second-harmonic generation...