Rapid communication Characterization of optical and nonlinear properties of periodically-poled RbTiOAsO4 in the mid-infrared range via difference-frequency generation

Tunable mid-infrared coherent radiation (3.25− 3.7 μm) is generated by quasi-phase-matched difference frequency generation in a multi-grating periodically-poled RbTiOAsO4 crystal. The spontaneous polarization and coercive field of flux-grown RbTiOAsO4 are determined by polarization switching measurements. The nonlinear interaction enables us to explore the optical and nonlinear properties of this material in the mid-infrared range, where data is scarce. The measurements are used to derive a midinfrared corrected dispersion equation for nz in RbTiOAsO4. This equation is in excellent agreement with previously published measurements of nonlinear interactions in periodically-poled RbTiOAsO4. The measured wavelength and temperature bandwidths are ≈ 48 nm cm and ≈ 29 ◦C cm, respectively. A relatively high temperature tuning slope of the phasematched idler wavelength, −1.27 nm/◦C, is measured. This may be useful for realizing temperature-tuned nonlinear devices. PACS: 42.70.Mp; 42.79.Nv; 78.20.Ci During the last few years, practical methods for modulating the nonlinear coefficient in ferroelectric materials by electric field poling have been developed. Quasiphase-matching (QPM) [1] has been demonstrated with several periodically-poled ferroelectric crystals, including LiNbO3 [2], KTiOPO4 (KTP) [3, 4], RbTiOAsO4 (RTA) [5, 6] and recently also KTiOAsO4 (KTA) [7]. The QPM method is particularly attractive at wavelength regions in which compact and efficient sources are scarce, e.g. in the midinfrared (mid-IR, 2–10 μm). Lately, we have explored the nonlinear and optical properties of periodically-poled KTP (PP-KTP) [8] and periodically-poled KTA (PP-KTA) [9] in the mid-IR range by performing QPM difference-frequency generation (DFG) experiments with these crystals, which are known to have a much higher damage threshold compared to periodicallypoled LiNbO3 (PP-LN). One of the important characteristics of the alkali metal titanyl arsenate crystals, such as KTA, is that in addition to maintaining most of the valuable properties of KTP, e.g. low coercive field and high damage threshold, they also lack the significant absorption that KTP exhibits around ≈ 3.3 μm. Moreover, they enjoy a longer cutoff wavelength of ≈ 5 μm, compared to ≈ 4 μm in KTP. Another attractive alkali metal titanyl arsenate crystal is RTA, which also exhibits a long cutoff wavelength,≈ 5.3 μm. Whereas flux-grown KTP and KTA crystals should be either cooled to a low temperature (≈ 170 K) [4] or pre-treated by chemical indifussion of Rb ions [10], RTA crystals have been poled near room temperature without any pretreatment [5]. Furthermore, since the poling field increases as the temperature decreases, the electric field required to pole RTA near room temperature is ≈ 2.5 kV/mm [5, 6], less than half of that required for KTP and KTA at 170 K. In addition, the poling field is approximately an order of magnitude lower than that of LiNbO3 [11]. Owing to that, fairly thick samples (≈ 3 mm) have been periodically-poled [12]. Up until now, PP-RTA crystals were only used in optical parametric oscillator (OPO) configurations, from continuouswave mode [13] and up to ultra-short pulses [14]. In this paper we report, for the first time to the best of our knowledge, DFG of mid-IR coherent radiation using a multi-grating PP-RTA crystal. This nonlinear interaction is used for characterizing the optical and nonlinear properties of PPRTA in the midIR range. One advantage of small signal DFG compared to OPO is that the DFG idler wavelength is fully determined by the choice of the pump and signal wavelengths. Furthermore, unlike in the case of OPO, there is no threshold and the interaction is less dependent on the losses of the interacting waves. Knowledge about the dispersion in the crystal is required for designing an appropriate modulation period for a specific QPM nonlinear interaction [1]. A design based on an inaccurate dispersion equation could result in significant mismatch and therefore less than optimum frequency conversion efficiency. Alternatively, it may cause constraints on the operating conditions, e.g. crystal temperature, pump and signal wavelengths, etc., in order to achieve phase-matching.