Temperature-dependent refractive index of potassium acid phthalate (KAP) in the visible and near-infrared K. Moutzouris a,⇑ , I. Stavrakas a , D. Triantis a , M. Enculescu b a Laboratory of Electric Characterization of Materials and Electronic Systems, Department of Electronics, Technological Educational Institution of Athens, Athens 12210, Greece b National Institute of Materials Physics, PO Box MG-7, Magurele-Bucharest 77125, Romania article info Article history: Received 15 November 2010 Received in revised form 21 December 2010 Accepted 31 December 2010 Available online 26 January 2011 Keywords: Nonlinear optical materials Temperature-dependent Sellmeier equations KAP abstract We report an investigation of the temperature-dependence of the refractive index of potassium acid phthalate (KAP) crystal. Refractive index measurements are based on a refractometer setup operating in a temperature range from 25 °C to 200 °C and using five different laser wavelengths in the visible and near-infrared. Temperature-dependent dispersion relations are proposed, with an average fitting quality that exceeds experimental accuracy, and compared to previous room-temperature Sellmeier equations. Consequently, the new relations are employed for the calculation of various linear and nonlin- ear optical properties of KAP. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Potassium acid phthalate (KAP), also referred to as potassium hydrogen phthalate, is a semi-organic material that attracted con- siderable attention over the last decades. KAP crystals may be eas- ily grown [1–5] and exhibit interesting dielectric, piezoelectric, pyroelectric, elastic, as well as linear and nonlinear optical proper- ties [6–9]. Several authors reported the development and charac- terization of various doped KAP crystals [10–15], aiming at the control of particular material properties such as transmittance, dielectric loss, and nonlinear coefficients. This material is also em- ployed in a variety of applications, ranging from X-ray fluorescence spectroscopy [16–18] and Raman wavelength shifting [19], to the growth of highly oriented films of nonlinear polymers [20,21], the fabrication of uniform nanostructures with controlled mor- phology [22,23] and the preparation of pH buffer solutions in chemistry [24]. To the best of our knowledge, despite the continuous interest in KAP crystals, there is only one room-temperature Sellmeier equa- tion for this material [8] available today. This two-term dispersion relation was built from experimental refractive index data obtained in 1970 by Belyaev et al. [25]. A more recent work on a Brewster-angle technique [26] determined a KAP refractive index (at a single wavelength and room temperature) that was in good agreement with the Sellmeier equation of Ref. [25]. It is well known that refractive index of crystals depends upon both wavelength and temperature, thus is a function n(k,T). Accu- rate knowledge of the temperature dependent chromatic disper- sion is important in order to evaluate numerous properties of the material, such as higher order dispersion, three-wave-mixing, spa- tial and temporal walk-off, and thermal expansion coefficients, as well as to properly design optical systems accounting for the actual operating temperatures. In this work, we report on experimental measurements of the refractive index of KAP at five different wavelengths in the visible and near-infrared and temperatures varying from 25 °C to 200 °C. We consequently use these data to construct generalized temper- ature-dependent dispersion equations for all three principal indices (n a , n b and n c ) of this biaxial crystal. The proposed general- ized equations reproduce measured data with an average accuracy better than measurement accuracy and exhibit reasonable agree- ment with the room-temperature relations of Belyaev et al. Finally, we employ these new equations in the evaluation of temperature- dependent related effects in KAP, such as thermo-optic coefficient, optical anisotropy and phase-matched second-harmonic-genera- tion (SHG). 2. Crystal growth and experimental setup Potassium acid phthalate with the chemical unit formula K(C 6 H 4 COOH–COO) is a semi-organic salt that belongs to the alkali acid phthalate series, has an orthorhombic symmetry with the space group Pca2 1 and shows a perfect cleavage along (0 1 0) plane. We obtained pure KAP crystals by the evaporation technique from 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.12.021 ⇑ Corresponding author. E-mail address: moutzouris@ee.teiath.gr (K. Moutzouris). Optical Materials 33 (2011) 812–816 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat