Eur. Phys. J. AP 17, 189–200 (2002) DOI: 10.1051/epjap:2002012 T HE EUROPEAN P HYSICAL JOURNAL APPLIED PHYSICS c  EDP Sciences 2002 Optical anisotropy and EO properties of ferroelectrics TGS and LHP single crystals using a HAUP technique C.Hern´andez-Rodr´ ıguez a , R. Guerrero-Lemus, and J. Herreros-Cedr´ es Departamento de F´ ısica B´asica, FacultaddeF´ ısica, Universidad de La Laguna, Avda.Francisco S´anchez s/n ◦ , 38206 La Laguna, Tenerife, Spain Received: 12 July 2001 / Received in final form: 7 November 2001 / Accepted: 20 November 2001 Abstract. Triglycine sulphate, (NH2CH2CO2H)3 · H2SO4 (TGS), is a ferroelectric crystal widely used in optical properties studies, phase transitions theory as well as test for experimental techniques. Lead mono- hydrogen phosphate, PbHPO4 (LHP), is a typical representative of the family of ferroelectric schultenites, suitable in testing microscopic theories of ferroelectricity. In this work the so-called high-accuracy univer- sal polarimeter (HAUP) has been used to determine the natural anisotropy in the birefringent crystalline sections (010), obtaining the temperature dependence of optical anisotropy magnitudes of the crystal: gy- rotropy, birefringence and optical indicatrix rotation. Our HAUP technique also permits to investigate the electro-optic (EO) coefficients associated to the phases above and below phase transition temperatures. PACS. 78.20.Ek Optical activity – 78.20.Jq Electrooptical effects – 07.60.Fs Polarimeters and ellipsometers 1 Introduction 1.1 Triglycine sulphate TGS is a monoclinic crystal that exhibits a second or- der phase transition from one centrosymmetric paraelec- tric phase (class 2/m) to another ferroelectric (class 2) at a Curie temperature, T c = 49.7 ◦ C [1]. Above this temper- ature (paraelectric phase), the material presents birefrin- gence and rotation of optical indicatrix around two fold axis (axis b). Below Tc (ferroelectric phase), TGS exhibits a second-order phase transition loosing the symmetry el- ement m as a result of the field of spontaneous polariza- tion. In this phase, triglycine sulphate, besides the two effects of optical anisotropy already mentioned, exhibits optical activity. The equation of state parameters of TGS ferroelectric for both phases near the critical point have been studied recently by Mierzwa et al. [2], where a single set of Landau equation-of-state parameters is not suit- able for describing the dielectric behaviour of TGS near the critical point. Triglycine sulphate single crystal is still an important material used for the fabrication of infrared detectors and target part of vidicons operating at room temperature [3]. To check the good operation of the instrument, we have carried out a test on the TGS. In spite this crystal has been researched, HAUP set-up has allowed a detailed study of the optical anisotropy [4–6]. Discrepancies about results of gyrotropy can be seen and source for deviations was commented by Kaminsky [7]. a e-mail: chdezr@ull.es On this material we have been carried out a wide study of optical anisotropy that includes the determination of the characteristic errors, rotation of the optical indicatrix, optical activity and the electro-optic coefficients in the ferroelectric and paraelectric phases. 1.2 Lead monohydrogen phosphate The ferroelectric crystal LHP undergoes a second-order phase transition at around 310 K [8]. Like TGS the crys- tal is monoclinic with two formula units per primitive cell and belongs to the space group Pc below Tc and P 2/c in the paraelectric phase [8]. Konak et al. [9] present the first experimental data on changes of optical properties (optical indicatrix rotation and birefringence) that appear at the ferroelectric transition in LHP. Other studies about opti- cal properties are reported by Zgonik et al. [10] where op- tical second harmonic generation was measured and Ohno et al. [11,12] show reflection and absorption spectra as well as luminescence properties. LHP crystals may be useful in transducers and in many linear and non-linear optical- mechanical devices. We performed a measurement of temperature depen- dence of optical indicatrix rotation and birefringence by using our HAUP method. Spontaneous electrooptic effect in this crystal also was studied. 2 HAUP technique In the HAUP method, the sample is placed between nearly crossed polarizers. Under conditions of small azimuths Θ