Investigation on growth of 4-N, N-dimethylamino-N′-methylstilbazolium p-chlorobenzenesulfonate: An efficient organic NLO crystal with potential THz properties

Abstract

The growth of an efficient organic nonlinear optical material 4-N, N-dimethylamino-N′-methylstilbazolium p-chlorobenzenesulfonate (DASC), a derivative of the stilbazolium tosylate family has been reported by employing the slope nucleation method coupled with slow solvent evaporation. The crystal system and lattice parameters are estimated by single crystal X-ray diffraction. The second harmonic conversion efficiency of DASC was determined using the Kurtz and Perry powder technique. The thermal stability of the compound is determined by thermo gravimetric/differential thermo gravimetric (TG/DTG) and differential scanning calorimetric (DSC) techniques. The hardness profile of the sample is investigated by Vickers microhardness test. The dielectric constant and dielectric loss of the crystal were studied as function of frequency. The surface properties of DASC are reported by undertaking scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies.

Introduction

Terahertz (THz) technology has entered into an unprecedented revolutionary era with ever growing applications in biology and medicine, covering pharmaceutical industry, medical imaging, material spectroscopy and sensing, security and high data rate communications. High power and reliable THz sources and high performance THz spectroscopy and imaging systems have been developed over the past few years making it possible to explore new areas in science and technology [1]. However, the large THz portion of the spectrum has not been well developed because there was neither convenient high power emitter to send out controlled THz signals nor efficient sensors to collect them and record information. The lack of high power along with low cost portable room temperature THz source is the most significant limitation of modern THz systems. Coverage of a wide frequency range requires several different source systems; however the use of an organic nonlinear optical (NLO) crystal has the feasibility of ultra wide tenability for breakthrough developing THz source [1], [2].

Nonlinear optical effects such as difference frequency generation (DFG) or optical rectification (OR) are widely used for generating coherent THz radiation. Appropriate source materials have a high nonlinearity optical susceptibility χ⁽²⁾ and a low refractive index in the THz frequency range. Recent research indicates that 4-N, N-dimethylamino-N′-methylstilbazolium p-toluenesulfonate (DAST) and its derivatives are promising candidates for THz wave generation devices [3]. By far, the generation of broadband high power THz waves from DAST single crystal has been realized up to 30 THz by DFG and the generation of sub-10 THz waves are achieved by OR [4], [5]. However, a number of absorption features in the THz spectra of DAST limits its application. Another major constraint for the development of DAST crystal is to overcome the formation of hydrated DAST co-crystals, which destroys the noncentrosymmetric crystal structure [6]. In this connection, researchers are aiming to develop alternate sources to generate THz waves of different frequency ranges and thereby explore the possibility of further expanding the areas of applications.

The studies carried out by Yang et al. and Okada et al. have provided important information on how a change of the counter anion will modify the crystal structure, and the second harmonic generation (SHG) activity of stilbazolium salts, thus offering almost unlimited design possibilities to obtain new materials by straightforwardly varying counter anions [7], [8], [9]. Various derivatives of DAST have been synthesized, among them; 4-N, N-dimethylamino-N′-methylstilbazolium p-chlorobenzenesulfonate (DASC), 4-N, N-dimethylamino-N′-methyl stilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS), 4-N, N-dimethylamino-N′-methyl stilbazolium p-methoxybenzenesulfonate and N, N-dimethylamino-N′-methylstilbazolium p-trifluoromethylbenzenesulfonate (DSPFS) are some of the prominent materials which are found to fulfil the purpose of better crystal growth properties than DAST [7], [10], [11], [12].

Among these crystals, the generation of broadband high power THz waves from bulk crystals of DASC has been recently demonstrated by Matsukawa et al. [4]. DASC has the same first hyperpolarizability β (β₀ = 370 × 10⁻³⁰ esu) as DAST because it has a stilbazolium cation moiety. DASC and DAST also have the same nonlinearity because they share the same crystal structure [13]. Further DASC was found to have superior transmission characteristics in the THz band than DAST [4].

Our research group reported on the growth DAST crystals in different modes [14], [15], [16], [17] and also made detailed study on 4-N, N-dimethylamino-4-N-methyl stilbazolium p-styrenesulfonate (DSSS) and 4-N, N-dimethylamino-4-N-methyl stilbazolium p-methoxybenzenesulfonate (DSMOS), which are identified as efficient stilbazolium derivatives. As a continuation of our ongoing research on novel organic THz materials, in this work, an attempt has made to grow bulk size crystals of DASC. The most striking feature of DASC comes from its ability to achieve THz generation better than that of DAST even with thinner sample. Another added advantage is that DASC has its melting point higher than that of DAST by 20 °C [13]. Keeping the importance attached the material and the need to develop efficient NLO and THz sources for wide variety of applications, the present work is focused on further improving the growth characteristics and carry out an in-depth study of various physical and chemical properties of the developed crystal. We explored the growth of DASC in a mixed solvent of water and methanol (1:1) by employing slope nucleation method coupled with slow evaporation (SNM-SE). Our study confirms the growth of DASC crystal close to cm length along with improved thickness. The growth of crystals with improved thickness is an added advantage in terms of the cost as well as quality of the grown crystal. Through proper cutting and polishing of the thicker crystal, it is possible to compare the THz properties of crystalline materials of identical quality but with varying thickness. Earlier work by Matsukawa et al. dealt with the growth of DASC crystal employing slow cooling technique [4]. Deviated from the earlier work, an attempt has been made to grow DASC crystal by slope nucleation method (SNM) in order to exploit the advantages of this method. The advantages include very simple and high yielding process, controlled nucleation position, growth in a specific orientation, thickness control by Teflon grooves and formation many good quality crystals at one parallel run [16]. This article presents the vital properties such as surface features, microhardness by Vickers hardness measurements and dielectric response of DASC crystal for the first time. The grown crystal of DASC was also subjected to various characterization techniques like single crystal X-ray diffraction, NLO test and thermal studies.