Exploration of sulfur compounds for nonlinear optical applications

Abstract

This thesis attempts to explore acentric inorganic sulfur or thiophosphate compounds for potential nonlinear optical (NLO) applications, and reports the synthesis, crystal growth, electronic structure studies, property measurements, and reviews crystal structures. The current commercially available benchmark mid-IR NLO materials have limitations which limit their usability in the mid-IR range, such as two photon absorption for $ZnGeP_2$ or low laser damage threshold (LDT) for $AgGaS_2$ (AGS) and $AgGaSe_2$. Thus, the continuation to contribute to this field of scientific study persists. Sulfur and thiophosphates exhibit good potential application as mid-IR NLO materials, but still need development to limit the shortcomings of the benchmark NLO materials. For the first compound studied, and based off search results, $La_2PbS_4$ is the first $A_2BS_4$ (A=Rare earth, B=divalent metals) compound, in the $1\bar{4}3d$ (no. 220) space group with a $TH_3P_4$ structure type, to have NLO properties measured prior to this work. $Th_3P_4$ exhibits a low second harmonic generation (SHG) response of 0.22 × AGS and is non-phase matchable. Next, thiophosphates recently have seen a resurgence of study for NLOs. Properties of $KMP_2S_7$ (M = Cr, In) were investigated. $KInP_2S_7$ became the first acentric $[P_2S_6]^{4-}$ unit containing compound to exhibit experimental SHG response, exhibiting a low SHG response of 0.24 × AGS, and being non-phase matchable. Magnetic measurements of $KCrP_2S_7$ verified an antiferromagnetic transition around 21 K. Lastly, $[P_2S_6]^{4-}$ compounds have recently shown great successes for IR NLO properties. Hence, two compounds, acentric $a-Ag_4P_2S_6$ and centrosymmetric $\beta-Ag_4P_2S_6$ were studied. The $a-Ag_4P_2S_6$ and $\beta-Ag_4P_2S_6$ phases were characterized with differential scanning calorimetry (DSC), which revealed their phase transition relationships. Each exhibited photocurrent response and a broad IR window while $a-Ag_4P_2S_6$ exhibited a moderate SHG response of 0.61 × AGS, which was phase matchable, and exhibited a large laser damage threshold (LDT) of 3.2 × AGS.