Negative Third Order Optical Susceptibilities of Squaraines
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R.F. SHI, Q.L. ZHOU, AND A.F. GARITO Department of Physics, University of Pennsylvania, Philadelphia, PA 19104 ABSTRACT Quantum many-electron calculations based on multiple-excited configuration interaction have been implemented on squaraine molecules. It is found that certain squaraines possess negative third order optical susceptibilities (-y) even in the zero frequency limit. Our calculations agree well with the experimental results obtained by Dirk and co-workers[1] for anilinium squarylium(BSQ) in terms of the sign and magnitude. High-lying two-photon states in squaraines are found to be always critically important. The sign of y is found to depend on the electron donating ability of the two side groups in squaraines. I. INTRODUCTION In the past few years, it has been found that in conjugated organic and polymeric materials, electron-electron correlation effects determine their large, nonresonant, ultrafast third order nonlinear optical responses[2]. Most materials studied so far show positive values of yijkl(-W4; W1 , W2 , W3 ) when all frequencies are below any optical transition, although there has been no physical reason forbidding a negative
7Yijkl(-w4;
W1, W2 , w3). Recently, in a ma-
jor study, Dirk and his co-workers[1] discovered that in some squarylium dyes, namely, the indole squarylium (ISQ) and anilinium squarylium (BSQ), < -y(-2w; W,w, 0) > is negative when both w and 2w lie below the first electronic transition. Typical squaraines contain a highly polar and electron withdrawing central squaraine moiety and two strong electron donating groups, in the form of D+ --+ A- +- D+[3]. They usually exhibit a strong absorption peak in the visible region with the molar extinction coefficient on the order of 10omol- 1 - liter, cm- 1 [4], the exact value and peak position depending on the detailed structures and solvents used[5]. This paper discusses the microscopic origin and features of the third order optical responses of squaraines. Detailed analysis on our model system, bis(4- aminophenyl) squaraine (SQR), will be presented. Our results on anilinium squarylium (BSQ) agree well with the experimental results of Dirk and his co-workers in terms of the sign and magnitude. Furthermore, we find that the correlated, high-lying two-photon states are always important in squaraines with -, as well as in other squaraines possessing positive y. II. MODEL SQUARAINE RESULTS Our study begins with bis(4-aminophenyl)squaraine (SQR), a highly polar squarylium dye that has D2h symmetry. Figure 1 shows a schematic diagram of its molecular structure. In our summation over states calculations, bond lengths and bond angles were taken from reference [3], The x-axis is in the chain direction, while the z axis is perpendicular to the molecular plane. The dominant -y component comes from the x direction, along which 7relectrons are delocalized. 7ycomponents involving the z-axis are identically zero as required by symmetry. For the same reason, ir-electron states for SQR must have Ag, Big, B 2U, or B 3u symmetry. 643 Mat. Res.
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