In Situ Measurement of the Second Harmonic Signal of Adsorbing Nonlinear Optical Ionically Self-Assembled Monolayers.
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In Situ Measurement of the Second Harmonic Signal of Adsorbing Nonlinear Optical Ionically Self-assembled Monolayers. C. Brands,1 P.J. Neyman,2 M.T. Guzy,3 S. Shah,3 K.E Van Cott,3 R.M. Davis,3 H. Wang,4 H.W. Gibson,4 and J.R. Heflin.1 1
Department of Physics, Virginia Tech Blacksburg, VA 24061 2 Department of Materials Science and Engineering, Virginia Tech Blacksburg, VA 24061 3 Department of Chemical Engineering, Virginia Tech Blacksburg, VA 24061 4 Department of Chemistry, Virginia Tech Blacksburg, VA 24061 ABSTRACT Ionically self-assembled monolayers (ISAMs) have recently been shown to spontaneously exhibit a polar ordering that gives rise to substantial second order nonlinear optical response. The deposition of ISAMs has been studied in situ via second harmonic generation. This is a particularly sensitive probe of the growth of nanometerthick films since the centrosymmetry of the immersion solutions, the substrate, and the container yields no SHG contribution from these bulk components. Upon immersion in the NLO-active polyelectrolyte solution, the SHG rises sharply over the first minute. When a film is immersed into salt water, the SHG decreases significantly only to be restored when the salt solution is replaced with deionized water. INTRODUCTION Ionically self-assembled monolayer (ISAM) films are grown, one monolayer at a time, by immersing a charged substrate alternately in anionic and cationic solutions. This has been shown to be an easy, economic, and fast method for creating laterally homogeneous, nanostructured thin films. [1,2] These films can be used to provide nanoscale control of thickness, composition and orientation in devices such as lightemitting diodes, photovoltaics, and electrochromics. One application we have been focusing on is the fabrication of films with a second order nonlinear optical response. These films show substantial χ (2) values with outstanding thermal and time stability. [37] To increase the understanding of the formation of the ISAM films it is beneficial to be able to measure the growth of the layer in situ. In this paper, we describe studies of the deposition process using second harmonic generation (SHG) as a probe. We also compare the deposition process of a polymer with that of a monomer. EXPERIMENTAL DETAILS The measurements were done with a standard SHG setup employing a 30 picosecond pulsewidth modelocked Nd:YAG laser. The fundamental wavelength is 1064 nm and is p-polarized. Typical values for beam radius and pulse energy values were 100 µm and 1 mJ/pulse respectively. The SHG data are averaged over 10 shots and 100 shots per data point for in situ and ex situ measurements, respectively. The sample holder was
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Figure 1. The second harmonic signal as a function of incidence angle. The solid line represents the glass slide, which has some SHG signal due to the fact that the interface is non-centrosymetric. The dashed and the dotted lin
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