NSOM PR-PDLC Studies in the Higgins Labs

For the past several years we have been studying photorefractive polymer-dispersed liquid crystal (PR-PDLC) films in the Higgins group. PR-PDLCs incorporate sub-micrometer sized LC droplets doped with perylene (a photoexcitable electron donor) and N,N-di(n-octyl)-1,4,5,8-naphthalene diimide (NDI) (an electron acceptor). The droplets are encapsulated in a thin, optically transparent polymer film. We use well-known bulk spectroscopic methods to verify their photorefractive properties. Our primary contribution has been in the use of near-field optical microscopy (NSOM) as a means to better understand the local mechanisms by which photorefractivity occurs in these materials. NSOM studies of photorefractive PDLCs were performed for the first time in our labs.

Bulk fluorescence quenching studies of perylene were performed to provide evidence that electron transfer occurs between perylene and NDI. Verification of photorefractivity in the samples was conducted by asymmetric two-beam coupling (TBC) experiments (see Fig. PR-1). Improvements to existing TBC methods were developed and used to directly monitor asymmetric beam coupling. Our methods involve modulating one of the beams and using a lock-in amplifier to obtain the gain induced modulation of the other. This method simplifies the determination of the beam-coupling ratio.

High-resolution topography and optical images of the PR-PDLCs were obtained for the first time with subdiffraction-limited spatial resolution using our NSOM. The topography images obtained show that the PR-PDLCs are morphologically complex, containing spheroidal and collapsed droplets (see Fig. PR-2). The optical images show that the dye is encapsulated primarily in the LC droplets. Dynamic NSOM studies were conducted on these samples by applying an electric field across the sample, using an electrified NSOM probe. Ion-dependent, field- induced LC reorientation in these materials was then monitored optically. Ion- dependent changes in the amplitude and phase characteristics of the liquid crystal response are readily observed (see Fig. PR-3).

Computer simulations were also developed in our labs to assist in the interpretation of the NSOM data. These studies show that the dynamics vary spatially within the individual droplets and are attributed to spatial differences in the relative importance of elastically-driven and ion-dependent LC reorientation.

The effects of permanent ions doped into the LC were also explored. Our results indicate that the inclusion of permanent ions increases the signals obtained by NSOM, suggesting an increase in photorefractivity. Bulk ion-dependent TBC experiments were then conducted and also demonstrated an increase in photorefractivity. These results suggest that including permanent ions may prove to be a unique means for enhancing photorefractivity in PR-PDLCs.

Related Publications

1. Daniel A. Higgins, Jeffrey E. Hall and Aifang Xie "Optical Microscopy Studies of Dynamics within Individual Polymer-Encapsulated Liquid Crystal Droplets" Acc. Chem. Res. in press.

2. Jeffrey E. Hall and Daniel A. Higgins "Enhanced Photorefractivity from Ion-Doped Polymer-Dispersed Liquid Crystals" J. Phys. Chem. B 2004, 108, 16050.

3. Jeffrey E. Hall and Daniel A. Higgins "Exploring Dynamics in Photorefractive Polymer-Dispersed Liquid Crystals Using Near-Field Scanning Optical Microscopy", Applications of Scanned Probe Microscopy to Polymers ACS Symposium Series. in press.

4. Jeffrey E. Hall and Daniel A. Higgins "Exploring the Photorefractive Effect in Polymer-Dispersed Liquid Crystals using Near-Field Scanning Optical Microscopy" J. Phys. Chem. B, 2003, 107, 14211.

5. Jeffrey E. Hall and Daniel A. Higgins, ≥Photorefractive Polymer-Dispersed Liquid Crystals Studied by Near-Field Scanning Optical Microscopy≤ Polymeric Materials Science and Engineering, 2003, 88, 186.

6. Jeffrey E. Hall and Daniel A. Higgins, ≥A Direct Method for Monitoring Two-Beam Coupling in Photorefractive Materials≤ Rev. Sci. Instrum., 2002, 73, 2103.