Near-Field and Multiphoton Excited Fluorescence Studies of Polymer-Dispersed Liquid Crystals

Polymer-dispersed liquid crystal (PDLC) thin films find a variety of applications in a range of optical devices. They are used in electrically switchable windows, optical shutters, flexible displays, diffractive optics, and photorefractive systems. PDLCs consist of sub-micrometer-sized birefringent LC droplets encapsulated within what is usually an optically transparent polymer. Figure 1 shows a schematic of a PDLC device and a polarized light micrograph of a PDLC film. In their native state, the optical axes of the droplets are usually randomly aligned, causing the materials to strongly scatter light. Pure polymer/liquid-crystal composites are translucent in this state. When polymers and LCs with proper refractive indicies are employed, application of an electric field causes the films to become optically transparent to normally incident light.

Figure 1.

Important attributes of PDLC-based optical devices include the ease by which they can be prepared. In our lab, PDLCs are made by emulsion techniques and by solvent-induced phase separation. Thin PDLC films are simply cast from solution onto suitable conductive, transparent substrates. In functional devices, only a few volts per micrometer of film thickness are needed to induce optical switching. As with more common LC devices, PDLC systems draw very little current in the powered state. Unlike these other systems, however, optical contrast in PDLCs comes predominantly from their inherent light scattering properties.

While commercial devices are already being produced from PDLCs, their chemical and physical properties are not yet completely understood. Areas of particular concern include droplet size and shape issues, as well as the understanding of electric-field-induced LC dynamics. We use novel optical microscopic techniques such as near-field scanning optical microscopy (NSOM) and multiphoton-excited fluorescence microscopy (MPEFM) to study individual LC droplets and their electric-field-induced dynamics. NSOM provides high-resolution optical images of the droplets and their dynamics, as well as topographic information. MPEFM methods provide complementary capabilities, yielding somewhat lower spatial resolution, but providing the ability to probe droplet organization and dynamics deep within functional PDLC devices.

Near-Field Scanning Optical Microscopy

We have used NSOM to characterize both pure and dye-doped PDLC films. Figure 2 shows a schematic of the aperture-based NSOM instrument employed. During imaging, the sample is illuminated with linearly polarized light from a subwavelength-sized aluminum-coated fiber optic probe. Images are acquired by raster scanning the sample beneath the probe. To obtain high-resolution optical images, the tip and sample must be maintained in close proximity, and hence, the sample topography must be followed. In optical imaging of PDLCs, the light exiting the probe passes through the birefringent LC droplets, which modify its polarization. Optical contrast is obtained by detecting the transmitted light under crossed polarization conditions. The simultaneous recording of optical and topographic information makes NSOM a particularly powerful tool for characterizing PDLC sample structures.

Figure 2.

In dynamic NSOM imaging studies, the aluminum coated probe is electrified, as is the electrically conductive, optically transparent substrate upon which the sample is cast. A modulated electric field is applied between the probe and sample, inducing reorientation of the LC. Changes in the optical birefringence signal due to LC reorientation are monitored using a lock-in amplifier or an oscilloscope and spatial and/or temporal resolution of the LC dynamics are readily obtained. Importantly, the use of the sharp NSOM probe for field application helps confine the induced dynamics to the near-field regime, even in optically thick samples.

Multiphoton Excited Fluorescence Microscopy

MPEFM is a nonlinear optical imaging method that allows for high-resolution images of functioning/functional devices to be obtained. Our MPEFM system (see Figure 2) is built upon a conventional epi-illumination microscope. Pulses of light from a Ti:sapphire laser (l > 800 nm) are directed into the microscope and focused to a diffraction-limited spot within the sample, using a high-numerical-aperture objective. In PDLCs employing cyanobiphenyl and terphenyl LCs, fluorescence arises from three-photon excitation of the molecules. Images are collected by raster scanning the sample, which is mounted above the objective on a piezo-electric stage.

The use of near IR light for fluorescence excitation has several advantages in studies of light-scattering, birefringent materials. First, light scattering is minimized, as are changes in the polarization state of the incident light. Perhaps most important from the perspective of optically thick PDLC devices, fluorescence is excited only in the focal volume of the microscope. MPEFM images are virtually background free and depth-profiling experiments are readily performed by simply changing the focus depth. Multiphoton excitation is also highly sensitive to molecular orientation, so MPEFM can also be used to study LC organization.

Dynamical information is obtained in MPEFM studies by preparing functional PDLC thin films sandwiched between electrically conductive glass substrates. An electric field was applied to the sample and changes in the LC fluorescence are recorded in time. Time-resolved movies of the reorientation process are then prepared.

PDLC Droplet Shape Characteristics

Droplet shape in PDLC devices is an important parameter as it can cause LC organizational changes, and plays a significant role in governing LC reorientation dynamics. Unfortunately, droplet size and shape attributes often vary substantially within and between different PDLC films.

NSOM and MPEFM methods have been applied in our labs to better understand droplet shape and LC organization in PDLCs. Recently, we have demonstrated a method for obtaining highly regular LC droplets that can also be used to prepare well-ordered hexagonal droplet arrays. In this method, templated voids are prepared in a polymer film and the voids are subsequently filled with LC. This method is outlined in Figure 3 and is based on well-known methods for producing polymer inverse opals.

Figure 3.

Figure 4 shows MPEFM images of hexagonally-ordered templated PDLC droplet arrays obtained by this method. Figure 4A was recorded using circularly polarized incident light. Such images most clearly depict the hexagonal droplet order. In subsequent studies, linearly polarized light was used to excite LC fluorescence in hopes of understanding LC organization within the templated droplets. Figure 4C shows an example of these images. An important discovery in these studies were long-range orientational correlations observed between neighboring droplets. Such correlations could arise from I) subtle compression of the void array along a particular direction, due to compression of the precursor sphere arrays, II) organizational ≥communication≤ between droplets through interconnecting channels between the voids, and/or III) alignment induced by physical features on the covering glass slide. The latter mechanism was verified by inducing LC alignment by rubbing polymer-coated cover slides prior to assembly of the devices. However, evidence for the participation of the other mechanisms also exists.

Figure 4.

Dynamics in PDLC Films

NSOM and MPEFM dynamics methods developed in our labs have been used to characterize the electric-field-induced dynamics in single LC droplets within PDLC films. Figure 5 presents example topography and dynamic optical NSOM images obtained from pure, bipolar PDLC droplets. The amplitude images shown in Figures 5B and 5C reflect the changes in LC alignment induced locally by the modulated electric field. Sub-diffraction limited resolution is clearly obtained, indicating the dynamics probed occur within the near field of the NSOM tip, even though the droplets are optically thick. The sensitivity of these measurements to near-field effects arises in part from concentration of the applied electric field near the end of the sharp aluminum-coated NSOM probe.

Phase images (see Figure 5D) provide a complementary view of the droplet dynamics. Clearly visible in Figure 5D are dramatic spatial variations in the phase signal. These phase images and complementary time-resolved data obtained from numerous bipolar droplets indicate that the LC dynamics are faster near the polymer-LC interface and relatively slower in central droplet regions. They also suggest that the LC reorientation process can be quite complex in central regions, especially near the equatorial plane of bipolar droplets and along their polar axes.

Figure 5.

A representative MPEFM movie of the dynamics in a bipolar droplet is shown in Figure 6, along with time transients recorded for specific droplet regions. Features associated with the equatorial plane and polar axis dynamics are clearly depicted. The polar axis in this droplet runs from the lower left corner of the images to the upper right. Unlike the NSOM data, these data were recorded under the influence of a relatively uniform applied electric field, indicating these dynamical features reflect inherent droplet properties variations. As observed by NSOM, the LC relaxation dynamics are fastest in the outer circumference (especially away from the poles) of the droplet, where relaxation is driven by orientationally ≥anchored≤ interfacial LC.

Figure 6.

Related Publications

(1) Mei, E.; Higgins, D. A. Local Dynamics in Polymer-Dispersed Liquid Crystals Studied by near-Field Scanning Optical Microscopy Appl. Phys. Lett. 1998, 73, 3515-3517.

(2) Mei, E.; Higgins, D. A. Near-Field Scanning Optical Microscopy Studies of Electric Field Induced Molecular Reorientation Dynamics J. Phys. Chem. A 1998, 102, 7558-7563.

(3) Mei, E.; Higgins, D. A. Polymer-Dispersed Liquid Crystal Films Studied by near-Field Scanning Optical Microscopy Langmuir 1998, 14, 1945-1950.

(4) Higgins, D. A.; Mei, E.; Liao, X. Electric-Field-Induced Molecular Reorientation Dynamics by near-Field Scanning Optical Microscopy Proc. SPIE 1999, 3607, 26.

(5) Mei, E.; Higgins, D. A. Electric-Field-Induced Ion Migration in Polymer-Dispersed Liquid-Crystal Films Observed by near-Field Scanning Optical Microscopy Appl. Phys. Lett. 1999, 75, 430-432.

(6) Higgins, D. A. Probing the Mesoscopic Chemical and Physical Properties of Polymer-Dispersed Liquid Crystals Adv. Mater. 2000, 12, 251-264.

(7) Mei, E.; Higgins, D. A. Nanometer-Scale Resolution and Depth Discrimination in near-Field Optical Microscopy Studies of Electric-Field Induced Molecular Reorientation Dynamics J. Chem. Phys. 2000, 112, 7839-7847.

(8) Springer, G. H.; Higgins, D. A. Toroidal Droplet Formation in Polymer-Dispersed Liquid Crystal Films J. Am. Chem. Soc. 2000, 122, 6801-6802.

(9) Higgins, D. A.; Liao, X.; Hall, J. E.; Mei, E. Simultaneous near-Field Optical Birefringence and Fluorescence Contrast Applied to the Study of Dye-Doped Polymer-Dispersed Liquid Crystals J. Phys. Chem. B. 2001, 105, 5874-5882.

(10) Higgins, D. A.; Mei, E. Scanning Probe Microscopy and Spectroscopy. Theory, Techniques, and Applications. In Near-Field Scanning Optical Microscopy; 2nd ed.; Bonnell, D., Ed.; Wiley-VCH: New York, 2001; pp 371.

(11) Luther, B. J.; Springer, G. H.; Higgins, D. A. Templated Droplets and Ordered Arrays in Polymer-Dispersed Liquid-Crystal Films Chem. Mater. 2001, 13, 2281-2287.

(12) Higgins, D. A.; Luther, B. J. Watching Molecules Reorient in Liquid Crystal Droplets with Multiphoton-Excited Fluorescence Microscopy J. Chem. Phys. 2003, 119, 3935-3942.

(13) Xie, A.; Higgins, D. A. Electric-Field-Induced Dynamics in Radial Liquid Crystal Droplets Studied by Multiphoton-Excited Fluorescence Microscopy Appl. Phys. Lett. 2004, 84, 4014-4016.

(14) Higgins, D. A.; Hall, J. E.; Xie, A. Optical Microscopy Studies of Dynamics within Individual Polymer-Dispersed Liquid Crystal Droplets Acc. Chem. Res. 2005, 38, 137-145.