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Optical Design & Engineering

Photoinduced reorientation of polymerizable liquid crystals for micro-optical elements

A flat microlens-array film prepared with dye-doped liquid crystals enhances the emission intensity of an electroluminescent device.
1 November 2010, SPIE Newsroom. DOI: 10.1117/2.1201009.003087

Planar, self-emitting technologies (such as inorganic and organic electroluminescent devices) have been actively investigated and commercialized in recent years. Their chief attraction is their potential application in flexible-panel displays and lighting. A drawback of these technologies is that only a small fraction of the light generated in the device can escape, owing to total internal reflection at the air/substrate interface. Consequently, low outcoupling efficiency has become one of the main limitations to practical implementation. A variety of approaches based either on extracting waveguide light or total reflection have been studied as a way around these problems, including mesa, micropatterned, and microcavity structures; silica microspheres; microlens arrays; low-refractive-index silica aerogels; and low-index grids. Among these techniques, the microlens is the most appealing because it can be fabricated simply and reliably.

Liquid crystals (LCs) are suitable photonic materials because they exhibit a large optical anisotropy (a direction-dependent property) that can be controlled by changing the alignment of the LC molecules.1 Photomanipulation of LC alignment has the advantage of a large photoinduced birefringence (i.e., a change in refractive index). It is well known that LC molecules tend to become aligned parallel to the light polarization when irradiated with linearly polarized light at high intensities.2 Moreover, the group of Jánossy has reported that a small amount of anthraquinone dye dissolved in LCs strongly enhances photoalignment behavior.3 When a laser beam with a Gaussian traversing intensity distribution is used as excitation source, the dye-doped LC system functions as a graded-index lens by so-called nonuniform director reorientation that corresponds to the traversing intensity profile. An LC microlens could be a new type of micro-optical component driven by an optical field. LC lenses have a flat surface that enables connection to other optical components. In addition, the procedure for preparing them is very simple, involving remote irradiation with a laser beam.

We recently reported that an oligothiophene derivative—5-5"-bis(5-butyl-2-thienylethynyl)-2,2':5',2"-terthiophene—acts as a highly efficient dye for photoinduced reorientation of LCs.4 Furthermore, we developed a novel method of making planar microlens arrays with polarization selectivity and successive photopolymerization using a single light source.5 This approach enables straightforward fabrication of various microlens arrays with both arrangement and polarization selectivity.

However, the fabricated lens arrays were colored with a dye, and their freestanding films could not be obtained because the main component was a low-molecular-weight LC. Further exploration would be required to apply LC microlens arrays to a wide range of optical applications. Here, we used photoinduced reorientation of a dye-doped polymerizable LC system to fabricate a new, colorless planar microlens-array film. We also investigated the potential of the microlens-array film as an optical element for planar, self-emitting devices.

We prepared a sample cell containing a guest dye, 2,5-bis(5-butyl-2-thienyl-ethynyl)thiophene (TR3); LC/LC monomers, 1-(4-pentylcyclohexyl)-4-cyanobenzene (PCH5):4-(4-pentylphenyl)-1-phenyl acrylate (A0T5):4-(4-propylcyclohexyl)-1-phenyl acrylate (A0PC3) =20:43:37mol; a photoinitiator, Irgacure184; and two parallel, transparent glass substrates. We coated the inner surfaces of the substrates with lecithin to obtain homeotropic (i.e., perpendicular to the substrate) alignment of LCs. We focused a linearly polarized 364nm Gaussian beam from an argon-ion laser normally onto the sample cell using a lens. Next, we moved the sample cell periodically. After repeating these two processes a few tens of times, we irradiated the whole cell with a high-pressure mercury lamp to stabilize the alignment in the non-irradiated area, producing a colorless freestanding film with microlens arrays (see Figure 1).

Figure 1. (a) Chemical structures of the compounds used in this study. (b) Top view of the optical setup. (c) Photograph of the colorless microlens-array film. S: Sulfur. He-Ne: Helium-neon. Hg: Mercury. λ: Wavelength. Ar+: Argon ion.

Figure 2 shows polarizing optical-micrograph images of the microlens array between crossed polarizers. The polarization direction of the 364nm beam was at ±45° with respect to the polarizer. When we rotated the sample so that the polarization direction of the beam was parallel to the polarizer's axis, the exposed part became dark, indicating that LC alignment was unidirectionally and homogeneously maintained. Thus, the microlens array exhibits polarization selectivity.

Figure 2. Polarizing optical micrographs of prepared microlens arrays. Crossed polarizers are set (a) 45 and (b) 0° to the polarization direction of the argon laser beam (blue arrow).

We then fabricated a layered structure incorporating an electroluminescent device to evaluate the microlens-array film as an optical element for improving device efficiency. We observed no spectral change from any monitored angle before or after fabrication of the microlens array (see Figure 3). We conclude that the LC microlens-array film does not cause interference or scattering of light, nor does it affect emission color. The emission intensity of the layered structure with the microlens array was higher than that without one.

Figure 3. (a) Evaluation set up for the change in emission intensities and spectra of the electroluminescent device and (b) electroluminescent spectra of the device with and without the microlens array film, respectively.

In summary, we prepared a colorless, flat microlens array with dye-doped LC mixtures using photoinduced reorientation of the LC system. The prepared microlens array enhanced the emission intensity of the electroluminescent device without any spectral change. This array is thus a promising candidate for both self-emitting devices and broad application as a key component in optical parallel-processing systems and large-scale free-space networks. We are currently working to develop an effective dye-doped system.

Motoi Kinoshita, Tomiki Ikeda, Tomohiro Kobayashi, Keisuke Takano, Yunmi Nam
Chemical Resources Laboratory
Tokyo Institute of Technology
Yokohama, Japan

Motoi Kinoshita is an assistant professor in the Polymer Chemistry Division.

Tomiki Ikeda is a professor of polymer chemistry and director of the Chemical Resources Laboratory.

Yunmi Nam is an assistant professor in the Polymer Chemistry Division.