Mechanical tuning of holographic polymer-dispersed liquid crystal reflection gratings

Scott A. Holmstrom, Lalgudi V. Natarajan, Vincent P. Tondiglia, Richard L. Sutherland, Timothy J. Bunning
2004 Applied Physics Letters  
Holographic polymer-dispersed liquid crystal reflective structures have been formed using a highly elastic photopolymer. The elasticity allows for mechanical tuning of the reflection notch by over 120 nm in the visible spectral range using compressive stress. The shift in the central position of the reflection notch is related to the strain in the system and an effective elastic modulus for the structure is obtained. Optical materials possessing a one-dimensional periodicity in the index of
more » ... in the index of refraction are of great technical importance for the control of reflection and transmission of light in optical systems. Holographic polymer-dispersed liquid crystal (H-PDLC) reflection gratings are periodic dielectric structures consisting of alternating PDLC and solid polymer layers. These materials have been of intense interest in recent years due, in part, to the ease in dynamic control of the refractive index modulation through the application of an electric field. 1 Recently, it has been shown that the optical properties of H-PDLC structures can be modified mechanically. Cairns et al. 2 stretched an H-PDLC film in a direction perpendicular to the index periodicity and observed a maximum shift in the central position of the reflection notch of 25 nm in the visible spectral range. Shear stress has also been used to increase reflectivity 3 and to tune a transmission grating. 4 In this work, we illustrate the mechanical versatility of H-PDLCs through the use of a highly elastic polymer which allows for the mechanical tuning of the reflection notch by greater than 120 nm in the visible spectral range. We model the system as a linear elastic composite and arrive at an effective elastic modulus for the H-PDLC structure. This work represents an observation of an H-PDLC grating tuned by compressive stress. Each H-PDLC structure starts as a viscous mixture of a photopolymer and a liquid crystal (LC). In this work, the photopolymer was a commercially available optical adhesive, NOA76 (Norland Products, Inc.) and the nematic LC was BLO38 (EM Industries, Inc.). The syrup had a composition of 70 wt. % NOA76 and 30 wt. % BLO38. In order to form an H-PDLC reflection grating, the prepolymer syrup was sandwiched between two glass slides separated by spacers. Polymerization was initiated by two mutually coherent laser beams ( = 363.8 nm, total power ϳ350 mW) incident simultaneously such that the light-intensity pattern varied sinusoidally along the direction perpendicular to the surfaces of the glass slides. The details of the experimental setup used in this work can be found in Ref. 5. The resulting structure consisted of alternating polymerrich and LC-rich regions. Polymerized NOA76 has an index of refraction of n p = 1.52. The LC itself is birefringent with n e = 1.799 and n o = 1.527 corresponding to light parallel and perpendicular to the long axis of the molecule, respectively. The LCs used in the work undergo homogeneous anchoring to the walls of the spherical inclusion in which they are contained (hereafter referred to as droplets). As such, each droplet is birefringent with a particular symmetry axis for the LC director. 6 However, there is no preferred alignment from one droplet to the next. The result is a multidomain stratum with a very small domain size (the size of an individual droplet). Light incident on this structure will see an isotropic index of n LC Ϸ 1.6 for the unstrained LC-rich strata. Figure 1 shows a bright-field transmission electron micrograph (TEM) of the elastic H-PDLC structures used in this work. The dark regions in the figure represent the LC droplets and the light regions represent the polymer. The thick, solid line in Fig. 2 represents a typical transmission spectrum for natural light incident in the direction perpendicular to the strata. The free space wavelength associated with the central position of the notch, 0 , can be related to the physical properties of the H-PDLC structure using the equation where n 0 = ͑n LC + n p ͒ / 2 is the average refractive index and ⌳ 0 is the spatial periodicity of the layers. Derivations of Eq. (1) can be found couched in the theory of one-dimensional photonic band-gap materials, 7 the coupled wave theory for thick hologram gratings, 8 or the matrix formulation of wave propagation in stratified media. 9 The approximation indicated in Eq. (1) is based on the condition that the modulation of the refractive index is much less than the average index. For the materials used in this work, the ratio of the modulation to the average is on the order of 10 −2 . The H-PDLC film used in the compression experiments was a disc with a radius of 0 = 2.84 mm and a thickness of L 0 = 0.12 mm. The periodic variation in the index of refraction was in the direction perpendicular to the plane of the a)
doi:10.1063/1.1790601 fatcat:bnwlenomajbkjc6ogra4urzlba