Liquid crystals, polymers, and electrically tunable optical components
Modern cell phones are the best examples of ‘smart’ consumer products that are indispensable in day-to-day life. Manufacturers continually strive to increase the appeal of such devices by adding new technological features. One of the most promising of such efforts is adaptive optical imaging (e.g., autofocus) for barcode reading and online commerce using cell-phone cameras. The cost of components in these products must be extremely low, which means that solutions, too, must be based on affordable parts and low-cost manufacturing processes. In this respect, organic materials such as liquid crystals (LCs) and polymers offer a key advantage over their inorganic counterparts. Many polymers are affordable, and manipulating them is easy and does not require high-temperature, costly vacuum processes. In addition, LCs provide electric variability (the capacity to change properties when an electric field is applied) without adding noticeable complexity. The appropriate combination of those two material systems may, consequently, enable very attractive solutions, ranging from flexible displays to adaptive imaging. Typical examples are polarization-free displays,1 tunable filters,2–4 and lenses.5
The basic idea of this approach can be illustrated by the following example. In traditional displays, ‘pure’ LC materials generate a polarization change in light, which requires additional polarizers for transformation into a change in intensity. Aside from the cost of polarizers, this significantly increases both light losses and energy consumption of the device. A proposed alternative approach uses polymer networks that ‘interpenetrate’ into the LC host matrices (materials) and change the intensity of light by controlling its scattering (without polarizers).1–4 These material systems are typically composed of 96wt% (weight percent) of LC, 3wt% of monomer, and 1wt% of photoinitiation complex. The standard procedure is to use a mixture of ingredients, fabricate thin films (in ‘sandwiched’ geometries), and then ‘program’ the system by exposing it to electric fields and photopolymerization. Our group has put substantial effort into using light and electromagnetic forces to ‘program’ the properties of composite materials in the early stages of their formation (a kind of ‘genetic modification’) that could be called optical molecular engineering.
Depending on the type of monomer, linear, crosslinked, dispersed, or aggregated networks can be fabricated.5,6 Accordingly, the materials obtained enable different optoelectronic components. For example, using a mixture of E7 (four different LC materials, from Merck) and monofunctional monomer SR-379 (Sartomer), we achieved optical films that scattered light strongly but could be ‘cleared’ by applying a few volts. Interestingly, these films can be made to possess anisotropy (polarization-direction-dependent) scattering, consequently allowing scattering-based (nonabsorbing) ‘polarizers’ with a high polarization-discrimination ratio (up to 600).7 Here, the LC reorientation process is defined by the character of the polymer network and also by its interaction with the LC host.6 Given that the formation of free radicals, and thus the local density of polymer chains, can be controlled by the photopolymerizing light intensity, we can use light to control the network's morphology (shape). Figure 1 shows the transmission of a probe light through a polymer-stabilized LC gradient (i.e., gradually changing in space) formed by exposing the original mixture (the left-hand corner above the diagonal) to polymerizing UV light through an amplitude mask.
An alternative application of such materials is an optical lens with an electrically variable focal distance.5 In fact, the polymer network may be formed in a centrally symmetric way by exposing the original film to light with a spherical gradient of intensity (i.e., decreasing from the center to the periphery). In this case, the polymer network will have a correspondingly ‘lenslike’ gradient. The application of a uniform electric field using uniform electrodes—much preferred by industry—generates various degrees of molecular reorientation in space, corresponding changes in the refractive index, and light focusing (see Figure 2). Such material systems have been used to build lenses that can change their focal distance from infinity to 10cm, again by applying a few volts. Periodic structures, such as Fresnel lenses, may also be fabricated using the same principle.8 Other types of LCs, such as smectic,9 ferroelectric,10 and cholesteric,11 may also be used to build composites, for example, with electrically controlled broadband spectral-reflection properties2–4 or tunable mirrorless lasers.11
It is important to point out that these materials do not have only advantages. In fact, many potential problems must be solved before the composites described here can find broad acceptance in industry. In particular, the ‘ease’ of dynamic control of properties by relatively weak electric fields (and the corresponding low power consumption so interesting to the mobile industry) obviously also entails increased sensitivity to external stimuli. Typical examples include environmental temperature and humidity. For this reason, appropriate packaging solutions are a prerequisite for practical application. In addition to these operational problems, regulations regarding lead-free manufacturing impose fairly high temperature steps for various volume-fabrication processes (on the order of 260°C). Finally, given standard, nonoperating storage conditions of between −40 and +85°C, the range of temperatures that must be supported extends to 300°C, which is very difficult for many organic materials to support. These are the key challenges facing the research and development community. The task is difficult, but not impossible, provided scientists team up with talented engineers and work together on specific problems. Substantial investment (financial and human) and dedicated, focused work have already yielded successful examples.12 In the mean time, we will continue our efforts to develop new ‘programmable’ material systems to address the challenges mentioned here.
Tigran Galstian received his PhD in quantum electronics in 1990 from the Special Department of Physics at Moscow Engineering Physics Institute (Russia). He joined Laval University as a professor in 1995. His main interests are optical materials and their application to optical-information technologies.
