Polymer Films As Photonic Devices.
This paper is the result of a research collaboration between a polymer chemistry group at Queen's University and a physics (optics) group at The Royal Military College in Kingston, ON. We are investigating the amazing properties of polymers containing azobenzenes, based on the very well-known photoisomerization of the azobenzene groups.
An example of azobenzene photoisomerization is shown in Figure 1. If donor (D) and acceptor (A) groups are present, the isomerization can be activated in both directions by a single wavelength. Such groups, incorporated into various polymer structures, produce a great variety of photoinduced motions, which in turn could be exploited in photonic applications. A brief summary of the studied phenomena is presented here.
Amorphous, randomly oriented polymer films, can be aligned using a linearly polarized laser, and the orientation can be destroyed by a circularly polarized laser, thus creating a reversible optical storage process in a millisecond time frame. While this is relatively slow to be used for real time storage, whole pages could be recorded as holograms, thus overcoming the time limitation. Another possible photonic use is waveguides. The aligned part of the film is birefringent, while the whole film is anisotropic. A line "written" on the film is capable to guide light within its confines, thus waveguides could be reversibly inscribed on the film.
Massive Material Movement
Under selected illumination, the polymer material can flow over long distances (microns) well below its glass transition temperature (Tg). Stable modified surfaces with design patterns can be produced in a few minutes. An example of surface relief gratings is shown in Figure 2. Such patterned surfaces can be used in three-dimensional holographic storage, as optical filters for various angles or various wavelengths, as polarization separators, or as couplers into and out of the waveguides previously mentioned.
Research into photorefractive polymers is very intense, mainly because these polymers can be used for holographic storage. Azobenzene polymers allow holographic storage by the two above-mentioned properties, but may -- with the right chemical structure -- have also photorefractive properties. We are studying carbazole and triphenylamine-based axobenzene polymers with high Tg and have demonstrated their photorefractivity. The intended use is as switching junctions on an all-polymer photonic device.
If the amorphous azobenzene polymer contains polar groups, they will move in concert with the photoinduced motion of the azobenxene, thus allowing a significant amplification of the motion for low absorbing materials. If the azobenzene polymer can form crystalline or liquid crystalline phases, the photo-orientation will also be greatly enhanced. The drawback in this last ease is the thermodynamic stability of the photoinduced orientation, which prevents proper optical erasure.
Liquid Crystal Alignment
Another very useful possible application of the massive material movement described above is creation of gratings to be used in liquid crystal alignment. We have demonstrated that liquid crystal cells can be photo-aligned either before or after assembly, if the wall of the cell is coated with an azobenxene polymer. The alignment is strong and stable and has none of the disadvantages of the rubbed polyimide alignment layers.
Thermochromism and Photochromism
A series of liquid crystalline azobenzene polymers show intriguing thermoebromic and photochromic properties. The film as cast, below Tg, is red. Above Tg, the azobenzene chromophores associate in antiparallel pairs, and the film colour changes irreversibly to orange. Annealing high above Tg creates a smectic A phase which is opaque red. Illumination with linearly polarized laser light on the orange film destroys the antiparallel association and reverts the colour of the exposed film to red. The resolution of the photochromic process is very good and masks can be used to create the appropriate contrast. One example is shown in Figure 3.
Photoinduced Liquid Crystallinity
Azobenzene groups have been previously used to trigger photochemical liquid crystalline to isotropic phase transitions, because the trans configuration is mesogenic, but the cis configuration disrupts the liquid crystalline phase. We have shown that the reverse transition: isotropic to liquid crystalline may also happen in special conditions, and one of the examples is on a polymer that has no known thermotropic or lyotropic behaviour.
Photoinduced Chirality and Switching
Finally, some of the newest results indicate the possibility to induce chirality in nonchiral liquid crystalline azobenzene polymers, even in the absence of any structural chiral center, simply by irradiating with appropriate circularly polarized light. The mechanism is assumed to be similar to the photoinduced anisotropy by linearly polarized light, i.e. a hole-burning process. The handedness of the circularly polarized light determines the optical rotation in the polymer film. The photoinduced chirality can be erased and even reversed by using circularly polarized light of opposite handedness, thus this phenomenon is another example of an optical switch.
Although azobenzenes and their photoisomerization have been known for a very long time, all the above mentioned phenomena are new and allow a better understanding of basic science and a variety of photonic applications. This is a very exciting field of polymer science.
Almeria Natansohu, FCIC is a professor in the department of chemistry, Queen's University Kingston, ON and Paul Rochon is a professor with the department of physics, Royal Military College, Kingston, ON.