Paul Rochon

Description of present research interests

The main thrust of my research is the study of the optical properties of polymer thin films containing azobenzene molecules that are attached to the main chain. The work consists of an investigation the fundamental properties of the azo polymers as well as the possible use of these polymers as to fabricate novel optical devices.

The key phenomenon of interest in this work is the optically induced isomerization of the azobenzene. During the isomerization the molecule changes shape from a linear optically anisotropic molecule to a more optically isotropic bent shape, then the molecule reverts back to the more stable linear shape due to absorption of light or heat. After this cycle the molecule will probably have adopted a new orientation unless it is greatly constrained to return to a fixed position by a rigid matrix. The re-orientation can be observed when linearly polarized light is used to induce birefringence and dichroism . The same mechanism can even lead to optically inscribed chiral molecular or domain arrangements in the film. We have studied these effects in the recent past and we intend to use this molecular orientation mechanism to alter the local refractive index on the polymer film in a controlled manner. The optically induced molecular motion also deposits thermal energy locally even if the average film temperature is not greatly affected and the motion (and/or energy) can act as a "motor" to produce various macroscopic effects resulting from the ensuing mass movement in the polymer matrix.

The present work concentrates on the study of optically induced mass movement in particular on induced surface relief gratings and patterns on thin films. At the same time, we study the use of azobenzene based optical devices for modulating light with light. In these studies the azobenzene films are used as optical devices that can be made or altered in house to study various optical designs to achieve optical modulation.

Optically induced Birefringence

The azobenzene molecule is usually in the trans form, which can be considered to be uniaxial. When this molecule is exposed to visible light it can undergo photoisomerization and change its form to the cis configuration. The molecule can then thermally or optically transform back to the trans form. After this cycle the molecule can find itself in a new orientation. This process can be used to control the macroscopic orientation of the molecules in the film. When linearly polarised light is used to activate the trans-cis photo-reorientation cycle only the molecules that are in the direction of polarization will absorb light and undergo the change of orientation. In this way the number of molecules parallel to the direction of polarization can be depleted and those perpendicular can be increased. The result is macroscopically observable dichroism and birefringence in the film in the region of elimination. In past years we have studied this effect in order to quantify the mechanism and to make polymer films that can record polarization holographic information and that can also be used in photonic applications. We have done systematic studies of side chain polymers and shown that a very stable optical birefringence can be induced optically and that this birefringence is also optically erasable. We are shown that the image is stable for temperatures up to be glass transition temperature of the polymer. We have recently demonstrated that a polarization hologram could be recorded and that polarization information could be recovered, for example we separated light according to its chirality. We have synthesised the number of polymers with the transition temperatures as high as 265C and shown that stable birefringence could be induced in these films. We have modelled the dynamics of the process and found that a bi-exponential kinematic model was sufficient to explain the results. We conclude from this that in non-liquid crystal polymers two mechanisms are responsible for the relaxation of the photoinduced birefringence. Although more sophisticated models do exist, the molecular environment in the polymer films is sufficiently complex that too many parameters are required to fit to the data and that no new information is gained. In some cases a stretched exponential it is required when effects such as liquid crystallinity come into play. We have also tried to enhance some of the properties of the azo polymer films by the proper design and synthesis of azo benzene containing polymers for specific needs. In order to increase the photoinduced birefringence, increase the image stability, and decrease the absorbance in the visible, we have been studying the co-operative movement of non-absorbing molecules that are placed in proximity to the azobenzene. We have shown using visible and infrared measurements that the non-absorbing molecules move in concert with the azobenzene molecules and that high, stable birefringence can be maintained while decreasing the absorbance of the film. We have also shown that polymers with high glass transition temperatures with the azobenzene in the main chain also exhibit photoinduced birefringence that is very stable. This was surprising since it was initially thought that the main chain inhibit be read orientation of the azobenzene. The results were confirmed both from measurements in the visible and in the infrared. We have also shown that by increasing the spacer between the azobenzene and the polymer main chain the polymer film can exhibit liquid crystalline phases. High birefringence with very high stability can be induced in these films. The image however is more difficult to erase. We have also embarked on a study of optically induced circular dichroism that has been recently observed in liquid crystal polymers. Our results show that circular dichroism can be directly controlled using circularly polarised light, chiral order can be directly induced or switched. This new phenomenon has yet to be satisfactorily explained. We have also been looking at the control of liquid crystal display using azobenzene as in solution or as surface control films. In both cases we have shown that the orientation of the liquid crystal could be altered with light directly.

Surface relief gratings

In 1994 we discovered that surface relief structures such as gratings could be optically inscribed onto as polymer films using medium power light.

Since then we have embarked on systematic studies to understand the mechanism is responsible for this phenomenon. We have proposed a model to explain the behaviour of the formation of the gratings. The model is separated into two parts, in the first place if one assumes the light can produce a force in the film that can lead to the formation of the gratings, the dynamics can be described using Navier Stokes equations. The second part of the study consists of identifying mechanisms that can generate an internal force that would lead to flow. The first part of the model seems to explain the dynamics reasonably well. In the last three years we have pushed the study of the formation of the surface relief using fluid flow models assuming that the light generates a sinusoidal force pattern in the polymer film. This study was undertaken in collaboration with Dr. Ulli Pietsch's group from Siegen, Germany. This group is involved in low angle x-ray scattering, fluid flow modeling and ,with us, light scattering. When modeling, and during experimentation, it was found that when the polymer flows it does not maintain uniform density but instead high density changes (as high as 15%) are seen in the regions of material accumulation and depletion. This means that when we inscribe surface relief gratings there is an accompanying volume density grating is being generated and this grating can diffract light with almost as much efficiency as the surface grating. Furthermore we have observed that when we heat the film above the glass transition temperature, surface tension is sufficient to make the material flow and erase the surface grating, on the other hand the density grating is seen to grow in polar azopolymers and we end up with a flat film containing a periodic density grating (this very reminiscent of fiber Bragg gratings but here the density change is 0.15 not .001). We have found that in our typical polymer where the azobenzene is attached to the side on the methacrylate chain the light tends to move the polymer away from the high intensity regions into the low intensity regions.

The opposite behavior was found when the spacer between the azo molecule and the chain was increased to allow more molecular motion. It was thought that this behavior may be the result of an aggregation mechanism taking place when the polar azomolecules are allowed to "crystallize".

We are also taking advantage of the fact that we optically write multiple gratings and structures on the same spot in house. This permits us to design, fabricate and test a number of optical configurations for new applications. We have successfully written waveguides with corrugated surfaces to generate resonant optical structures that act as couplers, filters and surface relief templates. We intend to concentrate on multiple grating structures. In particular we will superimpose two gratings on the same spot in order to generate two counter propagating waves in the underlying waveguide. In that case we will have generated a standing wave in the resonant structure, thereby increasing the local field strength and enhancing the devices sensitivity to non-linear effects and perturbations. The underlying waveguides will be made of a variety of materials for different applications. We will start with piezoelectric films for electric field modulations, with optically non-linear films for second harmonic generation and optical modulation, with fluorescent dye doped films for enhanced distributed feedback lasing structures. In addition the present work will continue to investigate using optically structured films as substrates or templates to control surface plasmon propagation on metal films deposited on the surface. In this context, we are particularly interested in enhancing surface plasmon based sensors for specific molecules deposited on the surface.