RESEARCH PROJECT

• Null Transmission Ellipsometry

• Differential Optical Reflectivity

• Surface Tension by Direct Force Measurement/Heat Capacity/Optical Reflectivity

• Surface Tension by Drumhead Resonance/Optical Reflectivity

• Surface Tension by Deflected String/Optical Reflectivity

• Depolarized Reflected Light Microscopy

In general, the ellipsometric parameters D and Y describe the orientation and ellipticity of polarized light.   Physically for free-standing film, Y represents the effect of the film on the orientation of polarized light and D is related to the biaxiality of the film.   The basic idea of ellipsometry is to measure the optical parameters, D and Y of the film.

Here the figure on the right shows real system picture.   Cartoons below show system schematic figures.

As shown in Figure (a), before the laser beam enters the oven, the linear polarized light passes through a 1/4 wave plate and becomes circular polarized.   Then the laser beam passes through the window, polarizer, compensator, sample, a second polarizer (called analyzer) and window.   Finally it reaches the detector.   The incident angle of the laser beam is 45^o.   The compensator is another 1/4 wave-plate with its fast axis fixed at 45^o to the axis.   Positive angles are defined as clockwise rotations when looking along the light propagating direction ().   The polarizer and analyzer are installed in two rotation stages.  Their orientations are controlled by a computer via a motion controller.   The resolution of the motion controller is 0.001^o.  

In Figure (b), and are the polarizations parallel and perpendicular to the incident plane of the beam traveling in the direction, respectively.   Based on these coordinates, the ellipsometric parameters are defined as follows:   D is the necessary phase lag between the p- and s-components of the incident light for the transmitted light to be polarized at an angle Y+90^o relative to the axis.

These two ellipsometric parameters, D and Y, are found by rotating the polarizer and analyzer until a null signal is registered at the detector.   With the configuration described above, the D= -2P_n + 90, Y=A_n, where P_n and A_n are the angles of the polarizer and analyzer relative to the axis.

Then by studying D and Y , molecular packing in the free-standing film can be figured out.   For example, for a two-layer film as shown left, D is different for the two cases in which the orientation of electric field is changed by 180^o.   By observing the difference of D with different orientation of electric field, different molecular structure can be notified.   Simulation will be performed to confirm the molecular arrangement using 4x4 matrix methods.   The figure below presents three distinct Y versus D curves corresponding to three different surface structures observed at different temperatures in one special liquid crystal compound.

Three curves describe three distinct surface structures (1) nonplanar, (2) anticlinic and (3) synclinic respectively.   Cartoon beside the figure shows these three structures.   Symbols are the data and solid lines are simulation results.