Polarization is one of the properties of light, in addition to intensity and wavelength.
Because human eye cannot distinguish this property it is often underrated, despite the fact it is one of the fundamental properties of light, and that mastering its manipulation opens a wide range of new applications.
Recently, techniques exploiting polarization are spreading rapidly, in fields spanning from LCD panels to optical telecommunications, from metrology to laser processing and so on.
Nowadays, “how to skillfully manipulate polarization” has become key in an increasing number of R&D and product development projects.
Using our photonic crystal is one way to get closer to an optimal use of light, for using its potential at its maximum.
a photonic crystal?
Crystals such as quartz have been used for a long time to manipulate polarization.
In crystals, the atoms making the crystal are arranged in an ordered way, along a regular lattice.
When this lattice is anisotropic, light going through the crystal experience a change in its polarization, due to the influence of the special orientations defined by the crystal lattice.
Natural crystals are usually monocrystals, made of only one domain having the same lattice, or polycrystals, which are a random aggregation of small monocrystal grains. In either case, it is very difficult to have fine control over the spatial distribution of crystal properties over its volume.
On the other hand, human-made photonic crystals are the result of a technique allowing building regular structures artificially.
Having control over crystalline properties at any point, one can design and manufacture a crystal with any desired pattern, if a manufacturing process adapted to the task is available.
In other words, the orientation of polarization along the crystal can be freely controlled, using photonic crystals.
This is the innovative process Photonic Lattice uses to realize actual photonic crystals for polarization control.
The auto-cloning method, as a practical way of generating photonic crystals, was proposed in 1996 by Dr. Kawakami, Photonic Lattice’s founder.
The method, which consists of depositing two different dielectric materials, one with a high refractive index, the other with a small refractive index, is realized by using modified sputtering techniques. The layers are deposited alternately on a corrugated substrate engraved with trenches, which serves as the basis for the corrugated layers, until the correct number of layers is achieved.
The reason for using a modified version of sputtering is that if we used a standard sputtering process, the layers would be gradually smoothed, as a growing layer of snow, resulting as a flat multilayer at the process end. To achieve structural anisotropy, which is key to the polarization properties, we use a modified version of the process preventing such smoothing and allowing the sloped shapes to be conserved at each layer. As each layer is “cloned” identically from the layer below, we ended calling this process “auto-cloning”.
Auto-cloned photonic crystals are, as explained above, an anisotropic stack of corrugated thin-films. With appropriate design, the photonic crystal obtained by this technique can realize familiar functions, such as polarizers and phase plates.
One of the most noticeable characteristics of the auto-cloning technique is that we can control the anisotropy axis direction (i.e., the transparency direction for polarizers or the retardance axis for phase plates) at any point of the crystal, as this direction results directly from the pattern that was engraved on the substrate.
For example, the integration of millions of micrometer size rectangular regions on the same chip is easy, by designing the corresponding substrate and performing normal auto-cloning over it.
In fact, the elements are not limited to having regions with uniform axis within them but can exhibit gradually changing axis orientation in curved designs. For example, axisymmetric polarizer and phase plates are possible.
Optical components manufactured with this technique are highly durable and resistant to high optical power, because they are made of inorganic materials and are just multilayered structures.
By their unprecedented patterning and integration capabilities, auto-cloned photonic crystals open a new world of innovative applications.
The original combination of image sensor with an auto-cloned photonic crystal array result is our polarization imaging sensor.
In contrast with conventional polarization measurement techniques, in which rotating filters are necessary, our sensor uses the high-integration power of photonic crystals to their advantage. Instead of rotating a unique bulky filter, small filters for every necessary direction are placed in front of each individual CCD sensor pixel, so measuring properties becomes a matter of appropriate image processing. With this sensor, the distribution of polarization properties is caught in one shot, at each point of the image, at high speed.
Polarization information is obtained by comparison of the intensity at adjacent pixels and simple curve fitting.
The range of polarization orientations is not, as the drawing may suggest, limited to multiples of 45 degrees, and in fact all intermediate polarizations in the continuous spectrum between 0 and 180 degrees can be detected by the camera.
By repeating the computation for each group of four pixels, the distribution of polarization properties over the whole sensor is achieved in a glimpse.