Flexible and Freestanding Photonic Crystals

Hank Hogan

Researchers from Università Politecnica delle Marche in Ancona, Italy, have created photonic structures in freestanding polymer films. The process of fabricating the films is relatively simple, and the resulting micron-resolution structures are uniform over large areas.

Scanning electron microscope images show flexible photonic crystals that were created by holographic recording in a photosensitive material. A direct (top) and an inverse (bottom) triangular lattice are depicted. Superimposed in the upper-right corner of each image are the simulated interference intensity patterns of the three laser beams used to create the pattern. Images reprinted with permission of the American Institute of Physics.

Because the films are freestanding, they can interact with the environment on two sides, effectively doubling their active area. Moreover, their flexibility makes them react to motion. When combined with an exploitation of their homogeneous pattern, those attributes could make the films commercially useful.

“There is a real possibility of using these materials as high-resolution biological, chemical or mechanical sensors,” said postdoctoral researcher Francesco Vita.

Scanning electron microscopy also shows a cross section of a 30-μm-thick sample of the freestanding lattice. The pattern is two-dimensional, and the voids between rows run through the sample.The inset shows a sample mounted between two metallic rings.

Vita noted that the group’s research is similar to that done by others in that it is based on holographic recording of photonic structures in polymer-dispersed liquid crystals. However, his group’s work yields freestanding and flexible films, not ones sandwiched between two glass slides.

The researchers spin-coated liquid crystals that were dispersed in a photosensitive monomer onto a transparent substrate, resulting in coatings of 5 to 30 μm. They created a regular pattern in the film through holographic recording, using three linearly polarized 514-nm laser beams produced by argon-ion lasers from Coherent Inc. of Santa Clara, Calif. The beams struck the film at the same angle with respect to the Z-axis, and interference created a regular structure in the X-Y plane because of the polymerization-induced phase separation of liquid crystal and polymer. The final structure consisted of a periodic pattern of liquid-crystal domains embedded in a polymer matrix.

Another scanning electron microscope image reveals that the flexible photonic crystal is highly homogeneous over a relatively large area. The inset shows the diffraction pattern created from a triangular freestanding photonic crystal, with the regular spacing of points clearly indicating that the pattern is uniform.

The scientists removed the liquid crystal and freed the film — the order of these steps a matter of convenience and quality. Finally, they supported the film on a structure, such as by suspending it across a ring.

By changing the angle of the lasers, they changed the spacing of the periodic structure. Altering the angle of the beams with respect to the X-axis changed the type of lattice created, allowing them to dial in a square, triangle or other shape.

With this technique, they created patterned films with a periodicity as low as 4 μm and with an area of 1 cm². Interferometric tests showed that the pattern was uniform in X and Y, whereas scanning electron microscope images revealed that the voids created in the film extend completely from one side to the other.

The films have commercial prospects in sensors and flexible displays as well as in telecommunication and micromechanical devices. Vita noted that the films also could be used to study the mechanical properties of microstructured materials.

“In this context, having a freestanding film — hence, not interacting with a substrate — allows a better isolation of the material properties,” he said. The group also is engaged in collaborative research in this area.

Applied Physics Letters, Sept. 3, 2007, Vol. 91, 103114.