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An introduction to phononic crystals (6)


Photons and phonons

We mentioned in the introduction that phononic crystals share a lot with photonic crystals. If optical frequencies (in the few hundreds of Terahertz range) are much larger than acoustic frequencies, it is perfectly feasible to match their wave lengths, close to a value of one micron, for instance. Thus, a given periodic micro structure can be simultaneously a photonic and a phononic band gap material, which leads to the term phoXonic crystal (in which the “X” replaces indifferently the “t” and the “n”). The interest of such an artificial crystal is to provide the possibility of confining simultaneously optical and acoustic energy within an extremely tiny volume, in the prospect of enhancing the interactions between the two kinds of waves.

Cross-section of a photonic crystal fiber

Cross-section of a photonic crystal fiber

Acousto-optics already has a long history, since it was born at the beginning of the twentieth century. Acousto-optical modulators are employed in many applications, notably for the steering of laser beams. However, the interaction of photons and phonons in micro structures is a subject that draws more and more attention from researchers. For instance, with colleagues of the Universities of Erlangen in Germany and Campinas in Brazil, we have recently studied the Brillouin effect in micro structured optical fibers, also called photonic fibers. This effect, discovered by the French physicist Léon Brillouin in 1922, designates the inelastic diffusion of a photon by a phonon. We have identified certain acoustic phonons guided inside a micron size solid core that occur in the Brillouin effect.

Brillouin acoustic phonon

Brillouin acoustic phonon

A photonic crystal fiber is an optical fiber containing micron size air holes running along its axis for hundreds of meters, or even kilometers. Picture (a) shows the cross-section of a silica photonic crystal fiber fabricated at the University of Bath in the UK, observed with a scanning electron microscope. The central core of this optical fiber is the equivalent of a glass rod with a diameter of 1.2 microns, or approximately one hundredth of a human hair. It can transport optical energy over very long distances. As we have shown recently, such a photonic fiber is also an efficient phonon trap. Picture (b) displays the energy distribution of an acoustic phonon guided by a phononic band gap effect along the core of the fiber. This phonon is involved in the stimulated Brillouin scattering effect, a non linear acousto-optical effect that limits the optical power that can be transported along an optical fiber, and hence has a strong economical impact in telecommunications.

Conclusion

The research and application perspectives are many and motivating for phononic crystals, a concept that emerged at the beginning of the 1990's. It may seem surprising that a technology based on the interference of waves has appeared so lately. However, the study of phononic crystals requires lots of theoretical and conceptual development, numerical simulations greedy with computer time, and clean room technologies to manufacture samples. It is only recently that equipments and knowledge have been made available for this purpose. Research on phononic crystals is still for the most part devoted to the exploration of the singular properties of these artificial materials and to proof-of-concept experiments. We however bet that the coming years will witness the first devices exploiting acoustic band gaps for signal processing. On the fundamental side, the exploitation of the simultaneous confinement of photons and phonons in phoXonic crystals has not yet delivered its secrets and is a promising research direction.

Please contact us for more information or to discuss possible collaborations on phononic crystals.

V. Laude, A. Khelif, S. Benchabane