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Plasmonics for efficient nonlinear components at telecom wavelengths

Main Researcher: Peter Debackere

Routing information through an all optical network is an important issue nowadays. Because of electro-optical conversions the switches are often the bottlenecks of the network. In order to solve this problem one can devise an all optical switch.

Optical switches are based on optical bistability, this basically means that depending on the prehistory of the component a certain input can lead to two different outputs. Practically this is achieved by placing a non-linear material inside a resonant cavity, the combination of feedback and non-linearity then gives rise to bistability.

Although the Kerr-effect is a very promising candidate for the non-linear section as it means a near-instantaneous intensity dependent refractive index change, the effect is quite weak in most everyday materials. This means that one needs large incoming intensities in order to observe bistability, intensities way too large for telecom applications.

These problems can be circumvented however by using a metamaterial consisting of small (nanometer scale) metallic particles embedded in a host dielectric medium. These metallic particles confine and enhance the incoming electric field in a very small region of space, so if we succeed in placing nonlinear material in that particular region of space we will have enhanced the Kerr effect significantly (enhancement factors up to 10000 have been reported)

The main reason for this giant field enhancement is the plasmon resonance of the metallic particles. The name plasmon is used for two different types of electromagnetic phenomena. A surface plasmon is an electromagnetic wave trapped on the surface because of its interaction with the free electrons of the conductor (strictly spoken they should be called surface plasmon polariton to reflect this hybrid nature). A particle plasmon on the other hand is a dipole or multipole resonance of the free electrons of a metallic particle. Because of the bound and non-radiative nature of these plasmons the electric and magnetic fields are strongly confined to the surface of the conductor.

The excitation of surface plasmons on infinitely extending metallic films has been known for over half a century and their properties are well understood. Depending on the metal used, surface plasmons can be excited over the entire visible spectrum, from ultra-violet to infrared, including the near-infrared at the optical telecommunication frequencies. However, much work remains to be done in the telecom part of the spectrum as most research thusfar has focused on the visual and ultraviolet range.

The main goal of this project is to create a metamaterial in which the Kerr effect is sufficiently strong to observe bistability at intensities custom to telecom. Once this has been achieved three possible applications will be looked into

  • Intrinsic Optical Bistability: this is optical bistabilty without the presence of an external resonating cavity. The non-linear behaviour is caused by the non-linear equation connecting the applied field with the local field.
  • Photonic Crystal Structure: the nonlinear metamaterial can be incorporated in a photonic crystal in order to successfully couple light in and out of the structure.
  • Plasmonic Waveguides: eventually we will devise a much smaller structure than photonic crystals. Surface plasmons enable us to confine light to structures much smaller than the diffraction limit. A lot of different designs for plasmonic waveguides have been proposed part of this project is to select the most promising candidate amongst them. Using these plasmonic waveguides one can devise a similar structure as before which has a non-linear zone, thus creating an all optical switch.

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