engleski

Particle shape influence on a plasmon hot spot

Thin metal island films (MIF) can show unusual optical phenomena such as near field enhancement, negative refraction and frequency selective absorption in the optical spectrum. All these effects are a consequence of the surface plasmon resonance (SPR). SPR is produced by coupling of the incoming electro-magnetic wave (light) into an evanescent surface wave of free electrons in the metal at the interface of the metal and dielectric. MIF can be made by the simple thermal evaporation process that can produce, either isolated, or clusters of metal nanoparticles with different morphological properties embedded in a dielectric matrix, depending on the deposition parameters (deposited mass of metal, substrate temperature, rate of deposition, etc.) [1]. Between such particles there is a large near field enhancement known as a "hot spot" (Fig. 1.) which is a result of a strong resonant process at the SPR and is very dependent on the shapes of such particles or islands. Such "hot spots" can be very useful for surface enhanced Raman spectroscopy (SERS) and various sensing applications [2]. Unfortunately, the geometrical shapes of such metallic nano-structures are usually very complex and therefore it is difficult to simulate their electro-magnetic behavior using classical Mie theory or similar analytical methods. One must use numerical algorithms such as FDTD (finite differences time domain) in order to get accurate results. The drawback of this method is a heavy computational load both on the computer resources and simulation time. In this work we study the effects of the mutual distance, size and shape on the field enhancement between two metal nano-particles using a FDTD numerical program. Results of this study are the first steps toward full wave simulations of large MIF on a dielectric substrate with a random distribution of positions, shapes and sizes of the islands. Such a simulation model would be very beneficial for optimization of the deposition process and could provide information on the near field of the MIF.

plasmonics; hot spot; near field;

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