Excitation and propagation of Surface Plasmon Polaritons in periodic structures of metallic cylinders
Surface plasmons have been studied in thin metallic films extensively over the past years. Their properties with respect to parameters as the material constants and film thicknesses are well understood. Over the past few decades, interest has grown in enhanced transmission through periodic metallic samples, such as hole arrays and deep metallic gratings. Recently this has been attributed to the resonant coupling of surface plasmon polaritons (SPPs) within the cavities of such samples causing the transmission of radiation through structures with cavity widths much smaller than the wavelength. In the THz spectral range structures with dimensions comparable to the wavelength can be fabricated easily and our experiments at comparable long wavelengths can be used as a model system for other wavelength ranges.
In a first set of experiments the excitation and propagation of SPPs in periodic structures of metallic cylinders was investigated. Already in 1902, Wood observed anomalies in the reflected spectra of ruled metallic gratings using a white light source, and these were later identified as surface plasmon polaritons. Since then many experiments have been conducted in order to fully understand and utilize this phenomenon. Excitation and emission studies were performed for different diameters of the cylinders, and for various materials and angles of excitation and detection. In order to determine the propagation speed of the SPPs, time-resolved measurements have been performed.
There is an excitation of surface plasmon polaritons if
The inverse grating vector can be calculated with
For erpendicular incidence the frequency νSPP of the surface plasmon is
For incidence at angle α the frequency νSPP is obtained from
Excitation frequencies are "missing" in the transmitted spectra, surface plasmon polaritons propagate in two directions.
The next figure shows the transmitted absorption and emission spectra of SPPs excited on a periodic structure of 1.5 mm diameter metallic rods under perpendicular incidence. The minima in absorption and maxima in emission crrespond to the frequencies of Wood's anomalies.
The shape of the absorption lines are caused by an interference between directly transmitted THz radiation (caused by a finite gap between the cylinders) and the emission of propagation SPPs along the surface of the array. For an incidence under an angle α both absorption and emission lines split corresponding to the excitation of SSps with different frequencies propagating in opposite directions.
Transmission and emission properties were simulated using classical electrodynamics:
There is excellent agreement between experiments and simulation:
Terahertz (THz), surface plasmons, periodic struktures, material constant, Wood's anomalies, thickness, transmission, wavelength
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