Nanoplasmonics and surface enhanced spectroscopy

Excitation of surface plasmon polaritons on aluminum-coated diffraction gratings formed on an inorganic chalcogenide photoresist: influence of surface relief profile
Indutnyi I. Z., Mynko V. I., Sopinskyy M. V., Danko V. A. and Lytvyn P. M.
V. Lashkaryov Institute of Semiconductor Physics Nat. Acad. of Sci. of Ukraine. 45 Prospect Nauky, 03028 Kyiv, Ukraine.  
E-mail: indutnyy@isp.kiev.ua 

Historically, the first and most studied plasmonic structures are metallic periodic structures (diffraction gratings), in which the excitation of surface plasmon polaritons (SPPs) was observed for the first time more than a century ago. The processes of SPP excitation in such structures have been intensively studied in recent decades. In a number of theoretical and experimental studies, it was found that the shape of SPP resonances in the angular dependence of the intensity of reflected light changes significantly when the grating groove profile deviates from a sinusoid. Therefore, in this work we experimentally studied the relationship between the groove profile of aluminum gratings and the shape of SPP resonances in the angular dependences of the intensities of specular reflection and diffracted radiation in the -1 order of diffraction. Gratings for studies with a period а = 700 nm were formed on films of As40S30Se30 chalcogenide photoresist by means of interference lithography and vacuum thermal deposition of an opaque aluminum layer about 80 nm thick. The grating groove profile was varied during their fabrication by varying the time of exposure and selective etching. 
It has been established that for the +1 order SPP resonance, an increase in the contribution of higher harmonics to the Fourier transform for the surface profiles of the gratings under study mainly affects the shape and amplitude of plasmon resonances in the angular dependence of diffracted radiation. At the same time, for -2 order SPP, a significant increase in the amplitude of SPP resonances is observed both in specular reflection and in diffracted radiation. This is due to the appearance in the Fourier transform of a component having a wave vector twice as large as the wave vector of the fundamental harmonic of the grating, and much higher efficiency of direct excitation of -2 order SPP through this component. It is shown that the integral efficiency of +1 order SPP excitation depends on the grating groove profile and decreases with increasing deviation of the grating relief profile from the sinusoid.