Study On Light Trapping Effect Of Subwavelength Metal Groove Array

Light trapping effect,also known as optical abnormal absorption,can limit the energy of photons irradiated to the structure surface to a small range.It has important applications in high conversion solar cells,high sensitivity optical detection and so on.When the electromagnetic field energy is confined near the structure surface,surface enhanced Raman scattering can also be achieved.In this paper,the rigorous coupled-wave analysis method is used to numerically simulate the light trap effect of the metal nano-groove array.The impacts of structural parameters on the light absorption efficiency of the metal groove array are explored and discussed.Physical mechanism of the effect is explained with the Fabry-Perot semi-analytical model.In this paper,it is proposed that the excitation of light trap effect for a metal slot array is the comprehensive contribution of the surface plasmon and the mode resonance in the grooves.The surface mode is responsible for coupling the energy that do not directly incident into the groove into the grooves,while Fabry-Perot mode resonance in the grooves is responsible for trapping the incident energy until it is transformed and absorbed.It should be emphasized that the energy coupling between the surface mode and the groove mode is bidirectional.Excitation of light trap effect requires that the intensity of mode resonance energy in the groove is higher than that of surface node to make sure more energy being coupled into the groove.However,surface mode with too strong intensity will compete for energy that was originally incident into the grooves,which might completely destroy the light trapping effect.Compared with other non-periodic structures in micro-nano optics,like discrete metal particles,the metal nano-groove array has many advantages like obvious physical mechanism and high repeatability.However,current micro-nano fabricaiton technology is difficult to obtain the right angle processing in rectangular grooves.So this paper explores the possibility of trapezoidal grooves arrays in replacing rectangular grooves arrays as optical trapping effect devices.Fabry-Perot model method in discussing the absorption enhancement by a rectangular groove array is introduced to analyze the absorption by a trapezoidal groove array.With the help of this model,the influence of side inclination angle on optical absorption rate by a trapezoidal grooves array is presented.Results show that the introduction of side inclination angle dose not attenuate the performance of the light trapping structure,however,it not only can reduce the processing difficulty,but also can provide theoretical research basis and machining parameter guidance for the design of light trapping devices based on a micro-nano grooves array.