Study of Infrared Transmission through Gratings with Various Metal
Date Issued
2007
Date
2007
Author(s)
Chen, Fu-Chuan
DOI
zh-TW
Abstract
Since 1902, Wood had found the anomalies of reflectance spectrum arose from electromagnetic wave impinging on the surface of metal grating. People never stop studying the transmission anomalies of metal grating. Most of them only simulate by software but do experiments rarely, which is not convictive to me. The main of this thesis is investigated in experiment, and we can build the theoretical model by analyzing the phenomenon which is observed from the data of experiments.
Because we need to make some gratings, whose line width is under micrometer scale, it’s difficult to implement with traditional optical lithography. So we use the e-beam lithography, which is used to design the pattern, RIE and thermal evaporator to make metal gratings. Finally, infrared transmission of metal grating is measured by FTIR. Grating is one dimension, which would make different polarization directions of light encounter different structures. To avoid these situations interfere with each other, we separate into TE and TM mode to study. First, we discuss the effect of line width, period and wavelength without changing metal. The modes of electromagnetic wave can exist in or out of the grating is deduced, which are used to explain the phenomenon we observed. We find that coupling between electromagnetic wave and gratings is the cause of transmission anomalies. Then we make gratings with various metal and find that the coupling between electromagnetic wave and metal grating with high conductivity is stronger. The coupling is corresponding to the electrons of metal surface, and we build the theoretical model.
There are many free electrons on the metal surface. When an electromagnetic wave impinging on metal, electrons will oscillate by the electric field, and the oscillating of electrons will provide electromagnetic field of certain frequency. The periodic structure of metal grating makes the electromagnetic field periodic, which can provide horizontal wave vector. The influences caused by electromagnetic waves which gain additional horizontal wave vector passing through gratings are different from those caused by directly impinging ones and this is why metal gratings have transmission anomalies. Different polarization directions of electromagnetic wave make electrons oscillate in different directions and encounter different metal structure, which make transmission different from each other.
Transmission enhancement can’t be explained sufficiently by surface plasma theorem, and it may be correspond to the evanescent wave. But now we only have a fundamental theoretical model, a serious mathematical model is still studied. Although we can’t explain everything we observe, we believe that our research can provide more applications of metal gratings.
Subjects
金屬光柵
電子束微影術
TE和TM模態
消散波
表面電漿波
metallic grating
E-beam lithography
TE and TM mode
evanescent wave
surface plasma
Type
thesis
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