THz photonic devices based on metallic rod arrays

• Main Authors: Chien-ChunPeng, 彭建竣
• Other Authors: Jia-Yu Lu
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/97407944255662952852

碩士 === 國立成功大學 === 光電科學與工程學系 === 100 === Terahertz (THz) radiations are electromagnetic waves sandwiched between visible lights and microwaves and their frequencies are ranging from 0.1THz to 3THz. THz wave has several unique properties such as to directly identify biological molecules and with a high data capacity for communication application. Recently, THz technology grows rapidly and various THz applications have been extensively developed, which are attributed to the great advances of THz generation and detection techniques. Especially, THz molecular sensing, bio-imaging, as well as development of various active and passive devices for THz communication application attract lots of attentions in recent years. In the thesis, we designed and successfully manufactured various periodic metallic rod arrays (MRA) with millimeter sizes for THz photonic devices and molecular sensing applications. Based on this structure, we successfully demonstrate a multifunctional THz filter which enables to act as a high pass filter and a band rejection filter. In addition, we utilized the MRA to successfully demonstrate minute gas molecules and nano-films detections. Firstly, we have successfully fabricated a three-dimensional dielectric cylindrical array with high-aspect ratios by using photo-polymers through a simple ultraviolet light microstereo-lithography (uSL) process. Based on surface plasmon effect, we demonstrated for the first time that the single and two rows of MRA are able to act as a THz high-pass filter under TE polarized excitation. The experimental cut-off frequency of the high pass filter is decreased when the period of the rod array becomes larger, and it is agree well with the theoretical result. The home-made THz high pass filter based on MRA has advantages including: the simple and fast fabrication due to the short UV exposure time, and with a high tunibility of cut-off frequency based on adjusting rod diameter. The maximum spectral shift of cut-off frequency is demonstrated over than 100GHz for 100um-long period variation. Secondly, we have successfully demonstrated a THz band rejection filter utilizing the multi-rows of MRA under TM polarized excitation. Based on the strong scattering between the metallic rods, the obvious bandgap caused by Bragg reflections has been observed and its best power rejection ratio is demonstrated greater than 23dB. Additionally, the central frequency and bandwidth of the Bragg forbidden band enable to be tunable significantly by changing the dimension of MRA such as the period, rod diameter, and array-columns. The experimental results show that the forbidden bandwidth is significantly increased with the occupied ratio of the metallic rods in the device chip due to the large scattering cross section, and the central frequency of forbidden band can be controlled by the period of metallic rods. The experimental results and the FDTD theoretical calculations make well a consistence. For molecular sensing applications, we have respectively demonstrated the minute vapors sensing and nanofilm detection utilizing the MRA operating at THz frequency ranges. For thin film sensing application, we have successfully demonstrated to sensitively detect the subwavelength-thick polypropylene film covered on the rod arrays based on the refractive index induced phase change, and the detection limit of the thickness variation can be down to 1um (~wavelength/544@520GHz) which is the highest sensitivity for thin film detection based on the THz photonic crystals devices. In addition, we also successfully identified two kinds of semiconductor nanofilm made by SiO2 and ZnO based on the MRA. For volatile vapors sensing application, we successfully identified various kinds and different concentrations of vapor molecules by using our THz high pass filter consisted of single row of metallic rods. The metallic rods in the THz high pass filter device are highly sensitive to the surrounded refractive indexes of materials, and it causes the spectral shift of the cut-off frequency while index change of the surrounding is generated. We have successfully both identified various concentrations of methanol vapors with the minimum detectable concentration variation of 11.6% equivalent to molecular density of 0.667 nano-mole/mm3, as well as distinguished different types of gas molecules such as alcohol and methanol. Based on the THz high pass filter device, the measured minimum detectable gas molecular density variation can reach 0.67nano-mole/mm3 which is the highest sensitivity of current THz gas sensors. The sensing results have potentials applied for future detection of dangerous gas leakage and biomolecular membrane.