Research On The Key Technology Of Broadband Terahertz Functional Devices

The advent of metamaterials has led the terahertz technology developing at a fast pace,since this kind of artificial composite structure has the extraordinary electromagnetic properties which natural materials don't have.They can manipulate the effective permittivity and permeability through tuning the structure,so they are widely used in the electromagnetic devices to arbitrarily tailor the electromagnetic properties.However since the traditional terahertz metamaterial functional devices are mostly based on metallic resonator which only function at their resonating frequency,the working bandwidths of the functional devices are thus greatly limited.In this article,we will make discussions on two terahertz functional devices: the terahertz Lüneburg lens and the terahertz absorber.We combined the specific theory to enlarge their working bandwidth and also verified our design through the experiments or the simulations.Firstly,we adopted the optics transformation theory and the effective medium theory to realize the functionality of the Lüneburg lens.Because there is no material with gradient refractive index in nature,we designed a subwavelength structure which can achieve the gradient refractive index by changing the diameter of the air hole inside it.These subwavelength structures were arrayed periodically to realize the functions of the terahertz broadband Lüneburg lens.Simulation results showed that the working bandwidth of such Lüneburg Lens is up to1.25 THz.In addition,we also designed a kind of terahertz broadband absorber based on the diffraction grating.Unlike the traditional terahertz broadband absorber based on metamaterials which achieved a relative wide absorption bandwidth through stacking several layers of metallic resonator layers,our design is much more simpler and easier to fabricate.Besides it can obtain an even wider absorption bandwidth.The grating layer in our design functioned as an anti-reflection film in the lower frequency,while in the higher frequency,the diffracted wave make the device fulfill the matching impedance condition and eliminate the reflection.Those will enhance the absorbance for a wide range of frequency.We also used the semiconductor technology to fabricate this absorber.Its performance is evaluated by the terahertz time domain spectroscopy(THz-TDS)system.The experimental results proved that the absorber is omnidirectional,polarization independent and can attain an over 95% absorbance from 0.9THz to 2.1THz.Based on this research,we also found that by changing the structure of the gratings or introducing the interference of the substrate can mergemore absorption peaks into the absorption spectrum,which will increase the absorption bandwidth.In summary,we listed two broadband terahertz functional devices,which all have huge potentials in application.The research and design of these devices have an important values in designing other broadband functional devices.