Absorption Characteristics And Applications Of Subwavelength Structural Devices

Subwavelength structure devices, as a new developing technology, have beenattracting much attention. When the feature size of the devices fall down to subwavelengthregion, many properties that cannot occur under macroscopic conditions will emerge. Newdevices within electromagnetic wavelength band with novel functions can be designed byusing the subwavelength structures. In this thesis, we propose and investigate twobroadband absorbers, one is for400nm-1100nm region (visible-near infrared) and the otheris for5GHz-15GHz (microwave). The absorption characteristics of the two devices areinvestigated with FDTD and FEM numerical simulation respectively. The detailed resultsare as follows:We first report a new broadband absorber for400nm-1100nm with a periodicallychirped subwavelength structure implemented in both one-dimensional (1-D) andtwo-dimensional (2-D) schemes. The subwavelength structure is composed of an array ofchirped metal-dielectric-metal resonators formed by top metal film and silicon layer lay ona thick bottom metal layer. Each resonator absorbs photons largely at its own wavelengthdue to localized surface plasmon resonance. Broadband absorption can be achieved byincorporating different plasmonic resonators with different dimensions in one unit cell. Theaverage absorptance of the device can be up to89.6%after the appropriate design of thesubwavelength structure and the optimization under the condition of phase-matching at therange of400nm to1100nm. The2-D chirped subwavelength structure is almostindependent of the light polarization due to its symmetrical structure. The short-circuitcurrent density of an ultra-thin crystalline silicon solar cell using the periodically chirpedsubwavelength structure is13.99mA/cm2, with an enhancement of~9.5than the bare solarcells (1.48mA/cm2). We also propose a broadband metamaterial absorber for5GHz-15GHz using atwo-dimensional (2-D) dually chirped periodical subwavelength structure and the averageabsorptance of the absorber can be up to97.8%. The subwavelength structure is composedof a periodic array of dually chirped metal-dielectric-metal resonators made of an array ofmetal-dielectric multilayered quadrangular frustum pyramids. Each resonator absorbselectromagnetic energy largely at its own frequency due to electromagnetic resonanceunder the condition of impedance-matching. Broadband absorption can be achieved byincorporating different resonators with different widths in one unit cell. The absorption inthe whole interested bandwidth can be optimized by incorporating different resonators withdifferent thicknesses. The2-D dually chirped subwavelength structure is almostindependent of the electromagnetic wave polarization due to that the surface of resonator issymmetric.