Design, fabrication and characterization of micromachined THz absorbers

Abstract

The purpose of this research is to develop a new class of THz absorbers for imaging and sensing applications that can provide improved functionality and perfor mance. These absorbers consist of periodic arrays of layered cells made of a back-plane metal layer, opaque to transmitted radiation, a dielectric substrate as a spacer, and a metallic patterned layer as resonating part. Once electromagnetic wave impinges, the structure reflects the incident wave, except in a specific frequency band determined by the absorber’s physical properties. In the presence of a blocking layer which eliminates transmitted power, absorption in that specific frequency band occurs. The effective per mittivity and permeability of absorber unit cell reach the same value and be negative simultaneously to have perfect absorption. Negative values of and µ do not happen in nature. At this point, ”metamaterials” are introduced. Metamaterials (MMs) are artificially engineered structures used in THz absorbers. MMs’ concept is derived from replacing the natural materials with engineered materials where their sizes are much smaller than the given wavelength. Intriguing properties of MM are achieved from the degree of the skillfulness of its structure (geometry, shape, orientation, and size), while its chemical construction plays an insignificant role. Currently, THz absorbers are in use as medical and security applications. Many types of MM based absorbers have been introduced based on their shapes, sizes, and configurations. This research has focused on absorbers with a low dependency of absorber’s performance on the exciting wave’s incidence and polarization angle. Besides, we demand these structures to operate in a wide range of frequency spectra designed to, with small dimensions of the patterned array compared to the wavelength; in other words, we concentrate on the design of wideband THz absorbers. In Chapter 1, we start with a brief introduction to THz radiation and its benefits and applications. It also comprises an overview of the history and theory of MMs. Var ious designs demonstrating the performance and operating frequency bands of MMs and their applications have been studied. A comprehensive study of wave propagation in the right and left-handed media is presented in Chapter 2. Basic models of proposed MM absorbers are introduced in Chapter 3. The principle of resonance is based on a lumped LC circuit and broadband operation provided in this chapter. Afterward, the design, Fabrication, and characterization of a MEMS-based THz metamaterial based absorber are described in Chapter 4. A new 3D design inspired by honeycomb structures as broadband and incident wave independent absorber has been proposed. Chapter 5 includes the design, simulation, fabrication process, and measurement re sults of this 3-dimensional MM based broadband absorber. The proposed absorber exhibits excellent absorption characteristics. The unique feature of the proposed ap proach is having very low sensitivity with respect to the incidence angle. The ability to maintain their absorption properties in THz frequencies, unlike their small feature sizes compared with traditional approaches, made these porous structures attractive for sensing applications. Throughout this dissertation, both physical and numerical interpretations of the spectral behavior of new designs are provided. Physical insight into the operating mechanism is the key to further improvements and creates more complex surfaces with desired frequency responses.