Metamaterials with Active Circuits
The focus of this dissertation is on embedding active circuits into metamaterials to mitigate some of their intrinsic problems such as losses and create new hybrid metamaterial devices. The dissertation can be divided into four distinct contributions.
For the first contribution, an experimental demonstration of loss compensation in metamaterials using embedded active transistor based circuits is described. Each unit cell of the metamaterial is embedded with a cross-coupled transistor pair based negative differential resistance circuit to cancel these losses. Design, simulation and experimental results for Split Ring Resonator (SRR) metamaterials with and without loss compensation are presented. Results indicate that the quality factor (Q) of the SRR improves by over 400% at 1.6GHz, showing the effectiveness of the approach.
In the second contribution, we demonstrate an example of a metamaterial absorber (MMA) detector array that enables room-temperature, narrow-band detection of gigahertz (GHz) radiation in the S-band (2-4 GHz). The system is implemented with a commercial printed circuit board process and we characterize the detector sensitivity and angular dependence. A modified MMA geometry allows for each unit cell to act as an isolated detector pixel and to collectively form a focal plane array (FPA). Each pixel can have a dedicated microwave receiver chain and functions together as a hybrid device tuned to maximize the efficiency of detected power. The demonstrated sub-wavelength pixel shows detected sensitivity of -77 dBm, corresponding to a radiation power density of 27 nW/m2, with pixel to pixel coupling interference below -14 dB at 2.5 GHz.
For the third contribution, a diode switchable metamaterial reflector/absorber is demonstrated as another application of metamaterials with active circuits. We embed diodes as active circuit elements within the metamaterial array to implement a switchable metamaterial reflector/absorber at microwave frequencies. Diodes are placed in series with the unit cells of the metamaterial array. This results in just a pair of control lines to actively tune all the diodes in an array. Tuning the diodes switches the function of the metamaterial array between a perfect absorber and a reflector. The design, simulation and experimental results of a switchable reflector/absorber in 2-6 GHz range is presented in this dissertation.
Lastly, we explore the utilization of CMOS technology for making metamaterials. CMOS is the mainstream fabrication technology featuring low cost, high yield and fast throughput. Continued scaling in CMOS process has resulted in available feature sizes of tens of nanometers. It offers all the elements needed to build new hybrid metamaterial devices including the multiple layers of metal, the vias connecting them, diodes, resistors, capacitors, transistors, etc. Several 2D and 3D metamaterial devices on CMOS are designed and simulation results are presented that verify the feasibility of implementing metamaterials on CMOS.
As proof of concept, a metamaterial enhanced mid-IR detector array on CMOS is fabricated and tested. We scale the metamaterial enhanced detector design down to μm dimensions to make it operate in the mid-IR band. Row/column decoder and readout circuits are implemented with the metamaterial enhanced detector array monolithically on the commercial 45nm CMOS chip. The simulation result verifies the design and the preliminary test results validate the electronic circuits are working.