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Abstract This study presents findings in the terahertz (THz) frequency spectrum for non-contact cardiac sensing applications. Cardiac pulse information is simultaneously extracted using THz waves based on the established principles in electronics and optics. The first fundamental principle is micro-Doppler motion effect. This motion based method, primarily using coherent phase information from the radar receiver, has been widely exploited in microwave frequency bands and has recently found popularity in millimeter waves (mmWave) for breathe rate and heart rate detection. The second fundamental principle is reflectance based optical measurement using infrared or visible light. The variation in the light reflection is proportional to the volumetric change of the heart, often referred as photoplethysmography (PPG). Herein, we introduce the concept of terahertz-wave-plethysmography (TPG), which detects blood volume changes in the upper dermis tissue layer by measuring the reflectance of THz waves, similar to the existing remote PPG (rPPG) principle. The TPG principle is justified by scientific deduction, electromagnetic wave simulations and carefully designed experimental demonstrations. Additionally, pulse measurements from various peripheral body parts of interest (BOI), palm, inner elbow, temple, fingertip and forehead, are demonstrated using a wideband THz sensing system developed by the Terahertz Electronics Lab at Arizona State University, Tempe. Among the BOIs under test, it is found that the measurements from forehead BOI gives the best accuracy with mean heart rate (HR) estimation error 1.51 beats per minute (BPM) and standard deviation 1.08 BPM. The results validate the feasibility of TPG for direct pulse monitoring. A comparative study on pulse sensitivity is conducted between TPG and rPPG. The results indicate that the TPG contains more pulsatile information from the forehead BOI than that in the rPPG signals in regular office lighting condition and thus generate better heart rate estimation statistic in the form of empirical cumulative distribution function of HR estimation error. Last but not least, TPG penetrability test for covered skin is demonstrated using two types of garment materials commonly used in daily life. 

We present a topology for suppressing quantization lobes in 1-bit reconfigurable reflective surfaces (RRSs). RRSs are planar surfaces that redirect the imping waves to the desired direction through phase modulation. For single-bit modulation, plane-wave illuminated RRSs exhibit quantization lobes due to the limited number of available phase bits. To eliminate such lobes, we randomize the quantization error by employing a fixed but random phase delay in every unit-cell of the RRS. Specifically, we focus on the fabrication and characterization of a mmWave single-layer, 1-bit, 30×30 randomized RRS designed at 222.5 GHz. The quasi-optical RCS characterization of the fabricated RRS demonstrates the successful suppression of the quantization lobe using the proposed technique. 

We present an analysis of graphene-loaded transmission line switches for sub-millimeter wave and terahertz applications. As such, we propose equivalent circuit models for graphene-loaded coplanar waveguides and striplines and examine the switching performance under certain parameters. Specifically, we identify the optimum design of graphene switches based on transmission line characteristic impedance, scaling factor, graphene shape, and topology (series or shunt). These parameters are varied to obtain the insertion loss and ON/OFF ratio of each switch configuration. The extracted results act as the design roadmap toward an optimum switch topology and emphasize the limitations with respect to fabrication challenges, parasitic effects, and radiation losses that are especially pronounced in the millimeter wave/terahertz bands. This is the first time that such an in-depth analysis is carried out on graphene-loaded transmission line switches, enabling the development of efficient millimeter wave/terahertz tunable topologies in terms of insertion loss and ON/OFF ratio. Specifically, the optimized switches can be integrated with antennas or employed for the development of tunable phase shifters, leading to the implementation of efficient reconfigurable reflective surfaces (e.g., reflectarrays) or coded phased arrays either for imaging or wireless communication applications. In our models, we use measured graphene values (sheet impedance) instead of theoretical equations, to obtain the actual switching performance. Moreover, the proposed study can be easily expanded to other thin film materials that can be characterized by a sheet impedance including vanadium dioxide and molybdenum disulfide. Finally, the proposed equivalent models are crucial for this in-depth study; since we simulated more than 2,000,000 configurations, a computationally challenging task with the use of full-wave solvers. 

We present a theoretical study on the performance of graphene-loaded coplanar waveguide switches for 5G and beyond applications. Therefore, we exploit the tunable properties of graphene to device cost-effective, large-scale, broadband sub- millimeter-wave switches. Given the sheet impedance of biased and unbiased graphene monolayers, the model provides the optimum switching ratio with respect to insertion loss, characteristic impedance of transmission line, and graphene geometry. Using measured graphene sheet resistance, we compute the optimum switching performance for series and shunt single- pole-single-though sub-millimeter-wave (220-330 GHz) switches. 

We investigate the propagation losses in terahertz (THz) non-line-of-sight (NLoS) imaging and compare the performance to the optical counterpart. NLoS imaging exploits the multiple reflections of electromagnetic waves from surrounding surfaces to reconstruct the geometry and location of hidden objects. THz and visible/infrared radiations are attractive for NLoS imaging due to the short wavelengths and practical apertures that can support this non-conventional imaging. However, the scattering mechanisms vary significantly and determine the quality of the reconstructed images. This work compares for the first time the free-space path loss and rough surface scattering losses of a simple THz and optical NLoS imaging topology. Because specular reflections are dominant in THz scattering while optical systems suffer from strong diffuse scattering, THz NLoS imaging systems can receive considerably stronger backscattered signals. 

We present a novel fabrication technique for large-scale, on-wafer graphene devices. With the proposed technique, large-area graphene apertures can be fabricated, enabling the proliferation of graphene-based reconfigurable devices, including metasurfaces. Such topologies require large-area high yield fabrication processes. To avoid graphene delamination during the chemical processes of the fabrication, we use a titanium sacrificial layer to protect the graphene monolayer. To evaluate the fabrication method, we present broadband in-plane graphene measurements in the 220-330 GHz band for the first time and compare the measured resistance sheet with previous works.