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Creators/Authors contains: "Trichopoulos, Georgios C."

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  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. We characterize a novel fabrication procedure for the implementation of large arrays of subwavelength graphene devices. With the proposed process, we can now integrate graphene layers on large substrate areas (> 4 cm2) and implement thousands of devices with high-yield (> 90 %). Examples of such systems include broadband THz phased arrays and metasurfaces that can be used in THz imaging and sensing. Current nano-fabrication processes hinder the proliferation of large arrays due to the fragile nature of graphene. Conversely, we use titanium sacrificial layers to protect the delicate graphene throughout the fabrication process. Thus, we minimize graphene delamination and enable multiple devices on large-area substrates with high-yield. In addition, we present a series of on-wafer measurement results in the 220-330 GHz band, verifying the robustness of our fabrication process.
  6. In this work, we propose a novel approach for high accuracy user localization by merging tools from both millimeter wave (mmWave) imaging and communications. The key idea of the proposed solution is to leverage mmWave imaging to construct a high-resolution 3D image of the line-of-sight (LOS) and non-line-of-sight (NLOS) objects in the environment at one antenna array. Then, uplink pilot signaling with the user is used to estimate the angle-of-arrival and time-of- arrival of the dominant channel paths. By projecting the AoA and ToA information on the 3D mmWave images of the environment, the proposed solution can locate the user with a sub-centimeter accuracy. This approach has several gains. First, it allows accurate simultaneous localization and mapping (SLAM) from a single standpoint, i.e., using only one antenna array. Second, it does not require any prior knowledge of the surrounding environment. Third, it can locate NLOS users, even if their signals experience more than one reflection and without requiring an antenna array at the user. The approach is evaluated using a hardware setup and its ability to provide sub-centimeter localization accuracy is shown