Woon-Gi Yeo

THz Transparent Metamaterials for Enhanced Spectroscopic and Imaging Measurements

We present a frequency-selective surfaces (FSS) transparent window exhibiting low losses for broadband spectroscopic and imaging measurements in terahertz (THz) frequencies. The intent is to replace existing samples holders (e.g., Z-cut crystal quartz) with transparent metamaterial windows and increase measurement sensitivity. To enable multiband and broadband responses, two designs of single-layer FSS with superstrate are demonstrated: circular slot and double square loop, and thus suppress losses in a given range of THz band.

New frontiers for commercial applications of terahertz

We present several preliminary experimental data to demonstrate the potential of terahertz imaging for various commercial applications. Specifically, terahertz images and signatures are presented using terahertz pulsed imaging (TPI™) for green leaves, lung cancer tissues, as well as farm and wild raised salmons. Raw data from these measurements provided the associated refractive indices and absorption coefficients. Time domain reflectometry (TDR) signatures were also collected at terahertz frequencies for integrated circuits. This was done using an electro-optic terahertz pulse reflectometry (EOTPR) system. In all cases, it is demonstrated that discriminatory information can be readily obtained from terahertz images or TDR signals for various organic and non-organic materials.

Phased array antenna with integrated MEMS phase shifters for Ka-band SATCOM

We present a Ka-band tightly coupled dipole array with micro electro-mechanical systems (MEMS) phase shifters directly integrated with the element feed structure. The dipole elements are printed on an alumina substrate with a twin coplanar waveguide (CPW) feed to ensure balanced feeding. An alternate feed structure in the form of Γ-matching circuit is also introduced to reduce the number of MEMS phase shifter required for the twin CPW feed.

Validation of CW THz spectral measurements

We present experimental data to validate the precision of our continuous wave (CW) THz spectrometer. Measurement data from this new CW spectrometer are compared with corresponding ones obtained using our Agilent vector network analyzer (VNA). Using these systems, terahertz transmission spectra are compared for wild and farm-raised salmon tissues. It is demonstrated that the CW spectrometer provides accurate measurement data comparable with the Agilent-VDI VNA system, but can be improved by increasing its dynamic range.

THz periodic surfaces to enhance spectroscopic measurements

We present a multi-layered frequency selective surfaces (FSS) exhibiting low losses for broad band spectroscopic measurements in THz frequencies. The intent is to replace existing samples holders (e. g. z-cut crystal quartz) with transparent metamaterial layers and increase measurement sensitivity. To enable multiband and broadband responses, a multilayered FSS is proposed, and thus suppress losses in a given range of THz bands.

THz spectroscopy and imaging of major human organ tissues for cancer margin assessment

Summary form only given. The sensitivity of THz waves to tissue hydration levels in excised specimen enables clinical, in-situ discrimination of cancerous vs. normal tissue regions (Z. D. Taylor et al., IEEE Trans. Terahertz Sci. Technol., 1(1), 201-219, 2011 and M. H. Arbab et al., J. Biomed. Opt., 18(7), 077004, 2013). However, previous studies on THz biomedical imaging of human tissue groups have been largely disconnected, with publications focusing on only a few tissue groups at a time. In addition, assessment of cancer margins to differentiate in-situ extent of disease has not been a major focus. As such, a more general consideration of the THz response of major human tissue groups has not been available. To address this shortcoming, we are focusing on a comprehensive assessment of THz spectroscopic properties of major human tissue groups and associated primary and metastatic malignancies using broadband time domain THz spectroscopy in the 60GHz-3THz band. In an effort to document the THz response of different tissue groups and malignancies as uniformly and as completely as possible, we employ a fixed protocol for the preparation and the characterization of all tissue specimens. Freshly excised tissues as well as formalin-fixed paraffin-embedded samples are characterized using a reflection-mode THz imaging system. Subsequently, we extract the refractive index and absorption properties of the samples. Moreover, using various locations in the samples, statistical averages for index and absorption coefficient of normal and cancerous regions are calculated. Among the studied tissue groups are freshly-excised tissues from human heart, pancreas, thyroid, liver, and stomach and formalin-fixed liver and brain tissues. Using formalinfixed liver tissue with metastatic colon cancer, we present a comprehensive study to differentiate cancer margins. In addition, a comparative study is performed to investigate the effect of formalin fixing on THz properties. In all cases, it is demonstrated that discriminatory information can be readily obtained from the THz images. We will present the THz reflection responses and cancer margin detection for the major human organ tissue groups.

Tightly coupled Ka-band phased array antenna with integrated MEMS phase shifters

Summary form only given. High-data-rate, beam-agile, low-profile, compact, and low-power transceiver systems are required to address the air-to-air and satellite communications (SATCOM) connectivity requirements of unmanned assets in the battlefield. To this end, the Ka-band offers a key advantage for smaller size and lower profile antennas over the traditional Xand Ku-band antennas and phased arrays. More importantly, unprecedented continuous bandwidth coverage of tightly-coupled arrays (e.g. J.P. Doane, K. Sertel, and J.L. Volakis, “A wideband, wide scanning tightly coupled dipole array with integrated balun”, IEEE TAP, vol. 61, no. 9, Sept. 2013) that have only recently been demonstrated for RF frequency bands can enable truly disruptive transceivers with continuous coverage of Xthrough Ka-band using a single aperture. Nonetheless, for UWB beam-agile phased-array operation, equally wideband phase-shifters are needed. In this work, we employ the micro electro-mechanical systems (MEMS) phaseshifter technology to develop switched phased-shifters that can be seamlesslyintegrated with the tightly-coupled dipole array elements. A tightly coupled Kaband (25~28 GHz) dipole array antenna with integrated MEMS phase-shifters from XCOM Wireless is presented. In particular, the dipole elements and the feed lines are printed on low-loss alumina (Al2O3) substrate and the phase shifters and the control circuitry are packaged directly into the array unit element, resulting in a compact and low-cost implementation. Each array element requires two phase shifters, in form of differential feed, however, both phase-shifters can be packaged into a single enclosure. To achieve optimum inter-element spacing and element dimensions, full-wave simulations were performed on the array unit cell and the phased-array performance is evaluated using finite array simulations with the integrated phase-shifters. Design and implementation details, as well as the integration approach and array performance will be presented.

Human tissue characterization and cancer margin assessment toward real-time THz imaging for clinical applications

THz waves are extremely sensitive to the degree of hydration, which is strongly correlated with electric properties and malignancies of various tissues due to their composition and morphological differences. As such, imaging in the THz band can provide a detailed picture of the malignant tumor with accurate assessment of tissue morphology and margins. Thus, here, we develop a database of the THz response of several human organ tissue groups using broadband time domain THz spectroscopy. We also investigate the use of real-time THz camera imaging to differentiate between benign and malignant tissues for the surgical margin identification in a clinical setting. For this purpose, we are developing a comprehensive study of THz spectroscopic characteristics and cancer margin assessment for human organ tissues.

Ka-band phased array antenna with integrated MEMS phase shifters

We present a wideband Ka-band phased array antenna with micro electromechanical systems (MEMS) phase shifters integrated directly into the feeding structure of each element. Traditionally, phased arrays use GaAs monolithic microwave integrated circuits (MMIC) phase shifters for beam-steering. However, as the number of array elements grow, MEMS-based phase-shifter technologies become more attractive due to lower insertion loss and very low power consumption. In particular, for a new class of phased arrays developed recently, the large number of elements necessitates use of MEMS-based phase-shifter to minimize total power.

Dual-band, Wide-incident-angle absorber for far-IR and THz frequencies

We present an electrically thin, dual-band, perfect absorber design that can be scaled to operate in the far infrared (FIR) or THz band. A dual-polarization frequency selective surface (FSS) consisting of crossed dipoles is modified using additional parasitic tip metallizations to achieve dual-band performance. The conducting FSS layer is placed on a silicon carbide (SiC) substrate backed by a metallic ground. The structure is optimized for absorption efficiency over a wide incidence angle range for both transverse electric (TE) and transverse magnetic (TM) polarizations.