Jose A. Hejase

Terahertz Characterization of Dielectric Substrates for Component Design and Nondestructive Evaluation of Packages

In this paper, terahertz (THz) characterization of dielectric substrates, THz planar and quasi-optical components, THz probing of planar devices, and THz nondestructive evaluation (NDE) are demonstrated. In particular, the goals of this paper are: 1) characterization of dielectric substrates for THz packaging applications; 2) design, fabrication, and evaluation of THz components built using some of these dielectric substrates; and 3) the use of the dielectric characterization approach and dielectric properties in NDE of electronic packages. The background theory for characterizing dielectric substrates using THz time-domain signals is provided. The Nelder and Mead modified simplex optimization algorithm is utilized in order to extract the dielectric properties of different packaging materials encompassing organic, inorganic, and composite materials. A planar THz power splitter, a dielectric probe, and a low-cost polymer-based quasi-optical band-stop interference filter are demonstrated. THz NDE of electronics packages is demonstrated for packaging delamination and moisture ingression in dielectric films.

Microwave artificially structured periodic media microfluidic sensor

In this paper, microstrip-based spiral structured artificial magnetic media (metamaterial) coupled with microfluidic channel is experimentally demonstrated for sensing applications. It is found that the resonant frequency and the amplitude changes due to dielectric loading from the introduction of chemical substances in the microfluidic channels. Different concentrations of water — methanol and water — isopropanol samples are used in the characterization of the sensor. For water — methanol mixtures, the resonant frequency shifts from 2.15 GHz to 2.0 GHz with change in dielectric constant from 25 to 75. Results show that the wave propagation in LH-media can be used for interrogation of minute volumes of samples with high sensitivity.

A Multiple Angle Method for THz Time-Domain Material Characterization

A multiple angle technique for Terahertz (THz) time-domain material characterization is described. In order to extract the properties of a material system, the technique utilizes the relations between a signal transmitted through a reference material at normal incidence and signals transmitted through the material system at different oblique angles of incidence. The material system may be composed of multiple dielectric layers. The extracted properties of the material system can include not only dielectric properties but also layer thickness. The method presented in this paper contributes to the available library of material characterization tools in the literature in that it provides the capability to carry out single and multiple dielectric layer material system characterization simultaneous with layer thickness extraction. Previous techniques that can characterize both constitutive parameters and layer thicknesses using THz time-domain signals have been scarce. The extraction procedure is simplified to solving a system of equations using a root finding algorithm. The data collection procedure and manipulation is uncomplicated. The method background theory, measurement procedure, forward solution from calculated and measured signals, inverse problem solution from calculated and measured signals, and current method limitations are presented.

Self-calibrating technique for terahertz time-domain material parameter extraction

This manuscript introduces a self-calibrating technique for terahertz (THz) time-domain material characterization in transmission mode. The self-calibrating technique utilizes the relations between the multiple transmissions of a transient plane wave through a sample under test in order to extract its material properties. The method presented in this paper is particularly useful in reducing the time required for simultaneous imaging and spectroscopy scans, since existing characterization methods require the knowledge of the waveform of both the plane wave transmitted through a reference sample and the waveform of the plane wave transmitted through the sample under test. The technique presented also ameliorates the problem caused by differences between sample and reference signals due to power and time drifts inherent in a THz system, hence reducing artifacts in the characterization results.

Metamaterial-inspired absorbers for Terahertz packaging applications

In this paper, thin metamaterial-inspired structures are investigated for wide-band absorption of stray signals for THz packages. The absorber itself consists of a low-index, low-loss dielectric sandwiched between a patterned, two-dimensionally periodic, thin metallic layer, and a metal backing. Numerical simulations were performed using both the finite element (FEM) and finite-difference time domain (FDTD) numerical methods. Design, fabrication and tests were carried out for absorbers having center frequencies of 0.2 THz and 0.4 THz. Ultra-wide bandwidth and strong absorption were obtained by taking advantage of skin-effect losses in metamaterial structures, and through multi-stacking of these structures. Absorbers having high absorption coefficients and bandwidth (> 1THz) can easily be fabricated using the approach demonstrated in this paper. Two measurement approaches are applied to characterize these structures, details of which are presented in this paper.

Design and test of wide-band terahertz dielectric sub-wavelength focusing probes

This manuscript presents two new wide-band terahertz (THz) dielectric sub-wavelength focusing probes for near-field measurements. The first design is characterized by being axially symmetric (conical) while the second is a rectangular based design (pyramidal). The axially symmetric design is specialized for doing spectroscopy and imaging due to its wideband and sub-wavelength focusing resolution. The rectangular design is characterized by its ability to preserve polarization. It will find applications in probing of terahertz planar devices and in polarization sensing and imaging. The probes have a wide bandwidth scanning approximately 0.1–1 THz. The probes were fabricated from low-loss high-density polyethylene. The axially symmetric probe has a sub-wavelength focusing resolution ranging from one fifth of a wavelength up to half a wavelength in a frequency band of about 150 GHz. In this paper, the design and properties of these two probes are presented. Applications of these probes are demonstrated by sub-wavelength near-field imaging, spectroscopy and probing of planar circuits.

Design study of electronically steerable half-width microstrip leaky wave antennas

A half-width microstrip leaky-wave antenna (HMLWA) having electronic beam steering capabilities is presented. Electronic beam scanning is demonstrated in X-, Ku- and Ka-bands through detailed design analysis and measurements. Two physical design approaches are studied; one requiring multi-layer processing and another requiring only a single layer metallization. In this paper, only the single layer design was implemented. Varactor diodes are edge coupled to the antenna element to achieve beam scanning as a function of the applied voltage at a fixed frequency. Beam scanning of 45-degrees for an 8 GHz design and 30-degrees for a 16 GHz antenna design are demonstrated. Details of design, fabrication and measurements are presented.

Nonlinear Eddy Current Technique for Characterizing Case Hardening Profiles

Industrial components are often case hardened to improve their strength and wear characteristics. Traditionally, component samples are collected from the production line at specific intervals and destructively tested for case depth profile assessment. This process is time-consuming, laborious, and can potentially allow an improperly treated component to escape detection. This paper presents a novel nonlinear eddy current technique for assessing the case hardening profile based on the premise that the magnetic characteristic of the case hardened region is different from that of the host material. A custom electromagnetic excitation-sensor array is used to both apply sinusoidal excitations to the component and measure the nonlinear response at multiple excitation frequencies and spatial locations, taking advantage of the different penetration regions due to the skin depth phenomenon. Each response signal obtained from the component under test is compared with that from a reference component subjected to the same excitation. Two pattern recognition algorithms (an artificial neural network and the Iterative Dichotomiser 3 (ID3) algorithm) are then used to process selected characteristics of the difference signal to determine the case depth profile of the component. The nonlinear eddy current technique has been applied to evaluate the case hardening profile of automotive bearing assemblies. This problem is challenging due to the variations in geometry across assemblies as well as the limited accessibility to the case hardened surface. In the neural network test, the system achieved a 95.77% accuracy. For the ID3 algorithm, the system achieved a 95.65% accuracy. These results demonstrate that the nonlinear eddy current inspection technique is highly promising in characterizing the case profile of induction hardened parts.

Terahertz packaging: Study of substrates for novel component designs

This paper will examine a new side to packaging: Terahertz (THz) Packaging. The goal of this paper is three fold: 1) characterizing dielectric materials for THz packaging applications; 2) Using these materials in the fabrication of THz passives (integrated and quasi-optical); and 3) Demonstrating non-destructive evaluation of packages using THz. In this manuscript, detailed characteristics of dielectric packaging materials in the THz spectral region are presented along with the theory used for the characterization procedure. THz non destructive evaluation (NDE) of electronics packages is observed in the form of delamination thickness detection and moisture content studies. Using the materials characterized, a planar THz power splitter and a quasi-optical THz bandstop interference filter are demonstrated. Furthermore, the power splitter is used as a THz microfluidic sensor.

A terahertz photonic crystal structure for sensing applications

This paper explores the use of a 1-D photonic crystal (PC), with and without a defect layer, for sensing in the THz spectral region. The PC consists of a stack of alternating high and low permittivity materials (BK-7 glass and air). Introducing a liquid or gaseous sample leads to a change in the transmission characteristics of the PC. An inverse problem method is used to characterize the dielectric properties of the sample under study. Such a configuration provides high sensitivity while requiring a small sample volume.