Qing Huo Liu

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Qing Huo Liu received his B.S. and M.S. degrees in physics from Xiamen University, and Ph.D. degree in electrical and computer engineering from the University of Illinois at Urbana–Champaign. He was a Research Scientist and the Program Leader with Schlumberger-Doll Research, Ridgefield, CT, USA from 1990 to 1995. From 1996 to May 1999, he was an Associate Professor with New Mexico State University, Las Cruces, NM, USA. Since June 1999, he has been with Duke University, Durham, NC, USA, where he is a Professor of electrical and computer engineering. He has been also the founder and chairman of Wave Computation Technologies, Inc. since 2005. In 2019-2020 he was a visiting professor at Kyoto University, Japan

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Higher Order Finite Element Method for Inhomogeneous Axisymmetric Resonators

Progress In Electromagnetics Research B, 2010

To analyze resonances in an axisymmetric inhomogeneous cavity, a higher-order finite element meth... more To analyze resonances in an axisymmetric inhomogeneous cavity, a higher-order finite element method (FEM) is developed. Mixed higher-order node-based and edge-based elements are applied to eigenvalue analysis for the azimuthal component and meridian components of the field, respectively. Compared with the lower-order FEM, the higher-order FEM can improve accuracy with the same number of unknowns and can reduce the CPU time and memory requirement for specified accuracy. Numerical results are given to demonstrate the validity and efficiency of the proposed method.

format_quoteThe new contribution of extending higher-order FEM to BOR cavities improves accuracy while reducing computational resource needs.format_quote

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A Fast Inverse Polynomial Reconstruction Method Based on Conformal Fourier Transformation

Progress In Electromagnetics Research, 2012

A fast Inverse Polynomial Reconstruction Method (IPRM) is proposed to efficiently eliminate the G... more A fast Inverse Polynomial Reconstruction Method (IPRM) is proposed to efficiently eliminate the Gibbs phenomenon in Fourier reconstruction of discontinuous functions. The framework of the fast IPRM is modified by reconstructing the function in discretized elements, then the Conformal Fourier Transform (CFT) and the Chirp Z-Transform (CZT) algorithms are applied to accelerate the evaluation of reconstruction coefficients. The memory cost of the fast IPRM is also significantly reduced, owing to the transformation matrix being discretized in the modified framework. The computation complexity and memory cost of the fast IPRM are O(M N log 2L) and O(M N), respectively, where L is the number of the discretized elements, M is the degree of polynomials for the reconstruction of each element, and N is the number of the Fourier series. Numerical results demonstrate that the fast IPRM method not only inherits the robustness of the Generalized IPRM (G-IPRM) method, but also significantly reduces the computation time and the memory cost. Therefore, the fast IPRM method is useful for the pseudospectral time domain methods and for the volume integral equation of the discontinuous material distributions.

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Planar Monopole Ultrawideband Antenna with Reduced Ground Plane Dependence

International Journal of Computer and Electrical Engineering, 2014

A technique to reduce ground plane effect on a printed wideband antenna is presented. The antenna... more A technique to reduce ground plane effect on a printed wideband antenna is presented. The antenna consists of a ground plane, and a wing-shaped monopole with an attached strip. The strip is attached to introduce a slight disturbance in the return loss of the antenna. Here, a slight peak in the form of a notch is introduced in the return loss at a specific frequency depending on the length of the attached strip. At this frequency, the antenna has reduced ground plane effect. The proposed antenna is etched on 11 × 15 mm 2 FR4 substrate and covers an operating frequency from 5-15 GHz (VSWR < 2.5) with a stable gain and good radiation properties. Experimental results show good antenna performance.

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GPR Landmine Imaging: 2D Migration and 3D Inverse Scattering in Layered Media

We present two methods for ground penetrating radar (GPR) imaging of landmines: 1) The migration ... more We present two methods for ground penetrating radar (GPR) imaging of landmines: 1) The migration technique has been successfully applied to processing field data collected at a test site and is shown to be a useful real-time imaging method; 2) A full 3D nonlinear inverse scattering algorithm has been developed in multilayered media and is demonstrated to reconstruct high-contrast buried

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Evaluation of TRM in the Complex Through Wall Environment

Progress In Electromagnetics Research, 2009

Practical interests arising from behind-the-wall target detection, surveillance and reconnaissanc... more Practical interests arising from behind-the-wall target detection, surveillance and reconnaissance et al. claim for high capability of imaging in complicated environments. Time Reversal Mirror (TRM) technique, making use of the principle of reciprocity, emerges as a promising way to deal with such complex problem. In this paper, we investigate TRM in the ultra-wideband (UWB) through wall radar imaging (TWRI) through numerical simulation. The probing region is a square room, with walls of rough surface and random media parameters. TRM is used to image the target settled in the room. We evaluate the degradation of the images when the aperture of the array is decreased or the received signals are contaminated by noises. The back projection (BP) algorithm is employed here as a comparison for imaging quality. For the case in which the random walls are changed between the forward and inverse phase of time reversal, we check the imaging stability and applied an averaged Green's function to improve the imaging quality. Finally, some interesting conclusions are drawn.

format_quoteUsing an averaged Green's function enhances imaging stability and quality when random wall mismatches occur in TRM applications.format_quote

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Iterative Time-Reversal Mirror Method for Imaging the Buried Object Beneath Rough Ground Surface

Progress In Electromagnetics Research, 2011

An iterative Time-Reversal Mirror (TRM) method is proposed to Detect and Image the buried target ... more An iterative Time-Reversal Mirror (TRM) method is proposed to Detect and Image the buried target beneath ground surface. Unlike the conventional TRM methods which treat the information of the ground as clutters and directly delete them, the iterative TRM imaging method proposed in this paper utilizes the information of rough ground surface as a useful knowledge. The new approach is consisted of two TRM procedures. In the first TRM procedure, it aims to image the rough surface where the propagation environment for electromagnetic wave is free space. The second TRM procedure aims to image the buried target. In this step, the information of the rough surface estimated by the first TRM procedure will be treated as newly updated propagation environment. Then conventional TRM is applied to image the buried target. By applying this iterative TRM method, the information of the rough ground can be well considered in the whole TRM procedure. Numerical simulations prove that this method performs significantly better image contrast comparing with the results obtained by using conventional TRM. 4-5 dB improvement on the imaging SNR has been achieved. Furthermore, the target can be located more accurately.

format_quoteBuried Object Detection (BOD) is crucial for civil and military applications, highlighting the need for high-resolution imaging methods like the iterative TRM.format_quote

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Microwave imaging for breast tumor: 2D forward and inverse methods

IEEE Antennas and Propagation Society International Symposium. 2001 Digest. Held in conjunction with: USNC/URSI National Radio Science Meeting (Cat. No.01CH37229), 2001

Microwave imaging is the method that used to detect breast cancer. Previous research is done by i... more Microwave imaging is the method that used to detect breast cancer. Previous research is done by inverse problem and forward problem solution, but the inverse problem is difficult to solve because there are many unknown constants. Thus in this paper, microwave imaging is solved by different way, i.e. using forward problem solution only with the aim to easier the imaging process without ignoring the resolution. So the malignant breast cancer is easier to detect and this technique can be used as an easy, safe, and cheap of early detection of the breast cancer. This process is done by illuminating normal and malignant breast tissue model using microwave at frequencies 5GHz, 10GHz, and 15GHz. Then the scattered field data are calculated by Method of Moment. The scattered field data is presented in 2D figure. The results show that normal and malignant breast tissue produce different scattered field patterns. The normal breast tissue tends to produce more curve pattern around object. This difference in patterns can be used as a reference to distinguish b etween normal and malignant breast tissue.

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Fast Inhomogeneous Plane Wave Algorithm for Analysis of Composite Bodies of Revolution

Progress In Electromagnetics Research, 2010

A fast inhomogeneous plane wave algorithm is developed for the electromagnetic scattering problem... more A fast inhomogeneous plane wave algorithm is developed for the electromagnetic scattering problem from the composite bodies of revolution (BOR). Poggio-Miller-Chang-Harrington-Wu (PMCHW) approach is used for the homogeneous dielectric objects, while the electric field integral equation (EFIE) is used for the perfect electric conducting objects. The aggregation and disaggregation factors can be expressed analytically by using the Weyl identity. Compared with the traditional method of moments (MoM), both the memory requirement and CPU time, are reduced for large-scale composite BOR problems. Numerical results are given to demonstrate the validity and the efficiency of the proposed method.

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Ultra Wide Band Response of an Electromagnetic Wave Shield Based on a Diode Grid

Progress In Electromagnetics Research, 2013

This paper investigates Ultra Wide Band (UWB) response of a self-actuated electromagnetic wave sh... more This paper investigates Ultra Wide Band (UWB) response of a self-actuated electromagnetic wave shield based on a diode grid both in frequency and time domain. The investigation is first carried out on a shield valid for an incident wave polarized at a specific direction only, then extended to a shield effective for an incident wave polarized at an arbitrary direction. In the frequency domain, two linear analysis methods are used to study the properties of the diode grid over the frequency range from 0.01 to 10 GHz. One method is the microwave network analysis. Another is simulating the diode grid by a linear equivalent circuit instead of a diode. In the time domain, the property of the shield is studied with respect to a broadband impulse, where the diode is described by its SPICE circuit model including the nonlinear property. The results show that the diode grid works well as a self-actuated electromagnetic power selective surface (PSS) in a certain frequency range. The diode grid is strongly frequency dependent. The operating frequency band relies on the reactive elements in the diode grid. In order to extend the operating frequency to a high band, smaller cell size and smaller junction capacitance should be employed.

format_quoteThe diode grid effectively prevents strong wave transmission at low frequencies, but transmission increases significantly above 7.0 GHz.format_quote

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Manipulating light absorption of graphene using plasmonic nanoparticles

Nanoscale, 2013

We present the incorporation of periodic gold nanoparticle arrays into graphene-based photodetect... more We present the incorporation of periodic gold nanoparticle arrays into graphene-based photodetectors to enhance and tune light absorption of graphene. By the use of electromagnetic simulations, we show that light absorption in graphene can be manipulated by tuning plasmonic resonance. A maximum absorption of 30.3% with a full width of 135 nm at half maximum is achieved through systematic optimization of nanostructures.

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A generalized recursive algorithm for wave-scattering solutions in two dimensions

IEEE Transactions on Microwave Theory and Techniques, 1992

A generalized recursive algorithm valid for both the E, and H, wave scattering of densely packed ... more A generalized recursive algorithm valid for both the E, and H, wave scattering of densely packed scatterers in two dimensions is derived. This is unlike previously derived recursive algorithms which have been found to be valid only for E, polarized waves [1]-[7]. In this generalized recursive algorithm, a scatterer is first divided into N subscatterers. The n-subscatterer solution is then used to solve the (n + n')-subscatterer solution. The computational complexity of such an algorithm is found to be of O(N2) in two dimensions, and meanwhile, providing a solution valid for all angles of incidence. This is better than the method of moments with Gaussian elimination which has an O (N 3) complexity.

$Fig. 2. The scattering solution of a square cylindrical dielectric scatterer of dimension 0.9 free-space wavelengths square illuminated by an H, po- larized wave from the x axis at 300 MHz. The relative permittivity of the cylinder is 2, and the field is computed at 10 free-space wavelengths from the center of the scatterer. The square dots represent the solution calculated with the extended boundary condition (EBC) method using 21 harmonics. The diamonds represent the solution calculated with 81 square subscatter- ers. The crosses represent the solution when the square subscatterers are replaced with circular subscatterers of the same cross-sectional area. The triangles are when the circular subscatterers are reduced in size with the help of Maxwell-Garnett mixing formulas (see (25)) with a fractional cross- sectional area of 80%. All subscatterers have three harmonics of scattered field.$

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Large-scale 3D finite-difference simulation of elastic wave propagation in borehole environments

The Journal of the Acoustical Society of America, 1994

Elastic wave propagation in realistic borehole environments is very complex because of the presen... more Elastic wave propagation in realistic borehole environments is very complex because of the presence of borehole, dipping beds, and other irregular scatterers. In order to understand this complex wave propagation phenomenon for sonic logging applications, a three-dimensional finite-difference (FD) method was used to simulate elastic wave propagation on a parallel computing architecture. The FD scheme solves the first-order elastic wave equations with central differencing in both space and time via staggered grids. Liao’s absorbing boundary condition is used to reduce artificial reflections from the finite computational domain. In this work, uniform grids in Cartesian coordinates are used to discretize the inhomogeneous medium. Because of the staircase approximation of the circular borehole, it is observed that the discretization requirement is quite different for monopole and dipole sources. Several methods are suggested to remedy this problem. The results from the FD method were val...

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Direct $Z$-Transform Implementation of the CFS-PML Based on Memory-Minimized Method

IEEE Transactions on Microwave Theory and Techniques, 2015

Efficient and unsplit-field-based stretched coordinate perfectly matched layer (PML) formulations... more Efficient and unsplit-field-based stretched coordinate perfectly matched layer (PML) formulations with the complex frequency-shifted (CFS) scheme that combine the memory-minimized method (Tri-M) and the direct-transform (DZT) method are proposed to truncate the finite-difference time-domain lattices. The proposed algorithm takes advantage of the minimized memory of the TriM method, the optimal accuracy of the DZT method, and the capacity of the PML for attenuating evanescent waves and reducing late-time reflections. Three 3-D numerical simulations have been carried out to validate the proposed formulations. Index Terms-Complex frequency-shifted perfectly matched layer (CFS-PML), direct-transform (DZT) method, finite difference time domain (FDTD), memory-minimized method (Tri-M). I. INTRODUCTION T HE STRETCHED coordinate perfectly matched layer (SC-PML) [1], presented firstly by Chew and Weedon, is a highly effective absorption boundary condition (ABC) material to truncate the finite-difference time-domain (FDTD) computational domains and has the advantage of simple implementation in the edges and corners of perfectly matched layer (PML) domains. However, the implementation of the presented SC-PML formulations in [1] needed splitting the field components. Next, based on the digital signal processing (DSP) technique and the auxiliary differential equation (ADE) method, two simple and efficient PML formulations, respectively, were proposed [2], [3] instead of the splitting of the field components. However, the common drawback of the proposed formulations in [2] and [3] are that two additional auxiliary variables are required per field component per cell in all corners and at some edges of the PML regions in three dimensions. Recently, based on the ADE and DSP techniques, another two SC-PML algorithms [4], [5] using the memory-minimized method (Tri-M) were presented and their main advantages are that only one auxiliary variable is needed in some edges and all

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An Image Correction Method Based on Electromagnetic Simulation for Microwave Induced Thermo-Acoustic Tomography System

Progress In Electromagnetics Research B, 2012

In microwave induced thermo-acoustic tomography (MI-TAT) system, radiation of an antenna is a nea... more In microwave induced thermo-acoustic tomography (MI-TAT) system, radiation of an antenna is a near field problem which gives rise to a non-uniform distribution of microwave radiation power in detection area. Due to this non-uniform distribution, the contrast of MITAT image which is proportional to the absorbed microwave energy will not reflect the real characteristics (dielectric properties) of biological tissues. In this paper, an image correction method based on electromagnetic simulation is proposed to correct the image contrast affected by the non-uniform microwave radiation distribution. First, the distribution of the microwave radiation power is simulated through a numerical simulation framework. Conventional time-reversal mirror (TRM) technique is applied to reconstruct the image. Then the microwave power distribution is applied to correct the image. The method is numerically demonstrated. The two samples with the same microwave absorption property and with different microwave absorption properties are experimentally investigated. Both numerical simulations and experimental results demonstrate the good performance of the proposed method.

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Resolving ambiguities in DOA estimation by optimizing the element orientations

2013 IEEE Antennas and Propagation Society International Symposium (APSURSI), 2013

ABSTRACT A method of resolving ambiguities in direction-of-arrival (DOA) estimation for sparse li... more ABSTRACT A method of resolving ambiguities in direction-of-arrival (DOA) estimation for sparse linear arrays with directional antennas is reported. The DOA estimation ambiguity comes from the linear dependence among the array manifold vectors of different DOAs. Optimizing the element orientations can break down the linearity. The element orientations are optimized by using particle swarm optimization (PSO) algorithm with the Weiss-Weinstein bound (WWB). This method is demonstrated through simulations of a 5-element collinear array of dipole antennas.

Design of a metasurface Luneburg lens antenna with flared structure

Proceedings of 2014 3rd Asia-Pacific Conference on Antennas and Propagation, 2014

This article demonstrates the design of a meta-surface Luneburg lens antenna with flared structur... more This article demonstrates the design of a meta-surface Luneburg lens antenna with flared structure. The lens antenna is composed of two flared metallic cones, a metasurface Luneburg lens and a reflection plate. The refraction index of Luneburg lens can be obtained by gradually changing the radius of patch arraying on the substrate. Such an antenna structure is easy to implement with low fabrication error. Some modified methods are applied to improve the radiation performance of this antenna. Simulation results show that the designed metasurface Luneburg lens antenna has a focused beam pattern with relatively low sidelobe level.

Fast computation of electromagnetic fields in anisotropic media layered both vertically and cylindrically using the numerical mode matching (NMM) method

2013 USNC-URSI Radio Science Meeting (Joint with AP-S Symposium), 2013

Summary form only given. A structure layered both vertically and horizontal cylindrically is a sp... more Summary form only given. A structure layered both vertically and horizontal cylindrically is a special 3D inhomogeneous medium with axial symmetry and has many engineering applications. An off-axis source will generate full three-dimensional electromagnetic fields in such a medium. Typical applications of such electromagnetic simulation include geophysical exploration, optical fiber communications and integrated optics. For example, in geophysical exploration, the induction well-logging tool consisting triaxial coils will produce nonaxisymmetric electromagnetic waves in a heterogeneous formation. Numerical mode matching (NMM) method is a powerful semi-analytical solver that can reduce a higher dimensional problem into a series of lower dimensional problems through numerical computation of eigenmodes and their reflection and transmission. Here the NMM method is applied to solve the 2.5D problem of an off-axis source radiating in the inhomogeneous medium layered vertically and cylindrically. The eigenmodes in each horizontal layer can be easily obtained by the 1D FEM. The discrete eigenmodes will be found if the structure is finite whereas continuous modes, i.e. radiation modes, will be observed for unbounded structure. The reflection and transmission matrices due to the multiple junction discontinuities can be found by matching boundary condition of tangential components of electric and magnetic fields. The excited total fields can be expressed in terms of summation of eigenmodes with different coefficients. The NMM method has previously been shown to be a fast and robust semianalytical solver for this 2.5D inhomogeneous medium. In this work, we have improved the NMM method with new capabilities: (1) The method has been extended to anisotropic media to expand its application range. (2) The perfectly matched layer (PML) has been added as an absorbing boundary condition in cylindrical coordinate system to make NMM more accurate in low conductivity media and applicable to lossless media. (3) Instead of using small radius to approach origin, this paper manipulates weak form of Maxwell's equation and impose the boundary condition to yield exact fields at origin. It is crucial in borehole well-logging system, which frequently measures the field along the Zaxis. (4) This work offers formulations for fields excited by both electric and magnetic dipole with arbitrary orientations. Numerical results will be shown to demonstrate the efficacy of this method.

An improved near-field imaging algorithm based on Stolt migration for single borehole radar with widely separated transceiver

2015 IEEE Radar Conference (RadarCon), 2015

In this paper, the Stolt migration algorithm is modified for impulse borehole radar in near-field... more In this paper, the Stolt migration algorithm is modified for impulse borehole radar in near-field imaging where the transceiver is widely separated with respect to the detection range. The original sample set is converted to a multidimensional virtual sample set. The relation between angular frequency and wavenumbers of transmitter and receiver is derived in the frame of virtual sample set, which provides the basis for the interpolation in angular frequency. By applying multidimensional inverse fast Fourier transform (IFFT) to the interpolated frequency-wavenumber spectrum of the virtual sample set, the energy of target responses will focus in some profile of the IFFT result, which can be extracted according to the separation between the transmitter and receiver. The proposed method is compared with the conventional Stolt migration algorithm, back projection method, and Kirchhoff migration algorithm on synthetic data. The results show that the developed Stolt migration is superior to the conventional methods in cross-range resolution, computational cost, and the ability to reconstruct locations and shapes of targets.

A Coaxial Measurement System for Electromagnetic Parameters at Microwave Frequencies

Advanced Materials Research, 2014

In this paper, an improved coaxial measurement system with a newly designed coaxial fixture is pr... more In this paper, an improved coaxial measurement system with a newly designed coaxial fixture is presented. The electromagnetic parameters of samples are retrieved from the scattering parameters measured by a vector network analyzer (VNA) at microwave frequencies. The measurements of air and PTFE in the range of 1~6 GHz were carried out to verify the reliability and the accuracy of this measurement system. By using frequency-sweep and data processing techniques, the multiplicity of roots is eliminated. The results show that this system can effectively be applied to measure the material electromagnetic parameters of absorbing materials with high accuracy in a wide frequency band.

Radio frequency interference suppression in cross-borehole impulse radar for well logging

2014 IEEE Radar Conference, 2014

ABSTRACT In this paper, the problem of radio frequency interference caused by leak energy from si... more ABSTRACT In this paper, the problem of radio frequency interference caused by leak energy from signal source in cross-borehole impulse radar is presented. These interferences reduce the focused image quality. A method based on the frequency difference between signal and clutter is put forward to improve signal-to-noise-ratio and the corresponding infinite impulse response band-pass filter is designed before the delay-and-sum processing. It is proved with field data that the filter improves the image result and makes the signal energy more focused.

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