Swee Ping Yeo

Microstrip Branch-Line Couplers for Crossover Application

The branch-line coupler may be redesigned for crossover application. The bandwidth of such a coupler can be extended by suitably incorporating additional sections into the composite design. Laboratory tests on microstrip prototypes have shown the return loss and isolation of the three- and four-section couplers to be better than 20 dB over bandwidths of 22% and 33%, respectively. The insertion losses and group delays vary by less than ±0.05 dB and ±1 ns, respectively, for both prototypes.

Theoretical realization of robust broadband transparency in ultrathin seamless nanostructures by dual blackbodies for near infrared light

We propose a counter-intuitive mechanism of constructing an ultrathin broadband transparent device with two perfect blackbodies. By introducing hybridization of plasmon modes, resonant modes with different symmetries coexist in this system. A broadband transmission spectrum in the near infrared regime is achieved through controlling their coupling strengths, which is governed by the thickness of high refractive index layer. Meanwhile, the transparency bandwidth is found to be tunable in a large range by varying the geometric dimension. More significantly, from the point view of applications, the proposed method of achieving broadband transparency can perfectly tolerate the misalignment and asymmetry of periodic nanoparticles on the top and bottom, which is empowered by the unique dual of coupling-in and coupling-out processes within the pair of blackbodies. Moreover, roughness has little influence on its transmission performance. According to the coupled mode theory, the distinguished transmittance performance is physically interpreted by the radiative decay rate of the entire system. In addition to the feature of uniquely robust broadband transparency, such a ultrathin seamless nanostructure (in the presence of a uniform silver layer) also provides polarization-independent and angle-independent operations. Therefore, it may power up a wide spectrum of exciting applications in thin film protection, touch screen techniques, absorber-emitter transformation, etc.

Manipulating DC currents with bilayer bulk natural materials.

A novel and general method for spatially manipulating DC currents has been proposed and experimentally verified by only using bilayer bulk natural conductive materials. Our approach shows distinctive advantages with respect to homogeneity, isotropy, and independence of complicated microfabrication techniques. Our design scheme can be readily extended to robust manipulations of magnetic fields, thermal heat, elastic mechanics, and matter waves.

Creation of a longitudinally polarized subwavelength hotspot with an ultra-thin planar lens: vectorial Rayleigh-Sommerfeld method

This letter shows how a longitudinally polarized hotspot can be created by a planar ultra-thin lens that beats the diffraction limit. On the imaging plane, a subwavelength optical resolution 0.39λ with almost purely longitudinal electric component has been demonstrated in air ambient. This novel paradigm addresses simultaneously both longitudinal polarization and deep sub-diffraction imaging, by a planar lens composed of ultra-thin opaque concentric annuli. The vectorial Rayleigh–Sommerfeld (VRS) approach, offering the advantage of significant reduction in computation, has been developed for a particular optimization of a flat lens with full control of polarization. Empowered by the robustness of VRS in dealing with polarization states, the proposed roadmap may be universally and efficiently integrated with other optimization algorithms to design super-resolution imaging with controlled polarization states at any wavelength without luminescence of the object. The lens, which is empowered by the proposed method, opens an avenue for the first time toward a highly integrated imaging system with advanced functionalities in far-field super-imaging, tailored polarization states and flat ultra-thin geometry simultaneously.

Six-Port Reflectometer Based on Modified Hybrid Couplers

Six-port reflectometers have traditionally employed hybrid couplers as their building blocks, but the instruments reported thus far in the literature do not comply with the optimum design specifications. This paper describes a six-port reflectometer based on four-port coplanar-waveguide couplers that have been modified so as to meet such design requirements. Additional design parameters have been incorporated so that adjustments may be made to correct for residual hardware imperfections. Laboratory tests have confirmed an operating bandwidth of 90% (from 1.1 to 2.9 GHz) for the resulting reflectometer.

A Compressive-Sensing-Based Phaseless Imaging Method for Point-Like Dielectric Objects

We construct a linear system for the intrinsically nonlinear problem of imaging point-like dielectric scatterers with phaseless measurement, so as to adapt it to the emerging framework of compressive sensing (CS), within which the problem is solved by convex programming. Numerical experiments show that the exact internal constitution of the domain of interest in homogeneous or heterogeneous background can be efficiently reconstructed with good resolution.

Subspace-based optimization method for reconstructing extended scatterers: transverse electric case

The subspace-based optimization method is generalized to the transverse electric (TE) case for reconstructing the relative-permittivity profiles of extended scatterers. The essence of the subspace-based optimization method is that part of the contrast source is determined from the spectrum analysis without recourse to optimization, whereas the rest is determined by using an optimization procedure. We show that the dimension of the signal subspace behaves somewhat as a regularization parameter. Due to the vectorial property of the TE wave, the reconstructed profile in TE case is smoother than that in the transverse magnetic (TM) case.

Subspace-Based Optimization Method for Inverse Scattering Problems Utilizing Phaseless Data

This paper presents a novel variation of the subspace-based optimization method (SOM) to reconstruct the scatterers permittivity profile by utilizing only phaseless measurements (i.e., intensity data of the total field with no phase information). Based on spectrum analysis, the contrast source is partitioned into two orthogonally complementary portions (viz., deterministic and ambiguous portions). The original SOMs procedure to obtain the deterministic portion has to be modified in order to accommodate the lack of phase information while the ambiguous portion is determined by another nonlinear optimization. The numerical results presented for the two examples of scatterers under transverse-electric incidence have demonstrated that the proposed method is capable of reconstructing complicated patterns with rapid rate of convergence and robust immunity to noise.

Comparison among the variants of subspace-based optimization method for addressing inverse scattering problems: transverse electric case.

The subspace-based optimization method (SOM) is an efficient approach to addressing the inverse scattering problem. In this paper, a comparative study, on the basis of numerical experiments, is conducted to evaluate the performances of variants of SOM, so as to find the optimal method for the determination of the ambiguous portion, which has a dominant influence on the computational cost and the reconstruction capability of the algorithm.

Extreme learning machine terrain-based navigation for unmanned aerial vehicles

Unmanned aerial vehicles (UAVs) rely on global positioning system (GPS) information to ascertain its position for navigation during mission execution. In the absence of GPS information, the capability of a UAV to carry out its intended mission is hindered. In this paper, we learn alternative means for UAVs to derive real-time positional reference information so as to ensure the continuity of the mission. We present extreme learning machine as a mechanism for learning the stored digital elevation information so as to aid UAVs to navigate through terrain without the need for GPS. The proposed algorithm accommodates the need of the on-line implementation by supporting multi-resolution terrain access, thus capable of generating an immediate path with high accuracy within the allowable time scale. Numerical tests have demonstrated the potential benefits of the approach.