Zhaolin Lu

Subwavelength imaging by a flat cylindrical lens using optimized negative refraction

We experimentally demonstrate subwavelength imaging by a “flat cylindrical” lens using negative refraction. A two-dimensional photonic crystal whose dispersion at the second band provides group velocity opposite to the phase velocity for electromagnetic waves is employed to realize the flat lens, and the working frequency is chosen so that the effective refractive index is approximately equal to −1.0. Experiment demonstrated the imaging of a point source in both amplitude and phase in the millimeter-wave regime. By measuring the field distributions in the object plane and image plane, we observed amplification of evanescent waves and subwavelength size image. The image of two incoherent sources with subwavelength distance showed two resolvable spots, which served to further verify subwavelength resolution.

Experimental demonstration of negative refraction imaging in both amplitude and phase

We studied a two-dimensional square-lattice photonic crystal with all-angle negative refraction at its first band. Using this photonic crystal, we designed and fabricated a flat lens functioning as a cylindrical lens by increasing the vertical dimension of the photonic crystal. Two-dimensional finite-difference time-domain simulation validated negative refraction imaging. To perform the experiment, a microwave imaging system was built based on a vector network analyzer. Field distributions were acquired by scanning the imaging plane and object plane. The experiment demonstrated negative refraction imaging in both amplitude and phase, and obtained an image with feature size, 0.77lambda0.

Three-dimensional photonic crystal flat lens by full 3D negative refraction

We present the experimental demonstration of imaging by all-angle negative refraction in a 3D photonic crystal flat lens at microwave frequencies. The flat lens is made of a body-centered cubic photonic crystal (PhC) whose dispersion at the third band results in group velocity opposite to phase velocity for electromagnetic waves. We fabricated the photonic crystal following a layer-by-layer process. A microwave imaging system was established based on a vector network analyzer, where two dipoles work as the source and the detector separately. By scanning the volume around the lens with the detector dipole, we captured the image of the dipole source in both amplitude and phase. The image of two incoherent sources separated by 0.44lambda showed two resolvable spots, which served to verify sub-wavelength resolution.

Total internal reflection–evanescent coupler for fiber-to-waveguide integration of planar optoelectric devices

We present a method for parallel coupling from a single-mode fiber, or fiber ribbon, into a silicon-on-insulator waveguide for integration with silicon optoelectronic circuits. The coupler incorporates the advantages of the vertically tapered waveguides and prism couplers, yet offers the flexibility of planar integration. The coupler can be fabricated by use of either wafer polishing technology or gray-scale photolithography. When optimal coupling is achieved in our experimental setup, the coupler can be packaged by epoxy bonding to form a fiber-waveguide parallel coupler or connector. Two-dimensional electromagnetic calculation predicts a coupling efficiency of 77% (- 1.14-dB insertion loss) for a silicon-to-silicon coupler with a uniform tunnel layer. The coupling efficiency is experimentally achieved to be 46% (-3.4-dB insertion loss), excluding the loss in silicon and the reflections from the input surface and the output facet.

Experimentally demonstrated filters based on guided resonance of photonic-crystal films

We demonstrate a guided resonance filter based on photonic crystals (PhCs), which are fabricated in a high-permittivity material. The resulting spectra from a three-dimensional analysis of the structure and experimental measurement results show sharp dips and flattop transmissions. These provide promising properties in constructing sensitive and compact wavelength-selective devices, such as wavelength-division multiplexers.

Experimental demonstration of self-collimation in low-index-contrast photonic crystals in the millimeter-wave regime

In this paper, we present the theoretical and experimental results for self-collimation in low-index-contrast photonic crystals (PhCs) in the millimeter-wave (MMW) region of the electromagnetic spectrum. The design of the PhCs is based on their equifrequency contours and the two-dimensional finite-difference time-domain simulation results. In the experiments, the MMW PhCs are fabricated in Rexolite slabs by a CNC micro-milling system. A MMW imaging system is built based on a vector network analyzer. The input source is launched either through a waveguide or a monopole, while the field distribution is acquired by scanning a monopole antenna over the surface of the photonic crystal to detect the profile of the evanescent waves. In both cases, we have observed and characterized the self-collimation effect for both the amplitude and phase of the propagating electromagnetic wave in low-index-contrast photonic crystals.

Electrical characterization of flip-chip interconnects formed using a novel conductive-adhesive-based process

Using conventional microfabrication techniques, we have developed a new, low-cost wafer bumping process that enables a high degree of control over patterning of conductive adhesive interconnects. This approach obviates the need for development of dispensing and scraping head equipment that may otherwise be required for mass fabrication of lithographically patterned adhesive bumps. Flip-chip interconnects formed using this new process offer better electrical performance as compared to those formed by squeegee-based definition techniques. This is inferred in this paper by experimentally demonstrating lower contact resistance with the polished bumps as compared to the squeegeed bumps. Furthermore, in order to study the high-speed electrical performance characteristics of these conductive adhesive bumps, a 10-GHz 1.55-mum p-i-n photodetector fabricated in the antimonide material system was used as case study. The results from the bandwidth characterization of the polymer flip-chip-integrated detector showed minimum degradation in the high-speed performance characteristics of the detector

Perfect lens makes a perfect trap

In this work, we present for the first time a new and realistic application of the “perfect lens”, namely, electromagnetic traps (or tweezers). We combined two recently developed techniques, 3D negative refraction flat lenses (3DNRFLs) and optical tweezers, and experimentally demonstrated the very unique advantages of using 3DNRFLs for electromagnetic traps. Super-resolution and short focal distance of the flat lens result in a highly focused and strongly convergent beam, which is a key requirement for a stable and accurate electromagnetic trap. The translation symmetry of 3DNRFL provides translation-invariance for imaging, which allows an electromagnetic trap to be translated without moving the lens, and permits a trap array by using multiple sources with a single lens. Electromagnetic trapping was demonstrated using polystyrene particles in suspension, and subsequent to being trapped to a single point, they were then accurately manipulated over a large distance by simple movement of a 3DNRFL-imaged microwave monopole source.

Self-collimation in 3D photonic crystals

In this paper we present self-collimation in three-dimensional (3D) photonic crystals (PhCs) consisting of a simple cubic structure. By exploiting the dispersive properties of photonic crystals, a cubic-like shape equi-frequency surface (EFS) is obtained. Such surfaces allow for structureless confinement of light. Due to the degeneracy of propagation modes in a 3D structure, self-collimation modes can be distinguished from other modes by launching a source with a particular polarization. To this end, we found that polarization dependence is a key issue in 3D self-collimation. The results hold promise for high-density PhCs devices due to the lack of structural interaction. Finally, a novel method for the fabrication of three-dimensional (3D) simple cubic photonic crystal structures using conventional planar silicon micromachining technology is presented. The method utilizes a single planar etch mask coupled with time multiplexed sidewall passivation and deep anisotropic reactive ion etching in combination with isotropic etch processes to create three-dimensional photonic crystal devices. Initial experimental results are presented.

Fabrication of terahertz two-dimensional photonic crystal lens on silicon-on-insulator

Using the special dispersion properties of photonic crystals (PhCs), we present a promising novel coupling device, the terahertz (THz) planar photonic crystal (PhC) lens. Three-dimensional finite-difference time-domain (3D-FDTD) calculations show a 90% power transfer from a 100 mm waveguide to a 10 mm waveguide, and experimental results confirm its high efficiency. Furthermore, the PhC lens couples the wave into a PhC line-defect waveguide is also reported. These achievements manifest the usefulness of the PhC lens as an effective approach to couple the wave into future THz circuits.