Roberto Coccioli

Novel 2-D photonic bandgap structure for microstrip lines

A new two-dimensional (2-D) photonic bandgap (PBG) structure for microstrip lines is proposed, in which a periodic 2-D pattern consisting of circles is etched in the ground plane of microstrip line. No drilling through the substrate is required. Three PBG circuits were fabricated with different circle radii to determine the optimum dimensions, as well as a PBG circuit with the compensated right-angle microstrip bend. Measurements show that deep and wide stopbands can be achieved using this method.

Aperture-coupled patch antenna on UC-PBG substrate

The recently developed uniplanar compact photonic bandgap (UC-PBG) substrate is successfully used to reduce surface-wave losses for an aperture-coupled fed patch antenna on a thick high dielectric-constant substrate. The surface-wave dispersion diagram of the UC-PBG substrate has been numerically computed for two different substrate thickness (25 and 50 mil) and found to have a complete stopband in the frequency range of 10.9-13.5 and 11.4-12.8 GHz, respectively. The thicker substrate is then used to enhance broadside gain of a patch antenna working in the stopband at 12 GHz. Computed results and measured data show that, due to effective surface-wave suppression, the antenna mounted on the UC-PBG substrate has over 3-dB higher gain in the broadside direction than the same antenna etched on a grounded dielectric slab with same thickness and dielectric constant. Cross-polarization level remains 13 dB down the co-polar component level for both E- and H-planes.

Light extraction from optically pumped light-emitting diode by thin-slab photonic crystals

We describe a promising thin-slab light-emitting diode (LED) design, which uses a highly efficient coherent external scattering of trapped light by a two-dimensional (2D) photonic crystal. The light generation region was an unpatterned heterostructure surrounded by the light extraction region, a thin film patterned as a 2D photonic crystal. A six-fold photoluminescence enhancement was observed compared to an unpatterned thin film LED. That corresponded to 70% external quantum efficiency.

Spontaneous emission extraction and Purcell enhancement from thin-film 2-D photonic crystals

Electromagnetic band structure can produce either an enhancement or a suppression of spontaneous emission from two-dimensional (2-D) photonic crystal thin films. We believe that such effects might be important for light emitting diodes. Our experiments were based on thin-film InGaAs-InP 2-D photonic crystals at ambient temperature, but the concepts would apply equally to InGaN thin films, for example. We show that the magnitude of Purcell enhancement factor, F/sub p//spl sim/2, for spatially extended band modes, is similar to that for a tiny mode in a three dimensional (3-D) nanocavity. Nonetheless, light extraction enhancement that arises from Zone folding or Bragg scattering of the photonic bands is probably the more important effect, and an external quantum efficiency >50% is possible. Angle resolved photoluminescence from inside the photonic crystal gives a direct spectral readout of the internal 2-D photonic band dispersion. The tradeoffs for employing various photonic crystal structures in high efficiency light-emitting diodes are analyzed.

A novel low-loss slow-wave microstrip structure

A low-loss slow-wave microstrip line using a periodic structure in the ground plane is presented. The periodic structure is realized with metal pads etched in the ground plane connected by thin lines to form a distributed LC network. The slow-wave factor is demonstrated to be 1.2-2.4 times larger than that of conventional microstrip lines over a wide frequency range. Due to the unique design of the structure, low insertion loss comparable to conventional microstrips has been achieved. The proposed structure is easier to fabricate than other slow-wave devices which require multilayer substrates or very fine features.

A global finite-element time-domain analysis of active nonlinear microwave circuits

An extension of the finite-element time-domain (FETD) method for mixed electromagnetic and circuit simulation of complex active microwave circuits is proposed in this paper. The passive distributed part of the circuit is analyzed using FETD and its interaction with lumped passive/active linear/nonlinear components is modeled via an equivalent-current generator whose value and internal capacitive admittance are computed at each time step by the FETD solver. Benchmark tests on a microwave amplifier and a injection-locked oscillator indicate that this extended FETD is not only superior in mesh flexibility, but also gives more accurate results than the similar FDTD-based algorithm.

Analysis of photonic crystals for light-emitting diodes using the finite-difference time domain technique

The Finite Difference Time Domain method has been used to analyze the dispersion diagram of a photonic crystal comprised of a perforated dielectric slab and the properties of a micro-cavity formed by introducing a defect into such a crystal. Computational requirements of the method, its advantages and disadvantages, and results for the structure analyzed are discussed.

Finite-element methods in microwaves: a selected bibliography

This paper reviews the history and present state of finite-element methods, as applied to electromagnetic-field problems in the microwave range (including element definitions and error estimation, guided wave propagation, scattering and antenna problems). It provides a selective bibliography, current as of the first months of 1996, with annotations and evaluative comments on the works surveyed. It briefly notes the capabilities of the finite-element method, and estimates how fully they have been realized to date. Some conjectures on the future of finite-element methods are also hazarded.

Characterization of dielectric materials with the finite-element method

A technique to characterize homogeneous and inhomogeneous dielectric materials based on measurements and computations is presented. The first step of the method proposed consists of measuring the resonant frequencies of an arbitrarily shaped cavity containing an arbitrarily shaped sample of the material under test. In the second step, the measured data are used in two different algorithms, both based on the finite-element method, to solve for the dielectric constant.

Radiation characteristics of a patch antenna on a thin PBG substrate

A periodically loaded substrate is employed to reduce excitation of surface waves by a patch antenna. The computed dispersion diagram of the substrates shows that surface wave propagation is forbidden along specific directions. By properly orienting the patch antenna on the substrate, this can result in good improvement of antenna performance. In agreement with the expected result, the measured radiation pattern shows a 10 dB reduction in the side lobes associated with power launched from surface waves.