ABSTRACT In this paper, the interplay of Bragg scattering and local resonance is theoretically st... more ABSTRACT In this paper, the interplay of Bragg scattering and local resonance is theoretically studied in a phononic crystal (PnC) structure composed of a silicon membrane with periodic tungsten pillars. The comparison of phononic band gaps (PnBGs) in three different lattice types (i.e., square, triangular, and honeycomb) with different pillar geometries shows that different PnBGs have varying degrees of dependency on the lattice symmetry based on the interplay of the local resonances and the Bragg effect. The details of this interplay is discussed. The significance of locally resonating pillars, specially in the case of tall pillars, on PnBGs is discussed and verified by examining the PnBG position and width in perturbed lattices via Monte Carlo simulations. It is shown that the PnBGs caused by the local resonance of the pillars are more resilient to the lattice perturbations than those caused by Bragg scattering.
We have designed interlayer grating couplers with single/double metallic reflectors for Si/SiO(2)... more We have designed interlayer grating couplers with single/double metallic reflectors for Si/SiO(2)/SiN multilayer material platform. Out-of-plane diffractive grating couplers separated by 1.6 μm thick buffer SiO(2) layer are vertically stacked against each other in Si and SiN layers. Geometrical optimization using genetic algorithm coupled with electromagnetic simulations using two-dimensional (2D) finite element method (FEM) results in coupler designs with high peak coupling efficiency of up to 89% for double- mirror and 64% for single-mirror structures at telecom wavelength. Also, 3-dB bandwidths of 40 nm and 50 nm are theoretically predicted for the two designs, respectively. We have fabricated the grating coupler structure with single mirror. Measured values for insertion loss and 3-dB bandwidth in the fabricated single-mirror coupler confirms the theoretical results. This opens up the possibility of low-loss 3D dense integration of optical functionalities in hybrid material plat...
The complete phononic band gap (PnBG) of a phononic crystal (PnC) slab is used to efficiently and... more The complete phononic band gap (PnBG) of a phononic crystal (PnC) slab is used to efficiently and simultaneously confine both flexural and extensional (longitudinal) elastic vibrations in a resonator. The PnC structure is fabricated by etching a hexagonal (honeycomb) array of air holes in a silicon (Si) slab. Flexural and extensional acoustic modes in the structure are directly excited by
ABSTRACT The existence of phononic bandgaps is shown in a phononic crystal structure composed of ... more ABSTRACT The existence of phononic bandgaps is shown in a phononic crystal structure composed of a silicon plate with stubbed tungsten pillars. Using the finite element method for numerical simulations, the evolution of bandgap opening with varying design parameters is studied. Several bandgaps have been observed for different lattice symmetries. It has been shown that bandgaps of up to 40% gap to mid-gap ratio can be achieved for the triangular and square lattice.
ABSTRACT In this paper, the interplay of Bragg scattering and local resonance is theoretically st... more ABSTRACT In this paper, the interplay of Bragg scattering and local resonance is theoretically studied in a phononic crystal (PnC) structure composed of a silicon membrane with periodic tungsten pillars. The comparison of phononic band gaps (PnBGs) in three different lattice types (i.e., square, triangular, and honeycomb) with different pillar geometries shows that different PnBGs have varying degrees of dependency on the lattice symmetry based on the interplay of the local resonances and the Bragg effect. The details of this interplay is discussed. The significance of locally resonating pillars, specially in the case of tall pillars, on PnBGs is discussed and verified by examining the PnBG position and width in perturbed lattices via Monte Carlo simulations. It is shown that the PnBGs caused by the local resonance of the pillars are more resilient to the lattice perturbations than those caused by Bragg scattering.
We have designed interlayer grating couplers with single/double metallic reflectors for Si/SiO(2)... more We have designed interlayer grating couplers with single/double metallic reflectors for Si/SiO(2)/SiN multilayer material platform. Out-of-plane diffractive grating couplers separated by 1.6 μm thick buffer SiO(2) layer are vertically stacked against each other in Si and SiN layers. Geometrical optimization using genetic algorithm coupled with electromagnetic simulations using two-dimensional (2D) finite element method (FEM) results in coupler designs with high peak coupling efficiency of up to 89% for double- mirror and 64% for single-mirror structures at telecom wavelength. Also, 3-dB bandwidths of 40 nm and 50 nm are theoretically predicted for the two designs, respectively. We have fabricated the grating coupler structure with single mirror. Measured values for insertion loss and 3-dB bandwidth in the fabricated single-mirror coupler confirms the theoretical results. This opens up the possibility of low-loss 3D dense integration of optical functionalities in hybrid material plat...
The complete phononic band gap (PnBG) of a phononic crystal (PnC) slab is used to efficiently and... more The complete phononic band gap (PnBG) of a phononic crystal (PnC) slab is used to efficiently and simultaneously confine both flexural and extensional (longitudinal) elastic vibrations in a resonator. The PnC structure is fabricated by etching a hexagonal (honeycomb) array of air holes in a silicon (Si) slab. Flexural and extensional acoustic modes in the structure are directly excited by
ABSTRACT The existence of phononic bandgaps is shown in a phononic crystal structure composed of ... more ABSTRACT The existence of phononic bandgaps is shown in a phononic crystal structure composed of a silicon plate with stubbed tungsten pillars. Using the finite element method for numerical simulations, the evolution of bandgap opening with varying design parameters is studied. Several bandgaps have been observed for different lattice symmetries. It has been shown that bandgaps of up to 40% gap to mid-gap ratio can be achieved for the triangular and square lattice.
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Papers by Reza Pourabolghasem