Reza Pourabolghasem

High-efficiency and wideband interlayer grating couplers in multilayer Si/SiO 2 /SiN platform for 3D integration of optical functionalities

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 platforms.

Physics of band-gap formation and its evolution in the pillar-based phononic crystal structures

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.

Experimental evidence of high-frequency complete elastic bandgap in pillar-based phononic slabs

We present strong experimental evidence for the existence of a complete phononic bandgap, for Lamb waves, in the high frequency regime (i.e., 800u2009MHz) for a pillar-based phononic crystal (PnC) membrane with a triangular lattice of gold pillars on top. The membrane is composed of an aluminum nitride film stacked on thin molybdenum and silicon layers. Experimental characterization shows a large attenuation of at least 20u2009dB in the three major crystallographic directions of the PnC lattice in the frequency range of 760 MHz–820u2009MHz, which is in agreement with our finite element simulations of the PnC bandgap. The results of experiments are analyzed and the physics behind the attenuation in different spectral windows is explained methodically by assessing the type of Bloch modes and the in-plane symmetry of the displacement profile.

Integrated phononic crystal resonators based on adiabatically-terminated phononic crystal waveguides

In this letter, we demonstrate a new design for integrated phononic crystal (PnC) resonators based on confining acoustic waves in a heterogeneous waveguide-based PnC structure. In this architecture, a PnC waveguide that supports a single mode at the desired resonance frequencies is terminated by two waveguide sections with no propagating mode at those frequencies (i.e., have mode gap). The proposed PnC resonators are designed through combining the spatial-domain and the spatial-frequency domain (i.e., the k-domain) analysis to achieve a smooth mode envelope. This design approach can benefit both membrane-based and surface-acoustic-wave-based architectures by confining the mode spreading in k-domain that leads to improved electromechanical excitation/detection coupling and reduced loss through propagating bulk modes.

High-frequency phononic crystal structures based on metallic pillars on piezzoelectric membranes (Presentation Video)

Phononic crystal (PnC) structures based on an array of metallic pillars on a piezzoelectric substrate will be discussed as a means for achieving PnBG at high (e.g., GHz) frequencies. In addition to operation at higher frequencies, the advantage of metallic-pillar-based structures in providing design flexibility for functional PnC-based devices will be discussed. Experimental evidence for the existence of the PnBG in these structures and the use of their relatively wide bandgap for the implementation of practical PnC devices (especially waveguides and resonators) will be theoretically and experimentally demonstrated, and the prospects of these structures for practical applications (e.g., sensing and wireless communications) will be discussed.

GHz Heterogeneous Phononic Crystal Slab Resonators

In this paper, we present a new design for waveguide-based phononic crystal (PnC) resonators in pillar-based piezoelectric membranes at the GHz frequency range based on mode-gap waveguide termination. The mode confinement in these resonators is achieved by a smooth transition from a phononic waveguide to another phononic waveguide that does not support (and therefore reflects) the guided modes of the first waveguide over a certain frequency range. These resonators can be utilized for applications including wireless communications and sensing [1, 2] where high-Q and high-frequency resonators are highly desirable.Copyright