Mehmet F. Su

Reduction in the thermal conductivity of single crystalline silicon by phononic crystal patterning.

Phononic crystals (PnCs) are the acoustic wave equivalent of photonic crystals, where a periodic array of scattering inclusions located in a homogeneous host material causes certain frequencies to be completely reflected by the structure. In conjunction with creating a phononic band gap, anomalous dispersion accompanied by a large reduction in phonon group velocities can lead to a massive reduction in silicon thermal conductivity. We measured the cross plane thermal conductivity of a series of single crystalline silicon PnCs using time domain thermoreflectance. The measured values are over an order of magnitude lower than those obtained for bulk Si (from 148 W m(-1) K(-1) to as low as 6.8 W m(-1) K(-1)). The measured thermal conductivity is much smaller than that predicted by only accounting for boundary scattering at the interfaces of the PnC lattice, indicating that coherent phononic effects are causing an additional reduction to the cross plane thermal conductivity.

Realization of a phononic crystal operating at gigahertz frequencies

We report on the experimental realization of a phononic crystal, designed to operate at gigahertz frequencies. Detailed studies of the structure have been performed using finite difference time domain method to determine effects of slab modes in finite-thickness slabs, thus enabling precise guidance of experimental efforts. In particular, we find the slab mode effects mitigated in ultrathin (thickness less than lattice periodicity) and ultrathick (thickness more than ten times lattice periodicity) slabs. Gigahertz-frequency phononic crystals are well poised to find usage as high-Q resonators, waveguides, and coupling elements in a variety of application areas including RF communications.

Thermal transport in phononic crystals and the observation of coherent phonon scattering at room temperature

Large reductions in the thermal conductivity of thin silicon membranes have been demonstrated in various porous structures. However, the role of coherent boundary scattering in such structures has become a matter of some debate. Here we report on the first experimental observation of coherent phonon boundary scattering at room temperature in 2D phononic crystals formed by the introduction of air holes in a silicon matrix with minimum feature sizes >100 nm. To delaminate incoherent from coherent boundary scattering, phononic crystals with a fixed minimum feature size, differing only in unit cell geometry, were fabricated. A suspended island technique was used to measure the thermal conductivity. We introduce a hybrid thermal conductivity model that accounts for partially coherent and partially incoherent phonon boundary scattering. We observe excellent agreement between this model and experimental data, and the results suggest that significant room temperature coherent phonon boundary scattering occurs.

Phononic crystals operating in the gigahertz range with extremely wide band gaps

Phononic crystals have numerous potential applications including use as filters and oscillators in communications systems and as acoustic isolators for resonant sensors such as gyroscopes. These applications are based on the ability of phononic crystals to exhibit elastic band gaps, frequency bands where the propagation of acoustic waves is forbidden. Here, we focus on solid-solid phononic crystals (solid inclusions in a solid matrix), since they typically exhibit wider band gaps than those observed with air-solid phononic crystals (air inclusions in a solid matrix). We present a micromachined solid-solid phononic crystal operating at 1.4 GHz center frequency with an ultrawide 800 MHz band gap.

A novel FDTD application featuring OpenMP-MPI hybrid parallelization

We have developed a high performance hybridized parallel finite difference time domain (FDTD) algorithm featuring both OpenMP shared memory programming and MPl message passing. Our goal is to effectively model the optical characteristics of a novel light source created by utilizing a new class of materials known as photonic band-gap crystals. Our method is based on the solution of the second order discretized Maxwells equations in space and time. This novel hybrid parallelization scheme allows us to take advantage of the new generation parallel machines possessing connected SMP nodes. By using parallel computations, we are able to complete a calculation on 24 processors in less than a day, where a serial version would have taken over three weeks. We present a detailed study of this hybrid scheme on an SGI origin 2000 distributed shared memory ccNUMA system along with a complete investigation of the advantages versus drawbacks of this method.

Molded transparent photopolymers and phase shift optics for fabricating three dimensional nanostructures.

This paper introduces approaches that combine micro/nanomolding, or nanoimprinting, techniques with proximity optical phase mask lithographic methods to form three dimensional (3D) nanostructures in thick, transparent layers of photopolymers. The results demonstrate three strategies of this type, where molded relief structures in these photopolymers represent (i) fine (<1 microm) features that serve as the phase masks for their own exposure, (ii) coarse features (>1 microm) that are used with phase masks to provide access to large structure dimensions, and (iii) fine structures that are used together phase masks to achieve large, multilevel phase modulations. Several examples are provided, together with optical modeling of the fabrication process and the transmission properties of certain of the fabricated structures. These approaches provide capabilities in 3D fabrication that complement those of other techniques, with potential applications in photonics, microfluidics, drug delivery and other areas.

Realization of optimal bandgaps in solid-solid, solid-air, and hybrid solid-air-solid phononic crystal slabs

We investigate the optimal conditions for bandgap formation in square-lattice phononic crystal (PnC) slabs composed of a solid matrix with solid or air inclusions. To ensure sufficient impedance mismatch (key for bandgap formation) and fabrication amenability, silicon and silica were chosen as candidate matrix materials with either air or tungsten inclusions. Solid-solid PnCs were found to exhibit larger bandgaps while relaxing the topological constraints as compared to solid-air PnCs for all but the largest filling fractions. We also demonstrate a hybridized lattice incorporating both air and solid inclusions in the matrix that further relaxes the constraints for realizing wide bandgaps.

Application of photonic crystals in submicron damage detection and quantification

We propose the use of photonic crystals (PC) for submicron damage detection and quantification. The idea is based on the inherent tie between PC topology and its corresponding spectral frequency response. We demonstrate using a simulation model that a PC sensor attached to a polymer substrate will experience significant changes in its band gap profile when microdamage is induced in the substrate. A damage metric, developed using principles of fuzzy pattern recognition, is used to quantify the change in the spectral response in relation to the level of induced damage. Finally, different damage scenarios demonstrating coincidence with results are examined and reported.

Realizing the frequency quality factor product limit in silicon via compact phononic crystal resonators

High-Q (quality factor) resonators are a versatile class of components for radio frequency micro-electromechanical systems . Phononic crystals provide a promising method of producing these resonators. In this article, we present a theoretical study of the Q factor of a cavity resonator in a two-dimensional phononic crystal comprised of tungsten rods in a silicon matrix. One can optimize the Q of a phononic crystal resonator by varying the number of inclusions or the cavity harmonic number. We conclude that using higher harmonics marginally increases Q while increasing crystal length via additional inclusions causes Q to increase by orders of magnitude. Incorporating loss into the model shows that the silicon material limit on Q is achievable using a two-dimensional phononic crystal design with a reasonable length. With five layers of inclusions on either side of the cavity, the material limit on Q is achieved, regardless of the harmonic number.

Thermal conductivity prediction of nanoscale phononic crystal slabs using a hybrid lattice dynamics-continuum mechanics technique

Recent work has demonstrated that nanostructuring of a semiconductor material to form a phononic crystal (PnC) can significantly reduce its thermal conductivity. In this paper, we present a classical method that combines atomic-level information with the application of Bloch theory at the continuum level for the prediction of the thermal conductivity of finite-thickness PnCs with unit cells sized in the micron scale. Lattice dynamics calculations are done at the bulk material level, and the plane-wave expansion method is implemented at the macrosale PnC unit cell level. The combination of the lattice dynamics-based and continuum mechanics-based dispersion information is then used in the Callaway-Holland model to calculate the thermal transport properties of the PnC. We demonstrate that this hybrid approach provides both accurate and efficient predictions of the thermal conductivity.