Ihab El-Kady

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.

Phononic band-gap crystals for radio frequency communications

We report on the experimental and theoretical observation of a phononic band-gap crystal operating in the megahertz regime. Our experimental data show over 25dB suppression of bulk acoustic waves, and our theoretical models predict almost linear scaling to the gigahertz frequencies, thus laying the foundation for the implementation of such devices in radio frequency communications. We further argue that cavities in such systems offer a unique opportunity to couple acoustic energy into a resonator utilizing piezoelectric materials, while at the same time allowing the realization of a resonance cavity in high-Q materials such as silicon oxide, silicon, and tungsten.

Three-dimensional photonic-crystal emission through thermal excitation

A three-dimensional tungsten photonic crystal is thermally excited and shown to emit light at a narrow band, lambda = 3.3-4.25 microm. The emission is experimentally observed to exceed that of the free-space Planck radiation over a wide temperature range, T = 475-850 K. It is proposed that an enhanced density of state associated with the propagating electromagnetic Bloch waves in the photonic crystal is responsible for this experimental finding.

Experimental observation of photonic-crystal emission near a photonic band edge

A three-dimensional tungsten photonic crystal is realized with a photonic band edge at λ∼4 μm wavelength. Its thermal emission is suppressed in the band gap regime and, at the same time, exhibits sharp peaks near the band edge. It is further observed that energy conversion efficiency from one side of the sample reaches η∼40%. This finding is attributed to a complete metallic photonic band gap in the infrared (λ⩾6 μm) and the enhanced density of photon states near the band edge of our tungsten photonic crystal.

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.

Origin of reduction in phonon thermal conductivity of microporous solids

Porous structures have strong tunable size effects due to increased surface area. Size effects on phonon thermal conductivity have been observed in porous materials with periodic voids on the order of microns. This letter explores the origin of this size effect on phonon thermal conductivity observed in periodic microporous membranes. Pore-edge boundary scattering of low frequency phonons explains the temperature trends in the thermal conductivity; further reduction in thermal conductivity is explained by the porosity.

Highly efficient light emission at λ = 1.5 μm by a three-dimensional tungsten photonic crystal

For what is believed to be the first time, a three-dimensional tungsten photonic crystal is demonstrated to emit light effectively at wavelength λ=1.5 μm . At a bias of V=7 V, the thermal emission exhibits a full width at half-maximum of Δλ=0.85 μm . Within this narrow band, the emitted optical power is 4.5 W and the electrical-to-optical conversion efficiency is ~22% per emitting surface. This unique emission is made possible by a large, absolute bandgap in the infrared λ and flat photonic dispersion near the band edges and in a narrow absorption band.

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.