Ali A. Eftekhar

High-Q micromechanical resonators in a two-dimensional phononic crystal slab

By creating line defects in the structure of a phononic crystal (PC) made by etching a hexagonal array of holes in a 15 μm thick slab of silicon, high-Q PC resonators are fabricated using a complimentary-metal-oxide-semiconductor-compatible process. The complete phononic band gap of the PC structure supports resonant modes with quality factors of more than 6000 at frequencies as high as 126 MHz. The confinement of acoustic energy is achieved by using only a few PC layers confining the cavity region. The calculated frequencies of resonance of the structure using finite element method are in a very good agreement with the experimental data. The performance of these PC resonator structures makes them excellent candidates for wireless communication and sensing applications.

Evidence of large high frequency complete phononic band gaps in silicon phononic crystal plates

We show the evidence of the existence of large complete phononic band gaps (CPBGs) in two-dimensional phononic crystals (PCs) formed by embedding cylindrical air holes in a solid plate (slab). The PC structure is made by etching a hexagonal array of air holes through a freestanding plate of silicon. A fabrication process compatible with metal-oxide-semiconductor technology is used on silicon-on-insulator substrate to realize the PC devices. Measuring the transmission of elastic waves through eight layers of the hexagonal lattice PC in the ΓK direction, more than 30dB attenuation is observed at a high frequency; i.e., 134MHz, with a band gap to midgap ratio of 23%. We show that this frequency region matches very well with the expected CPBG found through theoretical calculations.

High resolution on-chip spectroscopy based on miniaturized microdonut resonators.

We experimentally demonstrate a high resolution integrated spectrometer on silicon on insulator (SOI) substrate using a large-scale array of microdonut resonators. Through top-view imaging and processing, the measured spectral response of the spectrometer shows a linewidth of ~0.6 nm with an operating bandwidth of ~50 nm. This high resolution and bandwidth is achieved in a compact size using miniaturized microdonut resonators (radius ~2 μm) with a high quality factor, single-mode operation, and a large free spectral range. The microspectrometer is realized using silicon process compatible fabrication and has a great potential as a high-resolution, large dynamic range, light-weight, compact, high-speed, and versatile microspectrometer.

Simultaneous two-dimensional phononic and photonic band gaps in opto-mechanical crystal slabs

We demonstrate planar structures that can provide simultaneous two-dimensional phononic and photonic band gaps in opto-mechanical (or phoxonic) crystal slabs. Different phoxonic crystal (PxC) structures, composed of square, hexagonal (honeycomb), or triangular arrays of void cylindrical holes embedded in silicon (Si) slabs with a finite thickness, are investigated. Photonic band gap (PtBG) maps and the complete phononic band gap (PnBG) maps of PxC slabs with different radii of the holes and thicknesses of the slabs are calculated using a three-dimensional plane wave expansion code. Simultaneous phononic and photonic band gaps with band gap to midgap ratios of more than 10% are shown to be readily obtainable with practical geometries in both square and hexagonal lattices, but not for the triangular lattice.

Optimal sparse solution for fluorescent diffuse optical tomography: theory and phantom experimental results

We present a method to accurately localize small fluorescent objects within the tissue using fluorescent diffuse optical tomography (FDOT). The proposed method exploits the localized or sparse nature of the fluorophores in the tissue as a priori information to considerably improve the accuracy of the reconstruction of fluorophore distribution. This is accomplished by minimizing a cost function that includes the L1 norm of the fluorophore distribution vector. Experimental results for a milk-based phantom using a fiber-based cw FDOT system demonstrate the capability of this method in accurately localizing small fluorescent objects deep in the phantom.

Optimization of metallic microheaters for high-speed reconfigurable silicon photonics

The strong thermooptic effect in silicon enables low-power and low-loss reconfiguration of large-scale silicon photonics. Thermal reconfiguration through the integration of metallic microheaters has been one of the more widely used reconfiguration techniques in silicon photonics. In this paper, structural and material optimizations are carried out through heat transport modeling to improve the reconfiguration speed of such devices, and the results are experimentally verified. Around 4 micros reconfiguration time are shown for the optimized structures. Moreover, sub-microsecond reconfiguration time is experimentally demonstrated through the pulsed excitation of the microheaters. The limitation of this pulsed excitation scheme is also discussed through an accurate system-level model developed for the microheater response.

Vertical integration of high-Q silicon nitride microresonators into silicon-on-insulator platform

We demonstrate a vertical integration of high-Q silicon nitride microresonators into the silicon-on-insulator platform for applications at the telecommunication wavelengths. Low-loss silicon nitride films with a thickness of 400 nm are successfully grown, enabling compact silicon nitride microresonators with ultra-high intrinsic Qs (~ 6 × 10(6) for 60 μm radius and ~ 2 × 10(7) for 240 μm radius). The coupling between the silicon nitride microresonator and the underneath silicon waveguide is based on evanescent coupling with silicon dioxide as buffer. Selective coupling to a desired radial mode of the silicon nitride microresonator is also achievable using a pulley coupling scheme. In this work, a 60-μm-radius silicon nitride microresonator has been successfully integrated into the silicon-on-insulator platform, showing a single-mode operation with an intrinsic Q of 2 × 10(6).

Silicon nanophotonic devices for integrated sensing

The potentials of a nanophotonic platform, including compactness, low power consumption, integrability with other functionalities, and high sensitivity make them a suitable candidate for sensing applications. Strong light-matter interaction in such a platform enables a variety of sensing mechanisms, including refractive index change, fluorescence emission, and Raman scattering. Recent advances in nanophotonic devices include the demonstration of silicon and silicon-nitride microdisk resonators with high intrinsic Q values (0.5-2×106) for strong field enhancement and the realization of compact photonic crystal spectrometers (high spectral resolution at 100-µm length scales) for on-chip spectral analysis. These two basic building blocks, when combined with integrated fluidic channels for sample delivery, provide an efficient platform to implement different sensing mechanisms and architectures.

Fully reconfigurable compact RF photonic filters using high-Q silicon microdisk resonators.

We present a fully reconfigurable four-pole, four-zero SOI RF photonic filter with a tunable 3dB bandwidth of 0.9 - 5 GHz, more than 38 dB out-of-band rejection, FSR larger than 600 GHz, and compact size (total area 0.15 mm2) using high-Q resonator-based components.

Accurate post-fabrication trimming of ultra-compact resonators on silicon

We experimentally demonstrate an accurate post-fabrication trimming technique for the correction of the optical phase of silicon photonic devices using a single fabrication step. Using this technique, we reduce the random resonance wavelength variation of ultra-compact silicon resonators by a factor of 6 to below 50 pm.