Seyedhamidreza Alaie

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.

Sub-pg mass sensing and measurement with an optomechanical oscillator

Mass sensing based on mechanical oscillation frequency shift in micro/nano scale mechanical oscillators is a well-known and widely used technique. Piezo-electric, electronic excitation/detection and free-space optical detection are the most common techniques used for monitoring the minute frequency shifts induced by added mass. The advent of optomechanical oscillator (OMO), enabled by strong interaction between circulating optical power and mechanical deformation in high quality factor optical microresonators, has created new possibilities for excitation and interrogation of micro/nanomechanical resonators. In particular, radiation pressure driven optomechanical oscillators (OMOs) are excellent candidates for mass detection/measurement due to their simplicity, sensitivity and all-optical operation. In an OMO, a high quality factor optical mode simultaneously serves as an efficient actuator and a sensitive probe for precise monitoring of the mechanical eigen-frequencies of the cavity structure. Here, we show the narrow linewidth of optomechanical oscillation combined with harmonic optical modulation generated by nonlinear optical transfer function, can result in sub-pg mass sensitivity in large silica microtoroid OMOs. Moreover by carefully studying the impact of mechanical mode selection, device dimensions, mass position and noise mechanisms we explore the performance limits of OMO both as a mass detector and a high resolution mass measurement system. Our analysis shows that femtogram level resolution is within reach even with relatively large OMOs.

Design and characterization of a low temperature gradient and large displacement thermal actuators for in situ mechanical testing of nanoscale materials

The design, fabrication and characterization of a recently developed test platform for the characterization of nanoscale properties of thin films are presented. Platforms are comprised of a microfabricated cascaded thermal actuator system and test specimen. The cascaded thermal actuator system is capable of providing tens of microns of displacement and tens of milli-Newton forces simultaneously while applying a relatively low temperature gradient across the test specimen. The dimensions of the platform make its use possible in both the scanning/transmission electron microscope environments and on a probe station under an optical microscope. Digital image correlation was used to obtain similar accuracy (?10?nm) for displacement measurements in both a SEM and under an optical microscope. Proof of concept experiments were performed on freestanding 250?nm thick Pt thin films.

Effects of flexural and extensional excitation modes on the transmission spectrum of phononic crystals operating at gigahertz frequencies

Phononic crystals (PnCs) are a class of materials that are capable of manipulating elastodynamic waves. Much of the research on PnCs, both theoretical and experimental, focus on studying the transmission spectrum of PnCs in an effort to characterize and engineer their phononic band gaps. Although most studies have shown acceptable agreement between the theoretical and experimental bandgaps, perfect matches are elusive. A framework is presented wherein two and three dimensional harmonic finite element analyses are utilized to study their mechanical behavior for the purpose of more accurately predicting the spectral properties of PnCs. Discussions on a Harmonic Finite Elements Analysis formulation of a perfectly matched layer absorbing boundary and how reflections from absorbing boundaries can be inferred via standing wave ratios are provided. Comparisons between 2D and 3D analyses are presented that show the less computationally intensive 2D models are equally accurate under certain conditions. Finally, it...

Realization of a 33 GHz phononic crystal fabricated in a freestanding membrane

Phononic crystals (PnCs) are man-made structures with periodically varying material properties such as density, ρ, and elastic modulus, E. Periodic variations of the material properties with nanoscale characteristic dimensions yield PnCs that operate at frequencies above 10 GHz, allowing for the manipulation of thermal properties. In this article, a 2D simple cubic lattice PnC operating at 33 GHz is reported. The PnC is created by nanofabrication with a focused ion beam. A freestanding membrane of silicon is ion milled to create a simple cubic array of 32 nm diameter holes that are subsequently backfilled with tungsten to create inclusions at a spacing of 100 nm. Simulations are used to predict the operating frequency of the PnC. Additional modeling shows that milling a freestanding membrane has a unique characteristic; the exit via has a conical shape, or trumpet-like appearance.

The effect of stiffness and mass on coupled oscillations in a phononic crystal

Insight into phononic bandgap formation is presented using a first principles-type approach where phononic lattices are treated as coupled oscillators connected via massless tethers. The stiffness of the tethers and the mass of the oscillator are varied and their influences on the bandgap formation are deduced. This analysis is reinforced by conducting numerical simulations to examine the modes bounding the bandgap and highlighting the effect of the above parameters. The analysis presented here not only sheds light on the origins of gap formation, but also allows one to define design rules for wide phononic gaps and maximum gap-to-midgap ratios.

Microfabricated suspended island platform for the measurement of in-plane thermal conductivity of thin films and nanostructured materials with consideration of contact resistance

A technique based on suspended islands is described to measure the in-plane thermal conductivity of thin films and nano-structured materials, and is also employed for measurements of several samples with a single measurement platform. Using systematic steps for measurements, the characterization of the thermal resistances of a sample and its contacts are studied. The calibration of the contacts in this method is independent of the geometry, size, materials, and uniformity of contacts. To verify the technique, two different Si samples with different thicknesses and two samples of the same SiN(x) wafer are characterized on a single device. One of the Si samples is also characterized by another technique, which verifies the current results. Characterization of the two SiN(x) samples taken from the same wafer showed less than 1% difference in the measured thermal conductivities, indicating the precision of the method. Additionally, one of the SiN(x) samples is characterized and then demounted, remounted, and characterized for a second time. The comparison showed the change in the thermal resistance of the contact in multiple measurements could be as small as 0.2 K/μW, if a similar sample is used.

Ultra-high frequency, high Q/volume micromechanical resonators in a planar AlN phononic crystal

This paper presents the first design and experimental demonstration of an ultrahigh frequency complete phononic crystal (PnC) bandgap aluminum nitride (AlN)/air structure operating in the GHz range. A complete phononic bandgap of this design is used to efficiently and simultaneously confine elastic vibrations in a resonator. The PnC structure is fabricated by etching a square array of air holes in an AlN slab. The fabricated PnC resonator resonates at 1.117 GHz, which corresponds to an out-of-plane mode. The measured bandgap and resonance frequencies are in very good agreement with the eigen-frequency and frequency-domain finite element analyses. As a result, a quality factor/volume of 7.6 × 1017/m3 for the confined resonance mode was obtained that is the largest value reported for this type of PnC resonator to date. These results are an important step forward in achieving possible applications of PnCs for RF communication and signal processing with smaller dimensions.

Enhancing Mechanical Quality Factors of Micro-Toroidal Optomechanical Resonators Using Phononic Crystals

Fabrication of efficient optomechanical resonators with high mechanical Q-factors without degrading their optical Q-factors or reducing effective mass is challenging. One of the limiting sources of mechanical loss for silica micro-toroids is the clamping loss. Previous efforts on reducing this loss were focused on reducing the connectivity between the toroid and the pillar, which results in reduced effective mass and degraded phase noise. We explore a new approach for reducing the mechanical loss that relies on phononic crystals (PnCs). Since it is based on the geometrical modification of substrate, it does not affect the optical Q-factor or effective mass. First, we experimentally verify the performance of the proposed PnC patterns without a micro-toroid by measuring their acoustic transmission properties. This experiment is used to verify the effectiveness of the PnCs for blocking outgoing acoustic waves and our numerical model. Then, using the same numerical model, we study the effects of PnCs on the mechanical Q-factors of a micro-toroid previously investigated. Our results show that the PnCs improve the mechanical Q-factor about 150%. This technique can be used in conjunction with other previously demonstrated methods for the enhancement of the Q-factor for even greater improvements.

Measurement of the thermoelectric power factor of films over the 10-400 K range

The design and development of a novel apparatus for the simultaneous measurement of electrical resistivity and Seebeck coefficient of films is reported here. Mounting stage is integrated inside a cryostat chamber enabling measurements over the 10-400 K temperature range, intended for organic thermoelectrics. Finite element method was used to analyze the thermo-mechanical response of the sample holder. The apparatus was validated against high purity nickel film, and a very good agreement was found.