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.
We present fabrication, characterization, and experimental results describing electrical actuation and readout of the mechanical vibratory response of electrospun ZnO nanofibers. For a fiber with an approximate radius of 200 nm and a length of 70 μm, a resonance frequency around 3.62 MHz with a quality factor (Q) of about 235 in air at ambient conditions is observed. It is shown that the measured frequency of the resonance is consistent with results from finite element simulations. Also, the measurements were performed in an enclosed chamber with controlled levels of ethanol vapor. The adsorption of ethanol causes a shift in the resonance frequency of the fibers, which can be related to the concentration of ethanol vapor for sensing purposes.
Transfer and scattering matrix methods are widely in use for description of the propagation of waves in multilayered
media. When the profile of refractive index is continuous, however, a modified formulation of transfer matrices does
exist, which provides a complete analytical solution of the wave phenomena in such structures. Previously reported
variations of the so-called Differential Transfer Matrix Method (DTMM) had been limited to Cartesian geometry where
layered media form one-dimensional structures and plane waves are used as basis functions. In this work, we extend the
formalism to cylindrical geometry with radial symmetry, in which Bessel functions need to be employed as basis
functions. Hence, complete analytical formulation of the DTMM under radial and axial symmetry is described and
derived. This work could have applications in the analysis of propagation in optical fibers and motion of electrons in
nanowires and nanotubes.
In this paper we demonstrate the possibility of forming a new elastic filter structure based on
the coupled resonator waveguides in phononic crystal slabs (CRAW) with superior performance
over the conventional filters. The structures are made by etching a honeycomb array of holes
in a free standing slab. This phononic slab structure exhibits an absolute phononic band gap
for all polarizations of guided waves inside the slab including the Lamb and Love waves. We
present an analysis of a different family of waveguides in phononic-crystal slabs, and illustrate
the considerations that must be applied to achieve single-mode guided bands in these structures.
Consequently, an unusual family of selective elastic filters composed of several single resonators
that are coupled periodically through evanescent waves is obtained. The elastic energy is localized
in the extended defect formed by the collective coupled resonators. The frequencies of the filters
are sensitive to the geometrical parameters and to the separation distance between the indiviual
resonators. Numerical simulations are performed using the finite element method and considering
Zinc-Oxide slab.
By modifying the supporting structures of a conventional piezoelectric-on-substrate micromechanical (MM) resonator
using phononic crystal (PC) slab structures with complete phononic band gaps (PBGs) the support loss in
micromechanical resonators is suppressed and the quality factor of the fundamental extensional resonant mode is
improved from approximately 1,200 to approximately 6,000. The conventional MM resonator and the PC resonators are
both fabricated on the same chip and using the same fabrication process. The PC is made by etching a hexagonal
(honeycomb) array of holes in a 15μm-thick slab of silicon. The radii of the holes are approximately 6.4μm and the
spacing between the centers of the nearest holes is 15 μm. The conventional MM resonator is made of a rectangular
structure with dimensions of 600 μm by 60 μm and the fundamental flexural and extensional modes of the structure in
the smaller dimension are excited. In the third dimension, all the structures are made of a 15 μm silicon (Si) slab, a 100
nm layer of gold, a ~1 μm layer of zinc oxide, and a patterned 100 nm layer of aluminum electrodes stacked on top of
each other to serve as the resonant mass and the transduction medium. The significant improvement obtained using the
PC resonator structures makes them excellent candidates for next generation of MM resonators for wireless
communication and sensing applications despite some minor remaining challenges.
By creating line defects cavities in the structure of a phononic crystal (PC) made by etching a hexagonal (honeycomb)
array of holes in a 15μm-thick slab of silicon, high-Q PC resonators are fabricated and tested using a complimentary-metal-
oxide-semiconductor-compatible process. The radii of the holes are approximately 6.4μm and the spacing between
nearest holes is 15μm. We show that the complete phononic band gap of the PC structure supports resonant modes with
quality factors of more than 6000 at frequencies as high as 126MHz in the resonator structure. The very good
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.
We show that femtosecond optical pulses at optical communication wavelengths can be used for real time dispersion measurement of photonic crystal waveguides. Spectral resolutions on the order of one nanometer and bandwidths as large as tens of nanometers are demonstrated in real time measurements. Preliminary results are shown and discussed.
Compact on-chip wavelength demultiplexers and spectrometers are essential components for a variety of applications
including integrated optical information processing devices, optical communications, and integrated optical sensing.
Implementation of such devices requires strong dispersion in the optical materials, which can be realized using unique
dispersive properties of photonic crystals (PCs). Possibility of integration, compactness, and compatibility with different
host materials are the main advantages of PC based demultiplexers and spectrometers compared to other techniques.
Here, we show an implementation of superprism-based photonic crystal devices (using a diffraction compensation
scheme) that improves the performance of these devices compared to the conventional implementation. Structures
obtained through optimization have been fabricated in SOI wafers using e-beam writing and ICP etching, and spatial
separation of channels (with good isolation) in these superprism devices is experimentally demonstrated. The
performance of these superprism devices as general-purpose spectrometers and for locating spectral features in a
sensing platform will be also demonstrated and discussed. Further steps for improvement of these devices are
considered and the related implementation issues are investigated.
Wavelength demultiplexing is one of the major applications of unique dispersion properties of photonic crystals (PCs). Possibility of integration and compactness are two main advantages of PC based demultiplexers compared to other demultiplexing techniques for applications including compact spectrometers (for sensory applications) and WDM demultiplexers. Here, we show that resolution and size limitations of conventional superprism-based photonic crystal
demultiplexers are caused by the choice of configuration. We suggest an alternative implementation (combining superprism effect and focusing) that improves the performance compared to the conventional implementation in terms of being more compact and relaxing the requirement for divergence angle of the incident beam. We use effective index model to describe the beam behavior inside the photonic crystal region. Using this model, effective indices (second
order and third order) are calculated directly from the band structure and are used to find the optimal operation parameters for the demultiplexing device. Detailed calculations show that the required size of preconditioned superprism photonic crystal demultiplexers scales up as N5/2 (N being the number of channels which is proportional to the resolution of the device) which shows significant advantage over N4 dependence in conventional superprism-based devices, especially for high resolutions required in practical DWDM systems or spectroscopic applications. Structures obtained through optimization have been fabricated in SOI wafers using e-beam writing and ICP etching, and spatial separation of channels (with good isolation) in focusing superprism devices is experimentally demonstrated.
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