Biomedical subjects

B Djafari-Rouhani

Publications and source records attributed to B Djafari-Rouhani.

At least 19 recordsLinked to original sources

PubMed2005

Propagation and localization of electromagnetic waves in quasiperiodic serial loop structures.

We study the propagation of electromagnetic waves in one-dimensional quasiperiodic photonic band gap structures made of serial loop structures separated by segments. Different quasiperiodic structures such as Fibonacci, Thue-Morse, Rudin-Shapiro, and double period are investigated with special focus on the Fibonacci structure. Depending on the lengths of the two arms constituting the loops, one can distinguish two particular cases. (i) There are symmetric loop structures, which are shown to be equivalent to impedance-modulated mediums. In this case, it is found that besides the existence of extended and forbidden modes, some narrow frequency bands appear as defect modes in the transmission spectrum inside the gaps. These modes are shown to be localized within only one of the two types of blocks constituting the structure. An analysis of the transmission phase time enables us to derive the group velocity as well as the density of states in these structures. In particular, the stop bands (localized modes) may give rise to unusual (strong normal) dispersion in the gaps, yielding fast (slow) group velocities above (below) the velocity of light. (ii) There are also asymmetric loop structures, where the loops play the role of resonators that may introduce transmission zeros and hence additional gaps unnoticed in the case of simple impedance-modulated mediums. A comparison of the transmission amplitude and phase time of Fibonacci systems with those of other quasiperiodic systems is also outlined. In particular, it was shown that these structures present similar behaviors in the transmission spectra inside the regions of extended modes, whereas they present different localized modes inside the gaps. Experiments and numerical calculations are in very good agreement.

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PubMed2004

Band structure and omnidirectional photonic band gap in lamellar structures with left-handed materials.

We theoretically investigate the photonic band structure of one-dimensional superlattices composed of alternating layers of right-handed and left-handed materials (RHM and LHM). The dispersion curves are mainly studied by assuming that the dielectric permittivity and magnetic permeability are constant in each layer. It is shown that such structures can exhibit new types of electromagnetic modes and dispersion curves that do not exist in usual superlattices composed only of RHM. In particular, we emphasize the possibility of bands that originate from the interface modes localized at the boundary between a LHM and RHM or from confined modes in one type of layers. These waves are evanescent in both or in one constituent of the superlattice. One of the pass bands may lie below the light lines of the constituting material and go down to the static limit of a vanishing frequency omega, even at a value of the wave vector k(//) (parallel to the layers) that is different from zero. For a given value of the wave vector k(//), the dispersion curves omega versus k(z) (where k(z) is the Bloch wave vector of the periodic system along the axis of the superlattice) may exist only in a limited part of the superlattice Brillouin zone and exhibit a zigzag behavior instead of a monotonic behavior as in usual superlattices. With an appropriate choice of the parameters, we show that it is possible to realize an absolute (or omnidirectional) band gap for either transverse electric (TE) or transverse magnetic (TM) polarization of the electromagnetic waves. A combination of two multilayer structures composed of RHM and LHM is proposed to realize, in a certain range of frequency, an omnidirectional reflector of light for both polarizations.

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PubMed2004

Guided elastic waves along a rod defect of a two-dimensional phononic crystal.

It was shown that elastic waves propagating out-of-plane in a two-dimensional phononic crystal can experience full-band-gaps for nonzero values of the wave-vector component parallel to the rods. By further inserting a rod defect, it is demonstrated that modes propagating along the rod defect can be localized within the band-gaps of the phononic crystal. Such waveguide modes are exhibited for a tungsten/epoxy composite containing an aluminum nitride rod as the rod defect. It is expected that guided modes of such a structure can be excited and detected electrically owing to the piezoelectric effect.

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PubMed2004

Tunable filtering and demultiplexing in phononic crystals with hollow cylinders.

Acoustic band gap (ABG) materials constituted of steel hollow cylinders immersed in water can exhibit a tunable narrow pass band (NPB) located inside their gap. We theoretically investigate, using the finite difference time domain (FDTD) method, the properties of waveguides composed of a row of hollow cylinders in a two-dimensional (2D) phononic crystal made of filled steel cylinders. These waveguides exhibit NPB's at frequencies slightly higher than their infinite periodic ABG counterpart. The frequency of the waveguide's NPB can be selected by adjusting the inner radius of the hollow cylinders or by changing the nature of the fluid that fills them. We show that a waveguide constituted of a row of hollow cylinders with different inner radii can transport waves at two different frequencies. By selectively filling the cylinders with water or mercury we have created an active device that permits the transmission of waves at one, both, or neither of these frequencies. Finally, we examine the multiplexing and demultiplexing capabilities of Y shaped waveguides constituted of hollow cylinders.

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PubMed2004

Simple nanometric plasmon multiplexer.

We present a simple multiplexing structure made of two discrete plasmon wires coupled by two metal nanoclusters. We show that this simple nanosystem can transfer one plasmon wavelength from one wire to the other. Closed-form relations between the transmission coefficients and the nanocluster distances are given to optimize the desired directional plasmon ejection.

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PubMed2003

Out-of-plane propagation of elastic waves in two-dimensional phononic band-gap materials.

We have used a plane-wave-expansion model to study the out-of-plane propagation of elastic waves in a two-dimensional phononic band-gap material. The case of quartz rods embedded in an epoxy matrix has been computed. Band gaps for nonzero values of the wave-vector component parallel to the rods are shown to exist and are investigated. For wavelengths smaller than the period of the structure, modes are found that are localized in the epoxy intersites, and propagate perpendicularly to the plane of the structure.

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PubMed2003

Directional photon transfer between two wires.

The directional transfer of a single photon from one wire to another, leaving all other neighbor states unaffected, is of great importance. We present a simple coupling structure that makes such transfer possible, for any given photon wavelength and linewidth. We give closed-form expressions for the parameters necessary to build such a structure. An illustration of our analytic study is given for the directional transmission of a telecommunication signal between two lines.

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PubMed2002

Omnidirectional elastic band gap in finite lamellar structures.

This paper presents a comprehensive theoretical analysis of the occurrence of omnidirectional reflection in one-dimensional phononic crystal structures. We discuss the conditions for a one-dimensional layered structure, made of elastic materials, to exhibit total reflection of acoustic incident waves in a given frequency range, for all incident angles and all polarizations. The property of omnidirectional reflection can be fulfilled with a simple finite superlattice if the substrate from which the incident waves are launched is made of a material with high acoustic velocities (this is very similar to the case of omnidirectional optical mirror where the incident light is generated in vacuum). However, if the substrate is made of a material with low acoustic velocities, we propose two solutions to obtain an omnidirectional band gap, namely, the cladding of a superlattice with a layer of high acoustic velocities, which acts like a barrier for the propagation of phonons, or the association in tandem of two different superlattices in such a way that the superposition of their band structures exhibits an absolute acoustic band gap. We discuss the appropriate choices of the material and geometrical properties to realize such structures. The behavior of the transmission coefficients are discussed in relation with the dispersion curves of the finite structure embedded between two substrates. Both transmission coefficients and densities of states (from which we derive the dispersion curves) are calculated in the framework of a Green's function method.

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PubMed2002

Evidence of fano-like interference phenomena in locally resonant materials.

Sonic crystals consisting of three-dimensional arrays of units which exhibit localized resonances have been discovered recently. Here, it is shown that their two-dimensional counterparts behave in a similar manner. Particularly, it is observed that the transmittance spectra show very asymmetric peaks which are explained as a Fano-like interference phenomenon. A finite difference time domain method is employed to perform a comprehensive study of the resonance line shape as a function of the mass density of the structural units. Also, a simple analytical model is introduced to give an intuitive account of the origin of the interference phenomenon.

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PubMed2002

Phononic crystal with low filling fraction and absolute acoustic band gap in the audible frequency range: a theoretical and experimental study.

The propagation of acoustic waves in a two-dimensional composite medium constituted of a square array of parallel copper cylinders in air is investigated both theoretically and experimentally. The band structure is calculated with the plane wave expansion (PWE) method by imposing the condition of elastic rigidity to the solid inclusions. The PWE results are then compared to the transmission coefficients computed with the finite difference time domain (FDTD) method for finite thickness composite samples. In the low frequency regime, the band structure calculations agree with the FDTD results indicating that the assumption of infinitely rigid inclusion retains the validity of the PWE results to this frequency domain. These calculations predict that this composite material possesses a large absolute forbidden band in the domain of the audible frequencies. The FDTD spectra reveal also that hollow and filled cylinders produce very similar sound transmission suggesting the possibility of realizing light, effective sonic insulators. Experimental measurements show that the transmission through an array of hollow Cu cylinders drops to noise level throughout frequency interval in good agreement with the calculated forbidden band.

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PubMed2002

Theory of acoustic scattering by supported ridges at a solid-liquid interface.

We combine a general Green's function formalism and an approach due to Nyborg [W. L. Nyborg, in Acoustic Streaming, Physical Acoustics, edited by W. P. Mason (Academic, London, 1965), Vol. II B, Chap. 11] to calculate the first-order pressure and second-order pressure gradient fields in the vicinity of solid inhomogeneities at a solid/liquid interface. We treat the problem of scattering of an incident acoustic plane wave by a single ridge and two parallel ridges separated by a trench on a planar substrate. The calculated vibrational density of states shows the existence of resonances at low frequencies, especially in the case of a trench. Excitation of a trench resonant vibrational mode enhances the magnitude of the first-order pressure and of the second-order pressure gradient. The resonant frequencies of a trench decrease and the pressure enhancement increases with increasing aspect ratio of the ridges (height to width).

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PubMed2001

Stopping of acoustic waves by sonic polymer-fluid composites.

A two-dimensional periodic array of air cylinders in water is known to have giant acoustic stop bands [M.S. Kushwaha and B. Djafari-Rouhani, J. Appl. Phys. 84 (1998) 4677]. It is shown in the present paper that hollow cylinders made of an elastically-soft polymer containing air inside and arranged on a square lattice in water can still give rise to large acoustic band gaps. Similar properties can also be obtained with a close-packed array of tubes containing water when arranged on a honeycomb lattice in air. The transmission coefficient of films made of such polymer-fluid composites has been calculated by finite difference time domain method. With film thickness not exceeding 75 mm, a deep sonic attenuation band was found with, in the best cases, a lower limit below 1 kHz and an upper limit above 10 kHz.

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PubMed2001

Experimental and theoretical evidence for the existence of absolute acoustic band gaps in two-dimensional solid phononic crystals.

Experimental measurements of acoustic transmission through a solid-solid two-dimensional binary-composite medium constituted of a triangular array of parallel circular steel cylinders in an epoxy matrix are reported. Attention is restricted to propagation of elastic waves perpendicular to the cylinders. Measured transmitted spectra demonstrate the existence of absolute stop bands, i.e., band gaps independent of the direction of propagation in the plane perpendicular to the cylinders. Theoretical calculations of the band structure and transmission spectra using the plane wave expansion and the finite difference time domain methods support unambiguously the absolute nature of the observed band gaps.

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PubMed1996