Filippo Capolino | University of California, Irvine (original) (raw)
Papers by Filippo Capolino
Springer Proceedings in Physics, 2004
This paper summarizes the research activities conducted at the University of Siena during the pas... more This paper summarizes the research activities conducted at the University of Siena during the past six years on the truncated Floquet wave diffraction theory under the continuous guidance of Professor Felsen. During that period, we have analyzed and validated the diffraction phenomena in both the frequency and time domains pertaining to large planar phased arrays of different topologies and with quite general excitation profiles. The associated canonical dipole array Green's functions (AGFs) have been expressed in terms of constituents in a generalized high frequency asymptotic, periodicity-adapted, Floquet wave (FW)-modulated uniform geometrical theory of diffraction (UTD), which highlights the relevant wave physics. The present paper summarizes the essentials of these AGFs for large planar rectangular phased dipole arrays in free space, escalating sequentially from free-space geometries to multilayered media, with the eventual utilization of the Method of Moments (MoM) for practical applications. Particular emphasis is given to those aspects which are in progress, and have not yet appeared in print.
IEEE Antennas and Wireless Propagation Letters
We demonstrate how exceptional points of degeneracy (EPDs) are induced in a single transmission l... more We demonstrate how exceptional points of degeneracy (EPDs) are induced in a single transmission line (TL) directly by applying periodic space-time modulation to the per-unitlength distributed capacitance. In such space-time modulated (STM)-TL, two eigenmodes coalesce into a single degenerate one, in their eigenvalues (wavenumbers) and eigenvectors (voltagecurrent states) when the system approaches the EPD condition. The EPD condition is achieved by tuning a parameter in the space-time modulation, such as spatial or temporal modulation frequency, or the modulation depth. We unequivocally demonstrate the occurrence of the EPD by showing that the bifurcation of the wavenumber around the EPD is described by the Puiseux fractional power series expansion. We show that the first order expansion is sufficient to approximate well the dispersion diagram, and how this "exceptional" sensitivity of an STM-TL to tiny changes of any TL or modulation parameter enables a possible application as a highly sensitive TL sensor when operating at an EPD.
Physical Review A
The dispersion of a three-way waveguide is engineered to exhibit exceptional modal characteristic... more The dispersion of a three-way waveguide is engineered to exhibit exceptional modal characteristics. Two coupled waveguides with Parity-Time (PT) symmetry have been previously demonstrated to exhibit second order exceptional points of degeneracy (EPDs). In this work, we introduce and investigate a particular class of EPDs, applicable from radio frequency to optical wavelengths, whereby three coupled waveguides satisfy Glide-Time (GT) symmetry to exhibit a third order modal degeneracy with a real-valued wavenumber. GT symmetry involves glide symmetry of lossless/gainless components of the waveguide in addition to changing the sign of passive/active elements while applying a glide symmetry operation. This GTsymmetry condition allows three Floquet-Bloch eigenmodes of the structure to coalesce to a real-valued wavenumber at a single frequency, in addition of having one branch of the dispersion diagram with a purely real wavenumber. The proposed scheme may have applications including but not limited to distributed amplifiers, radiating arrays, and sensors, from radio frequency to optics.
Tenth International Conference on Antennas and Propagation (ICAP), 1997
The prediction of the power backscattered by the sea surface in the presence of rainfall is the o... more The prediction of the power backscattered by the sea surface in the presence of rainfall is the object of this paper. An electromagnetic (e.m.) model predicting Normalised Radar Cross Section (NRCS) of such surface, must conveniently account for both a large scale roughness roughness due to wind and a small scale roughness due to raindrop splashes. While several models are available in the literature for wind-induced roughness, only few experimental results or theoretical studies concerning additional rainfall-induced roughness are available. Here we describe an e.m. model that accounts for the effects of the raindrop splashes, and discuss some related results. An improved algorithm is then proposed here for retrieving rainfall rate profiles by means of spaceborne rain radars, which utilises the relationshp between sea NRCS and rainfall rate.
We investigate how a periodic array composed of achiral isotropic high-refractive index dielectri... more We investigate how a periodic array composed of achiral isotropic high-refractive index dielectric nanospheres generates nearfield over the array surface reaching helicity density very close to its upper bound. The required condition for an array of nanospheres to generate optimally chiral nearfield, which represents the upper bound of helicity density, is derived in terms of array effective electric and magnetic polarizabilities that almost satisfy the effective Kerker condition for arrays. The discussed concepts find applications in improving chirality detection based on circular dichroism (CD) at surface level instead of in the bulk. Importantly the array would not contribute to the generated CD signal when used as a substrate for detecting chirality of a thin layer of chiral molecules. This eliminates the need to separate the CD signal generated by the array from that of the chiral sample.
Applied Physics Letters, 2022
IEEE Transactions on Microwave Theory and Techniques, 2020
We present the general conditions to realize a fourth order exceptional point of degeneracy (EPD)... more We present the general conditions to realize a fourth order exceptional point of degeneracy (EPD) in two uniform (i.e., invariant along z) lossless and gainless coupled transmission lines (CTLs), namely, a degenerate band edge (DBE). Until now the DBE has been shown only in periodic structures. In contrast, the CTLs considered here are uniform and subdivided into four cases where the two TLs support combinations of forward propagation, backward propagation and evanescent modes (when neglecting the mutual coupling). We demonstrate for the first time that a DBE is supported in uniform CTLs when there is proper coupling between: (i) propagating modes and evanescent modes, (ii) forward and backward propagating modes, or (iii) four evanescent modes (two in each direction). We also show that the loaded quality factor of uniform CTLs exhibiting a fourth order EPD at k = 0 is robust to series losses due to the fact that the degenerate modes do not advance in phase. We also provide a microstrip possible implementation of a uniform CTL exhibiting a DBE using periodic series capacitors with very sub-wavelength unit-cell length. Finally, we show an experimental verification of the existence DBE for a microstrip implementation of a CTL supporting coupled propagating and evanescent modes.
We report on the possibility of adopting active gain materials (specifically, made of fluorescent... more We report on the possibility of adopting active gain materials (specifically, made of fluorescent dyes) to mitigate the losses in a 3D periodic array of dielectric-core metallic-shell nanospheres. We find the modes with complex wavenumber in the structure, and describe the composite material in terms of homogenized effective permittivity, comparing results from modal analysis and Maxwell Garnett theory. We then design two metamaterials in which the epsilon-near-zero frequency region overlaps with the emission band of the adopted gain media, and we show that metamaterials with effective parameters with low losses are feasible, thanks to the gain materials. Even though fluorescent dyes embedded in the nanoshells’ dielectric cores are employed in this study, the formulation provided is general, and could account for the usage of other active materials, such as semiconductors and quantum dots. ©2011 Optical Society of America OCIS codes: (250.5403) Plasmonics; (160.3918) Metamaterials; ...
The public reporting burden for this collection of information is estimated to average 1 hour per... more The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to the Department of Defense, Executive Service Directorate (0704-0188). Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.
We investigate local field enhancement phenomena using e-near-zero (ENZ) substrates for incident ... more We investigate local field enhancement phenomena using e-near-zero (ENZ) substrates for incident TM-polarized fields. When material losses are present a moderate field enhancement is achieved for a fairly wide frequency band and incident angles. In contrast, when material losses are partly compensated we observe much stronger field enhancement for narrower frequency band and incident angles. We show that the combination of low material losses, intended in the sense of small Im(�), and ENZ condition may lead to even higher absorption. However, this feature may be used to trigger and enhance low-threshold nonlinear phenomena despite increased pump losses.
In this paper, complex modes in a linear chain of gold nanospheres are analyzed, accounting for m... more In this paper, complex modes in a linear chain of gold nanospheres are analyzed, accounting for metal losses. Dispersion diagrams are computed for travelling modes with both longitudinal and transverse (with respect to the array axis) polarization states. The procedure outlined in this work allows for the description of single mode evolution varying frequency, thus the modal dispersion diagrams are composed by the superposition of all the different modes in the one dimensional array. Each nanoparticle is modeled as an electric dipole, by adopting the single dipole approximation, and the complex zeroes of the homogeneous equation characterizing the field in the periodic structure are computed. The Ewald method is employed to analytically continue the periodic Green’s function into the complex spectral domain and to achieve rapid convergence. Full characterization of the modes is provided in terms of their direction of propagation (forward/backward), their guidance and radiation prope...
IEEE Antennas and Propagation Society International Symposium. 1999 Digest. Held in conjunction with: USNC/URSI National Radio Science Meeting (Cat. No.99CH37010)
Journal of Applied Physics, 2020
We investigate how a periodic array composed of achiral isotropic high-refractive index dielectri... more We investigate how a periodic array composed of achiral isotropic high-refractive index dielectric nanospheres generates nearfield over the array surface reaching helicity density very close to its upper bound. The required condition for an array of nanospheres to generate "optimally chiral" nearfield, which represents the upper bound of helicity density, is derived in terms of array effective electric and magnetic polarizabilities that almost satisfy the effective Kerker condition for arrays. The discussed concepts find applications in improving chirality detection based on circular dichroism (CD) at surface level instead of in the bulk. Importantly the array would not contribute to the generated CD signal when used as a substrate for detecting chirality of a thin layer of chiral molecules. This eliminates the need to separate the CD signal generated by the array from that of the chiral sample.
IEEE Transactions on Antennas and Propagation, 2018
We demonstrate several new aspects of exceptional points of degeneracy (EPD) pertaining to propag... more We demonstrate several new aspects of exceptional points of degeneracy (EPD) pertaining to propagation in two uniform coupled transmission-line structures. We describe an EPD using two different approachesby solving an eigenvalue problem based on the system matrix, and as a singular point from bifurcation theory, and the link between these two disparate viewpoints. Cast as an eigenvalue problem, we show that eigenvalue degeneracies are always coincident with eigenvector degeneracies, so that all eigenvalue degeneracies are implicitly EPDs in two uniform coupled transmission lines. Furthermore, we discuss in some detail the fact that EPDs define branch points (BPs) in the complex-frequency plane; we provide simple formulas for these points, and show that parity-time (PT) symmetry leads to real-valued EPDs occurring on the real-frequency axis. We discuss the connection of the linear algebra approach to previous waveguide analysis based on singular points from bifurcation theory, which provides a complementary viewpoint of EPD phenomena, showing that EPDs are singular points of the dispersion function associated with the fold bifurcation. This provides an important connection of various modal interaction phenomena known in guided-wave structures with recent interesting effects observed in quantum mechanics, photonics, and metamaterials systems described in terms of the EPD formalism.
IEEE Transactions on Microwave Theory and Techniques, 2017
We experimentally demonstrate for the first time the degenerate band edge (DBE) condition, namely... more We experimentally demonstrate for the first time the degenerate band edge (DBE) condition, namely the degeneracy of four Bloch modes, in loaded circular metallic waveguides. The four modes forming the DBE represent a degeneracy of fourth order occurring in a periodic structure where four Bloch modes, two propagating and two evanescent, coalesce. It leads to a very flat wavenumber-frequency dispersion relation, and the finitelength structure's quality factor scales as N 5 where N is the number of unit cells. The proposed waveguide in which DBE is observed here is designed by periodically loading a circular waveguide with misaligned elliptical metallic rings, supported by a low-index dielectric. We validate the existence of the DBE in such structure using measurements and we report good agreement between full-wave simulation and the measured response of the waveguide near the DBE frequency; taking into account metallic losses. We correlate our finding to theoretical and simulation results utilizing various techniques including dispersion synthesis, as well as observing how quality factor and group delay scale as the structure length increases. Moreover, the reported geometry is only an example of metallic waveguide with DBE: DBE and its characteristics can also be designed in many other kinds of waveguides and various applications can be contemplated as high microwave generation in amplifiers and oscillators based on an electron beam interaction or solid state devices, pulse compressors and microwave sensors. Index Terms-Degenerate band edge (DBE), slow-wave structures, cavity resonators.
IEEE Transactions on Antennas and Propagation, 2017
We present a transmission line theory of exceptional points of degeneracy (EPD) in coupled-mode g... more We present a transmission line theory of exceptional points of degeneracy (EPD) in coupled-mode guiding structures, i.e., a theory that illustrates the characteristics of coupled electromagnetic modes under a special dispersion degeneracy condition, yet unexplored in the contest of gain and loss. We demonstrate the concept of Parity-Time ()-symmetry in coupled uniform waveguides with balanced and symmetric gain and loss and how this condition is associated with a second order EPD. We show that by introducing gain into naturally lossy structures provides for the conditions whereby exceptional points of non-Hermitian degeneracies can be manifested, such as in symmetric structures. Furthermore, we also demonstrate that symmetry, despite being the method often suggested for obtaining non-Hermitian degeneracies at optical frequencies, is not a necessary condition and indeed we show that EPD can be obtained with broken topological symmetry in uniform TLs. Operating near such special degeneracy conditions leads to potential performance enhancement in a variety of microwave and optical resonators, and devices such as distributed oscillators, including lasers, amplifiers, radiating arrays, pulse compressors, and Qswitching sensors.
ACS Photonics, 2016
The concept of magnetic nanoprobes (or magnetic nanoantennas) providing a magnetic near-field enh... more The concept of magnetic nanoprobes (or magnetic nanoantennas) providing a magnetic near-field enhancement and vanishing electric field is presented and investigated, together with their excitation. It is established that a particular type of cylindrical vector beams called azimuthally electric polarized vector beams yield strong longitudinal magnetic field on the beam axis where the electric field is ideally null. These beams with an electric polarization vortex and cylindrical symmetry are important in generating high magnetic to electric field contrast, i.e., large local field admittance, and in allowing selective excitation of magnetic transitions in matter located on the beam axis. We demonstrate that azimuthally polarized vector beam excitation of a photoinduced magnetic nanoprobe made of a magnetically polarizable nano cluster leads to enhanced magnetic near field with resolution beyond diffraction limit. We introduce two figures of merit as magnetic field enhancement and local field admittance normalized to that of a plane wave that are useful to classify magnetic nanoprobes and their excitation. The performance of magnetic nanoprobe and azimuthal polarized beams is quantified in comparison to other illumination options and with several defect scenarios.
Physical Review B, 2015
Photo-induced magnetic force between nanostructures, at optical frequencies, is investigated theo... more Photo-induced magnetic force between nanostructures, at optical frequencies, is investigated theoretically. Till now optical magnetic effects are not used in scanning probe microscopy because of the vanishing natural magnetism with increasing frequency. On the other hand, artificial magnetism in engineered nanostructures led to the development of measurable optical magnetism. Here, two examples of nanoprobes that are able to generate strong magnetic dipolar fields at optical frequency are investigated: first an ideal magnetically polarizable nanosphere and then a circular cluster of silver nanospheres that has a loop-like collective plasmonic resonance equivalent to a magnetic dipole. Magnetic forces are evaluated based on nanostructure polarizabilities, i.e. induced magnetic dipoles, and magnetic-near field evaluations. As an initial assessment on the possibility of a magnetic nanoprobe to detect magnetic forces, we consider two identical magnetically polarizable nanoprobes and observe magnetic forces in the order of piconewtons thereby bringing it within detection limits of conventional atomic force microscopes at ambient pressure and temperature. The detection of magnetic force is a promising method in studying optical magnetic transitions that can be the basis of innovative spectroscopy applications.
Springer Proceedings in Physics, 2004
This paper summarizes the research activities conducted at the University of Siena during the pas... more This paper summarizes the research activities conducted at the University of Siena during the past six years on the truncated Floquet wave diffraction theory under the continuous guidance of Professor Felsen. During that period, we have analyzed and validated the diffraction phenomena in both the frequency and time domains pertaining to large planar phased arrays of different topologies and with quite general excitation profiles. The associated canonical dipole array Green's functions (AGFs) have been expressed in terms of constituents in a generalized high frequency asymptotic, periodicity-adapted, Floquet wave (FW)-modulated uniform geometrical theory of diffraction (UTD), which highlights the relevant wave physics. The present paper summarizes the essentials of these AGFs for large planar rectangular phased dipole arrays in free space, escalating sequentially from free-space geometries to multilayered media, with the eventual utilization of the Method of Moments (MoM) for practical applications. Particular emphasis is given to those aspects which are in progress, and have not yet appeared in print.
IEEE Antennas and Wireless Propagation Letters
We demonstrate how exceptional points of degeneracy (EPDs) are induced in a single transmission l... more We demonstrate how exceptional points of degeneracy (EPDs) are induced in a single transmission line (TL) directly by applying periodic space-time modulation to the per-unitlength distributed capacitance. In such space-time modulated (STM)-TL, two eigenmodes coalesce into a single degenerate one, in their eigenvalues (wavenumbers) and eigenvectors (voltagecurrent states) when the system approaches the EPD condition. The EPD condition is achieved by tuning a parameter in the space-time modulation, such as spatial or temporal modulation frequency, or the modulation depth. We unequivocally demonstrate the occurrence of the EPD by showing that the bifurcation of the wavenumber around the EPD is described by the Puiseux fractional power series expansion. We show that the first order expansion is sufficient to approximate well the dispersion diagram, and how this "exceptional" sensitivity of an STM-TL to tiny changes of any TL or modulation parameter enables a possible application as a highly sensitive TL sensor when operating at an EPD.
Physical Review A
The dispersion of a three-way waveguide is engineered to exhibit exceptional modal characteristic... more The dispersion of a three-way waveguide is engineered to exhibit exceptional modal characteristics. Two coupled waveguides with Parity-Time (PT) symmetry have been previously demonstrated to exhibit second order exceptional points of degeneracy (EPDs). In this work, we introduce and investigate a particular class of EPDs, applicable from radio frequency to optical wavelengths, whereby three coupled waveguides satisfy Glide-Time (GT) symmetry to exhibit a third order modal degeneracy with a real-valued wavenumber. GT symmetry involves glide symmetry of lossless/gainless components of the waveguide in addition to changing the sign of passive/active elements while applying a glide symmetry operation. This GTsymmetry condition allows three Floquet-Bloch eigenmodes of the structure to coalesce to a real-valued wavenumber at a single frequency, in addition of having one branch of the dispersion diagram with a purely real wavenumber. The proposed scheme may have applications including but not limited to distributed amplifiers, radiating arrays, and sensors, from radio frequency to optics.
Tenth International Conference on Antennas and Propagation (ICAP), 1997
The prediction of the power backscattered by the sea surface in the presence of rainfall is the o... more The prediction of the power backscattered by the sea surface in the presence of rainfall is the object of this paper. An electromagnetic (e.m.) model predicting Normalised Radar Cross Section (NRCS) of such surface, must conveniently account for both a large scale roughness roughness due to wind and a small scale roughness due to raindrop splashes. While several models are available in the literature for wind-induced roughness, only few experimental results or theoretical studies concerning additional rainfall-induced roughness are available. Here we describe an e.m. model that accounts for the effects of the raindrop splashes, and discuss some related results. An improved algorithm is then proposed here for retrieving rainfall rate profiles by means of spaceborne rain radars, which utilises the relationshp between sea NRCS and rainfall rate.
We investigate how a periodic array composed of achiral isotropic high-refractive index dielectri... more We investigate how a periodic array composed of achiral isotropic high-refractive index dielectric nanospheres generates nearfield over the array surface reaching helicity density very close to its upper bound. The required condition for an array of nanospheres to generate optimally chiral nearfield, which represents the upper bound of helicity density, is derived in terms of array effective electric and magnetic polarizabilities that almost satisfy the effective Kerker condition for arrays. The discussed concepts find applications in improving chirality detection based on circular dichroism (CD) at surface level instead of in the bulk. Importantly the array would not contribute to the generated CD signal when used as a substrate for detecting chirality of a thin layer of chiral molecules. This eliminates the need to separate the CD signal generated by the array from that of the chiral sample.
Applied Physics Letters, 2022
IEEE Transactions on Microwave Theory and Techniques, 2020
We present the general conditions to realize a fourth order exceptional point of degeneracy (EPD)... more We present the general conditions to realize a fourth order exceptional point of degeneracy (EPD) in two uniform (i.e., invariant along z) lossless and gainless coupled transmission lines (CTLs), namely, a degenerate band edge (DBE). Until now the DBE has been shown only in periodic structures. In contrast, the CTLs considered here are uniform and subdivided into four cases where the two TLs support combinations of forward propagation, backward propagation and evanescent modes (when neglecting the mutual coupling). We demonstrate for the first time that a DBE is supported in uniform CTLs when there is proper coupling between: (i) propagating modes and evanescent modes, (ii) forward and backward propagating modes, or (iii) four evanescent modes (two in each direction). We also show that the loaded quality factor of uniform CTLs exhibiting a fourth order EPD at k = 0 is robust to series losses due to the fact that the degenerate modes do not advance in phase. We also provide a microstrip possible implementation of a uniform CTL exhibiting a DBE using periodic series capacitors with very sub-wavelength unit-cell length. Finally, we show an experimental verification of the existence DBE for a microstrip implementation of a CTL supporting coupled propagating and evanescent modes.
We report on the possibility of adopting active gain materials (specifically, made of fluorescent... more We report on the possibility of adopting active gain materials (specifically, made of fluorescent dyes) to mitigate the losses in a 3D periodic array of dielectric-core metallic-shell nanospheres. We find the modes with complex wavenumber in the structure, and describe the composite material in terms of homogenized effective permittivity, comparing results from modal analysis and Maxwell Garnett theory. We then design two metamaterials in which the epsilon-near-zero frequency region overlaps with the emission band of the adopted gain media, and we show that metamaterials with effective parameters with low losses are feasible, thanks to the gain materials. Even though fluorescent dyes embedded in the nanoshells’ dielectric cores are employed in this study, the formulation provided is general, and could account for the usage of other active materials, such as semiconductors and quantum dots. ©2011 Optical Society of America OCIS codes: (250.5403) Plasmonics; (160.3918) Metamaterials; ...
The public reporting burden for this collection of information is estimated to average 1 hour per... more The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to the Department of Defense, Executive Service Directorate (0704-0188). Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.
We investigate local field enhancement phenomena using e-near-zero (ENZ) substrates for incident ... more We investigate local field enhancement phenomena using e-near-zero (ENZ) substrates for incident TM-polarized fields. When material losses are present a moderate field enhancement is achieved for a fairly wide frequency band and incident angles. In contrast, when material losses are partly compensated we observe much stronger field enhancement for narrower frequency band and incident angles. We show that the combination of low material losses, intended in the sense of small Im(�), and ENZ condition may lead to even higher absorption. However, this feature may be used to trigger and enhance low-threshold nonlinear phenomena despite increased pump losses.
In this paper, complex modes in a linear chain of gold nanospheres are analyzed, accounting for m... more In this paper, complex modes in a linear chain of gold nanospheres are analyzed, accounting for metal losses. Dispersion diagrams are computed for travelling modes with both longitudinal and transverse (with respect to the array axis) polarization states. The procedure outlined in this work allows for the description of single mode evolution varying frequency, thus the modal dispersion diagrams are composed by the superposition of all the different modes in the one dimensional array. Each nanoparticle is modeled as an electric dipole, by adopting the single dipole approximation, and the complex zeroes of the homogeneous equation characterizing the field in the periodic structure are computed. The Ewald method is employed to analytically continue the periodic Green’s function into the complex spectral domain and to achieve rapid convergence. Full characterization of the modes is provided in terms of their direction of propagation (forward/backward), their guidance and radiation prope...
IEEE Antennas and Propagation Society International Symposium. 1999 Digest. Held in conjunction with: USNC/URSI National Radio Science Meeting (Cat. No.99CH37010)
Journal of Applied Physics, 2020
We investigate how a periodic array composed of achiral isotropic high-refractive index dielectri... more We investigate how a periodic array composed of achiral isotropic high-refractive index dielectric nanospheres generates nearfield over the array surface reaching helicity density very close to its upper bound. The required condition for an array of nanospheres to generate "optimally chiral" nearfield, which represents the upper bound of helicity density, is derived in terms of array effective electric and magnetic polarizabilities that almost satisfy the effective Kerker condition for arrays. The discussed concepts find applications in improving chirality detection based on circular dichroism (CD) at surface level instead of in the bulk. Importantly the array would not contribute to the generated CD signal when used as a substrate for detecting chirality of a thin layer of chiral molecules. This eliminates the need to separate the CD signal generated by the array from that of the chiral sample.
IEEE Transactions on Antennas and Propagation, 2018
We demonstrate several new aspects of exceptional points of degeneracy (EPD) pertaining to propag... more We demonstrate several new aspects of exceptional points of degeneracy (EPD) pertaining to propagation in two uniform coupled transmission-line structures. We describe an EPD using two different approachesby solving an eigenvalue problem based on the system matrix, and as a singular point from bifurcation theory, and the link between these two disparate viewpoints. Cast as an eigenvalue problem, we show that eigenvalue degeneracies are always coincident with eigenvector degeneracies, so that all eigenvalue degeneracies are implicitly EPDs in two uniform coupled transmission lines. Furthermore, we discuss in some detail the fact that EPDs define branch points (BPs) in the complex-frequency plane; we provide simple formulas for these points, and show that parity-time (PT) symmetry leads to real-valued EPDs occurring on the real-frequency axis. We discuss the connection of the linear algebra approach to previous waveguide analysis based on singular points from bifurcation theory, which provides a complementary viewpoint of EPD phenomena, showing that EPDs are singular points of the dispersion function associated with the fold bifurcation. This provides an important connection of various modal interaction phenomena known in guided-wave structures with recent interesting effects observed in quantum mechanics, photonics, and metamaterials systems described in terms of the EPD formalism.
IEEE Transactions on Microwave Theory and Techniques, 2017
We experimentally demonstrate for the first time the degenerate band edge (DBE) condition, namely... more We experimentally demonstrate for the first time the degenerate band edge (DBE) condition, namely the degeneracy of four Bloch modes, in loaded circular metallic waveguides. The four modes forming the DBE represent a degeneracy of fourth order occurring in a periodic structure where four Bloch modes, two propagating and two evanescent, coalesce. It leads to a very flat wavenumber-frequency dispersion relation, and the finitelength structure's quality factor scales as N 5 where N is the number of unit cells. The proposed waveguide in which DBE is observed here is designed by periodically loading a circular waveguide with misaligned elliptical metallic rings, supported by a low-index dielectric. We validate the existence of the DBE in such structure using measurements and we report good agreement between full-wave simulation and the measured response of the waveguide near the DBE frequency; taking into account metallic losses. We correlate our finding to theoretical and simulation results utilizing various techniques including dispersion synthesis, as well as observing how quality factor and group delay scale as the structure length increases. Moreover, the reported geometry is only an example of metallic waveguide with DBE: DBE and its characteristics can also be designed in many other kinds of waveguides and various applications can be contemplated as high microwave generation in amplifiers and oscillators based on an electron beam interaction or solid state devices, pulse compressors and microwave sensors. Index Terms-Degenerate band edge (DBE), slow-wave structures, cavity resonators.
IEEE Transactions on Antennas and Propagation, 2017
We present a transmission line theory of exceptional points of degeneracy (EPD) in coupled-mode g... more We present a transmission line theory of exceptional points of degeneracy (EPD) in coupled-mode guiding structures, i.e., a theory that illustrates the characteristics of coupled electromagnetic modes under a special dispersion degeneracy condition, yet unexplored in the contest of gain and loss. We demonstrate the concept of Parity-Time ()-symmetry in coupled uniform waveguides with balanced and symmetric gain and loss and how this condition is associated with a second order EPD. We show that by introducing gain into naturally lossy structures provides for the conditions whereby exceptional points of non-Hermitian degeneracies can be manifested, such as in symmetric structures. Furthermore, we also demonstrate that symmetry, despite being the method often suggested for obtaining non-Hermitian degeneracies at optical frequencies, is not a necessary condition and indeed we show that EPD can be obtained with broken topological symmetry in uniform TLs. Operating near such special degeneracy conditions leads to potential performance enhancement in a variety of microwave and optical resonators, and devices such as distributed oscillators, including lasers, amplifiers, radiating arrays, pulse compressors, and Qswitching sensors.
ACS Photonics, 2016
The concept of magnetic nanoprobes (or magnetic nanoantennas) providing a magnetic near-field enh... more The concept of magnetic nanoprobes (or magnetic nanoantennas) providing a magnetic near-field enhancement and vanishing electric field is presented and investigated, together with their excitation. It is established that a particular type of cylindrical vector beams called azimuthally electric polarized vector beams yield strong longitudinal magnetic field on the beam axis where the electric field is ideally null. These beams with an electric polarization vortex and cylindrical symmetry are important in generating high magnetic to electric field contrast, i.e., large local field admittance, and in allowing selective excitation of magnetic transitions in matter located on the beam axis. We demonstrate that azimuthally polarized vector beam excitation of a photoinduced magnetic nanoprobe made of a magnetically polarizable nano cluster leads to enhanced magnetic near field with resolution beyond diffraction limit. We introduce two figures of merit as magnetic field enhancement and local field admittance normalized to that of a plane wave that are useful to classify magnetic nanoprobes and their excitation. The performance of magnetic nanoprobe and azimuthal polarized beams is quantified in comparison to other illumination options and with several defect scenarios.
Physical Review B, 2015
Photo-induced magnetic force between nanostructures, at optical frequencies, is investigated theo... more Photo-induced magnetic force between nanostructures, at optical frequencies, is investigated theoretically. Till now optical magnetic effects are not used in scanning probe microscopy because of the vanishing natural magnetism with increasing frequency. On the other hand, artificial magnetism in engineered nanostructures led to the development of measurable optical magnetism. Here, two examples of nanoprobes that are able to generate strong magnetic dipolar fields at optical frequency are investigated: first an ideal magnetically polarizable nanosphere and then a circular cluster of silver nanospheres that has a loop-like collective plasmonic resonance equivalent to a magnetic dipole. Magnetic forces are evaluated based on nanostructure polarizabilities, i.e. induced magnetic dipoles, and magnetic-near field evaluations. As an initial assessment on the possibility of a magnetic nanoprobe to detect magnetic forces, we consider two identical magnetically polarizable nanoprobes and observe magnetic forces in the order of piconewtons thereby bringing it within detection limits of conventional atomic force microscopes at ambient pressure and temperature. The detection of magnetic force is a promising method in studying optical magnetic transitions that can be the basis of innovative spectroscopy applications.