Negative Index in Chiral Metamaterials under Conductive Loss and First-Order Material Dispersion Using Lorentzian, Condon and Drude Models

Emergence of negative index (NIM) in chiral materials with conductive loss using standard dispersive models is reported. Positive and negative phase and group indices are realized as expected for NIM behavior for sidebands with opposite polarities

Investigation of Electromagnetic Velocities and Negative Refraction in a Chiral Metamaterial with Second-order Material Dispersion using Spectral Analyses and Dispersive Models

In recent years, considerable research has been carried out relative to the electromagnetic (EM) propagation and refraction characteristics in metamaterials with emphasis on the origins of negative refractive index. Negative refractive index may be introduced in metamaterials via different methods; one such is the condition whereby the Poynting vector of the EM wave is in opposition to the group velocity in the material. Alternatively, negative refractive index also occurs when the group and phase velocities in the medium are in opposition. The latter phenomenon has been extensively investigated in the literature, including recent work involving chiral metamaterials with material dispersion up to the first order. This paper examines the possible emergence of negative refractive index in dispersive chiral metamaterials with material dispersion up to the second order. The motivation is to determine if using second- as opposed to first-order dispersion may lead to more practical negative index behavior. A spectral approach combined with a slowly time-varying phasor analysis is applied, leading to the analytic derivation of EM phase and group velocities, and the resulting phase and group velocities and the corresponding phase and group indices are evaluated by selecting somewhat arbitrary dispersive parameters. The results indicate the emergence of negative index (via negative phase indices along with positive group indices, as reported in the literature) or negative index material (NIM) behavior over information bandwidths in the low RF range. The second-order results are not significantly better than those for first-order results based on the theoretical analysis; however, greater parametric flexibility exists for the second-order system leading to the higher likelihood of achieving NIM over practical frequency bands. The velocities and indices computed using the Lorentzian and Condon models yield an NIM bandwidth around 200 − 400 Mrad∕ sec, about 2 orders of magnitude higher than that for the parametric approach; more importantly, NIM is found not to occur in the first order when using practical models.

Investigation of Negative Index in Dispersive, Chiral Materials via Contra-propagating Velocities under Second-order Dispersion (GVD)

Negative refractive index arises typically in metamaterials via multiple routes. One such avenue is the condition where the Poynting vector of the electromagnetic wave is in opposition to the group velocity in the material. An earlier work along this route in a chiral material led to the well-known result of requiring very large (non-realizable) chirality. Thereafter, a combination of chirality together with first-order dispersion was examined using plane wave electromagnetic analysis. To arrive at the conclusions in that approach, the three wave velocities (energy, group and phase) were derived under first-order dispersion in permittivity, permeability and chirality. Negative index in this approach was established under the condition of contra-propagating group and phase velocities. Regions of negative index were found analytically by assuming standard dispersive models (such as Condon). In this paper, we will re-visit the negative index problem under higher-order dispersion. In addition, we will re-examine the plane wave propagation model under parametric dispersion where each material parameter (ε, μ, κ) is dispersively expanded up to the second order in frequency. Such a physical effect may be traced to group velocity dispersion (GVD) in the material. Field solutions are then obtained under the GVD effect, and extended to the evaluation of the energy, phase and group velocities

Realization of Negative Index in Second-order Dispersive Metamaterials Using Standard Dispersion Models for Electromagnetic Parameters

In recent work, electromagnetic propagation velocities for plane waves in dispersive metamaterials were calculated assuming frequency dispersion up to the second order. The three velocities were expressed in terms of dispersive coefficients under certain simplifying constraints. Frequency domains were found to exist around resonances where group and phase velocities are in opposition, implying possible negative index behavior. In this paper, we incorporate in the derived equations physical models (including Debye, Lorentz and Condon) for material dispersion in permittivity, permeability and chirality in order to further examine the consequences of second-order dispersion leading to negative index for practical cases, and also evaluate the resulting phase and group indices

Examination of the Nonlinear Dynamics of a Chaotic Acousto-optic Bragg Modulator with Feedback under Signal Encryption and Decryption

An acousto-optic Bragg cell with first-order feedback, which exhibits chaotic behavior past the threshold for bistability, was recently examined for possible chaotic encryption and recovery of simple messages (such as low-amplitude periodic signals) applied via the bias input of the sound cell driver. We carry out a thorough examination of the nonlinear dynamics of the Bragg cell under intensity feedback for (i) dc variations of the feedback gain (β˜) and the phase shift parameter (α^ 0) and (ii) ac variations of α^ 0; total under signal encryption, investigating both from two different perspectives: (i) examining chaos in view of the so-called Lyapunov exponent derived recently by Ghosh and Verma and (ii) examining chaos in terms of the familiar bifurcation maps of intensity plotted against the feedback gain and the effective bias. It is shown that overall, the nonlinear dynamical results using the two approaches broadly agree, both for dc (fixed-parameter) analyses and, more importantly, when applied to the case of ac signal encryption cases. This affirms the effectiveness of the nonlinear dynamical theory in predicting and tracking the actual physical behavior of this system for message signal transmission and recovery under complex chaotic encryption

Emergence of Negative Index in a Lossy Chiral Metamaterial under First-order Material Dispersion

For a lossless dispersive chiral material, negative index (NIM) occurred only by use of parametric analysis (and not via practical models) to first-order. These findings are re-visited for the lossy problem and the results are compared

Examination of Chaotic Signal Encryption and Recovery for Secure Communication using Hybrid Acousto-optic Feedback

Generation of chaos from acousto-optic (A-O)Bragg cell modulators with an electronic feedback has been studied for over 3 decades. Since an acousto-optic Bragg cell with zeroth- and first-order feedback exhibits chaotic behavior past the threshold for bistability, such a system was recently examined for possible chaotic encryption of simple messages (such as a low-amplitude sinusoidal signal) applied via the bias input of the sound cell driver. Subsequent recovery of the message signal was carried out via a heterodyne-type strategy employing a locally generated chaotic carrier, with threshold parameters matched to the transmitting Bragg cell. In this paper, we present numerical results and detailed interpretations for signal encryption and recovery under hybrid A-O electronic feedback using a heterodyne strategy. Important features of this setup, such as the system robustness in terms of parameter matching (feedback gain, dc bias, and time delay) are also examined in some detail

Chaotic Bandgaps in Hybrid Acousto-optic Feedback and their Implications

The nonlinear dynamics of a hybrid acousto-optic device was examined from the perspective of the Lyapunov exponent (LE) and bifurcation maps. The plots for LE versus system parameters and bifurcation maps have recently been examined against known simulation results including chaotic encryption experiments [1]. It is verified that the loop gain (feedback gain (β) times incident light amplitude (Iin) needs to be greater than one as a necessary , but not sufficient condition for the onset of chaos. It is found that for certain combinations of β, Iin, net bias voltage (αtοt), and the initial value of the first-order scattered light (I1(0)), there are pronounced regions of chaos in the parameter field, while for others, chaos is minimal. It is also observed that in some cases, the negative spikes in the LE are far larger than its positive amplitudes, hence indicating a greater tendency to become non-chaotic. Additionally, we have examined the bifurcation plots versus the two most salient system parameters, αtοt and β. These maps have revealed behavior that is by no means uniformly chaotic. It is found that the system moves in and out of chaos within distinct bands along the αtοt and β axes. These results imply strong sensitivity vis-Ã -vis these parameters around the passbands and stopbands , and may indicate control of chaos by appropriate parameter adjustment. Such control may have applications in biological chaos, such as arresting malignant, chaotic cell multiplication. Overall, the dynamical results compare favorably with time-domain characteristics of encrypted chaotic waveforms in signal modulation and transmission applications

Deep Vein Thrombosis in a Patient with Negative Age-Adjusted D-Dimer Level

D-dimer level, along with a clinical probability tool that uses the Wells score, is commonly used to exclude deep vein thrombosis (DVT). Age-adjusted D-dimer values are routinely used in clinical practice to increase the negative predictive value and avoid unnecessary Doppler ultrasound imaging. We describe a patient with a low pre-test probability of DVT upon admission and a negative D-dimer level based on age-adjusted values who was later diagnosed with DVT. Our experiences with this case highlight that the geriatric population is unique and, at times, frail

Performance Measures in Acousto-optic Chaotic Signal Encryption System Subject to Parametric Variations and Additive Noise

Signal encryption and recovery using chaotic optical waves has been a subject of active research in the past 10 years. Since an acousto-optic Bragg cell with zeroth- and first-order feedback exhibits chaotic behavior past the threshold for bistability, such a system was recently examined for possible chaotic encryption using a low-amplitude sinusoidal signal applied via the bias input of the sound cell driver. Subsequent recovery of the message signal was carried out via a heterodyne strategy employing a locally generated chaotic carrier, with threshold parameters matched to the transmitting Bragg cell. The simulation results, though encouraging, were limited to relatively low chaos frequencies and sinusoidal message signals only. In this paper, we extend the previous work by (i) increasing the chaos frequency using appropriate parameter control; (ii) carefully examining the system sensitivity to three system parameters, viz., feedback delay, feedback gain, and dc bias level; (iii) examine signal recoverability relative to shifts in the three parameters mentioned above relative to the transmitter; and (iv) determining the robustness of such a system relative to the primary transmitter parameters. Additionally, we consider the effect of the additive bandpass noise (obtained from white Gaussian noise in the simulator) on signal recovery in such a system from a performance standpoint. It is also conjectured that signal recovery can be effected by passing the modulated light through a second sound cell in a matched chaotic regime. This aspect is also under investigation