Ashok Puri

Digital signal propagation in dispersive media

In this article, the propagation of digital and analog signals through media which, in general, are both dissipative and dispersive is modeled using the one-dimensional telegraph equation. Input signals are represented using impulsive, Heaviside unit step, Gaussian, rectangular pulse, and both unmodulated and modulated sinusoidal pulse type boundary data. Applications to coaxial transmission lines and freshwater signal propagation, for both digital and analog signals, are included. The analysis presented here supports the finding that digital transmission in dispersive media is generally superior to that of analog. The boundary data (input signals) give rise to solutions of the telegraph equation which contain propagating discontinuities. It is shown that the magnitudes of these discontinuities, as a function of distance, can be found without the need of solving the governing equation. Thus, for digital signals in particular, signal strength at a given distance from the input source can be easily determin...

An application of nonlinear supratransmission to the propagation of binary signals in weakly damped, mechanical systems of coupled oscillators

Abstract In the present Letter, we simulate the propagation of binary signals in semi-infinite, mechanical chains of coupled oscillators harmonically driven at the end, by making use of the recently discovered process of nonlinear supratransmission. Our numerical results—which are based on a brand-new computational technique with energy-invariant properties—show an efficient and reliable transmission of information.

Anomalous polarization of an electron in a finite quantum well

Abstract We study the behavior of an electron in a finite potential well in the presence of a static and homogenous electric field. As the electric field is increased, the ground state polarization of the electron increases monotonically but for large fields it shows a broad maximum. The excited states polarization is shown to be negative for weak fields and have a minimum as a function of the strength of the applied field.

Second and third harmonic generation in BBO by femtosecond Ti: sapphire laser pulses

Abstract Harmonic generation in barium metaborate (BBO) by femtosecond Ti: sapphire laser pulses is studied, taking into account both the group velocity mismatch and the lowest and second order group velocity dispersion (GVD). The lowest and second order GVD is calculated as a function of wavelength in the BBO crystal. Second and third harmonic radiation using femtosecond Ti: sapphire laser pulses is computed numerically by solving the coupled wave equations. The effects of the lowest and second order GVD on the fundamental and the harmonic pulses are analyzed. The compensation of group velocity mismatch in sum frequency processes for third harmonic generation is considered.

On the transmission of binary bits in discrete Josephson-junction arrays

In this work, we use supratransmission and infratransmission in the mathematical modeling of the propagation of digital signals in weakly damped, discrete Josephson-junction arrays, using energy-based detection criteria. Our results show an efficient and reliable transmission of binary information.

Tribological properties of fullerenes C60 and C70 microparticles

The frictional behaviors of fullerenes C-60 and C-70 were studied because they were speculated to be solid lubricants. For the sublimated pure C-60 films on Si(001), a high friction coefficient (0.55-0.8) was observed under different loads and pin materials. For the C-70 film, the friction coefficient showed a pin dependence, which changed from 0.5 with an Al2O3 pin to about 0.9 with a 440 stainless steel pin. The relatively high friction coefficients of C-60 and C-70 films were due to the tendency of the C-60 and C-70 particles to clump and compress into high shear strength layers rather than due to the impurities in the fullerenes. The benzene-solvated C-60. 4C(6)H(6) and C-70. xC(6)H(6) showed a lowered friction coefficient (0.25 for C-60. 4C(6)H(6) and 0.3 for C-70. xC(6)H(6)), which might result from the lowered shear strength of the hcp structure of C-60. 4C(6)H(6) and C-70. xC(6)H(6) molecular crystals in which the benzene molecules were intercalated.

Cyclotron frequency coupled enhancement of Kerr rotation in low refractive index-dielectric/magneto-optic bilayer thin-film structures

We investigate the enhancement of magneto-optic polar Kerr rotation over a broad range of optical frequencies. The Drude model for the dielectric tensor is considered. Resonance-like peaks in the Kerr rotation and ellipticity caused in the vicinity of a plasma edge is extended throughout the visible spectrum by deriving an expression for the cyclotron frequency such that Re(exx)∼1 at any given incident photon energy and plasma frequency of the material. The Kerr rotation obtained by use of this expression was studied for the case of InSb and further enhancement of Kerr rotation was achieved in the case of LiF/InSb bilayer thin-film structure, grown on a glass substrate. The numerical analysis was carried out using a 2×2 characteristic matrix, which takes into account multiple reflections and interface effects within the medium. In addition the role of various magneto-optic material parameters and layer thicknesses is investigated in determining the optical frequency at which the maximum Kerr rotation occu...

Aberration-free negative-refractive-index lens

The aberrations of a spherical lens composed of left-handed materials are studied in this letter. Five Seidel aberrations (spherical, coma, astigmatism, field curvature, and distortion) as a function of the refractive index n and shape factor q of the lens are considered. Our numerical calculations show that the negative refractive index gives much larger windows of small values of aberrations than the positive index, which will significantly enhance the flexibility for the design of an optical lens. Two possible regions with optimized aberrations are proposed: n=−1, q=−2.2 and n=−0.81 and q=0.83.

Qualitative results for solutions of the steady fisher-KPP equation☆

In this letter, the homogeneous Dirichlet problem involving the N-dimensional Fisher-KPP equation, a reaction-diffusion model which arises in study of population genetics, is investigated for a class of nonlinear polynomial growth laws. Existence and uniqueness conditions for positive (i.e., physically realistic), steady-state solutions on finite domains, or habitats, are noted and stability questions are addressed. Of particular interest are habitats that can be modeled as open balls. For these cases, two relatively recent and powerful theorems from nonlinear analysis are employed to ascertain important qualitative information. Specifically, these solutions are shown to be strictly decreasing and radially symmetric, as well as achieving a stationary maximum at the habitats center. In addition, the function spaces containing these solutions are determined. Last, the effects of the solution parameters are investigated numerically for the physically relevant cases of N = 2 and 3, the temporal evolution of a particular solution is illustrated, and connections to nuclear reactor science, as well as other fields, are noted.

Compensation for the soliton self-frequency shift and the third-order dispersion using bandwidth-limited optical gain

Abstract Soliton self-frequency shift and third-order dispersion are the forces destructive of pulse propagation in ultrahigh-bit-rate (>100 Gbits/s) optical transmission systems. These effects can be compensated for by the proper choice of finite optical gain and the dispersion parameters of the optical fiber. Numerical simulation of the conditions under which these effects can be compensated for leading to stable soliton propagation are given.