arXiv subjects

Masud Mansuripur

Publications and source records attributed to Masud Mansuripur.

At least 73 records · Page 4Linked to original sources

arXiv2014

Advances in Macromolecular Data Storage

We propose to develop a new method of information storage to replace magnetic hard disk drives and other instruments of secondary/backup data storage. The proposed method stores petabytes of user-data in a sugar cube (1 cm3), and can read/write that information at hundreds of megabits/sec. Digital information is recorded and stored in the form of a long macromolecule consisting of at least two bases, A and B. (This would be similar to DNA strands constructed from the four nucleic acids G,C,A,T.) The macromolecules initially enter the system as blank slates. A macromolecule with, say, 10,000 identical bases in the form of AAAAA....AAA may be used to record a kilobyte block of user-data (including modulation and error-correction coding), although, in this blank state, it can only represent the null sequence 00000....000. Suppose this blank string of A's is dragged before an atomically-sharp needle of a scanning tunneling microscope (STM). When electric pulses are applied to the needle in accordance with the sequence of 0s and 1s of a 1 kB block of user-data, selected A molecules will be transformed into B molecules (e.g., a fraction of A will be broken off and discarded). The resulting string now encodes the user-data in the form of AABABBA...BAB. The same STM needle can subsequently read the recorded information, as A and B would produce different electric signals when the strand passes under the needle. The macromolecule now represents a data block to be stored in a "parking lot" within the sugar cube, and later brought to a read station on demand. Millions of parking spots and thousands of Read/Write stations may be integrated within the micro-fabricated sugar cube, thus providing access to petabytes of user-data in a scheme that benefits from the massive parallelism of thousands of Read/Write stations within the same three-dimensionally micro-structured device.

physics.bio-ph↗

arXiv2014

The Charge-Magnet Paradoxes of Classical Electrodynamics

A number of charge-magnet paradoxes have been discussed in the literature, beginning with Shockley's famous 1967 paper, where he introduced the notion of hidden momentum in electromagnetic systems. We discuss all these paradoxes in a single, general context, showing that the conservation laws of linear and angular momenta can be satisfied without the need for hidden entities, provided that the Einstein-Laub laws of force and torque are used in place of the standard Lorentz law. Einstein and Laub published their paper in 1908, but the simplicity of the conventional Lorentz law overshadowed the subtle features of their formulation which, at first sight, appears somewhat complicated. However, that slight complication turns out to lead to subsequent advantages in light of Shockley's discovery of hidden momentum, which occurred more than a decade after Einstein had passed away. In this paper, we show how the Einstein-Laub formalism handles the underlying problems associated with certain paradoxes of classical electrodynamics involving a static distribution of electric charges and a magnet whose magnetization slowly fades away in time. The Einstein-Laub laws of electromagnetic force and torque treat these paradoxes with elegance and without contradicting the existing body of knowledge, which has been confirmed by more than one and a half century of theoretical and experimental investigations.

physics.class-ph↗

arXiv2014

DNA, Human Memory, and the Storage Technology of the 21st Century

The sophisticated tools and techniques employed by Nature for purposeful storage of information stand in stark contrast to the primitive and relatively inefficient means used by man. We describe some impressive features of biological data storage, and speculate on approaches to research and development that could benefit the storage industry in the coming decades.

q-bio.OT↗

arXiv2014

Reply to "Comment on 'Theoretical analysis of the force on the end face of a nano-filament exerted by an outgoing light pulse'"

We respond to a Comment on our paper [Physical Review A 80, 023823 (2009)], which appears to have stemmed from a misunderstanding of the various energy-momentum tensors of classical electrodynamics. It is shown that each stress tensor, when used in conjunction with the corresponding force-density and momentum-density expressions, yields results that are consistent with Maxwell's equations and with the conservation laws.

physics.optics↗

arXiv2014

The Lorentz force law and its connections to hidden momentum, the Einstein-Laub force, and the Aharonov-Casher effect

The Lorentz force of classical electrodynamics, when applied to magnetic materials, gives rise to hidden energy and hidden momentum. Removing the contributions of hidden entities from the Poynting vector, from the electromagnetic momentum density, and from the Lorentz force and torque densities simplifies the equations of the classical theory. In particular, the reduced expression of the electromagnetic force-density becomes very similar (but not identical) to the Einstein-Laub expression for the force exerted by electric and magnetic fields on a distribution of charge, current, polarization and magnetization. Examples reveal the similarities and differences among various equations that describe the force and torque exerted by electromagnetic fields on material media. An important example of the simplifications afforded by the Einstein-Laub formula is provided by a magnetic dipole moving in a static electric field and exhibiting the Aharonov-Casher effect.

physics.class-ph↗

arXiv2014

On the Foundational Equations of the Classical Theory of Electrodynamics

A close examination of the Maxwell-Lorentz theory of electrodynamics reveals that polarization and magnetization of material media need not be treated as local averages over small volumes - volumes that nevertheless contain a large number of electric and/or magnetic dipoles. Indeed, Maxwell's macroscopic equations are exact and self-consistent mathematical relations between electromagnetic fields and their sources, which consist of free charge, free current, polarization, and magnetization. When necessary, the discrete nature of the constituents of matter and the granularity of material media can be handled with the aid of special functions, such as Dirac's delta-function. The energy of the electromagnetic field and the exchange of this energy with material media are treated with a single postulate that establishes the Poynting vector S = ExH as the rate of flow of electromagnetic energy under all circumstances. Similarly, the linear and angular momentum densities of the fields are simple functions of the Poynting vector that can be unambiguously evaluated at all points in space and time, irrespective of the type of material media, if any, that might reside at various locations. Finally, we examine the Einstein-Laub force- and torque-density equations, and point out the consistency of these equations with the preceding postulates, with the conservation laws, and with the special theory of relativity. The set of postulates thus obtained constitutes a foundation for the classical theory of electrodynamics.

physics.optics↗

arXiv2014

Electromagnetic Stress Tensor in Ponderable Media

We derive an expression for the Maxwell stress tensor in a magnetic dielectric medium specified by its permittivity "epsilon" and permeability "mu." The derivation proceeds from the generalized form of the Lorentz law, which specifies the force exerted by the electromagnetic E and H fields on the polarization P and magnetization M of ponderable media. Our stress tensor differs from the well-known tensors of Abraham and Minkowski, which have been at the center of a century-old controversy surrounding the momentum of the electromagnetic field in transparent materials.

physics.optics↗

arXiv2014

Electromagnetic force and torque in ponderable media

Maxwell's macroscopic equations combined with a generalized form of the Lorentz law of force are a complete and consistent set of equations. Not only are these five equations fully compatible with special relativity, they also conform with conservation laws of energy, momentum, and angular momentum. We demonstrate consistency with the conservation laws by showing that, when a beam of light enters a magnetic dielectric, a fraction of the incident linear (or angular) momentum pours into the medium at a rate determined by the Abraham momentum density, ExH/c^2, and the group velocity V_g of the electromagnetic field. The balance of the incident, reflected, and transmitted momenta is subsequently transferred to the medium as force (or torque) at the leading edge of the beam, which propagates through the medium with velocity V_g. Our analysis does not require "hidden" momenta to comply with the conservation laws, nor does it dissolve into ambiguities with regard to the nature of electromagnetic momentum in ponderable media. The linear and angular momenta of the electromagnetic field are clearly associated with the Abraham momentum, and the phase and group refractive indices (n_p and n_g) play distinct yet definitive roles in the expressions of force, torque, and momentum densities.

physics.optics↗

arXiv2014

Radiation pressure and the linear momentum of the electromagnetic field in magnetic media

We examine the force of the electromagnetic radiation on linear, isotropic, homogeneous media specified in terms of their permittivity epsilon and permeability mu. A formula is proposed for the electromagnetic Lorentz force on the magnetization M, which is treated here as an Amperian current loop. Using the proposed formula, we demonstrate conservation of momentum in several cases that are amenable to rigorous analysis based on the classical Maxwell equations, the Lorentz law of force, and the constitutive relations. Our analysis yields precise expressions for the density of the electromagnetic and mechanical momenta inside the media that are specified by their (epsilon, mu) parameters. An interesting consequence of this analysis is the identification of an "intrinsic" mechanical momentum density, ExM/c^2, analogous to the electromagnetic (or Abraham) momentum density, ExH/c^2. (Here E and H are the magnitudes of the electric and magnetic fields, respectively, and c is the speed of light in vacuum.) This intrinsic mechanical momentum, associated with the magnetization M in the presence of an electric field E, is apparently the same "hidden" momentum that was predicted by W. Shockley and R.P. James nearly four decades ago.

physics.optics↗

arXiv2014

Radiation Pressure on Submerged Mirrors: Implications for the Momentum of Light in Dielectric Media

Radiation pressure measurements on mirrors submerged in dielectric liquids have consistently shown an effective Minkowski momentum for the photons within the liquid. Using an exact theoretical calculation based on Maxwell's equations and the Lorentz law of force, we demonstrate that this result is a consequence of the fact that conventional mirrors impart, upon reflection, a 180 degree phase-shift to the incident beam of light. If the mirror is designed to impart a different phase, then the effective momentum will turn out to be anywhere between the two extremes of the Minkowski and Abraham momenta. Since all values in the range between these two extremes are equally likely to be found in experiments, we argue that the photon momentum inside a dielectric host has the arithmetic mean value of the Abraham and Minkowski momenta.

physics.optics↗

arXiv2014

Radiation pressure on a dielectric wedge

The force of electromagnetic radiation on a dielectric medium may be derived by a direct application of the Lorentz law of classical electrodynamics. While the light's electric field acts upon the (induced) bound charges in the medium, its magnetic field exerts a force on the bound currents. We use the example of a wedge-shaped solid dielectric, immersed in a transparent liquid and illuminated at Brewster's angle, to demonstrate that the linear momentum of the electromagnetic field within dielectrics has neither the Minkowski nor the Abraham form; rather, the correct expression for momentum density has equal contributions from both. The time rate of change of the incident momentum thus expressed is equal to the force exerted on the wedge plus that experienced by the surrounding liquid.

physics.optics↗

arXiv2014

Radiation pressure and the linear momentum of light in dispersive dielectric media

We derive an exact expression for the radiation pressure of a quasi- monochromatic plane wave incident from the free space onto the flat surface of a semi-infinite dielectric medium. In order to account for the total optical momentum (incident plus reflected) that is transferred to the dielectric, the mechanical momentum acquired by the medium must be added to the rate of flow of the electromagnetic momentum (the so-called Abraham momentum) inside the dielectric. We confirm that the electromagnetic momentum travels with the group velocity of light inside the medium. The photon drag effect, in which the photons captured in a semiconductor appear to have the Minkowski momentum, is explained by analyzing a model system consisting of a thin absorptive layer embedded in a transparent dielectric.

physics.optics↗

arXiv2014

Radiation pressure and the distribution of electromagnetic force in dielectric media (II)

Using the Finite-Difference-Time-Domain (FDTD) method, we compute the electromagnetic field distribution in and around dielectric media of various shapes and optical properties. With the aid of the constitutive relations, we proceed to compute the bound charge and bound current densities, then employ the Lorentz law of force to determine the distribution of force-density within the regions of interest. For a few simple cases where analytical solutions exist, these solutions are found to be in complete agreement with our numerical results. We also analyze the distribution of fields and forces in more complex systems, and discuss the relevance of our findings to experimental observations. In particular, we demonstrate the single-beam trapping of a dielectric micro-sphere immersed in a liquid under conditions that are typical of optical tweezers.

physics.optics↗

arXiv2014

Angular momentum of circularly polarized light in dielectric media

A circularly polarized plane-wave is known to have no angular momentum when examined through Maxwell's equations. This, however, contradicts the experimentally observed facts, where finite segments of plane waves are known to be capable of imparting angular momentum to birefringent platelets. Using a superposition of four plane-waves propagating at slightly different angles to a common direction, we derive an expression for the angular momentum density of a single plane-wave in the limit when the propagation directions of the four beams come into alignment. We proceed to use this four-beam technique to analyze the conservation of angular momentum when a plane-wave enters a dielectric slab from the free space. The angular momentum of the beam is shown to decrease upon entering the dielectric medium, by virtue of the fact that the incident beam exerts a torque on the slab surface at the point of entry. When the beam leaves the slab, it imparts an equal but opposite torque to the exit facet, thus recovering its initial angular momentum upon re-emerging into the free-space. Along the way, we derive an expression for the outward-directed force of a normally incident, finite-diameter beam on a dielectric surface; the possible relationship between this force and the experimentally observed bulging of a liquid surface under intense illumination is explored.

physics.optics↗

arXiv2013

Maxwell's macroscopic equations, the energy-momentum postulates, and the Lorentz law of force

We argue that the classical theory of electromagnetism is based on Maxwell's macroscopic equations, an energy postulate, a momentum postulate, and a generalized form of the Lorentz law of force. These seven postulates constitute the foundation of a complete and consistent theory, thus eliminating the need for actual (i.e., physical) models of polarization P and magnetization M, these being the distinguishing features of Maxwell's macroscopic equations. In the proposed formulation, P(r,t) and M(r,t) are arbitrary functions of space and time, their physical properties being embedded in the seven postulates of the theory. The postulates are self-consistent, comply with the requirements of the special theory of relativity, and satisfy the laws of conservation of energy, linear momentum, and angular momentum. One advantage of the proposed formulation is that it side-steps the long-standing Abraham-Minkowski controversy surrounding the electromagnetic momentum inside a material medium by simply "assigning" the Abraham momentum density E(r,t) x H(r,t)/c^2 to the electromagnetic field. This well-defined momentum is thus taken to be universal as it does not depend on whether the field is propagating or evanescent, and whether or not the host medium is homogeneous, transparent, isotropic, dispersive, magnetic, linear, etc. In other words, the local and instantaneous momentum density is uniquely and unambiguously specified at each and every point of the material system in terms of the E and H fields residing at that point. Any variation with time of the total electromagnetic momentum of a closed system results in a force exerted on the material media within the system in accordance with the generalized Lorentz law.

physics.class-ph↗

arXiv2013

The Force Law of Classical Electrodynamics: Lorentz versus Einstein and Laub

The classical theory of electrodynamics is built upon Maxwell's equations and the concepts of electromagnetic field, force, energy, and momentum, which are intimately tied together by Poynting's theorem and the Lorentz force law. Whereas Maxwell's macroscopic equations relate the electric and magnetic fields to their material sources (i.e., charge, current, polarization and magnetization), Poynting's theorem governs the flow of electromagnetic energy and its exchange between fields and material media, while the Lorentz law regulates the back-and-forth transfer of momentum between the media and the fields. As it turns out, an alternative force law, first proposed in 1908 by Einstein and Laub, exists that is consistent with Maxwell's macroscopic equations and complies with the conservation laws as well as with the requirements of special relativity. While the Lorentz law requires the introduction of hidden energy and hidden momentum in situations where an electric field acts on a magnetic material, the Einstein-Laub formulation of electromagnetic force and torque does not invoke hidden entities under such circumstances. Moreover, the total force and the total torque exerted by electromagnetic fields on any given object turn out to be independent of whether force and torque densities are evaluated using the Lorentz law or in accordance with the Einstein-Laub formulas. Hidden entities aside, the two formulations differ only in their predicted force and torque distributions throughout material media. Such differences in distribution are occasionally measurable, and could serve as a guide in deciding which formulation, if either, corresponds to physical reality.

physics.class-ph↗

arXiv2013

Radiation Pressure and Photon Momentum in Negative-Index Media

Radiation pressure and photon momentum in negative-index media are no different than their counterparts in ordinary (positive-index) materials. This is because the parameters responsible for these properties are the admittance, sqrt(epsilon/mu), and the group refractive index n_g of the material (both positive entities), and not the phase refractive index, n=sqrt(epsilon*mu), which is negative in negative-index media. One approach to investigating the exchange of momentum between electromagnetic waves and material media is via the Doppler shift phenomenon. In this paper we use the Doppler shift to arrive at an expression for the radiation pressure on a mirror submerged in a negative-index medium. In preparation for the analysis, we investigate the phenomenon of Doppler shift in various settings, and show the conditions under which a so-called "inverse" Doppler shift could occur. We also argue that a recent observation of the inverse Doppler shift upon reflection from a negative-index medium cannot be correct, because it violates the conservation laws.

physics.optics↗

arXiv2013

Angular Momentum Exchange Between Light and Material Media Deduced from the Doppler Shift

Electromagnetic waves carry energy as well as linear and angular momenta. When a light pulse is reflected from, transmitted through, or absorbed by a material medium, energy and momentum (both linear and angular) are generally exchanged, while the total amount of each entity remains intact. The extent of such exchanges between light and matter can be deduced, among other methods, with the aid of the Doppler shift phenomenon. The main focus of the present paper is on the transfer of angular momentum from a monochromatic light pulse to spinning objects such as a mirror, an absorptive dielectric, or a birefringent plate. The fact that individual photons of frequency omega carry energy in the amount of h_bar*omega, where h_bar is Planck's reduced constant, enables one to relate the Doppler shift to the amount of energy exchanged. Under certain circumstances, the knowledge of exchanged energy leads directly to a determination of the momentum transferred from the photon to the material body, or vice versa.

physics.optics↗