Brandon A. Kemp

The observable pressure of light in dielectric fluids

By considering a perfect reflector submerged in a dielectric fluid, we show that the Minkowski formulation describes the optical momentum transfer to submerged objects. This result is required by global energy conservation, regardless of the phase of the reflected wave. While the electromagnetic pressure on a submerged reflector can vary with phase of the mirror reflection coefficient between twice the Abraham momentum and twice the Minkowski momentum, the Minkowski momentum is always restored due to the additional pressure on the dielectric surface. This analysis also gives further evidence for use of the Minkowski stress tensor at the boundary of a dielectric interface, which has been the subject of a long-standing debate in physics and the source of uncertainty in the modeling of optical forces on submerged particles.

Optical force on a cylindrical cloak under arbitrary wave illumination

The optical force distribution in the cylindrical cloak under arbitrary incident waves is presented. We show that on the inner surface of the cloak both the induced surface currents and polarization charges interact with the waves and give opposite radiation pressure onto the inner surface. The Lorentz force in the cloak can contribute to change the trajectory of the rays, while in some cases it may only reflect the rays having a tendency to decrease the total energy it carries. The force is symmetric and in balance. Therefore the total momentum transfer from the waves to the cylindrical cloak is zero.

Transfer of optical momentum: reconciliations of the Abraham and Minowski formulations

The correct form of electromagnetic wave momentum in matter has been debated in the literature for over a century, with the main candidates being the Minkowski and the Abraham formulations. Recently, a third momentum expression has been proposed, averaging the previous two, and has been argued to be the resolution of the debate. In this paper, we revisit the various formulations and show that the debate has been carried out on a wrong platform: the question is not which momentum expression is right and which is wrong, but which is measurable and which is not. In that regard, experiments have been overwhelmingly in favor of the Minkowski form, which, however, does not discredit the Abraham form as a theoretical concept. The third form of momentum, averaging the previous two, is here argued to be merely the result of a mathematical exercise, which physical assumptions need to be revisited.

Revisiting Mie’s scattering theory for the analysis of the plasmonic resonance of metal nanospheres

An alternate correction of Mie scattering theory for the analysis of plasmonics resonance of metal nanospheres is proposed. The analysis is based on the correction of Mie scattering theory originally developed for left-handed media or negative index media, which employs a solution using the Ricatti–Bessel functions in the calculation of Mie coefficients. It provides validation that the Mie solution for negative index media of spherical particles is also valid for resonance metal nanospheres. Therefore, the viewpoint of a time-reversal problem for the scattered fields in previous correction is not a necessary, fundamental correction to the Mie scattering theory for spherical metal particles. Our analytical solution is also verified by using finite element method.

Subsystem approach to the electrodynamics in dielectric fluids

A century has now passed since the origins of the Abraham-Minkowski controversy pertaining to the correct form of optical momentum in media. Since, the debate has come to reference the general debate over optical momentum, including a number of competing formulations. The pervasive modern view is that the Abraham momentum represents the optical momentum contained within the fields and the Minkowski momentum includes a material component which is coupled with the fields. A recently proposed resolution to the debate identified Abraham’s kinetic momentum as responsible for the overall center-of-mass translations of a medium and Minkowski’s canonical momentum as responsible for local translations of a medium within or with respect to another medium. Still, current literature reveals significant confusion as to how systems of light and matter should be modeled as to deduce the equations of motion when multiple material types are present. For example, the state-of-the-art model for optical dynamics of submerged particles assumes over damped systems such that the mass of the particles is ignored in the equations of motion. In this paper, we apply the subsystem approach to deduce the electrodynamics of such systems. We show that regardless of which electromagnetic momentum continuity law is applied, the equations of motion can be correctly deduced as long as the continuity law is consistent with Maxwells equations and the overall system is closed such that momentum is conserved. Because the closed system includes the material response, the model can be very complex. However, we demonstrate with simple, well-known examples.

Field and material stresses predict observable surface forces in optical and electrostatic manipulation

The momentum of light in media has been one of the most debated topics in physics over the past one hundred years. Originally a theoretical debate over the electrodynamics of moving media, practical applications have emerged over the past few decades due to interest in optical manipulation and nanotechnology. Resolution of the debate identifies a kinetic momentum as the momentum of the fields responsible for center of mass translations and a canonical momentum related to the coupled field and material system. The optical momentum resolution has been considered incomplete because it did not uniquely identify the full stress-energy-momentum (SEM) tensor of the field-kinetic subsystem. A consequence of this partial resolution is that the field-kinetic momentum could be described by three of the leading formulations found in the literature. The Abraham, Einstein-Laub, and Chu SEM tensors share the field-kinetic momentum, but their SEM tensors differ resulting in competing force densities. We can show now that the Abraham and Einstein-Laub formulations are invalid since their SEM tensors are not frame invariant, whereas the Chu SEM tensor satisfies relativistic principles as the field-kinetic formulation. However, a number of reports indicate that the force distribution in matter may not accurately represent experimental observations. In this correspondence, we show that the field-kinetic SEM tensor can be used along with the corresponding material subsystem to accurately predict experimental force and stress distributions. We model experimental examples from optical and static manipulation of particles and fluids.

A study of plasmonic field enhancement in bimetallic and active core-shell nanoparticles/nanorods

We theoretically demonstrate the field enhancement in both single and multiple bimetallic core-shell nanoparticles/nanorods at resonance condition. We have shown single core-shell bimetallic nanoparticle can be tuned by changing their core-to-shell ratio using both Rayleigh and Mie scattering theory. For multiple core-shell nanoparticles, we have used Foldy-Lax multi-scattering theory. In this case, the arrangement of the particles also plays a vital role for tunability beside the core-to-shell ratio. The analysis is also expanded for the multiple nanorods using the ellipsoidal approximation of polarizability. It is shown that the field can be enhanced in Au-Ag core-shell nanoparticles/nanorods due to the plasmonic resonance for all the cases. The analytical results are also validated by using finite element method.

Radiation pressure on core-shell nanoparticles in Rayleigh regime

The radiation pressure to the single and multiple core-shell nanoparticles using Rayleigh scattering theory is presented. The force on a single core-shell nanoparticle with different material compositions for core and shell is demonstrated. The metal shell having frequency dependent permittivity gives plasmonic resonance at a certain frequency. The force in both resonance and off-resonance condition of silver and gold shell is also shown. The total force on single core-shell nanoparticle is also validated using Mie scattering theory. For the case of multiple core-shell nanoparticles, the discrete dipole approximation (DDA) was used. The approximate force calculation on aggregates of multiple core-shell nanoparticles can be useful for wide variety of applications.

Negative force on free carriers in positive index nanoparticles

We theoretically demonstrate the reversal of optical forces on free charge carriers in positive refractive index nanostructures. Though optical momentum in positive refractive index materials is necessarily parallel to the local energy flow, reversal of optical momentum transfer can be accomplished by exploiting the geometry and size of subwavelength particles. Using the Mie scattering theory and separation of optical momentum transfers to the bound and free charges and currents, we have shown that metal nanoparticles can exhibit strong momentum transfer to free carriers opposite to the direction of incident electromagnetic waves. This can be explained for small particles in terms of a reversal of Poynting power inside the material resulting in a negative net force on free carriers in small particles. Two-dimensional simulations further illuminate this point by demonstrating the effect of incident wave polarization.

Physics of electromagnetic and material stresses in optical manipulation

Modeling the dynamics of optical manipulation experiments relies upon a precise mathematical representation of electromagnetic fields and the interpretation of optical momentum and stresses in materials. However, the momentum of light within media has been an issue of debate over the past century. Multiple energy-momentum models have been advanced, each, under certain conditions, agreeing with experimental observation and mathematically consistent with classical electromagnetism. The modern view is that the various formulations of electrodynamics represent different divisions of the total energy-momentum tensor, with the separation of field and matter being ambiguous. Recently, a proposed view of photon momentum identified two leading forms as the kinetic and canonical momenta. The Abraham momentum is responsible for the overall center-of-mass translation of a material, while the Minkowski momentum is responsible for translations with respect to the surrounding medium. However, the Abraham momentum corresponds to multiple, unique electromagnetic energy-momentum tensors that attempt to separate field from material responses (e.g. Abraham, Chu, and Einstein-Laub). However, only the form of the kinetic momentum density has been revealed, while the formulation that uniquely separates the kinetic stress tensor has remained ambiguous. In this correspondence, multiple formulations are considered within the framework of relativistic electrodynamics. We apply various mathematical techniques to identify the kinetic subsystem of electrodynamics. While optical manipulation is usually modeled using a stationary medium approximation, the lessons from relativistic electrodynamics reveal a specific distribution of electromagnetic stress in media. The physics of optical and static manipulation of dielectric particles are described within this framework.