Nanomechanical effects of light unveil photons momentum in medium OPEN (original) (raw)
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Nanomechanical effects of light unveil photons momentum in medium
Scientific Reports, 2017
Precision measurement on momentum transfer between light and fluid interface has many implications including resolving the intriguing nature of photons momentum in a medium. For example, the existence of Abraham pressure of light under specific experimental configuration and the predictions of Chau-Amperian formalism of optical momentum for TE and TM polarizations remain untested. Here, we quantitatively and cleanly measure nanomehanical dynamics of water surface excited by radiation pressure of a laser beam. We systematically scanned wide range of experimental parameters including long exposure times, angle of incidence, spot size and laser polarization, and used two independent pump-probe techniques to validate a nano-bump on the water surface under all the tested conditions, in quantitative agreement with the Minkowski's momentum of light. With careful experiments, we demonstrate advantages and limitations of nanometer resolved optical probing techniques and narrow down actual manifestation of optical momentum in a medium.
Macroscopic Theory of Optical Momentum
Light possesses energy and momentum within the propagating electromagnetic fields. When electromagnetic waves enter a material, the description of energy and momentum becomes ambiguous. In spite of more than a century of development, significant confusion still exists regarding the appropriate macroscopic theory of electrodynamics required to predict experimental outcomes and develop new applications. This confusion stems from the myriad of electromagnetic force equations and expressions for the momentum density and flux. In this review, the leading formulations of electrodynamics are compared with respect to how media are modeled. This view is applied to illustrate how the combination of electromagnetic fields and material responses contribute to the continuity of energy and momentum. A number of basic conclusions are deduced with the specific aim of modeling experiments where dielectric and magnetic media are submerged in media with a differing electromagnetic response. These conclusions are applied to demonstrate applicability to optical manipulation experiments.
What Do Experiments in Optics tell us about Photon Momentum in Media?
In order to get some insight in the intricacies of the Abraham-Minkowski energy-momentum problem in phenomenological electrodynamics, we briefly analyze eight different experimental situations in radiation optics. Six of the experiments are already existing, while the remaining two are suggestions for future endeavors. Among the first six, we distinguish between three which are incapable of informing about photon momentum in a medium, and the remaining three which are able to give us useful information. Our general conclusion is that the Abraham-Minkowski "problem" is essentially a matter of convenience. In actual physical situations it is the Minkowski expression which is most natural alternative to employ. Also, in a canonical context, it is the Minkowski energy-momentum tensor which is the most convenient alternative to work with, as this tensor is divergence-free causing the total radiation momentum and energy to make up a four-vector, although notably a spacelike one.
Studying the interface between condensed matter, atomic physics, optics, quantum optics, nanoscience, quantum information, and computing. View project Singularimetry of the Scalar Optical Field View project Radiation pressure has been known since Kepler's observation that a comet's tail is always oriented away from the sun, and in the past centuries this phenomenon stimulated remarkable discoveries in electromagnetism, quantum physics and relativity [1-3]. In modern terms, the pressure of light is associated with the momentum of photons, which plays a crucial role in a variety of systems, from atomic [4-7] to astronomical scales. Experience from these cases leads us to assume that the direction of the optical momentum and the radiation-pressure force are naturally aligned with the propagation of light, i.e., its wavevector. Here we report the direct observation of an extraordinary optical momentum and force directed perpendicular to the wavevector, and proportional to the optical spin (i.e., degree of circular polarization). This transverse spin-dependent optical force, a few orders of magnitude weaker than the usual radiation pressure, was recently predicted for evanescent waves [10] and other structured fields . Fundamentally, it can be associated with the enigmatic "spin momentum", introduced by Belinfante in field theory 75 years ago [12-14]. We measure this unusual transverse momentum using a nano-cantilever with extremely low compliance (capable of femto-Newton resolution), which is immersed in an evanescent optical field directly above the total-internal-reflecting glass surface.
Annals of Physics, 2017
A discussion is given on the interpretation and physical importance of the Minkowski momentum in macroscopic electrodynamics (essential for the Abraham-Minkowski problem). We focus on the following two facets: (1) Adopting a simple dielectric model where the refractive index n is constant, we demonstrate by means of a mapping procedure how the electromagnetic field in a medium can be mapped into a corresponding field in vacuum. This mapping was presented many years ago [I. Brevik and B. Lautrup, Mat. Fys. Medd. Dan. Vid. Selsk 38(1), 1 (1970)], but is apparently not well known. A characteristic property of this procedure is that it shows how natural the Minkowski energy-momentum tensor fits into the canonical formalism. Especially the spacelike character of the electromagnetic total four-momentum for a radiation field (implying negative electromagnetic energy in some inertial frames), so strikingly demonstrated in the Cherenkov effect, is worth attention. (2) Our second objective is to give a critical analysis of some recent experiments on electromagnetic momentum. Care must here be taken in the interpretations: it is easy to be misled and conclude that an experiment is important for the energy-momentum problem, while what is demonstrated experimentally is merely the action of the Abraham-Minkowski force acting in surface layers or inhomogeneous regions. The Abraham-Minkowski force is common for the two energy-momentum tensors and carries no information about field momentum. As a final item, we propose an experiment that might show the existence of the Abraham force at high frequencies. This would eventually be a welcome optical analogue to the classic low-frequency 1975 Lahoz-Walker experiment.
Universal Long-Range Nanometric Bending of Water by Light
Resolving mechanical effects of light on fluids has fundamental importance with wide applications. Most experiments to date on optofluidic interface deformation exploited radiation forces exerted by normally incident lasers. However, the intriguing effects of photon momentum for any configuration, including the unique total internal reflection regime, where an evanescent wave leaks above the interface, remain largely unexplored. A major difficulty in resolving nanomechanical effects has been the lack of a sensitive detection technique. Here, we devise a simple setup whereby a probe laser produces high-contrast Newton-ring-like fringes from a sessile water drop. The mechanical action of the photon momentum of a pump beam modulates the fringes, thus allowing us to perform a direct noninvasive measurement of a nanometric bulge with sub-5-nm precision. Remarkably, a <10 nm difference in the height of the bulge due to different laser polarizations and nonlinear enhancement in the bulge near total internal reflection is isolated. In addition, the nanometric bulge is shown to extend far longer, 100 times beyond the pump spot. Our high precision data validate the century-old Minkowski theory for a general angle and offer potential for novel optofluidic devices and noncontact nanomanipulation strategies.
Radiation Pressure on Submerged Mirrors: Implications for the Momentum of Light in Dielectric Media
2014
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.
arXiv (Cornell University), 2018
Though the interfacial tractor beam experiment supports the increase of photon momentum (i.e., Minkowski momentum), it is still a matter of investigation whether, inside matter, the photon momentum always increases. Considering the inhomogeneous or heterogeneous background, we have demonstrated that if the background and the half-immersed object are both non-absorbing, the transferred photon momentum to the object can be considered as the one of Minkowski exactly at the interface. In contrast, the presence of loss inside matter, either in the half-immersed object (i.e., plasmonic or lossy dielectric) or in the background, changes the whole situation. For such cases, our several demonstrations and proposed thought experiments have strongly supported the decrease of photon momentum instead of the usual perception of its increase. Although almost all the major radiation pressure experiments have so far supported the linear increase of photon momentum, our proposed simple experimental setups may introduce a novel way to observe and verify the exactly opposite proposal: the Abraham momentum of photon. Finally, as an interesting sidewalk, based on several parameters, a machine learning based system has been developed to predict the transferred momentum of photon within a very short time avoiding time-consuming full simulation.
Nonlinear Laser-Induced Deformations of Fluid-Fluid Interfaces
2008
Experimentally, it turns out that radiation forces from a cw-laser on a liquid-liquid interface are able to produce giant deformations (up to about 100 µm), if the system is close to the critical point where the surface tension becomes small. We present a new model for such a fingerlike deformation, implying that the system is described as an optical fiber. One reason for introducing such a model is that the refractive index difference in modern experiments, such as those of the Bordeaux group, is small, of the same order as in practical fibers in optics. It is natural therefore, to adopt the hybrid HE 11 mode, known from fiber theory, as the fundamental mode for the liquid system. We show how the balance between hydrodynamical and radiation forces leads to a stable equilibrium point for the liquid column. Also, we calculate the narrowing of the column radius as the depth increases. Comparison with experimental results of the Bordeaux group yields quite satisfactory agreement as regards the column width.
Light-induced deformation and instability of a liquid interface. I. Statics
Physical Review E, 2006
We study the dynamics of the deformation of a soft liquid-liquid interface by the optical radiation pressure of a focused cw gaussian laser beam. We measured the temporal evolution of both the hump height and the hump curvature by direct observation and by detecting the focusing effect of the hump acting as a lens. Extending the results of Yoshitake et al. [J. Appl. Phys. 97, 024901 (2005)] to the case of liquid-liquid interfaces and to the Bo 1 regime (Bo = (ω 0 / c ) 2 , where ω 0 is the beam waist and c the capillary length), we show that, in the Bo 1 and Bo 1