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Register a new userRadiation Pressure on a Diffractive Sailcraft
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Added by
Grover Swartzlander Jr.
Publication date
2017
fields
Physics
and research's language is
English
Authors
Grover A. Swartzlander Jr
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Advanced diffractive films may afford advantages over passive reflective surfaces for a variety space missions that use solar or laser in-space propulsion. Three cases are compared: Sun-facing diffractive sails, Littrow diffraction configurations, and conventional reflective sails. A simple Earth-to-Mars orbit transfer at a constant attitude with respect to the sun-line finds no penalty for transparent diffractive sails. Advantages of the latter approach include actively controlled metasails and the reuse of photons.
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Radiation pressure afforded by natural broadband sunlight upon a transmissive diffractive sail is theoretically and numerically investigated. A grating period of one micrometer is found to convert 83% of the solar black body spectrum into sailcraft momentum. Non-optimized orbit-raising trajectories for diffractive and reflective sails are compared. Potential advantages of diffractive sails are also described.
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 lights 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 Brewsters 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.
Deformations of horizontal liquid interfaces by optical radiation pressure are generally expected to display similar behaviors whatever the direction of propagation of the exciting laser beam is. In the present experiment we find this expectation to be borne out, as long as the cw laser illumination is moderate in strength. However, as a striking contrast in the case of high field strengths, we find that either a large stable tether can be formed, or else that a break-up of the interface can occur, depending on whether the laser beam is upward or downward directed. Physically, the reason for this asymmetry can be traced to whether total reflection can occur or not. We also present two simple theoretical models, one based on geometrical optics, the other on wave optics, that are able to illustrate the essence of the effect. In the case leading to interface disruption our experimental results are compared with those obtained by Zhang and Chang for water droplets under intense laser pulses [Opt. Lett. textbf{13}, 916 (1988)]. A key point in our experimental investigations is to work with a near-critical liquid/liquid interface. The surface tension becomes therefore significantly reduced, which thus enhances the magnitude of the stationary deformations induced.
We describe the experimental implementation of a superluminal ({it i.e.} faster than light {it in vacuo}) polarization current distribution that both oscillates and undergoes centripetal acceleration. Theoretical treatments lead one to expect that the radiation emitted from each volume element of such a polarization current will comprise a v{C}erenkov-like envelope with two sheets that meet along a cusp. The emission from the experimental machine is in good agreement with these expectations, the combined effect of the volume elements leading to tightly-defined beams of a well-defined geometry, determined by the source speed and trajectory. In addition, over a restricted range of angles, we detect the presence of cusps in the emitted radiation. These are due to the detection over a short time period (in the laboratory frame) of radiation emitted over a considerably longer period of source time. Consequently, the intensity of the radiation at these angles was observed to decline more slowly with increasing distance from the source than would the emission from a conventional antenna. The angular distribution of the emitted radiation and the properties associated with the cusps are in good {it quantitative} agreement with theoretical models of superluminal sources once the effect of reflections from the earths surface are taken into account.
Quantitative measurements of the vibrational eigenmodes in ultra-high-Q silica microspheres are reported. The modes are efficiently excited via radiation-pressure induced dynamical back-action of light confined in the optical whispering-gallery modes of the microspheres (i.e. via the parametric oscillation instability). Two families of modes are studied and their frequency dependence on sphere size investigated. The measured frequencies are in good agreement both with Lambs theory and numerical finite element simulation and are found to be proportional to the spheres inverse diameter.
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