Structured Cloud of Particles
- Paper number
IAC-05-A3.1.10
- Author
Mr. Julien Coyne, University of Cambridge, United Kingdom
- Year
2005
- Abstract
Developments in space based astronomical optics are aiming at extremely large and lightweight optics. Space based telescope optics based on Structured Cloud of Particles (SCoP) would clearly fulfil this goal (despite their moderate reflectivity): about 5 grams/ m 2.
The concept of SCoP is based on a simple physical principle: Several particles (of sizes and distances between them inferior to the observing wavelength) embedded in a medium induce a change in the refractive index of this medium. We focus on the two main aspects of SCoP: the implemented structure and the structuring field.
A (randomly organized) cloud of particles is merely a random phase screen. However if the particles concentration can be spatially modulated then the refractive index is spatially modulated. The Effective Medium Theory is a powerful tool to compute the local refractive index. The cloud can then be considered as a phase medium, and its optical properties directly studied. It is therefore possible to implement a structure in the cloud, such as a membrane mirror or a phase hologram.
Several concepts of structuring fields based on coherent and/or incoherent light sources, with one/several light sources, full/diluted aperture are proposed and compared. The basic idea is to use the radiation pressure from the light source(s) to trap the particles and to spatially modulate their concentration.
Two applications of the SCoP concept applied to space based telescope design are presented:
(Space Based) Laser Trapped Mirror: The concept of Laser Trapped Mirror (LTM), originally proposed by A. Labeyrie (1979), can be re-interpreted in the context of ScoP. A thin layer of particles (a few observing wavelengths thick) can act as the primary mirror of a telescope. It is shown that using nanoparticles made of glass, a reflectivity of a fraction of a percent is achievable. Optimization of the particles parameters (size, concentration, material...) is likely to provide an even better reflectivity. A 30 m diameter LTM would only require about 15 kg of glass nanoparticles (to be compared with the 2.4 m diameter and 824 kg of the Hubble Space Telescope primary mirror).
(Space Based) Thin Laser Trapped Hologram: A SCoP can also be structured as a thin Laser Trapped Hologram (thin LTH) The cloud is organized as a thin layer (a few observing wavelength thick), with the particles concentration spatially modulated to form a thin hologram. It is shown that using nanoparticles of glass it is possible to reach the maximum diffraction efficiency of a thin hologram: 33%. A holographic telescope can be implemented with a thin LTH. The chromatic effects can be either corrected to do imaging over a large bandwidth or directly used to observe simultaneously at several wavelengths (3D spectro-imaging). A 100 m diameter thin LTH could be used for exoplanet detection and characterization, or even to study the volcanic activity of Io without the need to send a (rather expensive) spacecraft to orbit the satellite.
A SCoP can theoretically implement basically almost anything one can do with holograms and mirrors such as: solar concentrator, holographic image processor, data storage, (laser trapped) solar sail...
The current status of experimental studies (in laboratory with silver halides holographic emulsions and in microgravity with nanoparticles) is finally briefly presented.
- Abstract document
- Manuscript document
IAC-05-A3.1.10.pdf (🔒 authorized access only).
To get the manuscript, please contact IAF Secretariat.