LESIA - Observatoire de Paris

Utilisation of Electromagnetic Angular Momentum in Radio Studies of Space (parts 3/4 and 4/4)

Monday 18 October 2010, by Bo Thidé (Swedish Institute of Space Physics, Uppsala)

Friday 22 October 2010 à 15h00 , Lieu : Salle de conférence du bât. 17

Two 45 minute seminars separated by a coffee break will be presented on this day.

Most of the information we obtain about the Universe is extracted from the electromagnetic radiation from space that reaches our telescopes and other sensor systems on Earth and on board spacecraft orbiting Earth.

Up till now, virtually all radiation from Nature has been analyzed only with respect to its intensity, spectral content, direction of arrival, and polarization. While polarization is a manifestation of the physical fact that electromagnetic radiation does not only carry linear momentum but also angular momentum, polarization measurements alone do not provide an exhaustive characterization of the total angular momentum state of electromagnetic radiation. Already in the early 1900s it was shown theoretically that collimated beams can carry angular momentum. In the 1930s and 1940s, elegant Einstein-de Haas type experiments for photons carried out by Beth in optics and by Carrara in radio showed that angular momentum can be transferred from electromagnetic beams to mechanical bodies. In the years that followed, very high angular momentum states of nuclei were discovered via their photon decays. Still, it was not until the 1990s that laser beams and microwave radio beams carrying both spin angular momentum (polarization) and orbital angular momentum (OAM) could be readily generated, controlled, and detected. Soon thereafter it was experimentally proved that individual photons can be endowed with OAM and be entangled in these states, showing that electromagnetic radiation can be characterized, analyzed, and utilized more fully than what was commonly known at the time. Recently the possibility of using the OAM degrees of freedom of light and radio in astrophysics and space physics has come to the fore.

It is namely reasonable to assume that electromagnetic OAM is radiated by some astrophysical sources or is imparted upon radiation through interaction with plasma in space, at least under certain conditions. The characterization of the OAM of light or radio beams intercepted by telescopes on Earth or in space can provide new and crucial information about the physical processes involved. Following this assumption, many authors have proposed studies with new instruments to detect and manipulate the electromagnetic OAM, henceforth referred to as POAM (photon orbital angular momentum) and also performed new experiments at the telescope. This represents a single face of a more complex evolution occurring now in astronomy. As described by Harwit, POAM permits new types of measurements and paves the way for the utilization of topological and more generic degrees of freedom in astronomy and space sciences.

These seminars describe and discuss some of these new opportunities, from theoretical foundations to applications of POAM in space physics.