Bo Thidé

Utilization of Photon Orbital Angular Momentum in the Low-Frequency Radio Domain

We show numerically that vector antenna arrays can generate radio beams that exhibit spin and orbital angular momentum characteristics similar to those of helical Laguerre-Gauss laser beams in paraxial optics. For low frequencies (< or = 1 GHz), digital techniques can be used to coherently measure the instantaneous, local field vectors and to manipulate them in software. This enables new types of experiments that go beyond what is possible in optics. It allows information-rich radio astronomy and paves the way for novel wireless communication concepts.

Orbital Angular Momentum in Radio—A System Study

Recent discoveries concerning rotating (helical) phase fronts and orbital angular momentum (OAM) of laser beams are applied to radio frequencies and comprehensive simulations of a radio OAM system are performed. We find that with the use of vector field-sensing electric and magnetic triaxial antennas, it is possible to unambiguously estimate the OAM in radio beams by local measurements at a single point, assuming ideal (noiseless) conditions and that the beam axis is known. Furthermore, we show that conventional antenna pattern optimization methods can be applied to OAM-generating circular arrays to enhance their directivity.

Ionospheric modification experiments in northern Scandinavia

Abstract The heating facility at Ramfjordmoen near Tromso, Norway, is briefly described, and a survey is given of the experiments performed with this facility until now. These experiments comprise D -region modification, polar electrojet modulation at VLF, ELF and ULF, HF absorption and backscatter due to short-scale field-aligned irregularities, stimulated radio wave emission of the modified ionospheric plasma, short-time scale HF absorption due to the parametric decay instability, airglow modification, excitation of large-scale irregularities, and F -region cross modulation.

Experimental verification of photon angular momentum and vorticity with radio techniques

The experimental evidence that radio techniques can be used for synthesizing and analyzing non-integer electromagnetic (EM) orbital angular momentum (OAM) of radiation is presented. The technique used amounts to sample, in space and time, the EM field vectors and digitally processing the data to calculate the vortex structure, the spatial phase distribution, and the OAM spectrum of the radiation. The experimental verification that OAM-carrying beams can be readily generated and exploited by using radio techniques paves the way to an entirely new paradigm of radar and radio communication protocols.

Twisting of light around rotating black holes

Kerr black holes are among the most intriguing predictions of Einsteins general relativity theory(1,2). These rotating massive astrophysical objects drag and intermix their surrounding space and time, deflecting and phase-modifying light emitted near them. We have found that this leads to a new relativistic effect that imprints orbital angular momentum on such light. Numerical experiments, based on the integration of the null geodesic equations of light from orbiting point-like sources in the Kerr black hole equatorial plane to an asymptotic observer(3), indeed identify the phase change and wavefront warping and predict the associated light-beam orbital angular momentum spectra(4). Setting up the best existing telescopes properly, it should be possible to detect and measure this twisted light, thus allowing a direct observational demonstration of the existence of rotating black holes. As non-rotating objects are more an exception than a rule in the Universe, our findings are of fundamental importance.

Downshifted maximum features in stimulated electromagnetic emission spectra

New and detailed experimental and theoretical results concerning the prominent downshifted maximum (DM) feature in spectra of stimulated electromagnetic emissions are reported. The experimental results were obtained at the Sura ionospheric modification facility in Russia by transmitting a powerful high-frequency ordinary mode pump wave into the ionospheric F region. We present detailed experimental results of the dependence of the DM on the pump frequency. Different frequency components of the DM have slightly different growth times after pump turn-on and are suppressed in different pump frequency ranges at harmonics of the electron cyclotron frequency. The pump frequency range near the cyclotron harmonics in which the DM does not develop, decreases significantly with increasing harmonic, from several kilohertz at the fourth harmonic to an extremely narrow range of 0.2 kHz or less at the seventh harmonic. We discuss conditions for plasma wave propagation to explain this dependence on the cyclotron harmonics.

Spectral structure of stimulated electromagnetic emissions between electron cyclotron harmonics

Experimental results concerning the spectrum of stimulated electromagnetic emissions (SEE) in the sidebands of a powerful high-frequency electromagnetic ordinary mode pump wave are presented. The pump wave was vertically injected into the ionospheric F region from the Sura ionospheric modification facility in Russia. We report detailed measurements of the commonly observed continuum, downshifted maximum, and broad upshifted maximum emissions for pump frequencies ranging from the fourth to above the seventh electron cyclotron harmonic in the ionospheric plasma as well as observations of a new wideband emission occurring primarily in the upper sideband. The dependence of the SEE spectral structure on the pump frequency in relation to electron cyclotron harmonics is emphasized. All spectral features in the sidebands of the pump exhibit cyclotron harmonic effects.

Stimulated Raman and Brillouin Backscattering of Collimated Beams Carrying Orbital Angular Momentum

We study theoretically the exchange of angular momentum between electromagnetic and electrostatic waves in a plasma, due to the stimulated Raman and Brillouin backscattering processes. Angular momentum states for plasmon and phonon fields are introduced for the first time. We demonstrate that these states can be excited by nonlinear wave mixing, associated with the scattering processes. This could be relevant for plasma diagnostics, both in laboratory and in space. Nonlinearly coupled paraxial equations and instability growth rates are derived.

New electron gyroharmonic effects in HF scatter from pump-excited magnetic field-aligned ionospheric irregularities

Experimental results of resonant high-frequency (HF) scattering from small-scale geomagnetic field-aligned plasma irregularities excited by electromagnetic HF pumping of the ionospheric F region from the Sura radio facility in Russia are reported. These results include the first observations of a minimum in the scattered signal intensity when the pump frequency was near the fourth electron gyroharmonic and of a significant broadening of the frequency spectrum of the scattered signal for pump frequencies above the gyroharmonic. After pump-off, the flanks of the broad spectrum decayed rapidly. These new observations link together several previous experimental results of gyroharmonic effects in phenomena attributed to the excitation of small-scale field-aligned irregularities and are therefore thought to be significant contributions to our understanding of the self-organization of the space plasma under the action of an external perturbation.

Reply to Comment on ‘Encoding many channels on the same frequency through radio vorticity: first experimental test’

Our recent paper (Tamburini et al 2012 New J. Phys. 14 033001), which presented results from outdoor experiments that demonstrate that it is physically feasible to simultaneously transmit different states of the newly recognized electromagnetic (EM) quantity orbital angular momentum (OAM) at radio frequencies into the far zone and to identify these states there, has led to a comment (Tamagnone et al 2012 New J. Phys. 14 118001). These authors discuss whether our investigations can be regarded as a particular implementation of the multiple-input–multiple-output (MIMO) technique. Clearly, our experimental confirmation of a theoretical prediction, first made almost a century ago (Abraham 1914 Phys. Z. XV 914–8), that the total EM angular momentum (a pseudovector of dimension length × mass × velocity) can propagate over huge distances, is essentially different from—and conceptually incompatible with—the fact that there exist engineering techniques that can enhance the spectral capacity of EM linear momentum (an ordinary vector of dimension mass × velocity). Our OAM experiments (Tamburini et al 2012 New J. Phys. 14 033001; Tamburini et al 2011 Appl. Phys. Lett. 99 204102–3) confirm the availability of a new physical layer for real-world radio communications based on EM rotational degrees of freedom. The next step is to develop new protocols and techniques for high spectral density on this new physical layer. This includes MIMO-like and other, more efficient, techniques.