Initial three-dimensional phase and intensity measurements of a 100 GHz l=±1 orbital angular momentum (OAM) vortex are presented. The vortex was generated by illuminating a polypropylene spiral phase plate. Measurements were taken with a three-dimensional field scanner operating in the W-band (75-100 GHz). Early analysis shows splitting of the OAM phase dislocation at the vortex center, resulting in a complex inner vortex intensity pattern.
q-plates are quasi-optical devices specifically designed to generate and detect the orbital angular momentum states of the light. It is possible to produce q-plates working at millimeter wavelengths by using a well-known and cheap manufacturing technique. The technique consists of creating inhomogeneous, artificial birefringent materials by machining grooves with specific geometries into normal dielectric materials. In this work, a q-plate working around 100 GHz has been designed, manufactured, and tested using a vector network analyzer. The experimental data validate the modeled intensity and phase for the transformation of an incident Gaussian beam.
We extend the theory of astrophysical maser propagation through a medium with a Zeemansplit molecular response to the case of a non-uniform magnetic field, and allow a component of the electric field of the radiation in the direction of propagation: a characteristic of radiation with orbital angular momentum. A classical reduction of the governing equations leads to a set of nine differential equations for the evolution of intensity-like parameters for each Fourier component of the radiation. Four of these parameters correspond to the standard Stokes parameters, whilst the other five represent the z-component of the electric field, and its coupling to the conventional components in the x − y-plane. A restricted analytical solution of the governing equations demonstrates a non-trivial coupling of the Stokes parameters to those representing orbital angular momentum: the z-component of the electric field can grow from a background in which only Stokes-I is non-zero. A numerical solution of the governing equations reveals radiation patterns with a radial and angular structure for the case of an ideal quadrupole magnetic field perpendicular to the propagation direction. In this ideal case generation of radiation orbital angular momentum, like polarization, can approach 100 per cent.
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