Spiral Phase Plates are a type of Beam Mode Convertor. Although only developed recently, they have already found a number of uses.
These have varied from ‘fundamental science’ experiments to improving the performance of mm-wave antennas.
A Fundamental Mode Gaussian Beam Mode has a spherical phasefront. Normally, beams with circular symmetry have a phase which varies radially, and along the beam, but not as you ‘go around’ the beam's circumference. A Spiral phase plate has a thickness which varies circumferally ‘around’ the plate, but is uniform radially. The plate is made of a dielectric material which is transparent. Hence a fundamental mode beam passing though the plate undergoes a phase-change which introduces a ‘spiral’ element into its phasefront. By choosing the appropriate plate geometry we can turn the beam phasefront into a “spiral staircase” pattern.
This mode conversion has two important effects:
- Firstly, the beam now has a non-zero angular momentum. (Note this is a property of the beam, not the individual photons.)
- Secondly, the beam must now have a null in the field amplitude on its axis. This is because, otherwise, a non-zero field would have to simultaneously have multiple phase values since the axis is at all possible circumferal angles!
The non-zero beam angular momentum has some interesting consequences. For example, rotating the source and spiral phase plate produces a change in the output beam's frequency — and its linear momentum!
The axial null in the beam's power profile can also reduce the sidelobe level when the beam is used in an ‘omnidirectional’ (in a plane) antenna.
For more information see:
Measurement of the Rotational Frequency Shift Imparted to a Rotating Light Beam Possessing Orbital Angular Momentum.
J. Courtal, K. Dholakia, D. A. Robertson, L. Allen, M. J. Padgett.
Phys Rev Letts Vol 80 Number 15 3217-9
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