Spiral Electric Fields Imposed on Laser Beam Creates Spiral Complex, Surface Micro-structures
Author: Walter Perrie
Affiliation: Laser Group, University of Liverpool, UK.
Scientists at the University of Liverpool have generated new polarisation states of light and imposed these on an ultrafast laser beam producing 10 picosecond (1ps = 10-12s) laser pulses . Linear polarisation states are familiar in physics where the electric field is uni-directional in space across the laser beam and output intensity often a “Gaussian” mode with an intensity maximum at centre. Much less familiar polarisation states are, for example, Radial and Azimuthal polarisations which are vector fields in which the electric field direction varies spatially in a fixed plane with radially pointed vectors over 0-360° (Radial polarisation) and the orthogonal state (Azimuthal polarisation) where the field vectors consist of concentric circles. Such states have an intensity and polarisation singularity at their centre and so have ring intensity distributions.
By creating superpositions of Radial and Azimuthal polarisation states, the resulting laser electric fields were logarithmic spirals, a natural spiral (first described by Descartes and admired by Bernoulli) describing, for example the spiral arms of galaxies. The electric field at a given point is given by E(r, φ) = a ek φ where a and k are constants.
Ph.D student Jinglie Ouyang and colleagues (led by Dr. W.Perrie and Dr.O.J Allegre) used these states to imprint Laser Induced Periodic Surface Structures (LIPSS) to create beautiful spiral grooved structures with 1 μm pitch on polished metals for the first time . These Plasmon structures develop orthogonal to the local electric field component and so elucidate the incident electric field distribution unambiguously. The spiral states are created by rotating an incident linear polarised laser beam on a specially nano-structured waveplate which generates Radial, Azimuthal and superpostion states of polarisation resulting in the spirals.
The scientists also added Optical Angular momentum (OAM) to these states by twisting the wavefronts so that each photon carries a z-component of angular momentum, Lz = h/2π per photon and focusing of these beams created a near Gaussian beam intensity distribution with circular polarisation (carrying spin angular momentum Lz = h/2π per photon) at the centre. This is an example of Orbital to Spin angular momentum conversion and creates even more complex microstructures.
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