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2Physics Quote:
"Lasers are light sources with well-defined and well-manageable properties, making them an ideal tool for scientific research. Nevertheless, at some points the inherent (quasi-) monochromaticity of lasers is a drawback. Using a convenient converting phosphor can produce a broad spectrum but also results in a loss of the desired laser properties, in particular the high degree of directionality. To generate true white light while retaining this directionality, one can resort to nonlinear effects like soliton formation."
-- Nils W. Rosemann, Jens P. Eußner, Andreas Beyer, Stephan W. Koch, Kerstin Volz, Stefanie Dehnen, Sangam Chatterjee
(Read Full Article: "Nonlinear Medium for Efficient Steady-State Directional White-Light Generation"
)

Sunday, June 28, 2015

Spiral Electric Fields Imposed on Laser Beam Creates Spiral Complex, Surface Micro-structures

Jinglie Ouyang, the Ph.D student of the Laser Group at Liverpool, who carried out this research led by Dr. Walter Perrie and Dr. Olivier Allegre. 


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 [1]. 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 [1]. 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.
Interestingly, these spiral fields were predicted theoretically by Professor Franco Gori of the University degli Studi Roma Tre in 2001 [2] and not observed until now. The above figure shows two of these spiral structures ablated on stainless steel with incident angles to the waveplate axis of 22.5° and 45° respectively with theoretical fits (red curves) which agree well.

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.

Relevant Papers:
[1] J. Ouyang, W. Perrie, O. J. Allegre, T. Heil, Y. Jin, E. Fearon, D. Eckford, S. P. Edwardson, G. Dearden, “Tailored Optical vector fields for ultrashort-pulse laser induced complex surface Plasmon structuring”. Optics Express, 23, 12562-12572 (2015). Abstract.
[2] Franco Gori, "Polarisation basis for vortex beams”. Journal of the Optical Society of America A 18, 1612 (2001). Abstract.
[3] Qiwen Zhan, “Cylindrical Vector Beams from mathematical concepts to applications”. Advances in Optics and Photonics, 1(1), 1-57 (2009). Abstract.
[4] Martynas Beresna, Mindaugas Gecevičius, Peter G. Kazansky, Titas Gertus, “Radially polarised optical converter created by femtosecond laser nanostructuring of glass”, Applied Physics Letters, 98, 201101 (2011). Abstract.
[5] Y.Jin, O. J. Allegre, W. Perrie, K. Abrams, J. Ouyang, E. Fearon, S. Edwardson, G.Dearden, “Dynamic modulation of spatially stuctured polarisation fields for real time control of ultrafast laser –material interactions”. Optics Express, 21, 25333 (2013). Abstract.

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