Laser synthesis for strong field physics

• Main Author: Arnold, Martin Oliver
• Other Authors: Zair, Amelle ; Marangos, Jon
• Published: Imperial College London 2017
• Subjects: 530
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.733219

The work of this thesis involves temporal and spatial synthesis of the laser beam for use in strong field applications, such as high harmonic generation. The results include simulations of the electron trajectories with a temporally synthesised electric field, particularly the sub cycle laser field. The synthesis is produced by mixing of single colour fields and varying the phase between the two pulses. This showed an extension of the high harmonic cutoff from the typical 3.17UP to 5.05UP, and is due to the longer time the electron spends in the continuum due to the subcycle temporal synthesis. A temporal synthesis of the pulse envelope was carried out using a birefringent plate and cube polariser. A pair of birefringent wedges at 45 degrees relative to the laser polarisation was used to vary the phase between the two generated pulses. The spectrum of the laser is shown to shift depending on the relative phase between the two pulses. Subsequent simulations of this effect is presented and compared to the experimental results of which there is agreement in the periodicity of the spectral shift and magnitude. High harmonic generation was also measured and studied to show the harmonic frequency shifting proportionally to the laser depending on the harmonic order. Spatial synthesis of the laser field was studied and also used for generation of high harmonics. A pair of concentric phase plates was used to generate a spatial flat top intensity profile in the focus of the laser beam. The flat top was found to extend plus/minus 1mm either side of the focus of 100 um. The beam waist at the focus increased from 45 um to 100 um during this synthesis, causing a drop in intensity such that harmonics were very hard to achieve.