Increasing the brightness of harmonic XUV radiation with spatially-tailored driver beams

Abstract

Bright high harmonic sources can be produced by loosely focussing high peak power laser pulses to exploit the quadratic scaling of flux with driver spot size at the expense of a larger experimental footprint. Here, we present a method for increasing the brightness of a harmonic source (while maintaining a compact experimental geometry) by spatially shaping the transverse focal intensity distribution of a driving laser from a Gaussian to supergaussian. Using a phase-only spatial light modulator we increase the size and order of the supergaussian focal profiles, thereby increasing the number of harmonic emitters more efficiently than possible with Gaussian beams. This provides the benefits of a loose focussing geometry, yielding a five-fold increase in harmonic brightness, whilst maintaining a constant experimental footprint. This technique can readily be applied to existing high harmonic systems, opening new opportunities for applications requiring bright, compact sources of coherent short wavelength radiation.

Adaptive shaping of laser beams for high-harmonic generation applications

“…Treacher et al (2020) demonstrated that an SLM can be used to create super-Gaussian spatial profiles to drive and improve the brightness of single harmonics [50]. The technique used imprinted a π-phase shift transversely onto a collimated Gaussian beam.…”

“…where a single phase step caused the centre and periphery of the beam to interfere at the focus, changing the transverse intensity profile. Further details on the π-shift shaping method for HHG can be found in references [48,50].…”

“…In chapter 5 beam shaping simulations are performed with a phase-only spatial light modulator (SLM). The performance of a learning algorithm to control the SLM is compared with a known π-shift method for super-Gaussian generation [49,50].…”

“…where I IR (r) is the tansverse intensity profile of the laser, q eff is the effective nonlinearity, q is the harmonic order, φ IR (r) is the phase from the fundamental beam, and φ atom (r) is the atomic dipole phase [48,50,75,170,171]. The atomic dipole phase is given by,…”

“…Simulations have been performed using the SDM and SFA to investigate the effects of using a driver laser with transverse intensity profiles of increasing super-Gaussian order. The practical benefit of super-Gaussian shaping using adaptive optics to lower divergence is the ability to irradiate smaller volumes of interest, and increase brightness [50]. Super-Gaussians drivers are also more efficient for HHG as there is more energy in the central region of the beam where the intensity is above the threshold for generating high harmonics.…”