Overcoming limits of pulse compression

The invention of chirped pulse amplification (CPA), awarded the physics Noble prize in 2018, has triggered a revolution in high-intensity laser development, resulting in an uninterrupted growth of achieved peak laser intensities for three decades. In turn, the availability of intense driver pulses has enabled the emergence of many new laser-driven applications, such as strong-field photo- dissociation and non-thermal phase transitions, charged and neutral particle acceleration, secondary radiation emission from THz to X-rays and gamma-rays and further prospective relativistic light matter interactions. Whereas the quest for increasing the intensity and peak power of laser pulses continues unabated, steady progress in the development of the CPA approach through improvement of laser materials and geometries only solves the challenges of scaling the pulse energy and average power of the laser source. To reduce the duration of the temporal pulse envelope and thus push up the peak power of the laser, one needs to broaden pulse bandwidths beyond those supported by the laser gain materials by use of external pulse compression outside the laser. Very recently, the Taiwanese Partner in this bilateral project has pioneered a ground-breaking multi- plate compression technique (MPC) that led to the generation of nearly single-cycle near-IR pulses and provided a solution to the beam and material break-up problem inevitably occurring if a conventional continuous single piece of bulk material is used. In a complementary effort in the mid-IR spectral range, the Austrian Partner has similarly shown the possibility to exceed the critical power of self-focusing in an anomalously dispersive bulk by many orders of magnitude whilst self-compressing the pulse to reach sub-terawatt peak power and preserving spatial beam coherence. The core idea of the proposed project is to join the knowhow and the infrastructure capabilities of the Taiwanese and Austrian teams with the aim of breaking the fundamental limits of pulse compression. As part of this effort, the teams will develop and demonstrate new prospective driver sources for strong field applications. One of the main challenges faced by any type of external compression geometry, waveguide or bulk, is the rapid, typically quadratic scaling of the setup size with the linear increase of the pulse energy. Another fundamental problem is the need to prevent beam breakup into uncontrolled multiple filaments. To meet these challenges, the partner teams will explore several strategies, based on compression stage cascading and 1-dimensional beam reshaping, working toward an extremely compact and stable pulse compression setup. Underway, pulse post-compression scenarios and nonlinear self-compression scenarios will be explored in both laboratories using a range of input pulse wavelengths, durations and energies.

The invention of chirped pulse amplification (CPA), awarded the physics Noble prize in 2018, has triggered a revolution in high-intensity laser development, resulting in an uninterrupted growth of achieved peak laser intensities for three decades. In turn, the availability of intense driver pulses has enabled the emergence of many new laser-driven applications, such as strong-field photo- dissociation and non-thermal phase transitions, particle acceleration, secondary radiation emission from THz to X-rays and gamma-rays and further prospective relativistic light matter interactions. To reduce the duration of the temporal pulse envelope and increase the laser peak power, one needs to broaden pulse bandwidth by use of external pulse compression outside the laser. Typically, such schemes rely self-phase modulation (SPM) as a process for spectral broadening in a bulk or gas nonlinear medium and are implemented either in a waveguide or in a free-space format, where the laser beam is repeatedly focused in or near the SPM medium. Within the completed FWF project, the TU Wien team and international partners have addressed the main challenges facing the extension of the SPM-based schemes to higher laser pulse energies. Conventionally, the size of the external compressor grows quadratically with the pulse energy to maintain a safe balance between optics damage, ionization losses and a suitable level of beam self-focusing. The TU Wien has pursued two strategies toward solving the scaling challenge. The first approach to obviate intensity limits was based on one-dimensional spatial and temporal scaling. The second approach included changing the pulse format from a single high-energy pulse to a short packet of pulses, a THz-frequency pulse burst, where the total energy was divided among 2-40 lower-intensity pulses. The research highlights of the completed project include the invention of a variant of the multi-pass gas cell compressor in which the one-dimensional beam scaling on the cell mirrors is achieved by dispersive spatial chirping of the input beam performed inside a modified grating-pair compressor of the CPA laser source. As a result, the separation between the curved mirror can remain fixed whereas the larger spot size on the mirror permits pulse energy scaling by an arbitrary factor. Another noteworthy insight, that has emerged from the in-depth study of the THz burst format, is that spectral interference features emerging in a highly regular periodic pulse burst survive nonlinear-optical spectral transformations (SPM and frequency conversion) with high fidelity. This finding has led to the proposal for a novel femtosecond pulse scheme for stimulated Raman scattering on molecular gases that is superior to a conventional approach based on two narrowband tunable pulses. The numerical feasibility proof assessment of signal levels and spectral resolution were performed and published within this project and serve as a groundwork for future experimental investigation.