Sarah L. Stebbings

Single-pass high-harmonic generation at 20.8 MHz repetition rate

We report on single-pass high-harmonic generation (HHG) with amplified driving laser pulses at a repetition rate of 20.8 MHz. An Yb:YAG Innoslab amplifier system provides 35 fs pulses with 20 W average power at 1030 nm after external pulse compression. Following tight focusing into a xenon gas jet, we observe the generation of high-harmonic radiation of up to the seventeenth order. Our results show that state-of-the-art amplifier systems have become a promising alternative to cavity-assisted HHG for applications that require high repetition rates, such as frequency comb spectroscopy in the extreme UV.

Generation of isolated attosecond extreme ultraviolet pulses employing nanoplasmonic field enhancement: optimization of coupled ellipsoids

The production of extreme ultraviolet (XUV) radiation via nanoplasmonic field-enhanced high-harmonic generation (HHG) in gold nanostructures at MHz repetition rates is investigated theoretically in this paper. Analytical and numerical calculations are employed and compared in order to determine the plasmonic fields in gold ellipsoidal nanoparticles. The comparison indicates that numerical calculations can accurately predict the field enhancement and plasmonic decay, but may encounter difficulties when attempting to predict the oscillatory behavior of the plasmonic field. Numerical calculations for coupled symmetric and asymmetric ellipsoids for different carrier-envelope phases (CEPs) of the driving laser field are combined with time-dependent Schrodinger equation simulations to predict the resulting HHG spectra. The studies reveal that the plasmonic field oscillations, which are controlled by the CEP of the driving laser field, play a more important role than the nanostructure configuration in finding the optimal conditions for the generation of isolated attosecond XUV pulses via nanoplasmonic field enhancement.

High-harmonic and single attosecond pulse generation using plasmonic field enhancement in ordered arrays of gold nanoparticles with chirped laser pulses

Coherent XUV sources, which may operate at MHz repetition rate, could find applications in high-precision spectroscopy and for spatio-time-resolved measurements of collective electron dynamics on nanostructured surfaces. We theoretically investigate utilizing the enhanced plasmonic fields in an ordered array of gold nanoparticles for the generation of high-harmonic, extreme-ultraviolet (XUV) radiation. By optimization of the chirp of ultrashort laser pulses incident on the array, our simulations indicate a potential route towards the temporal shaping of the plasmonic near-field and, in turn, the generation of single attosecond pulses. The inherent effects of inhomogeneity of the local fields on the high-harmonic generation are analyzed and discussed. While taking the inhomogeneity into account does not affect the optimal chirp for the generation of a single attosecond pulse, the cut-off energy of the high-harmonic spectrum is enhanced by about a factor of two.

Optimization and characterization of a highly-efficient diffraction nanograting for MHz XUV pulses.

We designed, fabricated and characterized a nano-periodical highly-efficient blazed grating for extreme-ultraviolet (XUV) radiation. The grating was optimized by the rigorous coupled-wave analysis method (RCWA) and milled into the top layer of a highly-reflective mirror for IR light. The XUV diffraction efficiency was determined to be around 20% in the range from 35.5 to 79.2 nm. The effects of the nanograting on the reflectivity of the IR light and non-linear effects introduced by the nanograting have been measured and are discussed.

Spatiotemporal phase-matching in capillary high-harmonic generation

We present a simple phase-matching model that takes into account the full spatiotemporal nature of capillary high-harmonic generation. Spectra predicted from the model are compared to experimental results for a number of gases and are shown to reproduce the spectral envelope of experimentally generated harmonics. The model demonstrates that an ionization-induced phase mismatch is limiting the energy of the generated harmonics in current capillary high-harmonic generation experiments. The success of this model shows that phase-matching processes play a dominant role in determining the emission from capillary high-harmonic generation.

Probing ultrafast nano-localized plasmonic fields via XUV light generation

We present a systematic investigation into the conditions required for the production of XUV light via nanoplasmonic enhanced high harmonic generation in metallic spheroids. Control over the temporal response of the plasmonic fields, and therefore the resulting XUV radiation, is achieved through the nanostructure configuration and the carrier envelope phase of the driving laser pulse. Coupled symmetric structures are shown to produce sufficient localized field enhancement and relatively long exponential plasmon decay times leading to the characteristic high harmonic spectra. In contrast coupled asymmetric structures have a much broader resonance and a highly non-uniform plasmon response in the temporal domain.

An Attosecond Soft x-ray Nanoprobe: New Technology for Molecular Imaging

The ability to image matter on the microscopic scale is of fundamental importance to many areas of research and development including pharmacology, materials science and nanotechnology. Owing to its generality, x-ray scattering is one of the most powerful tools available for structural studies. The major limitation however is the necessity of producing suitable crystalline structures – this technique relies upon many x-ray photons being scattered from a large number of molecules with identical orientations. As it is neither possible nor desirable to crystallise every molecule of interest, this has provided a huge drawback for most biotechnologies. Although improvements in both sources and detectors have had a strong impact in this area, driving down the required sample size, the need for macroscopic crystalline samples remains a fundamental bottleneck. Fortunately recent technological developments in the generation and sub-micron focusing of soft x-rays (SXRs) have provided a route for bypassing the need for a regular, crystalline structure. For the purposes of this chapter, SXRs are defined as electromagnetic radiation with wavelengths from 1 – 50 nm, which correspond to photon energies of 1.2 keV – 25 eV respectively. As their wavelengths are on a comparable scale to objects such as proteins, cells and quantum dots, SXRs are ideally suited for imaging these targets with a high spatial resolution. Furthermore water is transparent and carbon opaque to SXRs whose wavelengths lie between 2 – 4 nm, the so-called water window. This offers clear potential for the imaging of biological molecules within their native, aqueous environment, something that would be impossible using traditional x-ray crystallography experiments. Unsurprisingly there has been great interest in the production and application of SXRs across a wide range of scientific endeavours including, but not limited to, resolving electron motion (Drescher et al