An attosecond soft x-ray nanoprobe: new technology for molecular imaging

Abstract

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, somethingthat 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. 2002), production of isolated attosecond pulses (Goulielmakis et al., 2008) and x-ray diffraction microscopy (Sandberg et al., 2008). To date there are three major approaches employed to generate SXRs.
The free electron laser (FEL) such as the one located at DESY, Hamburg in Germany, exploits the interactions of electrons within an alternating magnetic field to produce SXR radiation. Electrons are accelerated up to relativistic speeds before being passed into undulator. The undulator consists of a series of magnets that produce an alternating field that causes the electrons to oscillate and emit SXR radiation. These electrons are then able to interact with the radiation to form micro bunches leading to a significant increase in the SXR intensity. Using this source, researchers have produced some impressive images via holographic (Rosenhahn et al., 2009) and diffraction techniques (Bogan et al., 2008). Due to the properties of the SXR source, the objects that were being imaged were destroyed after only one laser “shot”. This is unfortunate as it places a major limitation on the potential quality and reproducibility of the data.
A second approach is to employ a synchrotron source such as the Diamond light source at the Rutherford Appleton Laboratory in Oxfordshire, UK. Here electrons are accelerated up to relativistic speeds in a linear accelerator, booster synchrotron and storage ring. There are a series of bending magnets within the storage ring that control the electron trajectories and cause them to emit synchrotron radiation. This radiation typically ranges from the infrared (wavelength, λ = 700 nm) to gamma rays (λ = 10^-3 nm), easily encompassing the SXR range of the electromagnetic spectrum. Further arrays of magnets within the storage ring cause the electrons to wiggle in a similar manner to the undulator in a FEL, resulting in a more tuneable and intense light beam. The generality of this source has been demonstrated in recent work investigating the structure of metallic nanowires (Humphrey et al., 2008) and the characterisation of 3D molecular orbitals (Beale et al., 2009). In common with FELs, synchrotrons are multi-user, large-scale facilities whose cost and beam time can be restrictive to many researchers. Fortunately there is a third approach to producing SXRs that is a fraction of the cost and can fit in a standard size laboratory.
This chapter describes the development and implementation of such a source of sub-femtosecond (1 fs = 10^-15 seconds) SXR duration pulses that can be focused down to the nanometre (1 nm = 10^-9 metres) scale. Consequently this source has the potential to reach down in scale in both time and space that are of enormous benefit to a wide range of fields such as engineering, physical and biological sciences, significantly extending upon the generality of traditional x-ray scattering experiments. In contrast to the synchrotron and FEL sources, this source exploits the highly nonlinear interaction between an intense, femtosecond laser field with a gas medium such as argon in order to produce SXR radiation via a process known as laser-driven high harmonic generation.

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Identifiers

ORCID for Jeremy G. Frey: orcid.org/0000-0003-0842-4302

ORCID for W.S. Brocklesby: orcid.org/0000-0002-2123-6712

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