Optical and aerodynamic focusing of isolated particles for diffractive imaging experiments at X-ray free electron lasers

The highly brilliant, short and coherent pulses produced by X-ray free-electron lasers (XFELs) opened up a unique opportunity to image small objects in nature, such as protein macromolecules, clusters, viruses and nanoparticles with nearly atomic spatial resolution, and their dynamic processes can be probed with femtosecond temporal resolution. Single-particle imaging (SPI) is a coherent diffraction imaging technique at XFELs that consists of directing a stream of randomly oriented isolated bio-particles across the focus of the X-ray beam to construct high-resolution 3D structure from diffraction patterns of multiple identical particles. Since the intense X-ray pulses destroy every particle upon interaction, a constant replenishment of sample is required. However, efficiently and successively placing of the biological single particle, which are typically few tens of nanometer in size, onto the few hundred nanometer X-ray beam still remains a considerable challenge. The main objective of this dissertation is to demonstrate an alternative aerosol focusing mechanism that efficiently and precisely delivers isolated particles into the focus of the XFEL beam, intended primary for SPI experiments. To this end, two complementary techniques were studied. The first method dealt with aerodynamic focusing of aerosol particles using an improved aerosol injector design, which is based on a simple convergent nozzle geometry. Through high-speed optical imaging, it was shown that 300 nm virus particles can be focused down to a 3.5 μm FWHM spot, which is considerably smaller than the existing aerosol injectors can achieve. The use of this injector for SPI experiment was demonstrated in a micro-focused soft-X-ray FEL beam at the FLASH facility in Hamburg, delivering virus particles, and a hit fraction > 18 % was recorded. Furthermore, delivering macromolecular nanocrystals of Cydia pomonella granulosis virus (CpGV), in serial femtosecond crystallography (SFX) experiments at the LCLS CXI hard X-ray instruments, extremely low background Bragg diffraction has obtained. The second part of the dissertation reports, for the first time, the confining and concentrating of aerodynamically delivered high-speed particle beam using a structured laser illumination. In this proof-of-concept experiment, an aerosol beam from a capillary aerosol injector was delivered into vacuum against a counterpropagating laser beam. The beam was constructed with Laguerre-Gaussian or a slowly diverging Quasi-Bessel beam that has a dark core in the middle. We call such beam an optical funnel. The optical force, governed by the spatial profile of the optical funnel, axially decelerate and lateral confine the particles to guide them into a convergent trajectories. Using this technique, a CpGV particle beam moving with an axial velocity of 17.4 ± 0.93 m/s, was transversely confined by a factor of two from its original width and its central peak particle density was increased by more than four times from the reference (laser-off) condition. Prospectively, optical guiding combined with the aerodynamic focusing may further advance single particle imaging experiments, by solving the principal problem of the precise and efficient delivery of isolated single particles to the sub-micron X-ray focus.