Soft-X Lab on Chip

Motivations. Soft X ray sources generating ultrashort (100 as – 10 fs) pulses of radiation in the 10 eV – 500 eV spectral range are particularly interesting for applications to atomic, molecular and solid-state physics. Such sources enable the time-resolved spectroscopy of matter with extreme temporal resolution and, at the same time, with chemical sensitivity since different elements are characterized by different soft X absorption edges that can be selected in the spectral domain.

Laboratory-scale soft X ultrafast sources are based on High-order Harmonic Generation (HHG), an extremely nonlinear process occurring when an intense and ultrashort IR laser pulse is focused in gaseous media. Unfortunately, HHG is a very inefficient process; furthermore, beamlines exploiting this radiation extend over several meters since grazing-incidence optics are required to handle soft-X beams. These drawbacks hinder HHG-based applications.

Thesis goal. This thesis work aims at the realization of a miniaturized efficient soft X lab on chip for matter spectroscopy realized by Femtosecond Laser Micromachining, in order to overcome the limitations of present HHG technology.

Methods. A prototype of miniaturized HHG source is shown in the figure; it consists of a hollow optical waveguide realized in a fused silica chip. The IR laser beam (represented by a white arrow) is coupled to the waveguide; a gas, in which HHG is driven by the laser pulse, is introduced in the waveguide by smaller distribution channels that can be seen in the lower panel.

HHG spectra generated in helium inside the chip (blue line) are compared in the figure with spectra produced in a He jet expanding in vacuum (orange line, 10 times magnified), which is the standard generation technique.  The chip source outperforms the standard jet source both in terms of spectral extension (>200 eV versus 110 eV) and in terms of efficiency.     

Thesis activities and perspectives. The thesis work will be developed in the framework of a collaboration with IFN-CNR (prof. Roberto Osellame and dr. Rebeca Martinez Vazquez) which fabricate the microstructured devices and with the Department of Aerospace Science and Technology (prof. Aldo Frezzotti) which provides fluid dynamics modelling of the gas medium injected inside the chip; a fluid dynamics model of the chip prototype is shown in the figure. 

The thesis activities are experimental and concern (i) novel chip designs integrating additional functionalities inside a monolithic slab (IR radiation rejection, fluid sample interaction region etc.) and pushing forward the performances (spectral extension towards 500 eV, larger HHG efficiency by quasi-phase-matching strategies);

(ii) chip characterization using IR (800 nm) and mid-IR (1450-1700 nm) laser pulses;

(iii) applications to on-chip soft X spectroscopy in fluids.  

Involvement in fluid dynamics modelling is also available.