RESEARCH PROJECTS
Focal volume Control Using Structured Illumination Sources "FOCUSIS"
The resolution of optical imaging instruments is fundamentally limited by the diffraction of light. Increasing the numerical aperture, decreasing the imaging wavelengths or decreasing the focal volume dimensions could help optimise spatial resolution. Near-field optical methods could be suitable for controlling the focal volume dimensions as they confine the illuminating light to smaller dimensions.
Funded by the Marie Skłodowska-Curie programme, the FOCUSIS project will investigate methods for modifying and controlling the focal volume dimensions in an optical system combining the near-field optical phenomena induced by different illumination methods. The project will conduct experiments and simulations for further understanding of the propagation of shaped beams in a particular optical system.
Principal Investigator: Camilo Florian Baron
Duration: 2020-2023
Total budget: € 245,732.16
One of the challenges in materials processing with lasers is the modification of materials with highest spatial resolution. In this quest, the role of optics is to provide new methods to increase the resolution close to or beyond the natural diffraction limit of the optical system. In most laser-based micromachining techniques, the spatial resolution is increased by using high numerical aperture lenses in combination with short laser wavelengths, effectively shrinking the focal volume and thus the minimum feature size that can be inscribed. An alternative approach is based on near-field optical methods due to the extraordinary properties of the optical near field at extremely short scales, where the field is evanescent rather than propagating, allowing it to be concentrated to smaller volumes. Moreover, beam shaping techniques are powerful complementary strategies that offer new control parameters to define how light propagates and ultimately interacts with matter. In order to meet the challenge of processing materials with highest spatial resolution, the project FOCUSIS developed and applied a specific optical system composed by dielectric microspheres as focusing lenses placed on the material combined with advanced beam shaping employing a spatial light modulator. The goal was to shape femtosecond laser pulses incident onto the microspheres in order to achieve maximum resolution as well as axial and lateral positioning control of the modifications inscribed.
The overall objective required a complete control and characterization of the laser pulses implemented, in order to be able to modify and optimize the intensity distribution at user defined locations. Importantly, for the FOCUSIS project, user defined locations mean also axial positioning. This feature should be highlighted, since in this project we demonstrate that it is possible to modify how and where the maximum intensity is located by means of phase control over the incident laser beam, without requiring mechanical movement of the optical elements or the irradiated sample. This achievement has the potential to contribute to a resolution increase of well-established ultrahigh resolution imaging and marking techniques. At the same time, the advantage of allowing lateral and axial positioning control can be exploited in near-field point scanning microscopy techniques.
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In short, the findings obtained from the project FOCUSIS offer new approaches for controlling how light can be shaped to alter matter at will and benefit areas of science and applications including optics, biology, medicine, and materials science, to name a few.
The figure compiles a general overview of the main experiment in the project FOCUSIS. A phase modified femtosecond beam is focused through a dielectric microsphere. The final intensity maxima and its lateral and axial position can be defined by the implemented light phase, as confirmed by experiments and simulations. The scale bar on the right: 1 µm.
The project started in 2020 at Princeton University, in Princeton, New Jersey, USA, under the supervision of Prof. Craig B. Arnold. In his laboratory, experiments implementing optically trapped dielectric microspheres for the near-field irradiation of materials took place. Then, a secondment period at FEMTO-St, in Besançon, France under the supervision of Dr. François Courvoisier. Here, different microlens fabrication methods were developed, consisting in the selective melting of glass fibers. Importantly, experience with the use of Spatial Light Modulators was acquired during this phase. The final incoming phase at the home institution, starting in 2022, the Spanish National Research Council, in Madrid, Spain, under the supervision of Prof. Jan Siegel. It was here where the culmination of the project took place, allowing the demonstration of actual lateral and axial control in an optical system based on a spatial light modulator and a microsphere as last focusing element.
The project outcome includes 7 peer-reviewed publications, 1book for scientific dissemination, 2 book chapter contributions and 1 US patent application. Moreover, 20 scientific contributions were presented at different international conferences in the field of laser materials processing, optics and photonics. 2 of these contributions were invited talks and 1 prize was received for best paper. Strong emphasis was put on dissemination activities to non-scientific audiences in form of 4 seminars and participation in 3 outreach actions in USA, France and Spain.
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