Experimental high numerical aperture focusing with high contrast gratings

Area of Science:

• Optics and Photonics
• Micro-optics
• Diffraction Engineering

Background:

• Achieving high numerical aperture (NA) for three-dimensional (3D) focusing in free space is critical for micro-optical applications.
• Traditional optical elements face limitations in achieving the desired NA and precise focusing control at the microscale.

Purpose of the Study:

• To demonstrate high NA (up to NA~0.64) 3D focusing in free space using wavefront-engineered diffraction gratings.
• To tailor the optical response of grating lenses for micro-optical applications by controlling focal lengths around 100 μm.

Main Methods:

• Fabrication of diffraction gratings with spatially varying ridge and groove widths in two dimensions.
• Design of a phase profile incorporating multiple 2π phase jumps using an optimization algorithm for phase and amplitude.
• Experimental characterization of the focusing performance and spot size.

Main Results:

• Demonstration of high NA focusing capabilities (up to NA~0.64) in free space.
• Achievement of focal lengths in the order of 100 μm, suitable for micro-optical applications.
• Experimental measurement of a lateral spot size of 5 μm, closely matching the theoretical Airy disk size.

Conclusions:

• Wavefront-engineered diffraction gratings offer a viable solution for high NA 3D focusing in micro-optics.
• The developed grating lenses provide precise control over focal length and spot size.
• This technology has significant potential for advancements in micro-optical systems and applications.