Direct Laser Writing of 3-D Diffraction Gratings and Diffraction Optics

Integrated diffractive optics has many applications in beam shaping and control on the micro-scale. Fabrication using lithography is limited to planar or layered geometries. We demonstrate fabrication of diffractive elements via direct laser writing. We have tested 3D diffraction gratings and zone plates designed for operation at visible wavelengths. Direct laser writing is a promising technique to fabricate integrated 3D and multi-layer diffraction optics. We have previously developed a laser writing technique that enables fabrication of disconnected metal structures in a polymer matrix, which we now apply to 3D diffraction optics. Ultrafast laser pulses centered at 795 nm are focused into a thick (100-250 micrometer) polymer film doped with silver nitrate. Multi-photon absorption prompts reduction of metal ions and formation of silver structures at the focal point. Using a long-travel, high precision 3D translation stage and a 0.8-NA, long working distance objective, we have demonstrated feature sizes below 100 nm and resolution of 0.5 micrometers on samples spanning several millimeters. Features can be positioned freely in the horizontal and z-directions and we have produced structures with over 10 layers in the z-direction. The polymer is left in place after fabrication, providing a dielectric matrix transparent in the visible and near infrared wavelengths for the disconnected structures. We fabricate 3D gratings which are analogs of crystallographic structures (simple and body-centered cubic and structures with a two-atom basis with different scattering strengths). Scatterers are spaced by 5-40 micrometers in order to observe multiple diffraction orders using visible wavelength illumination. Samples span several millimeters in plane and consisted of 2-12 layers in the z-direction. Diffraction patterns measured in transmission using a 633-nm HeNe laser are in good agreement with calculated Laue diffraction patterns. Zone plates with focal lengths ranging from 4 to 50 micrometers at 633 nm are fabricated. Devices are tested in a transmission microscope using a white LED light source and focal spots for different wavelengths are imaged by adjusting the microscope focal plane. Wavelength selectivity using pairs of laser-written zone plates and pinholes has been observed, showing that multi-layer structures with flexible spacing in the z-direction can be successfully fabricated.