High-performance geometric phase elements in silica glass

Abstract

High-precision three-dimensional ultrafast laser direct nanostructuring of silica glass resulting in multi-layered space-variant dielectric metasurfaces embedded in volume is demonstrated. Continuous phase profiles of nearly any optical component are achieved solely by the means of geometric phase. Complex designs of half-wave retarders with 90% transmission at 532 nm and >95% transmission at >1 μm, including polarization gratings with efficiency nearing 90% and computer generated holograms with a phase gradient of ∼0.8π rad/μm, were fabricated. A vortex half-wave retarder generating a single beam optical vortex with a tunable orbital angular momentum of up to ±100ℏ is shown. The high damage threshold of silica elements enables the simultaneous optical manipulation of a large number of micro-objects using high-power laser beams. Thus, the continuous control of torque without altering the intensity distribution was implemented in optical trapping demonstration with a total of 5 W average power, which is otherwise impossible with alternate beam shaping devices. In principle, the direct-write technique can be extended to any transparent material that supports laser assisted nanostructuring and can be effectively exploited for the integration of printed optics into multi-functional optoelectronic systems.

Ultralow-loss geometric phase and polarization shaping by ultrafast laser writing in silica glass

“…Femtosecond laser writing [18][19][20][21][22] has emerged as a technologically attractive method for birefringence patterning via the formation of polarization-controlled self-assembled lamella structures (nanogratings, referred as type II modification) in various bulk and thin film transparent materials and silica glass in particular [23][24][25][26][27][28][29][30][31] . GPOEs and vector beam polarization converters (S-waveplates) have been fabricated by polarization control of the nanograting orientation and the related optical axis of the form birefringence 23 .…”

“…In particular, silica glass is an ideal optical material due to its wide transmission window, high chemical and thermal durability and high optical damage threshold 32 . However, a drawback of birefringent modification produced by nanogratings is the reduced transmission, especially in the visible and ultraviolet, which originates from fluctuations of the periodicity in the nanograting structures 24,28 . A higher transmission, of 90% at 532 nm, can be achieved by delivering a higher density of incident pulses per μm of scan path at a low writing speed, which allows the formation of more uniform nanogratings 24,33 .…”

“…However, a drawback of birefringent modification produced by nanogratings is the reduced transmission, especially in the visible and ultraviolet, which originates from fluctuations of the periodicity in the nanograting structures 24,28 . A higher transmission, of 90% at 532 nm, can be achieved by delivering a higher density of incident pulses per μm of scan path at a low writing speed, which allows the formation of more uniform nanogratings 24,33 . However, the losses are still significantly high compared with conventional optical elements due to the scattering loss, especially in the UV region.…”

Embedded nanograting-based waveplates for polarization control in integrated photonic circuits

“…However, we would like to stress that no efforts were made to reduce the losses. Other publications have shown that losses induced by nanogratings can be limited to below 0.5 dB [48].…”

Geometric phase circular gratings fabricated by ultrafast laser nanostructuring for symmetric simultaneous spatio-temporal focusing

“…Such a periodic assembly behaves as a uniaxial birefringent material with optical axis oriented parallel to the polarization of incident laser beam, and serves as a perfect candidate for designing geometric phase profiles of nearly any optical components. The performance of fabricated GPOE varies depending on the processing conditions reaching the efficiency and transmittance higher than 90%, as well as the phase gradient higher than  rad/µm [4]. The silica glass based optics with a damage threshold of 22.8 J/cm 2 demonstrate the potential of high-power applications.…”