High efficiency focusing double-etched SiN grating coupler for trapped ion qubit manipulation

Abstract

A one-dimensional focusing grating coupler array based in silicon nitride (SiN) was proposed for trapped ion qubit manipulation. By applying inverse design optimization with a double-etched grating structure, a directionality of 98% was achieved. A small beam diameter of 2.5 µm on the target ion with a low crosstalk of -36 dB was attained. Additionally, the impact of fabrication errors was investigated through a Monte Carlo simulation; within the accuracy of an electron beam lithography-based process, the output efficiency was maintained at 93%

Grating Design Methodology for Tailored Free-Space Beam-Forming

“…Rather than a strict choice, however, the methodology can be straightforwardly adapted to integrate existing optimization techniques. The clear partition between longitudinal and transverse design allows, for example, applying optimizationbased methods for the 2D longitudinal structure [25], [58], [24], before extruding to 3D following the transverse design procedure of this work. The direct longitudinal design presented in this work enables accurate design with flexibility and speed, but lacks direct control of possible target metrics, such as efficiency or side lobe reduction, which may be of primary interest in other applications.…”

“…5d) which will be a utilized later on to track the field's phase in the grating region. It would be straightforward to extend this same method to more complex grating structures such as those including multiple layers or etch depths [25].…”

“…Also commonplace are approximations for the effective index in the grating, assuming it either remains constant despite longitudinal apodization [17], [19] or follows a weighted average of the core/cladding indices [20]. Inverse design and topology optimization have been applied to grating outcoupler design [21], [22], [23], [24], [25], but computational requirements have limited applicability to 2D or simplified, small-area 3D structures.…”

Optical engineering for trapped ion quantum computing

“…To this end, we developed a high-efficiency focusing grating coupler. 13 A two-layer etched grating structure, combined with inverse design optimization provides a beam diameter of 2.5 µm which provides a crosstalk of -36dB. We are also investigating methods to lithographically align focusing optics with the trapping electrodes to improve both light collection and excitation.…”