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Abstract
We present the design and characterization of waveguide grating devices that couple visible-wavelength light at λ = 674 nm from single-mode, high index-contrast dielectric waveguides to free-space beams forming micron-scale diffraction-limited spots a designed distance and angle from the grating. With a view to application in spatially-selective optical addressing, and in contrast to previous work on similar devices, deviations from the main Gaussian lobe up to 25 microns from the focus and down to the 5 × 10−6 level in relative intensity are characterized as well; we show that along one dimension the intensity of these weak sidelobes approaches the limit imposed by diffraction from the finite field extent in the grating region. Additionally, we characterize the polarization purity in the focal region, observing at the center of the focus a low impurity <3 × 10−4 in relative intensity. Our approach allows quick, intuitive design of devices with such performance, which may be applied in trapped-ion quantum information processing and generally in any systems requiring optical routing to or from objects 10 s–100 s of microns from a chip surface, but benefitting from the parallelism and density of planar-fabricated dielectric integrated optics.
Highlights
A number of systems may employ integrated waveguiding optics, formed in a planar dielectric layer, that require directing light to objects external to the chip
Along the direction of propagation (y as labeled in Fig. 1), the emitted field profile is tailored via the local grating period (Λ) and duty cycle (DC), which together set the local angle of emission θ and grating strength α
Our observations indicate that these devices can produce beams with a high degree of polarization purity at the center of the focus
Summary
This and the waveguide width wc, together with the standard equations for Gaussian beam propagation give a prediction of the transverse focal height and width As shown below, this constraint on radii serves to reduce the strength of low-intensity sidelobes in the beam profile away from the focus as compared to devices in which this constraint on the radii was not imposed[2]. When the distance from the grating to the focus is larger than a Rayleigh range, curvature constrained as described above results in focuses positioned approximately along the vertical (z) above the start of the taper (with a height set by the longitudinal grating parameters and emission angle) This constraint is not required for focusing action in general, and methods related to those presented e.g. in refs 16 and 19 may be employed to choose curvatures to focus at other locations, though with a tradeoff in sidelobe suppression unless otherwise compensated. Reactive ion etching is performed with CHF3 and O2 gases, followed by PECVD cladding deposition of SiO2 using TEOS precursor
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