Abstract
In order to enlarge the capability for in-plane manipulation of the Bloch surface wave (BSW), we investigate 2D gradient index (GRIN) optical components using a finite-difference time-domain (FDTD) numerical method. To ease difficulties in fabrication to acquire a continuous index profile of GRIN optical components, we propose a stepwise index profile. For 2D surface wave devices, such discrete index steps can be achieved by stepwise structuring of the top layer, also called the device layer. For the demonstration of the stepwise GRIN optics concept, we consider a Luneburg lens, which is a good example of the GRIN optical component that produces a strong focal spot on the shadow-side curvature of the lens. The limited index contrast of the BSW systems loosens the confinement of the focal spot. A mitigation plan is to elongate the circular geometry to the prolate ellipse. BSW-based Luneburg lenses with a relatively small number of steps and an elliptical geometry are demonstrated with comparable performances to a standard Luneburg lens.
Highlights
Propagating electromagnetic surface waves are a key technological element for many near-field and nano-photonics devices
We have investigated the gradient index (GRIN) optics components on the Bloch surface wave (BSW) devices using a 2D finite-difference time-domain (FDTD)
Ourapproach approachisistotouse usea astepwise stepwiseindex indexprofile profileinstead instead continuous gradient index profile, which is difficult to achieve in fabrication
Summary
Propagating electromagnetic surface waves are a key technological element for many near-field and nano-photonics devices. The surface topology of the lens corresponds to the continuous and stepwise index profiles, respectively, where the surface wave propagates along the z-axis towards the lenses. In the EIS scheme, the step widthscheme at the rim of index the element gradually narrower, which reflects the steeper slope at the rim of the index profile This approach resembles the multitopology of the device;. Scheme and (d) the corresponding surface topology reflects the steeper slope at the rim of the index profile This approach resembles the multi-level level phase encoding of diffractive microlenses [16], the operation mechanism is different. The focal spot starts to show noticeable degradation large, e.g., N > 6, the response of the lens shows no difference form the case of the continuous index from N = 4, see Figure 3a.
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