Manipulating Bloch surface waves in 2D: a platform concept-based flat lens

At the end of the 1970s,it was confirmed that dielectric multilayers can sustain Bloch surface waves(BSWs).However,BSWs were not widely studied until more recently.Taking advantage of their high-quality factor,sensing applications have focused on BSWs.Thus far,no work has been performed to manipulate and control the natural surface propagations in terms of defined functions with two-dimensional(2D)components,targeting the realization of a 2D system.In this study,we demonstrate that 2D photonic components can be implemented by coating an in-plane shaped ultrathin(l/15)polymer layer on the dielectric multilayer.The presence of the polymer modifies the local effective refractive index,enabling direct manipulation of the BSW.By locally shaping the geometries of the 2D components,the BSW can be deflected,diffracted,focused and coupled with 2D freedom.Enabling BSW manipulation in 2D,the dielectric multilayer can play a new role as a robust platform for 2D optics,which can pave the way for integration in photonic chips.Multiheterodyne near-field measurements are used to study light propagation through micro-and nano-optical components.We demonstrate that a lens-shaped polymer layer can be considered as a 2D component based on the agreement between near-field measurements and theoretical calculations.Both the focal shift and the resolution of a 2D BSW lens are measured for the first time.The proposed platform enables the design of 2D all-optical integrated systems,which have numerous potential applications,including molecular sensing and photonic circuits.

Inverse photonic design of functional elements that focus Bloch surface waves

Bloch surface waves(BSWs)are sustained at the interface of a suitably designed one-dimensional(1D)dielectric photonic crystal and an ambient material.The elements that control the propagation of BSWs are defined by a spatially structured device layer on top of the 1D photonic crystal that locally changes the effective index of the BSW.An example of such an element is a focusing device that squeezes an incident BSW into a tiny space.However,the ability to focus BSWs is limited since the index contrast achievable with the device layer is usually only on the order ofΔn≈0.1 for practical reasons.Conventional elements,e.g.,discs or triangles,which rely on a photonic nanojet to focus BSWs,operate insufficiently at such a low index contrast.To solve this problem,we utilize an inverse photonic design strategy to attain functional elements that focus BSWs efficiently into spatial domains slightly smaller than half the wavelength.Selected examples of such functional elements are fabricated.Their ability to focus BSWs is experimentally verified by measuring the field distributions with a scanning near-field optical microscope.Our focusing elements are promising ingredients for a future generation of integrated photonic devices that rely on BSWs,e.g.,to carry information,or lab-on-chip devices for specific sensing applications.

Experimental evidence of Bloch surface waves on photonic crystals with thin-film LiNbO_3 as a top layer

Strong nonlinear, electro-optical, and thermo-optical properties of lithium niobate(LN) have gained much attention. However, the implementation of LiNbO_3 in real devices is not a trivial task due to difficulties in manufacturing and handling thin-film LN. In this study, we investigate an optical device where the Bloch surface wave(BSW) propagates on the thin-film LN to unlock its properties. First, access to the LN film from air(or open space) is important to exploit its properties. Second, for sustaining the BSW, one-dimensional photonic crystal(1DPhC) is necessary to be fabricated under the thin-film LN. We consider two material platforms to realize such a device: bulk LN and commercial thin-film LN. Clear reflectance dips observed in far-field measurements demonstrate the propagation of BSWs on top of the LN surface of the designed 1DPhCs.