Subwavelength Focusing of Bloch Surface Waves

Abstract

Micro-sized spheres can focus light into subwavelength spatial domains: a phenomena called photonic nanojet. Even though well studied in three-dimensional (3D) configurations, only a few attempts have been reported to observe similar phenomena in twodimensional (2D) systems. This, however, is important to take advantage of photonic nanojets in integrated optical systems. Usually, surface plasmon polaritons are suggested for this purpose, but they suffer notoriously from the rather low propagation lengths due to intrinsic absorption. Here, we solve this problem and explore, theoretically, numerically, and experimentally, the use of Bloch surface waves sustained by a suitably structured all-dielectric media to enable subwavelength focusing in an integrated planar optical system. Since only a low index contrast can be achieved while relying on Bloch surface waves, we perceive a new functional element that allows a tight focusing and the observation of a photonic nanojet on top of the surface. We experimentally demonstrate a spot size of 0.66λ in the effective medium. Our approach paves the way to 2D alldielectric photonic chips for nano-particle manipulation in fluidic devices and sensing applications.

Stepwise Luneburg Lens for Bloch Surface Waves

“…Investigations are carried out exclusively using a 2D FDTD simulation (CST microwave studio, at λ = 1555 nm). In our previous work [14], the validity of 2D numerical simulations was confirmed by showing a perfect agreement with the results of 3D simulations and scanning near-field optical microscopy measurements. Since the large number of height steps will be a time-consuming and challenging task, we will first determine the smallest number of index steps for optimal performances.…”

“…Therefore, the lower refractive power loosens the focusing effect, and eventually pushes the focal spot position away from the shadow-side curvature of the lens, as shown in Figure 7, where the conventional circular geometry Luneburg lenses with the ELS N = 5 and EIS N = 4 are simulated. This lower ∆n can be compensated for by increasing the optical path length of the component [14]. To do this, we elongate the circular geometry to a prolate ellipse.…”

Optical Effects Induced by Bloch Surface Waves in One-Dimensional Photonic Crystals

“…In several further works, the possibilities of BSW guiding using diffraction gratings of different geometries and visualization of the BSW by leakage radiation microscopy techniques are shown [18][19][20]. The prospect of the BSWs in photonic crystals for the development of flat optics elements was recently demonstrated in two-dimensional lens [21] and planar triangles [7].…”

“…The all-dielectric structure of photonic crystals leads to low Ohmic losses, BSW propagation lengths up to millimeters [5] and high Q-factor of the BSW resonances. The dispersion law of the BSWs can be chosen in wide spectral range from UV to IR by changing the photonic crystal parameters [6,7]. High sensitivity of the BSWs to boundary conditions provides the possibility to use them as bio-and gas sensors [8][9][10][11][12][13].…”

Bloch Surface Waves for MoS2 Emission Coupling and Polariton Systems

“…Thanks to these properties, BSWs are naturally suited for label-free, fluorescence sensing applications, and, in this context, they have widely expressed their capability in the last decades [1][2][3][4][5][6][7]. Moreover, recent papers have investigated their potentialities for on-chip guiding and manipulation of an optical signal propagating on a 1D photonic crystal [8][9][10][11][12][13][14]. Several studies have followed the lead of the first seminal paper on Bloch surface wave guided modes [8], engineering the top layer of the distributed Bragg reflector (DBR) stack to achieve optical manipulation of the in-plane propagating BSW.…”