Stepwise Luneburg Lens for Bloch Surface Waves

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

Axial phase measurements of light interacting with microstructures

We present an experimental method to study field structures of highly confined light after interaction with microstructures. A high-resolution interference microscope (HRIM) allows us to measure the three-dimensional (3D) amplitude and phase distributions of light emerging from the sample. While the amplitude fields represent conventional pictures of light confinements like a hotspot, the phase fields exhibit peculiar behaviors, which are of significant interest. Longitudinal-differential interferometry can directly visualize and quantify phase deviations in 3D space with respect to a plane wave of the same frequency serving as a reference. The phase fields near the confinement exhibits particular phase features, e.g., axial phase anomaly and superluminal phase velocity. As example of the light interaction with microstructures, two specific optical phenomena have been investigated here: Gouy phase anomaly in the photonic nanojet and superluminal phase propagation of the spot of Arago. For the first time, we could experimentally demonstrate high-resolution axial phase measurements of such phenomena generated by microstructures of wavelengthscale size and at visible light with 642-nm wavelength

Direct Visualization of the Axial Phase Evolution of Light Fields Emerging from Microstructures

We investigate the axial phase evolution of light emerging from microstructures. The high-resolution interference microscope (HRIM) allows to record three-dimensional (3D) phase distributions in differential and propagation modes along the longitudinal direction. We apply this differential-mode HRIM to study the axial phase evolution of particular cases of microstructures, for instance, the photonic nanojet generated by a diecectric microsphere and the spot of Arago created by a micrometer-size metallic dis

Near-field characterization of 2D disk resonator on Bloch surface wave platform

We experimentally and theoretically investigate the characteristics of a two-dimensional (2D) disk resonator, which is fabricated on a Bloch surface wave platform. Such a platform is exploited to manipulate the surface waves by patterning nano-thin 2D optical components on the to

Photonic Nanojet engineering: Focal point shaping with scattering phenomena of dielectric microspheres

We experimentally engineer Nanojets produced by dielectric spheres by varying the illumination and observe the effect with a high-resolution interference microscope (HRIM). Converging and diverging spherical wavefronts and Bessel- Gauss beams are considered. We find that the diverging wavefront pushes Nanojets away from the surface of the sphere without change of the spot size. This allows earning several micrometers of working distance contrary to the Nanojet confined at the sphere’s surface. When the radius of curvature of the incident wavefront is greater than about 5 times the sphere size, the Nanojet moves back to the sphere surface like it is found for plane wave incidence. On-axis Bessel- Gauss beam illumination with the central lobe covering the whole sphere leads to the same results as the plane wave case. Off-axis Bessel beam illumination can generate multiple-spot Nanojets. We demonstrate the separation of such spots of about 220 nm at 642 nm. This separation is smaller than the feature sizes defined by the diffraction limit at this wavelength. We discuss briefly applications of engineered Nanojets for nano-lithography and near-field sensing

Engineering photonic nanojets

Photonic Nanojets are highly localized wave fields emerging directly behind dielectric microspheres; if suitably illuminated. In this contribution we reveal how different illumination conditions can be used to engineer the photonic Nanojets by measuring them in amplitude and phase with a high resolution interference microscope. We investigate how the wavelength, the amplitude distribution of the illumination, its polarization, or a break in symmetry of the axial-symmetric structure and the illumination affect the position, the localization and the shape of the photonic Nanojets. Various fascinating properties are systematically revealed and their implications for possible applications are discussed

Phase anomalies in Talbot light carpets of selfimages

An interesting feature of light fields is a phase anomaly, which occurs on the optical axis when light is converging as in a focal spot. Since in Talbot images the light is periodically confined in both transverse and axial directions, it remains an open question whether at all and to which extent the phase in the Talbot images sustains an analogous phase anomaly. Here, we investigate experimentally and theoretically the anomalous phase behavior of Talbot images that emerge from a 1D amplitude grating with a period only slightly larger than the illumination wavelength. Talbot light carpets are observed close to the grating. We concisely show that the phase in each of the Talbot images possesses an anomalous axial shift. We show that this phase shift is analogous to a Gouy phase of a converging wave and occurs due to the periodic light confinement caused by the interference of various diffraction orders. Longitudinal-differential interferometry is used to directly demonstrate the axial phase shifts by comparing Talbot images phase maps to a plane wave. Supporting simulations based on rigorous diffraction theory are used to explore the effect numerically. Numerical and experimental results are in excellent agreement. We discover that the phase anomaly, i.e., the difference of the phase of the field behind the grating to the phase of a referential plane wave, is an increasing function with respect to the propagation distance. We also observe within one Talbot length an irregular wavefront spacing that causes a deviation from the linear slope of the phase anomaly. We complement our work by providing an analytical model that explains these features of the axial phase shift

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 of Δn≈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