Bloch surface waves, a 2D platform for planar optical integration

A novel platform suitable for fundamental investigations of light propagation through micro-and nanostructures will be discussed. This platform is based on a dielectric multilayer that sustains Bloch surface waves (BSWs). BSWs are electromagnetic surface waves excited at the interface between a truncated periodic dielectric multilayer and a surrounding media. The modification of the top surface to customize a complete 2D micro-system can be produced using e-beam writing, optical lithography or other patterning techniques. The results obtained confirm the possibility of developing a robust multilayer platform that would pave the way for integration of photonic components in photonic chips using standard wafer-scale production.

Near-field imaging

Libo Yu

Bloch surface waves (BSWs) are surface electromagnetic waves excited at the interface between a truncated periodic dielectric multilayer and a surrounding medium. This particular solution to the Maxwell equations in stratified media has been known since the late 1970s. However, few studies have been conducted till now on the manipulation and control of BSW propagation over 2D surfaces of photonic structures. There are several advantages provided by BSW propagation on almost flat surfaces. Due to the use of dielectric materials, losses are very low, thus allowing the propagation of BSWs over long distances. Another advantage in using BSWs is the possibility of operating within a broad range of wavelengths by properly designing a suitable multilayered structure. Furthermore, since the maximum intensity associated with BSWs can be tuned on the surface, a strong field intensity increased by several orders of magnitude can be achieved and can thereby enhance the field close to the structure's surface. This tunable localized field confinement is particularly attractive in fluorescence biosensing. In the following, we will use an exemplary BSW-sustaining multilayer as a general platform suitable for manipulating and controlling BSW propagation on its surface in the spectral range of telecom wavelengths. Several 2D optical photonic devices can be implemented starting from an ultra-thin (~ λ /15) polymer layer spun on the multilayer, which can be subsequently shaped as desired. The presence of the polymer modifies the local effective refractive index, enabling a direct manipulation of the BSWs. We experimentally and theoretically demonstrate that BSWs can be diffracted, focused, coupled and made resonating by locally shaping the geometries of 2D photonic devices—such as lenses, prisms, gratings, bended waveguides and waveguide couplers—on the multilayer. One of the main advantages is that these 2D photonic devices can have arbitrary shapes, which are difficult to obtain in 3D. The strong point of the platform concept is that a thin-film multilayer enables standard-wafer-scale production. The top surface can be modified to customize a 2D micro-system by, e.g., e-beam writing, optical lithography, stamping or other replication techniques. Since a BSW can be considered as a 2D wave and it is bounded at the surface of the multilayer, near-field imaging is one of the preferred tools to directly monitor and characterize the near field produced on the structured multilayered surface mentioned above. A multi-heterodyne scanning near-field optical microscope (MH-SNOM) developed in our lab (EPFL-OPT) makes near-field characterization possible. The operation wavelength is in the near infrared range (1460 nm–1580 nm). A polarization-retrieval method is developed in this thesis to measure and characterize the near-field polarization response for arbitrary polarization- sensitive photonic nano-devices, such as photonic crystal microcavities, waveguides, thin films, nanoparticles, spiral gratings and other near-field polarization-sensitive imaging applications. Polarization retrieval is easily accessible in far-field microscopy, but is challenging for the near field. This is because when a SNOM probe interacts with the near field and scatters the signal to the far field, the near-field polarization may be considerably altered. In this thesis, the developed method uses an isotropic region in the vicinity of the nanostructure as a calibration area, whose known polarization properties provide a global criterion to calibrate and quantify the polarization distortion induced by the detection system. It is then applied on the field measured above the structure of interest to compensate and reconstruct its complex field response. We experimentally show the effectiveness of the method on a silicon form- birefringent grating (FBG) with significant polarization diversity. Three spatial dimensional near-field measurements are in agreement with theoretical predictions obtained with rigorous coupled-wave analysis (RCWA). Pseudo-far- field (~10λ away from the surface) measurements are performed to obtain the effective refractive index of the FBG, emphasizing the validity of the proposed method. This reconstruction algorithm makes the MH-SNOM a powerful tool to analyze concurrently the polarization-dependent near-field optical response of any nanostructures with subwavelength resolution as long as a calibration area is available in close proximity.

EPFL2013

Investigation of Surface Electromagnetic Waves with Multi-Heterodyne Scanning Near-Field Optical Microscopy

Tristan Sfez

In this thesis, we present the development of a tunable Multi-Heterodyne Scanning Near-Field Optical Microscope (MH-SNOM). This instrument has been built and evaluated for the investigation of optical near fields in amplitude, phase and polarization. With this microscope, the response of a structure illuminated with two orthogonally polarized beams can be simultaneously measured both in amplitude and phase. Moreover, the integral state of polarization at the surface of a specimen can be retrieved under specific conditions. We demonstrate the capabilities of the system through a series of measurements involving Surface Electromagnetic Waves (SEWs). We have mainly focused our attention on a particular class of SEWs known as Bloch Surface Waves (BSWs). The propagation of BSWs on the outer surface of a silicon nitride multilayer has been studied in detail. Furthermore, we show that this propagation is affected by the presence of shallow dielectric corrugations such as a subwavelength grating or at the straight interface with a coated portion of the multilayer. In particular, we demonstrate that ultra-thin (thickness < λ/10) dielectric ridges may act as BSW waveguides. Combining the detection capabilities of the MH-SNOM with a numerical treatment of the experimental data, we are able to separate the transverse and longitudinal field components of the three modes propagating within a specific BSW waveguide. This new structure provides interesting opportunities in waveguide-based biosensing schemes in which the ridge is realized with functionalized molecular layers of nanometric thickness. Finally, we investigate a structure sustaining another type of SEW: Surface Plasmon Polaritons (SPPs). This structure is designed for the asymmetrical coupling of SPPs at normal incidence. Through a detailed analysis of the spatial spectra, we show that, in addition to SPPs, the field contains other near-field components. All these experiments demonstrate the expected MH-SNOM capabilities of measuring the amplitude, phase and polarization of optical near fields. The MH-SNOM therefore serves as a powerful tool for the investigation with subwavelength resolution of optical near fields generated in structures such as integrated optics, photonic crystals, cavities, resonators, etc.

EPFL2010

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

Elsie Barakat, Joab Di Francesco, Hans Peter Herzig, Lubos Hvozdara, Tristan Sfez, Libo Yu

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 (similar to lambda/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.