Observation of Bloch Surface Waves in the Mid-Infrared Spectral Range

We present the observation of Bloch Surface Waves in the mid-infrared spectral range. A dielectric multi-layer coating made of ZnSe and YbF 3 deposited on a CaF2 substrate is designed and fabricated specifically for that purpose. The structure supports the propagation of surface waves between 6.6 and 10.6 micrometres wavelength. A tunable quantum cascade laser setup is used to characterize the fabricated structure.

Observation of Bloch Surface waves in the mid-infrared spectral range

Nicolas Maxime André Descharmes, Hans Peter Herzig, Grégoire Maxime Smolik

We present the observation of Bloch Surface Waves in the mid-infrared spectral range. A dielectric multi-layer coating made of ZnSe and YbF3 deposited on a CaF2 substrate is designed and fabricated specifically for that purpose. The structure supports the propagation of surface waves between 6.6 and 10.6 micrometres wavelength. A tunable quantum cascade laser setup is used to characterize the fabricated structure.

IEEE2018

Advanced optical components for mid-infrared applications

Grégoire Maxime Smolik

The mid-infrared wavelength range, defined from 2.5 um to 25 um is of a high scientific and technological interest. One of its main application field is in spectroscopy, since these wavelengths coincidewith the fundamental rotational-vibrational modes of many molecules. While the typical optical sensing setup is composed of active elements - detectors and radiation sources- it also requires critical passive components such as lenses, filters and an interface with the medium to be probed. To this day, most of the scientific work in mid-infrared has been mostly focused on active elements such as the quantum cascade laser. While very modern detectors and lasers are now available, the experimental setups still rely on classical and bulky passive optics. The work presented in this thesis explores modern optical technologies for the realization of novel passive mid-infrared optical components. The fabrication of an external cavity tunable quantum cascade laser emitting from 7.6 um to 8.1 um is presented. This thesis describes the fundamentals of sub-wavelength structures for optical phase generation. Such structures allow local modifications of the effective refractive index of a dielectric material. Using these concepts, original designs of polarizers, anti-reflective and diffractive micro-optical elements are proposed for high-index dielectrics such as silicon or germanium. In particular, the fabrication and characterization of a novel binary grating in silicon that combines anti-reflective and diffractive functions around 7.85 um is presented. This component is characterized using the developed tunable light source setup. Another type of optical component is presented, based one dimensional photonic crystals. It consists of a ZnSe-YbF3 multi-layer deposited on top of a CaF2 substrate. This element allows to observe the first occurrence of Bloch surface waves in the mid-infrared, using the same tunable laser setup. The evanescent field of a surface-propagating wave can be used to probe the surrounding medium. Such structures are promising novel types of substrates for surface-based sensing applications. In addition, a quantum cascade laser-based setup for the detection of cocaine in liquid is presented. This experiment illustrates the great potential of quantum cascade lasers for compact chemical sensing setups.

EPFL2017

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.