Near-field characterization of Bloch surface waves based 2D optical components

Bloch surface waves (BSWs) are surface electromagnetic modes that propagate at the interface between a multilayer substrate and a homogeneous external medium. The optical field of the surface mode is confined near the surface of the multilayer. This vertical confinement as well as the low absorption inherent to the dielectric materials make the BSWs an interesting candidate for the development of 2D optical systems and sensors. Such a periodic multilayer structure is introduced as a platform on which many optical functions can therefore be created. In this thesis, two-dimensional optical components based on the Bloch wave platform are studied, in particular: a disk resonator, a Bessel-like beam generator and a waveguide grating as a Bragg mirror engraved in a waveguide. The optical properties of the components such as the resonance inside the disc, the "quasi non-diffracting" behavior and the reflection properties are presented. The 2D optical components are designed from a commercial FDTD program (CST Microwave studio). They are then characterized by a near-field scanning microscope with multi-heterodyne detection (MH-SNOM). Thanks to the MH-SNOM, it is possible to map the field distribution locally at the surface of the structures with a resolution lower than the wavelength. Simultaneous measurement of amplitude and phase allows a detailed reconstruction of the complex amplitude of the electric field. In a first part, the influence of a device layer of material with a high refractive index (TiO2) is studied. The impact of the thickness of the TiO2 layer on the propagation properties of the BSWs is presented. It is demonstrated that by adapting the thickness of the device layer, the BSW dispersion curve position can be moved within the photonic band gap and consequently the BSW mode propagation properties can be adapted. The propagation properties of the BSWs include, for example, the propagation length and the effective refractive index. Thanks to the low losses and the design of our multilayer platform, propagation lengths in the range of millimeter are obtained. In a second part, 2D optical components fabricated in a 60 nm (wavelength/25) device layer of TiO2 are presented, and initially, disk resonators. The latter are key elements in integrated optics systems. For a disk with a radius of 100 µm, an experimental quality factor of the order of 10^3 is obtained. A 2D Bessel-like beam generator is also studied. An isosceles triangle is used to generate such a beams. The main expected property of Bessel-type beams is their "quasi non-diffracting" nature. The optical properties of non-diffracting beams are measured in the near-field for different base/height ratios of the isosceles triangle. It is demonstrated that the beam propagates without significant spreading for considerable propagation distance, approximately 50 µm. Finally, the gratings engraved in a waveguide are studied. It is demonstrated that they perform as a Bragg mirror at a wavelength of 1550 nm. Thanks to the MH-SNOM, the interference fringes between the incident wave and the reflected wave are measured. The experimental reflectivity is obtained from the contrast of the interference fringes. The presented waveguide grating can be used as a Bragg reflector at telecom wavelengths for 2D optics systems.

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

Near Field Investigation of Bloch Surface Based Platform for 2D Integrated Optics

Elsie Barakat, Richa Dubey, Hans Peter Herzig

Dielectric multilayers sustaining Bloch surface waves (BSWs) are considered as a novel platform for two dimensional (2D) integrated optics. The dielectric platform is exploited to manipulate surface waves by patterning 2D dielectric optical components on top of it. BSW shows a potential of higher propagation length and large resonance strength because of low loss characteristics of dielectric materials. Taking the advantage of high ¯eld con¯nement on the surface, the platform has also applications in sensing. In order to excite a BSW, momentum of incoming beam should match with the momentum of the BSW. Therefore, we use Total internal re°ection con¯guration for this purpose, which consists of BK7-glass prism. The schematic of the con¯guration and the platform is presented in Figure 1. New design of multilayer platform consists of periodic stacks of alternative SiO2 and Si3N4 layers. It has been fabricated to work around the wavelengths of 1.5 ¹m. In this paper, we study the key parameters, propagation length and the e®ective refractive index (ne® ), of BSW. They play an important role in characterizing losses associated with the multilayer platform and propagation of surface modes. E®ective refractive index contrast (¢n) is introduced by depositing an additional layer of high refractive index material on the top of platform. It is basically the di®erence between ne® of additional layer and platform. It plays a key role in determining the optical properties of the 2D surface photonic devices and hence their capability to manipulate the BSW most essentially. High refractive index materials, as an active material to pattern 2D optical elements on the top of the platform, have been investigated in near ¯eld and far ¯eld. These material include titanium dioxide (TiO2), for the time being, and Graphene. We obtained propagation length of around 2mm for 15nm thickness of additional TiO2 layer with the aid of multi-heterodyne scanning near-¯eld optical microscopy (MH-SNOM). It is around 25 times longer compare to the recently obtained \Long-Range SPPs" studied by Lin et al. [1]. We achieved ¢n of around 0.2 measured in the far ¯eld with 100nm thickness of TiO2. It is around 3.5 times higher than the ¢n obtained for same thickness of Photoresist [2]. In near future, we aim to characterize di®erent optical components on the top of multilayer platform with the aid of MH-SNOM, for example, Ring resonators and Interferometers for the time being.

2015

Étude expérimentale des propriétés optiques des cristaux photoniques bidimensionnels et de leur accordabillité

Barbara Wild

Photonic crystals are periodic dielectric structures, where the periodicity varies in one, two or three dimensions. Analogous to the periodic potential of electrons in semiconductors, the periodic variation of the dielectric constant influences the electromagnetic properties. The energy of the light is separated in bandgaps, energy ranges in which the propagation of the light is forbidden for certain directions and energies. These properties suggest that photonic crystals may be suitable for fabrication of the components needed for integrated optics. Since the fabrication of three dimensional photonic crystals is still limited by complex fabrication problems we have studied two dimensional photonic crystals. These photonic crystals can be fabricated by standard microelectronic technology. The photonic crystals studied in this thesis consist of GaAs and InP based low index vertical waveguides with a matrix of holes etched into it. The fabrication of photonic crystals with good optical properties is an important factor for photonic crystal devices in integrated optics. In the first part we studied the optical properties of photonic crystal slabs and of Fabry-Pérot cavities. These structures were measured by the internal light source technique, which allows quantitative normalized transmission measurements. The photonic crystals were etched by three different dry etching technologies. Out-of plane scattering is described by a 2D-FDTD fit of the transmission spectra and by a semi-analytical model. The finite hole depth and the conical shape of the holes are important structural parameters. The detailed study of their effects yields an important feedback for the critical parameters of the fabrication process. Photonic crystal waveguides are another group of components in integrated optics. We measured two types of straight photonic crystal waveguides by the endfire technique. Here the light is coupled by optical fibers and ridge waveguides in the photonic crystal waveguides. The transmission spectra reveal fringes which are due to multiple interference of the light in the sample. This interference contains information about propagation losses of the waveguides and an analysis based on the Fourier transform of the transmission spectra enabled us to deduce the propagation losses of the photonic crystal waveguides. The necessity of tuning or trimming the optical properties of photonic crystals is an important issue either compensating fabrication imperfections (trimming) or controlling the optical properties on demand (tuning) for devices like filters. We have shown that it is possible to tune the optical properties of planar photonic crystal cavities by temperature. The experimental results were validated by theoretical calculations. Also we have shown that is possible to tune the optical properties of photonic crystals by infiltrating liquid crystals in the holes. Liquid crystals are a birefringent material whose optical axis can be tuned by external fields (electric, temperature, photonic source, etc.). Phase transitions exhibiting abrupt changes in the refractive index are important characteristics of liquid crystals. The holes of the photonic crystal are infiltrated by a reversible and reliable infiltration process. The infiltration efficiency and the orientation of the molecules in the holes were determined by polarization resolved internal light source measurements. We have shown that the frequency of the Fabry-Pérot resonance changes at the phase transitions of the liquid crystal.

EPFL2006