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
Nature Publishing Group, 2014
Article

Résumé

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.

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https://infoscience.epfl.ch/record/198113?ln=fr

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Elsie Barakat, Joab Di Francesco, Hans Peter Herzig, Lubos Hvozdara, Tristan Sfez, Libo Yu
Nature Publishing Group, 2014
Article

Résumé

Official source

https://infoscience.epfl.ch/record/198113?ln=fr

À propos de ce résultat

Concepts associés (15)

Concepts associés

Chargement

Publications associées (16)

Publications associées

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

Richa Dubey

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.

EPFL2017

Chargement

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

Chargement

Fabrication and Characterization of Freestanding Single Crystal Diamond and Silicon Microresonators

Teodoro Graziosi

Microresonators are fundamental building blocks of any photonic integrated circuit, as they can be used as filters, modulators, sensors, and to enhance light emission. If these resonators are suspended and are free to oscillate, we can exploit the interaction of the optical and mechanical resonance. Demonstration of the fundamental interactions between the optical and mechanical fields, as well as applications based on this physics, were already presented for other material systems such as silicon, silicon oxide, or silicon nitride. Single crystal diamond is a strong candidate to realise high quality resonators due to the excellent material properties. The mechanical properties, such as stiffness, low intrinsic damping, and high thermal conductivity, are critical for mechanical oscillators with high frequency and high quality factor, which is strongly correlated to the noise of the oscillator. The wide transparency range and the low absorption enable operation in a large wavelength range spanning from near ultraviolet to far infrared. Diamond can host very bright emitters, based on defects of the diamond lattice, which can be employed as single photon sources, quantum memories, or very sensitive magnetic sensors. Chemical resistance to most acids or bases permit operations in aggressive environments. For these reasons, single crystal diamond is an attractive platform for integrated photonics and micro- or nanomechanics, and it should be possible to realize low noise optomechanical resonators. However, compared to more established material systems, microfabrication of single crystal diamond is not easy. High quality single crystal diamond substrates are available, to date, only as bulk plates, therefore it is important to develop fabrication strategies that isolate optically and mechanically a thin diamond layer from the rest of the diamond substrate. Over the last years, several approaches have been proposed and in this work two methods will be employed: 3D milling using a focused ion beam to create micrometer sized disks supported by a narrow pillar; and etching of the bulk diamond using a plasma which is selective to particular crystal planes of the single crystal diamond. Using these processes, single crystal diamond microresonators were realized, measuring optical quality factors of 5700 and 42000 at telecom wavelengths, respectively, and of 3100 and 10500 at visible wavelengths, respectively. Single crystal silicon is similar in many aspects to single crystal diamond, with the material properties being slightly worse. However, silicon benefits from decades of knowledge developed for integrated electronic circuits and integrated silicon photonics circuits. Silicon photonics is now extensively used in telecommunications to interface the optical links to electronics. Commercial foundries started to offer silicon photonics services, either within the CMOS fabrication or with dedicated processes. Micro Electro Mechanical Systems represent a way to expand on the capability of photonic integrated circuits by providing low power and/or reconfigurable components. In this context, optomechanical oscillators were realized by postprocessing an IMEC iSiPP50G silicon photonics chip. Optical quality factors of 300000 were measured at telecom wavelengths. Mechanical oscillations are supported at 250 MHz, with quality factors of 1800 measured at ambient conditions.

EPFL2019

Chargement

Fabrication and Characterization of Freestanding Single Crystal Diamond and Silicon Microresonators

Teodoro Graziosi

EPFL2019

EPFL2013

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

Richa Dubey

EPFL2017