Quantification of scattering loss of III-nitride photonic crystal cavities in the blue spectral range

The mechanisms contributing to experimental quality factors of shortwavelength (lambda = 440-480 nm) III-nitride on silicon one-dimensional photonic crystal cavities were quantified. Fluctuations in fundamental and first-order cavity mode wavelength and quality factor were compared over sets of nominally identical cavities. Unlike at. = 1.5 mu m, experimental quality factors were not limited by fabrication disorder modeled as smooth, normally distributed hole size and position variations; after ruling out absorption losses, additional scattering losses were found to predominate at short wavelengths. Experimental quality factors were sensitive to conformal deposition of few nanometer thin films on the photonic crystal surface, suggesting that the additional scattering losses were linked to the surface.

III-Nitride Semiconductor Photonic Nanocavities on Silicon

Ian Michael Rousseau

Optical nanocavities enhance light-matter interaction due to their high quality factors (Q) and small modal volumes (V). The control of light-matter interaction lies at the heart of potential applications for integrated optical circuits, including optical communication, quantum computation, biophotonics, and optical projection. Many of these applications could benefit from wide band gap III-nitride semiconductor material, which already forms the basis of industrial lighting technology based on blue light-emitting diodes. Cleanroom fabrication and optical spectroscopy comprise the laboratory work in this thesis. Air-suspended III-nitride nanocavities are fabricated using electron beam lithography, reactive ion etching, and vapor phase etching from thin (< 300 nm) III-nitride epilayers grown on silicon (111) substrates by metal-organic vapor phase epitaxy. An embedded single InGaN/GaN quantum well serves as an internal light source during micro-photoluminescence experiments. Strain and heating effects are analyzed by micro-Raman spectroscopy. A dedicated quantum optics laboratory is constructed in order to study nanocavities and quantum emitters in III-nitrides at short wavelengths (lambda < 500 nm). Photonic crystal nanocavities exhibit the highest figure-of-merit for light-matter interaction enhancement, Q/V. However, in contrast to silicon and gallium arsenide nanocavities working at telecommunication wavelengths, III-nitride based nanocavities underperform theoretical Q estimates by nearly three orders of magnitude at short wavelengths (lambda = 430 - 500 nm). This thesis accounts for this discrepancy by combined experimental and theoretical studies of nanobeam fabrication statistics. Surface state absorption is found to be the primary factor limiting Q at short wavelengths. Surface effects are studied in the microdisk resonator geometry. UV photoinduced desorption of oxygen gas from the III-nitride surface redshifts and broadens whispering gallery mode resonances over minute timescales. Oxygen desorption incurs additional optical absorption losses up to 100 cm^{-1}. The redshift and broadening is nearly reversible upon the introduction of oxygen into the measurement environment, implicating oxygen in passivation of the III-nitride surface. Finally, optimized surface passivation techniques using rapid thermal processing and conformal chemical vapor deposition more than double state-of-the-art Q of GaN-based microdisk resonators to beyond Q=10,000 in the blue spectral range. The results demonstrate the capabilities and limitations of III-nitride materials for optical nanocavities and photonic integrated circuits at short wavelengths.

EPFL2018

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

Blue surface plasmon propagation along thin gold film-gas interface and its use for sensitive nitrogen dioxide detection

Giovanni Dietler

We demonstrate effective surface plasmon propagation along thin gold film in a blue spectral range: the measured 0.6 degrees angular width of the plasmon excitation peak corresponds to the propagation length around 7 mu m. For this end, we prepared a specially designed photonic crystal (PC; 18-layer 1D structure of altering transparent dielectric layers with relatively large and low refraction indices deposited onto a silica support) coated by a 8 nm thick gold layer. For given blue light wavelength (405 nm) and certain light incidence angle this 1D PC structure supports ultralong surface plasmon propagation along the gold nanofilm. This structure has been tested as a surface plasmon resonance sensor to detect small concentrations of nitrogen dioxide in air. This sensor exhibited the sensitivity similar to earlier works where surface plasmon resonance in red spectral range has been used for this purpose. (C) 2013 Elsevier B.V. All rights reserved.