Shining Diamond: from Micro- and Nano-Engineering to Quantum Photonics

The discovery of room-temperature single photon emission from color centers in diamond has in recent years boosted research interest in this renowned material in photonics and quantum community and beyond. Following understanding some basic principles of how the color centers work, new challenges arose: designing and fabricating real devices that leverage the unrivaled properties that diamond possesses. Due to this material's extreme physical hardness and its chemical resistance to etchants, the established semiconductor technologies cannot straightforwardly answer to the call for scalable and reliable fabrication of micro- and nano-photonic devices in single crystal diamond. In this thesis, we first demonstrate an unconventional polishing protocol with ion beam etching, which makes possible wafer-scale fine finishing of diamond substrates, the starting point for any subsequent processing. Following this we present the experimental observation of self-organized nanotextures on diamond induced by ion beam irradiation, a lithography-free approach for nanoengineering periodic structures. Regarding device fabrication, we obtained free-standing micro- and nano-photonic structures using angled reactive ion etching, facilitated by a novel and reproducible Faraday cage design. We also propose possible designs to interconnect individual components in hope of realizing a photonic integrated circuit in the future. Last but not least, we present spectroscopic characterization of single and ensemble color centers either grown by microwave plasma chemical vapor deposition, or fabricated by ion implantation with subsequent high-temperature annealing, which are the fundamental building blocks for quantum applications. Photoluminescence spectra that, to our knowledge, are not seen in literature were recorded for ensemble silicon-vacancy centers at cryogenic temperature, signifying potentially unknown mechanism for tuning the optical transitions, and unexplored possibilities with this exciting material platform for quantum technologies.

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

Light Confinement and Nonlinear Light-Matter Interaction in Semiconductor Photonic Crystal Cavities

Mohamed Sabry Abdel-Aliem Mohamed

The ability to control and manipulate light is a fundamental aspect that is at the very core of the development of integrated photonic circuits. It is desirable to achieve such control down to a scale that is comparable to or smaller than the wavelength of light, to enable highly efficient interaction with matter, which can eventually go down to single photon levels. Photonic crystals are particularly appealing in that respect, by providing optical bandgaps and facilitating dispersion shaping, through the precise patterning of layers. This is to be accomplished ideally in compact designs that are well-suited for fabrication with current technology and by utilizing material that has the capacity for integration. By exploiting existing light-matter interaction mechanisms, the available toolset for component design can be expanded, allowing to build optical chips with a wide range of functionality. In the current work, a platform for integrated III-nitride photonic components on silicon is designed and implemented. A refined fabrication process for III-nitride micro- and nanostructures is developed, leading to the realization of suspended optical devices with both high performance and structural stability. Thorough optical and material characterization of the semiconductor layers is conducted to identify potential limitations, particularly with epitaxial GaN on Si technology, with respect to current crystal growth techniques. AlN layers based on a sputter-deposition process are proposed as an alternative for passive optical devices. Two-dimensional photonic crystals are designed in gallium nitride and aluminum nitride, targeting operational wavelengths in the near-infrared telecom range. Optimized photonic crystal cavities are considered for maximizing light confinement in the semiconductor layers. By utilizing resonance enhancement, second and third harmonic generation are demonstrated in gallium nitride photonic crystal cavities featuring record-high quality factor values. This is achieved through a far-field coupling approach under low-power continuous-wave operation. High conversion efficiencies are attained in gallium nitride, exceeding previous demonstrations in the material. Propagation of the harmonic signals in the semiconductor layer is analyzed using finite-element time-domain modeling for the feasibility of an extraction mechanism. Moreover, slow light coupled cavity waveguides are developed in a silicon platform, relying on optimized cavity designs that feature wideband operation in the near-infrared telecom range. A record-high value for group-index bandwidth product is achieved in the devices through improved design implementation. An end-fire based Fourier-space imaging technique is applied for the characterization of the optical structures, through which spatially-selective reconstruction of dispersion maps enabled accurate extraction of slow light properties. The influence of finite-size effects on slow light behavior is elucidated by implementing extended cavity chains. Furthermore, light transport regimes across the dispersion band are identified including the transition towards diffusive light transport and subsequent localization. The performance of coupled-cavity waveguides in the presence of disorder is quantified, revealing constraints on slow light transport, considering state-of-the-art fabrication.

EPFL2018

Integrated photonic devices in single crystal diamond

Teodoro Graziosi, Sichen Mi, Niels Quack

The field of diamond photonics is reviewed, with a focus on recent experimental demonstrations of photonic integrated devices in a single crystal diamond. This field leverages the outstanding material properties of diamond with the aim to establish large-scale integrated photonics for applications in sensing, information and communication technologies, and optomechanics. Accordingly, this review introduces recent progress in scalable micro- and nano-fabrication techniques for single crystal diamond photonic integrated devices, and provides quantitative comparative evaluation of the performance of the state of the art devices. The review concludes with an outlook of the potential of photonic integrated circuits in single crystal diamond.

IOP PUBLISHING LTD2020

III-Nitride Semiconductor Photonic Nanocavities on Silicon

Ian Michael Rousseau

EPFL2018

Light Confinement and Nonlinear Light-Matter Interaction in Semiconductor Photonic Crystal Cavities

Mohamed Sabry Abdel-Aliem Mohamed

EPFL2018