Fabrication and Characterization of Freestanding Single Crystal Diamond and Silicon Microresonators

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.

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

Advanced Diamond Microfabrication for Microoptics and Photonics

Marcell Kristof Kiss

Diamond is an exceptional material - hard, stiff, transparent, which makes it ideal for the fabrication of optical and mechanical systems that take advantage of these properties. Diamond is not only "better", but it offers the possibility of integrating bright colour centres. These optically active defects can be exploited for a variety of applications, including the study of fundamental science, magnetometry, biosensing and new types of lasers. Despite the attractive properties of diamond for optics, no standard platform exists to create photonic circuits and devices. This thesis shows the realisation of a diamond-on-insulator photonics platform, that aims to combine the versatility of the silicon-on-insulator photonics with the capability of performing diamond science in single crystal guided-wave devices. The diamond-on-insulator substrate is achieved via ion implantation-based cut of a single crystal membrane and bonding of the membrane to an insulator layer. This approach provides a monolithic fabrication process that scales to commercial exploitation, bringing convenient access to the study of diamond light-matter interactions without requiring custom substrate fabrication. Furthermore, the possibility of releasing the diamond devices adds access to freestanding structures, opening the way to diamond micro-opto-electro-mechanical systems and optomechanics. Diamond also makes an excellent microoptics substrate, due to its high laser damage threshold, transparency and high refractive index, which is an attractive combination for high-power, compact optical systems operating in the UV, visible and near-infrared. Different diamond etching techniques are developed and investigated in this thesis, that enable the realisation of unique features. These techniques are employed to create diamond diffractive microoptical components, which are then characterised, showing high-quality surfaces that closely match the designed features, indicating reliable fabrication and resulting in excellent optical performance. These devices have applications in high power beamsplitters, beam shapers and compact spectrometers.

EPFL2019

Functional properties of III-V nanowires addressed by Raman spectroscopy

Francesca Amaduzzi

In recent years, semiconductor nanowires have attracted considerable attention as a result of their unique properties and potential applications in many fields. In particular, they can be very attractive materials for certain optoelectronic and electronic devices, such as lasers, detectors and solar cells, which benefit from the photonic properties of nanowires. In order for these future technologies to become a reality, a good understanding of the functional properties of nanowires is fundamental. In this thesis we have investigated the optical and the electrical properties of III-V semiconductors nanowires by the means of Raman spectroscopy. Thanks to its non-destructive and spatial resolution, Raman spectroscopy is a powerful contact-less tool for the characterization of semiconductor nanowires. Raman spectroscopy can provide information about crystallinity, orientation, size and chemical composition. In polar semiconductors it is also possible to characterize the free carriers, through the coupling of plasmons with longitudinal optical modes. Due to the small size and particular morphology of nanowires, the interaction of light can be more complex than in thin films. In particular, the existence of photonic modes alters significantly the corresponding light-matter interaction. In this thesis we exploit the use of photonic modes for the compositional mapping of nanowire core-shell heterostructures and also to circumvent the macroscopic selection rules. In the first part of this thesis, we have performed Raman scattering measurements on GaAs/AlGaAs core/shell nanowires. We have shown that it is possible to select and characterize regions of the structure with different Aluminum content, by performing the measurements at different laser wavelengths. Then, we have shown that the photonic modes can be modified by suspending the nanowires on a trench. We have shown that in this case it is possible to enhance the response of the longitudinal optical phonon mode. We have then applied this configuration for the characterization of the hole concentration on p-type GaAs nanowires in back-scattering geometry. The second part of the thesis focused on the assessment of free carriers by Raman spectroscopy in systems with an expected high electron mobility: GaAs nanowires with a modulation doped structure and InAs(Sb) nanowires. Raman measurements were performed as a function of the temperature on modulation doped GaAs/AlGaAs nanowire. By characterizing the coupling between free carrier and the LO phonons, we have extracted the concentration and the mobility of carriers. We have found that Si donors are almost ionized for a temperature above 50 K. We have shown that the mobility is limited by interface scattering, with values of 400 cm^2/Vs at room temperature and 2700 cm^2/Vs at low temperature. Finally, we investigated the electronic properties of InAs(Sb) nanowires as a function of the temperature. The effect of a dielectric coating on the electronic properties was also studied. We have found an increase of mobility and electron concentration with the antimony content , moving from 5100 cm^2/Vs for InAs nanowires, to 17500 cm^2/Vs for InAsSb with 35% of antimony at 14K. Moreover, we have shown that in the case of InAs electrons are located in the accumulation layer at the surface, while for InAsSb our measurements are consistent with the carriers located in the nanowire core.

EPFL2017