Optomechanical coupling in photonic crystal supported nanomechanical waveguides

We report enhanced optomechanical coupling by embedding a nano-mechanical beam resonator within an optical race-track resonator. Precise control of the mechanical resonator is achieved by clamping the beam between two low-loss photonic crystal waveguide couplers. The low insertion loss and the rigid mechanical support provided by the couplers yield both high mechanical and optical Q-factors for improved signal quality

Stepped-height ridge waveguide MQW polarization mode converter monolithically integrated with sidewall grating DFB laser

We report the first demonstration of a 1555 nm stepped-height ridge waveguide polarization mode converter monolithically integrated with a side wall grating distributed-feedback (DFB) laser using the identical epitaxial layer scheme. The device shows stable single longitudinal mode (SLM) operation with the output light converted from TE to TM polarization with an efficiency of >94% over a wide range of DFB injection currents (IDFB) from 140 mA to 190 mA. The highest TM mode purity of 98.2% was obtained at IDFB=180 mA. A particular advantage of this device is that only a single step of metalorganic vapor-phase epitaxy and two steps of III-V material dry etching are required for the whole integrated device fabrication, significantly reducing complexity and cost

Stepped-height ridge waveguide MQW polarization mode converter monolithically integrated with sidewall grating DFB laser

We report the first demonstration of a 1555 nm stepped-height ridge waveguide polarization mode converter monolithically integrated with a side wall grating distributed-feedback (DFB) laser using the identical epitaxial layer scheme. The device shows stable single longitudinal mode (SLM) operation with the output light converted from TE to TM polarization with an efficiency of >94% over a wide range of DFB injection currents (IDFB) from 140 mA to 190 mA. The highest TM mode purity of 98.2% was obtained at IDFB=180 mA. A particular advantage of this device is that only a single step of metalorganic vapor-phase epitaxy and two steps of III-V material dry etching are required for the whole integrated device fabrication, significantly reducing complexity and cost

Integrated polarisation rotators

The ability to control and manipulate the state of polarisation of optical signals is becoming an increasingly desirable feature in numerous applications including integrated optical circuits, semiconductor optical amplifiers (SOAs) and optical communication systems. This thesis introduces the design, optimisation, fabrication and operation of two novel integrated reciprocal single-section passive polarisation converter devices based upon mode-beating. The converter designs consist of asymmetric profiled waveguides, which were realised in a single reactive ion dry-etch process step. An in-situ custom built sample holder was utilised to place the samples at a predetermined angle to the incoming ions, which resulted in waveguide profiles with sloped sidewalls. This fabrication technique also allowed the incorporation of adiabatic taper sections within the device design. The converter section waveguide profile of the first design consists of two sloped sidewalls. Devices realised on a GaAs/AlGaAs material structure achieved a converted transverse magnetic (TM) polarisation purity of 81.4% at a device length of 30 μm for a transverse electric (TE) polarised input signal at an operating wavelength of λ = 1064 nm. The convention used is that TE refers to light polarised in the plane of the wafer and TM refers to light polarised perpendicular to the plane of the wafer. The total optical loss imposed by this device was evaluated to be 1.72 dB. This design was also used for the monolithic integration of a passive polarisation converter incorporated within a Fabry-Perot semiconductor laser diode on an unstrained GaAs/AlGaAs double quantum well heterostructure material system. A predominantly TM polarised optical output from the converter facet of greater than 80% is demonstrated for a converter length of 20 μm at an emitting wavelength of 867.1 nm. The about 1.4 mm long fabricated device has a current threshold level of 100 mA and a side mode suppression ratio (SMSR) of 25 dB. The second converter design is based on the modification of an already existing stripe waveguide structure. The converter section is defined by applying the above mentioned angled dry-etch process on a certain length of the stripe waveguide. The fabricated asymmetric waveguide core profile consists of a sloped undercut. A TM polarisation purity of 90% at a device length of 55 μm for a TE polarised input signal at an operating wavelength of λ = 1064 nm was achieved at the output. The total optical loss imposed by this device was evaluated to be 0.47 dB

Monolithically integrated polarisation mode convertor with a semiconductor laser

In this thesis, the design, optimisation, fabrication and operation of waveguide based semiconductor lasers, integrated with polarisation mode convertors (PMCs), is described. Devices are fabricated in the GaAs/AlGaAs and InP/AlGaInAs material systems, using two types of structures; single PMC and back-to-back PMCs. The convertor designs are based upon air trenches, of sub-wavelength dimensions, being introduced into waveguide structures in order to achieve an asymmetric cross-sectional profile, resulting in wave-pate functionality. The GaAs/AlGaAs PMCs are fabricated using reactive ion etching (RIE), and the phenomena of RIE lag technique is also exploited for obtaining the required asymmetric waveguide profile in a single etch step. These are then integrated with semiconductor lasers. The InP/AlGaInAs PMCs are fabricated using a combination of RIE and inductively coupled plasma (ICP) etching and are integrated with semiconductor lasers and also differential phase shifter (DPS) sections to realise devices with active polarisation control. Integrated devices fabricated on InP/AlGaInAs material system with a semiconductor laser, a PMC followed by a DPS section yields ~40 % polarisation mode conversion whilst the DPS section is held at the transparency condition. Greater than 85 % polarisation mode conversion was also obtained with back to back PMCs, which was complement to the devices fabricated with a single PMC. Furthermore, a first active polarisation controller, monolithically integrated with a semiconductor laser is reported. High speed modulation of the integrated device with 300 Mbps is also demonstrated via current injection to the phase shifter section of the device

Silicon based hybrid photonic structures and their applications to biosensing

Porous silicon has been studied as an effective host, efficient emitter, sensor and recently, a candidate for photonic crystals. Convenient surface modification chemistry not only counteracts the drawbacks of surface instability that comes with nanostructured morphology, it also adds useful functionalities such as specific target capture for sensing utility. Structurally novel devices can also be realised through manipulating porous silicon multilayers. This thesis extends on the effort to explore the optical properties and applications of porous silicon through the construction of functional porous silicon structures assisted by surface chemistry. In addition, extraordinary optical properties resulting from peculiar behaviour of light in porous silicon multilayers as photonic crystals are investigated and exploited to broaden the understanding of structural novelty and practically, showing attractive prospects for biosensing applications. Quantum dot doped porous silicon one dimensional microcavity structures have been fabricated by incorporating colloidal II-VI compound quantum dots into the microcavity assembled from two separately anodised Bragg mirrors. The formation of microcavity structures is facilitated by strong affinity between biomolecules. High quality microcavity structures built on quantum dots at 565 nm, 625 nm and 780 nm with this technique exhibit well defined stop bands and resonant modes with line-widths less than 3.5 nm. Enhancement of photoluminescence emission, spectral and spatial modification by the microcavities is observed. Tunable emission from the microcavities also suggests the potential applications in biosensing. The outermost truncation of regular Bragg reflectors creates a new type of novel structure sustaining Bloch surface waves with promising capability for biosensing. The structure is passivated and functionalized using established surface chemistry. Biosensing capability of the structures is demonstrated by protease-catalytic cleavage reaction of grafted gelatin. A detection limit of 0.37 nM protease is obtained. The possibility of kinetics study is explored. Fabrication and characterisation of high quality protein spaced porous silicon microcavities for sensing purposes are summarised. Gelatin is incorporated as the central layer of the microcavity structure and, as the sensing element in biosensing operation. Some constraints to the engendering protease sensing are identified and possible solutions to these problems, proposed