One of the main advantages of rod-type PhC cavity is allowing optimised coupling between the cavity mode and a 2D emitter which increases light-matter interaction and provides enhancement in the light extraction from these emitters. Performing computer simulations that can best predict experimental results can only be done by including silicon’s absorption in the simulations. This could be done by assigning complex refractive indices to the high dielectric material that resemble silicon. Absorption by the dielectric rods and the substrate can lead to reduced cavity Q-factor and affect the extraction efficiency ratio. In addition, placing a 2D monolayer above the cavity can cause perturbation to the dielectric symmetry of the cavity structure, taking this into account may lead to reduction in the achieved Q-factor, despite the fact that the perturbation might be very small.
In chapter 4, simulations were carried out to investigation the effect of varying the emitter’s height in the cavity on the achieved enhancement. While emission from artificially created defects is promoted when the monolayer is maximally strained, simulating changes in the extraction efficiency as the emitter’s position is varied in the x-y plane is useful for a comprehensive study of the cavity. Finite element methods of the dipping of the emitter can be utilised to help predicting the dipping of a monolayer in the cavity.
Vertical light confinement is necessary to allow light guiding on-chip with minimum losses. One method of achieving vertical confinement with rod-type PhCs can be accomplished by surrounding the rods with an optically transparent material which could be realised by spin coating the chip or bottom up growth of the material using chemical vapour deposition. The refractive index contrast between the transparent material with air on top and the dielectric substrate at the bottom causes total internal reflection. This technique has never been used with rod-type PhC structures but is expected to increase
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the cavity Q-factor, encouraging a wider use of rod-type PhCs for integrated photonic applications. This allows integrating QKD systems using rod-type PhCs. Future plans also involve controlling the dipping of a monolayer inside a cavity using probe techniques.
Having fabricated nanowire sites and successfully managed to isolate single and arrays of nanowires, work has recently started in growing p-i-n junction GaAs nanowires for photodetection applications. Work in this direction has already been carried out using InAs as composition material of the nanowires and results have been published showing mid-infrared photodetection. The path toward growing p-i-n junction nanowires with GaAs should be sufficiently clear, considering similar work was done with InAs nanowires. Following the growth, free standing nanowire will biased by fabricating contacts using the approach explained in chapter 5 as this approach is perfectly suitable for spatial coupling to integrated PhC waveguides. The mask for fabricating the contact pads used to bias the nanowire grid arrays is shown in Figure 7.1. The large pads at the bottom of the figure are contact pads, while the pads at the top of the figure are used to probe the nanowire that grow in the hole arrays (black squares and dots in the figure). Studies can be then follow to investigate the efficiency of photodetection using a single and an array of nanowires.
Figure 7.1: A schematic diagram of the designed mask for fabricating contact pads for biasing the nanowire grid arrays. Enclosed pink regions indicate areas that will be exposed to UV light. The top end of the pads align with the nanowire arrays while the bottom pads are used for making probe contact or wire bonds.
Probe pad N an o wire Co n tac t p ad
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Integrating a QKD system in hand held devices could only be achieved by miniaturizing single photon detectors to the chip-level. For example light from Bob’s photonic circuit can be coupled to an external off-the-shelf Si APD using beam out-couplers. The out-coupler directs the light from the waveguide toward the detector chip which is typically situated adjacent to the measurement chip. The layer structure of a QKD chip can also be grown so that both the Si APD detector layers and those required for fabricating photonic crystals are done by molecular beam epitaxy or chemical vapour deposition. Fabricating the detector’s contacts and the PhC can then follow using conventional lithography techniques. Another approach to integrating the detectors used in QKD could emerge from successfully realising nanowire based photodetectors. To achieve this, research is needed to be carried out to tune the operation wavelength to match that of InAs QD’s emission.
Further work can be carried out to optimise light guiding in hole-type PhC waveguides by making modification to its design. Increasing the confinements of these waveguides and reducing their losses is essential for many applications other than QKD. Waveguides with lower losses can increase the QKD bit rate in QKD and allows fabricating long phase modulators, reducing the voltage required to establish high electric fields for short waveguides. This reduces heat resultant from operating at high switching frequencies.
Successful growth and fabrication of the full QKD system will be followed by many measurements such as photoluminescence, coupling efficiencies between the different optical components, losses by the different components and key generation.
The QKD system discussed in this thesis uses the differential phase shift keying, however, polarisation encoding QKD can also have strong integration potential on a chip scale. Each of these schemes have advantages and disadvantages in their fabrication complexity, bit rate, achieved distance, etc, and the best scheme should be chosen based on the QKD application the system is designed for.