Mapping the optical properties of slab-type two-dimensional photonic crystal waveguides

We report on systematic experimental mapping of the transmission properties of two-dimensional silicon-on-insulator photonic crystal waveguides for a broad range of hole radii, slab thicknesses and waveguide lengths for both TE and TM polarizations. Detailed analysis of numerous spectral features allows a direct comparison of experimental data with 3D plane wave and finite-difference time-domain calculations. We find, counter-intuitively, that the bandwidth for low-loss propagation completely vanishes for structural parameters where the photonic band gap is maximized. Our results demonstrate that, in order to maximize the bandwidth of low-loss waveguiding, the hole radius must be significantly reduced. While the photonic band gap considerably narrows, the bandwidth of low-loss propagation in PhC waveguides is increased up to 125nm with losses as low as 8$\pm$2dB/cm.Comment: 10 pages, 8 figure

Transmission of Slow Light through Photonic Crystal Waveguide Bends

The spectral dependence of a bending loss of cascaded 60-degree bends in photonic crystal (PhC) waveguides is explored in a slab-type silicon-on-insulator system. Ultra-low bending loss of (0.05+/-0.03)dB/bend is measured at wavelengths corresponding to the nearly dispersionless transmission regime. In contrast, the PhC bend is found to become completely opaque for wavelengths range corresponding to the slow light regime. A general strategy is presented and experimentally verified to optimize the bend design for improved slow light transmission.Comment: 4 pages, 3 figures; submitted to Optics Letter

New Zealand Guideline for the Connection of PV Solar Power and Determining Hosting Capacity for PV Solar Power

Small-scale distributed generation (DG) in New Zealand, particularly photovoltaic (PV) generation, has been growing steadily over the past few years. In the last year alone to 31 March 2016, installed PV generation of all capacities has grown by a factor of about 1.6 to reach 37 MW. Approximately 90% (33 MW) of this installed PV capacity is made up of small-scale, single phase residential grid-tied systems with ratings below 10 kW. This corresponds, on average, to approximately 300-400 new PV systems being installed each month within low voltage (LV) distribution networks. Traditionally, the flow of power in electricity distribution networks has been largely unidirectional. However, distributed generation introduces reverse power flows into the LV network when the power produced by DG systems is greater than what can be consumed locally. This introduction of reverse power flows and the dynamic behavior of DG system inverters can negatively impact the electricity network, causing issues such as over-voltage, phase imbalance, overloading of conductors and transformers, and create unique safety challenges. As such, each DG connection application received by electricity distribution businesses (EDBs) presently needs to be carefully considered for its impact on the electricity network. The resourcing demand imposed by larger numbers of connection applications, and the difficulty of technical assessment including congestion evaluation, are likely to increase substantially as DG uptake intensifies. This has prompted the Electric Power Engineering Centre (EPECentre) via its GREEN Grid programme, with the assistance of the electricity industry based Network Analysis Group (NAG), to develop a small-scale inverter based DG connection guideline for New Zealand EDBs. This has been developed on behalf of the Electricity Engineers’ Association (EEA) specifically for the connection of inverter energy systems (IES) of 10 kW or less. This paper summarizes key aspects of this guideline. This includes a streamlined connection application evaluation process that enables EDBs to efficiently categorize DG applications into three groups. These groups vary from those with minimal or moderate network impact that can be autoassessed, to those most likely to cause network congestion that require manual assessment. These categories are determined by looking at the DG hosting capacity specific to the LV network that the DG is connecting to. For two of these categories, mitigation measures for connection, are prescribed. It is also shown how DG hosting capacity can be used to simply evaluate LV network congestion in order to satisfy Electricity Industry Participation Code (EIPC) Part 6 requirements. Key technical requirements for all IES, appropriate for New Zealand conditions, are also summarized

Evanescent near-field optical lithography : overcoming the diffraction limit.

Concepts of optical resolution limits have been transformed in the past two decades with the development of near-field optical microscopy. Resolutions of λ/40 have been demonstrated by taking advantage of additional information present the near field of an object. These resolutions are far higher than what diffraction-limited lens-based optical systems are capable of. Attempts have been made to replicate these resolutions for lithography using a scanning probe based optical equivalent, but these systems suffer from low throughput owing to their serial nature. A desirable alternative would be replication of all the patterns within a field in a single flood exposure in a manner similar to how optical projection lithography replicates the field of a mask, but with the additional resolution available from working in the near field. This is the basis of evanescent near-field optical lithography, the subject of this thesis. Evanescent near-field optical lithography (ENFOL) brings traditional contact lithography into the near-near field using a combination of conformable masks and ultra-thin photoresists. This thesis describes a study of ENFOL both experimentally and via electromagnetic simulations to evaluate what the resolution limit might be. The fabrication of membrane masks is described, a key component for the ENFOL exposure. The characteristics of an ENFOL exposure using broad-band light are investigated from exposures into thick resist. These exposures demonstrate the trend of decreasing depth of field as the period of grating structures is reduced. ENFOL's requirement of a thin imaging photoresist for high resolution lithography complicates the pattern transfer step essential to translate the photoresist image into a useful material for devices. The development of an additive pattern transfer process is described, that utilises a trilayer resist scheme to enable lift-off metallisation. NiCr gratings with periods down to 270nm have been fabricated using this process subsequent to an ENFOL exposure. Wire-grid polarisers consisting of 270nm-period NiCr gratings on glass substrates have been fabricated and their polarisation properties measured at visible wavelengths. Simulation results of exposures of sub-wavelength grating structures are presented that investigate the fundamental limit to resolution for contact lithography techniques such as ENFOL. A full-vector, rigorous electromagnetic simulation technique, the multiple multipole program is used to provide information about the near field of subwavelength gratings. The potential for λ/20 resolution is indicated; a tantalising prospect for optical lithography and well below the diffraction limit of conventional optical projection-based lithographies. Perhaps the most critical parameter for an evanescent exposure, the depth of field, was characterised and a linear relationship shown between the depth of field and grating period. The effect of parameters such as grating duty cycle, absorber material and thickness on the exposure are observed with the intention to optimise the experimental setup. Interesting interference phenomena are observed in simulation results for exposures. where the effective exposure wavelength is equivalent to the grating period. In particular a period halving occurs in the transverse magnetic polarisation due to interference of the first diffracted orders. A novel interference technique - evanescent interference lithography is proposed that takes advantage of an enhanced period halving at an exposure wavelength corresponding to a grating resonance

Effects of time - scale on householder PV economic analyses: over - estimation of self - consumption

The economic analysis of when solar roof-top photovoltaic (PV) systems become viable is of great interest to many householders. It is well known that buy-back rates for solar energy are low and in general a PV system only becomes financially attractive if households consume a high proportion of the energy generated, in order to benefit from avoiding the cost of buying electricity at retail rates. Battery storage for most householders remains uneconomic (although prices are trending downwards) making power consumption patterns critical to determining financial viability. Currently householders have access to their power consumption from their retailer in half-hour time periods. If this information is combined with PV generation at the householder’s location, a good estimate of the financial viability can be determined. However, using a half-hour analysis period implies that the power generated and load consumed in the half hour period is constant. The reality can be very different though, as both loads and generation can spike or dip and it is the instantaneous load and generation that determines when power is exported or imported from the grid. To determine the effect of time-scale on self-consumption, values were calculated from one-minute generation and load data for a number of New Zealand installations and compared with calculations for half-hour and hourly data. All households analysed showed some over-estimation of self-consumption when compared with the one-minute baseline data. For a typical 3.5kW PV system, the over-estimation varied between 1% and 8%, depending on the base level of self-consumption and characteristics of the load profile. In addition, the impact of using median load profiles, such as those used by the EECA EnergywiseTM Solar Calculator on self-consumption values was investigated. The process of creating median load profiles further smooths the load, similar to the effect of using longer-time-scales. When using median load profiles, the self-consumption over-estimation was found to be in the order of 5% for the majority of householder profiles for the various regions specified in the Solar Calculator

The Economics and Potential Uptake of PV Solar Power by Region and PV System Cost

In 2015 the GREEN Grid project examined the economics of Photovoltaic solar power (PV) to residential users, commercial users, and utility companies. They found that for a certain number of residential households, PV was already financially viable. They also found that the financial viability was highly dependent on local irradiation, nature of the household load, access to capital, and cost of the PV system. Given that people are more likely to install solar when it becomes financial viable to do so, by exploring the factors affecting financial viability, potential uptake can be inferred. This paper extends the economics of PV to residential users to all regions of New Zealand, to understand the local irradiation and retail electricity pricing components, uses a greater sample of households’ load profiles, and looks at the potential uptake based on PV system cost. The paper also looks at the sensitivity of potential uptake based on PV system cost. It concludes that if all homes for which PV is financially viable under existing distribution pricing regimes install PV, the PV system cost is close to a point in most regions where the installed capacity of PV will rise very rapidly. From the analysis, the PV system cost needs to fall to about 2.3 $/Wp for the New Zealand wide median NPV to be at zero (i.e. where PV is financially viable for 50subject to assumptions. This is an important conclusion, as the PV system cost in New Zealand is not far from this already, and worldwide PV prices are forecast to continue to fall. The paper also concludes that for some regions, such as Marlborough and Gisborne, this point occurs at a higher system cost, around 3$/Wp (i.e. those regions are already close to the point where PV is financially viable for around 50% of the population). The reasons for this relate mainly to high irradiation and variable retail prices in those regions. The same conclusion could not be made for the Nelson and Tasman regions due to lack of load profile data