Flat-top temperature tuning response in periodically-poled nonlinear crystals

Second harmonic generation via periodically-poled nonlinear materials offers an efficient means of generating high-quality visible light that would be otherwise unattainable with traditional laser sources. While this technology has the potential for implementation in many mass-industrial applications, temperature stability requirements of 0.1 deg.C can make packaging with a pump source problematic. Using our high fidelity poling technique we have achieved precise placement of poled domains in Lithium Niobate based on the resulting mathematical models. These initial devices provide more than 4 deg.C flat-top temperature stability, albeit with a corresponding loss in operational efficiency. Our aim is to implement improved designs in magnesium-doped Lithium Niobate for packaging with near-room temperature diode-based pump sources, as could be applied towards RGB TV and projector applications

Gouy phase compensation in quasi-phase matching

In any focussed nonlinear interaction the focus induced phase shift, known as the Gouy phase shift, provides an imperfection in phase matching for any linearly invariant material. However, using an appropriately designed quasi-phase matched structure it is theoretically possible to compensate for the deleterious effects of the Gouy phase shift, allowing a symmetric frequency response and tighter optimal focussing than in a uniform material

Direct optical observation of walls and disclination effects in active photonic devices

Liquid crystal tunable Bragg Gratings defined in planar substrates via a laser patterning technique exhibit complex wavelength tuning. This tuning displays threshold points and hysteresis. These tuning features are shown to be a manifestation of physical processes occurring in the confined geometry of our tunable devices. Such physical processes include the formation and removal of line disclinations and an associated wall. We discuss the effect of walls in the liquid crystal with regards to voltage tuning characteristics and whether they may allow faster wavelength tuning

100 GHz electrically tunable planar Bragg grating via nematic liquid crystal overlay towards reconfigurable WDM networks

Novel liquid crystal-based integrated optical devices with >140GHz electrical tuning are presented for application towards reconfigurable wavelength division multiplexing (WDM) networks. Initial results with Bragg wavelength tuning covering five 25GHz WDM channel spacing have been achieved with 170V (peak-to-peak) sinusoidal voltages applied across electro-patterned ITO-covered glass electrodes placed 60µm apart. These prototype devices were fabricated using direct UV grating writing, with an evanescent field coupling into a liquid crystal overlay through an etched window. Electrically controlled liquid crystal birefringence modifies the waveguide effective index, resulting in Bragg wavelength shift. Merck 18523 nematic liquid crystals are used, exhibiting compatible refractive index values to that of silica (no=1.44, ne=1.49 at lambda=1550nm). Homeotropic alignment of the liquid crystal is provided by application of a surfactant layer.The inherent refractive index sensitivity of our etched direct-UV-written structures allows observation of previously unreported liquid crystal surface-behaviour, such as multi-threshold points during variation of the applied field. Continued optimisation based on evanescent field penetration, electrode layout, and surface interaction will allow implementation towards a variety of novel liquid crystal applications and devices. For example, a cascaded architecture of these integrated liquid crystal devices operating at different Bragg wavelengths would pave the way towards true colorless add/drop modules for dense optical networks

P-type conductivity in Sn-doped Sb2Se3

Antimony selenide (Sb2Se3) is a promising absorber material for thin-film photovoltaics. However, certain areas of fundamental understanding of this material remain incomplete and this presents a barrier to further efficiency gains. In particular, recent studies have highlighted the role of majority carrier type and extrinsic doping in drastically changing the performance of high efficiency devices [1]. Herein, Sndoped Sb2Se3 bulk crystals are shown to exhibit p-type conductivity using Hall effect and hot-probe measurements. The measured conductivities are higher than those achieved through native defects alone, but with a carrier density (up to 7.4 × 1014 cm−3) several orders of magnitude smaller than the quantity of Sn included in the source material. Additionally, a combination of ultraviolet, X-ray and hard X-ray photoemission spectroscopies are employed to obtain a non-destructive depth profile of the valence band maximum, confirming p-type conductivity and indicating a majority carrier type inversion layer at the surface. Finally, these results are supported by density functional theory calculations of the defect formation energies in Sn-doped Sb2Se3, showing a possible limit on the carrier concentration achievable with Sn as a dopant. This study sheds light on the effectiveness of Sn as a p-type dopant in Sb2Se3 and highlights avenues for further optimisation of doped Sb2Se3 for solar energy devices

QPM grating design for novel PPLN structures

This thesis describes a series of theoretical and experimental studies into modifying the phasematching characteristics of nonlinear parametric interactions, specifically second harmonic generation, using quasi-phase-matched structures. The use of quasi-phase-matching by periodic poling affords a flexibility in designing tailored phase-matching characteristics not offered by alternative techniques. In this work phase matching characteristics are modified to provide enhanced acceptance bandwidths, compensation for focusing effects and high power operation. The first result of this work describes the design and manufacture of 20mm long LiNbO3 aperiodic quasi-phase matched devices for the generation of stable second harmonic power across wide temperature ranges. Theoretical simulations have demonstrated constant power output over a range of 9C. Providing over 35 times the bandwidth of equivalent length periodic structures, whilst offering almost an order of magnitude efficiency enhancement over periodic devices with the same bandwidth. Experimental verification of these devices has shown that stable power can be obtained across wide temperature ranges with only slight deviation from theory. Additionally, an investigation into the effects of focusing on second harmonic generation is undertaken. In this work the Gouy phase of a focused beam has been analytically identified as the source of dephasing in bulk nonlinear interactions, causing such effects as back conversion, reduced efficiency and errors in the phase matching condition. A method to negate these effects, using a modified QPM structure has been proposed and experimentally demonstrated. Finally, simultaneous compensation of both the Gouy phase and focused intensity variation has been applied to aperiodic, wide temperature bandwidth devices. Removal of these deleterious effects has been shown theoretically to correct the experimentally observed bandwidth errors, resulting in the focused interactions performing identically to plane-wave simulations

Improved SHG phase matching response for focused Gaussian beams in Gouy compensated quasi-phase-matched structures

Nonlinear interactions such as second harmonic generation (SHG) [1] and sum and difference frequency generation are routinely used for the generation of laser wavelengths. With high power lasers it is necessary to use bulk crystals and tight beam focusing to achieve the maximum conversion efficiency. The use of Quasiphase-matched (QPM) materials such as PPLN and PPKTP are now routinely reported; however, as we will show in this paper, a simple linear QPM crystal does not fully optimize the conversion response. We will report theoretical and experimental results showing that by careful control of the crystal design it is possible to fully compensate for the phase errors associated with the focusing induced Gouy shift

268nm Period Bragg gratings and integrated circuits produced by direct UV writing

We demonstrate 268nm period planar Bragg gratings and Mach-Zehnder interferometers fabricated by direct UV-writing. Grating reflectivities of ~30dB and FWHM of ~0.16nm were measured at operational wavelengths around 800nm

Towards high-speed liquid crystal electrically tunable planar Bragg gratings for integrated optical networks

Liquid crystal-based integrated optical devices offer the potential for high speed and dynamically tunable optical switches in modern telecommunications networks. Here, electrically tunable devices have major advantages over their thermal counterparts, with superior response times and low operating voltages (~100V). Our approach to achieving such devices is to fabricate planar optical waveguides with integrated Bragg gratings via direct UV writing1 into silica-on-silicon samples with evanescent field coupling into a liquid crystal overlay through an etched window (Fig. 1(a)). Such electrically tunable devices work on the principle of shifting the Bragg wavelength by modifying the effective index of a waveguide in a multilayer substrate. Electrically controlled liquid crystal birefringence modifies the waveguide effective index, producing a Bragg wavelength shift. In our early samples, Merck 18523 nematic liquid crystal is used as it has a compatible refractive index to silica (n=1.49 at lambda=1550nm). Homeotropic alignment of the liquid crystal is provided by application of a surfactant layer

Not Available

Not AvailableRice production in aerobic conditions holds promise in Asia especially in the era of increasing irrigation water scarcity. Field experiments were conduceted on aerobic rice systems at the research farm of the Directorate of Water Management (ICAR), Bhubaneswar, India, during dry seasons of 2007-2008 to 2009-2010 to evaluate different irrigation regimes and water saving potential in aerobic rice compared to traditional flooded rice, varietal performance and fertilizer N-rates. Results revealed that the rice varieties viz. ‘Surendra’, ‘Apo’ and ‘Lalat’ showed the highest yield potential between 3.9 to 4.6 t ha-1 under aerobic conditions with soil moisture at 80-90% of field capacity throughout the growing season. These varieties were suitable for growing under aerobic condition because of favorable physiological characteristics and crop yield. Water input as a pre-sowing irrigation was estimated as 54-62 mm for aerobic rice, and 362-401 mm for wet land preparation for traditional flooded rice. On average, water input during crop growth stage was 506 mm for aerobic rice and 882 mm for traditional flooded rice. In total, saving potential of water input was 42-60% with aerobic rice when compared to traditional flooded rice. The grain yield was 2.39-3.36 t ha-1 under aerobic irrigation regimes, with the highest being with irrigation at 80-90% of field capacity of soils. Results showed a reduction in yield under aerobic conditions as compared to traditional flooded. This yield reduction was 16% with irrigation at 80-90% of field capacity. However, estimated water productivity, with respect to rainfall and irrigation water input, increased in aerobic rice (4.71 kg grain ha-1 mm-1) compared to traditional one (3.04 kg grain ha-1mm-1). Studies on irrigation x N interaction revealed that a highest grain yield of 4.4 t ha-1 was obtained with N rate of 120 kg ha-1 receiving 780 mm irrigation for rice variety ‘Surendra’. The next best combination viz. N rate of 80 kg ha-1 with 780 mm irrigation (3.84 t ha-1) and N rate of 120 kg ha-1 and 660 mm irrigation (3.61 t ha-1) were statistically similar. Hence, based on availability of irrigation water, N rate needs to be decided. The study on varietal (viz. ‘Apo’, ‘Lalat’ and ‘Surendra’) response to N rates showed that, irrespective of variety, aerobic rice with 120 N kg ha-1 with 780 mm irrigation gave the highest grain and straw yield of 4.24 and 6.63 t ha-1 with grain and straw N-uptake of 52.17 and 52.63 kg ha-1, respectively.Not Availabl