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

Green-pumped, picosecond MgO:PPLN optical parametric oscillator

We investigate the performance of a magnesium-oxide-doped periodically poled lithium niobate crystal (MgO:PPLN) in an optical parametric oscillator (OPO) synchronously-pumped by 530nm, 20ps, 230MHz pulses with an average power of up to 2W from a frequency-doubled, gain-switched laser diode seed and a multi-stage Yb:fiber amplifier system. The OPO produces ~165mW (signal, 845nm) and ~107mW (idler, 1421nm) of average power for ~1W of pump power and can be tuned from ~800nm to 900nm (signal) and 1.28µm to 1.54µm (idler). Observations of photo-refraction and green-induced infrared absorption (GRIIRA) in different operational regimes of the MgO:PPLN OPO are described and the role of peak intensity and average power are investigated, both with the aim to find the optimal operating regime for pulsed systems

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

Chemical etching of Sb2Se3 solar cells: surface chemistry and back contact behaviour

The effect of (NH4)2S and CS2 chemical etches on surface chemistry and contacting in Sb2Se3 solar cells was investigated via a combination of x-ray photoemission spectroscopy (XPS) and photovoltaic device analysis. Thin film solar cells were produced in superstrate configuration with an absorber layer deposited by close space sublimation. Devices of up to 5.7% efficiency were compared via current–voltage measurements (J–V) and temperature-dependent current–voltage (J–V–T) analysis. XPS analysis demonstrated that both etching processes were successful in removing Sb2O3 contamination, while there was no decrease in free elemental selenium content by either etch, in contrast to prior work. Using J–V–T analysis the removal of Sb2O3 at the back surface in etched samples was found to improve contacting by reducing the potential barrier at the back contact from 0.43 eV to 0.26 eV and lowering the series resistance. However, J–V data showed that due to the decrease in shunt resistance and short-circuit current as a result of etching, the devices show a lower efficiency following both etches, despite a lowering of the series resistance. Further optimisation of the etching process yielded an improved efficiency of 6.6%. This work elucidates the role of surface treatments in Sb2Se3 devices and resolves inconsistencies in previously published works

Natural Band Alignments and Band Offsets of Sb2Se3 Solar Cells

Sb2Se3 is a promising material for use in photovoltaics, but the optimum device structure has not yet been identified. This study provides band alignment measurements between Sb2Se3, identical to that used in high-efficiency photovoltaic devices, and its two most commonly used window layers, namely, CdS and TiO2. Band alignments are measured via two different approaches: Anderson’s rule was used to predict an interface band alignment from measured natural band alignments, and the Kraut method was used in conjunction with hard X-ray photoemission spectroscopy to directly measure the band offsets at the interface. This allows examination of the effect of interface formation on the band alignments. The conduction band minimum (CBM) of TiO2 is found by the Kraut method to lie 0.82 eV below that of Sb2Se3, whereas the CdS CBM is only 0.01 eV below that of Sb2Se3. Furthermore, a significant difference is observed between the natural alignment- and Kraut method-determined offsets for TiO2/Sb2Se3, whereas there is little difference for CdS/Sb2Se3. Finally, these results are related to device performance, taking into consideration how these results may guide the future development of Sb2Se3 solar cells and providing a methodology that can be used to assess band alignments in device-relevant systems

Band alignment of Sb2O3 and Sb2Se3

Antimony selenide (Sb2Se3) possesses great potential in the field of photovoltaics (PV) due to its suitable properties for use as a solar absorber and good prospects for scalability. Previous studies have reported the growth of a native antimony oxide (Sb2O3) layer at the surface of Sb2Se3 thin films during deposition and exposure to air, which can affect the contact between Sb2Se3 and subsequent layers. In this study, photoemission techniques were utilized on both Sb2Se3 bulk crystals and thin films to investigate the band alignment between Sb2Se3 and the Sb2O3 layer. By subtracting the valence band spectrum of an in situ cleaved Sb2Se3 bulk crystal from that of the atmospherically contaminated bulk crystal, a valence band offset (VBO) of −1.72 eV is measured between Sb2Se3 and Sb2O3. This result is supported by a −1.90 eV VBO measured between Sb2O3 and Sb2Se3 thin films via the Kraut method. Both results indicate a straddling alignment that would oppose carrier extraction through the back contact of superstrate PV devices. This work yields greater insight into the band alignment of Sb2O3 at the surface of Sb2Se3 films, which is crucial for improving the performance of these PV devices

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