School of Physics - Theses
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ItemAstigmatic phase retrieval of lightfields with helical wavefronts( 2012)The controlled use of coherent radiation has led to the development of a wide range of imaging methods in which aspects of the phase are enhanced through diffraction and propagation. A mathematical description of the propagation of light allows us to determine the properties of an optical wavefield in any plane. When a sample is illuminated with coherent planar illumination and its diffracted wavefield is recorded in the far-field of propagation, a direct inverse calculation of the phase can be quickly performed through computational means – the fast Fourier transform. Algorithmic processing is required, however, because only the intensity of the diffracted wavefield can be recorded. To determine structural information about the sample, some other information must be known about the experimental system. What is known, and how it is processed computationally, has led to the development and successful application of a broad spectrum of phase reconstruction iterative algorithms. Vortices in lightfields have a helical structure to their wavefront, at the core of which exists, necessarily, a screw-discontinuity to their phase. They have a characteristic intensity distribution comprising a radially symmetric bright ring around a dark core which, for either handedness of the rotation of the vortex, appears identical. Observation of a vortex is, therefore, ambiguous in its ability to determine its true direction of rotation. The ubiquitous presence of vortices in all lightfields hinder the success of phase reconstruction methods based on planar illumination and, if successful, render any reconstruction of the phase non-unique, due to the ambiguity associated to their helicity. The presence of a controlled spherical phase distortion can break the symmetry of the appearance of the vortices and, hence, remove the ambiguity from the system and drive algorithms to a solution. For the pathological case of an on-axis vortex, however, spherical distortion will not break the radial symmetry. The astigmatic phase retrieval method separates the spherical distortion into cylindrical distortion in two orthogonal directions. This form of phase distortion breaks the symmetry of a vortex allowing a unique determination of the phase. The incorporation of such use of cylindrical distortion into an iterative phase reconstruction algorithm forms the basis for the astigmatic phase retrieval (APR) method. Presented in this thesis is the creation and propagation of lightfields with helical wavefronts, produced through simulation and experiment. Observation of the effects of cylindrical distortion on vortices is explored in detail, particularly for split high-charge vortices where their positions can inform the type and strength of the applied phase distortion. Experimentally, onaxis vortices are created and distorted for the purposes of astigmatic phase retrieval in both visible light and X-ray wavefields. This thesis presents the first experimental demonstration of the astigmatic phase retrieval (APR) method, successfully applied optically with a simple test sample. The method is also applied to lightfields with helical wavefronts. The successful unambiguous reconstruction of on-axis chargeone and charge-two visible light vortices are presented, which is the first experimental demonstration on the unique phase reconstruction of an on-axis vortex from intensity measurements alone. Experiments are then performed to apply the method to vortices created in X-ray wavefields. The parameters of the experiment and the data have not, however, allowed for a successful reconstruction in this case. It is demonstrated through extensive simulation analysis that the APR method is a fast and robust imaging method. It is also shown that, through observation of the error metric, experimental parameters can be corrected or even determined, making the method successful even if there is no a priori knowledge of the experimental system. The application of the APR method as a general imaging technique for use in high-resolution X-ray diffraction experiments is, therefore, is a logical extension of the work of this thesis.
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ItemErbium for optical modulation and quantum computation( 2017)Erbium (Er) is a lanthanide element, mainly used in its trivalent ionic form (Er3+), as an active dopant in optical devices, for light amplification or generation. The luminescence of Er3+ lies within the conventional wavelength band, 1530-1565 nm, for fiber-optic communication. The low noise, linear response, and stability of the optical gain provided by the Er3+ luminescence are ideal for applications in photonic systems that operate in the fiber-optic frequency. While much research has been done to understand the Er3+ luminescence in various lasing media, few studies have been conducted to tap the potential of Er for applications other than amplifiers or lasers. This thesis delves into two new areas, namely optical modulation and quantum computation, where the Er3+ luminescence may be able to be applied in a novel way. By incorporating Er3+ into a switchable optical material, an optical modulator could potentially be made that is capable of not only switching but also amplifying signal transmission or sustaining the signal intensity from propagation losses. This integrated approach could reduce device footprint and latency for on-chip as well as synchronous applications. Successful integration of Er, however, has never been demonstrated in conventional optical modulators because their reliance on electro-optic effects conflicts with the carrier-sensitive mechanism of the Er3+ luminescence. The compatibility between Er and a recently advocated optical material, namely vanadium dioxide (VO2), is examined in the first part of this thesis. VO2 exhibits a hysteretic, bistable phase transition that is accompanied by a high-contrast optical switching in infrared, including the fiber-optic, wavelength band. The phase transition can be triggered thermally as well as optically. When triggered optically, it can occur in picosecond timescale, making VO2 a promising material for ultrafast optical switching applications. Experimental characterizations of the Er3+ luminescence and the optical switching were performed on selectively prepared thin-film samples of VO2. The Er3+ luminescence could be observed after the samples were implanted with Er and then annealed between 800*C and 1000*C. The optical switching could also be measured in the implanted and annealed samples as they were thermally heated up and then cooled down past the critical temperature of the phase transition. The Er-implanted samples, however, were found to have broader hysteresis and lower switching contrasts than the pure VO2 samples. It is concluded that although Er-implanted VO2 could probably work as a combined optical switch and amplifier, the poorer switching qualities do not guarantee that a device based on the material could provide better utility than a separated system of optical switches and Er amplifiers. The Er3+ luminescence could also be utilized for quantum frequency conversion, for implementation in interconnects that interface superconducting quantum computers to a fiber-optic quantum network. For two superconducting quantum computers to be able to communicate over a fiber-optic quantum network, the frequency of the signals transmitted from either computer needs to be converted into the fiber-optic frequency, and then back into the microwave frequency upon receipt at the other computer. Early proposals suggested that the interconnect be at least comprised of Er3+ ions, a microwave resonator, and an optical resonator. The realization of this system has been attempted recently in basic experiments, but the conversion efficiency was found to be too low. The weak couplings between the Er3+ ions and the two resonators were identified as one of the main reasons for the low conversion efficiency. One way to mitigate the weak coupling in the microwave part is to have a superconducting flux qubit bridge the interaction between the Er3+ ions and the microwave resonator. The second part of this thesis presents a theoretical and simulation study of the dynamics of a coupled system consisting of Er3+ ions, a superconducting microwave resonator, an optical resonator, and a superconducting flux qubit. It is shown that quantum information can be exchanged between the Er3+ ions and the microwave resonator with a high fidelity via the qubit coupling, and the exchange process is controllable by changing the frequency of the qubit. The frequency conversion between the microwave and the optical regime is shown to be infeasible to be realized at the limit where the number of optical excitations (n) is much less than the number of ions (N). A high-efficiency frequency down-conversion is demonstrated to be achievable in the case where there is no decoherence, and both n and N are small. However, the time it takes to complete the down-conversion is very long, leaving the efficiency prone to decoherence. It is argued that for the frequency conversion to be able to be accomplished in a typical decoherent environment, both n and N need to be large. The study of the dynamics, in this case, is left for future research.
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ItemUltrafast spectroscopy of nanostructures( 2017)This thesis presents studies of ultrafast laser spectroscopy of semiconductor and gold nanostructures, aiming to advance our understanding of, and consequently control, photoinduced charge carrier dynamics in nanostructures to further improve their performance in practical applications. Artificial nanostructures have drawn significant attention in applications such as optoelectronic devices, photo-catalysts, and solar cells. Compared to bulk materials, nanostructures provide unique optical properties, which more importantly can be directly and easily tailored through changing size or shapes of the structures, during their synthesis procedures. Photoinduced charge carrier dynamics in the nanostructures play an important role in the photon conversion processes. However, in contrast to the fast development of nanostructure-based devices, the mechanisms of these processes are still being experimentally unravelled. In this study, a range of ultrafast optical spectroscopy methods have been applied to investigate the carrier dynamics, with a focus on the electron transfer (ET) process. Semiconductor nanoparticles, or quantum dots (QDs), of core/shell heterostructures are promising for their good photostability and high photoluminescence quantum yields. The ET dynamics from the 1S$_\mathrm{e}$ electron state to adsorbed methyl viologen electron acceptors, in CdSe/CdS and CdSe/CdS/ZnS QDs, were studied using femtosecond transient absorption and time-resolved photoluminescence spectroscopy. By changing shell thickness or alloying the shell interface, significant modulation of the ET dynamics was observed. In CdSe/CdS QDs, the 1S$_\mathrm{e}$ ET dynamics exhibited a hole-coupled effect, which is ascribed to the Auger-assisted ET process. In CdSe/CdS/ZnS QDs, the formation of alloyed shell interfaces at elevated shelling temperatures reduced the shell potential barrier, leading to an observed greater ET rate. Photoinduced ET processes from gold nanorod and nanowire structures to TiO$_{2}$ were also investigated, using a visible pump-NIR probe transient absorption spectroscopy method. Partially embedded Au nanorods on a TiO$_{2}$ layer exhibited an enhanced but directional ET process. An Au nanowire grating supported on a TiO$_{2}$ layer structure underwent the plasmon-waveguide hybridisation mechanism. The ET dynamics from the split states showed a dependence on the light-matter coupling effect that can be varied with the Au grating period. In summary, this thesis shows the great ability of ultrafast optical spectroscopy to reveal photoinduced processes in nanostructures. Results indicate ways for rational design of nanostructure-based devices. A greater understanding in underlying physics leads to better control of the performance of these nano-systems in potential practical applications.
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ItemNon-invasive imaging techniques for the investigation of cultural objects( 2017)Laser speckle is used in cultural materials conservation to study changes in the physical condition and movement on the surface and within the structure of objects. This thesis investigates three speckle methods, Laser Speckle Contrast Imaging (LSCI), Dynamic Speckle Imaging (DSI) and Electronic Speckle Pattern Interferometry (ESPI) to: image obscured text, examine paint application methods, record physical transformations during paint drying processes, and monitor canvas strain due to climate variations in- situ. It extends the boundaries of use of speckle-based methods in conservation through a set of four case studies that cover a broad range of cultural conservation problems faced in Australian collections. These studies were contextualised within an examination the properties of viscoelastic materials, a mathematical description of speckle, an explanation of the speckle-based methods and the experimental equipment and design considerations that provide information about the challenges and considerations required when deploying speckle-based methods. In the first case study, LSCI was used to image obscured text on books that had been repaired with layers of paper. The quality of the results, which was dependent on the weight of the paper, colour of the subsurface medium, and temperature of the object, was similar to that of Infrared Photography. In the second case study LSCI was used to monitor the curing process of modified alkyd resins over 120 hours to investigate whether the application method used by the artist was robust to typical ageing conditions. The results correlated well with traditional gravimetric results, which only provide information about the mass changes to the samples with no spatial information. LSCI was able to demonstrate where and when the alkyd resin surface was moving, most likely due to off gassing processes. The third case study used DSI to monitor the drying process of artists’ acrylic paints, allowing for the novel non-invasive detection of when the paint was touch dry, a task that normally involves destructive methods or has only been applied to rapid drying industrial paints. The final case study demonstrated the in-situ application of ESPI to monitor two archetypical colonial easel paintings in assessing physical movement due to variations in the display environment. ESPI was used in-situ to record changes in the strain of the canvas. Furthermore, it was shown that DSI analysis could be applied to ESPI data to monitor for vibrational disturbances that can decorrelate results The speckle-based techniques used in this thesis were shown to satisfy six criteria for preferred conservation methods: all information is gathered non-invasively and can be analysed in a variety of ways; off the shelf optical components are used, thus it is highly suitable for fields in which specialised optical components are not available; no prior analysis about the sample is required; experimental challenges can be diagnosed and overcome through the analysis of data sets and/or computer simulations; the techniques can be applied to the investigation of a range of materials with different optical properties; and multiple speckle-based methods can be combined to provide complementary information. The case-studies explored in this thesis further demonstrated that speckle is capable of non-invasive characterisation of many physical properties and can be applied both in the laboratory and in-situ. Through the assessment of theory and case studies this work has extended the application of speckle analysis within conservation studies, delivering practical outcomes to aid conservation efforts.
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ItemClassical processing for topological quantum error correction( 2017)This thesis deals with a series of issues implementing the classical software stack to control a large-scale quantum computer. Due to the relatively high error rates of quantum hardware expected for the foreseeable future, high threshold error correction schemes such as the topological cluster state and the surface code, will be essential to performing robust quantum computation. The work begins by bringing new functionality to the Autotune software library to support the presence of runtime qubit loss when using the topological cluster state for error correction. Through simulation, it is shown that <1% runtime loss is required for minimal space-time overhead. The thesis continues by constructing new software, LogiQSim, which enables fully-fault-tolerant quantum simulation and error analysis of logical quantum gates. Through the implementation of byproduct operator tracking and the placement of defects and correlation fragments, it was possible to demonstrate a full quantum simulation of a working logical controlled NOT gate (CNOT), as well as estimate its logical error rate directly through simulation. This approach was further improved to allow the simulation of the Clifford portion of A-state distillation, significant as this circuit is the predominant source of overhead in quantum computation when using the topological cluster state. The simulated overhead of the distillation process was compared to estimates calculated from the results of the logical CNOT simulations in order to determine the accuracy of estimating a larger circuit using the simulations of a smaller one. Furthermore, we found that the relationship between the structure of a correlation fragment and its contribution to the logical error rate is complex. This is due to an asymmetry in the frequency of logical errors that are the result of error chains connecting two defects as compared to rings of errors around defects. The work presented in this thesis makes substantial progress towards the understanding and development of a practical classical software stack that can be used to run a topologically error corrected quantum computer, and utilizes this (in the absence of physical hardware) to simulate and provide more accurate overhead estimates for key logical operations.
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ItemNano-electronics at graphene-electrolyte interfaces for biological application( 2017)Research of graphene for life science has attracted significant attention from not only the scientific community but also the general public. Graphene possesses many exciting properties which are beneficial for biological applications, such as biocompatibility, tunable chemical properties, atomic thickness, among others. Detection of biomolecules in liquids, such as the electrolytes encountered in biological environments, is a key application. However, the physical processes at the graphene-electrolyte interface are not fully understood. This thesis presents a theoretical and experimental study of this system, and demonstrates its application in bio-sensing. Electrical double layer (EDL) is a critical component necessary for the investigation of the processes at solid-electrolyte interfaces. A new ionic vibration model for the EDL as an improvement to the conventional Gouy-Chapman-Stern (GCS) theory is proposed in chapter 3, as the GCS fails to describe the EDL capacitance over a wider range of electrode potentials. This new theory elucidates the mechanism behind the ionic dynamics at the interface of solid-electrolyte systems, as ions are found to vibrate near the electrode surface in response to the applied electric field. The calculated results are consistent with measurements, and this provide useful insight not only important for biosensing (as we may need to change the potential on graphene over a wide range), but also for energy storage, neuron stimulation, among others. Since some of these applications (e.g.: neuron stimulation) require large capacitance, and this theory indicates that the EDL capacitance decreases with the electrode potential, one of the approaches is to increase the surface area of the electrode material (graphene). Conformally coating graphene onto a nanoporous three-dimensional (3D) structure gives 3D graphene with very large surface area. A novel method of directly fabricating 3D graphene by plasma assisted graphitization is presented in chapter 4. The obtained material shows very large electrochemical capacitance values, as high as 2.1 mF for a sample of 10 mm$^3$. It also exhibits excellent chemical stability, providing a good platform for electrochemical applications (e.g.: energy storage, neuron stimulation, etc.). This sample is then used to capacitively detect a protein (bovine serum albumin) as a non-destructive label-free method. The results can inspire the work on developing bio-sensors which are able to work as bio-filters simultaneously in future, due to the nano-porous structure. However, the unclear sensing mechanism (whether EDL capacitance or quantum capacitance is dominant), low sensitivity, and long response time of the sensor raise the task for searching for different sensing mechanisms. Based on the discussion of electron transport of graphene in solution in chapter 5, a new method for the detection of biomolecules at very low concentrations using Hall effect measurements on liquid gated graphene based devices is proposed in chapter 6. The results of L-histidine detection suggests high sensitivity in the pM range, exceeding the performance of the conventional amperometric and potentiometric techniques under the same conditions. Through the determination of changes of the mobility and the charge carrier density in graphene we provide new clues for understanding different mechanisms of molecular detection on graphene-based devices in liquids. Theoretical modeling of the experiments indicates that the asymmetric electron-hole mobilities can explain the enhanced detection sensitivity. Quantum capacitance is only dominant near the "Dirac" point, while EDL capacitance dominates at higher potential. This explains the drastic changes of the measured electronic properties close to the "Dirac" point. The detailed theoretical and experimental studies presented in this thesis provide a deep understanding for graphene-electrolyte systems, which are important not only for biological applications (such as biosensing, neuron stimulation and recording, bionic implants, etc), but also for energy storage systems.
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ItemInvestigation of B+ mesons decay to K+K−π+ at the Belle experiment( 2017)Charmless decays of B mesons to three charged hadrons are suppressed in the Standard Model, and thus provide an opportunity to search for physics beyond the Standard Model. An unexpected excess and a large CP asymmetry in the low invariant mass spectrum of the K+K− system for the decay B+ → K+K−π+ were observed by BaBar and LHCb in recent years. We present the measurements of branching fraction and direct CP asymmetry of the charmless decay B+ → K+K−π+. This analysis is performed on a data sample of 772 × 10^6BB pairs produced at the Υ(4S) resonance by the KEKB asymmetric-energy e+e− collider and collected by the Belle detector. We perform a blind analysis, examining signal reconstruc- tion and background suppression with Monte Carlo simulated samples, and extract signal yield and direct CP asymmetry with a 2D extended maximum likelihood fit to the data. The measured branching fraction and direct CP asymmetry are B(B+ → K+K−π+) = (5.38 ± 0.40 ± 0.35) × 10^−6 and ACP = −0.170 ± 0.073 ± 0.017, respectively, where the first uncertainties are statistical and the second are systematic. These results are in agreement with the current world average. We extract the branching fraction and direct CP asymmetry as a function of the K+K− invariant mass. The K+K− invariant mass distribution of the signal candidates shows an excess in the region below 1.5 GeV/c^2, which is consistent with the previous studies from BaBar and LHCb. Strong evidence of a large direct CP asymmetry of −0.90 ± 0.17 ± 0.03 with 4.8σ significance is found in the K+K− low-invariant-mass region.
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ItemElectron diffraction using a cold atom source( 2017)Observing matter on atomic length and time scales simultaneously is now routinely achieved in ultrafast electron and X-ray imaging techniques, but continued advances in both approaches promise to deliver huge leaps in our understanding of all kinds of atomic structures and processes. Both technologies rely on the generation of ultrabright, ultrashort duration electron bunches, with these bunches being used directly to probe the sample in electron diffraction, or to generate ultrabright X-ray pulses in X-ray free electron lasers. The Cold Atom Electron and Ion Source (CAEIS) was conceived as a possible way to generate ultrafast electron bunches that are brighter than can currently be produced, with the aim of enabling next-generation structural determination techniques, particularly those based on electron diffraction. The CAEIS generates electrons by near-threshold photoionisation of an atomic gas, which has been shown to produce electron bunches with temperature as low as 10K. Such cold electron bunches have the potential to be much brighter than those generated from solid photocathode sources, which typically have temperatures in the thousands of Kelvin range. Extremely cold ions are also generated in the CAEIS, which show great potential for use in ion microscopy and milling. This thesis presents work on a number of different aspects of the continued development of the cold atom electron and ion source, with a particular emphasis on progress towards ultrafast single-shot electron diffraction based experiments. The brightness degrading effects of space-charge repulsion are investigated using nanosecond duration ion bunches as analogues of ultrafast, picosecond duration electron bunches. Ion bunch shaping was achieved through tailoring of the spatial profile of lasers used in ionisation of the atomic gas. It was found that atomic fluorescence could substantially reduce the fidelity with which the ion bunch profile could be controlled, but methods were developed to circumvent the fluorescence problem. The improved shaping procedures allowed generation of uniformly filled ellipsoidal bunches, which theoretically will not suffer emittance degradation under space-charge expansion. Emittance measurements following space-charge driven expansion showed that these uniformly filled ellipsoids did indeed have reduced emittance growth compared to other profiles. Photoexcitation and field-ionisation processes involved in generation of cold electrons on ultrafast timescales were investigated, with the aim of determining the mechanisms that affect the ultimate electron bunch duration. Bunch duration was measured for a range of excitation conditions, with the finding that previously assumed ultrafast excitation pathways in fact generated fairly slow nanosecond long bunches. Ionisation time could also be a million times slower than assumed if atoms were excited below the classical ionisation threshold. Identification of the conditions required for ultrafast excitation and ionisation ultimately allowed generation of ultrafast cold electron bunches with duration of tens of picoseconds. Electron diffraction using nanosecond long electron bunches was achieved in both transmission and reflection modes for a variety of large samples of inorganic crystals. Bunches were of sufficiently high charge to allow identification of features of a crystal structure using only a single shot. Bragg peaks could also be identified by averaging together many images formed using ultrafast, but low-charge bunches. Simulations were performed to determine the feasibility of using electrons generated in the CAEIS for electron coherent diffractive imaging of nanoscale apertures. It was found that it should be possible to successfully reconstruct the object plane wavefield, even taking into account realistic experimental parameters for partial coherence and noise.
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ItemFabrication of single crystal diamond membranes for nanophotonics and nanomechanics( 2016)There is an intense interest in exploiting diamond’s remarkable combination of properties in many fields of science and technology. However, diamond fabrication and processing remains a major challenge in the application of diamond as a platform for quantum, nanophotonics, sensing and nanoelectromechanics. These challenges include: difficulty of scalable fabrication of thin single crystal diamond, lack of efficient colour centre-cavity coupling, and immature nanofabrication techniques. Although diamond is well known to display excellent properties for quantum information processing, realization of its potential in various practical applications requires the availability of thin (down to sub-micron thickness), high-quality single crystal diamond membranes. Scalability of such diamond platform for photonic structures is still in its infancy compared with traditional photonic platforms such as silicon-based technology. This thesis seeks the opportunity to overcome these obstacles. Successful outcomes will allow a diamond platform that will accelerate the transition from quantum science to applied quantum technology. The aim of this thesis is to design, fabricate, and characterize single crystal diamond membranes and use these membranes to create high quality factor nanophotonic and nanomechanical devices. In this work, a novel scalable method for the fabrication of single crystal diamond membrane windows down to 300nm thickness has been presented. A suite of integrated photonic structures with optical quality factor for telecom wavelength have been demonstrated. Furthermore the drum head resonators and cantilevers with high mechanical quality factor of in air. The demonstrated single crystal diamond membranes and devices provide immense opportunity toward a platform for wide range of applications that could lead to a significant advancement of quantum technologies.
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ItemNovel plasmonic elements for the generation, manipulation and detection of polarised light( 2017)Plasmonics provides an opportunity to develop nanoscale optical devices, where the spectral, angular and polarisation response can be tailored. The aim of this research is to determine which designs prove suitable as polarisation sensitive features, how novel nanofabriction techniques can be employed to scale up the production and in what settings can we use these scalable plasmonic polarising devices to address current nanophotonics challenges. Presented here is a study of potential plasmonics based methods of manipulating polarisation, including converting the polarisation state of a beam from linear to circular with carefully designed cross nanoapertures in a metallic film, strong filtering of left and right circular polarised light using 2D chiral geometries as well as creating a compact nanoantenna-enabled metal--semiconductor--metal photodetector to determine the polarisation state of a beam. To ensure the economic feasibility of the devices, special attention is also paid to novel scalable nanofabrication techniques. The cost of a feature is of great concern, as it is of little use to have an expensive feature for applications in consumer photonics. To that end a direct imprinting technique for the low cost production of plasmonic metasurfaces is investigated, including a study of the optical phenomena achievable and some potential applications are discussed. Finally, altering the quantum properties of emitters coupled to these scalable plasmonic features is investigated. Particular attention is paid to increasing the emission rate and polarising or focussing light from quantum sources using plasmonic nanocavities. These plasmon-exciton devices could see a reduction in the energy requirements of LED displays.