Tuning localized plasmons in nanostructured substrates for surface-enhanced Raman scattering

Comprehensive reflectivity mapping of the angular dispersion of nanostructured arrays comprising of inverted pyramidal pits is demonstrated. By comparing equivalently structured dielectric and metallic arrays, diffraction and plasmonic features are readily distinguished. While the diffraction features match expected theory, localised plasmons are also observed with severely flattened energy dispersions. Using pit arrays with identical pitch, but graded pit dimensions, energy scaling of the localised plasmon is observed. These localised plasmons are found to match a simple model which confines surface plasmons onto the pit sidewalls thus allowing an intuitive picture of the plasmons to be developed. This model agrees well with a 2D finite-difference time-domain simulation which shows the same dependence on pit dimensions. We believe these tuneable plasmons are responsible for the surface-enhancement of the Raman scattering (SERS) of an attached layer of benzenethiol molecules. Such SERS substrates have a wide range of applications both in security, chemical identification, environmental monitoring and healthcare

Compact strain-sensitive flexible photonic crystals for sensors

A promising fabrication route to produce absorbing flexible photonic crystals is presented, which exploits self-assembly during the shear processing of multi-shelled polymer spheres. When absorbing material is incorporated in the interstitial space surrounding high-refractive-index spheres, a dramatic enhancement in the transmission edge on the short-wavelength side of the band gap is observed. This effect originates from the shifting optical field spatial distribution as the incident wavelength is tuned around the band gap, and results in a contrast up to 100 times better than similar but nonabsorbing photonic crystals. An order-of-magnitude improvement in strain sensitivity is shown, suggesting the use of these thin films in photonic sensors

The new physics of non-equilibrium condensates: insights from classical dynamics

We discuss the dynamics of classical Dicke-type models, aiming to clarify the mechanisms by which coherent states could develop in potentially non-equilibrium systems such as semiconductor microcavities. We present simulations of an undamped model which show spontaneous coherent states with persistent oscillations in the magnitude of the order parameter. These states are generalisations of superradiant ringing to the case of inhomogeneous broadening. They correspond to the persistent gap oscillations proposed in fermionic atomic condensates, and arise from a variety of initial conditions. We show that introducing randomness into the couplings can suppress the oscillations, leading to a limiting dynamics with a time-independent order parameter. This demonstrates that non-equilibrium generalisations of polariton condensates can be created even without dissipation. We explain the dynamical origins of the coherence in terms of instabilities of the normal state, and consider how it can additionally develop through scattering and dissipation.Comment: 10 pages, 4 figures, submitted for a special issue of J. Phys.: Condensed Matter on "Optical coherence and collective phenomena in nanostructures". v2: added discussion of links to exact solution

Eliminating irreproducibility in SERS substrates

Irreproducibility in surface-enhanced Raman spectroscopy (SERS) due to variability among substrates is a source of recurrent debate within the field. It is regarded as a major hurdle towards the widespread adoption of SERS as a sensing platform. Most of the literature focused on developing substrates for various applications considers reproducibility of lower importance. Here, we address and analyse the sources of this irreproducibility in order to show how these can be minimised. We apply our findings to a simple substrate demonstrating reproducible SERS measurements with relative standard deviations well below 1% between different batches and days. Identifying the sources of irreproducibility and understanding how to reduce these can aid in the transition of SERS from the lab to real world applications.Isaac Newton Trust Leverhulme Trust Winton Programme for the Physics of Sustainability Trinity College, University of Cambridg

Few-emitter lasing in single ultra-small nanocavities

Funding: We acknowledge support from EPSRC grants EP/G060649/1, EP/L027151/1, EP/G037221/1, EP/T014032/1, EPSRC NanoDTC, and from the European Research Council (ERC) under Horizon 2020 research and innovation programme PICOFORCE (Grant Agreement No. 883703), THOR (Grant Agreement No. 829067) and POSEIDON (Grant Agreement No. 861950). O.S.O acknowledges the support of a Rubicon fellowship from the Netherlands Organisation for Scientific Research.Lasers are ubiquitous for information storage, processing, communications, sensing, biological research, and medical applications. To decrease their energy and materials usage, a key quest is to miniaturize lasers down to nanocavities. Obtaining the smallest mode volumes demands plasmonic nanocavities, but for these, gain comes from only single or few emitters. Until now, lasing in such devices was unobtainable due to low gain and high cavity losses. Here, we demonstrate a form of “few emitter lasing” in a plasmonic nanocavity approaching the single-molecule emitter regime. The few-emitter lasing transition significantly broadens, and depends on the number of molecules and their individual locations. We show this non-standard few-emitter lasing can be understood by developing a theoretical approach extending previous weak-coupling theories. Our work paves the way for developing nanolaser applications as well as fundamental studies at the limit of few emitters.Publisher PDFPeer reviewe

Using Nanocavity Plasmons to Improve Solar Cell Efficiency

Although in principle very promising, photovoltaic technology has so far failed to deliver robust high efficiency modules at affordable prices. Despite considerable research, high efficiency silicon based cells remain expensive, while the more recent organic photovoltaics are still struggling with low efficiencies and short lifetimes. Meanwhile, over the last few years, the study of localized plasmons [1,2] has also received great attention due to the high field enhancements associated with confined fields , with a wide range of applications possible, from optical switches to substrates for surface enhanced Raman spectroscopy (SERS). Here we discuss how combining the structures normally used in photovoltaic devices with metallic cavities supporting localized plasmons can lead to considerable improvements in the performance of solar cells. In particular we show how by changing the shape and size of spherical voids on a metallic surface, one can tune the plasmon modes to obtain significant absorptions across the solar spectrum [3]. By coating one such nanocavity surface with a sub 100 nm-layer of semiconductor, we can create a nanostructured solar cell, where the localised Mie modes efficiently couple light into the semiconductor layer. As the plasmons electric field enhancement is largest very close to the surface, significant absorption can be maintained even when the semiconductor thickness is reduced to below the typical exciton diffusion length. In addition minority carrier transport is improved. That means we can beat the usual balance between light absorption and exciton recombination loses, and so significantly increase the overall efficiency of the photovoltaic devices. Keywords: plasmons, solar cells, nanostructured surfaces

Pulsed Molecular Optomechanics in Plasmonic Nanocavities: From Nonlinear Vibrational Instabilities to Bond-Breaking

Small numbers of surface-bound molecules are shown to behave as would be expected for opto-mechanical oscillators placed inside plasmonic nano-cavities that support extreme confinement of optical fields. Pulsed Raman scattering reveals superlinear Stokes emission above a threshold, arising from the stimulated vibrational pumping of molecular bonds under pulsed excitation shorter than the phonon decay time, and agreeing with pulsed optomechanical quantum theory. Reaching the parametric instability (equivalent to a phonon laser or ‘phaser’ regime) is however hindered by motion of gold atoms and molecular reconfiguration at phonon occupations approaching unity. We show how this irreversible bond breaking can ultimately limit the exploitation of molecules as quantum mechanical oscillators, but accesses optically-driven chemistry

Using candidacy theory to explore unemployed over-50s perceptions of suitability of a welfare to work programme: a longitudinal qualitative study

Welfare to work interventions seek to move out‐of‐work individuals from claiming unemployment benefits towards paid work. However, previous research has highlighted that for over‐50s, particularly those with chronic health conditions, participation in such activities are less likely to result in a return to work. Using longitudinal semi‐structured interviews, we followed 26 over‐50s during their experience of a mandated welfare to work intervention (the Work Programme) in the United Kingdom. Focusing on their perception of suitability, we utilise and adapt Candidacy Theory to explore how previous experiences of work, health, and interaction with staff (both in the intervention, and with healthcare practitioners) influence these perceptions. Despite many participants acknowledging the benefit of work, many described a pessimism regarding their own ability to return to work in the future, and therefore their lack of suitability for this intervention. This was particularly felt by those with chronic health conditions, who reflected on difficulties with managing their conditions (e.g., attending appointments, adhering to treatment regimens). By adapting Candidacy Theory, we highlighted the ways that mandatory intervention was navigated by all the participants, and how some discussed attempts to remove themselves from this intervention. We also discuss the role played by decision makers such as employment‐support staff and healthcare practitioners in supporting or contesting these feelings. Findings suggest that greater effort is required by policy makers to understand the lived experience of chronic illness in terms of ability to RTW, and the importance of inter‐agency work in shaping perceptions of those involved

Role of Changing Job Demands and Control in Incapacity Claims

Abstract It remains a puzzle as to why incapacity claims rose in many OECD countries when life expectancy was increasing. While potentially due to hidden unemployment and policy failure, this paper tests a further explanation: that work has become more difficult for disabled workers. It focuses on the UK as a 'most likely' case, given evidence of intensification and declining control at work. To get a more objective measure of working conditions, the models use average working conditions in particular occupations, and impute this into the British Household Panel Survey. The results show that people in low-control (but not high-demands) jobs are more likely to claim incapacity benefits in the following year, a result that is robust to a number of sensitivity analyses. Deteriorating job control seems to be a part of the explanation for rising incapacity, and strategies to cut the number of incapacity claimants should therefore consider ways to improve job control. Given the challenges in changing job characteristics, however, an equally important implication is that high levels of incapacity should not just be seen as a result of poor policies and a lack of jobs, but also as a result of the changing nature of work