Fluorescence lifetime imaging for explosive detection

In this Letter, a time-resolved 120 × 128 pixel single-photon avalanche diode (SPAD) sensor is used in conjunction with an array of organic semiconductor films as a means of detecting the presence of explosive vapors. Using the spatial and temporal resolution of the sensor, both fluorescence intensity and fluorescence lifetime can be monitored on a pixel-by-pixel basis for each of the polymer films arranged in a 2 × 2 grid. This represents a significant improvement on similar systems demonstrated in the past, which either offer spatial resolution without the temporal resolution required to monitor lifetime or offer only a single bulk measurement of lifetime and intensity without the spatial resolution. The potential of the sensing system is demonstrated using vapors of DNT, and differing responses for each of the four polymer films is observed. This system has clear applications as the basis of a portable chemical fingerprinting tool with applications in humanitarian demining and security.

Thermal control of organic semiconductors for trace detection of explosives

Organic semiconductors can be applied as ultra-sensitive fluorescent sensors for detecting trace vapours of explosives. The detection of explosives is manifest by the fluorescence quenching of the sensors. However, for many organic fluorescent sensors, the fluorescence quenching is irreversible and imposes a limitation in terms of reusability. Here we present a study of the thermal control of thin-film fluorescent sensors made from the commercial fluorescent polymer Super Yellow (SY). Thermal control of the sensor's temperature results in the desorption of the absorbed analytes, nitroaromatic explosives (2,4-DNT and DNB), and a taggant molecule (DMDNB). The amount of photoluminescence (PL) quenching and the desorption temperature of analytes provides a route to discriminate between the analytes, and additonally make the SY sensors reusable.

Dominance of photo over chromatic acclimation strategies by habitat-forming mesophotic red algae

Red coralline algae are the deepest living macroalgae, capable of creating spatially complex reefs from the intertidal to 100+ m depth with global ecological and biogeochemical significance. How these algae maintain photosynthetic function under increasingly limiting light intensity and spectral availability is key to explaining their large depth distribution. Here, we investigated the photo- and chromatic acclimation and morphological change of free-living red coralline algae towards mesophotic depths in the Fernando do Noronha archipelago, Brazil. From 13 to 86 m depth, thalli tended to become smaller and less complex. We observed a dominance of the photo-acclimatory response, characterized by an increase in photosynthetic efficiency and a decrease in maximum electron transport rate. Chromatic acclimation was generally stable across the euphotic-mesophotic transition with no clear depth trend. Taxonomic comparisons suggest these photosynthetic strategies are conserved to at least the Order level. Light saturation necessitated the use of photoprotection to 65 m depth, while optimal light levels were met at 86 m. Changes to the light environment (e.g. reduced water clarity) due to human activities therefore places these mesophotic algae at risk of light limitation, necessitating the importance of maintaining good water quality for the conservation and protection of mesophotic habitats.

Electrically driven organic laser using integrated OLED pumping

Organic semiconductors are carbon-based materials that combine optoelectronic properties with simple fabrication and the scope for tuning by changing their chemical structure1-3. They have been successfully used to make organic light-emitting diodes2,4,5 (OLEDs, now widely found in mobile phone displays and televisions), solar cells1, transistors6 and sensors7. However, making electrically driven organic semiconductor lasers is very challenging8,9. It is difficult because organic semiconductors typically support only low current densities, suffer substantial absorption from injected charges and triplets, and have additional losses due to contacts10,11. In short, injecting charges into the gain medium leads to intolerable losses. Here we take an alternative approach in which charge injection and lasing are spatially separated, thereby greatly reducing losses. We achieve this by developing an integrated device structure that efficiently couples an OLED, with exceptionally high internal-light generation, with a polymer distributed feedback laser. Under the electrical driving of the integrated structure, we observe a threshold in light output versus drive current, with a narrow emission spectrum and the formation of a beam above the threshold. These observations confirm lasing. Our results provide an organic electronic device that has not been previously demonstrated, and show that indirect electrical pumping by an OLED is a very effective way of realizing an electrically driven organic semiconductor laser. This provides an approach to visible lasers that could see applications in spectroscopy, metrology and sensing.

Manipulation of the Structure and Optoelectronic Properties through Bromine Inclusion in a Layered Lead Bromide Perovskite

One of the great advantages of organic-inorganic metal halides is that their structures and properties are highly tuneable and this is important when optimizing materials for photovoltaics or other optoelectronic devices. One of the most common and effective ways of tuning the electronic structure is through anion substitution. Here, we report the inclusion of bromine into the layered perovskite [H3N(CH2)6NH3]PbBr4 to form [H3N(CH2)6NH3]PbBr4·Br2, which contains molecular bromine (Br2) intercalated between the layers of corner-sharing PbBr6 octahedra. Bromine intercalation in [H3N(CH2)6NH3]PbBr4·Br2 results in a decrease in the band gap of 0.85 eV and induces a structural transition from a Ruddlesden-Popper-like to Dion-Jacobson-like phase, while also changing the conformation of the amine. Electronic structure calculations show that Br2 intercalation is accompanied by the formation of a new band in the electronic structure and a significant decrease in the effective masses of around two orders of magnitude. This is backed up by our resistivity measurements that show that [H3N(CH2)6NH3]PbBr4·Br2 has a resistivity value of one order of magnitude lower than [H3N(CH2)6NH3]PbBr4, suggesting that bromine inclusion significantly increases the mobility and/or carrier concentration in the material. This work highlights the possibility of using molecular inclusion as an alternative tool to tune the electronic properties of layered organic-inorganic perovskites, while also being the first example of molecular bromine inclusion in a layered lead halide perovskite. By using a combination of crystallography and computation, we show that the key to this manipulation of the electronic structure is the formation of halogen bonds between the Br2 and Br in the [PbBr4]∞ layers, which is likely to have important effects in a range of organic-inorganic metal halides.

Red algae acclimate to low light by modifying phycobilisome composition to maintain efficient light harvesting

BACKGROUND

Despite a global prevalence of photosynthetic organisms in the ocean's mesophotic zone (30-200+ m depth), the mechanisms that enable photosynthesis to proceed in this low light environment are poorly defined. Red coralline algae are the deepest known marine benthic macroalgae - here we investigated the light harvesting mechanism and mesophotic acclimatory response of the red coralline alga Lithothamnion glaciale.

RESULTS

Following initial absorption by phycourobilin and phycoerythrobilin in phycoerythrin, energy was transferred from the phycobilisome to photosystems I and II within 120 ps. This enabled delivery of 94% of excitations to reaction centres. Low light intensity, and to a lesser extent a mesophotic spectrum, caused significant acclimatory change in chromophores and biliproteins, including a 10% increase in phycoerythrin light harvesting capacity and a 20% reduction in chlorophyll-a concentration and photon requirements for photosystems I and II. The rate of energy transfer remained consistent across experimental treatments, indicating an acclimatory response that maintains energy transfer.

CONCLUSIONS

Our results demonstrate that responsive light harvesting by phycobilisomes and photosystem functional acclimation are key to red algal success in the mesophotic zone.

Ecosystem engineer morphological traits and taxon identity shape biodiversity across the euphotic-mesophotic transition

The euphotic-mesophotic transition is characterized by dramatic changes in environmental conditions, which can significantly alter the functioning of ecosystem engineers and the structure of their associated communities. However, the drivers of biodiversity change across the euphotic-mesophotic transition remain unclear. Here, we investigated the mechanisms affecting the biodiversity-supporting potential of free-living red coralline algae-globally important habitat creators-towards mesophotic depths. Across a 73 m depth gradient, we observed a general decline in macrofaunal biodiversity (fauna abundance, taxon richness and alpha diversity), but an increase in beta-diversity (i.e. variation between assemblages) at the deepest site (86 m depth, where light levels were less than 1% surface irradiance). We identified a gradient in abundance decline rather than distinct ecological shifts, driven by a complex interaction between declining light availability, declining size of the coralline algal host individuals and a changing host taxonomy. However, despite abundance declines, high between-assemblage variability at deeper depths allowed biodiversity-supporting potential to be maintained, highlighting their importance as coastal refugia.

Honeybee-based biohybrid system for landmine detection

Legacy landmines in post-conflict areas are a non-discriminatory lethal hazard and can still be triggered decades after the conflict has ended. Efforts to detect these explosive devices are expensive, time-consuming, and dangerous to humans and animals involved. While methods such as metal detectors and sniffer dogs have successfully been used in humanitarian demining, more tools are required for both site surveying and accurate mine detection. Honeybees have emerged in recent years as efficient bioaccumulation and biomonitoring animals. The system reported here uses two complementary landmine detection methods: passive sampling and active search. Passive sampling aims to confirm the presence of explosive materials in a mine-suspected area by the analysis of explosive material brought back to the colony on honeybee bodies returning from foraging trips. Analysis is performed by light-emitting chemical sensors detecting explosives thermally desorbed from a preconcentrator strip. The active search is intended to be able to pinpoint the place where individual landmines are most likely to be present. Used together, both methods are anticipated to be useful in an end-to-end process for area surveying, suspected hazardous area reduction, and post-clearing internal and external quality control in humanitarian demining.

Comparison of UV-A photolytic and UV/TiO2 photocatalytic effects on Microcystis aeruginosa PCC7813 and four microcystin analogues: A pilot scale study

To date, the high cost of supplying UV irradiation has prevented the widespread application of UV photolysis and titanium dioxide based photocatalysis in removing undesirable organics in the water treatment sector. To overcome this problem, the use of UV-LEDs (365 nm) for photolysis and heterogeneous photocatalysis applying TiO₂ coated glass beads under UV-LED illumination (365 nm) in a pilot scale reactor for the elimination of Microcystis aeruginosa PCC7813 and four microcystin analogues (MC-LR, -LY, -LW, -LF) with a view to deployment in drinking water reservoirs was investigated. UV-A (365 nm) photolysis was shown to be more effective than the UV/TiO₂ photocatalytic system for the removal of Microcystis aeruginosa cells and microcystins. During photolysis, cell density significantly decreased over 5 days from an initial concentration of 5.8 × 10⁶ cells mL^-1 until few cells were left. Both intra- and extracellular microcystin concentrations were significantly reduced by 100 and 92 %, respectively, by day 5 of the UV treatment for all microcystin analogues. During UV/TiO₂ treatment, there was great variability between replicates, making prediction of the effect on cyanobacterial cell and toxin behavior difficult.

Biomonitoring for wide area surveying in landmine detection using honeybees and optical sensing

Humanitarian demining is a worldwide effort and the range of climates and environments prevent any one detection method being suitable for all sites, so more tools are required for safe and efficient explosives sensing. Landmines emit a chemical flux over time, and honeybees can collect the trace residues of explosives (as particles or as vapour) on their body hairs. This capability was exploited using a passive method allowing the honeybees to freely forage in a mined area, where trace explosives present in the environment stuck to the honeybee body, which were subsequently transferred onto an adsorbent material for analysis by a fluorescent polymer sensor. Potential false positive sources were investigated, namely common bee pheromones, the anti-varroa pesticide Amitraz, and the environment around a clean apiary, and no significant response was found to any from the sensor. The mined site gave a substantial response in the optical sensor films, with quenching efficiencies of up to 38%. A model was adapted to estimate the mass of explosives returned to the colony, which may be useful for estimating the number of mines in a given area.

A kinetic model of thin-film fluorescent sensors for strategies to enhance chemical selectivity

Thin film chemical sensors are widely used in environmental and industrial applications due to their scalable fabrication and high sensitivity, however they often suffer from low specificity limiting their ability to discriminate between analytes. In this paper we analyse the influence of molecular diffusion and binding interactions on the optical response of thin film fluorescent chemical sensors. We use a computational model to calculate the dynamics of fluorescence quenching due to sorption and desorption of analyte molecules, and compare this with experimental measurements of a conjugated polymer sensor for nitroaromatic vapour. We find that to increase selectivity, such sensors should use thinner films, analyses should concentrate on the recovery dynamics, and sensor materials should be chosen to provide sensor-analyte combinations where diffusion is hindered by strong sensor-analyte binding interactions.

Organic photovoltaics for simultaneous energy harvesting and high-speed MIMO optical wireless communications

We show that organic photovoltaics (OPVs) are suitable for high-speed optical wireless data receivers that can also harvest power. In addition, these OPVs are of particular interest for indoor applications, as their bandgap is larger than that of silicon, leading to better matching to the spectrum of artificial light. By selecting a suitable combination of a narrow bandgap donor polymer and a nonfullerene acceptor, stable OPVs are fabricated with a power conversion efficiency of 8.8% under 1 Sun and 14% under indoor lighting conditions. In an optical wireless communication experiment, a data rate of 363 Mb/s and a simultaneous harvested power of 10.9 mW are achieved in a 4-by-4 multiple-input multiple-output (MIMO) setup that consists of four laser diodes, each transmitting 56 mW optical power and four OPV cells on a single panel as receivers at a distance of 40 cm. This result is the highest reported data rate using OPVs as data receivers and energy harvesters. This finding may be relevant to future mobile communication applications because it enables enhanced wireless data communication performance while prolonging the battery life in a mobile device.

Explosives detection by swabbing for improvised explosive devices

Swabs taken from the surface of a suspicious object are a standard method of identifying a concealed explosive device in security-conscious locations like airports. In this paper we demonstrate a sensitive method to collect and detect trace explosive residues from improvised explosive devices using swabs and an optical sensor element. Swabs coated with a commercial fluoropolymer are used to collect material and are subsequently heated to thermally desorb the explosives, causing the quenching of light emission from a thin film luminescent sensor. We report the sorption and desorption characteristics of swabs loaded with 2,4-DNT tested with Super Yellow fluorescent sensors in a laboratory setting, with detection that is up to three orders of magnitude more sensitive than standard colorimetric tests. The method was then applied in field tests with raw military-grade explosives TNT, PETN and RDX, on various objects containing the explosives, and post-blast craters. We show for the first time results using organic semiconductors to detect sub-milligram amounts of explosive sorbed onto a substrate from real explosives in the field, giving a promising new approach for IED detection.

Organic semiconductors for visible light communications

Organic semiconductors are an important class of optoelectronic material that are widely studied because of the scope for tuning their properties by tuning their chemical structure, and simple fabrication to make flexible films and devices. Although most effort has focused on developing displays and lighting from these materials, their distinctive properties also make them of interest for visible light communications (VLCs). This article explains how their properties make them suitable for VLC and reviews the main uses that have been explored. On the transmitter side, record white VLC communication has been achieved by using organic semiconductors as colour converters, while direct modulation of organic light-emitting diodes is also possible and could be of interest for display-to-display communication. On the receiver side, organic solar cells can be used to harvest power and data simultaneously, and fluorescent antennas enable fast and sensitive receivers with large field of view. This article is part of the theme issue 'Optical wireless communication'.

245 MHz bandwidth organic light-emitting diodes used in a gigabit optical wireless data link

Organic optoelectronic devices combine high-performance, simple fabrication and distinctive form factors. They are widely integrated in smart devices and wearables as flexible, high pixel density organic light emitting diode (OLED) displays, and may be scaled to large area by roll-to-roll printing for lightweight solar power systems. Exceptionally thin and flexible organic devices may enable future integrated bioelectronics and security features. However, as a result of their low charge mobility, these are generally thought to be slow devices with microsecond response times, thereby limiting their full scope of potential applications. By investigating the factors limiting their bandwidth and overcoming them, we demonstrate here exceptionally fast OLEDs with bandwidths in the hundreds of MHz range. This opens up a wide range of potential applications in spectroscopy, communications, sensing and optical ranging. As an illustration of this, we have demonstrated visible light communication using OLEDs with data rates exceeding 1 gigabit per second.

A hybrid organic-inorganic polariton LED

Polaritons are quasi-particles composed of a superposition of excitons and photons that can be created within a strongly coupled optical microcavity. Here, we describe a structure in which a strongly coupled microcavity containing an organic semiconductor is coupled to a second microcavity containing a series of weakly coupled inorganic quantum wells. We show that optical hybridisation occurs between the optical modes of the two cavities, creating a delocalised polaritonic state. By electrically injecting electron-hole pairs into the inorganic quantum-well system, we are able to transfer energy between the cavities and populate organic-exciton polaritons. Our approach represents a new strategy to create highly efficient devices for emerging 'polaritonic' technologies.

Design of Linear and Star-Shaped Macromolecular Organic Semiconductors for Photonic Applications

One of the most desirable and advantageous attributes of organic materials chemistry is the ability to tune the molecular structure to achieve targeted physical properties. This can be performed to achieve specific values for the ionization potential or electron affinity of the material, the absorption and emission characteristics, charge transport properties, phase behavior, solubility, processability, and many other properties, which in turn can help push the limits of performance in organic semiconductor devices. A striking example is the ability to make subtle structural changes to a conjugated macromolecule to vary the absorption and emission properties of a generic chemical structure. In this Account, we demonstrate that target properties for specific photonic applications can be achieved from different types of semiconductor structures, namely, monodisperse star-shaped molecules, complex linear macromolecules, and conjugated polymers. The most appropriate material for any single application inevitably demands consideration of a trade-off of various properties; in this Account, we focus on applications such as organic lasers, electrogenerated chemiluminescence, hybrid light emitting diodes, and visible light communications. In terms of synthesis, atom and step economies are also important. The star-shaped structures consist of a core unit with 3 or 4 functional connection points, to which can be attached conjugated oligomers of varying length and composition. This strategy follows a convergent synthetic pathway and allows the isolation of target macromolecules in good yield, high purity, and absolute reproducibility. It is a versatile approach, providing a wide choice of constituent molecular units and therefore varying properties, while the products share many of the desirable attributes of polymers. Constructing linear conjugated macromolecules with multifunctionality can lead to complex synthetic routes and lower atom and step economies, inferior processability, and lower thermal or chemical stability, but these materials can be designed to provide a range of different targeted physical properties. Conventional conjugated polymers, as the third type of structure, often feature so-called "champion" properties. The synthetic challenge is mainly concerned with monomer synthesis, but the final polymerization sequence can be hard to control, leading to variable molecular weights and polydispersities and some degree of inconsistency in the properties of the same material between different synthetic batches. If a champion characteristic persists between samples, then the variation of other properties between batches can be tolerable, depending on the target application. In the case of polymers, we have chosen to study PPV-type polymers with bulky side groups that provide protection of their conjugated backbone from π-π stacking interactions. These polymers exhibit high photoluminescence quantum yields (PLQYs) in films and short radiative lifetimes and are an important benchmark to monodisperse star-shaped systems in terms of different absorption/emission regions. This Account therefore outlines the advantages and special features of monodisperse star-shaped macromolecules for photonic applications but also considers the two alternative classes of materials and highlights the pros and cons of each class of conjugated structure.

Preconcentration techniques for trace explosive sensing

Trace sensing of explosive vapours is a method in humanitarian demining and Improvised Explosive Device (IED) detection that has received increasing attention recently, since accurate, fast, and reliable chemical detection is highly important for threat identification. However, trace molecule sampling in the field can be extremely difficult due to factors including weather, locale, and very low vapour pressure of the explosive. Preconcentration of target molecules onto a substrate can provide a method to collect higher amounts of analyte for analysis. We used the commercial fluoropolymer Aflas as a preconcentrator material to sorb explosive molecules to the surface, allowing subsequent detection of the explosives via the luminescence quenching response from the organic polymer Super Yellow. The preconcentration effect of Aflas was confirmed and characterised with 2,4-DNT, prior to field sampling being conducted at a test minefield in Croatia by placing preconcentration strips in the entrance of beehives, where honeybees have collected explosive materials during free-flying. In this work we show for the first time a method for confirmation of landmines combining honeybee colonies containing a preconcentration material and subsequent monitoring of luminescence quenching.

Transfer-printed micro-LED and polymer-based transceiver for visible light communications

Visible light communications (VLC) is an emerging technology that uses LEDs, such as found in lighting fixtures and displays, to transmit data wirelessly. Research has so far focused on LED transmitters and on photoreceivers as separate, discrete components. Combining both types of devices into a single transceiver format will enable bi-directional VLC and offer flexibility for the development of future advanced VLC systems. Here, a proof of concept for an integrated optical transceiver is demonstrated by transfer printing a microsize LED, the transmitter, directly onto a fluorescent optical concentrator edge-coupled to a photodiode, the receiver. This integrated device can simultaneously receive (downlink) and transmit (uplink) data at rates of 416 Mbps and 165 Mbps, respectively. Its capability to operate in optical relay mode at 337 Mbps is experimentally demonstrated.

An Organic Vortex Laser

Optical vortex beams are at the heart of a number of novel research directions, both as carriers of information and for the investigation of optical activity and chiral molecules. Optical vortex beams are beams of light with a helical wavefront and associated orbital angular momentum. They are typically generated using bulk optics methods or by a passive element such as a forked grating or a metasurface to imprint the required phase distribution onto an incident beam. Since many applications benefit from further miniaturization, a more integrated yet scalable method is highly desirable. Here, we demonstrate the generation of an azimuthally polarized vortex beam directly by an organic semiconductor laser that meets these requirements. The organic vortex laser uses a spiral grating as a feedback element that gives control over phase, handedness, and degree of helicity of the emitted beam. We demonstrate vortex beams up to an azimuthal index l = 3 that can be readily multiplexed into an array configuration.

Strong coupling in a microcavity containing β-carotene

We have fabricated an open-cavity microcavity structure containing a thin film of the biologically-derived molecule β-carotene. We show that the β-carotene absorption can be described in terms of a series of Lorentzian functions that approximate the 0-0, 0-1, 0-2, 0-3 and 0-4 electronic and vibronic transitions. On placing this molecular material into a microcavity, we obtain anti-crossing between the cavity mode and the 0-1 vibronic transition, however other electronic and vibronic transitions remain in the intermediate or weak-coupling regime due to their lower oscillator strength and broader linewidth. We discuss the consequences of strong-coupling for the possible modification of photosynthetic processes, or a re-ordering of allowed and optically-forbidden states.

Ormosil-coated conjugated polymers for the detection of explosives in aqueous environments

A fluorescence-based sensor for detecting explosives, based on a conjugated polymer coated with an ormosil layer, has been developed for use in aqueous environments. The conjugated polymer Super Yellow was spin-coated onto glass substrates prior to a further spin-coating of an MTEOS/TFP-TMOS-based ormosil film, giving an inexpensive, solution-based barrier material for ruggedization of the polymer to an aqueous environment. The sensors showed good sensitivity to 2,4-DNT in the aqueous phase at micromolar and millimolar concentrations, and also showed good recovery of fluorescence when the explosive was removed.

Green Perovskite Distributed Feedback Lasers

A visible perovskite distributed feedback laser is fabricated for the first time. Through the use of nanocrystal pinning, highly luminescent methylammonium lead bromide films are used to produce stable lasers emitting at 550 nm, with a low threshold of 6 µJcm-2. The lasers were able to support multiple polarisations, and could be switched between transverse magnetic and transverse electric mode operation through simple tuning of the distributed feedback grating period.

Nanoimprinted distributed feedback lasers of solution processed hybrid perovskites

Hybrid perovskite materials have considerable potential for light emitting devices such as LEDs and lasers. We combine solution processed CH₃NH₃PbI₃ perovskite with UV nanoimprinted polymer gratings to fabricate distributed feedback (DFB) lasers. The lead acetate deposition route is shown to be an effective method for fabricating low-loss waveguides (loss coefficient ~6 cm-1) and highly compatible with the polymer grating substrates. The nanoimprinted perovskite exhibited single-mode band-edge lasing, confirmed by angle-dependent transmission measurements. Depending on the excitation pulse duration the lasing threshold shows a value of 110 μJ/cm2 under nanosecond pumping and 4 μJ/cm2 under femtosecond pumping. We demonstrate further that this laser has excellent stability with a lifetime of 10⁸ pulses.

Self-trapping and excited state absorption in fluorene homo-polymer and copolymers with benzothiadiazole and tri-phenylamine

Excited state absorption (ESA) is studied using time-dependent density functional theory and compared with experiments performed in dilute solutions.

Synthesis and properties of novel star-shaped oligofluorene conjugated systems with BODIPY cores

Star-shaped conjugated systems with varying oligofluorene arm length and substitution patterns of the central BODIPY core have been synthesised, leading to two families of compounds, T-B1–T-B4 and Y-B1–Y-B4, with T- and Y-shaped motifs, respectively. Thermal stability, cyclic voltammetry, absorption and photoluminescence spectroscopy of each member of these two families were studied in order to determine their suitability as emissive materials in photonic applications.

LED pumped polymer laser sensor for explosives

A very compact explosive vapor sensor is demonstrated based on a distributed feedback polymer laser pumped by a commercial InGaN light-emitting diode. The laser shows a two-stage turn on of the laser emission, for pulsed drive currents above 15.7 A. The 'double-threshold' phenomenon is attributed to the slow rise of the ∼30 ns duration LED pump pulses. The laser emits a 533 nm pulsed output beam of ∼10 ns duration perpendicular to the polymer film. When exposed to nitroaromatic model explosive vapors at ∼8 ppb concentration, the laser shows a 46% change in the surface-emitted output under optimized LED excitation.

Nanoimprinted polymer lasers with threshold below 100 W/cm2 using mixed-order distributed feedback resonators

Organic semiconductor lasers were fabricated by UV-nanoimprint lithography with thresholds as low as 57 W/cm(2) under 4 ns pulsed operation. The nanoimprinted lasers employed mixed-order distributed feedback resonators, with second-order gratings surrounded by first-order gratings, combined with a light-emitting conjugated polymer. They were pumped by InGaN LEDs to produce green-emitting lasers, with thresholds of 208 W/cm(2) (102 nJ/pulse). These hybrid lasers incorporate a scalable UV-nanoimprint lithography process, compatible with high-performance LEDs, therefore we have demonstrated a coherent, compact, low-cost light source.

Nanoimprinted organic semiconductor laser pumped by a light-emitting diode

An organic semiconductor laser, simply fabricated by UV-nanoimprint lithography (UV-NIL), that is pumped with a pulsed InGaN LED is demonstrated. Molecular weight optimization of the polymer gain medium on a nanoimprinted polymer distributed feedback resonator enables the lowest reported UV-NIL laser threshold density of 770 W cm(-2) , establishing the potential for scalable organic laser fabrication compatible with mass-produced LEDs.

Dynamics of fluorescence depolarisation in star-shaped oligofluorene-truxene molecules

Star-shaped molecules are of growing interest as organic optoelectronic materials. Here a detailed study of their photophysics using fluorescence depolarisation is reported. Fluorescence depolarisation dynamics are studied in branched oligofluorene-truxene molecules with a truxene core and well-defined three-fold symmetry, and are compared with linear fluorene oligomers. An initial anisotropy value of 0.4 is observed which shows a two-exponential decay with time constants of 500 fs and 3-8 ps in addition to a long-lived component. The femtosecond component is attributed to exciton localisation on one branch of the molecule and its amplitude reduces when the excitation is tuned to the low energy tail of the absorption spectrum. The picosecond component shows a weak dependence on the excitation wavelength and is similar to the calculated rate of the resonant energy transfer of the localised exciton between the branches. These assignments are supported by density-functional theory calculations which show a disorder-induced splitting of the two degenerate excited states. Exciton localisation is much slower than previously reported in other branched molecules which suggests that efficient light-harvesting systems can be designed using oligofluorenes and truxenes as building blocks.

Laser chemosensor with rapid responsivity and inherent memory based on a polymer of intrinsic microporosity

This work explores the use of a polymer of intrinsic microporosity (PIM-1) as the active layer within a laser sensor to detect nitroaromatic-based explosive vapors. We show successful detection of dinitrobenzene (DNB) by monitoring the real-time photoluminescence. We also show that PIM-1 has an inherent memory, so that it accumulates the analyte during exposure. In addition, the optical gain and refractive index of the polymer were studied by amplified spontaneous emission and variable-angle ellipsometry, respectively. A second-order distributed feedback PIM-1 laser sensor was fabricated and found to show an increase in laser threshold of 2.5 times and a reduction of the laser slope efficiency by 4.4 times after a 5-min exposure to the DNB vapor. For pumping at 2 times threshold, the lasing action was stopped within 30 s indicating that PIM-1 has a very fast responsivity and as such has a potential sensing ability for ultra-low-concentration explosives.

Effect of exciton self-trapping and molecular conformation on photophysical properties of oligofluorenes

Electronic absorption and fluorescence transitions in fluorene oligomers of differing lengths are studied experimentally and using density functional theory (DFT) and time-dependent DFT. Experimental values are determined in two ways: from the measured molar absorption coefficient and from the radiative rate deduced from a combination of fluorescence quantum yield and lifetime measurements. Good agreement between the calculated and measured transition dipoles is achieved. In both theory and experiment a gradual increase in transition dipoles with increasing oligomer length is found. In absorption the transition dipole follows an approximately n(0.5) dependence on the number of fluorene units n for the range of 2 < or = n < or = 12, whereas a clear saturation of the transition dipole with oligomer length is found in fluorescence. This behavior is attributed to structural relaxation of the molecules in the excited state leading to localization of the excitation (exciton self-trapping) in the middle of the oligomer for both twisted and planar backbone conformations. Twisted oligofluorene chains were found to adopt straight or bent geometries depending on alternation of the dihedral angle between adjacent fluorene units. These different molecular conformations show the same values for the transition energies and the magnitude of the transition dipole.

Semiconducting polymer waveguides for end-fired ultra-fast optical amplifiers

A method to fabricate conjugated polymer waveguides with well defined edge facets is demonstrated. The utility of the approach is explored for application as end-fired ultrafast optical amplifiers based on poly(9,9'-dioctylfluorene-co-benzothiadiazole). An internal gain of 19 dB was achieved on a 760 microm long waveguide at 565 nm wavelength. This fabrication procedure may be applied to a wide range of conjugated polymers and organic light-emitting devices, providing an important step towards future applications of organic integrated photonics.

Fluidic fibre dye lasers

We report the demonstration of compact fluidic fibre lasers based on capillary tubes and photonic crystal fibres, featuring single channel and multiple laterally integrated fluidic lasers respectively. Their preparation was based on capillary action and lasing occurred without the need for external mirrors or lithographically defined microstructures. The fibre lasers were found to be tunable by varying the chromophore density in the liquid core and a functional wavelength selectivity mechanism inherent in both types of lasers provided a long free spectral range that does not correspond to the length of the fibres. The enhanced mode spacing is attributed to a Vernier resonant effect.

Diode pumped distributed Bragg reflector lasers based on a dye-to-polymer energy transfer blend

We report the demonstration of a compact, all-solid-state polymer laser system comprising of a Gallium Nitride (GaN) semiconductor diode laser as the pump source. The polymer laser was configured as a surface emitting, distributed Bragg reflector laser (DBR), based on a novel energy transfer blend of Coumarin 102 and the conjugated polymer poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene). In this configuration, diode pumping was possible both due to the improved quality of the resonators and the improved harvesting of the diode laser light.

Optical amplification in a first-generation dendritic organic semiconductor

We report a study of a new class of organic semiconductor as an optical gain medium. We demonstrate amplification of violet light by use of stimulated emission in a solution of a first-generation bis-fluorene-cored semiconducting dendrimer. Amplification is also observed in the solid state by means of amplified spontaneous emission in an optically pumped dendrimer planar waveguide. Gains of 36 dB cm(-1) at 420 nm and 26 dB cm(-1) at 390 nm in solution and 350 dB cm(-1) in the solid state are obtained. These results show that semiconducting dendrimers have potential as visible laser and amplifier materials.

Degradation of substituted phenylurea herbicides by Arthrobacter globiformis strain D47 and characterization of a plasmid-associated hydrolase gene, puhA

Arthrobacter globiformis D47 was shown to degrade a range of substituted phenylurea herbicides in soil. This strain contained two plasmids of approximately 47 kb (pHRIM620) and 34 kb (pHRIM621). Plasmid-curing experiments produced plasmid-free strains as well as strains containing either the 47- or the 34-kb plasmid. The strains were tested for their ability to degrade diuron, which demonstrated that the degradative genes were located on the 47-kb plasmid. Studies on the growth of these strains indicated that the ability to degrade diuron did not offer a selective advantage to A. globiformis D47 on minimal medium designed to contain the herbicide as a sole carbon source. The location of the genes on a plasmid and a lack of selection would explain why the degradative phenotype, as with many other pesticide-degrading bacteria, can be lost on subculture. A 22-kb EcoRI fragment of plasmid pHRIM620 was expressed in Escherichia coli and enabled cells to degrade diuron. Transposon mutagenesis of this fragment identified one open reading frame that was essential for enzyme activity. A smaller subclone of this gene (2.5 kb) expressed in E. coli coded for the protein that degraded diuron. This gene and its predicted protein sequence showed only a low level of protein identity (25% over ca. 440 amino acids) to other database sequences and was named after the enzyme it encoded, phenylurea hydrolase (puhA gene).

Extended mode-hop-free tuning by use of a dual-cavity, pump-enhanced optical parametric oscillator

We report extended mode-hop-free tuning in a continuous-wave, pump-enhanced optical parametric oscillator (PE-OPO). We employ a dual-cavity configuration to allow independent control of the resonant pump and signal fields, and so we can suppress frequent mode hops in the signal as the pump is tuned in frequency. With the signal field clamped in frequency by an uncoated etalon, the idler field can be scanned smoothly through a range of 10.8 GHz. The PE-OPO outputs can also be tuned coarsely from 1.01 to 1.18 mum in the signal and from 2.71 to 3.26 mum and 4.07 to 5.26 mum in the idler. We find that increased idler absorption only slightly increases the oscillation threshold.

Doubly resonant continuous-wave optical parametric oscillator pumped by a single-mode diode laser

The performance characteristics of a doubly (signal and idler) resonant continuous-wave optical parametric oscillator based on periodically poled lithium niobate and pumped by a 100-mW single-mode laser diode at 810 nm are reported. Pump power thresholds as low as 16 mW and wavelength tuning over the range 1.15-1.25 microm at the signal and 2.31-2.66 microm at the idler were achieved through variation of crystal temperature, pump wavelength, and grating period. Up to 5 mW of signal output was obtained with the single-mode diode pump, and signal powers of up to 39 mW were obtained when pumping with a 400-mW injection-locked broad-area diode laser.

Continuous-wave singly resonant optical parametric oscillator based on periodically poled RbTiOAsO(4)

We report a continuous-wave optical parametric oscillator (OPO) based on periodically poled RbTiOAsO(4) (PPRTA). The singly resonant OPO, which is located within a Ti:sapphire laser, has a high-finesse signal cavity and delivers a maximum output power of 270 mW to the nonresonant idler wave at 2.92mum , through a 4.5-mm PPRTA crystal. For room-temperature operation and a crystal with a 30-mu;m grating period, pump tuning over 838-848 nm results in OPO tuning over 1.13-1.27mum (signal) and 2.53-3.26mum (idler), limited by the bandwidth of optical coatings. PPRTA exhibits thermal properties superior to those of periodically poled LiNbO(3) .