Skip to main content

Active Organic Surface Plasmon Enhanced Nanophotonic Devices

Final Report Summary - ACTOSPED (Active Organic Surface Plasmon Enhanced Nanophotonic Devices)

Technical summary

To address the objective of this project - to enhance the performance and improve the electrical integration potential of organic nanoscale photonic devices through the control of surface plasmon-mediated enhancement of absorption and emission processes in the organic material - gold nanowire antennas have been fabricated and employed to modify the spontaneous emission spectrum, polarisation direction and emission lifetime of monolithically-coupled organic poly(3-hexylthiophene) (P3HT), light-emitting nanostructures. The P3HT nanostructure has been integrated onto the end of the gold nanowire antenna, where highly localised, longitudinal surface plasmon mode fields are strongest. Comprehensive optical characterisation and theoretical modelling have been employed to demonstrate plasmonic nanoantenna-mediated light emission from P3HT; a radiative emission-rate enhancement of up to a factor of 7 from P3HT have been determined at the resonance wavelength of the gold nanowire antenna.

To further improve metal-organic polymer light-matter interaction and increase radiative decay rate, we have designed more complex split-dipole nanoantenna heterostructures whereby P3HT is directly incorporated into the slot region of split gold nanoantennas. Using this approach, it has been shown that P3HT radiative emission rates could be enhanced by a factor of up to 550 in theory. Additionally, apparent theoretical luminescence quantum efficiency has been shown to increase from 1 % to 45 % for optimised nanoantenna parameters. The gold-P3HT-gold nanoantenna heterostructures have been fabricated by a refined templating route in ultra-thin nanoporous alumina templates grown on conductive substrates. Experimental measurements of photoluminescence intensity and the lifetime of the fabricated nanoantenna heterostructures as a function of nanoantenna length were carried out and enhancements in luminescence intensity and radiative decay rate of 12 and 29 were determined, respectively.

A monopole configuration consisting of a vertically orientated gold nanowire antenna placed on an organic P3HT thin film on gold was also explored in theory as a means to further enhance light-emitting efficiency and radiative decay rates from organic P3HT layers. Simulations indicated that this configuration would result in an event of greater radiative decay rate enhancements. In addition, this metal-organic heterostructure configuration is more amenable to large-area optoelectronic device integration which would enable the enhanced properties of the system to be exploited in macroscope devices.

In addition to the metal-polymer nanowire heterostructures described above, fabrication and electrical integration of arrays of silver nanoparticles with conjugated polymer layers has been investigated to start exploring how metal-polymer nanoscale effects could be transferred to large-area, opto-electronic areas. The optical characteristics of sub-50 nm P3HT semiconductor thin films deposited on Ag nanoparticle array electrodes were studied both experimentally and theoretically. Nanostructured electrodes are fabricated by the evaporation of Ag through nanoporous alumina masks, and the subsequent removal of the alumina. The evolution of reflected light spectra from P3HT coated Ag nanoparticle array electrodes as a function of Ag nanoparticle height has been studied experimentally using finite difference time domain (FDTD) simulations. The results show that light absorption in the active layer is enhanced by factors of between 5 and 10 in the 570 to 750 nm wavelength range in the red area of the P3HT absorption band using nanoparticles with height:diameter aspect ratios of more than 1. Additionally, the enhanced absorption has been shown to lead to improved red response in the photocurrent spectra of fabricated large-area test devices.

Socio-economic input of the project

Surface plasmon and photonic mode propagation in gold nanotubes

This work arose from mentorship of an undergraduate researcher during the outgoing phase of the project and has led to the preparation of a manuscript for publication. Gold nanotube arrays were synthesised with a range of wall thickness (15 nm to > 140 nm) and inner diameters of ~ 200 nm using a hard-template method. A red spectral shift with decreasing wall thickness was observed in dark-field spectra of nanotube arrays and single nanowire/nanotube heterostructures (> 0.39 eV). FDTD simulations showed that nanotubes in this size regime supported propagating surface plasmon modes as well as surface plasmon ring resonances at visible wavelengths (the latter was observed only for excitation directions normal to the nanotube long axis with transverse polarisation). The energy of the surface plasmon modes decreased with decreasing wall thickness and was attributed to an increase in mode coupling between propagating modes in the nanotube core and outer surface, and the circumference-dependence of ring resonances. Correlations were made between the experimentally observed changes in dark-field spectra, and the changes in surface plasmon mode properties observed in simulations for the various gold nanotube wall thicknesses and excitation conditions.

Strong-coupling in metal-dielectric-metal cavities

This work arose from an involvement in group research activities during the return phase of the fellowship. The modification of energy transfer efficiency between donor and acceptor dye molecules in a polymer host was studied when placed in a metal-dye-metal cavity with resonance tuned to the acceptor absorption band. The aim of the study was to modify the absorption energy of the acceptor molecules via strong exciton-photon coupling in the cavity. It was proposed that, in the strong coupling regime, the absorption energy of the dye molecules would split into two absorption bands, one with a lower energy and one with a higher energy than the original absorption transition. In the donor-acceptor system, creating a modified energy state may promote more efficient energy transfer between excited donor molecules and acceptor molecules that may not otherwise exhibit Förster energy transfer. The results of this work consisted of transmission, absorption and photoluminescence exploratory measurements of donor-acceptor dye blends in poly(methylmethacrylate), and host films placed in a silver Fabry-Pérot cavity consisting of two 30 nm thick silver films on either side of the dye-host film.

Broader impacts of the fellowship project

This work is expected to lead to the demonstration of plasmonic nanoantenna-mediated light emission and absorption in semiconducting conjugated polymer materials. The potential use is in the exploitation of nanoscale phenomena in both integrated nanoscale photonic devices (such as free-space optical frequency communications) and in macroscale optoelectronic devices (such as light-emitting and photovoltaic devices) to improve device-operating speeds and light-management in the active organic semiconducting layer.

Broader impacts of the fellowship project on the researcher

The purpose of this fellowship is to train the applicant to become a world-class researcher in the field of nanophotonics, specialising in the area of active organic surface plasmon-enhanced nanophotonic devices. The research fellow has developed the following complementary skills and knowledge during the outgoing phase.

Knowledge transfer

By executing the planned work packages in the Atwater Group at Caltech to date, the researcher has developed technical experience in metal/light-emitting polymer nanostructure synthesis and device behaviour, as well as many other aspects of plasmonics and surface plasmon (SP)-enhanced light emission. The researcher is now in a strong position to further develop optimised metal/light-emitting polymer heterostructural devices, specifically optical antennas, for integrated nanophotonics.

As well as executing the specific technical goals of the ACTOSPED project, the researcher has had the opportunity to participate in and contribute to other research projects in the Atwater Group primarily on 1. synthesis and optical characterisation of gold nanotube arrays with varying wall thicknesses for refractive index sensing, 2. development of optical characterisation techniques for silicon-silver core-shell plasmonic nanoresonators, and 3. fabrication of surface plasmon-enhanced conjugated polymer photovoltaic devices.

The researcher's participation in weekly group meetings and in formal and informal technical workshops, tutorials and clubs in plasmonics at Caltech has been invaluable for knowledge transfer and for her ongoing career development, and has supplemented prior research skills in the field of nanophotonics and organic light-emitting polymer semiconductors.

As part of the Atwater Group the researcher has gained experience in full-field electromagnetic simulation tools, which are invaluable for the design, optimisation and understanding of plasmonic devices.

Throughout the first 18 months of the project, the researcher contributed to laboratory management, equipment trouble-shooting and maintenance, and user training activities, and the researcher learnt responsible, independent behaviour in the management of the scientific research project.

As a result of the work carried out in the Ebbesen Group at the University of Strasbourg during the second project period, the researcher developed technical experience in polymer-nanoantenna optical characterisation, the study of metal-molecule interactions and focused on ion beam milling nanostructure fabrication techniques, as well as many other aspects of plasmonics and surface plasmon (SP)-enhanced light emission.

As well as executing the specific technical goals of the ACTOSPED project, the researcher has had the opportunity to participate in and learn about other research projects in the Ebbesen group, primarily on strong-coupling in metal-molecule-metal cavities and hole-arrays.

Participation in regular group meetings and participation in informal technical journal clubs in plasmonics at the University of Strasbourg was useful for knowledge transfer and for the ongoing career development of the researcher, and supplemented her research skills in the field of nanophotonics and organic light-emitting polymer semiconductors.

Research dissemination/networking

The researcher completed and submitted two papers that arose from the work directly related to the ACTOSPED project during the return phase, and presented and attended a number of international and specialist conferences during that phase, including the Materials Research Society, Gordon Research Specialist Conference in Plasmonics, and European Photovoltaic Solar Energy Conference, which were important in disseminating the results of the project to the broader research community and developing networks in the fields of plasmonics and photovoltaics.

The researcher has also given invited talks at the Caltech Kavli Nanoscience Institute (KNI) and the Microdevices Laboratory (MDL) at JPL Joint Monthly Seminar Series and at the University of California, Berkeley, Electrical Engineering Department, Photonics and Plasmonics Seminar Series, which has aided in discussions and dissemination of the project work to a wider audience.

Teaching/mentoring

The researcher has been involved in mentoring and assisting undergraduate and graduate students in a scientific advisory and project management capacity. The researcher has also been involved with developing summer undergraduate research proposals, establishing scientific goals and research project reporting (SURF 2008: Micha Fireman, 'Large-area vertical arrays of luminescent metal-organic nanowire composites'; SURF 2009: Adrianne Collopy, 'Nanostructured metal electrodes for the enhancement of charge carrier collection efficiency in organic polymer solar cells'). The researcher has also been on the selection committee for Caltech undergraduate summer researchers in the Atwater Group.

The researcher has attended and followed teaching workshops : 'How to write and teaching statement' and 'Teaching a class? Just stay CALM... ', organised by the Caltech Project for Effective Teaching, which has been useful for furthering an academic career.

Integration activities

The researcher has attended numerous seminars as part of the Caltech Applied Physics seminar series (with invited speakers in the fields of optoelectronics, plasmonics, nanowire synthesis, photovoltaics, etc.). In addition, the researcher has had the opportunity to learn about and take part in green energy generation and energy conservation seminars and activities, which are actively promoted on the Caltech campus.

In addition to plasmonics, the Atwater Group has a very established and active photovoltaic research agenda. Through group meetings, working groups and informal interaction with group members, the researcher has had the opportunity to gain awareness of the direction of next-generation solar research and assess how the researcher's skills and knowledge in the field of nanotechnology and polymer semiconductors could be employed as part of future green energy technologies. Exposure to different project ideas outside the researcher's own experience, as well as the dynamics of a large research group, have been highly beneficial towards developing a balanced and well informed research ethic and will be invaluable for attaining future independence and success as a researcher.

The researcher attended numerous seminars as part of the Institut de Science et d’Ingénierie Supramoléculaire (ISIS), University of Strasbourg seminar series (with invited speakers in the fields of optoelectronics, plasmonics, photovoltaics, etc.), which helped to further the researcher's knowledge of research in interdisciplinary fields.

Through group meetings and informal interaction with Ebbesen Group members, the researcher had the opportunity to gain exposure to different project ideas outside the researcher's own experience, as well as the dynamics of a European research group. These experiences were beneficial to the researcher in developing a balanced and well-informed research ethic and contributed towards the development of an independent research career on completion of the fellowship.

Prospects of the researcher after completion of the fellowship

This fellowship was an excellent opportunity for the researcher to gain research experience outside her home country of Ireland. The two host groups in the United States of America and Europe are world-class experts in the field of plasmonics and the researcher had excellent exposure to the field. As a result, the researcher's background in engineering, conjugated polymer materials and nanoscale photonic devices was supplemented by her knowledge gained in the field of plasmonics. This led to project results that were interdisciplinary and novel. Additionally, the knowledge transfer, networks and teaching/mentoring experiences gained by the researcher during the fellowship were deemed invaluable to her pursuit of an independent research career as a research group leader. Despite her desire to return to her home country, Ireland, to pursue an independent group leader position, the economic circumstances of her home country in 2009/2010 resulted in a lack of research group leader positions. As a result, she applied to English-speaking group leader positions in Europe and the USA during the final year of the fellowship. The researcher obtained one job interview at Rutgers University for a tenure-track faculty position in the USA, which resulted in a job offer enabling the researcher to pursue her independent research career goals. Although the researcher does not remain in Europe while starting her independent research career, many links and collaborations exist between the researcher and her home country, which are likely to persist and lead to fruitful US-EU collaborative research projects. Opportunities to re-integrate into her home country will be explored in the future as independent research group leader positions become more viable again in years to come.