Strong light-matter coupling in organic microcavities : investigating the fundamental principles of strong coupling in strongly disordered materials experimentally

Strong light–matter coupling gives rise to intriguing possibilities like Bose-Einstein condensation at room temperature. In this context, organic semiconductors are particularly attractive because they combine large oscillator strengths with high exciton binding energies and thus readily enable large light–matter coupling strengths up to room temperature. Yet, in these commonly strongly disordered materials, the applicability of fundamental predictions developed for systems of high order needs to be verified. Hence, the validity of the theoretically predicted form of the coupling strength and of the coupled oscillator model is tested for strongly coupled organic microcavities in this thesis. Experimental investigations of metal-clad microcavities confirm the coupling strength to be proportional to the electric field with which the excitons interact and to the square root both of the oscillator strength of the material and of the number of chromophores inside the microcavity. Systematically varying these parameters demonstrates a non-zero threshold for the onset of the strong coupling regime for the first time, which confirms the applicability of the coupled oscillator model also for strongly disordered systems. Moreover, the effect of the coupling strength on the photoluminescence from organic microcavities is investigated. For metal-clad cavities, but not for microcavities with dielectric mirrors, an increase of the luminescence intensity with increasing coupling strength was found. For the latter system, a systematic study aimed to determine the properties of the cavity and of the organic material which are crucial for polariton lasing. However, experiments did not yield polariton lasing, for which two potential reasons are identified: (1) the vanishing of modes close to resonance and (2) pronounced bimolecular quenching in the studied material. Since organic microcavities are complex, systematic studies as presented in this thesis are an important step towards a more profound understanding of organic polaritons."This work was supported by the Engineering and Physical Sciences Research Council [grant number EP/L015110/1]" -- Acknowledgement

Investigating the onset of the strong coupling regime by fine-tuning the Rabi splitting in multilayer organic microcavities

This research was financially supported by the European Research Council under the European Union’s Horizon 2020 Framework Programme (FP/2014-2020)/ERC Grant Agreement No. 640012 (ABLASE), the Volkswagen Foundation (93404), and the Scottish Funding Council (through SUPA). L.T. acknowledges the studentship funding through the EPSRC CM-CDT (EP/L015110/1).Given the prevalence of disorder in many organic semiconductors, the applicability of simple models to describe their behavior in the strong coupling regime, like the two-level coupled oscillator, is not evident. Here, the validity of the two-level coupled oscillator model and the simple dependence of the coupling strength on the number of absorbers and the electric field is tested experimentally in metal-clad microcavities containing a disordered film of small molecules. Multi-layer microcavities are produced by combining different thin film deposition techniques. These allow for isolating the relevant parameters and thus to confirm the coupling strength is proportional to (1) the square root of the number of absorbers and (2) the amplitude of the electric field. By changing either of these two parameters, the microcavities are shifted from the weak to the strong coupling regime. Moreover, careful analysis reveals that there is a threshold coupling strength for the onset of the Rabi splitting. Two independent investigations show that this threshold is comparable to the losses in the cavities. These results validate the coupled two-level Hamiltonian for microcavities containing disordered organic semiconductors, even though the assumption of a single exciton level represents a strong simplification for these systems.PostprintPeer reviewe

Near-infrared exciton-polaritons in strongly coupled single-walled carbon nanotube microcavities

This research was financially supported by the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement No. 306298 (EN-LUMINATE) and under the European Union’s Horizon 2020 Framework Programme (FP/2014-2020)/ERC Grant Agreement No. 640012 (ABLASE), by EPSRC through the CM-DTC (EP/L015110/1) and by the Scottish Funding Council through SUPA. J.Z. thanks the Alfried Krupp von Bohlen und Halbach-Stiftung via the “Alfried Krupp Förderpreis für junge Hochschullehrer” for general support.Exciton-polaritons form upon strong coupling between electronic excitations of a material and photonic states of a surrounding microcavity. In organic semiconductors the special nature of excited states leads to particularly strong coupling and facilitates condensation of exciton-polaritons at room temperature, which may lead to electrically pumped organic polariton lasers. However, charge carrier mobility and photo-stability in currently used materials is limited and exciton-polariton emission so far has been restricted to visible wavelengths. Here, we demonstrate strong light-matter coupling in the near infrared using single-walled carbon nanotubes (SWCNTs) in a polymer matrix in a planar metal-clad cavity. By exploiting the exceptional oscillator strength and sharp excitonic transition of (6,5) SWCNTs, we achieve large Rabi splitting (> 110 meV), efficient polariton relaxation and narrow band emission (< 15 meV). Given their high charge carrier mobility and excellent photostability, SWCNTs represent a promising new avenue towards practical exciton-polariton devices operating at telecommunication wavelengths. Publisher PDFPublisher PDFPeer reviewe

An exciton-polariton laser based on biologically produced fluorescent protein

We thank A. Clemens (TU Dresden, Germany) for technical support with protein preparation and C. Murawski (U St Andrews, UK) for support with TDAF deposition. We acknowledge support from the ERC Starting Grant ABLASE (640012), the Scottish Funding Council (via SUPA), the European Union Marie Curie Career Integration Grant (PCIG12-GA-2012-334407), studentship funding through the EPSRC CM-DTC (EP/L015110/1) and the EPSRC Hybrid Polaritonics program grant (EP/M025330/1). S.H. gratefully acknowledges support by the Royal Society and the Wolfson Foundation and M.S. gratefully acknowledges support from a MSCA IF (659213).Under adequate conditions, cavity-polaritons form a macroscopic coherent quantum state, known as polariton condensate (PC). Compared to Wannier-Mott polaritons in inorganic semiconductors, the localized Frenkel polaritons in organic emitter materials show weaker interaction with each other but stronger coupling to light, which recently enabled the first realization of a PC at room temperature. However, this required ultrafast optical pumping which limits the applications of organic PCs. Here, we demonstrate room-temperature PCs of cavity-polaritons in simple laminated microcavities filled with the biologically produced enhanced green fluorescent protein (eGFP). The unique molecular structure of eGFP prevents exciton annihilation even at high excitation densities, thus facilitating PCs under conventional nanosecond pumping. Condensation is clearly evidenced by a distinct threshold, an interaction-induced blueshift of the condensate, long-range coherence and the presence of a second threshold at higher excitation density which is associated with the onset of photon lasing.Publisher PDFPeer reviewe

Influence of optical material properties on strong coupling in organic semiconductor based microcavities

The optical properties of organic semiconductors are generally characterised by a number of material specific parameters, including absorbance, photoluminescence quantum yield, Stokes shift, and molecular orientation. Here, we study four different organic semiconductors and compare their optical properties to the characteristics of the exciton-polaritons that are formed when these materials are introduced into metal-clad microcavities. We find that the strength of coupling between cavity photons and excitons is clearly correlated with the absorptivity of the material. In addition, we show that anisotropy strongly affects the characteristics of the formed exciton-polaritons

Electrical pumping and tuning of exciton-polaritons in carbon nanotube microcavities

This research was financially supported by the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement No. 306298 (EN-LUMINATE) and under the European Union’s Horizon 2020 Framework Programme (FP/2014-2020)/ERC Grant Agreement No. 640012 (ABLASE) and by the Scottish Funding Council (through SUPA). L.T. thanks the EPSRC for support through the CM-DTC (EP/L015110/1). J.Z. thanks the Alfried Krupp von Bohlen und Halbach-Stiftung via the “Alfried Krupp Förderpreis für junge Hochschullehrer” for general support.Exciton-polaritons are hybrid light–matter particles that form upon strong coupling of an excitonic transition to a cavity mode. As bosons, polaritons can form condensates with coherent laser-like emission. For organic materials, optically pumped condensation was achieved at room temperature but electrically pumped condensation remains elusive due to insufficient polariton densities. Here we combine the outstanding optical and electronic properties of purified, solution-processed semiconducting (6,5) single-walled carbon nanotubes (SWCNTs) in a microcavity-integrated light-emitting field-effect transistor to realize efficient electrical pumping of exciton-polaritons at room temperature with high current densities (>10 kA cm−2) and tunability in the near-infrared (1,060 nm to 1,530 nm). We demonstrate thermalization of SWCNT polaritons, exciton-polariton pumping rates ~104 times higher than in current organic polariton devices, direct control over the coupling strength (Rabi splitting) via the applied gate voltage, and a tenfold enhancement of polaritonic over excitonic emission. This powerful material–device combination paves the way to carbon-based polariton emitters and possibly lasers.PostprintPostprintPeer reviewe

Distributed feedback lasers based on green fluorescent protein and conformal high refractive index oxide layers

Funding: European Research Council (ERC StG ABLASE, 640012). M.K. and A.M. acknowledge funding from the EPSRC DTG (EP/M506631/1 and EP/M508214/1). L.T. acknowledges studentship funding through the EPSRC CM-CDT (EP/L015110/1). M.S. acknowledges funding from the Royal Society (Dorothy Hodgkin Fellowship, DH160102). I.D.W.S. acknowledges funding from a Royal Society Wolfson research merit award.Fluorescent proteins have emerged as an attractive gain material for lasers, especially for devices requiring biocompatibility. However, due to their optical properties, integration with distributed feedback (DFB) resonators is not readily achievable. Here, a DFB laser with enhanced green fluorescent protein (eGFP) as the gain material is demonstrated by incorporating a thin (65 nm), high refractive index (n = 2.12) ZrO2 interlayer as waveguide core. Deposition of ZrO2 via atomic layer deposition yields a smooth and conformal film as required to minimize optical losses. Lasing emission is obtained from 2D second‐order DFB eGFP lasers at pump power densities above 56.6 kW cm–2 and a wavelength tuning range of Δλ = 51.7 nm is demonstrated. Furthermore, it is shown that in contrast to conventional organic DFB lasers, both transverse electric (TE) and transverse magnetic (TM) modes are accessible. The effective refractive index of these modes can be predicted accurately through optical modelling. Using far‐field imaging, the laser beam profile is studied and TE and TM modes are distinguished.Publisher PDFPeer reviewe

Ultrastrong coupling of electrically pumped near-infrared exciton-polaritons in high mobility polymers

This research was financially supported by the European Research Council under the European Union's Seventh Framework Programme (FP/2007- 2013)/ERC Grant Agreement No. 306298 (EN -LUMINATE) and under the European Union’s Horizon 2020 Framework Programme (FP/2014- 2020)/ERC Grant Agreement No. 640012 (ABLASE) and by the EPSRC Programme Grant EP/P030017/1. L.T. thanks EPSRC for support through the CM -DTC (EP/L015110/1).Exciton-polaritons are quasiparticles with hybrid light–matter properties that may be used in new optoelectronic devices. Here, electrically pumped ultrastrongly coupled exciton-polaritons in a high-mobility donor–acceptor copolymer are demonstrated by integrating a light-emitting field-effect transistor into a metal-clad microcavity. Near-infrared electroluminescence is emitted exclusively from the lower polariton branch, which indicates efficient relaxation. A coupling strength of 24% of the exciton transition energy implies the system is in the ultrastrong coupling regime with a narrow and almost angle-independent emission. The lower polariton energy, which can be adjusted by the cavity detuning, strongly influences the external quantum efficiency of the device. Driving the transistors at ambipolar current densities of up to 4000 A cm−2 does not decrease the coupling strength or polariton emission efficiency. Cavity-integrated light-emitting field-effect transistors thus represent a versatile platform for polariton emission and polaritonic devices.Publisher PDFPeer reviewe

Narrowband organic light-emitting diodes for fluorescence microscopy and calcium imaging

Funding: Leverhulme Trust (RPG-2017-231), the EPSRC NSF-CBET lead agency agreement (EP/R010595/1, 1706207), the DARPA NESD program (N66001-17-C-4012) and the RS Macdonald Charitable Trust. C.M. acknowledges funding from the European Commission through a Marie Skłodowska Curie individual fellowship (703387). A.M. acknowledges funding through an individual fellowship of the Deutsche Forschungsgemeinschaft (404587082). Y.D. acknowledges support from the Chinese Scholarship Council (CSC). L.T. acknowledges studentship funding through the EPSRC CM-CDT (EP/L015110/1). M.S. acknowledges funding by the Royal Society (Dorothy Hodgkin Fellowship, DH160102).Fluorescence imaging is an indispensable tool in biology, with applications ranging from single‐cell to whole‐animal studies and with live mapping of neuronal activity currently receiving particular attention. To enable fluorescence imaging at cellular scale in freely moving animals, miniaturized microscopes and lensless imagers are developed that can be implanted in a minimally invasive fashion; but the rigidity, size, and potential toxicity of the involved light sources remain a challenge. Here, narrowband organic light‐emitting diodes (OLEDs) are developed and used for fluorescence imaging of live cells and for mapping of neuronal activity in Drosophila melanogaster via genetically encoded Ca2+ indicators. In order to avoid spectral overlap with fluorescence from the sample, distributed Bragg reflectors are integrated onto the OLEDs to block their long‐wavelength emission tail, which enables an image contrast comparable to conventional, much bulkier mercury light sources. As OLEDs can be fabricated on mechanically flexible substrates and structured into arrays of cell‐sized pixels, this work opens a new pathway for the development of implantable light sources that enable functional imaging and sensing in freely moving animals.PostprintPeer reviewe

Low-threshold polariton lasing in a highly disordered conjugated polymer

Funding. China Scholarship Council; Engineering and Physical Sciences Research Council (EP/L015110/1, EP/M025330/1).Low-threshold, room-temperature polariton lasing is crucial for future application of polaritonic devices. Conjugated polymers are attractive candidates for room-temperature polariton lasers, due to their high exciton binding energy, very high oscillator strength, easy fabrication, and tunability. However, to date, polariton lasing has only been reported in one conjugated polymer, ladder-type MeLPPP, whose very rigid structure gives an atypically narrow excitonic linewidth. Here, we observe polariton lasing in a highly disordered prototypical conjugated polymer, poly(9,9-dioctylfluorene), thereby opening up the field of polymer materials for polaritonics. The long-range spatial coherence of the emission shows a maximum fringe visibility contrast of 72%. The observed polariton lasing threshold (27.7  μJ/cm2, corresponding to an absorbed pump fluence of 19.1  μJ/cm2) is an order of magnitude smaller than for the previous polymer polariton laser, potentially bringing electrical pumping of such devices a step closer.Publisher PDFPeer reviewe