R. F. Mahrt

Conjugated Polymers: Lasing and Stimulated Emission

Abstract The discovery of gain narrowing in electroluminescent conjugated polymers has launched a new field of research aiming towards polymer-based injection lasers. Especially poly( para -phenylene vinylene)- (PPV) and poly( para -phenylene)- (PPP) -type polymers have been applied as active materials. Time-resolved measurements have unravelled the photophysics for low and high excitation levels. If suited resonator configurations of the device are used (e.g. microcavity, microring, microdisc, distributed feedback and distributed Bragg resonators) optically pumped single mode laser emission can be observed for moderate excitation levels. These results represent an important step towards the realisation of future ‘all-plastics’ injection lasers. Conjugated polymer laser devices might lead to novel applications in measurement instrumentation and information technology which rely on large area emitters and geometries not accessible to conventional inorganic laser materials.

Femtosecond dynamics of stimulated emission and photoinduced absorption in a PPP-type ladder polymer

Abstract The dynamics of optical excitations in a PPP-type ladder polymer have been investigated by means of femtosecond pump-probe experiments. Stimulated emission (SE) is observed around the S 1 → S 0 transition while photoinduced absorption (PA) dominates the differential transmission at lower energies. The stimulated emission is due to singlet exciton recombination. Comparative studies of the temporal behaviour of the SE and the PA demonstrate that the SE and the PA signals originate from different species. Our results provide evidence that the PA is due to spatially separated electron-hole pairs (indirect excitons).

Observation of strong exciton-photon coupling in an organic microcavity.

Abstract An optical organic semiconductor microcavity showing the strong coupling regime was prepared. The dye 1,1 ′ -diethyl-2,2 ′ -cyanine (PIC) forming J-aggregates was used as optically active material in a planar λ /2-microcavity configuration. Stable thin layers of homogeneously distributed aggregates without any matrix were prepared by spin-coating of a specific salt of the dye [(PIC + ) 2 B 10 H 10 2− ]. Strong exciton–photon coupling was demonstrated by the anticrossing of exciton and photon modes. A Rabi-splitting of more than 50 meV was observed in transmission measurements. The asymmetric normal mode splitting could be explained by the asymmetric absorption spectrum of the J-aggregate.

The optical gain mechanism in solid conjugated polymers

We report on stimulated emission from a solid conjugated methyl-substituted ladder-type poly(paraphenylene). The threshold for stimulated emission as well as the spectral narrowing is measured. Upon exciting within the low energy tail of the S1-S0 (0–0) absorption band narrow emission lines offset from the excitation by the energies of the dominant vibrational modes are observed. From these measurements we conclude that vibronic sublevels play a major role in the optical gain mechanism. Hence, the gain mechanism can be explained in terms of molecularly excited states.

Picosecond time resolved photoluminescence spectroscopy of a tetracene film on highly oriented pyrolytic graphite: Dynamical relaxation, trap emission, and superradiance

A detailed time resolved investigation of the photoluminescence of a thin tetracene film deposited on highly oriented pyrolytic graphite is presented. In agreement with Lim et al. [Phys. Rev. Lett. 92, 107402 (2004)], we find strong evidence for superradiance: an increase of the relative intensity of the pure electronic transition with respect to the vibronic sideband and a concomitant decrease of the radiative lifetime from 10 to 1.83 ns upon cooling from 300 to 4 K. For lower temperatures, a redshift (approximately 200 cm(-1)) of the free exciton is observed. Previously, this shift was attributed to a structural phase transition. Our time resolved spectra reveal that the spectral shift is related to a dynamical relaxation process which occurs within the first 50 ps.

The dynamics of gain-narrowing in a ladder-type pi-conjugated polymer

Abstract Amplified spontaneous emission (ASE) indicated by photoluminescence spectral-narrowing is observed from a ladder-type conjugated polymer at high excitation densities. The narrowed emission shows a distinct blue-shift with respect to the photoluminescence emitted at lower densities. The line-narrowing is examined in detail using picosecond luminescence spectroscopy in conjunction with femtosecond pump–probe spectroscopy. The experiments give insight into the fundamental processes that lead to optical gain in conjugated polymers. Our results provide conclusive evidence that gain-narrowing occurs simultaneously with an energetic relaxation of localized excitations. The spectral properties of the ASE are closely linked to the dynamics of the excited states.

Exciton versus band description of the absorption, luminescence and electro-absorption of poly(phenylphenylenevinylene) and poly(dodecylthiophene)

Abstract Arguments are presented in favour of an exciton description of the optical absorption of poly(phenylenevinylenes) (PPVs) and polythiophenes (PTs). (i) Absorption spectra can be deconvoluted into a series of inhomogeneously broadened vibronic transitions characteristic of chromophores no longer than about 10 repeat units. (ii) Site-selective fluorescence studies are in accord with the concept of spectral diffusion among chromophores differing in excitation energy. (iii) Electro-absorption measurements bear out a quadratic Stark effect acting on an exciton the spatial extent of which is 2–3 repeat units only; no evidence for transitions among delocalized states is found. (iv) Action spectra of intrinsic photoconductivity can be interpreted in terms of disorder-promoted expansion of singlet excitons to form geminate e-h pairs that can subsequently dissociate. (v) By comparison with the absorption spectra of chemically prepared anions of oligo-and poly(phenylenevinylenes) the photoinduced absorption of PPV is assigned to the absorption of monovalent ordered polymer segments of finite length.

Two-photon fluorescence and femtosecond two-photon absorption studies of MeLPPP, a ladder-type poly(phenylene) with low intra-chain disorder

Abstract MeLPPP is a ladder-type conjugated poly(phenylene) with particularly low intra-chain disorder and inhomogeneous broadening of only 20 meV. The two-photon excited state (mA g ), investigated by two-photon fluorescence excitation spectroscopy, is much broader [ E (mA g )≈3.4±0.2 eV] and without well-resolved vibronic structure. This agrees with a broad underlying feature in the electroabsorption spectra of MeLPPP. Femtosecond pump–probe measurements reveal transient absorption into the mA g state within the duration of the pump pulse (140 fs) and rapid subsequent decay ( τ ≈150 fs) to the 1B u exciton, with photoluminescence clearly visible 600 fs after excitation. A value of the nonlinear absorption coefficient β ≈11 cm 2 /GW is calculated for the two-photon absorption, mA g ←1A g .

Hampered Excimer Formation in a Perylene Derivative with Bulky Functional Groups

The optical properties of N′N′Bis (2,6-xylyl)perylene-3,4:9,10-bis(dicarboximide) (DPP-PTCDI) thin films of various thicknesses have been studied. While the absorption spectra are redshifted compared to the monomer absorption spectra in solution, the photoluminescence spectra show an additional inhomogeneously broadened unstructured band, which increases in intensity with increasing film thickness. Photoluminescence measurements reveal that this broadband emission originates from defect states that exist in the ground state. The bulky functional group of the DPP-PTCDI molecule inhibits excimer formation, otherwise typical for perylene derivatives. Time-resolved photoluminescence (TRPL) measurements show that the molecular decay is directly related to the film thickness due to an increasing density of defects with increasing film thickness.

OBSERVATION OF INTERFACE EXCITONS AND ENERGY TRANSFER PROCESSES IN AN OLIGO-THIOPHENE MULTI-LAYER STRUCTURE

Abstract The optical properties of a three-layer organic heterostructure consisting of tetrathiophene(T4)/hexathiophene(T6)/tetrathiophene(T4) are investigated by a variety of optical techniques. The optical spectra give strong evidence for the formation of interface exciton states at the interface between adjacent T4 and T6 layers. To our knowledge this is the first time that interface excitons have been experimentally observed in an oligothiophene multilayer structure. Furthermore, evidence of energy transfer from T4 excited states to interface excitons and T6 states is found. Electroluminescence experiments indicate charge confinement and efficient electron-hole recombination in the central T6 layer.