J-aggregate Films Research Articles

Intracavity optical pumping of J-aggregate microcavity exciton polaritons

We demonstrate intracavity optical pumping of J-aggregate microcavity exciton polaritons. The use of ultrathin layer-by-layer J-aggregate thin films as the strongly coupled exciton medium allows for inclusion of a thermally evaporated luminescent cavity spacer layer, through which the lower-branch exciton-polariton states are resonantly pumped. We present a measurement of the lower-branch exciton-polariton occupation in room-temperature microcavity devices containing J-aggregated molecular thin films under low-density steady-state excitation. The observed exciton-polariton occupation shows a Maxwell-Boltzmann distribution at $T=300\text{ }\text{K}$, indicating thermalization of exciton polaritons in the lower energy branch. This device design enables us to propose a new type of ``polariton laser'' architecture for microcavity exciton polaritons.

Chiral Sum Frequency Spectroscopy of Thin Films of Porphyrin J-Aggregates

Thin films of chiral porphyrin J-aggregates have been studied by vibrationally and electronically doubly resonant sum frequency generation (SFG) spectroscopy. It was revealed that the chiral supramolecular structures of porphyrin aggregates in solutions were retained in the thin film samples, and their chirality was determined by using chiral vibrational SFG spectroscopy. Electronic resonance profiles of some vibrational bands in achiral and chiral SFG were different from each other, and both were distinct from electronic absorption spectra. To account for these peculiar profiles, we have proposed interference effects of Raman tensor components in achiral and chiral SFG susceptibilities, which is analogous to that of resonance Raman scattering.

Nanometer films of polymethine dyes in optical memory and nonlinear optics

We present a review of experimental investigations in the synthesis and characterization of thin solid films of polymethine dyes in monomer and aggregate forms, which have been performed at the Institute of Organic Chemistry and the Institute of Automation and Electrometry (Siberian Branch of the Russian Academy of Sciences, Novosibirsk) for the past 15 years. The films, obtained by spin-coating technique on glass substrates, were intended for use in optical memory disks of the WORM (write once read many) type and as nonlinear optical media with cubic nonlinearity. The synthesized symmetric thioflavylium dyes with tert-butyl substituents form non-crystallized films of optical quality, which are suited for the creation of recording layers of WORM disks. Thermal, optical, and nonlinear optical properties of thin films based on J-aggregates of pseudoisocyanine derivatives with long alkyl substituents, the films of such dyes doped with cluster anions of higher boron hydrides, and the films of an aggregated thiocarbocyanine dye are considered. Nanometer films of stable J-aggregates of pseudoisocyanine derivatives exhibit the absolute values of the cubic optical nonlinear susceptibility on the order of 10−5 esu at room temperature and 10−4 esu at liquid nitrogen temperature. Thin films of J-aggregates of organic dyes can also be used as saturable absorbers of laser radiation.

Photophysical property of the J-aggregate thin film of an oxacyanine dye prepared by the spin-coating method and enhancement of its photostability by use of polydimethylsilane

The J-aggregate thin film of an oxacyanine dye has been prepared by the spin-coating method on a quartz plate and its photophysical property has been studied. Coherent size of the J-aggregate was estimated to be ca. 4–7 by means of the fluorescence radiative lifetime. Photostability of the J-aggregate thin film and J-aggregate/poly(dimethylsilane) composite thin film has also been examined. Poly(dimethylsilane) vacuum deposited on the J-aggregate thin film was found to markedly enhance the photostability. This stabilization mechanism has been discussed in terms of the electron transfer from poly(dimethylsilane).

Simulation of J-aggregate microcavity photoluminescence

We have developed a model in order to account for the photoexcitation dynamics of J-aggregate films and strongly coupled J-aggregate microcavities. The J aggregates are described as a disordered Frenkel exciton system in which relaxation occurs due to the presence of a thermal bath of molecular vibrations. The correspondence between the photophysics in J-aggregate films and that in J-aggregate microcavities is obtained by introducing a model polariton wave function mixing cavity photon modes and J-aggregate super-radiant excitons. With the same description of the material properties, we have calculated both absorption and luminescence spectra for the J-aggregate film and the photoluminescence of strongly coupled organic microcavities. The model is able to account for the fast relaxation dynamics in organic microcavities following nonresonant pumping and explains the temperature dependence of the ratio between the upper polariton and the lower polariton luminescence.

Solid state cavity QED: Strong coupling in organic thin films

We review our research efforts to develop solid state integrated devices that operate in the strong coupling limit of cavity quantum electrodynamics (QED) for eventual application in high speed optical switching, optical computing, and quantum computing. Our devices contain J-aggregates of (organic) cyanine dyes which, by virtue of their molecular arrangement and strong dipolar coupling, exhibit a collective narrow linewidth high oscillator strength optical transition. Using J-aggregates, the strong coupling limit can be reached at room temperature with large coupling strengths (Rabi-splitting >250 meV) in exciton–polariton microcavity structures. We demonstrate that high quality nanoscale thick J-aggregate films can be uniformly deposited over macroscopic substrates, engineered at the molecular level, and patterned into single or multi-dimensional photonic bandgap structures. Our unique methods for depositing J-aggregates enabled us to structure light emitting devices that demonstrated the first ever electrically pumped polariton emission, uniquely accomplished in room temperature operation. Additionally, we demonstrated critically coupled resonators that concentrate nearly all of the incident light into 5 nm thick J-aggregate films, yielding a record high effective absorption constant of 6.8 · 10 6 cm � 1 for films with thickness that is less than 1% of the incident light wavelength. Such remarkable optical properties, enabled by scalable deposition techniques, suggest that J-aggregates are a unique materials platform on which to demonstrate integrated exciton–polariton devices with the far reaching properties of polaritons in the optical domain. � 2007 Elsevier B.V. All rights reserved. PACS: 71.36.+c; 71.35.Aa; 85.60.Jb; 78.40.Me

Self-Modulation of Scattering Intensity from a Silica Sphere Coated with a Sol-Gel Film Doped with J-Aggregates

A micrometer-sized nonlinear optical resonator constituted of a silica microsphere coated with J-aggregates has been fabricated by the sol-gel process. We determined the scattering light spectrum and discuss the application of the observed effect for the realization of an optical switching element. Optical evaluation of the sphere has been performed in the attenuated-total-reflection (ATR) configuration. We excited a whispering gallery mode and measured the nonlinear ATR signal dependence on excitation intensity. The experimental result agrees very well with the simulation results obtained using the Mie theory and the finite-difference time-domain method taking into account optical Kerr nonlinearity. The permittivity of the film of J-aggregates was measured by ATR method at several wavelengths and the value at the desired wavelength was determined by extrapolation using the Lorentz function.

J-Aggregate Formation of Spiropyran Derivatives in LB and Vapor-Deposited Thin Films

ABSTRACT J-aggregate formation in the films of spiropyran derivatives (SP1822 and SP18) prepared with Langumuir-Blodegett (LB) and vapor-deposition methods was investigated. In the films of SP1822, J-aggregate formation occurred with UV light illumination and thermal treatment. In the case of SP18 film, on the other hand, only photoisomerization occurred. When the PMC-form of SP18 was transferred onto a glass plate, J-like aggregate formation occurred after UV light illumination. By UV light illumination during evaporation process, SP1822 formed J-aggregate. Finally, SP1822 J-aggregate film oriented one-dimensionally was fabricated by using a PTFE layer as a substrate.

Study of the broad-band saturable absorption of indocyanine green J-aggregates in polymeric films using 10-fs laser pulses

Thin films of indocyanine green J-aggregates in polymers have been prepared and their saturable absorption properties studied in a high intensity regime using a unique ultrashort pulse Ti:sapphire laser (10 fs) with a very broad bandwidth (over 200 nm at -20 dB). The samples showed a surprisingly high stability to irradiation, strong absorption bleaching, and wavelength dependent differential absorption spectra.

New organic materials and microcavity structures for strong exciton‐photon coupling

We discuss the properties of a selection of cyanine dyes that form J-aggregates with narrow absorption bands, suitable for studying strong exciton-photon coupling in optical microcavities at room temperature. In particular, we show how the optical properties of J-aggregate films can be tailored for microcavity applications. Microcavities are fabricated by lamination of the organic material between 2 dielectric mirrors. Such cavities contain an optical wedge, so the relative tuning between the organic exciton and the cavity mode can be realized by changing the sample position with regard to the light beam. Transmission measurements performed at room temperature show well resolved anticrossing behaviour with Rabi splitting of around 80 meV. Moreover, photoluminescence measurements allow us to study polariton emission under conditions of non-resonant excitation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical nonlinearity of J-aggregates in vapor-deposited films of a bisazomethine dye

The third-order optical nonlinearity of the J-aggregates of nonionic dye in vapor-deposited films has been investigated in terms of electroabsorption measurement. Compared with that of a monomer film of the dye, the nonlinearity was enhanced by a factor of 4000. However, the number of aggregates in the J-aggregate film was estimated to be only a few. The obtained enhancement is mainly caused by the concentration of oscillator strength on the J-band and not by the development of the coherence size of excitons. Therefore, a much larger optical nonlinearity can be expected in highly ordered J-aggregates of the nonionic dye, and it is demonstrated that J-aggregate formation is an effective strategy for nonionic dye to improve its optical nonlinearity.

Layer‐by‐Layer J‐Aggregate Thin Films with a Peak Absorption Constant of 106 cm–1

Layer-by-layer assembly is used to produce thin films of J-aggregates of cyanine dye molecules with an absorption constant of 106 cm–1. The films display high reflection and absorption at resonance (see Figure, ∼50 nm films) and nanoscale features that make them well suited for use in strong-coupling optoelectronic applications, enabling the creation of exciton–polariton optoelectronic devices that operate at room temperature.

PASSIVE MODE LOCKING OF A Nd3+:YAG LASER WITH A SATURABLE ABSORBER IN THE FORM OF THIN FILMS OF J-AGGREGATES

A novel type of a saturable absorber for passive mode locking was proposed. As the novel saturable absorber, we studied the thin solid films of the molecular J-aggregates of polymethyn dyes. The films of the J-aggregates were prepared using different polymethyn dyes. Linear and nonlinear optical properties of these films were investigated. Passive mode locking using these films as the saturable absorber was performed in a Nd 3+: YAG laser. Stable generation of short pulses (pulse duration 13 ps) was achieved.

Hybrid polaritons in strongly coupled microcavities: experiments and models

We describe the fabrication of one-dimensional quantum microcavities containing two different layers of molecular J-aggregates. We show that strong coupling can occur between the confined cavity photon mode and the two different molecular exciton modes, leading to the appearance of new ‘hybrid’ polaritonic modes. Such hybrid states can be described in terms of a superposition of the cavity photon and the two excitonic states. We characterise such cavities as a function of external viewing angle by measuring both the white-light reflectivity and photoluminescence emission following non-resonant optical excitation. We apply a simple model to describe the photoluminescence emission from the cavity by assuming a transfer of population between the different hybrid-polariton modes. We describe the predictions of our model and show that it provides a reasonable qualitative description of the emission. We then use a transfer matrix scattering model to calculate the reflectivity spectra of a cavity, based on an organic semiconductor layer composed of a thin film of J-aggregates deposited onto an inorganic heterostructure containing three InGaP quantum wells. Our results demonstrate that optical hybridisation may be expected between the organic and inorganic excitons in suitably designed structures.

Enhancement of resonant bleaching of J-aggregates upon lengthening of an exciting radiation

Thin films of J-aggregates of a new amphiphilic thiacarbocyanine dye of the benthiazole series are prepared and the nonlinear optical response of molecular J-aggregates is studied for femto-and nanosecond exciting radiation pulses. It is found that the nonlinear optical response of J-aggregates exhibits substantial enhancement upon an increase in the pulse duration, which cannot be described by the saturation effect in the model of a two-level system. This effect is considered using a three-level model taking into account the formation of self-trapped exciton states in molecular J-aggregates.

Exciton-polariton dynamics in a transparent organic semiconductor microcavity

Exciton-polariton dynamics are studied in organic semiconductor microcavities operating in the strongly coupled polariton regime, with thin films of cyanine J-aggregates sandwiched between low-loss dielectric mirrors. The delocalized excitons are subject to polariton normal mode splittings which are modulated by near resonant femtosecond excitation pulses, while orientational disorder effects on the formation of polariton states play an important role in polariton dynamics.

Passive mode locking of a Nd 3+:YAG laser with a thin film of J-aggregates as a saturable absorber

Thin films of molecular J-aggregates of polymethyn dyes were obtained. Linear and nonlinear optical properties of the films were investigated. It was demonstrated for the first time that such a film of J-aggregates can be successfully used as a saturable absorber for passive mode locking of a Nd 3+:YAG laser. The reproduced generation of ultra short pulses with a duration of 13 ps was obtained for two dyes forming J-aggregates.

Femtosecond timing measurement and control using ultrafast organic thin films

We show a femtosecond timing measurement and control technique using a squarylium dye J-aggregate film, which is an organic thin film that acts as an ultrafast two-dimensional optical switch. Optical pulse timing is directly mapped to space-domain position on the film, and the large area and ultrafast response offer a femtosecond-resolved, large dynamic range, real-time, multichannel timing measurement capability. A timing fluctuation (jitter, wander, and skew) reduction architecture is presented and experimentally demonstrated.

Morphology of self-assembled merocyanine J-aggregates in films studied by scanning near-field optical microscope

Morphology of 6-methyl merocyanine J-aggregates self-assembled in polyvinyl alcohol (PVA) films by a spin-coating method has been investigated by means of scanning near-field optical microscope and confocal laser scanning microscope. Photoluminescence image measurements show that J-aggregates form different domain structures with a grain-shape and a fiber-shape depending on the concentrations of merocyanine and arachidic acid in the PVA film. From the polarization-dependent measurements, we have found that the 6-methyl merocyanine molecules construct a hierarchical structure of J-aggregates in which a J-aggregate domain is composed of coherent aggregates with the same transition dipole orientation.

Nonlinear optical properties of one-dimensional organic molecular aggregates in nanometer films

Nanometer films of molecular J-aggregates of pseudoisocyanine (PIC) with various degrees of both static and dynamic disorder were prepared. The amount of static disorder was varied by modifying the local environment of the J-aggregates in the films, while the dynamic disorder was varied by changing the film temperature. Through these variations, the delocalization length N del of the excitons was controlled, as could be monitored from the width of the exciton absorption line. This has allowed us to establish the scaling of the nonlinear optical susceptibility per aggregate molecule, χ (3)/ N, with the delocalization length as χ (3)/ N ∝ N del γ , with γ=2.3±0.2. This behavior turned out to be independent of the detailed nature of disorder, and agrees very well with the theoretically predicted size-enhancement of the cubic susceptibility in J-aggregates.