Three-dimensional photonic confinement in imprinted liquid crystalline pillar microcavities

Abstract

We demonstrate the feasibility of a thermal imprint technology capable of structuring organic thin films with liquid crystalline properties forming feature sizes on a several micrometer scale. The imprint technique can directly be applied onto a variety of substrates including dielectric mirrors. The so fabricated three-dimensional microcavities have lateral extensions up to 20 µm and heights between 1 and 5 µm. Exemplarily, pillar microcavities were produced wherein three-dimensional photonic confinement is observed by the formation of 0D cavity mode patterns. The imprint technique further favors the formation of hemispherical pillar geometries rather than cylindrical pillars resulting in equidistant mode spacings of transversal cavity modes.Although organic dyes have already been used as gain medium in laser cavities for more than fifty years, microcavities with active organic medium are still in the spotlight of modern research activities as they hold great potential for emergent applications such as lab-on-a-chip architectures or sensing devices 1 . This is first and foremost related to the versatility and flexibility that is offered by organic compounds. They not only provide spectral tunability and chemical customizability, they can also be fabricated at low costs and with simple processing techniques. The simplest organic cavity design is the planar microcavity that is composed of the active organic layer sandwiched between two highly reflective mirrors (either metallic or dielectric). In this planar geometry, organic photon lasing 2,3 as well as polariton condensation 4-6 was already demonstrated. More elaborated cavity concepts include distributed feedback (DFB) gratings 7 , whispering gallery resonators 8 , photonic crystals 9 or hemispherical cavities 10 . While those provide multi-dimensional photonic confinement, planar cavity designs achieve confinement only in the out-of-plane direction. In order to decrease the mode volume for planar cavities anyhow, it is necessary to provide a photonic modulation in plane by either structuring the excitonic or dielectric part of the organic microcavity. So far, this has been achieved by incorporating additional metal layers 11 into the cavity or by laser writing 12 .In this letter, we present a method to create threedimensional photonic confinement in an organic microcavity system: thermal imprint technology using a liquid crystalline dye. Imprinted structures were so far limited to a few hundreds of nanometers in feature size. Here, a) Electronic mail: christof.dietrich@physik.uni-wuerzburg.de we make use of the liquid-crystalline nature of the recently developed organic compound MEH-PBI (bearing twelve 2-ethylhexyl substituents at the perylene bisimide core) 13 . In the liquid crystalline phase, a pre-patterned stamp is pressed into a layer of the organic material creating micropillars with micrometer-sized dimensions. Imprinting directly on dielectric mirrors and capping the micropillars by a thin gold layer creates three-dimensional photonic confinement. Th...

Self-assembly of multi-stranded perylene dye J-aggregates in columnar liquid-crystalline phases

“…The twist in the PBI cores and the steric demand of the dendrons play a key role in the columnar assembly modes, which in turn have an impact on the exciton couplings between dyes and on spectroscopic properties such as absorption and emission maxima of the J-aggregates. These results are of great importance for the fine-tuning of properties of tailor-made materials for photonic applications as it was recently demonstrated for MEH-PBI in imprinted LC pillar microcavities 39 .

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“…In this vein, we recently reported an unprecedented organization of PBIs into LC triple-stranded helical columnar structures in which the PBI cores are oriented parallel to the columnar axis 38 . This material was subsequently utilized in photonic microcavities 39 . Our molecular design consists of a tetra-bay functionalized MEH-PBI with 1,2-ethylhexyl (EH) substituents and free NH groups at the imide positions.…”

Optical‐Mode Structure of Micropillar Microcavities Containing a Fluorescent Conjugated Polymer

“…Using near‐field optical imaging techniques, it was shown that such structures were able to modify spontaneous emission rates. Organic micropillars have also previously been fabricated using a thermal imprint technology to pattern a liquid‐crystalline molecular dye into pillar shapes on the surface of a dielectric mirror . By coating such structures with a thin film of gold, the hemispherical pillar‐geometry cavities created were shown to support several families of quantized optical modes.…”

Supramolecularly Engineered J-Aggregates Based on Perylene Bisimide Dyes

“…PBI J-aggregates are ideal candidates because they not only fulfill this essential criterion but also show hyperchromicity and fluorescence in the solid state . Accordingly, J-aggregates of PBIs 3d and 6d could be implemented in all-polymer photonic microcavities, pillar microcavities, and open microcavities . For the former, a film of PBI 3d J-aggregates in amorphous polypropylene (aPP) was prepared between two polymerically distributed Bragg reflectors (DBR) to afford a 5-fold increase in photoluminescence intensity compared to that of a reference sample and a narrow fwhm of 6.7 nm at the cavity mode wavelength (725 nm) (Figure , bottom right) …”