Molding Photonic Boxes into Fluorescent Emitters by Direct Laser Writing

Abstract

There is a rapidly increasing interest in the photonics research community to develop methods for immediate and on-demand fabrication of complex photonic structures. This is motivated by a desire to tailor the properties of components like interconnects, waveguides or cavities to specific but varying requirements. Due to their versatility and flexibility, organic materials are highly promising candidates for achieving this goal and a number of fabrication methods such as direct laser writing by two-photon polymerization [1][2][3] or direct circuit printing [4] have been suggested and are now developing rapidly.Biologically produced fluorescent proteins (FPs) represent a special class of organic emitter materials. Due to their unique molecular structure, which comprises a highly fluorescent chromophore unit surrounded by a protective molecular bumper (β-barrel), [5,6] the chromophores are well protected from their surrounding and FPs are largely immune to concentration quenching. [7] This has allowed demonstration of low threshold lasers [8][9][10][11] and more recently formation of exciton-polariton condensation at room temperature under nanosecond excitation. [12] Here, we demonstrate an approach for direct on-demand writing of photonic structures in thin films of FPs. Our approach exploits the usually unwanted effect of photobleaching which occurs in organic emitters upon excitation with high power lasers. In addition to the local loss of fluorescence, the bleaching also induces a local change in refractive index of the active organic material which allows us to introduce inplane spatial confinement and guiding of photons in the active organic layer. Based on this principle, we demonstrate stimulated emission in three-dimensionally confined photonic modes and achieve lasing thresholds one order of magnitude lower than in planar unstructured reference devices. As the active FP in our microcavities, we use tdTomato, a tandem dimer protein, in which each molecule is formed by four β-barrels, each containing a chromophore at its center. The four barrels are arranged in pairs connected by a linker. tdTomato is one of the most widely used FPs and due to its large absorption coefficient (138 000 m −1 cm −1 ) and high fluorescence quantum yield (0.6) [13] it offers high brightness and has proven to be well suited to generate lasing. [7] Figure 1a shows the fluorescence of a tdTomato thin film (thickness, 500 nm) on a glass substrate when excited by an optical parametric oscillator (OPO) system with 530 nm nanosecond pulses of light with increasing pulse energy (2 µm spot size). There is a clear linear increase of the fluorescence with increasing pump pulse energy over a range of more than four orders of magnitude (from 1 nJ to 10 µJ per pulse). This proves that tdTomato films offer excellent photostability in the range required for lasing from tdTomato (reported lasing thresholds range between 1 and 1000 nJ). [7] However, for excitation energies larger than 10 µJ, the excitation-dependent fluorescence flattens and abo...

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

“…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. Other work has used an optical writing technique to selectively change the refractive index of a thin film of a biologically produced fluorescent protein by photobleaching . Such films were then used as the active layer of a micropillar, with lateral photonic confinement leading to a reduction in lasing thresholds compared to an unpatterned control.…”

“…Other work has used an optical writing technique to selectively change the refractive index of a thin film of a biologically produced fluorescent protein by photobleaching. [32] Such films were then used as the active layer of a micropillar, with lateral photonic confinement leading to a reduction in lasing thresholds compared to an unpatterned control. Further approaches to fabrication of laterally quantized states in organic photonic structures include the use of advanced shadow-masking techniques.…”

Defect‐State Lasing in Photonic Lattices of Metal–Organic Microcavities

“…Resulting from the micron‐scale trapping of light, discrete modes in angular‐resolved spectra of MCs with individual 0D boxes [ 7 ] and individual 1D wires [ 8 ] have been analyzed and utilized for low‐threshold lasing. [ 9,10 ] In addition, photonic bands and BGs of chains of boxes [ 11,12 ] or arrays of wires [ 13–15 ] have been facilitated and observed, thus creating another photonic crystal in the lateral defect plane. Such structures are often fabricated by etching micropillars into MCs via electron beam lithography and reactive‐ion etching [ 12,16 ] or direct laser writing.…”

“…Such structures are often fabricated by etching micropillars into MCs via electron beam lithography and reactive‐ion etching [ 12,16 ] or direct laser writing. [ 10 ] In our case, in‐plane (lateral) photonic confinement is realized by adding a thin silver layer that is deposited in between the bottom DBR and the active layer. Subsequently, structuring this silver layer via photolithography into parallel stripes facilitates the interaction of cavity photons and Tamm plasmon polaritons (TPPs).…”

Three-dimensional photonic confinement in imprinted liquid crystalline pillar microcavities

“…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.…”