Revealing the Origin of Fast Delayed Fluorescence in a Donor Functionalized Bisterpyridine.

A new carbazole-substituted bisterpyridine with pronounced delayed fluorescence is presented. While the molecular donor-acceptor-donor design suggests the origin of this to be thermally activated delayed fluorescence (TADF), results from various photophysical characterizations, OLED characteristics, temperature-dependent NMR spectroscopy, and DFT calculations all point against the involvement of triplet states. The molecule exhibits blue emission at about 440 nm with two or more fast decay channels in the lower nanosecond range in both solution and thin films. The delayed emission is proposed to be caused by rotational vibrational modes. We suggest that these results are generally applicable, especially for more complex molecules, and should be considered as alternative or competitive emissive relaxation pathways in the field of organic light emitting materials.

Snapshot hyperspectral imaging of intracellular lasers.

Intracellular lasers are emerging as powerful biosensors for multiplexed tracking and precision sensing of cells and their microenvironment. This sensing capacity is enabled by quantifying their narrow-linewidth emission spectra, which is presently challenging to do at high speeds. In this work, we demonstrate rapid snapshot hyperspectral imaging of intracellular lasers. Using integral field mapping with a microlens array and a diffraction grating, we obtain images of the spatial and spectral intensity distribution from a single camera acquisition. We demonstrate widefield hyperspectral imaging over a 3 × 3 mm2 field of view and volumetric imaging over 250 × 250 × 800 µm3 (XYZ) volumes with a lateral (XY) resolution of 5 µm, axial (Z) resolution of 10 µm, and a spectral resolution of less than 0.8 nm. We evaluate the performance and outline the challenges and strengths of snapshot methods in the context of characterizing the emission from intracellular lasers. This method offers new opportunities for a diverse range of applications, including high-throughput and long-term biosensing with intracellular lasers.

Mesogenic Groups Control the Emitter Orientation in Multi-Resonance TADF Emitter Films.

The use of thermally activated delayed fluorescence (TADF) emitters and emitters that show preferential horizontal orientation of their transition dipole moment (TDM) are two emerging strategies to enhance the efficiency of OLEDs. We present the first example of a liquid crystalline multi-resonance TADF (MR-TADF) emitter, DiKTa-LC. The compound possesses a nematic liquid crystalline phase between 80 °C and 110 °C. Importantly, the TDM of the spin-coated film shows preferential horizontal orientation, with an anisotropy factor, a, of 0.28, which is preserved in doped poly(vinylcarbazole) films. Green-emitting (λEL =492 nm) solution-processed OLEDs based on DiKTa-LC showed an EQEmax of 13.6 %. We thus demonstrate for the first time how self-assembly of a liquid crystalline TADF emitter can lead to the so-far elusive control of the orientation of the transition dipole in solution-processed films, which will be of relevance for high-performance solution-processed OLEDs.

Cortical cell stiffness is independent of substrate mechanics.

Cortical stiffness is an important cellular property that changes during migration, adhesion and growth. Previous atomic force microscopy (AFM) indentation measurements of cells cultured on deformable substrates have suggested that cells adapt their stiffness to that of their surroundings. Here we show that the force applied by AFM to a cell results in a significant deformation of the underlying substrate if this substrate is softer than the cell. This 'soft substrate effect' leads to an underestimation of a cell's elastic modulus when analysing data using a standard Hertz model, as confirmed by finite element modelling and AFM measurements of calibrated polyacrylamide beads, microglial cells and fibroblasts. To account for this substrate deformation, we developed a 'composite cell-substrate model'. Correcting for the substrate indentation revealed that cortical cell stiffness is largely independent of substrate mechanics, which has major implications for our interpretation of many physiological and pathological processes.

Cell Force-Driven Basement Membrane Disruption Fuels EGF- and Stiffness-Induced Invasive Cell Dissemination from Benign Breast Gland Acini.

Local basement membrane (BM) disruption marks the initial step of breast cancer invasion. The activation mechanisms of force-driven BM-weakening remain elusive. We studied the mechanical response of MCF10A-derived human breast cell acini with BMs of tuneable maturation to physical and soluble tumour-like extracellular matrix (ECM) cues. Traction force microscopy (TFM) and elastic resonator interference stress microscopy (ERISM) were used to quantify pro-invasive BM stress and protrusive forces. Substrate stiffening and mechanically impaired BM scaffolds induced the invasive transition of benign acini synergistically. Robust BM scaffolds attenuated this invasive response. Additional oncogenic EGFR activation compromised the BMs' barrier function, fuelling invasion speed and incidence. Mechanistically, EGFR-PI3-Kinase downstream signalling modulated both MMP- and force-driven BM-weakening processes. We show that breast acini form non-proteolytic and BM-piercing filopodia for continuous matrix mechanosensation, which significantly push and pull on the BM and ECM under pro-invasive conditions. Invasion-triggered acini further shear and compress their BM by contractility-based stresses that were significantly increased (3.7-fold) compared to non-invasive conditions. Overall, the highest amplitudes of protrusive and contractile forces accompanied the highest invasiveness. This work provides a mechanistic concept for tumour ECM-induced mechanically misbalanced breast glands fuelling force-driven BM disruption. Finally, this could facilitate early cell dissemination from pre-invasive lesions to metastasize eventually.

Spectroscopic near-infrared photodetectors enabled by strong light-matter coupling in (6,5) single-walled carbon nanotubes.

Strong light-matter coupling leads to the formation of mixed exciton-polariton states, allowing for a rigorous manipulation of the absorption and emission of excitonic materials. Here, we demonstrate the realization of this promising concept in organic photodetectors. By hybridizing the E11 exciton of semiconducting (6,5) single-walled carbon nanotubes (SWNTs) with near-infrared cavity photons, we create spectrally tunable polariton states within a photodiode. In turn, we are able to red-shift the detection peak that coincides with the lower polariton band. Our photodiodes comprise a metal cavity to mediate strong coupling between light and SWNTs and utilize P3HT and PC70BM as the electron donor and acceptor, respectively. The diodes are formed either via mixing of SWNTs, P3HT, and PC70BM to create a bulk heterojunction or by sequential processing of layers to form flat heterojunctions. The resulting near-infrared sensors show tunable, efficient exciton harvesting in an application-relevant wavelength range between 1000 nm and 1300 nm, with optical simulations showing a possible extension beyond 1500 nm.

Inexpensive Methods for Live Imaging of Central Pattern Generator Activity in the Drosophila Larval Locomotor System.

Central pattern generators (CPGs) are neural networks that produce rhythmic motor activity in the absence of sensory input. CPGs produce 'fictive' behaviours in vitro which parallel activity seen in intact animals. CPG networks have been identified in a wide variety of model organisms and have been shown to be critical for generating rhythmic behaviours such as swimming, walking, chewing and breathing. Work with CPG preparations has led to fundamental advances in neuroscience; however, most CPG preparations involve intensive dissections and require sophisticated electrophysiology equipment, making export to teaching laboratories problematic. Here we present an integrated approach for bringing the study of locomotor CPGs in Drosophila larvae into teaching laboratories. First, we present freely available genetic constructs that enable educators to express genetically encoded calcium indicators in cells of interest in the larval central nervous system. Next, we describe how to isolate the larval central nervous system and prepare it for live imaging. We then show how to modify standard compound microscopes to enable fluorescent imaging using 3D printed materials and inexpensive optical components. Finally, we show how to use the free image analysis programme ImageJ and freely available features in the signal analysis programme DataView to analyse rhythmic CPG activity in the larval CNS. Comparison of results to those obtained on research equipment shows that signal-to-noise levels are comparable and core features of larval CPG activity can be observed. Overall, this work shows the viability of exporting live imaging experiments to low cost environments and paves the way for new teaching laboratory exercises revolving around optical imaging of CPG activity.

Developing Next-generation Brain Sensing Technologies - A Review.

Advances in sensing technology raise the possibility of creating neural interfaces that can more effectively restore or repair neural function and reveal fundamental properties of neural information processing. To realize the potential of these bioelectronic devices, it is necessary to understand the capabilities of emerging technologies and identify the best strategies to translate these technologies into products and therapies that will improve the lives of patients with neurological and other disorders. Here we discuss emerging technologies for sensing brain activity, anticipated challenges for translation, and perspectives for how to best transition these technologies from academic research labs to useful products for neuroscience researchers and human patients.

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

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.

Organic Lasers: Recent Developments on Materials, Device Geometries, and Fabrication Techniques.

Organic dyes have been used as gain medium for lasers since the 1960s, long before the advent of today's organic electronic devices. Organic gain materials are highly attractive for lasing due to their chemical tunability and large stimulated emission cross section. While the traditional dye laser has been largely replaced by solid-state lasers, a number of new and miniaturized organic lasers have emerged that hold great potential for lab-on-chip applications, biointegration, low-cost sensing and related areas, which benefit from the unique properties of organic gain materials. On the fundamental level, these include high exciton binding energy, low refractive index (compared to inorganic semiconductors), and ease of spectral and chemical tuning. On a technological level, mechanical flexibility and compatibility with simple processing techniques such as printing, roll-to-roll, self-assembly, and soft-lithography are most relevant. Here, the authors provide a comprehensive review of the developments in the field over the past decade, discussing recent advances in organic gain materials, which are today often based on solid-state organic semiconductors, as well as optical feedback structures, and device fabrication. Recent efforts toward continuous wave operation and electrical pumping of solid-state organic lasers are reviewed, and new device concepts and emerging applications are summarized.

Integration of spectral coronagraphy within VIPA-based spectrometers for high extinction Brillouin imaging.

VIPA (virtually imaged phase array) spectrometers have enabled rapid Brillouin spectrum measurements and current designs of multi-stage VIPA spectrometers offer enough spectral extinction to probe transparent tissue, cells and biomaterials. However, in highly scattering media or in the presence of strong back-reflections, such as at interfaces between materials of different refractive indices, VIPA-based Brillouin spectral measurements are limited. While several approaches to address this issue have recently been pursued, important challenges remain. Here we have adapted the design of coronagraphs used for exosolar planet imaging to the spectral domain and integrated it in a double-stage VIPA spectrometer. We demonstrate that this yields an increase in extinction up to 20 dB, with nearly no added insertion loss. The power of this improvement is vividly demonstrated by Brillouin imaging close to reflecting interfaces without index matching or sample tilting.

Broadband Tunable, Polarization-Selective and Directional Emission of (6,5) Carbon Nanotubes Coupled to Plasmonic Crystals.

We demonstrate broadband tunability of light emission from dense (6,5) single-walled carbon nanotube thin films via efficient coupling to periodic arrays of gold nanodisks that support surface lattice resonances (SLRs). We thus eliminate the need to select single-walled carbon nanotubes (SWNTs) with different chiralities to obtain narrow linewidth emission at specific near-infrared wavelengths. Emission from these hybrid films is spectrally narrow (20-40 meV) yet broadly tunable (∼1000-1500 nm) and highly directional (divergence <1.5°). In addition, SLR scattering renders the emission highly polarized, even though the SWNTs are randomly distributed. Numerical simulations are applied to correlate the increased local electric fields around the nanodisks with the observed enhancement of directional emission. The ability to control the emission properties of a single type of near-infrared emitting SWNTs over a wide range of wavelengths will enable application of carbon nanotubes in multifunctional photonic devices.

Lasing within Live Cells Containing Intracellular Optical Microresonators for Barcode-Type Cell Tagging and Tracking.

We report on a laser that is fully embedded within a single live cell. By harnessing natural endocytosis of the cell, we introduce a fluorescent whispering gallery mode (WGM) microresonator into the cell cytoplasm. On pumping with nanojoule light pulses, green laser emission is generated inside the cells. Our approach can be applied to different cell types, and cells with microresonators remain viable for weeks under standard conditions. The characteristics of the lasing spectrum provide each cell with a barcode-type label which enables uniquely identifying and tracking individual migrating cells. Self-sustained lasing from cells paves the way to new forms of cell tracking, intracellular sensing, and adaptive imaging.

Cellular dye lasers: lasing thresholds and sensing in a planar resonator.

Biological cell lasers are promising novel building blocks of future biocompatible optical systems and offer new approaches to cellular sensing and cytometry in a microfluidic setting. Here, we demonstrate a simple method for providing optical gain by using a variety of standard fluorescent dyes. The dye gain medium can be located inside or outside a cell, or in both, which gives flexibility in experimental design and makes the method applicable to all cell types. Due to the higher refractive index of the cytoplasm compared to the surrounding medium, a cell acts as a convex lens in a planar Fabry-Perot cavity. Its effect on the stability of the laser cavity is analyzed and utilized to suppress lasing outside cells. The resonance modes depend on the shape and internal structure of the cell. As proof of concept, we show how the laser output modes are affected by the osmotic pressure.

Lasing from fluorescent protein crystals.

We investigated fluorescent protein crystals for potential photonic applications, for the first time to our knowledge. Rod-shaped crystals of enhanced green fluorescent protein (EGFP) were synthesized, with diameters of 0.5-2 µm and lengths of 100-200 µm. The crystals exhibit minimal light scattering due to their ordered structure and generate substantially higher fluorescence intensity than EGFP or dye molecules in solutions. The magnitude of concentration quenching in EGFP crystals was measured to be about 7-10 dB. Upon optical pumping at 485 nm, individual EGFP crystals located between dichroic mirrors generated laser emission with a single-mode spectral line at 513 nm. Our results demonstrate the potential of protein crystals as novel optical elements for self-assembled, micro- or nano-lasers and amplifiers in aqueous environment.

Coherent mode coupling in highly efficient top-emitting OLEDs on periodically corrugated substrates.

Bragg scattering at one-dimensional corrugated substrates allows to improve the light outcoupling from top-emitting organic light-emitting diodes (OLEDs). The OLEDs rely on a highly efficient phosphorescent pin stack and contain metal electrodes that introduce pronounced microcavity effects. A corrugated photoresist layer underneath the bottom electrode introduces light scattering. Compared to optically optimized reference OLEDs without the corrugated substrate, the corrugation increases light outcoupling efficiency but does not adversely affect the electrical properties of the devices. The external quantum efficiency (EQE) is increased from 15 % for an optimized planar layer structure to 17.5 % for a corrugated OLED with a grating period of 1.0 µm and a modulation depth of about 70 nm. Detailed analysis and optical modeling of the angular resolved emission spectra of the OLEDs provide evidence for Bragg scattering of waveguided and surface plasmon modes that are normally confined within the OLED stack into the air-cone. We observe constructive and destructive interference between these scattered modes and the radiative cavity mode. This interference is quantitatively described by a complex summation of Lorentz-like resonances.

High-Sensitivity Detection of Changes in Local Refractive Index and Absorption by Analyzing WGM Microlaser Emission via a 2D Dispersion Spectrometer.

Microlasers have been widely used in biosensing applications because of their high sensitivity to changes in local conditions. However, in most applications, the sensitivity limit is not dictated by the microlaser line width but rather by the much worse spectral resolution of the detection system, typically a grating spectrometer. To address this issue, we built and characterized a two-dimensional (2D) dispersion spectrometer with a virtually imaged phase array etalon and a diffraction grating. The spectrometer can analyze microlaser emission with a spectral resolution of better than 0.300 pm, which enables high-precision measurements of spectral shifts in laser peak emission wavelength and sufficient resolution to detect changes in peak line width. Using commercial fluorescent microspheres as the microlasers, the 2D dispersion spectrometer demonstrated a detection limit for the refractive index change of a liquid medium of 1.37 × 10-5 RIU and a detection limit for absorption changes of less than 0.02 cm-1.

High-Brightness Blue Polariton Organic Light-Emitting Diodes.

Polariton organic light-emitting diodes (POLEDs) use strong light-matter coupling as an additional degree of freedom to tailor device characteristics, thus making them ideal candidates for many applications, such as room temperature laser diodes and high-color purity displays. However, achieving efficient formation of and emission from exciton-polaritons in an electrically driven device remains challenging due to the need for strong absorption, which often induces significant nonradiative recombination. Here, we investigate a novel POLED architecture to achieve polariton formation and high-brightness light emission. We utilize the blue-fluorescent emitter material 4,4'-Bis(4-(9H-carbazol-9-yl)styryl)biphenyl (BSBCz), which exhibits strong absorption and a highly horizontal transition-dipole orientation as well as a high photoluminescence quantum efficiency, even at high doping concentrations. We achieve a peak luminance of over 20,000 cd/m2 and external quantum efficiencies of more than 2%. To the best of our knowledge, these values represent the highest reported so far for electrically driven polariton emission from an organic semiconductor emitting in the blue region of the spectrum. Our work therefore paves the way for a new generation of efficient and powerful optoelectronic devices based on POLEDs.

Wireless magnetoelectrically powered organic light-emitting diodes.

Compact wireless light sources are a fundamental building block for applications ranging from wireless displays to optical implants. However, their realization remains challenging because of constraints in miniaturization and the integration of power harvesting and light-emission technologies. Here, we introduce a strategy for a compact wirelessly powered light-source that consists of a magnetoelectric transducer serving as power source and substrate and an antiparallel pair of custom-designed organic light-emitting diodes. The devices operate at low-frequency ac magnetic fields (~100 kHz), which has the added benefit of allowing operation multiple centimeters deep inside watery environments. By tuning the device resonance frequency, it is possible to separately address multiple devices, e.g., to produce light of distinct colors, to address individual display pixels, or for clustered operation. By simultaneously offering small size, individual addressing, and compatibility with challenging environments, our devices pave the way for a multitude of applications in wireless displays, deep tissue treatment, sensing, and imaging.

Deformable microlaser force sensing.

Mechanical forces are key regulators of cellular behavior and function, affecting many fundamental biological processes such as cell migration, embryogenesis, immunological responses, and pathological states. Specialized force sensors and imaging techniques have been developed to quantify these otherwise invisible forces in single cells and in vivo. However, current techniques rely heavily on high-resolution microscopy and do not allow interrogation of optically dense tissue, reducing their application to 2D cell cultures and highly transparent biological tissue. Here, we introduce DEFORM, deformable microlaser force sensing, a spectroscopic technique that detects sub-nanonewton forces with unprecedented spatio-temporal resolution. DEFORM is based on the spectral analysis of laser emission from dye-doped oil microdroplets and uses the force-induced lifting of laser mode degeneracy in these droplets to detect nanometer deformations. Following validation by atomic force microscopy and development of a model that links changes in laser spectrum to applied force, DEFORM is used to measure forces in 3D and at depths of hundreds of microns within tumor spheroids and late-stage Drosophila larva. We furthermore show continuous force sensing with single-cell spatial and millisecond temporal resolution, thus paving the way for non-invasive studies of biomechanical forces in advanced stages of embryogenesis, tissue remodeling, and tumor invasion.