Metamaterial near-field sensor for deep-subwavelength thickness measurements and sensitive refractometry in the terahertz frequency range

We present a metamaterial-based terahertz (THz) sensor for thickness measurements of subwavelength-thin materials and refractometry of liquids and liquid mixtures. The sensor operates in reflection geometry and exploits the frequency shift of a sharp Fano resonance minimum in the presence of dielectric materials. We obtained a minimum thickness resolution of 12.5 nm (1/16000 times the wavelength of the THz radiation) and a refractive index sensitivity of 0.43 THz per refractive index unit. We support the experimental results by an analytical model that describes the dependence of the resonance frequency on the sample material thickness and the refractive index.Comment: 10 pages, 5 figure

Cells as active particles in asymmetric potentials: Motility under external gradients

Cell migration is a crucial event during development and in disease. Mechanical constraints and chemical gradients can contribute to the establishment of cell direction, but their respective roles remain poorly understood. Using a microfabricated topographical ratchet, we show that the nucleus dictates the direction of cell movement through mechanical guidance by its environment. We demonstrate that this direction can be tuned by combining the topographical ratchet with a biochemical gradient of fibronectin adhesion. We report competition and cooperation between the two external cues. We also quantitatively compare the measurements associated with the trajectory of a model that treats cells as fluctuating particles trapped in a periodic asymmetric potential. We show that the cell nucleus contributes to the strength of the trap, whereas cell protrusions guided by the adhesive gradients add a constant tunable bias to the direction of cell motion

Aktive Gele in vivo : Muster und Dynamiken in zytokinetischen Ringen und ihre Funktionen in der Zellteilung

Actomyosin structures are involved in many cell functions. Understanding their organization and collective behavior is still challenging. We study the cytokinetic ring in mammalian cells and in fission yeasts, by orienting cells in microcavities. This allows seeing the ring in a single plane of focus. With this setup, we reveal new structures and distinct dynamics for both cellular systems. In mammalian cells we find a pattern of regular clusters of myosin and formin. The characteristics of this pattern are stable throughout closure and its formation coincides with the onset of constriction. We propose that its characteristic is an inherent property of the actomyosin network and that its formation leads to an increase in stress generation. These hypotheses are supported by our theoretical mean field model. In contrast, fission yeast rings show rotating inhomogeneities (speckles), i.e. rotations of actin, myosin, cell wall building proteins (Bgs) and other proteins. Myosin speckles dynamic is unchanged, if wall growth is inhibited. However, the inhibition of speckle motion leads to stalled closure. We propose that the ring closure is driven by the rotation of actin and myosin, which pull Bgs thereby building the septum. This model is supported by our calculations and by simulations. We suggest that actomyosin systems in vivo can be regulated by transition between states of different orders and dynamics.Aktomyosinstrukturen sind involviert in viele Zellfunktionen. Ihre Organisation und kollektives Verhalten zu verstehen, ist immer noch eine Herausforderung. Wir studieren den zytokinetischen Ring in Säugetierzellen und Spalthefe, indem wir Zellen in Mikrovertiefungen orientieren, die es uns erlauben, den Ring in einer einzigen Ebene zu fokussieren. Damit zeigen wir neue Strukturen und Dynamiken in beiden Systemen. In Säugetierzellen finden wir ein Muster aus regelmäßigen Anhäufungen von Myosin und Formin. Die Charakteristiken des Musters sind stabil und die Entstehung des Musters fällt mit dem Beginn der Teilung zusammen. Wir schlagen vor, dass die Charakteristiken des Musters inhärente Eigenschaften des Aktomyosinnetzwerkes sind und dass das Muster zu einer erhöhten Spannungserzeugung führt. Diese Hypothesen werden von unserer Molekularfeldheorie-Beschreibung unterstützt. Ringe der Hefe zeigen rotierende Inhomogenitäten in Aktin, Myosin, zellwandbildenden Proteinen (Bgs) und anderen Proteinen. Die Myosindynamik ist unverändert, wenn die Wandbildung inhibiert ist. Hemmung der Myosinrotation, führt jedoch zu einem Anhalten der Teilung. Wir schlagen vor, dass die Schließung des Ringes durch die Rotationen von Aktin und Myosin angetrieben wird, die wiederum Bgs ziehen. Unsere Berechnungen und Simulationen unterstützen dieses Modell. Wir schlagen vor, dass Aktomyosinsysteme reguliert werden können durch Übergänge von Zuständen von unterschiedlicher Struktur oder Dynamik

Gels actifs in vivo : structures et dynamiques dans l'anneau de cytokinètique et leurs fonctions dans la division cellulaire

Actomyosin structures are involved in many cell functions. Understanding their organization and collective behavior is still challenging. We study the cytokinetic ring in mammalian cells and in fission yeasts, by orienting cells in microcavities. This allows seeing the ring in a single plane of focus. With this setup, we reveal new structures and distinct dynamics for both cellular systems. In mammalian cells we find a pattern of regular clusters of myosin and formin. The characteristics of this pattern are stable throughout closure and its formation coincides with the onset of constriction. We propose that its characteristic is an inherent property of the actomyosin network and that its formation leads to an increase in stress generation. These hypotheses are supported by our theoretical mean field model. In contrast, fission yeast rings show rotating inhomogeneities (speckles), i.e. rotations of actin, myosin, cell wall building proteins (Bgs) and other proteins. Myosin speckles dynamic is unchanged, if wall growth is inhibited. However, the inhibition of speckle motion leads to stalled closure. We propose that the ring closure is driven by the rotation of actin and myosin, which pull Bgs thereby building the septum. This model is supported by our calculations and by numerical simulations. We suggest that the transition between states of different orders and dynamics might be a way to regulate actomyosin systems in vivo.Les structures d'acto-myosine sont impliquées dans de nombreuses fonctions cellulaires. Comprendre leur organisation et leur comportement collectif est toujours difficile. Nous avons étudié l'anneau cytokinétique dans les cellules de mammifères et dans les levures de fission, en orientant les cellules dans les microcavités, ce qui permet de voir l'anneau dans un seul plan focal. Avec cette configuration, nous révélons de nouvelles structures et des dynamiques distinctes pour les deux systèmes cellulaires. Dans les cellules de mammifères, nous trouvons des motifs réguliers de la myosine et la formine. Les caractéristiques de ces motifs sont stables tout au long de sa fermeture et leur apparition coïncide avec la constriction. Nous proposons que ce phénomène est une propriété inhérente du réseau d'acto-myosine et que la formation de ces motifs entraîne une augmentation du stress. Ces hypothèses sont confirmées par notre modèle en champ moyen. Par contraste, l'anneau de levure de fission montre des inhomogénéités tournantes de l'actine, de la myosine, des protéines de la construction de la paroi (Bgs) et d'autres protéines. La dynamique des inhomogénéités de myosine est inchangée, si la croissance de la paroi est inhibée. Cependant, l'inhibition du mouvement des inhomogénéités conduit à l'arrêt de la fermeture. Nous proposons que la fermeture de l'anneau est entraînée par la rotation de l'actine et de la myosine qui tirent des protéines Bgs, lesquelles construisent ainsi le septum. Cette hypothèse est confirmée par nos calculs et par des simulations numériques. Nous suggérons que la transition entre les états de différents ordres et dynamiques pourrait être une façon de réguler in vivo les systèmes d'acto-myosine

Terahertz thin film and refractive index sensing with a metamaterial near-field sensor

We present a metamaterial-based sensor for use at terahertz (THz) frequencies that is suitable for the measurement of the thickness and the refractive index of a dielectric sample material. The sensor is designed to operate in reflection geometry in the frequency range between 1 THz and 1.6 THz. Deep-subwavelength sample thicknesses as small as 1/16000 of the operating wavelength can be resolved as well as refractive index differences of 0.01

Cells as active particles in asymmetric potentials: Motility under external gradients

Cell migration is a crucial event during development and in disease. Mechanical constraints and chemical gradients can contribute to the establishment of cell direction, but their respective roles remain poorly understood. Using a microfabricated topographical ratchet, we show that the nucleus dictates the direction of cell movement through mechanical guidance by its environment. We demonstrate that this direction can be tuned by combining the topographical ratchet with a biochemical gradient of fibronectin adhesion. We report competition and cooperation between the two external cues. We also quantitatively compare the measurements associated with the trajectory of a model that treats cells as fluctuating particles trapped in a periodic asymmetric potential. We show that the cell nucleus contributes to the strength of the trap, whereas cell protrusions guided by the adhesive gradients add a constant tunable bias to the direction of cell motion