Brownian Motion of Boomerang Colloidal ParticlesWe investigate the Brownian motion of boomerang colloidal particles confined between two glass plates. Our experimental observations show that the mean displacements are biased towards the center of hydrodynamic stress (CoH), and that the mean-square displacements exhibit a crossover from short time faster to long time slower diffusion with the short-time diffusion coefficients dependent on the points used for tracking. A model based on Langevin theory elucidates that these behaviors are ascribed to a superposition of two diffusive modes: the ellipsoidal motion of the CoH and the rotational motion of the tracking point with respect to the CoH.Brownian motion as a general phenomenon of the diffusion processes has inspired extensive research [1][2][3][4][5][6][7][8][9][10][11][12] due to both its interesting physics and practical applications such as in microrheology [13][14][15][16], selfpropelled microswimmers [17] and particle and molecular separation [18][19][20]. Inspired by the diverse geometric shapes of biological macromolecules, Brenner and others have extended the hydrodynamic theory of Brownian motion to particles with irregular shapes [21][22][23][24][25][26]. A set of hydrodynamic centers are introduced, which include the center of hydrodynamic stress (CoH) where the coupling diffusion matrix becomes zero, the center of reaction where the coupling resistance matrix becomes symmetric and the center of diffusion where the coupling diffusion matrix becomes symmetric [22,24,27]. For screw-like or skewed particles, the translational and rotational motions are intrinsically coupled, therefore, the CoH does not exist and the centers of diffusion and reaction differ from each other. By contrast, for non-skewed particles, there always exists a unique point at which these three hydrodynamic centers coincide.Thus far, experimental studies of Brownian motion have been focused primarily on spherical particles; it was only recently that the Brownian motion of lowsymmetry particles was explored in experiments [5,[28][29][30][31][32][33][34]. Particle shapes are critical to various applications such as self-propelled microswimmers and particle/molecular separations [17,35]. By engineering particle shapes, microswimmers may be tailored to perform circular, spinning-top or other types of motion [35][36][37]. Understanding the hydrodynamics of chiral particles may lead to new avenues towards separation of particle or molecular enantiomers [38].In this letter we study the Brownian motion of boomerang-shaped colloidal particles under quasi twodimensional confinements. The boomerang particles with C 2v mirror symmetry represent an attractive system for studying the Brownian motion of low symmetry particles because their CoM and CoH do not coincide and both lie outside the body. Especially, the location of the CoH is unknown before the motion of any tracking point (TP) is analyzed. Boomerang particles are also an interesting model system for active microswimmers [37], the elec...

We implement microfabricated boomerang particles with unequal arm lengths as a model for nonsymmetric particles and study their Brownian motion in a quasi-two-dimensional geometry by using high-precision single-particle motion tracking. We show that because of the coupling between translation and rotation, the mean squared displacements of a single asymmetric boomerang particle exhibit a nonlinear crossover from short-time faster to long-time slower diffusion, and the mean displacements for fixed initial orientation are nonzero and saturate out at long times. The measured anisotropic diffusion coefficients versus the tracking point position indicate that there exists one unique point, i.e., the center of hydrodynamic stress (CoH), at which all coupled diffusion coefficients vanish. This implies that in contrast to motion in three dimensions where the CoH exists only for high-symmetry particles, the CoH always exists for Brownian motion in two dimensions. We develop an analytical model based on Langevin theory to explain the experimental results and show that among the six anisotropic diffusion coefficients only five are independent because the translation-translation coupling originates from the translation-rotation coupling. Finally, we classify the behavior of two-dimensional Brownian motion of arbitrarily shaped particles into four groups based on the particle shape symmetry group and discussed potential applications of the CoH in simplifying understanding of the circular motions of microswimmers.

Production of multicharged ions and behavior of microwave modes in an electron cyclotron resonance ion source directly excited in a circular cavity resonator Rev. Sci. Instrum. 77, 03A336 (2006); 10.1063/1.2163330 All-metamaterial-based subwavelength cavities ( λ ∕ 60 ) for ultrathin directive antennas Appl. Phys. Lett. 88, 084103 (2006); 10.1063/1.2172740Resonantly coupled surface plasmon polaritons in the grooves of very deep highly blazed zero-order metallic gratings at microwave frequencies

In this paper, we demonstrate arrays of optical patch nanoantennas can convert light polarization through reflection. By breaking the azimuthal symmetry, elliptical plasmonic patch nanoantennas exhibit both even and odd cavity modes, which can be expressed by Mathieu functions. It is shown that by properly orienting the incident polarization, a linearly polarized light in resonance with one cavity mode can be converted into an elliptical or circular polarization after reflection. Since the major cavity modes can be excited at all incident angles, the polarization conversion by these elliptical patch nanoantennas can be realized with wide range of incident angles. V