13 research outputs found
On Mechanical Metamaterials
Mechanical Metamaterials are artificial materials which give access to elastic properties beyond natural materials. Within this work linear elastic material properties are tailored to exploit a Poisson\u27s ratio of close to 0.5 as well as approaching -1. These material properties are computed and the density is adjusted to specific values to control e.g. the impedance. At last two different cloaking concepts are realized with these materials and elastostatic cloaking is achieved
Hall-effect sign-inversion in a realizable 3D metamaterial
In 2009, Briane and Milton proved mathematically the existence of
three-dimensional isotropic metamaterials with a classical Hall coefficient
which is negative with respect to that of all of the metamaterial constituents.
Here, we significantly simplify their blueprint towards an architecture
composed of only a single constituent material in vacuum/air, which can be seen
as a special type of porosity. We show that the sign of the Hall voltage is
determined by a separation parameter between adjacent tori. This qualitative
behavior is robust even for only a small number of metamaterial unit cells. The
combination of simplification and robustness brings experimental verifications
of this striking sign-inversion into reach.Comment: 9 figures, 7 page
On three-dimensional dilational elastic metamaterials
Dilational materials are stable three-dimensional isotropic auxetics with an
ultimate Poisson's ratio of -1. We design, evaluate, fabricate, and
characterize crystalline metamaterials approaching this ideal. To reveal all
modes, we calculate the phonon band structures. On this basis, using cubic
symmetry, we can unambiguously retrieve all different non-zero elements of the
rank-4 effective metamaterial elasticity tensor, from which all effective
elastic metamaterial properties follow. While the elastic properties and the
phase velocity remain anisotropic, the effective Poisson's ratio indeed becomes
isotropic and approaches -1 in the limit of small internal connections. This
finding is also supported by independent static continuum-mechanics
calculations. In static experiments on macroscopic polymer structures
fabricated by three-dimensional printing, we measure Poisson's ratios as low as
-0.8 in good agreement with theory. Microscopic samples are also presented.Comment: 8 figure
Phonon band structures of three-dimensional pentamode metamaterials
Three-dimensional pentamode metamaterials are artificial solids that
approximately behave like liquids, which have vanishing shear modulus.
Pentamodes have recently become experimental reality. Here, we calculate their
phonon band structures for various parameters. Consistent with static continuum
mechanics, we find that compression and shear waves exhibit phase velocities
that can realistically be different by more than one order of magnitude.
Interestingly, we also find frequency intervals with more than two octaves
bandwidth in which pure single-mode behavior is obtained. Herein, exclusively
compression waves exist due to a complete three-dimensional band gap for shear
waves and, hence, no coupling to shear modes is possible. Such single-mode
behavior might, e.g., be interesting for transformation-elastodynamics
architectures.Comment: 5 figure
On the feasibility of pentamode mechanical metamaterials
Conceptually, all conceivable three-dimensional mechanical materials can be
built from pentamode materials. Pentamodes also enable to implement
three-dimensional transformation acoustics - the analogue of transformation
optics. However, pentamodes have not been realized experimentally to the best
of our knowledge. Here, we investigate inasmuch the pentamode theoretical ideal
suggested by Milton and Cherkaev in 1995 can be approximated by a metamaterial
with current state-of-the-art lithography. Using numerical calculations
calibrated by our fabricated three-dimensional microstructures, we find that
the figure of merit, i.e., the ratio of bulk modulus to shear modulus, can
realistically be made as large as about 1,000.Comment: 4 pages and 5 figure
Hybrid 2D-3D optical devices for integrated optics by direct laser writing
Integrated optical chips have already been established for application in optical communication. They also offer interesting future perspectives for integrated quantum optics on a chip. At present, however, they are mostly fabricated using essentially planar fabrication approaches like electron-beam lithography or UV optical lithography. Many further design options would arise if one had complete fabrication freedom in regard to the third dimension normal to the chip without having to give up the virtues and the know-how of existing planar fabrication technologies. As a step in this direction, we here use three-dimensional dip-in direct-laser-writing optical lithography to fabricate three-dimensional polymeric functional devices on pre-fabricated planar optical chips containing Si3N4 waveguides as well as grating couplers made by standard electron-beam lithography. The first example is a polymeric dielectric rectangular-shaped waveguide which is connected to Si3N4 waveguides and that is adiabatically twisted along its axis to achieve geometrical rotation of linear polarization on the chip. The rotator’s broadband performance at around 1550 nm wavelength is verified by polarization-dependent grating couplers. Such polarization rotation on the optical chip cannot easily be achieved by other means. The second example is a whispering-gallery-mode optical resonator connected to Si3N4 waveguides on the chip via polymeric waveguides. By mechanically connecting the latter to the disk, we can control the coupling to the resonator and, at the same time, guarantee mechanical stability of the three-dimensional architecture on the chip
Supplement 1: Transient behavior of invisibility cloaks for diffusive light propagation
Originally published in Optica on 20 February 2015 (optica-2-2-84