Jordi Prat-Camps

Controlling magnetism, essential for a wide range of technologies, is impaired by the impossibility of generating a maximum of magnetic field in free space. Here, we propose a strategy based on negative permeability to overcome this stringent limitation. We experimentally demonstrate that an active magnetic metamaterial can emulate the field of a straight current wire at a distance. Our strategy leads to an unprecedented focusing of magnetic fields in empty space and enables the remote cancellation of magnetic sources, opening an avenue for manipulating magnetic fields in inaccessible regions.

Precise control of ultrasonic acoustic waves with frequencies f ≳ 20 kHz is useful in a range of applications from ultrasonic scanners to nondestructive testing and consumer haptic devices. A spatial sound modulator (SSM) is the acoustic analogy to the spatial light modulator (SLM) in optics and is highly sought after by acoustics researchers. A spatial sound modulator is constrained by very distinct practical conditions. Namely, it must be a reconfigurable device which modulates sound arbitrarily from a decoupled source. Here a reflective phase modulating device is realized, whose local units can be tuned to imprint a phase signature to an incoming wave. It is manually reconfigurable and consists of 1024 rigidly ended square waveguides with sliding bottom surfaces to provide variable phase delays. Experiments demonstrate the ability of this device to focus ultrasonic waves in air at different points in space, generate accurate pressure landscapes, and perform multiplane holography. Moreover, thanks to the subwavelength nature of the unit cells, this device outperforms state‐of‐the‐art phased‐array transducers of the same size in the quality and energy distribution of generated acoustic holographic images. These results pave the way for the construction of electronically controlled reflective SSMs.

The control of magnetic fields, essential for our science and technology, is currently achieved by magnetic materials with positive permeability, including ferromagnetic, paramagnetic, and diamagnetic types. Here we introduce materials with negative static permeability as a new paradigm for manipulating magnetic fields. As a first step, we extend the solutions of Maxwell magnetostatic equations to include negative-permeability values. The understanding of these new solutions allow us to devise a negative-permeability material as a suitably tailored set of currents arranged in space, overcoming the fact that passive materials with negative permeability do no exist in magnetostatics. We confirm the theory by experimentally creating a spherical shell that emulates a negative-permeability material in a uniform magnetic field. Our results open new possibilities for creating and manipulating magnetic fields, which can be useful for practical applications.

We theoretically show that a dipole externally driven by a pulse with a lower-bounded temporal width, and placed inside a cylindrical hollow waveguide, can generate a train of arbitrarily short and focused electromagnetic pulses. The waveguide encloses vacuum with perfect electric conducting walls. A dipole driven by a single short pulse, which is properly engineered to exploit the linear spectral filtering of the cylindrical hollow waveguide, excites longitudinal waveguide modes that are coherently refocused at some particular instances of time, thereby producing arbitrarily short and focused electromagnetic pulses. We numerically show that such ultrafocused pulses persist outside the cylindrical waveguide at distances comparable to its radius.

Magnetic sensors are key elements in our interconnected smart society. Their sensitivity becomes essential for many applications in fields such as biomedicine, computer memories, geophysics, or space exploration. Here we present a universal way of increasing the sensitivity of magnetic sensors by surrounding them with a spherical metamaterial shell with specially designed anisotropic magnetic properties. We analytically demonstrate that the magnetic field in the sensing area is enhanced by our metamaterial shell by a known factor that depends on the shell radii ratio. When the applied field is non-uniform, as for dipolar magnetic field sources, field gradient is increased as well. A proof-of-concept experimental realization confirms the theoretical predictions. The metamaterial shell is also shown to concentrate time-dependent magnetic fields upto frequencies of 100 kHz.

Metamaterials have made possible many breakthroughs like electromagnetic cloaks or perfect lenses. In an interesting line of research, metamaterials have been used to simulate celestial objects that were thought to be impossible to replicate at the human scale. For example, the analogous of optical black holes has been theoretically and experimentally realized based on metamaterials. One of the most interesting celestial objects is a wormhole. In cosmology, a wormhole can be understood as a cosmic object that can connect two distant regions of the universe. Constructing such an object would require large amounts of negative gravitational energy, which obviously prevents its realization in a lab. A seminal work by Greenleaf et al. [1] presented a theoretical proposal for designing a wormhole that could work for for electromagnetic waves. Such an electromagnetic wormhole could allow electromagnetic wave propagation between two points in space through an invisible tunnel. However, the recipe given by transformation optics to construct such an object required the use of bulk metamaterials with complicated permeability and permittivity parameters. Here we present the design and experimental realization of a wormhole for the static magnetic field. It consist of a tunnel for transferring magnetic field lines from one point in space to another through a path that is magnetically undetectable. We make use of the unique properties that magnetostatic metamaterials offer in terms of magnetic field manipulation. First, we use a magnetic hose made of a ferromagnetic cylinder to guide to field of a magnetic source through a desired path to another location. And, second, we design a magnetic cloak that can surround the whole structure, thus rendering it magnetically undetectable. This latter requirement is an important feat in itself, since we need to construct an actual 3D spherical magnetic cloak to operate passively in free space. This is achieved by a careful combination of a superconducting shell and a ferromagnetic metasurface. The theory and numerical simulations are experimentally confirmed by constructing an actual device using soft ferromagnets and high-temperature superconductors, demonstrating all the required properties for the magnetic wormhole.

Metamaterials have introduced many relevant new possibilities in the manipulation of light, microwaves, sound waves, and other systems. In many cases, the design and optimization of metamaterials has been achieved in conjunction with the mathematical technique of transformation optics, which allows one to guide rays of light or electromagnetic fields along desired directions. This is achieved by tailoring the electromagnetic properties of materials, the recipe of which is provided by the transformation optics technique. A particularly active application of metamaterials has been in the magnetostatic case, where magnetic fields have been cloaked, concentrated in a spatial region or transferred at a distance. Here we use magnetic metamaterials, specially designed with transformation optics technique, to demonstrate two new important effects. First, on a general basis, we analytically demonstrate that the magnetic mutual inductance between two coils is greatly enhanced when surrounding them with our magnetic metamaterials, without increasing their self-inductances. Second, this property is further exploited to boost the wireless transmission of power between distant circuits, because such a transmission depends on the magnetic coupling between emitter and receiver. We experimentally demonstrate that the efficiency of the wireless power transmission is largely increased when using the magnetic metamaterials. Different from previous proposals, in our case the enhancement is produced while keeping the gap of air between emitter and receiver free. Also, the metamaterials are constructed from widely available conventional materials, using alternating layers of soft ferromagnets and conductors, not requiring any special property such as low temperature (unlike previous proposals that involved the use of superconducting materials). This combination of feasibility and power indicates the relevance that magnetic metamaterials can have in future technologies.

A novel and broadly applicable way to increase magnetic coupling between distant circuits in the quasistatic regime is introduced. It is shown how the use of magnetic metamaterials enhances the magnetic coupling between emitting and receiving coils. Results are experimentally demonstrated by measuring a boost on the efficiency of the wireless transmission of power between distant circuits.