Magnetoplasmonics Lab

Archives February 2019

With a Few Tweaks, a Near-Perfect Absorber Can Become a Time-Reversed Laser

DURHAM, N.C., Feb. 19, 2019 — With small adjustments, a near-perfect absorber of electromagnetic waves can be changed into a coherent perfect absorber (CPA), a device that absorbs coherent light and shows near-zero reflectance and high absorption. A CPA, also known as a time-reversed laser, absorbs all of the energy from two identical electromagnetic waves in synchrony. The waves are absorbed as they enter the material from either side at precisely the same time. 

The width, height, and spacing of the cylinders depicted here dictate how the metamaterial described in the new paper absorbs electromagnetic energy. Courtesy of Kebin Fan, Duke University.

This metamaterial features a zirconia ceramic built into a surface dimpled with cylinders, like the face of a Lego brick. After computationally modeling the metamaterial’s properties, the researchers found that they could create a basic CPA from the metamaterial by altering the cylinder size and spacing. 

Traditional “reverse lasers” can only absorb energy when the incoming electromagnetic waves are perfectly aligned, as in the top example. Courtesy of Kebin Fan and Willie Padilla, Duke University.

In contrast to existing CPAs, which work in one mode only, the CPA created by the Duke team has two overlapping modes, enabling it to absorb both aligned and misaligned waves. By changing the material’s parameters so that the two modes no longer overlapped, the researchers were able to create a CPA just like the CPAs currently described in the literature, but with more versatility. “Typical CPAs have only one variable, the material’s thickness,” said professor Kebin Fan. “We have three: the cylinders’ radius, height, and periodicity. This gives us a lot more room to tailor these modes and put them in the frequency spectrum where we want them, giving us a lot of flexibility for tailoring the CPAs.”  By increasing the cylinder height in the metamaterial from 1.1 to 1.4 mm, the researchers gave the device the ability to switch between absorbing all phases of electromagnetic waves and absorbing only waves occurring in sync with each other. The team believes that it could be possible to engineer a material that can make this switch dynamically. “We haven’t done that yet. It is challenging, but it’s on our agenda,” said professor Willie Padilla.  In principle, the researchers said, a device could be engineered that measures not just the intensity of incoming light like a normal camera, but also its phase. “If you’re trying to figure out the properties of a material, the more measurements you have, the more you can understand about the material,” Padilla said. “And while coherent detectors do exist … they’re extremely expensive to build through other technologies.” 
The demonstrated system and theory could open the way to a new class of absorbers for future applications in hyperspectral imaging and energy harvesting. 

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A New Way to Fabricate High-Performance Optical Metasurfaces for Use in Photonic Circuits

LAUSANNE, Switzerland, Feb. 13, 2019 — A way to produce glass metasurfaces that can be either rigid or flexible, developed by engineers from the EPFL Laboratory of Photonic Materials and Fiber Devices, could be used to fabricate all-dielectric optical metasurfaces quickly, at low temperatures, and with no need for a cleanroom. These metasurfaces could be used to build next-generation photonic circuits. Optical circuits, which are 10 to 100 times faster than electronic circuits and more energy-efficient, could transform the performance of many devices.

The new method employs dewetting, a natural process that occurs when a thin film of material is deposited on a substrate and then heated. The heat causes the film to retract and break apart into tiny nanoparticles.

The EPFL engineers used dewetting to create dielectric glass metasurfaces, rather than metallic metasurfaces. First, they created a substrate textured with the desired architecture. Then, they deposited the material — chalcogenide glass — in thin films just tens of nanometers (nm) thick. The substrate was heated for a couple of minutes until the glass became fluid and nanoparticles began to form in the sizes and positions dictated by the substrate’s texture. 
The engineers demonstrated the ability to tailor the position, shape, and size of nano-objects with feature sizes below 100 nm and with interparticle distances down to 10 nm. They used their method to generate optical nanostructures over rigid and soft substrates that were several centimeters in size, with optical performance and resolution comparable to traditional lithography-based processes. The metasurfaces are highly sensitive to changes in ambient conditions, thus able to detect the presence of very low concentrations of bioparticles, the team said.
Metasurfaces could enable engineers to make flexible photonic circuits and ultrathin optics for a host of applications, ranging from flexible tablet computers to solar panels with enhanced light-absorption characteristics. They could also be used to create flexible sensors to be placed directly on a patient’s skin, for example, to measure things such as pulse and blood pressure or to detect specific chemical compounds. 

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Our new paper in journal of magnetism and magnetic materials

Congratulations to our new paper ” Experimental study and micro-magnetic modeling of magnetization dynamics in L1⁠0-FePt thin film” by M. Shafei, M. M. Tehranchi, H. Falizkaran Yazdi, S. M. Hamidi, R. Yusupov, S. Nikitin

Among different magnetic thin films, L10 FePt due to high magnetocrystalline anisotropy is attracting much attention for applications in new generation of magnetic recording media. In this work, switching time and switching mechanism of magnetization as essential properties of L10 FePt film was studied by magneto-optical Kerr effect (MOKE) and time-resolved magneto-optical Kerr effect (TR-MOKE). For this purpose, static in plane and out of plane magnetic hysteresis loop of a L10 FePt film on (100) MgO was measured and modeled using polar and longitudinal MOKE and mumax code respectively. Furthermore, the switching time of magnetization was studied using laser induced ultrafast demagnetization and relaxation of the sample by TR-MOKE, in which for the first time, the magnetic field was applied in the plane of the sample for this measurement.

Surface Plasmon Resonance in a metallic nanoparticle embedded in a semiconductor matrix: exciton-plasmon coupling

They consider the effect of electromagnetic coupling between localized surface plasmons in

a metallic nanoparticle (NP) and excitons or weakly interacting electron-hole pairs in a semiconductor

matrix where the NP is embedded.

An expression is derived for the NP polarizability renormalized by this coupling and two possible situations are analyzed, both compatible with the conditions for Fano-type resonances:

  • a narrow-bound exciton transition overlapping with the NP surface plasmon resonance (SPR), and
  •  SPR overlapping with a parabolic absorption band due to electron-hole transitions in the semiconductor.

The absorption band line shape is strongly non-Lorentzian in both cases and similar to the typical Fano spectrum in the case (i).

However, it looks differently in the situation (ii) that takes place for gold NPs embedded in a CuO film and the use of the renormalized polarizability derived in this work permits to obtain a very good fit to the experimentally measured LSPR line shape.

Our new paper in journal of superconductivity and novel magnetism

Congratulations to our new paper ” Switching time Probing in electric field assisted magnetization of PbZrTiO3/Cobalt structure ” by M. Shafei, M. M. Tehranchi, S. M. Hamidi

Electric field assisted full magnetization switching in a multiferroic heterostructure composed of a PbZrTiO3 (PZT) substrate and 100nm Cobalt (Co) layer was investigated. For this, by measuring magnetic in plane anisotropy of the sample, using magneto-optical Kerr effect (MOKE), it was shown that the sample has a uniaxial anisotropy. In addition, the coercive field of the Co layer can be tuned by applying an electric field to the PZT which can be used in electric field assisted magnetization reversal in the Co layer. Direct measurement reveals that electric field assisted magnetization switching in layers take place in about 100 µs that is in compatibility with domain wall motion. Our measurement is a promising technique for probing of switching time in electric field assisted magnetization switching elements.