Our new paper in Optical and Quantum electronics

Congratulations for the publication of paper” Demonstration of tunable complex refractive index of graphene covered one dimensional photonic crystals”, in journal of  optical and quantum electronics, by  S. M. Hamidi, M. Mahboubi, S. M. Mohseni, B. Azizi, A. Ghaderi,  S. Javadi.


Scientists create diodes made of light

Photonics researchers at the National Physical Laboratory (NPL) have achieved the extra-ordinary by creating a diode consisting of light that can be used, for the first time, in miniaturised photonic circuits, as published in Optica.

Dr. Pascal Del’Haye and his team at NPL have created an optical version of a diode that transmits light in one direction only, and can be integrated in microphotonic circuits. This small-scale integration has been a major challenge in photonics because existing optical diodes require bulky magnets.

NPL’s ground-breaking work has overcome the limitation of diodes based on bulky magnets, by using light stored in tiny chip-based glass rings to form a diode.

Diodes are well known in electronic circuits. They transmit electric current in one direction but block the current in the backward direction. Diodes are essential components of nearly every electronic circuit and are used, for example, in battery chargers.

The novel technique was created by sending lots of light into a microresonator – a glass ring on a silicon chip, about the width of a human hair – and harnessing the circulating optical power to generate the diode effect.

Dr. Jonathan Silver, Higher Research Scientist at NPL, explains: “To create the optical diodes we used microrings that can store extremely large amounts of light. This meant that, even though we were only sending small amounts of light into these glass rings, the circulating power was comparable to the light generated by the flood lights in a whole football stadium—but confined into a device smaller than a human hair. The light intensities enable the formation of a diode via a light-with-light interaction called the Kerr effect.”

In their experiments, they have shown that the electromagnetic field of clockwise circulating light in these glass rings effectively blocks any counterclockwise circulating light.

Pascal Del’Haye, Principal Research Scientist of the project emphasises: “These diodes will, for the first time, open the door to cheap and efficient optical diodes on microphotonic chips, and will pave the way for novel types of integrated photonic circuits which could be used for optical computing.

“They could also have significant impact on future optical telecommunication systems, for more efficient use of telecom networks.”

Leonardo Del Bino, Doctoral Student on the project, said: “A remarkable property of this novel diode is that the performance improves if the forward propagating light field is increased. This is very important, for example, when using the diode to protect chip-integrated laser diodes from back reflections.”

Beyond the use for optical diodes, NPL’s research on interaction of counterpropagating light can enable new types of optical rotation sensors and optical memories.

More information: Leonardo Del Bino et al. Microresonator isolators and circulators based on the intrinsic nonreciprocity of the Kerr effect, Optica (2018). DOI: 10.1364/OPTICA.5.000279

Piezomagnetic material changes magnetic properties when stretched

Piezoelectric materials, which generate an electric current when compressed or stretched, are familiar and widely used: think of lighters that spark when you press a switch, but also microphones, sensors, motors and all kinds of other devices. Now a group of physicists has found a material with a similar property, but for magnetism. This “piezomagnetic” material changes its magnetic properties when put under mechanical strain.

“Piezomagnetic materials are rarely found in nature, as far as I’m aware,” said Nicholas Curro, professor of physics at UC Davis and senior author of a paper on the discovery published March 13 in the journal Nature Communications.

Curro and colleagues were studying a barium-iron-arsenic compound, BaFe2As2, that can act as a superconductor at temperatures of about 25 Kelvin when doped with small amounts of other elements. This type of iron-based superconductor is interesting because although it has to be kept pretty cold to work, it could be stretched into wires or cables.

BaFe2As2 is what is called a “nematic” crystal because its structure goes through a phase transition before it becomes superconducting. In the case of BaFe2As2, its crystal structure goes from a square to a rectangular configuration.

Curro and graduate students Tanat Kissikov and Matthew Lawson were attempting to study the material by nuclear magnetic resonance (NMR) imaging while stretching it, to see if they could force it into the rectangular configuration. To their surprise, the magnetic properties of BaFe2As2 changed as they stretched it.

The material is not a bulk magnet – the spins of its atoms point in alternating opposite directions, making it an antiferromagnet. But the direction of those magnetic spins does change in a measurable way when under stress, they found.

“The real surprise is that it appears that the direction of magnetism can change and come out of plane,” Curro said.

At this point, there’s no theory to explain these results, Curro said. His lab is looking to see if other materials can show the same behavior and if mechanical strain can affect the superconducting properties of the material (these experiments were not carried out at temperatures where BaFe2As2 is a superconductor).

The discovery could have applications in new ways to look for strain within materials such as aircraft components, Curro said.

More information: T. Kissikov et al, Uniaxial strain control of spin-polarization in multicomponent nematic order of BaFe2As2, Nature Communications (2018). DOI: 10.1038/s41467-018-03377-8


Our new paper in physica C

Congratulations for the publication of paper”Sensitivity optimization of Bell-Bloom magnetometers by manipulation of atomic spin synchronization”, in journal of  Physica C, by  Malihe Ranjbaran, Mohammad Mehdi Tehranchi, Seyedeh Mehri Hamidi, Mohammad Hossein Khalkhali.

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