A team of optics researchers from the University of Central Florida has demonstrated the first-ever nonmagnetic topological insulator laser, a finding that has the potential to substantially improve the efficiency, beam quality, and resilience of semiconductor laser arrays.

These results are presented in two research papers, one describing the theory of topological lasers and the other experiments, published in Science.

The project, led by Professors Mercedeh Khajavikhan and Demetrios Christodoulides of the College of Optics and Photonics (CREOL) and their graduate students Steffen Wittek, Midya Parto and Jinhan Ren, was conducted in conjunction with a team from Technion - Israel that includes Moti Segev, Miguel Bandres, and Gal Harari. The theoretical component of the work was initiated by the Technion team, while the experimental part was carried out at CREOL.

The teams were interested in solving a long-standing problem in laser physics that perplexed scientists for the past 40 years: how to create a high-power, ultimately focusable, and single frequency semiconductor laser array that does not lose efficiency even when its sub-elements fail or malfunction. Such a laser is expected to find applications in numerous areas of science and technology. The answer to this problem came from a rather unexpected place.

"We were inspired by the developments in topological insulator materials," said Christodoulides.

"It was less than two years ago that the Nobel Prize in Physics was awarded to the theoretical physicists whose work established the role of topology in understanding these exotic forms of matter."

Topological insulators are one of the most innovative and promising areas of physics in recent years, providing new insight into the basic understanding of protected transport. These are special materials that conduct a "super-current" on their surface while being insulators in their interior. The current on the surface is not affected by defects, sharp corners or disorder; it continues unidirectionally without being scattered.

Two years ago, the team wondered if there could be a way to utilize notions from topological science into laser physics. This could lead to altogether new families of lasers with improved performance characteristics.

The team decided to build a topological insulator for photons. However, this is a rather challenging task since photons, unlike electrons, do not have a charge. Moreover, magnetic fields do not substantially affect semiconductor light emitting materials.

"To solve these problems, we came up with clever designs to trick photons to feel as if they are experiencing a magnetic field and having a spin," said Khajavikhan, the lead experimentalist.

The team used an array of microring resonators that are arranged in such a way to mimic the presence of a magnetic field. By pumping only the rings on the periphery of the array, they excited the laser to emit in the topological edge mode. This mode, traveling along the edge, can most efficiently use all the available pump power to generate a coherent, single-mode, high power beam.

The study leads to a new class of active topological photonic devices that could be readily integrated with sensors, antennas and other photonic devices. The work demonstrated that not only are topological insulator lasers theoretically possible and experimentally feasible, but also marked the first practical application using such topological principles in optics.

"There is a great pleasure to see how a line of fundamental research can address such tangible and practical problems," Christodoulides said.

related report Hydrogen transfer: One thing after the other
Washington DC (SPX) Feb 15 - Hydride transfer is an important reaction for chemistry (e.g., fuel cells), as well as biology (e.g., respiratory chain and photosynthesis). Often, one partial reaction involves the transfer of a hydride ion (H(-)). But does this hydride transfer involve one step or several individual steps? In the journal Angewandte Chemie, scientists have now provided the first proof of stepwise hydride transfer in a biological system.

An important step in the biosynthesis of chlorophyll is the light-dependent hydrogenation of protochlorophyllide to chlorophyllide. This involves the reduction of a double bond between carbon atoms 17 and 18 in this complex ring system to a single bond as both carbon atoms bind to an additional hydrogen atom. This step is catalyzed by the enzyme protochlorophyllide oxireductase and requires irradiation with light.

Technically speaking, however, this reaction does not add one hydrogen atom to each carbon. Instead, there is first addition of a hydride ion (H(-)) to C17 and then addition of a proton (H(+)) to C18. The first partial reaction, the hydride transfer, requires the cofactor nicotinamide adenine dinucleotide phosphate (NADPH). NADPH serves as a source for two electrons and a proton (H(+)), the equivalent of a hydride anion, H(-).

Hydride transfer reactions play a key role in many biological systems. However, their mechanism is still disputed. Do the three elementary steps - transfer of an electron, a proton, and another electron from NADPH to the substrate - occur simultaneously, or stepwise?

Because of the short lifetime of the intermediates, direct proof of a stepwise mechanism has not previously been possible. Light-dependent reactions - such as the hydrogenation that occurs in the biosynthesis of chlorophyll - that can be triggered by a short laser pulse have solved this problem. By using time-resolved absorption and emission spectroscopy, researchers working with Roger J. Kutta and Nigel S. Scrutton at the University of Manchester (UK) have been able to characterize the mechanism of this hydride transfer.

In addition to excited states of protochlorophyllide, the researchers were able to resolve three discrete intermediates that are consistent with a partially stepwise mechanism: an initial electron transfer from NADPH to protochlorophyllide that has been excited (to the singlet state) by light is followed by the coupled transfer of a proton and an electron. As expected, the final step is transfer of the second proton.

Interestingly, the researchers found different intermediates for the wild type of the enzyme and a mutated version (C226S): While the initial hydride binds to C17 in the wild type, it is transferred to C18 in the mutant version. However, the end result is the same chlorphyllide stereoisomer.

The insights gained from these experiments provide a deeper understanding of how light energy can be used for chemical reactions involving hydrogen transfer, particularly with regard to the design of light-activated catalysts.

Stepwise hydride transfer in the biosynthesis ofchlorophyll

Research paper

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A frequency-doubling unit for transportable lasers
Berlin, Germany (SPX) Feb 03, 2018
The Physikalisch-Technische Bundesanstalt (PTB) is known for providing time e.g. for radio-controlled clocks. For this purpose, it operates some of the best cesium atomic clocks in the world. At the same time, PTB is already developing various atomic clocks of the next generation. These clocks are no longer based on a microwave transition in cesium, but they rather operate with other atoms that are excited using optical frequencies. Some of these new clocks can even be transported to other locatio ... read more