From Institute for Basic Science [기초과학연구원](KR): “Seeing stable topology using instabilities”

From Institute for Basic Science [ 기초과학연구원](KR)

Scientists decipher fingerprints of topological phases of light in nonlinear optical instabilities.

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Figure 1.The spatial intensity profile of a laser beam propagating in a nonlinear medium spontaneously becomes nonuniform due to the process of modulational instability.

We are most familiar with the four conventional phases of matter: solid, liquid, gas, and plasma. Changes between two phases, known as phase transitions, are marked by abrupt changes in material properties such as density. In recent decades a wide body of physics research has been devoted to discovering new unconventional phases of matter, which typically emerge at ultra-low temperatures or in specially-structured materials. Exotic “topological” phases exhibit properties that can only change in a quantized (step-wise) manner, making them intrinsically robust against impurities and defects.

In addition to topological states of matter, topological phases of light can emerge in certain optical systems such as photonic crystals and optical waveguide arrays. Topological states of light are of interest as they can form the basis for future energy-efficient light-based communication technologies such as lasers and integrated optical circuits.

However, at high intensities light can modify the properties of the underlying material. One example of such a phenomenon is the damage that the high-power lasers can inflict on the mirrors and lenses. This in turn affects the propagation of the light, forming a nonlinear feedback loop. Nonlinear optical effects are essential for the operation of certain devices such as lasers, but they can lead to the emergence of disorder from order in a process known as modulational instability, as is shown in Figure 1. Understanding the interplay between topology and nonlinearity is a fascinating subject of ongoing research.

Daniel Leykam, Aleksandra Maluckov, and Sergej Flach at the Center for Theoretical Physics of Complex Systems (PCS) within the Institute for Basic Science (IBS, South Korea), along with their colleagues Ekaterina Smolina and Daria Smirnova from the Institute of Applied Physics, Russian Academy of Sciences [Росси́йская акаде́мия нау́к; (РАН) Rossíiskaya akadémiya naúk](RU) and the Australian National University(AU), have proposed a novel method to characterize topological phases of light using nonlinear instabilities exhibited by bright beams of light. This research was published in Physical Review Letters.

In this work, the researchers addressed the fundamental question of how topological phases of light in nonlinear optical media undergo the process of modulational instability. It was shown theoretically that certain features of the instability, such as its growth rate, can differ between different topological phases. The researchers performed numerical simulations of the modulational instability and demonstrated that it can be used as a tool to identify different topological phases of light. An example of this idea is shown in Figure 2: While the light beams generated by the instability have seemingly-random patterns of intensity, they exhibit hidden order in their polarization in the form of robust vortices. The number of vortices appearing as a result of the instability is quantized, and they can be used to distinguish different topological phases.

The most common way to identify topological phases of light has been to look at the edges of the material, where certain optical wavelengths become localized. However, a complete characterization requires measuring the bulk properties of the material, which is a much harder task. The light in the bulk material undergoes complicated wave interference and is highly sensitive to defects, which obscures its topological properties. Counterintuitively, the researchers have shown how nonlinear instabilities may be used to tame this unwanted interference and spontaneously encode the bulk topological properties of the material into beams of light. This approach provides a simpler way to probe and perhaps even generate topological states of light.

The next step will be to test this proposal in an experiment. For example, optical waveguide arrays inscribed in a glass will be an ideal platform for this purpose. By shining a bright pulsed laser beam into the glass, it should be possible to directly observe the modulational instability and thereby measure the topological properties of the waveguide array. The research group is currently discussing possible designs for the experimental verification of their theory with collaborators.

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Figure 2. Far-field polarization profile of laser beams after the modulational instability has developed. Trivial and nontrivial topological phases can be distinguished by counting the number of vortices in the polarization angle. There are no vortices in the trivial phase (left), and a pair of oppositely-charged vortices in the nontrivial phase (right).

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Institute for Basic Science [ 기초과학연구원](KR) pursues excellence in basic science research. The goal of IBS is to advance the frontiers of knowledge and to train the leading scientists of tomorrow.
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IBS was established in November 2011 as Korea’s first dedicated basic science research institute. By studying the fundamental principles of nature, basic science is essential in creating new knowledge from which significant societal transformations are derived. IBS promotes the highest quality of research that will increase the national basic science capacity and generate new opportunities for this nation.

IBS specializes in long-term projects that require large groups of researchers. As research in the 21st century requires more interdisciplinary collaborations from larger groups of people, scientists at IBS work together in the same laboratory base with a long-term perspective on research. We promote autonomy in research. IBS believes scientists unleash their creative potential most effectively when they conduct research in an autonomous environment with world-class research infrastructure, including RISP, the rare isotope accelerator, to enable major scientific advances. By developing strong synergies from outstanding talents, autonomous research support systems, and world-class infrastructure, IBS is steadily growing into a major basic research institute that meets the global standards of excellence.
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By pursuing excellence in research, IBS has selected global leading scientists as directors of Centers. These directors are operating 31 Centers of which research proposals are evaluated superior in the IBS peer review process. The review is carried out by a Review Panel composed of independent and expert scientists from Korea and abroad. Directors choose the themes of their research and allocate funds accordingly. Generally, Centers operate projects with no fixed term for their duration as long as the quality of research is verified in evaluations. New Centers receive an initial evaluation five years after its launch, followed by three-year interval evaluations.

IBS has been inviting top scientists from around the globe and providing them full support for their relocations. Young scientists also enjoy unique research opportunities to collaborate with world renowned scientists and to organize and operate their own research groups, broadening their professional expertise. IBS brings together outstanding talents throughout all career levels to grow and inspire each other through close collaborations.

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IBS welcomes scientists from Korea and abroad seeking to work in a collaborative research environment. IBS’ faculty researcher program and IBS’s affiliation with the founding body of University of Science and Technology (UST) help IBS scientists to reach out to and foster young talent outside the institution. Centers serve as a catalyst for research collaboration with universities and other government-funded research institutions through joint research and the sharing of research equipment. Other efforts are also underway to stimulate collaborations, including overseas training programs and visiting scientist programs.

To disseminate research findings, IBS holds “IBS Conferences” and develops a global network with the world’s prominent research institutions including the Max Planck Society [Max Planck Gesellschaft](DE) and the Royal Society (UK). We expect our work to make transformative changes outside as well as inside the institution. To realize this exciting vision, IBS will serve as a national R&D platform and accelerate the creation and use of new knowledge to support universities, research institutions, and businesses. As a driving force for dynamic research collaborations, IBS will continually develop and refresh its science, while always remaining receptive to outside talents and ideas.
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IBS shares the same passion as other great minds to investigate the origin of the universe, nature, and life for the development of humanity, as shown in its vision Masking Discoveries for Humanity & Society. We are committed to realizing this vision through a phased endeavor as outlined in our Five-year Plan (2013 – 2017). We aim to:

Become a national hub for basic science research by 2017
Complete the construction of the rare isotope accelerator by 2021
Evolve into one of the world’s top 20 basic research institution by 2030 (measured in terms of impact on research).

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