Therefore, high levels of configuration entropy in HEGSs imply inescapable phonon broadening. The voids can be occupied by ions of different types and the presence of several atomic components widens the frequency of the collective vibrations. In particular, tetrahedral and octahedral voids are formed through close packing of O 2. is designed on the close packing of oxides. How is this possible? The answer lies in the role played by phonon broadening. The outcome is really surprising and is complemented by the discovery that spatial coherence occurs at a low excitation threshold. Instead, Zhang and collaborators find a novel and unexpected result: in HEGSs, there exists enough correlation to induce coherence. On a more negative note, it must be underlined that spectral broadening is usually the precursor of incoherent optical phenomena and, as such, there is no correlation between absorption and emission. It represents a mechanical channel through which energy is dispersed and its unfavorable effect appears in the broadening of infrared absorption. On the other hand, phonon broadening has seemingly an adverse effect when combined with the absorption and emission of radiation in ordinary materials. This manifestation is very common and carries information about thermal and transport characteristics of HEMs 9, 10, 11. ![]() In glassy materials with multiple atomic components, vibrations (or phonons) propagating through the host deviate from the ideal quasi-monochromatic limit and vibrational energy is dispersed over many frequencies (phonon broadening). The phenomenon is rather intricate and needs some explaining. In their contribution, fundamental questions that HEGSs pose are explored and much emphasis is given to the role of phonon broadening in the coherent build-up. The route is indeed arduous and presents itself with several unknowns.Ī contribution aimed at exploring high-entropy glass systems (HEGSs) in view of laser operation comes from the work by Zhang et al. The choice guarantees the successful realization of HEMs with great optical potentialities and, for obvious reasons, laser operation is captivating although challenging. Such systems mimic the optical medium used in known solid-state lasers (e.g., Nd:Glass 7). The solution to the conundrum is within the reach of conventional laser physics and points towards glassy materials with optically active centers. Understandably, the optical regime requires the propagation of electromagnetic fields at visible or infrared wavelengths and opacity is detrimental to any ambitious goal of turning HEMs into novel optical media. Nonetheless, on the downside, these structures are not transparent and we cannot take advantage of their robust peculiarities when it comes to optical applications. Under such a circumstance, being the relative abundance of such atomic components similar in space, the macroscopic picture is captured by a material where the evenly distributed disorder emulates a single-phase system 6. They are also nearly equal in numbers and the final distribution determines a significant increase in the configuration entropy 5. To fulfill the objective, the trick resides in a random distribution of multiple atomic components filling the crystal lattice. ![]() These features result from the accurate design of such structures. To name them, good structural stability, high strength, and hardness, outstanding wear resistance, limited softening due to high temperatures, and low sensitivity to deterioration caused by corrosion and oxidation. They are very attractive for a number of reasons. ![]() The first question to ask is why should we care about materials of that kind? High-entropy materials (HEMs) are usually made of alloys 2, 3, 4. It regards the generation of optically coherent emission in materials where the entropic disorder nears its maximum. Here, we present another innovation that holds the promise of more interesting applications. However, more scientific and technological advancements are to come as though optical coherence was a wonder that never ceases to amaze us. Not to mention how the laser is an invaluable asset to science at large. One example among many is the barcode reader which has become an indispensable tool for industrial and commercial purposes. Their popularity touches any fragment of our society. Since then, uncountable applications and developments have been flooding the full extent of human activities. Maiman 1 unveiled the power of optical coherence (i.e., the fundamental feature of laser action) and gave birth to one of the most important scientific and technological revolutions in human history. More than 60 years ago, the first laser operation at the hands of Theodore H.
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