#optical properties

Pleochromism

Taking a quick break from describing the types of chromism, the topic of this post is pleochromism—which, despite the name, is not actually a form of chromism. Pleochromism is the optical phenomenon that results when a materials has a different appearance/color when viewed at different angles, or different angles of polarized light. It is not the process of a color change caused by an external stimuli, but rather the inherent property of a material.

Pleochromism results from anisotropy in crystal structures. Isotropic crystals do not exhibit pleochromism. Different crystal orientations in a material absorb and transmit different wavelengths of light, which can lead to both subtle or strong differences in visual appearances. This is most commonly seen in minerals, some exhibiting merely a change in the intensity of their coloration, others shifting colors entirely.

Some known pleochromic minerals include tourmaline, pictured above, alexandrite, corundum, and apatite. Commercial applications of pleochromism are rare, but scientists can use the effect to help identify minerals and gemstones.

Sulfur-containing polymer generates high refractive index and transparency

Researchers reported a novel technology enhancing the high transparency of refractive polymer film via a one-step vapor deposition process. The sulfur-containing polymer (SCP) film produced by Professor Sung Gap Im's research team at KAIST's Department of Chemical and Biomolecular Engineering has exhibited excellent environmental stability and chemical resistance, which is highly desirable for its application in long-term optical device applications. The high refractive index exceeding 1.9 while being fully transparent in the entire visible range will help expand the applications of optoelectronic devices.

The refractive index is a ratio of the speed of light in a vacuum to the phase velocity of light in a material, used as a measure of how much the path of light is bent when passing through a material. With the miniaturization of various optical parts used in mobile devices and imaging, demand has been rapidly growing for high refractive index transparent materials that induce more light refraction with a thin film.

Supercomputer predicts optical and thermal properties of complex hybrid materials

Materials scientists at Duke University computationally predicted the electrical and optical properties of semiconductors made from extended organic molecules sandwiched by inorganic structures.

These types of so-called layered "hybrid organic-inorganic perovskites"—or HOIPs—are popular targets for light-based devices such as solar cells and light-emitting diodes (LEDs). The ability to build accurate models of these materials atom-by-atom will allow researchers to explore new material designs for next-generation devices.

The results appeared online on October 4 in Physical Review Letters.

"Ideally we would like to be able to manipulate the organic and inorganic components of these types of materials independently and create semiconductors with new, predictable properties," said David Mitzi, the Simon Family Professor of Mechanical Engineering and Materials Science at Duke. "This study shows that we are able to match and explain the experimental properties of these materials through complex supercomputer simulations, which is quite exciting."

A material that can switch between multiple phases that have distinct electronic, optical and magnetic properties

A large team of researchers with members from China, the U.K., the U.S. and Japan has developed a material that can switch between multiple phases with distinct electronic, optical and magnetic properties. In their paper published in the journal Nature, the team describes how they made their material, how it can be caused to switch properties and possible uses for it. Shriram Ramanathan, with Purdue University offers a News & Views piece on the work done by the team in the same journal issue and adds some additional background on the search for functional materials.

As Ramanathan points out, humans have been searching for functional materials for centuries—we want more out of our materials than simply bearing loads. As he further notes, many such materials have been developed due to clearly directed efforts, but some have also come about by changing a material that has already been discovered. In this new effort, the researchers have taken the latter approach—they have modified an existing material to make it more useful by causing it to have different properties depending on how it is used.

To create the new material, the researchers created a thin layer of ceramic material in the traditional way, on top of a substrate. But instead of cooking it, as has been done historically, they covered the surface of the material with an ionic gel-like liquid. To provide for additional functionality, the liquid was an electrical insulator and able to conduct ions. It also held dissolved oxide ions and hydrogen ions. When electricity was applied to the material, the result depended on the polarity of the voltage—ions from either the hydrogen or oxide ions were driven into the ceramic material below. Reversing the voltage induced the reverse, which quite obviously meant that the system was reversible, as well.