Optics and photonics underpin much of today?s technological world.  Information technology, consumer electronics, communications, and modern healthcare form an incomplete list of sectors that would not exist in their current form without the foundation of optics and condensed matter physics, the latter being the branch of science that studies and explains the behavior of electrons in all materials and underpins all of today?s photonics.  Light is an electromagnetic wave and is the central entity in our project.  Like any wave, light is characterized by the wavelength ? the distance between its two nearest crests.  Our project focused on a specific range of the wavelengths of light ? terahertz or far-infrared.  These wavelengths are not visible to the human eye and can easily pass through some materials, such as clothing and plastics, but they are completely blocked by metals.  Our goal was to examine how different materials interact with terahertz light and how terahertz light can be used to alter materials? properties.  

One materials category that we studied was multiferroics, which can be simultaneously magnetic and ferroelectric. Magnetism is a property that has been used for data storage and recording.  Ferroelectricity is another property that is used for data storage by using the direction of ferroelectric polarization.  In multiferroic materials, magnetism and ferroelectricity tend to be resonant with terahertz light, which also makes them good candidates for property manipulation by very strong pulses of terahertz light.  One discovery that resulted from this line of work is an optical diode effect that occurs at high temperature ? much higher than the ferromagnetic ordering temperature.  In optical diode effect, a sample of material can transmit light in one direction and completely block the light traveling in the opposite direction.  This property can be very useful in photonic devices.  Such optical nonreciprocity only happens in materials with special symmetry conditions: the material must not possess the center of symmetry (center of inversion) and must not obey time reversal symmetry.  We also set out to create artificial materials that fulfil the symmetry conditions for optical nonreciprocity.  Such artificial materials are called metamaterials and consists of ordered arrays of light scatterers of subwavelength dimensions.  The optical properties of metamaterials can be designed to perform a specific function, and we created two metamaterial designs that exhibit optical nonreciprocity.  While working on this topic, we also discovered a way to create a one-way mirror ? if such a mirror were hung on a wall, you could see your friend?s reflection in it, but your friend would not see you.

As another part of our project, we explored how different materials can be altered by intense pulses of very strong terahertz light.  For example, we found a way to accurately describe how semiconductors become more transparent when illuminated by such intense terahertz pulses.  We also explored how some transparent crystals become birefringent when illuminated by intense terahertz light.  In the most exciting finding, we observed that the crystal KTaO₃ enters a new hidden state when it interacts with intense terahertz light.  We call this state hidden because it is not achievable with other means, for example by changing the temperature or pressure on the material.  In this hidden state, KTaO₃ crystal develops birefringence that persists long after the terahertz pulse leaves the crystal.  The birefringence can potentially be used for terahertz-frequency neuromorphic computing devices.

Among the societal benefits that go beyond the described scientific advances, we report that this project served as the research training for four graduate students who obtained their PhD degrees.  Two PhD students graduated in 2018 and the other two graduated in 2022.  All four students have now entered the US workforce as highly educated and value-producing workers.  As part of our educational commitment, our Tulane laboratory sponsored and conducted workshops for middle school boys and girls as part of the GiST (Girls in STEM at Tulane) and BATS (Boys at Tulane in STEM) programs.  These programs are designed to allow boys and girls to meet and work with role models in STEM (science, technology, engineering, and mathematics) fields and encourage them to investigate and discover in a hands-on scientific setting.  The long-term goal is to promote US student participation in STEM fields.  Our workshop titled ?Fluids, bubbles, and slime? consistently attracted full enrollments of both girls and boys.  

Last Modified: 06/28/2022
Modified by: Diyar Talbayev