During the project period we demonstrated that the strong interaction between infrared light and the lattice vibration in quantum paraelectric materials can enable microscale photonic devices in the frequency range of 5 – 15 THz. This frequency range is a notable technology gap where there are no good commercial solutions to control electromagnetic waves, because atomic bond vibration in materials tends to absorb light. Interestingly, further increasing the interaction between atoms and light can be used to control light propagation. In quantum paraelectric materials, some atoms sit in unstable positions and exhibit prominent quantum fluctuations, i.e. large uncertainty in their position. The average position of these atoms is also easily shifted by electrical force, leading to very strong coupling with electromagnetic waves. The coupled atomic oscillation and electromagnetic waves form a new particle called “phonon-polariton”. This particle gives negative dielectric function, i.e., the current caused by the ionic motion is opposite to the external electric field, in a wide frequency range. Therefore, it can stop the propagation of electromagnetic waves and capture the light energy at the surface of the material. We theoretically and experimentally proved the concept that SrTiO3 phonon-polariton devices can focus infrared light to a much tighter space than the wavelength, maintaining the short duration and the polarization of an ultrafast pulse.

To study the strong infrared light field in a small area and its effects on quantum materials, we developed a unique time-resolved terahertz microscopy imaging setup. Using a pulse with much shorter wavelength and duration, we showed that the electric field of the infrared pulses can be measured inside any material as a function of space and time. Moreover, we can also measure the local magnetic effect of circularly polarized infrared light pulses. Such pulses carry atoms to rotate in materials, breaking time-reversal symmetry, and can cause strong ultrafast effective magnetic fields in some rare earth compounds. We found that these materials exhibit very strong interactions between the electronic spins and atomic rotations (termed chiral phonons). The magnetic field can in principle reach 20 Tesla, especially when the atomic motion is enhanced by phonon-polariton structures. We also found that the quantum uncertainty of the atomic motion may be sufficient to impact the spin properties in some two-dimensional magnetic materials. When the frequency of the spin wave is close to that of atomic oscillation, the spin and atom can hybridize into a new particle. This particle can become topological under a magnetic field, meaning that it can move one-way along the edge of the material, which is not possible for either the spin wave or the atomic oscillation alone.

Overall, we established new methodologies to understand atomic motion in quantum materials, which is important for designing and operating solid-state quantum devices. We also improved optical nonlinear microscopy as infrastructure to characterize quantum materials. Our results have been disseminated in 15 peer-reviewed journal publications and several presentations in international conferences. The research has been incorporated in graduate workforce development, education, and undergraduate research experience at Rice University.

Last Modified: 11/14/2024
Modified by: Hanyu Zhu