CAREER: Probing Quantum Materials Modified by Terahertz Quantum Fluctuations

Nontechnical description

Vacuum is not truly empty but contains light waves that are constantly fluctuating according to quantum mechanics. Similarly, materials cooled to near-absolute zero temperature are not truly frozen but contain constantly fluctuating atomic motions. Although these fluctuations are usually negligible in daily life, theoretically they can grow large when a standing wave is compressed into a very small volume, called a cavity, with dimensions compared with the wave’s wavelength.

Under such circumstances, according to some recent theoretical and experimental research, the fluctuating wave may be strong enough to change the properties of materials immersed in the wave, including the atomic structure, electrical conductivity, and magnetic properties. This research attempts to answer a few major open questions in this new paradigm of cavity modified materials based on a specific system: the mixing of electromagnetic waves and atomic vibrations that naturally happens in many ionic crystals.

Near the vibrational resonance, which is in the terahertz frequencies, the wavelength of the mixed wave shrinks, so the quantum fluctuation is expected to enhance. The research team plans to directly measure the quantum fluctuation of the light and matter inside small cavities, and track the modified energy evolution and materials properties. The results promise insights into optimizing materials by harvesting quantum forces for free.

The project also supports the dissemination of the basic concepts of quantum materials to audiences at different levels of education. The principal investigator plans to formulate a modular graduate course to bridge the knowledge gap between existing materials education and the societal need for building quantum infrastructure, to offer an undergraduate research opportunity targeting underrepresented groups in community colleges, and to work with high school teachers in the Houston Independent School District to design a lesson about materials in quantum technology.

These efforts are part of NSF’s Big Idea of Quantum Leap and the National Quantum Initiative to build a quantum workforce for the future. Technical description

Emerging experimental evidence and theoretical models open a possibility for cavity-enhanced quantum fluctuations of bosonic modes, including photons and phonons, to significantly alter materials’ structural, transport, and magnetic behaviors in both the excited states and the ground state. The quantum fluctuations thus may serve as a control knob to transform quantum materials without the need for active input.

However, experimental validation is scarce about a possibility of the amplified fluctuations in terahertz frequencies or the transfer of properties between cavity-coupled bright polaritons and much more numerous cavity-decoupled localized states. This research provides the much-needed experimental knowledge on the magnitude and consequence of the quantum fluctuations of ultra-strongly coupled phonon-polaritons in sub-wavelength cavities, which may guide future efforts to realize various functional quantum materials through cavity engineering, possibly including superconductors, ferromagnets, ferroelectrics, and topological electronic materials.

The principal investigator and research team plans to develop sensitive electro-optic microscopy in the terahertz frequencies to quantify the fluctuating electric fields and atomic displacements. The measurements can provide a firmer ground to the theoretical analysis of cavity quantum electrodynamics in realistic, lossy quantum materials. The quantum microscopy combined with time-resolved spectroscopy reveal the coherence and population dynamics of phonon-polariton excitations, which shed light on possible cavity-induced structural transitions.

Finally, the acquired knowledge facilitates the design and realization of hybrid terahertz cavity-materials systems to modify the spin and charge states in two-dimensional layered materials and heterostructures, where the electronic interactions are strong and externally tunable.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.