Science

Overview

Controlling the outcome of chemical processes remains a long-standing goal of chemistry. The proposition of energizing a specific chemical bond to promote a desired reaction pathway gave birth to the field of "mode-selective chemistry" decades ago. Most, if not all, examples have involved small molecules or clusters in the gas phase. There have been enormous efforts to extend this scheme to selective chemical reactions in the liquid or solid phase by resonant vibrational excitation with infrared light at the absorption resonance, i.e., interaction with the imaginary part of the dielectric function. However, it has been deemed unsuccessful due to the prevailing influence of intramolecular vibrational relaxation.

At Shi Group, we develop novel nonresonant excitation protocols that leverage the real part of the dielectric function to selectively catalyze key chemical processes with a direct impact on clean energy and quantum information science. We build upon terahertz-to-mid-infrared (THz-to-MIR) pulses that can polarize electrons (Field), impart impulsive forces (Raman) across multiple superposed modes to access transition states, and synchronize Raman mode displacements (Optomechanics) across complex potential surfaces to modify equilibrium positions. This strategy could address major challenges in clean energy and quantum science:

Research Directions

The omnipresence of quantum mechanics at the nanoscale advocates an emergent field of quantum information science with quantum computing, cryptography, and metrology applications. A central building block is single quantum emitters with near-perfect optical coherence while fundamentally hampered by many factors, especially universal fluorescence blinking. A non-invasive control of blinking and emission coherence is highly desirable while still under development after decades of effort. We intend to achieve a unified understanding and all-optical control of blinking in quantum emitters (e.g., quantum dots) by engineering an ultrafast device architecture combining nonresonant THz/MIR excitation and electrochemical gating. THz/MIR pulses could control charge-mediated blinking via field-driven electronic polarization and tunneling without inducing excitonic excitation, akin optical scalpels for electrons.

Long-range ionic motions in the solid electrolyte have enabled many technological breakthroughs, including safe and energy-dense batteries for renewable power storage. Superionic conductors hold great potential to surpass liquid-phase conductivity through the couplings between mobile ions and the lattice phonon vibrations, but enhancing their conductivity remains a grand challenge. We focus on implementing nonresonant schemes to excite multiple collective vibrational modes via THz/MIR-driven impulsive stimulated Raman scattering and accelerate ion conduction in solid electrolytes. Further mechanistic studies with ultrafast spectroscopies can characterize beyond the driven lattice motions to the many-ion correlation and time-dependent entropy growth that are closely linked to optimizing heating-free high-entropy power storage with enhanced durability.

Nanoparticles with reduced dimensions exhibit unique catalytic properties that depend sensitively on their morphologies, e.g., size, shape, and crystallography. Controlling nanoparticle morphology is a goal in chemistry research with myriad applications, including solar energy conversion. We aim to leverage a novel nonresonant optomechanical excitation to selectively transform nanoparticles to metastable morphology with enhanced photocatalytic water-splitting efficiency. The proposed mechanism centers on exerting optomechanical forces on collective atomic motions, similar to that realized by optical tweezers for atoms. Besides conversion rate characterization via gas chromatography and photocurrent, we also involve probing energy flow in these exotic nanoparticles that underline their photocatalytic and photovoltaic activities through ultrasfast single-particle spectroscopy.

We anticipate the nonresonant excitation strategies above can be extended to molecular systems in liquid and gas phases. The field, Raman and optomechanical forces could selectively guide many photochemical reactions at conical intersections or ground-state reactions with tantalizing applications in molecular electronics, clean energy production, and quantum science. These experiments transform Raman from spectroscopy into forces.