Prof. Keith A Nelson

Research is aimed at time-resolved optical study and control of condensed matter structural changes and the collective modes of motion through which they occur. How do phase transitions or other collective structural rearrangements in crystalline solids occur, where huge numbers of molecules or ions move cooperatively into new positions? What are the dynamics, and how are they mediated by the lattice vibrational modes? Can we guide or control them with ultrashort optical pulse sequences? In crystalline chemical reactions, how do the neighbors surrounding the reacting species cooperatively accommodate fragments? What are the interactions between the reactive molecular modes and the lattice vibrations, and how do these influence reaction dynamics and outcomes? Can the complex structural relaxation dynamics of viscoelastic fluids and polymers, starting on subpicosecond time scales and sometimes extending for seconds, be understood in terms of a comprehensive statistical mechanical theory? Can we gain experimental access to the collective motions involved and measure them over all of the relevant time scales?

Much of the progress, and much of the “art” in our efforts, comes through understanding how light – especially in the form of short pulses – interacts with matter, and how the interactions can be exploited for improved material characterization or control. For example, we have developed novel methods for recording complete femtosecond time-resolved spectroscopy measurements in a single laser shot, with the objective of observing ultrafast, irreversible structural and chemical changes in solids that are permanently altered during the measurement. We are optimizing coherent spectroscopy methods for use on samples in diamond anvil high-pressure cells, and even for samples under extreme conditions of shock loading. We have invented femtosecond pulse shaping techniques for multiple-pulse excitation and coherent control of crystal lattice vibrations whose motions are involved in ferroelectric phase transitions, and for generation of ultrahigh-frequency acoustic waves whose motions are involved in structural relaxation of viscoelastic fluids and polymers. In the ferroelectric crystals that we study, the terahertz (THz) frequency lattice vibrations that we generate are strongly coupled to THz-frequency light, and they form mixed electromagnetic/vibrational “polariton” waves that move through the crystals at light-like speeds! Nevertheless, although they may run fast, they can’t hide: multiple femtosecond pulses arrive at different times and sample locations to manipulate them as they move, and the interferences of many of these waves generated from many sample locations are used to produce specified, coherent lattice responses. Integrated waveguides and other structures that we fabricate through femtosecond laser machining provide further control over these unique THz waves. And their generation, control, and propagation through the crystals is all monitored directly through real-space imaging, yielding “movies” that reveal their complete temporal and spatial evolution. In the process of developing methods for study and control of ferroelectric phases, we have developed a THz signal generation and processing platform.

The Group is extending these and other methods and applying them toward study of and control over condensed matter structural change. This is an interdisciplinary effort centered in chemical physics and cutting across condensed matter physics, materials science, and photonics. Our development of novel optical methods and our work with advanced materials, as well as our cross-disciplinary collaborations, sometimes give rise to practical applications as well as fundamental understanding, and these are selectively pursued.