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Assistant Professor Yaguo Wang, recipient of an NSF Early CAREER Award.

Assistant Professor Yaguo Wang, recipient of an NSF Early CAREER Award.

Assistant Professor Yaguo Wang, who joined Mechanical Engineering Department in Jan. 2013, has recently been selected by the National Science Foundation for an Early CAREER Award, which carries with it $400,000 in start-up research funding for five years.

The grant was designed to help assistant professors (tenure track) jump-start a successful research program. Wang is the only Mechanical Engineering professor in the department to win one this year, but the 20th department faculty member to win since 1992. The other three winners from the Cockrell School are assistant professors, Nanshu Lu (Aerospace and Engineering Mechanics), Evdokia Nikolova (Electrical and Computer) and Evdokia Nikolova (Civil, Architectural and Environmental).

Her award is for

Optical Signal Showing Collective Atomic Movements

Optical reflectivity change shows the collective movements of atoms inside the Bismuth crystal. The time period of this oscillation is about 266 femtosecond. 

Optical reflectivity change shows the collective movements of atoms inside the Bismuth crystal. The time period of this oscillation is about 266 femtosecond.  View larger image.

Her research focuses on the study of ultrafast carrier dynamics and micro/nano-scale heat transfer in nanomaterials with optical spectroscopy. The physics of macroscopic heat transfer breaks down at nanoscale, and currently it is extremely difficult to measure the atomic movement in nanostructures. The optical techniques will allow her team to investigate the behavior of individual carrier in nanomaterials. They have been building their own optical devices with femtosecond, nanosecond and continuous-wave lasers. Their short pulse lasers are non-invasive detectors and can detect the movement of atoms at 10-15s (seconds). Anything over 10-12s is considered

Dr. Wang's NSF project will study the behavior of heat carriers called phonons, which are responsible for heat transport in semiconductor materials like silicon. Phonons represent the vibrations of atoms in a crystal (graphic). She will address the fundamental heat transfer problems encountered in a wide variety of disciplines, such as thermoelectrics, quantum cascade lasers, infrared detectors, and nanoelectronics.

Phonons are not actual particles, but quasiparticles — a collective excitation of atoms or molecules in solids and some liquids. (The now famous Higgs boson is a quasiparticle, although a different type). The excitations of these groups of atoms are the lattice vibrations in crystalline structures. The two images below showcase different movement of phonons. The movement of the top figure illustrates the random movement of atoms without using the laser. The movement in the bottom figure was generated by using short pulse lasers. That movement is the measurable movement they are studying.

Animated gif file showing random atomic movement Animated gif file showing wavelike atomic movement

In most cases, atoms move randomly inside the crystal around a certain point. (First figure) Under the intensive field of short laser pulse, atoms can be disciplined to vibrate collectively. (Second Figure) Even though it is very rare, this collective motion of atoms gives us a good physical model to study heat transport by individual phonon.

Bridging the gap between simulation and actual measurement

Her research will bridge the gap between simulation studies of microscopic heat transfer and measurements of macroscopic thermal properties. She will use the short laser pulse to stimulate collaborative movements of atoms in crystalline (solids), not in the random atomic structure of amorphous solids, which would not produce a wavelike movement, but a random movement. Simulations can predict properties of individual phonons and explain some experimental observations. However, these results often cannot be validated directly because experimental data on individual phonon modes is usually not available. Assumptions have to be made to enable comparisons with experiment results.

Dr. Wang will use her short laser pulse to stimulate collective movements of atoms in nanostructures and determine the effect on heat carriers from various structure designs. They will measure lots of different type configurations so that material developers will be able to use this data to design customized semi-conductor nano-structures (which can be made in different shapes). The different particle designs will allow scientists to control the phonons to achieve the specific properties desired in different materials.

Graduate Research Assistants Wanted

Dr. Wang is actively recruiting for two more graduate research assistants in the fall for her lab. Please see the Wang Research Group web site for more information on this work. If you are an incoming graduate student interested in materials, physics and/or heat transfer, please contact Dr. Wang. Currently, there are two students [Feng He and Brandon (Khai Ta) Nguyen] and one visiting professor (Wenzhi Wu, from Heilongjiang University, China) working in her lab, but there is much more work to do, allowing you to make important scientific contributions.

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