Optical excitations and ultrafast carrier dynamics in nanomaterials
Carrier relaxation, diffusion and trapping at defect sites all occur within the first tens to hundreds of picoseconds following photoexcitation. All these processes are critical to the operation of nanomaterial-based photonic devices, such as solar and photoelectrochemical cells, and it is important to understand them in order to engineer efficient devices.
Time-resolved terahertz spectroscopy (TRTS) is an all-optical, contact-free technique that is uniquely suited for exploring carrier dynamics and conductivity in materials with sub-picosecond time resolution and on length scales of 1-100 nm.
We interested in investigating ultrafast carrier transport in various novel materials for solar energy conversion application. Ultrafast THz Physics Lab is a part of WPI Energy Research Center. We work with researchers from WPI, including WPI NanoEnergy Lab (PI - Pratap Rao), Grimmgroup (PI - Ron Grimm) and others, as well as with researcher in other universities in US and abroad.
Figure: TRTS applied to study photoexcited carrier dynamics in silicon nanocrystals embedded in silicon dioxide. (a) Example of a THz pulse waveform. The corresponding Fourier amplitude spectrum is shown in the inset. (b) Change in transmission of the main peak of the THz probe pulse as a function of time delay with respect to a 400 nm, 100 fs pump pulse for a silicon nanocrystal film with Si volume fraction of 51%. Change in THz peak transmission is proportional to the time-dependent photoconductivity induced by the pump pulse. Inset – schematic diagram of the optical-pump/THz-probe experiment. (c) Real (solid red squares) and imaginary (open blue squares) components of the complex conductivity of a silicon nanocrystal film measured 50 ps after photoexcitation. (d) Complex conductivity of photoexcited bulk single crystal silicon.