WPI - Ultrafast Terahertz Physics Laboratory

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. 

From L.V. Titova, T.L. Cocker, D.G. Cooke, X. Wang, A. Meldrum, and F.A. Hegmann, “Ultrafast Percolative Transport Dynamics in Silicon Nanocrystal Films,” Phys. Rev. B. 83, 085403 (2011).

THz technology holds tremendous promise of providing doctors and medical researchers with new non-invasive diagnostic and treatment tools.

Growing evidence suggests that intense THz pulses of picosecond duration, peak powers as high as 1 MW, and peak electric fields that reach MV/cm elicit molecular and cellular responses. In a collaboration with researchers from Universities of Alberta and Lethbridge in Canada, we have shown that exposure of human tissue to intense THz pulses can induce double-strand DNA breaks, initiate a DNA repair, change the levels of cell-cycle regulatory proteins, and alter the expression of genes associated with cancer and carcinogenesis. The apparent ability of intense THz pulses to induce cellular responses suggests possible new therapeutic applications. At the same time, THz radiation from intense sources may be a new powerful new tool for probing biological systems.

In our lab, we use intense THz pulses with energies of up to 1.5 microjoules per pulse to understand how intense THz radiation interacts with cells and cellular constituents.

The dynamical signatures of intermolecular vibrational, librational and damped rotational dynamics of many liquids, organic materials, as well as biomolecules fall predominantly into the THz range and the relevant time scales are in picosecond and sub-picosecond range. THz time-domain spectroscopy (THz-TDS) enables direct measurement of the complex dielectric function of a sample by analyzing the sample-induced changes in a transmitted or reflected broadband THz pulse. It has already been used to uncover significant variations in dynamical properties of bulk, interfacial, nano-confined, and solvation shell water,to assess the extent of the solvation shell around certain biomolecules.

We are interested in using THz spectroscopy to understand intermolecular interactions in complex media, from organic semiconductors, to liquid-air and liquid-solid interfaces, to biomolecules in crystal form and in physiological conditions.