Principal Investigator Keith Nelson
Project Website http://nelson.mit.edu/blog/nonlinear-terahertz-spectroscopy
In recent years there has been great progress in the exploration of the electromagnetic spectrum between 0.1 and 10 THz, also known as the THz gap. This region gives access to many interesting physical properties of semiconductors, molecular crystals, ferroelectrics, gas molecules, superconductors and biological objects, whose spectroscopic signatures are largely to be discovered in the THz range. In our lab, we are focused on development of generation of broadband/tunable narrow band THz radiation with high field strengths and characterization of various material systems' nonlinear and collective behaviors induced by the intense THz fields.
High field THz generation -- Nonlinear THz spectroscopy requires broad bandwidth/tunable narrow band, and high electric field strengths. We generate single- and multi-cycle THz pulses using optical rectification in LiNbO3 crystals via noncollinear phase matching with tilted pulse front excitation from a Ti:Sapphire amplifier system. THz pulses are characterized by electro-optic (EO) sampling in EO crystals such ZnTe. The single-cycle THz pulses we generated typically have electric field strengths exceeding 100kV/cm and spectra ranging through 0.1-3 THz. We also developed the chirp and delay method to generate multiple-cycle THz pulses with flexible tunability and high field strength. The THz electric fields can be further enhanced by an order of magnitude using metamaterial structures called split-ring resonators. These generation methods open up new possibilities of observation of nonlinear collective materials responses.
Orientation of gas phase molecules by THz fields -- The interaction of a molecule with an electromagnetic field depends on the relative angle between the molecule and the field polarization. In samples lacking of long range order, as in the gas phase, one measures spectroscopic signals which are averages over all possible molecular orientations. This can be avoided by ‘ordering’ the molecular sample prior to its spectroscopic interrogation, by rotating all the molecules toward a desired direction in the lab frame.
THz induced dynamics of Cooper pairs in High-Tc superconductors -- THz time-dependent spectroscopy (THz-TDS) has been used to study thin films of YBCO, which is a high-temperature super conductor with a d-type Fermi surface. THz-TDS provides the amplitude and phase of the transmitted electric field, and enables us to extract real and imaginary conductivities of the YBCO thin film. Power-dependent transmission measurements have shown that a strong THz electric field induces partial transparency, which is consistent with a transient reduction of superconducting electrons. Dynamics of this phenomenon have been studied with THz pump / THz probe experiments that reveal a decay of the induced transmission on the time scale of a few picoseconds due to Cooper pair regeneration. After the initial decay, transmission remains above baseline values, which may indicate the relaxation process involves additional nonthermal dynamics that reestablish superconductivity in the thin film.
Nonlinear 2D THz Magnetic Resonance Spectroscopy of Magnons -- Magnons are quantized low-energy excitations of electron spins. In many ferromagnetic (FM) and antiferromagnetic (AFM) materials, intrinsic magnetic fields in the same range put collective spin waves (magnons) in the THz range. Current ESR spectroscopy remains limited at THz frequencies because the weak sources used only permit measurements of free-induction decay (FID) signals that are linearly proportional to the excitation magnetic field strength. Here, we explore the nonlinearity of magnons using time-delayed intense THz pulse pairs. We develop 2D THz magnetic resonance spectroscopy, which can be understood in terms of multiple field-spin interactions that generate the nonlinear signal fields. Magnons are resonantly excited without promoting electrons to excited states (as in most spintronics excitation) and hence the observed nonlinearities are of purely magnetic origin. The material under study is single-crystal YFeO3 (YFO). The ground state has canted AFM order with Two THz-active magnon modes, the quasi-AFM (AF) and quasi-FM (F) modes, can be constructed based on different cooperative motions of sublattice spins.