Nonlinear terahertz spectroscopy of every phase of matter

Tabletop generation of intense terahertz (THz) pulses with peak field amplitudes in the 0.1-1 MV/cm range and field enhancement up to 10 MV/cm and beyond has enabled nonlinear responses in solids, liquids, and gases to be driven by THz fields [1,2]. This has enabled nonlinear THz spectroscopy and THz coherent control of a wide variety of samples. In semiconductors and other samples, acceleration of carriers to multi-eV energies by THz excitation pulses has resulted in strong changes in carrier mobility and, in some cases, impact ionization that can lead to dramatic changes in conductivity [3,4]. Tunneling ionization has resulted in insulator-metal phase transitions with associated structural phase transitions [5]. In liquids and gases, THz pulses have produced molecular alignment and orientation [6,7]. Coherent control over molecular rotational motion involving multiple rotational levels has been demonstrated [8]. Recent nonlinear THz spectroscopy and prospects for more extensive THz coherent control over molecules and materials will be discussed.\\[4pt] [1] ``Generation of 10 $\mu $J ultrashort THz pulses by optical rectification,'' K.-L. Yeh, M.C. Hoffmann, J. Hebling, and K.A. Nelson, \textit{Appl. Phys. Lett}. \textbf{90}, 171121 (2007).\\[0pt] [2] ``Generation of high power tunable multicycle terahertz pulses,'' Z. Chen, X. Zhou, C.A. Werley, and K.A. Nelson, \textit{Appl. Phys. Lett}., \textbf{99}, 071102 (2011).\\[0pt] [3] ``Impact ionization in InSb probed by THz pump -- THz probe spectroscopy,'' M.C. Hoffmann, J. Hebling, H.Y. Hwang, K.-L. Yeh, and K.A. Nelson, \textit{Phys. Rev. B} \textbf{79}, 161201 (R) (2009).\\[0pt] [4] ``Nonlinear terahertz metamaterials via field-enhanced carrier dynamics in GaAs,'' \textit{Phys. Rev. Lett}. \textbf{110}, 217404 (2013).\\[0pt] [5] ``Terahertz-field-induced insulator-to-metal transition in vanadium dioxide metamaterial,'' M. Liu, et al., \textit{Nature} \textbf{487}, 345 (2012).\\[0pt] [6] ``Terahertz Kerr effect,'' M.C. Hoffmann, N.C. Brandt, H.Y. Hwang, K.-L. Yeh, and K.A. Nelson, \textit{Appl. Phys. Lett.} \textbf{95}, 231105 (2009). \\[0pt] [7] ``Molecular orientation and alignment by intense single-cycle THz pulses,'' S. Fleischer, Y. Zhou, R.W. Field, and K.A. Nelson, \textit{Phys. Rev. Lett}. \textbf{107}, 163603 (2011).\\[0pt] [8] ``Commensurate two-quantum coherences induced by time-delayed THz fields," S. Fleischer, R.W. Field, and K.A. Nelson, \textit{Phys. Rev. Lett.} \textbf{109}, 123603 (2012).