Session T14: Invited Session: Novel Nonlinear Spectroscopic Techniques for Understanding Material Structure and Function

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).   

Ultrafast Nonlinear Optics in the Tunneling Junction

Coupling of the electromagnetic radiation to the tip-sample junction of a scanning tunneling microscope (STM) offers exciting opportunities in molecular adsorbate identification, high-resolution dopant profiling, studies of the molecular motion and detection of dynamic changes in the electronic structure of the materials. Microwave spectral region is of particular interest because it encompasses rotational, magnetic and other resonances of molecular and solid state systems. However, previous works have either used external microwave sources or generated microwave radiation by a nonlinear mixing of the outputs from two continuous-wave lasers in a tunneling junction. In both cases, the usable spectrum was limited to a single or few frequencies. On the other hand, the regular train of pulses from a mode-locked ultrafast laser has a spectrum which represents an optical frequency comb, with a series of narrow lines (modes) spaced by the pulse repetition frequency. Here, we will show that the nonlinear response of the tunneling junction of an STM to the field of ultrashort laser pulses results in an intermode mixing that produces microwave frequency comb (MFC) with harmonics up to n = 200 (14.85 GHz) on both semiconducting and metallic surfaces. The observed dependence of the microwave power on the harmonic number reveals adverse effects of the tunneling gap capacitance but also shows that the roll-off at higher microwave frequencies should be negligible within the tunneling junction itself leading to intrinsic MFC spread up to THz region. We also demonstrate that MFC generation on semiconductor surface might have the same origin as THz generation in a surface depletion field. Generation of the broadband microwave signals within the tunneling junction should reduce the extraneous effects and provide significantly higher coupling efficiency. With improved frequency response, the described MFC-STM may find broad range of applications in nanoscale characterization of dynamic electronic and magnetic response of the materials in a wide frequency range.   

Multimodal and multispectral nano-imaging: accessing structure, function, and dynamics on the molecular scale

Structure, function, and dynamics of many soft-matter systems, including polymer heterostructures, organic photovoltaics, or biomembranes are typically defined on the mesoscopic few nm to sub-micron scale. Tip-enhanced and scattering scanning near-field optical microscopy (s-SNOM) have already demonstrated their ability to spectroscopically access that relevant spatial regime. I will demonstrate how in combination with advanced linear, broad-band, and ultrafast IR-vibrational spectroscopy s-SNOM provides ultrahigh spatial, spectral, and dynamic molecular structural information. From studying with nanometer spatial resolution vibrational dynamics, solvatochromism, and spectral Stark shifts, we gain microscopic insight into structure and intra- and intermolecular interactions in polymer and biological heterostructures. The approach provides access to understanding and ultimately controlling the interplay between structure, function, and dynamics in heterogeneous functional soft matter.   

A two-dimensional view of electron dynamics and coherent coupling in semiconductors

Understanding coherent interaction among multiple electronic states is a prerequisite to controlling material properties at the level of electrons and is a challenge that is ubiquitous in material science. Specifically, the presence or absence of coherent coupling among excitons significantly influence energy transfer, photon emission statistics, and even quantum-logic operations in semiconductor heterostructures such as quantum wells, quantum wires, and quantum dots. This problem is also relevant for a broader range of materials including natural/artificial photosynthetic systems and conjugated polymers. We have investigated coherent coupling among exciton resonances in disordered quantum wells. We articulate how strong coherent coupling occurs between certain types of excitons but is missing between other types of excitons using a powerful spectroscopy tools known as the electronic two-dimensional Fourier transform spectroscopy. In simple terms, the distinctive nature of excitons results in different spatial overlap and different coupling strength. If time permits, we will also present our most recent results on monolayer transition metal dichalcogenides.\\[4pt] [1] Yuri Glinka, Zheng Sun, Mikhail Erementchouk, Michael Leuenberger, Alan Bristow, Alan Bracker, \textbf{Xiaoqin Li} ``Coherent Coupling among Exciton Resonances Governed by Disorder Potentials,'' Phys. Rev. B, 88, 075316, 2013.\\[0pt] [2] Yuri Glinka, Mikhail Erementchouk, Chandriker K. Dass, Michael N. Leuenberger, Alan Bristow, Alan Bracker, \textbf{Xiaoqin Li} ``Nonlocal Coherent Coupling Between Excitons in a Disordered Quantum Well,'' New Journal of Physics, 15, 075026, 2013.   

Probing Lattice Dynamics in Quantum-Confined Materials on Ultrafast Timescales

Experimental measurement of lattice dynamics in few nanometer semiconductor nanocrystals present difficulty owing to low frequency phonon modes and the possibility of rapid dynamics. We utilize recently developed femtosecond stimulated Raman spectroscopy in order to characterize longitudinal optical (LO) phonon production and dissipation throughout the process of confinement-enhanced, ultrafast intraband carrier relaxation. Prior to photoexcitation, CdSe nanocrystals produce a stimulated Raman spectral shape that resembles the spontaneous Raman spectrum. Upon excitation, we observe a decrease in stimulated Raman amplitude and note a size-independent LO phonon formation time. Mode softening is observed as is evidence of phonon down-conversion processes. Furthermore, spectrally and temporally resolved photoluminescence suggest evidence of acoustic phonon dissipation times that follow diffusive transport, which we can manipulate.