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1.
Nat Commun ; 13(1): 1055, 2022 Feb 25.
Artigo em Inglês | MEDLINE | ID: mdl-35217665

RESUMO

Plasticity is ubiquitous and plays a critical role in material deformation and damage; it inherently involves the atomistic length scale and picosecond time scale. A fundamental understanding of the elastic-plastic deformation transition, in particular, incipient plasticity, has been a grand challenge in high-pressure and high-strain-rate environments, impeded largely by experimental limitations on spatial and temporal resolution. Here, we report femtosecond MeV electron diffraction measurements visualizing the three-dimensional (3D) response of single-crystal aluminum to the ultrafast laser-induced compression. We capture lattice transitioning from a purely elastic to a plastically relaxed state within 5 ps, after reaching an elastic limit of ~25 GPa. Our results allow the direct determination of dislocation nucleation and transport that constitute the underlying defect kinetics of incipient plasticity. Large-scale molecular dynamics simulations show good agreement with the experiment and provide an atomic-level description of the dislocation-mediated plasticity.

2.
Phys Rev Lett ; 123(9): 097601, 2019 Aug 30.
Artigo em Inglês | MEDLINE | ID: mdl-31524450

RESUMO

Complex systems, which consist of a large number of interacting constituents, often exhibit universal behavior near a phase transition. A slowdown of certain dynamical observables is one such recurring feature found in a vast array of contexts. This phenomenon, known as critical slowing-down, is well studied mostly in thermodynamic phase transitions. However, it is less understood in highly nonequilibrium settings, where the time it takes to traverse the phase boundary becomes comparable to the timescale of dynamical fluctuations. Using transient optical spectroscopy and femtosecond electron diffraction, we studied a photoinduced transition of a model charge-density-wave (CDW) compound LaTe_{3}. We observed that it takes the longest time to suppress the order parameter at the threshold photoexcitation density, where the CDW transiently vanishes. This finding can be captured by generalizing the time-dependent Landau theory to a system far from equilibrium. The experimental observation and theoretical understanding of dynamical slowing-down may offer insight into other general principles behind nonequilibrium phase transitions in many-body systems.

3.
Sci Rep ; 9(1): 3689, 2019 Mar 06.
Artigo em Inglês | MEDLINE | ID: mdl-30842469

RESUMO

We demonstrate single-shot multi-frame imaging of quasi-2D cylindrically converging shock waves as they propagate through a multi-layer target sample assembly. We visualize the shock with sequences of up to 16 images, using a Fabry-Perot cavity to generate a pulse train that can be used in various imaging configurations. We employ multi-frame shadowgraph and dark-field imaging to measure the amplitude and phase of the light transmitted through the shocked target. Single-shot multi-frame imaging tracks geometric distortion and additional features in our images that were not previously resolvable in this experimental geometry. Analysis of our images, in combination with simulations, shows that the additional image features are formed by a coupled wave structure resulting from interface effects in our targets. This technique presents a new capability for tabletop imaging of shock waves that can be extended to experiments at large-scale facilities.

4.
Top Curr Chem (Cham) ; 376(1): 6, 2018 Jan 23.
Artigo em Inglês | MEDLINE | ID: mdl-29362935

RESUMO

Multidimensional spectroscopy in the visible and infrared spectral ranges has become a powerful technique to retrieve dynamic correlations and couplings in wide-ranging systems by utilizing multiple correlated light-matter interactions. Its extension to the terahertz (THz) regime of the electromagnetic spectrum, where rich material degrees of freedom reside, however, has been progressing slowly. This chapter reviews some of the THz-frequency two-dimensional (2D) spectroscopy techniques and experimental results realized in recent years. Examples include gas molecule rotations, spin precessions in magnetic systems, and liquid molecular dynamics studied by 2D THz or hybrid 2D THz-Raman spectroscopy techniques. The methodology shows promising applications to different THz-frequency degrees of freedom in various chemical systems and processes.


Assuntos
Espectroscopia Terahertz , Simulação de Dinâmica Molecular
5.
Chem Sci ; 8(11): 7312-7323, 2017 Nov 01.
Artigo em Inglês | MEDLINE | ID: mdl-29163882

RESUMO

Zero-field splitting (ZFS) parameters are fundamentally tied to the geometries of metal ion complexes. Despite their critical importance for understanding the magnetism and spectroscopy of metal complexes, they are not routinely available through general laboratory-based techniques, and are often inferred from magnetism data. Here we demonstrate a simple tabletop experimental approach that enables direct and reliable determination of ZFS parameters in the terahertz (THz) regime. We report time-domain measurements of electron paramagnetic resonance (EPR) signals associated with THz-frequency ZFSs in molecular complexes containing high-spin transition-metal ions. We measure the temporal profiles of the free-induction decays of spin resonances in the complexes at zero and nonzero external magnetic fields, and we derive the EPR spectra via numerical Fourier transformation of the time-domain signals. In most cases, absolute values of the ZFS parameters are extracted from the measured zero-field EPR frequencies, and the signs can be determined by zero-field measurements at two different temperatures. Field-dependent EPR measurements further allow refined determination of the ZFS parameters and access to the g-factor. The results show good agreement with those obtained by other methods. The simplicity of the method portends wide applicability in chemistry, biology and material science.

6.
Opt Express ; 25(14): 16140-16150, 2017 Jul 10.
Artigo em Inglês | MEDLINE | ID: mdl-28789123

RESUMO

We demonstrate a self-referencing method to reduce noise in a single-shot terahertz detection scheme. By splitting a single terahertz pulse and using a reflective echelon, both the signal and reference terahertz time-domain waveforms were measured using one laser pulse. Simultaneous acquisition of these waveforms significantly reduces noise originating from shot-to-shot fluctuations. We show that correlation function based referencing, which is not limited to polarization dependent measurements, can achieve a noise floor that is comparable to state-of-the-art polarization-gated balanced detection. Lastly, we extract the DC conductivity of a 30 nm free-standing gold film using a single THz pulse. The measured value of σ0 = 1.3 ± 0.4 × 107 S m-1 is in good agreement with the value measured by four-point probe, indicating the viability of this method for measuring dynamical changes and small signals.

7.
Phys Rev Lett ; 118(20): 207204, 2017 May 19.
Artigo em Inglês | MEDLINE | ID: mdl-28581810

RESUMO

We report a demonstration of two-dimensional (2D) terahertz (THz) magnetic resonance spectroscopy using the magnetic fields of two time-delayed THz pulses. We apply the methodology to directly reveal the nonlinear responses of collective spin waves (magnons) in a canted antiferromagnetic crystal. The 2D THz spectra show all of the third-order nonlinear magnon signals including magnon spin echoes, and 2-quantum signals that reveal pairwise correlations between magnons at the Brillouin zone center. We also observe second-order nonlinear magnon signals showing resonance-enhanced second-harmonic and difference-frequency generation. Numerical simulations of the spin dynamics reproduce all of the spectral features in excellent agreement with the experimental 2D THz spectra.

8.
Proc Natl Acad Sci U S A ; 113(42): 11800-11805, 2016 10 18.
Artigo em Inglês | MEDLINE | ID: mdl-27702903

RESUMO

Ultrafast 2D spectroscopy uses correlated multiple light-matter interactions for retrieving dynamic features that may otherwise be hidden under the linear spectrum; its extension to the terahertz regime of the electromagnetic spectrum, where a rich variety of material degrees of freedom reside, remains an experimental challenge. We report a demonstration of ultrafast 2D terahertz spectroscopy of gas-phase molecular rotors at room temperature. Using time-delayed terahertz pulse pairs, we observe photon echoes and other nonlinear signals resulting from molecular dipole orientation induced by multiple terahertz field-dipole interactions. The nonlinear time domain orientation signals are mapped into the frequency domain in 2D rotational spectra that reveal J-state-resolved nonlinear rotational dynamics. The approach enables direct observation of correlated rotational transitions and may reveal rotational coupling and relaxation pathways in the ground electronic and vibrational state.

9.
Opt Express ; 24(5): 5057-5068, 2016 Mar 07.
Artigo em Inglês | MEDLINE | ID: mdl-29092334

RESUMO

We present a novel method for THz generation in lithium niobate using a reflective stair-step echelon structure. The echelon produces a discretely tilted pulse front with less angular dispersion compared to a high groove-density grating. The THz output was characterized using both a 1-lens and 3-lens imaging system to set the tilt angle at room and cryogenic temperatures. Using broadband 800 nm pulses with a pulse energy of 0.95 mJ and a pulse duration of 70 fs (24 nm FWHM bandwidth, 39 fs transform limited width), we produced THz pulses with field strengths as high as 500 kV/cm and pulse energies as high as 3.1 µJ. The highest conversion efficiency we obtained was 0.33%. In addition, we find that the echelon is easily implemented into an experimental setup for quick alignment and optimization.

10.
Opt Express ; 23(11): 14876-96, 2015 Jun 01.
Artigo em Inglês | MEDLINE | ID: mdl-26072845

RESUMO

We explored the use of the optically transparent semiconductor indium tin oxide (ITO) as an alternative to optically opaque metals for the fabrication of photonic structures in terahertz (THz) near-field studies. Using the polaritonics platform, we confirmed the ability to clearly image both bound and leaky electric fields underneath an ITO layer. We observed good agreement between measured waveguide dispersion and analytical theory of an asymmetric metal-clad planar waveguide with TE and TM polarizations. Further characterization of the ITO revealed that even moderately conductive samples provided sufficiently high quality factors for studying guided and leaky wave behaviors in individual transparent THz resonant structures such as antennas or split ring resonators. However, without higher conductive ITO, the limited reflection efficiency and high radiation damping measured here both diminish the applicability of ITO for high-reflecting, arrayed, or long path-length elements.

11.
Rev Sci Instrum ; 86(5): 051301, 2015 May.
Artigo em Inglês | MEDLINE | ID: mdl-26026507

RESUMO

Multidimensional spectroscopy at visible and infrared frequencies has opened a window into the transfer of energy and quantum coherences at ultrafast time scales. For these measurements to be performed in a manageable amount of time, one spectral axis is typically recorded in a single laser shot. An analogous rapid-scanning capability for THz measurements will unlock the multidimensional toolkit in this frequency range. Here, we first review the merits of existing single-shot THz schemes and discuss their potential in multidimensional THz spectroscopy. We then introduce improved experimental designs and noise suppression techniques for the two most promising methods: frequency-to-time encoding with linear spectral interferometry and angle-to-time encoding with dual echelons. Both methods, each using electro-optic detection in the linear regime, were able to reproduce the THz temporal waveform acquired with a traditional scanning delay line. Although spectral interferometry had mediocre performance in terms of signal-to-noise, the dual echelon method was easily implemented and achieved the same level of signal-to-noise as the scanning delay line in only 4.5% of the laser pulses otherwise required (or 22 times faster). This reduction in acquisition time will compress day-long scans to hours and hence provides a practical technique for multidimensional THz measurements.

12.
J Phys Chem B ; 117(36): 10444-61, 2013 Sep 12.
Artigo em Inglês | MEDLINE | ID: mdl-23931283

RESUMO

Intermolecular vibrations of amino acid crystals occur in the THz, or far-infrared, region of the electromagnetic spectrum. We have measured the THz and Raman spectra of DL-leucine as well as two polymorphs of DL-valine, the spectroscopic properties of which have not previously been compared. Theoretical modeling of intermolecular vibrations in hydrophobic amino acids is challenging because the van der Waals interactions between molecules are not accounted for in standard density functional theory. Therefore, to calculate the vibrational modes, we used a recently developed approach that includes these nonlocal electron correlation forces. We discuss methods for comparing results from different theoretical models using metrics other than calculated vibrational frequency and intensity, and we also report a new approach enabling concise comparison of vibrational modes that involve complicated mixtures of inter- and intramolecular displacements.


Assuntos
Leucina/química , Valina/química , Cristalização , Ligação de Hidrogênio , Interações Hidrofóbicas e Hidrofílicas , Análise Espectral Raman , Eletricidade Estática , Estereoisomerismo , Vibração
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