Abstract-Two-Dimensional Spectroscopy at Terahertz Frequencies

Jian Lu, Xian Li, Yaqing Zhang, Harold Y. Hwang, Benjamin K. Ofori-Okai, Keith A. Nelson

https://link.springer.com/article/10.1007%2Fs41061-018-0185-4

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.

Abstract-Coherent Two-Dimensional Terahertz Magnetic Resonance Spectroscopy of Collective Spin Waves

Jian Lu, Xian Li, Harold Y. Hwang, Benjamin K. Ofori-Okai, Takayuki Kurihara, Tohru Suemoto, and Keith A. Nelson

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.207204

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.

• 
• 
• 
• 

Abstract-Nonlinear two-dimensional terahertz photon echo and rotational spectroscopy in the gas phase

1. Jian Lua, 
2. Yaqing Zhanga, 
3. Harold Y. Hwanga, 
4. Benjamin K. Ofori-Okaia, 
5. Sharly Fleischerb, and 
6. Keith A. Nelsona,1

http://www.pnas.org/content/113/42/11800

1. aDepartment of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139;
2. bDepartment of Chemical Physics, Tel-Aviv University, Tel Aviv 69978, Israel

Molecular rotations of small molecules provide a useful testbed for examining light−matter interactions with quantum mechanical systems, but the methods of modern spectroscopy have been largely unavailable in the terahertz frequency range where most of the rotational states that are thermally populated at ordinary temperatures absorb light. Applying a pair of strong terahertz pulses, we excite molecular rotations coherently, interrogate thermally populated rotational states, manipulate the rotational motions nonlinearly, and observe connections between different rotational states spectroscopically. The method is applicable to polar molecules in flames and other reactive conditions, and it enables enhanced control over molecular motion with light.

Abstract-Nonlinear two-dimensional terahertz photon echo and rotational spectroscopy in the gas phase

1. Jian Lua, 
2. Yaqing Zhanga, 
3. Harold Y. Hwanga, 
4. Benjamin K. Ofori-Okaia, 
5. Sharly Fleischerb, and 
6. Keith A. Nelsona,1

Author Affiliations

1. aDepartment of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139;
2. bDepartment of Chemical Physics, Tel-Aviv University, Tel Aviv 69978, Israel

1. Edited by Michael D. Fayer, Stanford University, Stanford, CA, and approved September 2, 2016 (received for review June 15, 2016)

Molecular rotations of small molecules provide a useful testbed for examining light−matter interactions with quantum mechanical systems, but the methods of modern spectroscopy have been largely unavailable in the terahertz frequency range where most of the rotational states that are thermally populated at ordinary temperatures absorb light. Applying a pair of strong terahertz pulses, we excite molecular rotations coherently, interrogate thermally populated rotational states, manipulate the rotational motions nonlinearly, and observe connections between different rotational states spectroscopically. The method is applicable to polar molecules in flames and other reactive conditions, and it enables enhanced control over molecular motion with light.

Abstract-Two-dimensional terahertz magnetic resonance spectroscopy of collective spin waves

Jian Lu, Xian Li, Harold Y. Hwang, Benjamin K. Ofori-Okai, Takayuki Kurihara, Tohru Suemoto, Keith A. Nelson

(Submitted on 20 May 2016)

http://arxiv.org/abs/1605.06476

Nonlinear manipulation of nuclear and electron spins is the basis for all advanced methods in magnetic resonance including multidimensional nuclear magnetic and electron spin resonance spectroscopies, magnetic resonance imaging, and in recent years, quantum control over individual spins. The methodology is facilitated by the ease with which the regime of strong coupling can be reached between radiofrequency or microwave magnetic fields and nuclear or electron spins respectively, typified by sequences of magnetic pulses that control the magnetic moment directions. The capabilities meet a bottleneck, however, for far-infrared magnetic resonances characteristic of correlated electron materials, molecular magnets, and proteins that contain high-spin transition metal ions. Here we report the development of two-dimensional terahertz magnetic resonance spectroscopy and its use for direct observation of the nonlinear responses of collective spin waves (magnons). The spectra show magnon spin echoes and 2-quantum signals that reveal pairwise correlations between magnons at the Brillouin zone center. They also show resonance-enhanced second-harmonic and difference-frequency signals. Our methods are readily generalizable to multidimensional magnetic resonance spectroscopy and nonlinear coherent control of terahertz-frequency spin systems in molecular complexes, biomolecules, and materials.

Abstract-Tunable multi-cycle THz generation in organic crystal HMQ-TMS

Jian Lu,1 Harold Y. Hwang,1 Xian Li,1 Seung-Heon Lee,2 O-Pil Kwon2 and Keith A. Nelson1,*

 1 Deartment of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA 2 Department of Molecular Science and Technology, Ajou University, Suwon 443-749, South Korea * kanelson@mit.edu

 https://www.osapublishing.org/view_article.cfm?gotourl=https%3A%2F%2Fwww%2Eosapublishing%2Eorg%2FDirectPDFAccess%2F0129DC5F-F642-F10F-6A6F73D0AFC3655B_324474%2Foe-23-17-22723%2Epdf%3Fda%3D1%26id%3D324474%26seq%3D0%26mobile%3Dno&org=

Abstract: We report on the generation of continuously tunable multi-cycle THz pulses with center frequencies from 0.3 to 0.8 THz in the organic nonlinear crystal, HMQ-TMS [2-(4-hydroxy-3-methoxystyryl)-1- methylquinolinium 2,-4,-6-trimethylbenzenesulfonate], by collinearly phase matched optical rectification of temporally shaped 800 nm pulses. The generation of harmonic frequency components inherent in the pulse shaper is selectively suppressed by varying the generation crystal thickness. THz pulses generated from HMQ-TMS show up to 20 times higher pulse energies compared to the benchmark inorganic THz generator ZnTe under identical conditions. The THz energy conversion efficiencies are measured to be on the order of 10−5 .

©2015 Optical Society of America OCIS codes: (160.4890) Organic materials; (190.0190) Nonlinear optics; (300.6495) Spectroscopy, terahertz. 

Abstract-A review of non-linear terahertz spectroscopy with ultrashort tabletop-laser pulses

DOI:

10.1080/09500340.2014.918200

Harold Y. Hwang, Sharly Fleischer, Nathaniel C. Brandt, Bradford G. Perkins Jr., Mengkun Liu, Kebin Fan, Aaron Sternbach, Xin Zhang, Richard D. Averitt & Keith A. Nelson,

Publishing models and article dates explained

• Received: 2 Oct 2013
• Accepted: 20 Apr 2014
• Published online: 12 Jun 2014

http://www.tandfonline.com/doi/abs/10.1080/09500340.2014.918200?journalCode=tmop20#.U5nYW5RdV8E

Over the past decade, breakthroughs in the generation and control of ultrafast high-field terahertz (THz) radiation have led to new spectroscopic methodologies for the study of light-matter interactions in the strong-field limit. In this review, we will outline recent experimental demonstrations of non-linear THz material responses in materials ranging from molecular gases, to liquids, to varieties of solids – including semiconductors, nanocarbon, and correlated electron materials. New insights into how strong THz fields interact with matter will be discussed in which a THz field can act as either a non-resonant electric field or a broad bandwidth pulse driving specific resonances within it. As an emerging field, non-linear THz spectroscopy shows promise for elucidating dynamic problems associated with next generation electronics and optoelectronics, as well as for demonstrating control over collective material degrees of freedom.

Abstract-Nonlinear Terahertz Metamaterials via Field-Enhanced Carrier Dynamics in GaAs

Kebin Fan¹, Harold Y. Hwang², Mengkun Liu³, Andrew C. Strikwerda³, Aaron Sternbach³, Jingdi Zhang³, Xiaoguang Zhao¹, Xin Zhang¹, Keith A. Nelson², and Richard D. Averitt^3 
1Department of Mechanical Engineering, Boston University, 110 Cummington Street, Boston, Massachusetts 02215, USA
²Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

We demonstrate nonlinear metamaterial split ring resonators (SRRs) on GaAs at terahertz frequencies. For SRRs on doped GaAs films, incident terahertz radiation with peak fields of ∼20–160  kV/cm drives intervalley scattering. This reduces the carrier mobility and enhances the SRR LC response due to a conductivity decrease in the doped thin film. Above ∼160  kV/cm, electric field enhancement within the SRR gaps leads to efficient impact ionization, increasing the carrier density and the conductivity which, in turn, suppresses the SRR resonance. We demonstrate an increase of up to 10 orders of magnitude in the carrier density in the SRR gaps on semi-insulating GaAs. Furthermore, we show that the effective permittivity can be swept from negative to positive values with an increasing terahertz field strength in the impact ionization regime, enabling new possibilities for nonlinear metamaterials.

© 2013 American Physical Society