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Top Curr Chem (Z) (2018) 376:6
Compared to the well-established research areas of multidimensional spectroscopy in the visible and IR frequency ranges, the methodology and application of 2D
THz spectroscopy are still in a nascent stage. In recent years, there have been developments of 2D THz techniques that led to the first demonstrations of 2D multi-THz
and 2D THz spectroscopies in the study of mostly nonresonant electronic and some
lattice vibrational responses with strong nonlinearities in condensed-matter systems.
Examples include carrier dynamics in graphene [27], correlations between carriers
and phonons in quantum wells [27, 28], THz nonlinear frequency mixing in LiNbO 3
[29], and coherent cyclotron resonance nonlinear mixing in a 2D electron gas [30].
Demonstrations of 2D THz spectroscopy more directly relevant to chemistry include
2D THz rotational spectroscopy of molecules in the gas phase [31, 32], 2D THz
vibrational spectroscopy of phonons in a semiconductor [33, 34], and 2D THz magnetic resonance spectroscopy of magnons [35, 36]. In the first of these, THz photon echo signals have been revealed for the first time in the gas phase, and the full
set of THz third-order (χ
(3)
) nonlinear responses has been mapped into 2D spectra.
In the second example, the first demonstration of 2D THz spectroscopy using three
separate pulses has been realized, which reveals strong nonresonant nonlinearity
beyond χ
(3)
responses associated with the two-phonon quantum coherences in the
system. The last example, though conducted on magnons in an antiferromagnetic
crystal, is directly relevant for the development of 2D THz electron paramagnetic
resonance (EPR) spectroscopy and the study of chemical and biologic systems that
have spin resonances in the THz regime. THz pulses combined with optical excitation and detection through nonresonant Raman processes have enabled demonstrations of hybrid 2D THz-Raman spectroscopies. Hybrid 2D THz-Raman studies have
revealed couplings among the intramolecular vibrational modes in halogenated liquids [37, 38], and the intermolecular dynamics of the hydrogen-bond network in
water [39] and aqueous salt solutions [40] through THz photon echoes.
This chapter is organized as follows. In Sect. 2, the basic techniques for intense
THz pulse generation and time-domain signal detection are reviewed. The methods
involving 2D THz spectroscopy in collinear geometry and typical pulse sequences
are discussed. As there are key differences between the techniques used in 2D THz
and 2D IR or visible spectroscopies, we will elaborate the 2D THz methods in detail
and frequently make comparisons with their analogs in the IR and visible. In what
follows, we will review 2D THz rotational spectroscopy, 2D THz vibrational spectroscopy, 2D THz-Raman spectroscopies and 2D THz magnetic resonance spectroscopy in detail. Lastly, a summary and a brief outlook of the directions to which 2D
THz spectroscopy can lead are presented.
2 Methods
2.1 THz Pulse Generation and Signal Detection
As 2D spectroscopies involve nonlinear signal generation and detection, experiments require strong excitation sources and sensitive detection schemes. Intense
THz pulse generation mainly relies on OR of strong fs laser pulses in a nonlinear
277
Reprinted from the journal
Top Curr Chem (Z) (2018) 376:6
Compared to the well-established research areas of multidimensional spectroscopy in the visible and IR frequency ranges, the methodology and application of 2D
THz spectroscopy are still in a nascent stage. In recent years, there have been developments of 2D THz techniques that led to the first demonstrations of 2D multi-THz
and 2D THz spectroscopies in the study of mostly nonresonant electronic and some
lattice vibrational responses with strong nonlinearities in condensed-matter systems.
Examples include carrier dynamics in graphene [27], correlations between carriers
and phonons in quantum wells [27, 28], THz nonlinear frequency mixing in LiNbO 3
[29], and coherent cyclotron resonance nonlinear mixing in a 2D electron gas [30].
Demonstrations of 2D THz spectroscopy more directly relevant to chemistry include
2D THz rotational spectroscopy of molecules in the gas phase [31, 32], 2D THz
vibrational spectroscopy of phonons in a semiconductor [33, 34], and 2D THz magnetic resonance spectroscopy of magnons [35, 36]. In the first of these, THz photon echo signals have been revealed for the first time in the gas phase, and the full
set of THz third-order (χ
(3)
) nonlinear responses has been mapped into 2D spectra.
In the second example, the first demonstration of 2D THz spectroscopy using three
separate pulses has been realized, which reveals strong nonresonant nonlinearity
beyond χ
(3)
responses associated with the two-phonon quantum coherences in the
system. The last example, though conducted on magnons in an antiferromagnetic
crystal, is directly relevant for the development of 2D THz electron paramagnetic
resonance (EPR) spectroscopy and the study of chemical and biologic systems that
have spin resonances in the THz regime. THz pulses combined with optical excitation and detection through nonresonant Raman processes have enabled demonstrations of hybrid 2D THz-Raman spectroscopies. Hybrid 2D THz-Raman studies have
revealed couplings among the intramolecular vibrational modes in halogenated liquids [37, 38], and the intermolecular dynamics of the hydrogen-bond network in
water [39] and aqueous salt solutions [40] through THz photon echoes.
This chapter is organized as follows. In Sect. 2, the basic techniques for intense
THz pulse generation and time-domain signal detection are reviewed. The methods
involving 2D THz spectroscopy in collinear geometry and typical pulse sequences
are discussed. As there are key differences between the techniques used in 2D THz
and 2D IR or visible spectroscopies, we will elaborate the 2D THz methods in detail
and frequently make comparisons with their analogs in the IR and visible. In what
follows, we will review 2D THz rotational spectroscopy, 2D THz vibrational spectroscopy, 2D THz-Raman spectroscopies and 2D THz magnetic resonance spectroscopy in detail. Lastly, a summary and a brief outlook of the directions to which 2D
THz spectroscopy can lead are presented.
2 Methods
2.1 THz Pulse Generation and Signal Detection
As 2D spectroscopies involve nonlinear signal generation and detection, experiments require strong excitation sources and sensitive detection schemes. Intense
THz pulse generation mainly relies on OR of strong fs laser pulses in a nonlinear
277
Reprinted from the journal