Top Curr Chem (Z) (2018) 376:6
1 3
biologic systems such as water [5], proteins [6], and DNA [7]. With visible light,
multidimensional electronic spectroscopies [8] have been applied to probe the highorder correlations of excitons in quantum wells [9, 10] and light-harvesting complexes [11–13]. Recently, 2D electronic spectroscopy has been extended across the
visible spectrum and into the ultraviolet regime [8, 14]. In the most common 2D
spectroscopy experiments, three resonant interactions between incident light fields
and the sample generate a nonlinear signal field that is fully characterized through
heterodyne mixing with a reference or local-oscillator field and either time- or frequency-resolved measurement of the superposition. By varying the relative time
delays and relative phases between incident fields and measuring the effects on
the signal field, dynamical information about sample coherences and populations,
couplings between different modes, and a great deal more can be learned. Variation of the time interval between two phase-coherent incident fields provides a second dimension along which the nonlinear signal can be measured, usually displayed
along a second frequency axis after Fourier transformation of the signal as a function of the relative (inter-pulse) delay. The results often reveal features that remain
hidden in conventional 1D linear spectra.
The terahertz (THz) regime of the electromagnetic spectrum is overlapped with
a rich variety of material degrees of freedom including rotations of polar gas molecules, lattice vibrations in solids, molecular dynamics of liquids, spin dynamics in
materials with magnetic order and/or high magnetic anisotropy, and many others [15].
Linear THz time-domain spectroscopy allows access to both the amplitude and phase
of the THz electric field after its interaction with the sample, from which the real and
imaginary parts of the dielectric function can be retrieved simultaneously without
referring to the Kramers-Kronig relation [15]. Linear THz time-domain spectroscopy
has found wide-ranging applications in the characterization of molecular and material
systems relevant to chemistry, biology, and physics. In such experiments, THz pulses
are typically generated by femtosecond laser pulses that either undergo optical rectification (OR) in nonlinear optical crystals, such as zinc telluride (ZnTe), gallium phosphate (GaP), or gallium arsenide (GaAs), or that generate ultrafast photocurrent in
biased photoconductive antennas [15]. The achievable electric field strength is generally less than 10 kV/cm [16], which in most samples precludes any significant nonlinear light-matter interactions. The absence of strong tabletop THz sources placed early
THz spectroscopy exclusively in the linear response region [17–19].
With the advent of strong tabletop THz sources, the subfield of nonlinear THz
spectroscopy has been growing rapidly in the past decade. Nowadays, intense singlecycle THz pulses spanning the low THz region between 0.1 and 3 THz with pulse
energies of several μJ and electric field strengths as high as 1 MV/cm (corresponding to magnetic field strengths of 0.33 T) have been routinely generated by OR of
Ti:sapphire laser pulses in lithium niobate (LiNbO 3 ) crystals [20, 21]. THz pulses
of 10 MV/cm electric fields (corresponding to multi-tesla magnetic fields), higher
center frequencies, and broader bandwidths have been recently demonstrated by
OR of near-IR fs pulses with center wavelengths between 1.2 and 1.5 μm in novel
organic nonlinear optical crystals [22–24]. The rapid development of intense THz
sources has enabled coherent THz spectroscopy and control over electronic, orbital,
lattice, and spin degrees of freedom in various systems [25, 26].
276
Reprinted from the journal
1 3
biologic systems such as water [5], proteins [6], and DNA [7]. With visible light,
multidimensional electronic spectroscopies [8] have been applied to probe the highorder correlations of excitons in quantum wells [9, 10] and light-harvesting complexes [11–13]. Recently, 2D electronic spectroscopy has been extended across the
visible spectrum and into the ultraviolet regime [8, 14]. In the most common 2D
spectroscopy experiments, three resonant interactions between incident light fields
and the sample generate a nonlinear signal field that is fully characterized through
heterodyne mixing with a reference or local-oscillator field and either time- or frequency-resolved measurement of the superposition. By varying the relative time
delays and relative phases between incident fields and measuring the effects on
the signal field, dynamical information about sample coherences and populations,
couplings between different modes, and a great deal more can be learned. Variation of the time interval between two phase-coherent incident fields provides a second dimension along which the nonlinear signal can be measured, usually displayed
along a second frequency axis after Fourier transformation of the signal as a function of the relative (inter-pulse) delay. The results often reveal features that remain
hidden in conventional 1D linear spectra.
The terahertz (THz) regime of the electromagnetic spectrum is overlapped with
a rich variety of material degrees of freedom including rotations of polar gas molecules, lattice vibrations in solids, molecular dynamics of liquids, spin dynamics in
materials with magnetic order and/or high magnetic anisotropy, and many others [15].
Linear THz time-domain spectroscopy allows access to both the amplitude and phase
of the THz electric field after its interaction with the sample, from which the real and
imaginary parts of the dielectric function can be retrieved simultaneously without
referring to the Kramers-Kronig relation [15]. Linear THz time-domain spectroscopy
has found wide-ranging applications in the characterization of molecular and material
systems relevant to chemistry, biology, and physics. In such experiments, THz pulses
are typically generated by femtosecond laser pulses that either undergo optical rectification (OR) in nonlinear optical crystals, such as zinc telluride (ZnTe), gallium phosphate (GaP), or gallium arsenide (GaAs), or that generate ultrafast photocurrent in
biased photoconductive antennas [15]. The achievable electric field strength is generally less than 10 kV/cm [16], which in most samples precludes any significant nonlinear light-matter interactions. The absence of strong tabletop THz sources placed early
THz spectroscopy exclusively in the linear response region [17–19].
With the advent of strong tabletop THz sources, the subfield of nonlinear THz
spectroscopy has been growing rapidly in the past decade. Nowadays, intense singlecycle THz pulses spanning the low THz region between 0.1 and 3 THz with pulse
energies of several μJ and electric field strengths as high as 1 MV/cm (corresponding to magnetic field strengths of 0.33 T) have been routinely generated by OR of
Ti:sapphire laser pulses in lithium niobate (LiNbO 3 ) crystals [20, 21]. THz pulses
of 10 MV/cm electric fields (corresponding to multi-tesla magnetic fields), higher
center frequencies, and broader bandwidths have been recently demonstrated by
OR of near-IR fs pulses with center wavelengths between 1.2 and 1.5 μm in novel
organic nonlinear optical crystals [22–24]. The rapid development of intense THz
sources has enabled coherent THz spectroscopy and control over electronic, orbital,
lattice, and spin degrees of freedom in various systems [25, 26].
276
Reprinted from the journal