INTRODUCTION

Terahertz (THz) radiation (0.1–30 THz) is between the microwave and infrared electromagnetic ranges and have photon energies close to the Fermi level and the peak electric and magnetic strengths about or above megavolts per centimeter and teslas, respectively [1, 2]. Furthermore, its picosecond/subpicosecond time resolution allows its application in time-resolved spectroscopy and imaging. Thus, THz radiation can be considered as a unique source of “cold light” and opens a new field of studying the interaction of light with matter.

Terahertz frequencies correspond to phonon and magnon vibrations of numerous strongly correlated electron and magnetic systems. Thus, intense THz fields at a certain frequency can coherently and resonantly transfer the energy to vibrations of the crystal lattice of a material, thus exciting new electronic configurations and opening possibilities to obtain new states of matter. This initiated a new investigation field known as lightwave quantum electronics, which is oriented to applications in quantum information processing. In addition, THz pulses can orient molecules and, therefore, control numerous catalytic processes in chemical technologies [3]. Terahertz pulses with a strong field can flip the spin of an electron and ensure the nonlinear control of the spin, providing a foundation for future ultrafast spintronics devices [4]. In combination with scanning probe microscopy, intense field pulses of THz radiation can generate a tunneling current at the tip of a scanning tunnel microscope, allowing one to overcome the diffraction limit of a light wave in the THz range and providing a new powerful tool to control the state of new nanomaterials [5, 6]. Intense THz field pulses can accelerate, compress, and control the parameters of electron bunches in several dimensions, which is expected to ensure the development of compact THz accelerators for application in compact attosecond sources of X-rays [7, 8]. Furthermore, intense THz radiation is used to establish biological safety protocols in the THz range, which is particularly relevant in biological applications [9].

The interaction of laser radiation with matter has for a long time been at the focus of attention in laser physics and nonlinear optics. Most of the nonlinear phenomena in the THz regime at lower frequencies are optical responses such as nonlinear transmission, reflection, and absorption in materials excited by intense THz pulses. In this respect, nonlinear THz studies differ only slightly from optical studies. However, in contrast to the optical range, where nonlinear phenomena is predominantly due to the charge distribution (polarization), the polarization of the medium is not a decisive factor in the THz range because of a longer oscillation cycle. Since certain THz frequencies can allow one to control low-frequency motions such as the rotation of molecules and vibrations of the crystal lattice by means of the interaction with the ion, electron, or spin degrees of freedom, THz radiation can resonantly excite these specific modes under the excitation of a strong THz field. In addition, when the peak amplitude of 1-THz fields exceeds 0.3 MV/cm, the ponderomotive energy of the electron can reach 1 eV, which is higher than the ionization energy of impurity electrons in a semiconductor and can induce impact ionization [2, 10] and the tunneling effect [11].

However, the absence of economic and efficient sources of intense THz radiation with controlled polarization limits the development of nonlinear terahertz photonics. The energy of a single pulse, peak field strength, and peak power of fields of THz radiation sources are still much lower than the respective parameters of superintense femtosecond laser sources of visible and near infrared radiation. The efficiency of generation of intense THz pulses can be significantly increased by using near-and mid-infrared radiation for generation in specially oriented nonlinear organic crystals [1, 12], in single-frequency fields through the ionization mechanism of perturbation of low-frequency photocurrents [13], and under the violation of the process symmetry in the two-color scheme of double-frequency generation in air [14]. In the first case, the generation of THz radiation is the most efficient when 1.2- to 1.5-μm pump radiation is used. In the second case, the use of 2- to 3-μm radiation increases the efficiency of the two-color generation of THz radiation by several orders of magnitude to 5–7% [15, 16].

Thus, since the development of methods for the efficient generation of intense THz radiation with controlled spectral–temporal properties is relevant, intense THz radiation is generated in this work in organic crystals pumped by 1.24-μm pump femtosecond laser radiation of a chromium forsterite laser system, the spectral–temporal distributions of radiation field are analyzed, it is shown that the spectrum of THz radiation can be controlled by chirping the pump pulse, and this effect is explained in the numerical simulation of the generation process.

SIMULATION OF THE GENERATION OF TERAHERTZ RADIATION

To describe the experimental results obtained for the generation of terahertz radiation in the DAST optical crystal pumped by chirped laser radiation, we performed a numerical simulation based on the solution of the system of truncated equations describing three-wave interaction. Sum and difference generation processes were considered in the slowly varying amplitude approximation [17–19] and the plane wave approximation taking into account material dispersion and absorption in the DAST organic crystal. We used a model including second-order nonlinear processes responsible for difference frequency generation (optical rectification), in particular, the cascade effect, as well as the mismatch and dispersion of the group velocities of interacting pulses. The linear and three-photon absorption of optical pump radiation and the linear absorption of THz radiation in the crystal were also taken into account [20].

The optical and THz fields in the slowly varying amplitude approximation can be represented in the form

$$\left\{ \begin{gathered} {{E}_{{{\text{op}}}}}(\omega ,z) = {{A}_{{{\text{op}}}}}(\omega ,z) \cdot {{e}^{{ - ik(\omega )z}}}, \hfill \\ {{E}_{{{\text{THz}}}}}(\Omega ,z) = {{A}_{{{\text{THz}}}}}(\Omega ,z) \cdot {{e}^{{ - ik(\Omega )z}}}. \hfill \\ \end{gathered} \right.$$
(1)

In this case, THz radiation generation in the DAST organic crystal based on optical rectification is described by the system of truncated equations

$$\left\{ \begin{gathered} \frac{{d{{A}_{{{\text{op}}}}}(\omega ,z)}}{{dz}} = - \frac{{{{\alpha }_{{{\text{op}}}}}(\omega ){{A}_{{{\text{op}}}}}(\omega ,z)}}{2} - {{P}_{1}}(\omega ,z) - {{P}_{2}}(\omega ,z), \hfill \\ \frac{{d{{A}_{{{\text{THz}}}}}(\Omega ,z)}}{{dz}} = - \frac{{{{\alpha }_{{{\text{THz}}}}}(\Omega ){{A}_{{{\text{THz}}}}}(\Omega ,z)}}{2} - {{P}_{3}}(\Omega ,z), \hfill \\ \end{gathered} \right.$$
(2)

where \({{A}_{{{\text{op}}}}}\) and \({{A}_{{{\text{THz}}}}}\) are the amplitudes of the optical pump wave and difference frequency wave of the THz range, respectively; \({{\alpha }_{{{\text{THz}}}}}\) and \({{\alpha }_{{{\text{op}}}}}\) are the absorption coefficients of THz and optical pump radiation, respectively; and terms P1, P2, and P3 describing the nonlinear interaction of optical and THz waves have the form

$$\left( {\begin{array}{*{20}{c}} {{{P}_{1}}(\omega ,z) = }&{\frac{{i\omega {{d}_{{{\text{eff}}}}}({{\omega }_{0}})}}{{{{n}_{{{\text{op}}}}}c}}} \\ {}&{ \times \;\int\limits_0^\infty {{{A}_{{{\text{op}}}}}} (\omega + \Omega ,z)A_{{{\text{THz}}}}^{*}(\Omega ,z)} \\ {}&{ \times \;{{e}^{{ - i\left( {k(\omega + \Omega ) - k(\omega ) - k(\Omega )} \right)z}}}d\Omega {\kern 1pt} {\kern 1pt} ,} \\ {}&{} \\ {{{P}_{2}}(\omega ,z) = }&{\frac{{i\omega {{d}_{{{\text{eff}}}}}({{\omega }_{0}})}}{{{{n}_{{{\text{op}}}}}c}}} \\ {}&{ \times \;\int\limits_0^\infty {{{A}_{{{\text{op}}}}}} (\omega - \Omega ,z){{A}_{{{\text{THz}}}}}(\Omega ,z)} \\ {}&{ \times \;{{e}^{{ - i\left( {k(\omega - \Omega ) - k(\omega ) + k(\Omega )} \right)z}}}d\Omega {\kern 1pt} {\kern 1pt} ,} \\ {}&{} \\ {{{P}_{3}}(\Omega ,Z) = }&{\frac{{i\Omega {{d}_{{{\text{eff}}}}}({{\omega }_{0}})}}{{{{n}_{{{\text{THz}}}}}c}}} \\ {}&{ \times \;\int\limits_0^\infty {{{A}_{{{\text{op}}}}}} (\omega + \Omega ,z)A_{{{\text{op}}}}^{*}(\omega ,z)} \\ {}&{ \times \;{{e}^{{ - i\left( {k(\omega + \Omega ) - k(\omega ) - k(\Omega )} \right)z}}}d\omega {\kern 1pt} {\kern 1pt} ,} \end{array}} \right.$$
(3)

where \({{n}_{{{\text{op}}}}}\) and \({{n}_{{{\text{THz}}}}}\) are the refractive indices of optical and THz waves, respectively; \({{d}_{{{\text{eff}}}}}\) is the effective nonlinear coefficient; ω and Ω are the frequencies of the optical pump wave and the difference frequency of the THz range, respectively; \(k(\omega )\) and \(k(\Omega )\) are the respective wavenumbers; c is the speed of light in vacuum; and z is the longitudinal coordinate along the radiation propagation axis.

The simulation was carried out under the assumption that a Gaussian pulse is fed to the input of the crystal with the following initial and boundary conditions:

$$\left\{ \begin{gathered} {{E}_{{{\text{op}}}}}(z = 0,t) = \frac{1}{2}{{E}_{{{{0}_{{{\text{op}}}}}}}}{{e}^{{ - 2\ln (2){{{\left( {\frac{t}{{\Delta {{t}_{{{\text{op}}}}}}}} \right)}}^{2}}}}} + {\text{c}}{\text{.c}}., \hfill \\ {{E}_{{{\text{op}}}}}(t = - T{\text{/}}2) = 0, \hfill \\ {{E}_{{{\text{op}}}}}(t = T{\text{/}}2) = 0, \hfill \\ \end{gathered} \right.$$
(4)

where \({{E}_{{{\text{op}}}}}\) is the field of the optical pulse wave, \(\Delta {{t}_{{{\text{op}}}}}\) is the pulse duration, and T is the width of the time window. The equations were solved numerically using the fourth-order Runge–Kutta method with a spatial resolution of 1 μm [18].

The main characteristics of the DAST crystal used in the simulation were calculated as follows. The refractive index for the 1.24-μm pump wave [21] was calculated by the Sellmeier formula and the refractive index for the difference frequency wave of the THz range was evaluated using the Lorentz oscillator model [22]. The effective nonlinear coefficient \({{d}_{{{\text{eff}}}}}\) used in the simulation was expressed in terms of the refractive index and the electro-optical coefficient. The coefficient \({{d}_{{{\text{eff}}}}}\) in the case of 1.24-μm pump radiation of the chromium–forsterite laser is 314 pm/V. The electro-optical coefficient responsible for optical rectification was taken from [23]. It was assumed that the crystal was oriented to the generation maximum; for this reason, \({{r}_{{{\text{eff}}}}} = {{r}_{{{\text{111}}}}}\) was taken for the electro-optical coefficient \({{r}_{{{\text{eff}}}}}\).

To study the effect of the chirp of the generating pulse on the parameters of THz radiation, this model allowed us to chirp the initial pump pulse by introducing the group delay dispersion into the spectral phase of the pulse, which corresponds to linear chirping [24]. In this case, the Gaussian pulse at the input of the crystal in the time representation has the form

$$E(t) = \frac{{{{E}_{0}}}}{{2{{\gamma }^{{1/4}}}}}{{e}^{{ - \frac{{{{t}^{2}}}}{{4\beta \gamma }}}}}{{e}^{{i\left( {{{\omega }_{0}}t + a{{t}^{2}} - \varepsilon } \right)}}},$$
(5)

where \(\Delta t\) is the initial duration of the bandwidth-limited pulse, \(\phi {\kern 1pt} ''\) is the group delay dispersion, a is the linear chirp, and

$$\left\{ \begin{gathered} \beta = \frac{{\Delta {{t}^{2}}}}{{8{\text{ln}}(2)}}, \hfill \\ \gamma = 1 + \frac{{\phi {\kern 1pt} ''{}^{2}}}{{4{{\beta }^{2}}}}, \hfill \\ a = \frac{{\phi {\kern 1pt} ''}}{{8{{\beta }^{2}}\gamma }}, \hfill \\ \varepsilon = \frac{1}{2}{\text{atan}}\left( {\frac{{\phi {\kern 1pt} ''}}{{2\beta }}} \right). \hfill \\ \end{gathered} \right.$$
(6)

Thus, the input parameters for the simulation of THz radiation generation in the organic crystal were the energy, duration, beam diameter, chirp, and central frequency of the pump pulse, as well as the effective nonlinear coefficient \({{d}_{{{\text{eff}}}}}\), refractive indices calculated by the Sellmeier formula and with the Lorentz oscillator model, and thickness of the studied DAST crystal. The output result was the spectrum of generated THz radiation obtained by means of the Fourier transform of the calculated field of the THz pulse.

RESULTS AND DISCUSSION

A multigigawatt chromium–forsterite laser complex (wavelength of 1.24 μm, energy of 3.5 mJ, and FWHM intensity pulse duration of 100 fs) was used for THz radiation generation in DAST (thickness × diameter = 0.5 × 10 mm), DSTMS (0.12 × 6 mm), OH1 (0.87 × 6 mm), and BNA (0.8 × 6 mm) crystals. Terahertz radiation was recorded by a Gentec QE8SP-B-BC-D0 pyroelectric detector with an ~8 × 8-mm receiving aperture, which guaranteed the detection of the total energy of THz radiation at focusing on a spot with an \(1{\text{/}}{{e}^{2}}\) intensity diameter of ~300–1000 μm on the receiving part. Terahertz radiation was separated from optical radiation using a Tydex LPF23.4 filter. The spectrum of THz radiation was detected by means of the Michelson interferometer scheme with a Tydex HRFZ-Si high-resistivity silicon plate used as a beam splitter.

COMPARISON OF TERAHERTZ RADIATION SPECTRA UNDER GENERATION IN ORGANIC CRYSTALS

Figures 1 and 2 show the first-order correlation function (field correlation function obtained in the Michelson interferometer scheme) and the THz radiation spectrum, which is the Fourier transform of the correlation function. As seen in Fig. 2, the THz radiation spectrum from the DAST, DSTMS, and BNA crystals is much wider (up to 5–6 THz) than that from the OH1 crystal, which is concentrated predominantly near 1 THz. It is noteworthy that THz radiation spectra from the DAST and DSTMS crystals hardly demonstrate a characteristic dip near 1 THz caused by phonon absorption, which can be due to a limited spectral resolution. This dip is more pronounced in spectra from the DAST crystal, which are presented in the next section. It is also noteworthy that the spectra from the DAST, DSTMS, and OH1 crystals are wider than those obtained in [12] for these crystals pumped by chromium–forsterite laser radiation: about 2.5 versus 2 THz, 2.8 versus 0.6 THz, and 1.2 versus 0.8 THz, respectively. According to the results presented below, this effect can be explained by a higher residual chirp of pump radiation in the cited work.

Fig. 1.
figure 1

(Color online) Correlation functions of THz radiation obtained in organic crystals pumped by the multigigawatt chromium–forsterite laser system.

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Fig. 2.
figure 2

(Color online) Terahertz radiation spectra obtained in organic crystals pumped by the multigigawatt chromium–forsterite laser system.

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The Gaussian approximation of the autocorrelation function of the THz radiation field made it possible to determine its FWHM, which was ΔtACF ~ 750 and 670 fs for the DAST and DSTMS crystals, respectively, which corresponds to the duration of the unchirped THz pulse ΔtTHz = ΔtACF/1.41 ~ 530 and 470 fs, respectively (Table 1). The duration of one field cycle of THz radiation with a central frequency of ~3 THz generated in these crystals is estimated at ~330 fs, which indicates that THz radiation with an extremely short duration close to one field cycle is generated in the DAST and DSTMS organic crystals. The duration of the THz pulse from the OH1 crystal was estimated at 1.1 ps, and the central frequency in the THz radiation spectrum for this crystal was 1.5 THz, which corresponds to the period of field oscillations of 670 fs. This indicates that a radiation pulse with a duration close to one field cycle is generated in the OH1 crystal, as well as in the DAST and DSTMS crystals, but with a longer oscillation period. The duration of the THz pulse obtained in the BNA crystal is estimated at 660 fs, which indicates the generation of an about two-cycle field pulse taking into account a central frequency of the spectrum of 3.5 THz and a corresponding field period of 290 fs.

Table 1. Consolidated table with the conversion efficiency and temporal characteristics of generated THz radiation for DAST, DSTMS, OH1, and BNA crystals
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Multigigawatt 1.24-μm laser radiation was used for the first time to pump the BNA crystal and the conversion efficiency reached 0.7%. The measured radiation spectrum from the BNA crystal (Fig. 2) is in good agreement with the results obtained in [25], where the signal wave from a parametric amplifier was used as a pump wave.

Thus, the rough spectral–temporal control of the THz radiation field can be ensured by using the DAST, DSTMS, OH1, and BNA organic crystals for generation.

CONTROL OF THE TERAHERTZ RADIATION SPECTRUM BY CHIRPING THE PUMP PULSE

For a more precise control of the spectral–temporal profile of the THz radiation field generated in crystals, the variation of the chirp of pump radiation can be used. To verify this approach, we carried out experiments on THz radiation generation in the DAST crystal by chirped femtosecond radiation of the chromium–forsterite laser system. The DAST crystal was chosen for the experiment with the chirped pump pulse because it provides both a wide spectrum and the highest conversion efficiency among the used crystals, which ensures maximizing the signal-to-noise ratio in measurements.

The crystal with a wide aperture (10 mm) allowed us to reach a conversion efficiency of about 2% in the efficiency saturation regime, which corresponded to the energy of the THz pulse of about 70 μJ (Fig. 3) immediately behind the crystal taking into account the absorption of a fraction of the energy in the Tydex LPF23.4 filter (~50%).

Fig. 3.
figure 3

(Color online) Output energy and conversion efficiency versus the pump energy density at a wavelength of 1.24 μm for the DAST crystal.

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The broadening of the THz radiation beam using off-axial parabolas and its subsequent focusing made it possible to obtain the beam waist diameter of 330 μm by the level of 1/e2 of intensity (see Fig. 4), which indicates an electric field of about 15 MV/cm at the beam waist with the use of the total energy (70 μJ) of the THz pulse. Taking into account the absorption of the THz pulse energy in the Tydex LPF23.4 filter (transmission of ~50% for the generated frequency spectrum) and absorption in a dried (a moisture of 30%) atmospheric path ~0.5 m in length (transmission of ~45%), the pulse energy in the waist was ~15 μJ, which corresponds to an attained field of ~7 MV/cm.

Fig. 4.
figure 4

(Color online) Diameter of the terahertz radiation beam versus the longitudinal z coordinate under focusing measured by the knife-edge method [26].

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The achievement of this strong THz radiation field was also confirmed experimentally by the detected blooming effect in a ~300-μm-thick silicon sample with an electron conductivity with a carrier density of ~5 × 1014 cm–3 (Fig. 5). The effect was detected using the z-scan technique [10], where a sample was displaced in the longitudinal direction near the waist by a micrometer stage and the energy of THz radiation transmitted through the sample was measured at each sample position. According to Fig. 5, transmission in the position of the sample at the waist was more than doubled from 4.5 to 10%. Thus, the detected induced transperency effect confirms that an intense THz radiation field, which is enough to observe nonlinear optical effects in the THz wavelength range, is achieved under the generation in the DAST crystal.

Fig. 5.
figure 5

(Color online) Transmittance of the silicon sample with the electron conductivity versus the longitudinal z coordinate of the sample near the waist.

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A change in the shape of the THz radiation spectrum under the variation of the chirp of the pump pulse at a wavelength of 1.24 μm is presented in Fig. 6. The experimental spectra in Fig. 6 indicate that the chirping of the pump pulse affects the width and central wavelength of the THz radiation spectrum. A positive chirp and the corresponding change in the duration of the laser pulse from spectrally limited (~100 fs) to chirped (~360 fs) lead to the change in both the central wavelength of THz radiation from 3 to 1.5 THz and the spectral width from ~2.5 to ~1 THz. To explain this effect, we performed the numerical simulation based on the solution of the system of truncated equations describing three-wave mixing processes (see Fig. 7, details are given above).

Fig. 6.
figure 6

(Color online) Experimental THz radiation spectra generated in the optical rectification of the 1.24-μm chirped chromium–forsterite laser pulse in the DAST crystal.

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Fig. 7.
figure 7

(Color online) Terahertz radiation spectra generated under the optical rectification of the 1.24-μm chirped chromium–forsterite laser pulse in the DAST crystal calculated using truncated equations for three-wave mixing.

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The simulation of THz radiation generation shows that THz radiation in the range up to 8 THz with two spectral maxima at 0.4 and 2 THz can be generated in the 500-μm-thick DAST crystal pumped by the spectrally limited pulse of the chromium–forsterite laser (~100 fs). Radiation at frequencies above 6–8 THz is strongly absorbed in atmospheric paths; for this reason, the intensity of radiation generated in this frequency range predicted in the simulation is higher than the experimental value. The 2-THz maximum can be due to a better match of the velocities of the pump and 2-THz radiation pulses because the coherence length in this case is equal to ~500 μm, which coincides with the length of the used crystal, and the dip near 1 THz can be attributed to the absorption line in this frequency range in the DAST crystal. The chirping of the pump pulse reduces both the width of the spectrum of generated THz radiation and the spectral amplitude of the 2-THz component, which is observed in both simulated and experimental spectra. A similar evolution of the spectrum of generated THz radiation was observed in the case of negative chirping (introduction of a negative group delay dispersion) to 300 fs. Achieved agreement between the theory and experiment indicates that the observed effects are due to a change in the phase matching width of degenerate difference frequency generation because the spectral width of THz radiation in the case of chirped pump pulses is determined by the instantaneous time overlapping of spectral components of pump pulses in the nonlinear crystal.

The spectra in Fig. 6 indicate that chirping reduces the spectral intensity at all frequencies of the radiation spectrum. The decrease in the spectral intensity is larger at high frequencies, which leads to a change in the shape of the spectrum, which is narrowed and its central frequency is shifted. In other words, since the degree of suppression of conversion depends on the frequency in the THz range under the chirping of pump radiation, the shape of the THz radiation spectrum changes, which allows the spectral rearrangement.

Although such a rearrangement reduces the energy of the THz pulse, the chirping of the pump pulse to control the shape of the THz radiation spectrum makes practical sense even despite the possibility of controlling the spectral composition of radiation by means of THz filters. First, a significant advantage of the chirping-induced spectral rearrangement is the possibility of a smooth change in the shape of the spectrum, which is hardly possible with THz filters. Second, a decrease in the transformation efficiency under the chirping of the pump pulse can be smaller than that in the case of spectral control by THz filters. Indeed, the chirping of the pump pulse to 360 fs reduced the width of the spectrum by a factor of ~2.5 (Fig. 6) and the pulse energy by a factor of ~6. The same narrowing of the spectrum by a factor of 2.5 with the use of a commercially available Tydex LPF3.2 low-frequency filter with the minimum available cutoff frequency of 3.2 THz (94 μm) would be accompanied by the reduction of the energy by a factor of ~2–2.5 because of the symmetry of the spectrum with respect to 3 THz and additionally by a factor of ~6 because of the imperfect spectral transmission curve; thus, the energy of THz radiation under such narrowing of the spectrum decreases in total by a factor of 12–15, which is ~2–2.5 times larger than that in the case of the use of the chirped pump pulse. The chirping of the pump pulse for the rearrangement of the THz radiation spectrum makes practical sense because it ensures a smaller decrease in the output energy of THz radiation than commercially available low-frequency filters and allows one to smoothly change the shape of the spectrum.

CONCLUSIONS

To conclude, the spectra of terahertz radiation generated in the DAST, DSTMS, OH1, and BNA organic crystals pumped by 1.24-μm femtosecond laser radiation of a chromium–forsterite laser have been comparatively analyzed. Terahertz radiation in the BNA organic crystal pumped by multigigawatt 1.24-μm radiation has been generated for the first time and a transformation efficiency of 0.7% has been reached. It has been shown that the indicated crystals allow one to roughly control the spectral–temporal properties of generated terahertz radiation and to vary the width and position of the spectrum of generated radiation, as well as the duration and number of cycles of the THz pulse field.

It has also been shown that a finer control of the spectral–temporal properties of intense THz radiation can be ensured by chirping the pump pulse, which results in the shift and narrowing of the spectrum of generated THz radiation because of a change in the phase matching width of the degenerate generation of difference terahertz frequency.

To summarize, approaches to the generation of intense tunable few-cycle THz radiation in organic crystals have been proposed for the first time. In combination with nonlinear optical methods of the broadening of the spectrum of laser radiation [27–29], they provide the foundation for the creation of a superbroadband source of coherent radiation from the ultraviolet to terahertz range, which will open a way to time-resolved multispectral spectroscopy of transient processes in matter in the femtosecond to attosecond time range. The generated intense terahertz field also allows the investigation of the generation of high- and low-order harmonics by femtosecond near- and mid-infrared radiation [30, 31] in the presence of an intense quasistatic terahertz field.