1. Introduction

High harmonic generation (HHG) in atoms and molecules has been explored broadly in the past decade. Many new physical phenomena in the high-field interaction are discovered, and a robust ultrafast coherent radiation source in the XUV region has been developed [1,2]. In the semiclassical three-step model of HHG, active electrons first tunnel through the potential barrier, are then accelerated by laser field, and finally recombine with parent ions to emit high-energy photons. This process can provide an approach to probe the electronic and nuclear dynamics of atoms and molecules with the unprecedentedly temporal resolution, and also offer an insight into the transient molecular structure [3–7] since the harmonic emission depends on the molecular structure.

The theoretical calculations [8,9] show that the molecular HHG is sensitive to the laser-induced vibrational motion, which represents that the more intense harmonics are generated in the heavier isotopes and the harmonic yield from the fixed nuclear is more intense than from the moving nuclear in molecules. With the development of the high-energy and broadband-tunable infrared parametric source [10–12] for HHG, the driving laser wavelength can be tuned to the infrared absorption wavelength of the molecular vibration, and it gives a new approach to investigate the nuclear dynamics in the molecular HHG. In our previous work [13], we have experimentally observed that the harmonic yield from CH₄ molecules is much weaker and more sensitive to the driving laser wavelength around the resonant absorption than that from Xe atoms with the comparable ionization potential at the same laser condition. It shows that the molecular HHG is too weak to identify the cut-off energy and the spectrum peak when the driving wavelength is near or above the infrared absorption wavelength. As comparison, the atomic HHG at the same laser condition is strong and normal. In order to verify the wavelength effect on the molecular HHG, we further investigate the molecular HHG by using a two-color scheme.

In this work, we experimentally investigate the HHG from CH₄ molecules and Xe atoms in a two-color field by controlling the time delay between the two color laser pulses or tuning the laser intensity of the longer wavelength component. The suppression of the HHG from CH₄ molecules is researched, and the enhancement of the HHG from Xe atoms at the same laser condition is also observed as comparison. These results indicate that the longer wavelength component around the molecular infrared absorption can destructively suppress the HHG from CH₄ molecules. Finally, we experimentally demonstrate the wavelength effect on the molecular HHG in the two-color field by tuning the longer wavelength component from 1500nm to 1900nm.

2. Experiment and discussion

2.1 Experimental setup

The experimental setup is shown in Fig. 1(a) . A commercial Ti:sapphire femtosecond laser (Coherent Inc.) is used in this experiment, which can produce 8mJ/1kHz laser pulses at 800nm center wavelength with 40fs pulse duration. The output pulses are split into two beams. One beam (6mJ/1kHz) is used to pump a home-build infrared optical parametric amplifier (OPA) [12], which can produce 1.0mJ/1kHz tunable pulses as the longer wavelength component. The central wavelength of the longer wavelength component can be tuned from 1500nm to 1900nm with ~45fs pulse duration each, and the full spectral width at half maximum intensity is about 120~180nm, which is shown in Fig. 1(b). The other beam (2mJ/1kHz) with 800nm central wavelength is used as the main driving pulses for generating the high harmonics. After a time delay line, the two color beams with the same parallel polarization are collimated and combined collinearly to be focused into a gas cell (the focal length of the lens is 150mm, and the length of the gas cell is ~2.5mm) located in a high-vacuum interaction chamber. The focal size of the tunable longer wavelength component is ~150µm and the Raleigh length is ~5mm, with the average intensity estimated to be ~0.6 × 10¹⁴ W/cm² in the interaction region within the gas. The intensity of the tunable longer wavelength component is too weak to generate HHG in gas sell. The focal size of the other 800nm component is ~100µm and the Raleigh length is ~2.5mm, with the average intensity estimated to be ~1.2 × 10¹⁴ W/cm² in the interaction region. The two focal spots are overlapped in space by adjusting the optical telescope. The stagnation pressure in the gas cell is controlled around 100 torr. The generated high-order harmonics are detected by a home-made flat-field grating spectrometer equipped with a soft-x-ray charge-coupled device camera (CCD, Princeton Instruments, SX 400), and Al foil with 500nm thickness is used in the spectrometer to stop the driving laser. The entire spectrum is imaged to the CCD array and averaged over multiple laser pulses.

 

Fig. 1 (a) Experimental setup. The two output beams are collinearly focused with variable time delay into a gas cell located in a high-vacuum interaction chamber. The generated high-order harmonics are detected by a home-made flat-field grating spectrometer equipped with a soft-x-ray CCD. (b) The tunable OPA spectra from 1500nm to 1900nm.

Download Full Size | PDF

2.2 Experimental results

We first measure the harmonic yields from CH₄ molecules and Xe atoms as functions of the time delay between the two color pulses at the equal gas density driven by the same two-color field with the longer wavelength component tuned to 1700nm. The measured harmonic yields are shown in Figs. 2(a) , 2(b). The x-axis is the time delay between the two color pulses, and the positive value means that the tunable longer wavelength component is in prepropagation. The y-axis is the harmonic spectrum. The harmonic spectra shown in Figs. 2(c), 2(d) are the lineouts of Figs. 2(a), 2(b) at the time delay of −100fs (red dotted lines, beyond the superposition of the two color pulses), −50fs (green dashed lines, at the edge of the superposition) and 0fs (black solid lines, at the center of the superposition). We unambiguously observe that the harmonic yield from CH₄ molecules is suppressed within the superposition of the two color pulses (from −50fs to + 50fs) while the harmonic yield from Xe atoms at the same laser condition is enhanced. Moreover, we measure the harmonic yield using only one laser beam respectively to ensure that the harmonic signal is mainly generated by the 800nm beam, and the laser intensity of the longer wavelength beam is too weak to generate the HHG in Xe or CH₄ gas.

 

Fig. 2 Measured harmonic yields as functions of the time delay between the two color pulses from (a) CH₄ molecules and (b) Xe atoms at the equal gas density driven by the same two-color field with the longer wavelength component tuned to 1700nm. (c)-(d) The harmonic spectra are the lineouts of (a)-(b) at the time delay of −100fs (red dotted lines), −50fs (green dashed lines) and 0fs (black solid lines).

Download Full Size | PDF

In order to further investigate the effect from the longer wavelength component, we also measure the harmonic yields from CH₄ molecules and Xe atoms as functions of the laser intensity of the longer wavelength component at the time delay of 0fs within the superposition of the two color pulses shown in Fig. 3 . The laser intensity of the 800nm component is kept constant. It is shown that the HHG from CH₄ molecules become weak and being suppressed with the increasing laser intensity of the longer wavelength component, while the HHG from Xe atoms is being enhanced and broadened with the increasing laser intensity of the longer wavelength component. They indicate that the 1700nm component can destructively suppress the HHG from CH₄ molecules.

 

Fig. 3 Measured harmonic yields as functions of the laser intensity of the 1700nm component at the time delay of 0fs from (a) CH₄ molecules and (b) Xe atoms. The laser intensity of the 800nm component is kept constant.

Download Full Size | PDF

In our previous work, we have investigated the wavelength effect on the HHG from CH₄ molecules driven by only one laser field of the tunable infrared source [13], which shows that the HHG from CH₄ molecules is sensitive to the driving infrared wavelength. In the following experiment, we further demonstrate the wavelength effect on the molecular HHG by using a two-color scheme (the 800nm component and the tunable longer wavelength component from 1500nm to 1900nm) shown in Fig. 4 . They also show that the harmonic yield from CH₄ molecules is sensitive to the longer wavelength component. The wavelength dependences at the different relative time delay of 0fs, 50fs and 100fs are shown in Figs. 4(d)–4(f).

 

Fig. 4 Measured harmonic yields as functions of the time delay between the two color pulses from CH₄ molecules driven by the longer wavelength component tuned to (a) 1500nm, (b) 1700nm, and (c) 1900nm. (d)-(f) The wavelength effect on the harmonic spectra at the different time delay of (d) 0fs, (e) 50fs and (f) 100fs, which are the lineouts of (a)-(c).

Download Full Size | PDF

2.3 Discussion

The infrared absorption is attributed to the stretching and bending vibration of methane together with the interactions between the stretching and the bending modes. The maximum absorption peak of methane is the characteristic absorption peak of 3300nm, and the other higher absorption peaks are mainly focused on the wavelength from 1600nm~1800nm, such as the vibrational excitations of the 2v ₁, 2v ₃ and v ₁ + v ₃ mode [14]. Therefore, there is a resonance between the infrared photon energy and one of the molecular vibrational energies when the CH₄ molecules are driven by the OPA laser tunable from 1500nm to 1900nm. The infrared laser around the absorption resonance can excite the higher molecular vibrational state even dissociation [15]. The excitation and the dissociation maybe together affect the HHG process in our experiment.

Macroscopically, we mainly compare the suppression of the molecular HHG with the enhancement of the atomic HHG at the same laser condition, and focus on the phenomenon that the longer wavelength component around the molecular infrared absorption can suppress the molecular HHG. The differences between CH₄ molecules and Xe atoms are obvious in these experiments. For CH₄ molecules, we have observed the suppression of the molecular HHG not only through the temporal overlap shown in Fig. 2 but also through the increasing intensity of the longer wavelength component shown in Fig. 3. For Xe atoms, the two-color field effect on the harmonic yield is normal and expected [16–20], and the enhancement and broadening of the atomic HHG can be attributed to the two-color field effect and the increasing electric field intensity.

Microcopically, the detailed features of the atomic HHG are complex and unstable when the relative time delay or the laser intensity is varied. There are many factors affecting the detailed features of the atomic HHG, especially in the two-color field with multi-cycle duration. The harmonic yield is related not only to the laser intensity but also to the ionization ratio and the relative phase-matching between the two color fields. The optimal laser intensity, the optimal ionization ratio and the optimal relative phase-matching wouldn't together appear at the perfect temporal overlap of 0fs. In our experiment, one can see that the maximum harmonic yield appears at the relative time delay of ~50fs shown in Fig. 2(b). For the broadening and enhancement features, the ionization ratio and the relative phase-matching may play key roles on such features; for the continuum feature, it would be attributed to the multi-cycle averaging and the temporal averaging due to that our pulse duration contains multiple cycles. Furthermore, when the two color pulses are beyond the superposition, the harmonic yield is mainly from the 800nm laser field (the photon energy of the HHG is nhυ₀); when the two color pulses are close to the superposition, the harmonic yield is generated by the compositive two-color field (the photon energy of the HHG is n₁hυ₀ + n ₂hυ_IR, so there exhibits more harmonic orders). Although the detailed features in atomic HHG are complex, they don’t affect the conclusion obtained from CH₄ molecules through the comparison between molecules and atoms.

These observations are helpful for the future experiment on such molecular system and important for the theoretical study on this topic. According to the calculations of the nuclear motion effect on the molecular high-order harmonics [8,9], one knows that the molecular HHG should be sensitive to the nuclear motion induced by the laser field. Furthermore, the nuclear resonance induced by the infrared absorption is an intensive nuclear motion, and this intensive motion must play a key role on the molecular HHG. Our theoretical work in this field has made progress on the simplest molecule H₂ which shows that the molecular HHG is intensively dependent on the driving laser wavelength [21]. However, due to the complexity of CH₄ molecule, we give a qualitative explanation and attribute such phenomena to the nuclear resonance induced by the molecular infrared absorption. A more comprehensive theory, considering the wavelength effect and the molecular infrared absorption, is necessary for fully account for the underlying physics.

3. Summary

In conclusion, we experimentally compare the harmonic yield from CH₄ molecules with that from Xe atoms in the two-color field with the same laser condition, and simultaneously observe the suppression of the HHG from CH₄ molecules and the enhancement of the HHG from Xe atoms through by controlling the temporal overlap of the two color pulses or tuning the laser intensity of the longer wavelength component. The wavelength effect on the molecular HHG in the two-color field is also demonstrated by tuning the longer wavelength component from 1500nm to 1900nm. The results indicate that the longer wavelength component around the molecular infrared absorption can destructively suppress the molecular HHG.