Carrier-envelope phase-stable mid-infrared pulses spanning from 5 μm to 11 μm with a pulse energy of 5 μJ were produced by difference frequency generation of two-color near-infrared pulses that were produced in a novel inline optical parametric amplifier. The mid-infrared electric waveform was characterized by electro-optic sampling using 6.5-fs pulses at 620 nm.
© 2016 Optical Society of America
Recent progress in the development of intense long-wavelength sources has opened the doorway to strong-field sub-cycle physics in solids [1,2]. By using intense low-frequency optical fields with photon energy much lower than the band gap energies of solids, extreme nonlinear responses and novel transient phases are expected to emerge in strongly-driven electronic states. Although intense THz pulses can produce a field strength of more than 1 MV/cm , mid-infrared (MIR) pulses can produce much stronger fields because of their better focusability and possibly shorter pulse duration. Thus far, the generation of carrier-envelope phase (CEP)-stable pulses in the MIR region with a field strength of more than 100 MV/cm has been reported by employing difference frequency generation (DFG) in a GaSe crystal of two-color near-infrared (NIR) pulses from independent optical parametric amplifiers (OPAs) . However, fluctuation of the relative path lengths from the OPAs to the DFG crystal degrades the CEPs of the MIR pulses.
Here, we report on a novel inline method to produce intense two-color NIR pulses in a broadband OPA, which we call a dual-wavelength OPA, followed by DFG. The electric waveform of the MIR pulses was characterized by using electro-optic sampling (EOS) with 6.5-fs pulses at 620 nm as a sub-cycle probe .
2. Dual-wavelength OPA
Figure 1 illustrates concept of the dual-wavelength OPA. When broadband seed pulses pass through a dispersive material with dominant third-order dispersion (TOD), the temporal distribution of the seed spectrum becomes parabolic. When such seed pulses are amplified in an OPA with short pump pulses, two discrete spectral components are amplified at the same time. The spectral separation of the amplified components can be controlled by varying the relative delay between the seed and pump pulses. CEP-stable MIR pulses can be generated by using DFG between these two amplified components [6–9]. The inline geometry of this scheme intrinsically ensures the CEP stability as well as the spectral tunability across the gain bandwidth of the OPA.
3.1 Generation of CEP-stable MIR pulses
Figure 2 shows a schematic of the dual-wavelength OPA followed by DFG and the EOS setup. The Ti:sapphire chirped-pulse amplifier system produced 7-mJ, 40-fs pulses at a repetition rate of 1 kHz with a shot-to-shot energy stability of 1.9%, which were divided into four pulses.
One of the four pulses with a pulse energy of 1 mJ was focused into a krypton-filled gas cell to broaden its spectrum by generating a filament. Spectral components from 530 to 700 nm were selected and compressed using several dielectric mirrors, resulting in 6.5-fs pulses with a pulse energy of 5 μJ with a shot-to-shot energy stability of less than 2%. These pulses were used as a sub-cycle probe in the EOS. The other three pulses were used to generate a white light continuum in a 4-mm-thick YAG crystal and pump two-stage OPAs. The NIR components of the white light continuum, which extended to >1800 nm was used as a seed for the following OPA stages. The seed pulses were sent through 300 mm fused silica. Since fused silica has zero group delay dispersion at 1270 nm as shown in Fig. 3(a) , mainly TOD-dominant group delays were induced on the NIR seed pulses. The highly dispersed seed pulses were then amplified as a signal in the two-stage OPAs that employed 1-mm-thick BiB3O6 (BiBO) crystals, which have a broadband gain bandwidth from 1200 to 2200 nm when the pump wavelength is approximately 790 nm [11,12]. Because of the parabolic group delays introduced by the fused silica, two spectral components were selectively amplified by the OPAs that were pumped by 40-fs pulses at 800 nm as shown in Fig. 3(b). An output energy of 500 μJ was obtained after the second OPA stage with a shot-to-shot energy stability of 3.9%. We evaluated the influence of the timing jitter between the pump and seed pulses on the spectral fluctuation of two components. The spectral stability of the short and long spectral components were measured to be 1.1 and 2.9 nm, respectively, which corresponded to a timing jitter of 3.8 fs. A pair of broadband NIR chirped mirrors  was used to compensate the group delay difference between the two spectral components. To achieve type II phase matching for DFG, a special wave plate was used to rotate the polarization of the component at 1200 nm by 90° while retaining the polarization of the 1400-nm component. The NIR pulses were then fed into a 1-mm-thick LiGaS2 (LGS) crystal for DFG [14,15]. The focused intensity at the LGS crystal was estimated to be 200 GW/cm2. A dielectric-coated Ge filter, indicated as LPF in Fig. 2, was placed behind the crystal to remove the co-propagating NIR pulses. The MIR pulse energy after the filter was measured to be 5 μJ. As shown in Fig. 3(a), the tunable range of this method is from 4.6 to 11 μm. The shorter wavelength limit is determined by gain bandwidth of BiBO where the parametric gain drops below 1.2 μm, while the longer wavelength limit is determined by the transmission range of the LGS crystal.
3.2 Characterization of the MIR waveform
The MIR pulses were focused by an off-axis parabolic mirror (f = 100 mm) onto a 15-μm-thick LGS crystal and characterized by EOS using the 6.5-fs pulses at 620 nm. The gray curve in Fig. 4(a) shows a measured waveform without calibration. We calibrated this waveform in the spectral domain by considering two factors: (i) the temporal resolution determined by the pulse duration of the probe pulses and (ii) the temporal walk-off between the probe and MIR pulses inside the LGS crystal. These factors were considered by defining an effective window function w(t) asFig. 4(b). After the Fourier transform of the calibrated spectrum to the time domain without changing the spectral phase, we obtained a calibrated waveform, shown as the red curve in Fig. 4(a). The blue curve in Fig. 4(b) shows the group delay of the MIR waveform. The calibrated spectrum (the red curve in Fig. 4b) agrees well with the spectrum (the black curve in Fig. 4b) measured by a MIR multichannel spectrometer (Infrared Systems Development Corporation, FPAS-6416), which validates the calibration procedure. From the calibrated waveform, the pulse duration was determined to be 70 fs (the full width at half maximum of the intensity distribution). The radius of 1/e2 intensity of the MIR beam was measured by a knife-edge method to be 32 μm, corresponding to a peak electric field amplitude of 56 MV/cm at the focal point. This spot size was about 1.5x diffraction limit.
We also measured the MIR waveforms repeatedly up to ~6 hours by EOS to test the long-term stability. Each scan took 12 minutes, and totally 31 scans were taken. Figure 5(a, b) shows the time evolution of the MIR waveforms and the waveforms of the first and last scans. The MIR waveforms show the drift that corresponds to a time shift of ~5 fs over 6 hours. As can be seen in Fig. 5(b), the two waveforms are almost identical except the time shift. It is likely that this time shift is due to the timing jitter between the MIR and visible probe pulses. This time shift corresponds to the effective CEP shift of 0.57π rad assuming no timing drift in EOS. In the case of DFG between the outputs from two OPAs , the long-term CEP stability was reported to be more than π rad over 1 hour without feedback control. By comparing this work, our inline method has better long-term CEP stability by a factor of ~10.
We have demonstrated the generation of CEP-stable MIR pulses (5-11 μm, 5 μJ, 70 fs, 1 kHz) by intra-pulse DFG of two-color NIR pulses from a dual-wavelength OPA. The MIR electric waveform was measured by EOS using 6.5-fs probe pulses in the visible region. The peak electric field was estimated to be 56 MV/cm. The whole setup can be used to explore strong-field phenomena in solids at fields exceeding 10 MV/cm with a capability of sub-cycle probing using ultrashort optical pulses in the visible.
This research was partially supported by JSPS KAKENHI Grant Numbers 23226003, 25790063, 15K13375, Program for Leading Graduate Schools (MERIT) by Japan Society for the Promotion of Science, and Photon and Quantum Basic Research Coordinated Development Program from MEXT.
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