# Development and Application of Sub-Cycle Mid-Infrared Source Based on Laser Filamentation
## 5 Conclusions  
For several years since the publication of the sub-cycle MIR pulse generation, some intriguing applications of the light source have already been demonstrated by several scientific groups. Thanks to the new waveform characterization scheme, the field oscillation of the sub-cycle pulse has become clear. The details of the generation and characterization of the sub-cycle MIR pulse are summarized in a review paper published by the authors [28].  
In pump-probe experiments, the broadband MIR pulse has been used as a probe pulse so far. The next stage of the application of the light source is to use the pulse as a pump pulse, which would  
Figure 1: Snapshots of the mid-infrared absorption spectrum of acetic acid during the chemical reaction with metallic magnesium.  
Figure 2: **(a)** A typical electric field of the sub-cycle mid-infrared pulse measured by the use of frequency-resolved optical gating capable of carrier-envelope phase determination; **(b)** When the delay between the fundamental and second harmonic pulses is changed by \(\sim\)200 nm, the phase of the field changes by \(\pi\).  
initiate interesting nonlinear phenomena. The well-characterized sub-cycle-oscillation field is ideal for studies of field-sensitive attosecond phenomena. In particular, high harmonic generation in solids is one of the most interesting applications of the light source.  
Needless to say, increase in the pulse energy of the sub-cycle pulse is important for using the pulse as a pump pulse. Unfortunately, the intensity of the MIR pulse (\(\sim\)0.5 uJ) is too low for most of the experiments related to high-field physics in general. In particular, the ring-shaped beam profile makes it difficult for high intensity to be the focus.  
However, if the beam for the generation of the sub-cycle pulse has a wavelength longer than that of Ti:sapphire lasers, the beam profile should be improved. According to our simple calculations, better beam quality should be achieved due to the smaller phase mismatch for the longer pump wavelength [28]. At the same time, the smaller phase mismatch will result in better conversion efficiency. Recently, ultrabroadband MIR supercontinuum generation though filamentation based on a Yb laser system was demonstrated [29]. The beam profile was still ring shaped. If we can use even longer wavelength, namely \(\sim\)2 uJ, the beam profile and conversion efficiency would become better.  
We believe that the light source has great potential for various applications and can contribute to the progress in ultrafast laser science and technology.  
The authors thank [PERSON] of the Institute for Molecular Science in Japan and [PERSON] of Chimie ParisTech in France for supporting the measurement and analysis of the data concerning the ATR experiment. This work was supported by Consortium for Photon Science and Technology.  
[PERSON] and [PERSON] designed and performed the ATR experiment. [PERSON], [PERSON] and Y.N. contributed to the FROG-CEP experiment. All the authors discussed the results and contributed to the final manuscript.  
The authors declare no conflict of interest.