# Development and Application of Sub-Cycle Mid-Infrared Source Based on Laser Filamentation
## 3 Chirped Pulse Upconversion  
Single-shot detection of the ultrabroadband MIR supercontinuum with reasonable resolution is useful for advanced MIR spectroscopies. One of the most straightforward methods is to use a dispersive MIR spectrometer consisting of a grating and a multichannel MIR detector. However, the bandwidth of this method has been limited to \(\sim\)500 cm\({}^{-1}\) due to the low pixel numbers and low sensitivity of the multichannel MIR detector. In addition, stray light from higher-order diffraction of the grating seriously disturbs broadband detection [17].  
An alternative approach to detect the MIR supercontinuum with a single-shot is optically converting the spectra into the visible region and recording them with a visible spectrometer, which has much higher performance than the MIR spectrometers. Several groups demonstrated single-shot detection of the MIR supercontinuum by the use of chirped-pulse upconversion (CPU) with solid crystals [8; 18; 19; 20; 21]. Although it is possible to have efficient frequency conversion with solid crystals, the bandwidth of the detection is still limited to \(\sim\)600 cm\({}^{-1}\) by the phase-matching condition.  
In order to detect the entire spectrum of the ultrabroadband MIR supercontinuum with single-shot detection, the authors proposed to use an FWM process in a gas medium for the upconversion of the MIR spectrum. An MIR pulse (\(\omega_{0}\)) and a chirped pulse from a Ti:sapphire chirped-pulse amplifier system (\(\omega_{1}\)) are focused into a gas medium and the MIR spectrum is upconverted to the visible (\(\omega_{2}\)) through an FWM process (\(\omega_{1}+\omega_{1}-\omega_{0}\rightarrow\omega_{2}\)). By using the scheme, single-shot detection of the full MIR spectrum spread from 1.7 to 50 \(\upmu\)m (from 200 to 6000 cm\({}^{-1}\)) has been achieved [22].  
The authors combined the chirped-pulse upconversion technique with the femtosecond pump-probe spectroscopy [12; 14] and attenuated total reflection spectroscopy (ATR) [23; 24]. As a demonstration of the ATR combined with CPU, the MIR absorption spectra of acetic acid (CH\({}_{3}\)COOH, >99%) on the ATR prism after adding some magnesium (Mg) were monitored. One thousand spectra were recorded with the interval of 100 ms. Figure 1 shows the measured absorption spectra. The intensities of the absorption lines due to C=O (1700 cm\({}^{-1}\)) and OH (3000 cm\({}^{-1}\)) decrease. In contrast, the absorption lines due to COO\({}^{-}\) stretching (symmetric: 1430 cm\({}^{-1}\), asymmetric: 1550 cm\({}^{-1}\)) and the weak bonding between Mg and acetate (640 cm\({}^{-1}\)) increase. Therefore, it is obvious that the simple chemical reaction of acetic acid with metallic magnesium which causes the formation of magnesium acetate along with the release of hydrogen gas is clearly observed in real-time by monitoring the change of the MIR spectrum. It is also important to note that the authors had to neither repeat the experiment nor average the signal.  
We believe that this unique system is effective for advanced studies in various scientific fields. In the near future, we would like to apply the ATR system to investigate the oxygen generation process of the metallic complex and anion-binding reaction dynamics of proteins.