Optical processes in a resonant three-level system, using quantum interference as a key, resulted in a wide variety of interesting phenomena, ranging from electromagnetically induced transparency (EIT), through ultraslow light, to manipulation of quantum coherence at the single-photon level. The extention of an equivalent three-level system to far-off resonance will allow various applications to use it as the light source including collinear generation of broad sidebands, ultrashort pulse generation, and the like.

　The aim of this project is to generate an ultrahigh-repetition-rate train of pulses at a practical level by producing a high-density molecular ensemble （〜10²⁰cm^-3） based on the far off-resonant three-level system that can vibrate and/or rotate coherently. By applying this principle, we have produced a 10-THz-repetition-rate train of ultrashort pulses with an effective duration of 12 fs and a high peak power of greater than 2 MW.

　By further development of these technologies, we also aim to generate a 10-THz-repetition-rate train of monocycle pulses with a fixed career-envelope-offset and a career-envelope-phase. We expect that these technologies will open the way to research areas centering on high-precision, high-repeatability optical pulses in Quantum Electronics field, such as adiabatic production of high-density coherent phonons, quantum localization in excited Rydberg states, and the generation of monochromatic terahertz wave with a high efficiency.

Figure 1 Figure 1 shows spectra of high-order Raman sidebands generated in parahydrogen by applying the principle discussed above. This clearly demonstrates that spectra of ultra-broadband sidebands from infrared to near vacuum-ultraviolet can be generated efficiently and collinearly without restriction by the phase-matching condition. Figures 1(a), 1(b), and 1(c) show, respectively, excitation of the pure vibrational transition (125 THz) of parahydrogen, the pure rotational transition (10 THz), and both vibrational (125 THz) and rotational (10 THz) transitions simultaneously.

Figure 1.

Figure 2 Figure 2 shows the autocorrelation trace of 10-THz-repetition train of ultrashort pulses realized by Fourier-synthesizing the Raman sidebands shown in Figure 1(b). It should be emphasized that the observed trace is not averaged. The thin solid line and inserted graph show, respectively, the autocorrelation trace given by the intensity waveform (poulse duration: 12 fs) and the beam pattern obtained. These results clearly demonstrate that an ultrahigh-repetition-rate train of ultrashort pulses with a high beam quality can be produced thorough this method.

Figure 2.