## 1 Introduction  
The generation of subfemtosecond pulses would extend the horizon of ultrafast measurements to the time scale of electronic motion. Remarkable progress has been made toward generation and characterization of ever-shorter pulses in the short-wavelength spectral region. For example, a single isolated attosecond (as) pulse of 67 as was composed from an extreme UV supercontinuum covering 55-130 eV generated by the double optical gating technique [1]. On the other hand, a few-femtosecond pulse in the optical region would have a great deal of potential in the research of ultrafast science and technology [2, 3]. There have been several techniques developed to achieve sub-cycle ultrashort pulses. For example, a subcycle field transient, which spans the infrared, visible, and ultraviolet spectral regions, was produced with a 1.5-octave three-channel optical field synthesizer by [PERSON] _et al._[4]. The 2.4 fs transient was focused into a krypton-gas cell to trigger sub-femtosecond electron motion. In another approach, based on cascaded four wave mixing (CFWM), [PERSON]'s group reported a high energy multicolored femtosecond pulse generated in a hydrogen-filled gas cell and hollow fiber [5]. Moreover, [PERSON]'s group reported a multi-watt broadband continuous-wave multi-harmonic optical comb based on a frequency division-by-three singly-resonant optical parametric oscillator [6]. Another light source that delivers sub-fs pulses with a spectrum centered on the visible region has been developed and ultimately may lead to optical arbitrary waveform generation (OAWG) [7]. The light source is based on what has been called \"molecular modulation\" [8, 9], which shares visual similarities with the approach for the generation of ultrashort optical pulses proposed by [PERSON] and [PERSON] [10]. It has been predicted that coherent molecular oscillations can produce laser frequency modulation (FM), with a total bandwidth extending over the infrared, visible, and ultraviolet spectral regions, and with the possibility of sub-femtosecond pulse compression [8]. The technique utilizes ideas of electromagnetically induced transparency (EIT) and relies on adiabatic preparation of maximal molecular coherence. The coherence is established by driving the molecular transition with two single-mode laser fields, slightly detuned from the Raman resonance so as to excite a single molecular eigenstate. Molecular oscillation, in turn, modulates the driving laser frequencies, causing the collinear generation of a very broad FM-like spectrum. This broadband light is inherently coherent and allows for sub-femtosecond (attosecond) pulse compression in the visible-UV range. Although attosecond pulses with wavelengths in the extreme ultraviolet and soft-X-ray pulses have been obtained by high-harmonic generation (HHG), the molecular modulation technique has the potential for generating high-energy sub-femtosecond pulses in the soft UV range, which will enable new experiments that exploit electronic resonances in molecules. In recent years, several groups have made substantial advances in molecular modulation. For example, [PERSON]'s group achieved absolute phase control of five discrete optical harmonics (two pump beams and three generated Raman sidebands from \(\mathrm{H_{2}}\) gas), and thus demonstrated the synthesis and measurement of ultrafast waveforms such as square and saw-tooth fields [7]. [PERSON]'s group reported the carrier envelope offset control of octave-spanning Raman comb by using dual-frequency laser radiation locked on a single laser cavity and, simultaneously, its second harmonic [11]. Broadband spectra based on multifrequency Raman generation were also studied in photonic crystal fiber [12] and hollow fibers filled with SF6 [13]. Meanwhile, the molecular modulation method has been extended to the continuous-wave (CW) domain. For example, [PERSON]'s group has studied CW--stimulated Raman scattering (SRS) inside a high-finesse cavity and demonstrated a continuous-wave optical modulator at 90 THz [14].