Lasers in the infrared (IR) band could be easily realized. In contrast, lasers in the blue and UV bands involve significantly larger intervals of energy levels. Disturbed by the intermediate energy levels, it is more challenging to generate lasers of blue and UV bands directly from laser media. Frequency up-conversion can convert long-wavelength laser to short-wavelength laser through a nonlinear process [1]; the four-wave mixing (FWM) process is one of the excellent choices. In atomic and atomic-like ensemble, phase control between FWM and six-wave mixing channels [2], FWM dipole solitons [3], multi-order fluorescence, and spontaneous parametric four-wave mixing (PFWM) [4] were studied theoretically and experimentally, using an extended-cavity diode laser with highly narrow linewidth (∼MHz), electromagnetically induced transparency would be created. Such studies can have important applications in high-precision measurements, coherence quantum control, quantum information processing, and all-optical signal processing. Another typical example is the generation of collimated blue light based on the FWM effect in alkali metal vapors. Due to the low absorption by water [5], [6], blue light can be used for underwater optical communication, underwater target detection, etc. [7]. Considering the advantage of having no resonant cavity, collimated blue rays generated using the FWM process in alkali vapor have attracted extensive attention from researchers [8], [9], [10], [11]. Vernier et al. used 780 nm and 776 nm pump lasers to excite the rubidium (Rb) atoms to the 52D5/2 level; optimizing pump polarizations and frequencies, improved conversion efficiency, and a 1 mW 420 nm collimated light was achieved [12]. Brekke et al. utilized a 778 nm continuous-wave (CW) laser to excite Rb atoms with two-photon and obtained 420 nm collimated light [13]. Schultz et al. used 852 nm and 917 nm dual-wavelength lasers to excite Cs atoms and achieved a 455 nm collimated light [14]. Merlemis et al. used two-photon excitation of K atoms to 6S level and generated a 404 nm collimated violet light through the FWM process [15]. However, FWM is a nonlinear process and requires rigorous phase-matching conditions to achieve frequency up-conversion [16]. The conversion efficiency of FWM can be enhanced by adding a ring cavity [17], [18], but the conversion efficiency is still limited.

In fact, the generation of collimated blue light in the FWM process requires mid-infrared photons from the transition between highly excited states. Akulshin et al. used pump lasers at 780, and 776 nm to stepwise excite the Rb atoms to the 52D5/2 level and a 420 nm collimated light were generated; a 5.23μm amplified spontaneous emission (ASE) was also found. Additionally, the directional IR radiation at 1.37μm (62S1/2 →52P3/2) was also detected in the co-propagating direction, which was generated by the PFWM process with a transition loop of 52P3/2 →52D5/2 →62P3/2 →62S1/2 →52P3/2 [19]. Subsequently, the characteristic of 5.23μm ASEs in the forward and backward directions were investigated, and their spectral dependences were found to be different; this suggested that 5.23μm ASE was a new type of temporally and spatially coherent radiation [20]. Sebbag et al. demonstrated that 5.23μm coherent mid-IR light was generated via PFWM by precisely tuning the frequency of a CW diode laser to excite Rb atoms [21]. Sulham et al. used a ∼778 nm pulsed laser to excite Rb atoms to 7S and 5D levels by two-photon absorption and detected mid-IR radiation (3.85-5.2μm); they also used two-photon excitation of Cs atoms to 9S and 7D levels, when 455 nm collimated blue light was produced, IR radiations of 1.94–2.43μm were also obtained [22]. By further exciting the alkali metal atoms to the high-lying Rydberg states [23], the transition between the highly excited states can also generate far-IR light and even terahertz (THz) radiations [24]. In our previous studies, ∼1.1 THz radiation was generated through Cs atoms, and THz signal features were investigated by collimated yellow and ultraviolet (UV) lights via the FWM process, respectively [25], [26].

Since the alkali metal atoms are excited to a highly excited state, the adjacent energy levels below the upper pump level have no population, then IR or THz ASE are easy to be generated by population inversion. Through the FWM process, multichannel frequency up-conversions could be achieved [27]. When the population of THz and IR ASEs comes from an ordinary upper energy level, they have a direct competitive relationship. If THz frequency was the target radiation, the related IR emission would directly impact the generation of THz radiation, so it would be essential to investigate the competitive mechanism. Direct detection of THz and far-IR lights requires a special detector. In contrast, the collimated UV light generated using the FWM process could be used to study the competition mechanism of THz and far-IR radiations indirectly.

In this work, an experimental study was conducted on the competitive mechanism among multichannel frequency up-conversions generated using the FWM processes. The generation mechanisms of THz and IR radiations and the competition among corresponding collimated UV lights were analyzed. The local competition between THz and IR lights was investigated by intensity variations of collimated UV lights in the phase-matching conditions. This study may have potential applications in all-optical control infrared, terahertz, and ultraviolet devices.