Lind Alexander J, Kowligy Abijith, Timmers Henry, Cruz Flavio C, Nader Nima, Silfies Myles C, Allison Thomas K, Diddams Scott A
Time and Frequency Division, National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305, USA.
Department of Physics, University of Colorado, 2000 Colorado Avenue, Boulder, Colorado 80309, USA.
Phys Rev Lett. 2020 Apr 3;124(13):133904. doi: 10.1103/PhysRevLett.124.133904.
The mid-infrared atmospheric window of 3-5.5 μm holds valuable information regarding molecular composition and function for fundamental and applied spectroscopy. Using a robust, mode-locked fiber-laser source of <11 fs pulses in the near infrared, we explore quadratic (χ^{(2)}) nonlinear optical processes leading to frequency comb generation across this entire mid-infrared atmospheric window. With experiments and modeling, we demonstrate intrapulse difference frequency generation that yields few-cycle mid-infrared pulses in a single pass through periodically poled lithium niobate. Harmonic and cascaded χ^{(2)} nonlinearities further provide direct access to the carrier-envelope offset frequency of the near infrared driving pulse train. The high frequency stability of the mid-infrared frequency comb is exploited for spectroscopy of acetone and carbonyl sulfide with simultaneous bandwidths exceeding 11 THz and with spectral resolution as high as 0.003 cm^{-1}. The combination of low noise and broad spectral coverage enables detection of trace gases with concentrations in the part-per-billion range.
3至5.5微米的中红外大气窗口包含有关分子组成和功能的宝贵信息，对基础光谱学和应用光谱学都很重要。利用一个强大的、锁模光纤激光源产生近红外波段脉宽小于11飞秒的脉冲，我们探索二次（χ(2)）非线性光学过程，以在整个中红外大气窗口产生频率梳。通过实验和建模，我们展示了通过周期性极化铌酸锂单次通过实现的脉内差频产生，从而产生少周期中红外脉冲。谐波和级联χ(2)非线性进一步提供了直接获取近红外驱动脉冲序列的载波包络偏移频率的途径。利用中红外频率梳的高频率稳定性对丙酮和羰基硫进行光谱分析，同时带宽超过11太赫兹，光谱分辨率高达0.003厘米-1。低噪声和宽光谱覆盖范围的结合使得能够检测浓度在十亿分之一范围内的痕量气体。
