No abstract

Mid-infrared laser frequency combs are compelling sources for precise and sensitive metrology with applications in molecular spectroscopy and spectro-imaging. The infrared atmospheric window between 3-5.5 µm in particular provides vital information regarding molecular composition. Using a robust, fiber-optic source of few-cycle pulses in the near-infrared, we experimentally demonstrate ultra-broad bandwidth nonlinear phenomena including harmonic and difference frequency generation in a single pass through periodically poled lithium niobate (PPLN). These χ (2) nonlinear optical processes result in the generation of frequency combs across the mid-infrared atmospheric window which we employ for dual-comb spectroscopy of acetone and carbonyl sulfide with resolution as high as 0.003 cm −1 . Moreover, cascaded χ (2) nonlinearities in the same PPLN directly provide the carrier-envelope offset frequency of the near-infrared driving pulse train in a compact geometry.Coherent laser sources in the mid-infrared (MIR, 3-25 µm) have long been recognized as important tools for both fundamental and applied spectroscopy and sensing. Recently, significant interest has focused on developing laser frequency combs in the MIR spectral region [1]. Of many promising applications, molecular spectroscopy using optical frequency combs benefits from a unique combination of high spectral resolution and broad bandwidth. This is particularly useful for the simultaneous measurement of spectral absorption fingerprints for a wide range of molecular compounds. The infrared atmospheric window between 3-5.5 µm exhibits reduced atmospheric attenuation while demonstrating strong absorption coefficients for greenhouse gases and pollutants such as methane, ethane, carbon dioxide, and formaldehyde [2-9], making this spectral range useful for climate research and atmospheric monitoring. Further, the same spectral window contains important molecular structure information pertaining to the C-H and O-H functional groups which can be used in the characterization of complex biochemical molecules [10,11] and spectro-imaging of biological samples [12,13].Based on these motivations, multiple approaches to MIR frequency comb generation have been pursued. Examples include optical parametric oscillators (OPOs) [14][15][16][17][18], supercontinuum generation [19][20][21][22], difference frequency generation (DFG) [23][24][25][26], direct generation with quantum cascade lasers (QCL) [27][28][29], mode-locked fiber lasers [30][31][32], and microresonator frequency combs [33]. Despite significant progress, many of these frequency comb sources require additional resonant cavities (OPOs) or careful spatio-temporal alignment of two femtosecond pulses (DFG) that increases complexity. Others lack absolute frequency calibration and have large modespacings (microresonator combs and QCLs) that are mis-matched to the spectroscopy of small molecules.In this Letter, we introduce a simple and powerful method for generating frequency combs across the MIR atmospheric window using int...

Broadband ultrafast optical spectroscopy methods, such as transient absorption spectroscopy and 2D spectroscopy, are widely used to study molecular dynamics. However, these techniques are typically restricted to optically thick samples,...