Mid-Infrared Frequency Comb Generation and Spectroscopy with Few-Cycle Pulses and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>χ</mml:mi><mml:mrow><mml:mo stretchy="false">(</mml:mo><mml:mn>2</mml:mn><mml:mo stretchy="false">)</mml:mo></mml:mrow></mml:msup></mml:math> Nonlinear Optics
Alexander J. Lind, Abijith S. Kowligy, Henry Timmers, Flávio C. Cruz, Nima Nader, Myles Silfies, Thomas K. Allison, Scott A. Diddams
Abstract
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.