Lamb-dip saturated-absorption cavity ring-down rovibrational molecular spectroscopy in the near-infrared
Roberto Aiello, Valentina Di Sarno, Maria Giulia Delli Santi, M. De Rosa, I. Ricciardi, G. Giusfredi, Paolo De Natale, Luigi Santamaria Amato, P. Maddaloni
Abstract
The high-detection-sensitivity saturated-absorption cavity ring-down (SCAR) technique is extended to Lamb-dip spectroscopy of rovibrational molecular transitions in the near-infrared region. Frequency-comb-referenced sub-Doppler saturation measurements, performed on the acetylene <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m1"> <mml:mo stretchy="false">(</mml:mo> <mml:msub> <mml:mi>ν</mml:mi> <mml:mn>1</mml:mn> </mml:msub> <mml:mo>+</mml:mo> <mml:msub> <mml:mi>ν</mml:mi> <mml:mn>3</mml:mn> </mml:msub> <mml:mo>+</mml:mo> <mml:msub> <mml:mi>ν</mml:mi> <mml:mn>4</mml:mn> </mml:msub> <mml:mo>←</mml:mo> <mml:msub> <mml:mi>ν</mml:mi> <mml:mn>4</mml:mn> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:math> R(14)e line at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m2"> <mml:mrow> <mml:mn>6562</mml:mn> <mml:mtext> </mml:mtext> <mml:msup> <mml:mrow> <mml:mi>cm</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> , are analyzed by a SCAR global line profile fitting routine, based on a specially developed theoretical model. Compared to a conventional cavity ring-down evaluation, our approach yields dip profiles with a linewidth freed from saturation broadening effects, reduced by 40%, and a signal-to-noise ratio increased by 90%. Ultimately, an overall (statistical and systematic) fractional uncertainty as low as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m3"> <mml:mrow> <mml:mn>7</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>12</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> is achieved for the absolute line-center frequency. At the same time, our method is also able to accurately infer the linear (non-saturated) behavior of the gas absorption, providing Lamb-dip-based line strength measurements with a relative uncertainty of 0.5%.