Concepedia

Ultrasensitive detections in atomic and molecular physics: demonstration in molecular overtone spectroscopy

Journal of the Optical Society of America B · 1998 · 392 citations · 44 references

Open access

TL;DR

We consider several highly sensitive techniques commonly used in detection of atomic and molecular absorptions. The study summarizes and compares the operating principles and performances of these techniques, and presents new results on ultrasensitive detection of molecular overtone transitions using cavity‑enhanced frequency‑modulation spectroscopy. The method employs an external cavity to amplify the molecular response and uses frequency‑modulation spectroscopy with sideband frequency matched to the cavity free spectral range, enabling shot‑noise‑limited, laser‑frequency‑noise‑insensitive detection that bypasses the cavity bandwidth limit. Using a 1.064‑µm Nd:YAG laser, sub‑Doppler overtone resonances of C₂HD, C₂H₂, and CO₂ were observed, achieving a detection sensitivity of 5 × 10⁻¹³ integrated absorption (1 × 10⁻¹⁴ cm⁻¹) over a 1‑s averaging time.

Abstract

We consider several highly sensitive techniques commonly used in detection of atomic and molecular absorptions. Their basic operating principles and corresponding performances are summarized and compared. We then present our latest results on the ultrasensitive detection of molecular overtone transitions to illustrate the principle and application of the cavity-enhanced frequency-modulation (FM) spectroscopy. An external cavity is used to enhance the molecular response to the light field, and an FM technique is applied for shot-noise-limited signal recovery. A perfect match between the FM sideband frequency and the cavity free spectral range makes the detection process insensitive to the laser-frequency noise relative to the cavity, and, at the same time, overcomes the cavity bandwidth limit. Working with a 1.064-μm Nd:YAG laser, we obtained sub-Doppler overtone resonances of C2HD,C2H2, and CO2 molecules. A detection sensitivity of 5×10-13 of integrated absorption (1×10-14/cm) over 1-s averaging time has been achieved.