High-optical-depth, sub-Doppler-width absorption lines at telecom wavelengths in hot, optically driven rubidium vapor

TL;DR

This paper demonstrates a method to suppress Doppler broadening in hot rubidium vapor, achieving high optical depth and narrow absorption lines at telecom wavelengths, which simplifies high-resolution spectroscopy without laser cooling.

Contribution

The study introduces a novel technique to reduce Doppler broadening in hot atomic vapors using optical dressing, enabling high-resolution spectroscopy at telecom wavelengths without cooling.

Findings

Achieved Doppler width reduction by an order of magnitude.

Observed high optical depth (~4) with narrow linewidth (~17 MHz).

Spectra agree well with theoretical models.

Abstract

Doppler broadening presents a major limitation for high-resolution spectroscopy and nonlinear optics in room-temperature atomic vapors. Here, we demonstrate the suppression of Doppler broadening accompanied by pronounced absorption on the upper transition of a three-level ladder system, achieved by dressing the intermediate state with a strong control field. As a concrete realization, we study a hot vapor of $^{87}$Rb where the lower transition is driven by a strong control field resonant with the D2 line at a wavelength of 780 nm, while a weak counter-propagating probe field at the telecom C-band wavelength of 1529 nm ($5P_{(3/2)}\leftrightarrow 4D_{(5/2)}$) interrogates the dressed states. We observe absorption features with a resonant optical depth of approximately 4 and a full width at half maximum of about 17 MHz. Remarkably, this corresponds to an order-of-magnitude reduction relative to the Doppler width, while the optical depth on the upper transition of the ladder scheme exceeds that of the Doppler-broadened lower transition. The measured spectra are in good agreement with theoretical modeling. Combining high optical density with sub-Doppler-width absorption lines typically requires laser-cooled atoms, while our approach profits from the experimental simplicity of a hot-vapor platform.