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 ${}^{87}\mathrm{Rb}$ where the lower transition is driven by a strong control field resonant with the $D_2$ line at a wavelength of 780 nm, while a weak counterpropagating 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.

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• Atomic spectra
• Light-matter interaction
• Quantum optics