# Selective reflection from a Potassium atomic layer with a thickness as small as $\lambda /13$

## TL;DR

This paper demonstrates a derivative of selective reflection technique from a nanocell for nearly Doppler-free potassium atomic spectroscopy, achieving high spectral resolution and measuring atom-surface interactions at nanometer scales.

## Contribution

It introduces a robust method using the derivative of selective reflection signals for high-resolution atomic spectroscopy in nanocells, including the first measurement of the van der Waals coefficient for potassium.

## Key findings

- Achieved 18 times narrower linewidth than Doppler broadening.
- Observed sign oscillations of the reflected signal with a periodicity of λ/2.
- Measured van der Waals coefficient C_3 = 1.9 ± 0.3 kHz·μm^3.

## Abstract

We demonstrate that a method using the derivative of the selective reflection signal from a nanocell is a convenient and robust tool for atomic laser spectroscopy, achieving a nearly Doppler-free spectral resolution. The recorded linewidth of the signal from a potassium-filled cell, whose thickness $\ell$ lies in the range $350-500$ nm, is 18 times smaller than the Doppler linewidth ($\sim 900$ MHz full width at half maximum) of potassium atoms. We also show experimentally a sign oscillation of the reflected signal's derivative with a periodicity of $\lambda/2$ when $\ell$ varies from 190 to 1200~nm confirming the theoretical prediction. We report the first measurement of the van der Waals atom-surface interaction coefficient $C_3 = 1.9\pm 0.3$ kHz$\cdot\mu$m$^3$ of potassium $4S_{1/2} \rightarrow 4P_{3/2}$ transitions with the nanocell's sapphire windows, demonstrating the usefulness and convenience of the derivative of selective reflection technique for cell thicknesses in the range $60 -120~$nm.