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arXiv:2502.08446 · Optics Letters

High-resolution rapid-scanning Fourier transform spectroscopy of ultracold atoms

Friedemann Landmesser, Nicolai Gölz, Ulrich Bangert, Tobias Sixt, Lukas Bruder

Published Jun 9, 2025• 2 citations
arXivHTMLPDF

Abstract

Femtosecond (fs) interferometry combined with acousto-optical phase modulation is an effective approach to implement various types of coherent nonlinear and multidimensional spectroscopy schemes. Here, we focus on gaseous and ultracold samples and compare directly the performance and spectral resolution between two experimental implementations of the phase modulation technique, which are stepwise and continuous rapid scanning of the underlying Fourier transform (FT) interferometers. We show the performance advantage of the rapid-scanning approach and demonstrate a spectral resolution of 250 MHz in the spectroscopy of laser-cooled Li atoms. This is a 10-fold resolution improvement compared to previous experiments.

DOI
10.1364/ol.565541

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Femtosecond (fs) interferometry combined with acousto-optical phase modulation is an effective approach to implement various types of coherent nonlinear and multidimensional spectroscopy schemes. Here, we focus on gaseous and ultracold samples and compare directly the performance and spectral resolution between two experimental implementations of the phase modulation technique, which are stepwise and continuous rapid scanning of the underlying Fourier transform (FT) interferometers. We show the performance advantage of the rapid-scanning approach and demonstrate a spectral resolution of 250 MHz in the spectroscopy of laser-cooled Li atoms. This is a 10-fold resolution improvement compared to previous experiments.
Femtosecond coherent multidimensional spectroscopy is demonstrated for an ultracold gas. A setup for phase modulation spectroscopy is used to probe the 3 2 S 1/2 –2 2 P 1/2,3/2 transition in an 800 μ K-cold sample of 7 Li atoms confined in a magneto-optical trap. The observation of a double quantum coherence response, a signature of interparticle interactions, paves the way for detailed investigations of few- and many-body effects in ultracold gases using this technique. The experiment combines a frequency resolution of 3 GHz with a potential time resolution of 200 fs, which allows for high-resolution studies of ultracold atoms and molecules both in the frequency and in the time domain.
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In recent years, nonlinear quantum interferometry has been widely explored for applications like spectroscopy, optical coherence tomography, and imaging [1]. Quantum infrared spectroscopy (QIRS) uses quantum interference of generation processes of frequency-entangled visible-IR photon pairs to perform infrared spectroscopy using visible light sources and silicon-based detectors [2]. This technique utilizes a nonlinear quantum interferometer configuration where the generated IR photons pass through the sample, and the correlated visible photons are detected. The change in the detected visible photon flux with and without the sample helps extract the complex transmittance of the sample in the IR region.
Multidimensional spectroscopy with femtosecond phase-related pulse sequences, in which the relative delays and optical phases of pulses are scanned during spectroscopic observation, is demonstrated on electronic transitions in atomic potassium vapor. The pulse sequences were constructed using emerging femtosecond pulse-shaping techniques that allow for automated generation of complex ultrafast waveforms. Sequences of two and three pulses with specified optical phase relationships and interpulse delays were used to prepare specified coherent states.
Abstract Two-photon excitation spectroscopy is a nonlinear technique that has gained rapidly in interest and significance for studying the complex energy-level structure and transition probabilities of materials. While the conventional spectroscopy based on tunable classical light has been long established, quantum light provides an alternative way towards excitation spectroscopy with potential advantages in temporal and spectral resolution, as well as reduced photon fluxes. By using a quantum Fourier transform that connects the sum-frequency intensity and N00N-state temporal interference, we present an approach for quantum interferometric two-photon excitation spectroscopy. Our proposed protocol overcomes the difficulties of engineering two-photon joint spectral intensities and fine-tuned absorption-frequency selection. These results may significantly facilitate the use of quantum interferometric spectroscopy for extracting the information about the electronic structure of the two-photon excited-state manifold of atoms or molecules without any requirement for precise and complicated scanning in the spectral domain. This may be particularly relevant for photon-sensitive biological and chemical samples.