title: Real-time spectral interferometry probes the internal dynamics of femtosecond soliton molecules.
abstract: Solitons, particle-like excitations ubiquitous in many fields of physics, have been shown to exhibit bound states akin to molecules. The formation of such temporal soliton bound states and their internal dynamics have escaped direct experimental observation. By means of an emerging time-stretch technique, we resolve the evolution of femtosecond soliton molecules in the cavity of a few-cycle mode-locked laser. We track two- and three-soliton bound states over hundreds of thousands of consecutive cavity roundtrips, identifying fixed points and periodic and aperiodic molecular orbits. A class of trajectories acquires a path-dependent geometrical phase, implying that its dynamics may be topologically protected. These findings highlight the importance of real-time detection in resolving interactions in complex nonlinear systems, including the dynamics of soliton bound states, breathers, and rogue waves.
authors.affiliation: IV. Physical Institute-Solids and Nanostructures, University of Göttingen, Göttingen, Germany. claus.ropers@uni-goettingen.de.|Department of Electrical Engineering, University of California, Los Angeles, CA, USA.
authors.forename: G
authors.identifier: http://orcid.org/0000-0001-6649-534X
authors.initials: G
authors.lastname: Herink
authors.affiliation: IV. Physical Institute-Solids and Nanostructures, University of Göttingen, Göttingen, Germany.
authors.forename: F
authors.identifier: http://orcid.org/0000-0003-0811-5200
authors.initials: F
authors.lastname: Kurtz
authors.affiliation: Department of Electrical Engineering, University of California, Los Angeles, CA, USA.
authors.forename: B
authors.initials: B
authors.lastname: Jalali
authors.affiliation: IV. Physical Institute-Solids and Nanostructures, University of Göttingen, Göttingen, Germany.|Department of Electrical Engineering, University of California, Los Angeles, CA, USA.
authors.forename: D R
authors.identifier: http://orcid.org/0000-0001-5901-2768
authors.initials: DR
authors.lastname: Solli
authors.affiliation: IV. Physical Institute-Solids and Nanostructures, University of Göttingen, Göttingen, Germany. claus.ropers@uni-goettingen.de.|International Center for Advanced Studies of Energy Conversion (ICASEC), University of Göttingen, Göttingen, Germany.
authors.forename: C
authors.identifier: http://orcid.org/0000-0002-9539-3817
authors.initials: C
authors.lastname: Ropers