Superfluidity of Light and Its Breakdown in Optical Mesh Lattices

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Superfluidity of Light and Its Breakdown in Optical Mesh Lattices

Martin Wimmer, Monika Monika, Iacopo Carusotto, Ulf Peschel, and Hannah M. Price

Phys. Rev. Lett. 127, 163901 – Published 11 October 2021

See Viewpoint: Photons Get Slippery

Abstract

Hydrodynamic phenomena can be observed with light thanks to the analogy between quantum gases and nonlinear optics. In this Letter, we report an experimental study of the superfluid-like properties of light in a ( $1 + 1$ )-dimensional nonlinear optical mesh lattice, where the arrival time of optical pulses plays the role of a synthetic spatial dimension. A spatially narrow defect at rest is used to excite sound waves in the fluid of light and measure the sound speed. The critical velocity for superfluidity is probed by looking at the threshold in the deposited energy by a moving defect, above which the apparent superfluid behavior breaks down. Our observations establish optical mesh lattices as a promising platform to study fluids of light in novel regimes of interdisciplinary interest, including non-Hermitian and/or topological physics.

Received 11 August 2020
Accepted 30 July 2021

DOI:https://doi.org/10.1103/PhysRevLett.127.163901

Physics Subject Headings (PhySH)

Nonlinear optics

Quantum fluids & solids Superfluids

Atomic, Molecular & Optical

Viewpoint

Photons Get Slippery

Published 11 October 2021

Researchers have turned light into a superfluid by using a “synthetic” dimension, which is created by using temporal degrees of freedom to mimic spatial degrees of freedom.

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Authors & Affiliations

Martin Wimmer¹, Monika Monika ¹, Iacopo Carusotto ², Ulf Peschel¹, and Hannah M. Price ³

¹Institute of Condensed Matter Theory and Optics Friedrich-Schiller-University Jena, Max-Wien-Platz 1, Jena D-07743, Germany
²INO-CNR BEC Center and Dipartimento di Fisica, Università di Trento, Povo I-38123, Italy
³School of Physics and Astronomy, University of Birmingham, Edgbaston Park Road, West Midlands B15 2TT, United Kingdom

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Issue

Vol. 127, Iss. 16 — 15 October 2021

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Images

Figure 1
(a) Optical pulses propagating in two nonlinear, coupled fiber loops of slightly different lengths, are used to explore nonlinear light evolution in the $(1 + 1) D$ lattice, shown schematically in (b). In this mapping, the light intensity is a function of the discrete position in the lattice, $n$ , and evolves with respect to the discrete time step, $m$ . Completing a round-trip in the short (long) loop in the real system in (a) corresponds to traveling from northeast (northwest) to southwest (southeast) in the effective lattice in (b). Acousto-optical modulators (AOM) and erbium doped fiber amplifiers (EDFA) are used to compensate for losses. A phase modulator (PM) in each loop allows us to induce arbitrarily designed space- and time-dependent potentials. (c) The corresponding photonic bands in the linear ( $Γ = 0$ ) regime. (d),(e) The Bogoliubov dispersions (2) on top of a condensate located at $Q = 0$ in the lower band [circle in (c)] for (d) linear and (e) nonlinear ( $Γ I_{0} = 0.2$ ) systems. The slope of the straight blue dashed line indicates the speed of sound (3). The red (black) color of each curve indicates the positive (negative) value of the band’s Bogoliubov norm.
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Figure 2
(a) Experimentally observed propagation of perturbations induced by imposing a short and localized phase defect onto a Gaussian fluid of light for two different values of the effective initial nonlinearity, $Γ_{eff} \propto Γ I_{0}$ . The colorscale represents the differential intensity, $Δ I$ , between the perturbed and unperturbed experiments. All parameters are stated in the main text. (b) The estimated speed of sound $v_{S} = v_{M} - v_{E}$ grows from around zero with increasing nonlinearity. Also plotted are $v_{M}$ , the average speed of the innermost minima at late times obtained by fitting the results in (a), and $v_{E}$ , the average expansion speed of the unperturbed fluid of light at the minima positions, from which the sound speed was extracted. For clarity, error bars for $v_{M}$ and $v_{E}$ are shown in the Supplemental Material [29]. Lines are included as a guide to the eye.
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Figure 3
(a) Spatiotemporal plot of the measured intensity in the short loop for a trapped nonlinear fluid of light ( $Γ I \approx 0.5$ in the center of the trap) excited by a sinusoidally moving defect at various speeds $s$ , applied to the optical field from $m = 0$ to $m = 200$ . Defect and trap parameters are as given in the main text. (b) Late-time average of the potential energy (5) for experiments in the linear and nonlinear regime [nonlinear regime: power levels as in (a), linear regime: power levels $1 / 5$ of (a)]. In the linear regime, we observe that the potential energy increases steadily with the defect speeds, while in the nonlinear regime, it remains approximately constant for low $s$ and shows a marked upward kink at $s \approx 0.07$ .
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