Steady-state superfluidity of light in a tunable cavity at room temperature (2012.13463v1)

Abstract: Light in a nonlinear cavity is expected to flow without friction -- like a superfluid -- under certain conditions. Until now, part-light part-matter (i.e., polariton) superfluids have been observed either at liquid helium temperatures in steady state, or at room temperature for sub-picosecond timescales. Here we report signatures of superfluid cavity photons (not polaritons) for the first time. When launching a photon fluid against a defect, we observe a suppression of backscattering above a critical intensity and below a critical velocity. Room-temperature and steady-state photon superfluidity emerges thanks to the strong thermo-optical nonlinearity of our oil-filled cavity. Numerical simulations qualitatively reproduce our experimental observations, and reveal how a viscous photon fluid reorganizes into a superfluid within the thermal relaxation time of the oil. Our results establish thermo-optical nonlinear cavities as platforms for probing photon superfluidity at room temperature, and offer perspectives for exploring superfluidity in arbitrary potential landscapes using structured mirrors.

• The paper reveals steady-state photon superfluidity at room temperature using an oil-filled Fabry-Pérot cavity with pronounced thermo-optical nonlinearity.
• The study employs a continuous laser beam to drive photon flows that bypass a mirror defect without scattering, indicating frictionless behavior.
• Numerical simulations based on the driven-dissipative Gross-Pitaevskii equation validate the presence of a critical velocity that marks the transition from viscous to superfluid flow.

Steady-State Superfluidity of Light in a Tunable Cavity at Room Temperature

The paper by G. Keijsers et al. explores the fascinating phenomenon of photon superfluidity within a thermo-optical cavity filled with oil, achieving steady-state conditions at room temperature. Traditionally, superfluidity—a macroscopic quantum effect characterized by frictionless flow—has been observed in polaritons under specific temperature and temporal constraints. This work is significant in that it demonstrates superfluid properties in cavities containing purely photons, without the polariton component.

Experimental Framework and Observations

In the experimental configuration, the authors utilized a Fabry-Pérot cavity, filled with olive oil, which exhibits strong thermo-optical nonlinearity. Through this setup, the photon-photon interactions necessary for achieving superfluidity are mediated. The authors explain that, by using these cavities, a continuous laser beam induces photon flows across the cavity. Central to the paper is the interaction of these photon flows with a naturally occurring defect in one of the mirrors.

Notable observations were made at specific conditions where light seemingly flowed around the defect without scattering, indicating superfluid behavior. The interplay between the nonlinearity due to thermo-optical properties and the photon lifetime is manipulated to reveal the conditions under which such superfluid behavior emerges. A critical laser power threshold delineates the transition from viscous to superfluid flow, where the defect does not interrupt the flow of light as it would in a non-superfluid state.

Theoretical Implications and Numerical Simulations

The work is underpinned by numerical simulations based on a driven-dissipative Gross-Pitaevskii equation, coupled to a thermal field. This theoretical approach provides a qualitative understanding of the experimental observations, validating the emergence of superfluidity even within the constraints of a viscous fluid medium governed by nonlocal, non-instantaneous thermo-optical effects.

Crucial to this paper is the concept that while thermal relaxation times are much longer than photon lifetimes, the emergence of a steady-state superfluid profile is not hindered. Instead, it suggests that thermo-optical nonlinearities could be beneficial for observing photon fluid dynamics over timescales accessible by conventional imaging techniques.

Critical Velocity

Another key aspect of the paper is the detection of a critical velocity ($v_c$), above which superfluidity is lost. This phenomenon mirrors traditional superfluid systems, where exceeding a critical velocity results in a breakdown of the superfluid state, reintroducing viscous properties. Adjusting the cavity length permitted control over the velocity of photon flows, further strengthening the parity between photon fluid dynamics in this system and known superfluid systems.

Future Directions and Conclusion

The implications of this research extend into potential investigations of photon superfluidity in more complex potential landscapes, achievable by microstructuring cavity mirrors. Furthermore, with the foundation of thermo-optical nonlinearity's role in photon superfluidity established, future work can explore dynamic control methods, enabling studies in novel regimes of quantum fluid dynamics.

Overall, Keijsers et al. have presented a thorough examination and observation of photon superfluidity in a thermo-optical cavity system at room temperature. This paper not only broadens the horizon for superfluid research beyond matter-based systems but also opens up a plethora of possibilities for exploring quantum fluids under realistic, ambient conditions.