Quantum Fluids of Light achieves an unprecedented new breakthrough

Experiments probing quasiparticles in semiconductor microcavities provide insight into the quantum hydrodynamics of light.

Superfluidity is the ability of a fluid to flow frictionlessly and is not restricted to systems described by hydrodynamics. More than a decade ago, optical researchers became interested in superfluidity and other quantum fluids: as they realized that light propagating in nonlinear media could exhibit quantum hydrodynamic characteristics.

To date, two platforms have emerged to study these "fluids of light": semiconductor microcavities in which photons are confined and the geometry of photon propagation in bulk media. Both configurations allow photons to acquire effective mass and experience effective interactions, properties that can also make them collectively behave as a quantum fluid.

However, our understanding of these exotic states is limited by experiment, in particular by the difficulty of detecting the collective excitations that are the hallmark of quantum fluid behavior. Now, Ferdinand Claude and his colleagues at the Laboratoire Kastler-Brossel (LKB) of the Sorbonne, France, provide an unprecedented detail of quantum fluids: quasiparticles generated by the strong coupling of photons and excitons in semiconductor microcavities. Their approach holds promise for exploring new systems of quantum fluids, including some that can serve as analog models of gravity.

The research results were published in the journal Physical review B on May 8 under the title Spectrum of collective excitations of a quantum fluid of polaritons.

Semiconductor microcavities provide a powerful platform to observe photonic hydrodynamic effects. When such a cavity is irradiated by an electromagnetic wave with a frequency matching the resonance of the cavity, the wave vector component perpendicular to the plane of the cavity becomes quantized. As a result, the relationship between this wave vector component and the photon frequency shows a quadratic dependence, thus giving the photon an effective mass.

At the same time, laser irradiation produces bound hole-electron states called excitons. The coupling between photons and excitons in the cavity produces quasiparticles called polaritons, which inherit the properties of photons and excitons. The masses of these polaritons are determined by the exciton effective mass and the photon effective mass, and they interact through exciton-exciton coupling. Thus, they can collectively behave as a large-mass, interacting particle stream, i.e., as a quantum fluid. In the last decade, polariton systems have indeed been shown to display quantum fluid behavior ranging from Bose-Einstein condensates to superfluids.

The similarity between cavity polaritons and two-dimensional quantum fluids goes beyond this qualitative description, since the time evolution of both systems is governed by the same mathematical formalism: the so-called Gross-Pitaevskii equation. A hallmark of quantum fluid behavior is the presence of collective excitation (collectively excitation), in particular small density perturbations propagating on the surface of a stationary fluid. This propagation is described by a Bogoliubov-like dispersion relation with a sound-like region (linear energy-momentum relation on large length scales) and a free particle-like region (parabolic relation on small length scales.) Claude and his colleagues focused on the quantitative measurement of these collective excitations, also known as Bogoliubov waves.

Bogoliubov dispersion curves in this experiment

Unlike the geometry of propagation, the cavity-polariton system requires an extension of the Bogoliubov theory; this is a consequence of the non-equilibrium nature of the polariton - it is generated by laser light excitation and has a finite lifetime. This discrepancy implies a challenge in obtaining and exploiting experimental data. Measurements of Bogoliubov excitation involve excitation of polaritons with a "pump" laser and then detection of the photoluminescence produced as the polaritons decay.

In earlier studies, the frequency of the pump laser was far from the cavity resonance. This facilitated the separation of the pump photon from the photoluminescent photon. However, the non-resonant excitation produced a wide range of polaritons, some of which were not part of the quantum fluid. Their presence distorts the measured spectrum - especially in those regions where superfluidity features are expected (low wave number). Another approach is to illuminate the cavity with a near-resonant or through-resonant pump (which must be filtered out from the photoluminescent photons). However, this method does not have enough energy resolution to observe many subtle features of the Bogoliubov dispersion curve.

Thanks to an innovative technique based on coherent-probe spectroscopy, previously developed by the group, the researchers were able to overcome these limitations. In this technique, the pump pulse is followed by a tunable laser field for probing Bogoliubov's excitation. The probe laser allows the signal to be separated from the background emission of the fluid, giving the device the ability to acquire features of the polarized fluid with unprecedented spatial and spectral resolution.

Experimental setup

Through a series of experiments, Claude and his colleagues have provided a comprehensive characterization of the dispersion of the collective excitation of a fluid. For a given pump energy, they measured the reflectivity of the probe beam at different angles, in the cavity; for each angle, the reflectivity showed a depression when the probe resonated with the collective polariton excitation, which allowed the researchers to characterize Bogoliubov excitations with different wave vectors and thus reconstruct the dispersion relation. In addition, they used the Gaussian shape of the beam to match their experimental results with theoretical predictions explaining this shape: a process that allowed them to extract the speed of sound in the polarized fluid.

Claude's team used a "pump-probe" device to describe a polarized sub-fluid in a semiconductor cavity. A "pump" laser pulse (red) generates polaritons by optical excitation. By measuring the cavity reflectance at different incidence angles of the probe pulse, the team obtained the polariton dispersion profile.

In superfluidity, the Bogoliubov dispersion relation has two branches, one for normal dispersion and one for negative dispersion - also known as the "ghost branch". The latter branch is named because it is very challenging in terms of excitation and therefore also for observation. The new work provides a significant improvement in the characterization of these two branches, especially for regions of the dispersion curve that until now have not been well characterized (such as those corresponding to low wave numbers). The team was also able to observe new details of how fluid density and other parameters affect the speed of sound and describe the onset of various fluid instabilities.

Thanks to this research, the framework of quantum fluids has gained a degree of experimental control, paving the way for more extensive quantitative studies of polarized fluids. By detecting small deviations from the behavior of standard quantum fluids, the device will provide unprecedented insight into quantum hydrodynamics. More importantly, it may enable the polarization system to be used as an optical analogue of gravity that can be used to model difficult-to-detect phenomena related to astrophysics, cosmology, and quantum gravity.

Reference links:

[1] https://baijiahao.baidu.com/s?id=1765384417759850622

[2]https://journals.aps.org/prb/abstract/10.1103/PhysRevB.107.174507

[3]https://physics.aps.org/articles/v16/74