Semiconductor lasers, foundational technology since the 1960s, operate based on population inversion of electrons, amplifying light emission within a resonator. This suggests a departure from thermal equilibrium. However, steady-state optical gain arises from quasi-thermal equilibrium of electrons and holes. Electromagnetic radiation can also achieve thermal equilibrium with surroundings, demonstrated previously with organic dyes in optical cavities. This work explores whether photons in an inorganic semiconductor microcavity, using a continuous-wave laser, can also achieve thermal equilibrium and undergo BEC at room temperature. The advantages of this approach compared to organic dyes include continuous, indefinitely sustained condensates and the lack of dark states that would otherwise trap carriers. In contrast to exciton-cavity polaritons, light condensates are only weakly coupled to their environment, reducing the need for low temperatures and stringent excitation conditions. The use of III-V materials, despite their low exciton binding energies, is enabled by these advantages, allowing exploration of the physics and applications of quantum statistical condensates.

The paper references previous work on BEC in various boson systems, including atoms, magnons, solid-state excitons, surface plasmon polaritons, and exciton-polaritons. It highlights the distinction between light condensates and lasers, noting that condensates operate robustly in their ground state, exhibit nonlinear and many-body physics, and don't require carrier inversion, displaying critical behavior below the laser threshold. Existing literature on photon BEC in organic dye-filled microcavities is referenced, serving as a basis for comparison and contrast with the presented inorganic semiconductor approach. The authors also cite theoretical works supporting the possibility of BEC in semiconductor cavities.

The experimental setup involves an open-access microcavity composed of a GaAs/AlAs heterostructure distributed Bragg reflector (DBR), an InGaAs quantum well, and a commercially manufactured DBR on a concave glass substrate. The cavity length is precisely controlled and stabilized interferometrically. Measurements were conducted at a longitudinal mode number of q = 9. The cavity loss rate (κ) was estimated from the laser threshold and reflection measurements. The microcavity's two-dimensional photon mode spectrum is described by a given equation, incorporating factors like photon mass, cavity trapping frequency, and nonlinear refraction. The semiconductor absorption is controlled using heterostructures, with a single InGaAs quantum well selected. Initial characterization involved optical excitation with a continuous 785 nm laser to determine emission and absorption spectra. The van Roosbroeck-Shockley (vRS) relation, relating absorption and emission spectra at thermal equilibrium, is used to verify thermalization of electrons and phonons. A detailed balance equation, considering modal emission, absorption, and cavity loss, is developed to model the photon production rate. This is used to derive expressions for the photon number spectrum under different excitation conditions, showing the transition to a Bose-Einstein distribution with increasing carrier density. The experimental data included spectral measurements, spatial distribution imaging, coherence time measurement using a Michelson interferometer, and excitation beam position variation. The influence of repulsive interactions within the condensate is analyzed by studying changes in condensate radius and wavelength as a function of intracavity condensate intensity.

The study reveals that photons within the inorganic semiconductor microcavity thermalize and undergo BEC at room temperature. The transition from thermal cloud to condensate is observed by increasing the pump intensity. Above a critical intensity, the ground-state population dominates, leading to the formation of a condensate. The spatial distribution images show a clear transition from a diffuse thermal cloud at low pump intensity to a concentrated condensate at higher intensity. Coherence time measurements exceed 500 ps, significantly greater than the cavity lifetime. The condensate exhibits polarization aligned with the pump beam. A phase diagram is established, mapping the spatial emission patterns (thermal cloud, single-mode condensate, multimode condensate, and lasing) as functions of cavity cut-off wavelength and excitation intensity. The observed repulsive interactions within the condensate are quantified through a dimensionless interaction parameter (g̃), which is found to be significantly larger than those previously reported for dye-based condensates. Measurements of condensate radius and wavelength shifts with increasing intracavity intensity provide consistent estimates of this parameter. The origin of the interaction strength is attributed to the dense electron-hole plasma within the semiconductor, although further research is required to detail the precise mechanisms involved.

The findings address the research question by demonstrating that photon BEC can be achieved in an inorganic semiconductor microcavity, providing a novel and robust platform for exploring the physics and applications of quantum statistical condensates. The successful observation of BEC in this system validates the theoretical predictions of photon condensation in semiconductor cavities. The larger interaction parameter compared to dye-based systems opens opportunities for investigations into many-body effects and quantum phenomena. The ability to achieve continuous and stable condensation at room temperature significantly enhances the potential for practical applications.

This work establishes a new and robust system for studying photon BEC using an inorganic semiconductor quantum well microcavity. The direct control over cavity length allows for precise tuning of thermalization, revealing the transition from BEC to lasing. The observed strong photon-photon interactions and the capacity for continuous operation of the condensate pave the way for investigations into superfluidity of light, vortex formation, and the Josephson effect, potentially leading to advancements in quantum technology at ambient conditions.

The study focuses on a specific semiconductor material and cavity design. The generalizability of the findings to other materials and configurations needs further investigation. The exact mechanisms behind the observed photon-photon interactions require more detailed analysis. The influence of heating on the condensate properties is a factor that needs further consideration and optimization.