Population Inversion Research Articles (Page 1)

We present a two-mode squeezed Jaynes–Cummings model, built upon the formalism of pair coherent states (PCSs), to investigate the dynamics of a two-level atom interacting with a two-mode quantized field. By solving the time-dependent Schrödinger equation under the rotating-wave approximation, we elucidate the system’s quantum evolution, with particular emphasis on how the squeezing degree and photon number difference modulate atomic population inversion and entanglement. We further quantify the nonclassical traits of the two-mode squeezed PCSs via Mandel’s parameter and the violation of the Cauchy–Schwarz inequality, highlighting their sensitivity to model parameters. These findings illuminate the subtle interplay of squeezing, photon statistics, and entanglement in advanced quantum optical systems.

Mode matching between the pump beam and the cavity eigenmodes plays a critical role in determining the beam quality and conversion efficiency of laser systems. However, compared to population inversion lasers, research on mode matching in Raman lasers has received less attention. Here, we investigate mode-matching characteristics in an external-cavity diamond Raman laser (DRL), leveraging the superior thermal properties of diamond. By employing quasi-continuous pump sources exhibiting significantly different beam qualities, high beam quality first Stokes laser output was achieved across a wide range of cavity length adjustments designed to modify the intra-cavity mode matching. The dynamic cavity length adjustment range reached 6 mm. Under the condition of a pump beam M 2 factor of 4.5, a beam quality improvement exceeding fourfold was attained. The compatible mode-matching ratio range between the pump beam and the resonant modes was found to be 0.54 to 1.20. Furthermore, the impact of different cavity lengths on the stable laser output power was analyzed. This work demonstrates the outstanding performance of the DRL in generating high beam quality output, along with its high tolerance to variations in pump beam quality and cavity length. The investigation of Raman cavity mode matching provides crucial guidance for laser design.

In recent years, the H + F2 reaction has attracted much attention because of its important role in theory and in chemical lasers. The aim of this study was to report a highly accurate potential energy surface (PES) for this reaction and carry out a product-state resolved reaction dynamics study of the H + F2 (v0 = 0, j0 = 0, 1, 2) → HF + F reaction in a collision energy range [0.0, 1.0] eV with the time-dependent wave packet method. The HF2 PES was constructed using the permutation invariant polynomial neural network method with thousands of energy points calculated by the MRCI-F12+Q method with the AVTZ basis sets. The calculated results suggest that the rotational excitation of low-lying states of reactant F2 has little effect on the reaction. The vibrational level population inversion of the product HF is significant, and the HF product is most to be produced in the v' = 4-7 vibrational states. At lower collision energy, the product HF preferred to be populated in highly excited rotational states, but at higher collision energies, the rotational distributions roughly exhibit Gaussian function distributions. The calculated reaction rate constants agree with the experiments well but with a little underestimation. This study suggests that the reaction H + F2 is a wonderful prototype for chemical lasers, just like the more famous H2 + F reaction, agreeing well with the previous findings.

This paper presents the implementation of a real-time nonlinear state observer applied to an erbium-doped fiber laser system. The observer is designed to estimate population inversion, a state variable that cannot be measured directly due to the physical limitations of measurement devices. Taking advantage of the fact that the laser intensity can be measured in real time, an observer was developed to reconstruct the dynamics of population inversion from this measurable variable. To validate and strengthen the estimate obtained by the observer, a Recurrent Wavelet First-Order Neural Network (RWFONN) was implemented and trained to identify both state variables: the laser intensity and the population inversion. This network efficiently captures the system’s nonlinear dynamic properties and complements the observer’s performance. Two metrics were applied to evaluate the accuracy and reliability of the results: the Euclidean distance and the mean square error (MSE), both of which confirm the consistency between the estimated and expected values. The ultimate goal of this research is to develop a neural control architecture that combines the estimation capabilities of state observers with the generalization and modeling power of artificial neural networks. This hybrid approach opens up the possibility of developing more robust and adaptive control systems for highly dynamic, complex laser systems.

Organic-inorganic perovskite materials have garnered widespread academic attention owing to their remarkable optical characteristics. Nonetheless, it is imperative to minimize the laser threshold and non-radiative recombination losses for developing perovskite lasers with superior performance. In this work, an innovative perovskite vertical-cavity surface-emitting laser (VCSEL) has been developed by integrating gold nanorods (Au NRs) into the resonant cavity to manipulate the light field energy distribution and optical confinement factor, significantly reducing the threshold of perovskite lasers through the localized surface plasmon resonance (LSPR) effect. The incorporation of Au NRs induces an efficient exciton-plasmon coupling effect, which can enhance the absorption and photoluminescence quantum yield while effectively facilitating the radiative recombination of carriers and population inversion, thereby facilitating a high optical gain essential for laser oscillation. Consequently, the Au NR-embedded perovskite VCSEL demonstrates an impressive threshold of 0.99 μJ cm-2 and a line width of 0.89 nm, outperforming the reference device with a threshold of 4.12 μJ cm-2 and a line width of 2.0 nm. Furthermore, the incorporation of Au NRs into the perovskite VCSEL can improve the stability of the laser output due to the strengthened exciton-photon coupling and the diminished intracavity losses. This study provides an effective strategy for constructing perovskite VCSELs with low thresholds and excellent stability.

Er3+ activated gadolinium phosphate oxyfluoride glasses have been prepared via melt-quenching process and measured the absorption and luminescence spectra. The absorption spectra of glasses have shown in UV-VIS and Near-infrared region. The absorption intensity of glasses was enhanced as a function of Er2O3 concentrations. With laser diode excitation at 980 nm, the emission in near infrared region has shown a prominent peak at 1536 nm (4I13/2→4I15/2) which is important peak for transmission windows. The absorption and emission measurements led to calculate the stimulated emission cross-sections by McCumber analysis. It was found to be 2.66×10–20 cm2 for 2.0 mol% of Er2O3. Furthermore, the positive gain cross-section of glass is identified when the population inversion (P) is more than 0.4. It is pointed out the flat gain bandwidth in wavelength range 1400 - 1700 nm will be achieved when the P exceeds 40% which covered S, C and L bands.

Thought experiments like Maxwell’s Demon or the Smoluchowski–Feynman Ratchet can help in pursuing the microscopic origin of the Second Law of Thermodynamics. Here we present a more sophisticated mechanical system than a ratchet, consisting of a Hamiltonian (non-Brownian) active particle which can harvest energy from an environment which may be in thermal equilibrium at a single temperature. We show that while a phenomenological description would seem to allow the system to operate as a Perpetual Motion Machine of the Second Kind, a full mechanical analysis confirms that this is impossible, and that perpetual energy harvesting within a mechanical system can only occur if the environment has an energetic population inversion similar to a lasing medium.

Achieving low-threshold lasing under multiphoton excitation is challenging due to weak nonlinear absorption and the requirement for population inversion. We address this by employing colloidal quantum wells (CQWs). They exhibit large three-photon absorption (3PA) cross sections, strong optical gain, and robust excitonic properties, enabling exciton-photon strong coupling and inversion-free polariton lasing. Under 1030 nm femtosecond excitation, CdSe core and CdSe/CdS core/crown CQWs show photoluminescence and amplified spontaneous emission via 3PA. Embedding CdSe/CdS CQWs in Fabry-Pérot cavities yields pronounced polariton dispersions (Rabi splitting of ∼202 meV) and a sharp, blue-shifted emission peak at k = 0 under higher excitation, evidencing exciton-polariton lasing via 3PA. Compared to photonic 3PA lasers, our polaritonic system features a reduced threshold, enabled by bosonic final-state stimulation. This work presents the first upconverted exciton-polariton lasing in colloidal semiconductors, establishing CQWs as a powerful platform for low-threshold, upconverted lasers and nonlinear polaritonic devices.

Yellow Dy-doped ZBLAN fiber lasers emitting in the 560600 nm window is highly desirable for biomedical imaging, laser display, lidar and atmospheric sensing, yet their performance is still limited by sub-optimal gain design. Here we present an optimization of the Dy: ZBLAN fiber laser that simultaneously tunes the dopant concentration and the linear-cavity length. A four-level rate-equation model was established and coupled with pump-power and signal-power propagation equations; the system was numerically solved in MATLAB with experimentally reported spectroscopic parameters. Parametric sweeps show that increasing the Dy concentration from51024m-3to21025m-3and adjusting the cavity to 5 m maximizes population inversion while keeping re-absorption loss low. At the identified optimum the laser reaches an external slope efficiency of 38 % and produces 3.77 W of continuous-wave output at 574 nm when pumped with 10 W at 453 nm; the lasing threshold is about equal to 1 W, and no backward pump is required. The model also predicts a nearly linear outputpump characteristic and negligible backward signal, confirming good unidirectional operation. These results provide clear design rules for watt-level, narrow-band yellow fiber lasers and lay the groundwork for compact sources in advanced photonic applications.

The fluxonium qubit is a promising platform for quantum operations due to its large anharmonicity and long coherence time. However, conventional resonant driving methods often require long operation times and complex implementation. As an alternative, nonadiabatic transitions induced by a time-dependent external flux ϕext(t) can enable faster and simpler control by exploiting quantum interference between multiple transition paths. Existing approaches typically impose periodicity or symmetry on ϕext(t), which can limit control efficiency. We propose a design strategy for ϕext(t) that relaxes these constraints. We demonstrate the effectiveness of this approach in two key scenarios: population inversion and adiabatic passage. Numerical simulations show that our method achieves higher control efficiency than existing schemes, emphasizing the advantage of breaking periodicity and symmetry. Furthermore, by incorporating decoherence effects using the Lindblad master equation, we confirm that the proposed scheme remains robust under realistic conditions.

In topology, averaging over local geometrical details reveals robust global features. These are crucial in physics for understanding quantized bulk transport and exotic boundary effects of linear wave propagation in (meta-)materials. Beyond linear Hamiltonian systems, topological physics strives to characterize open (non-Hermitian) and interacting systems. Here, we establish a framework for the topological classification of driven-dissipative nonlinear systems by defining a graph index for their Floquet semiclassical equations of motion. Our index builds upon the topology of vector flows and encodes the particle-hole nature of excitations around all out-of-equilibrium stationary states. Thus, we uncover the topology of nonlinear resonator's dynamics under external and parametric forcing. Our framework sheds light on the topology of driven-dissipative phases, including under- to overdamped responses and symmetry-broken phases linked to population inversion. We therefore expose the pervasive link between topology and nonlinear dynamics, with broad implications for interacting topological insulators, topological solitons, neuromorphic networks, and bosonic codes.

Abstract This study explores the dynamics of a two two-level atomic system interacting with a parity-deformed field, modeled as a coherent state. By varying the parity deformation parameter and incorporating the effects of intensity-dependent coupling, we examine the time evolution of key quantum phenomena. These include atomic population inversion, von Neumann entropy, concurrence, quantum Fisher information, and the Mandel parameter, where each serves as a critical indicator. Our analysis reveals how the interplay between the deformation parameter and coupling effects governs the quantum dynamics, influencing entanglement, parameter estimation, and statistical properties of the field. These insights contribute to a more comprehensive understanding of how to control and optimize quantum phenomena, highlighting the model’s relevance for quantum information processing technologies.

This study presents a comprehensive study of a driven degenerate Λ-type three-level laser system utilizing a squeezed vacuum reservoir to enhance its performance. The interaction between the laser medium and the squeezed vacuum field is analyzed, revealing significant improvements in coherence and output characteristics. By employing a Λ-type configuration, we explore the population inversion dynamics and the role of squeezed states in reducing quantum noise, ultimately leading to enhanced laser stability and efficiency. Theoretical models are developed to describe the laser dynamics, incorporating the effects of external pumping and the non-classical properties of the squeezed vacuum. Numerical simulations demonstrate that the incorporation of squeezed states leads to a notable increase in the laser’s output power and spectral purity compared to traditional systems. These findings suggest that squeezed vacuum reservoirs can serve as a powerful tool for advancing the capabilities of three-level laser systems, with potential applications in quantum optics, communication technologies, and precision measurement.

Mid-infrared lasers at 3 μm hold many important applications in biological tissue ablation, gas detection, and so on. At present, holmium ion (Ho3+) can achieve efficient laser oscillation at this wavelength by the 5I6→5I7 transition. However, in the traditional high-phonon-energy oxide crystals (e.g., YAG), it is quite difficult to realize ∼3 μm continuous-wave (CW) lasing owing to the strong non-radiative relaxation and inherent self-terminating effect. Here, for the first time, we investigated the growth, spectroscopy, and mid-infrared lasing properties of Ho,Pr:YGG crystal. The growth of high-quality laser crystals was achieved using the optical floating zone method. Compared to the YAG host, YGG has a reduced phonon energy, which is beneficial for decreasing the probability of non-radiative relaxation and strengthening the ∼3 μm emission intensity. Then, Pr3+ ions were introduced as deactivators to reduce the lifetimes of 5I7 low-level and promote the population inversion of Ho3+ activators. As a result, Ho,Pr:YGG yields the stable ∼3 μm CW lasing, delivering a maximum output power of 548 mW and a slope efficiency of 5.88%. As far as we are aware, this is the highest reported CW output at ∼3 μm among Ho3+-doped oxide-based laser crystals.

Even before its role in electroweak symmetry breaking, the Anderson-Higgs mechanism was introduced to explain the Meissner effect in superconductors. Spontaneous symmetry-breaking yields massless phase modes representing the low-energy excitations of the Mexican-Hat potential. Only in superconductors the phase mode is shifted towards higher energies owing to the gauge field of the charged condensate. This results in a low-energy excitation spectrum governed by the Higgs mode. Consequently, the Bardeen-Cooper-Schrieffer-like Meissner effect signifies a macroscopic quantum condensate in which a photon acquires mass, representing a one-to-one analogy to high-energy physics. We report on an innovative spectroscopic technique to study symmetries and energies of the Higgs modes in the high-temperature superconductor Bi2Sr2CaCu2O8 after a soft quench of the Mexican-Hat potential. Population inversion induced by an initial laser pulse leads to an additional anti-Stokes Raman-scattering signal, which is consistent with polarization-dependent Higgs modes. Within Ginzburg-Landau theory, the Higgs-mode energy is connected to the Cooper-pair coherence length. Within a Bardeen-Cooper-Schrieffer weak-coupling model we develop a quantitative and coherent description of single-particle and two-particle channels. This opens the avenue for Higgs Spectroscopy in quantum condensates and provides a unique pathway to control and explore Higgs physics.

Chromosomal inversions are a type of structural variant that have long interested evolutionary biologists because of their potential role in local adaptation and speciation. However, direct experimental evidence for the fitness consequences of inversions is rare, limiting our ability to dissect the evolutionary forces associated with the spread and maintenance of inversions in natural populations. We tackle this knowledge gap by studying the fitness effects of three chromosomal inversions that consistently differ between marine and freshwater populations of threespine sticklebacks (Gasterosteus aculeatus). Using controlled laboratory crosses, we tested whether inversion genotype influences fitness (measured as survival, standard length, and body condition) across two salinity treatments (freshwater vs saltwater). In both the freshwater and the saltwater treatments, there were no deviations from Mendelian ratios at any of the three inversions. This suggests that there are no intrinsic deleterious effects of these inversions, in contrast to observations from other systems. Overall, there was no effect of inversion genotype on standard length or body size across the two salinity treatments for the chromosome XI and XXI inversions. For the chromosome I inversion, heterozygotes had a slightly lower body condition in the freshwater treatment. Together, these results suggest that the fitness effects of these inversions are not strongly influenced by salinity and that other selective forces might be involved in their evolution. More broadly, these findings highlight the importance of performing empirical tests of fitness effects of chromosomal inversions to better explain their spread and maintenance in nature.

Genomic structural variants (SVs) contribute substantially to genetic diversity and human diseases1-4, yet remain under-characterized in population-scale cohorts5. Here we conducted long-read sequencing6 in 1,019 humans to construct an intermediate-coverage resource covering 26 populations from the 1000 Genomes Project. Integrating linear and graph genome-based analyses, we uncover over 100,000 sequence-resolved biallelic SVs and we genotype 300,000 multiallelic variable number of tandem repeats7, advancing SV characterization over short-read-based population-scale surveys3,4. We characterize deletions, duplications, insertions and inversions in distinct populations. Long interspersed nuclear element-1 (L1) and SINE-VNTR-Alu (SVA) retrotransposition activities mediate the transduction8,9 of unique sequence stretches in 5' or 3', depending on source mobile element class and locus. SV breakpoint analyses point to a spectrum of homology-mediated processes contributing to SV formation and recurrent deletion events. Our open-access resource underscores the value of long-read sequencing in advancing SV characterization and enables guiding variant prioritization in patient genomes.

Generating short and strong pulses in solid-state lasers is usually achieved with Q-switching, whereby the quality of the laser cavity is switched from a low-quality value, used during a time interval for pumping, to a high-quality value, used to initiate short-pulse emission. The pumping time interval is long so that large population inversion can be achieved. This method does not work for semiconductor lasers, owing to the fast radiative and non-radiative recombination time. We propose the method of lifetime switching (L-switching), which is analogous to Q-switching, but solves the problem of fast recombination during the pump phase. L-switching is achieved via an externally applied electric field (capacitive storage). We introduce the concept and illustrate its functionality by numerically evaluating the system's electrostatic properties, using non-polar InGaN as an example. We address the simultaneous requirements of large lifetime reduction and optical gain, and the effect that excitation-induced screening has on these requirements. The corresponding laser dynamics will be discussed in a future publication.

Abstract In this work, we introduce a model for a two-atom (T–A) system interacting with a field mode initially prepared in a coherent state of a Para Bose–field (P–F). The T–As are initially represented by a Bell state, and we present the quantum framework for the whole system by solving the dynamical equations. We investigate the time-dependent behavior of essential quantum resources relevant to various tasks in quantum optics and information science, including atomic population inversion, T–As entanglement, T–As-P–F entanglement, and the statistical properties of the P–F as they relate to the model parameters. Our analysis reveals how these quantum resources are affected by different parameters in the T–As-P–F model. Finally, we illustrate the evolving interdependencies among these quantum resources within the system.

Indium phosphide is the leading material for commercial applications of colloidal quantum dots. To date, however, the community has failed to achieve successful operation under strong excitation conditions, contrasting sharply with other materials. Here, we report unusual photophysics of state-of-the-art InP-based quantum dots, which makes them unattractive as a laser gain material despite a near-unity quantum yield. A combination of ensemble-based time-resolved spectroscopy over timescales from femtoseconds to microseconds and single-quantum-dot spectroscopy reveals ultrafast trapping of hot charge carriers. This process reduces the achievable population inversion and limits light amplification for lasing applications. However, it does not quench fluorescence. Instead, trapped carriers can recombine radiatively, leading to delayed—but bright—fluorescence. Single-quantum-dot experiments confirm the direct link between hot-carrier trapping and delayed fluorescence. Hot-carrier trapping thus explains why the latest generation of InP-based quantum dots struggle to support optical gain, although the quantum yield is near unity for low-intensity applications. Comparison with other popular quantum-dot materials—CdSe, Pb–halide perovskites, and CuInS2—indicate that the hot-carrier dynamics observed are unique to InP.