Laser Theory Research Articles

Phase space theory for open quantum systems with local and collective dissipative processes

In this article we investigate driven dissipative quantum dynamics of an ensemble of two-level systems given by a Markovian master equation with collective and local dissipators. Exploiting the permutation symmetry in our model, we employ a phase space approach for the solution of this equation in terms of a diagonal representation with respect to certain generalized spin coherent states. Remarkably, this allows to interpolate between mean field theory and finite system size in a formalism independent of Hilbert-space dimension. Moreover, in certain parameter regimes, the evolution equation for the corresponding quasiprobability distribution resembles a Fokker–Planck equation, which can be efficiently solved by stochastic calculus. Then, the dynamics can be seen as classical in the sense that no entanglement between the two-level systems is generated. Our results expose, utilize and promote techniques pioneered in the context of laser theory, which can be powerful tools to investigate problems of current theoretical and experimental interest.

Supercritical Poincaré–Andronov–Hopf Bifurcation in a Mean-Field Quantum Laser Equation

We deal with the dynamical system properties of a Gorini–Kossakowski–Sudarshan–Lindblad equation with mean-field Hamiltonian that models a simple laser by applying a mean-field approximation to a quantum system describing a single-mode optical cavity and a set of two-level atoms, each coupled to a reservoir. We prove that the mean-field quantum master equation has a unique regular stationary solution. In case a relevant parameter $$C_\mathfrak {b} $$ , i.e., the cavity cooperative parameter, is less than 1, we prove that any regular solution converges exponentially fast to the equilibrium, and so the regular stationary state is a globally asymptotically stable equilibrium solution. We obtain that a locally exponential stable limit cycle is born at the regular stationary state as $$C_\mathfrak {b} $$ passes through the critical value 1. Then, the mean-field laser equation has a Poincare–Andronov–Hopf bifurcation at $$C_\mathfrak {b} =1 $$ of supercritical-like type. Namely, we derive rigorously, at the level of density matrices—for the first time—the transition from a global attractor quantum state, where the light is not emitted, to a locally stable set of coherent quantum states producing coherent light. Moreover, we establish the local exponential stability of the limit cycle in case a relevant parameter is between the first and second laser thresholds appearing in the semiclassical laser theory. Thus, we get that the coherent laser light persists over time under this condition. In order to prove the exponential convergence of the quantum state, as the time goes to $$+ \infty $$ , we develop a new technique for proving the exponential convergence in open quantum systems that is based on a new variation of constant formula, which is obtained by combining probabilistic techniques with classical arguments from the semigroup theory. Furthermore, applying our main results we find the long-time behavior of the von Neumann entropy, the photon number statistics, and the quantum variance of the quadratures.

Solving the spherical p -spin model with the cavity method: equivalence with the replica results

The spherical p-spin is a fundamental model for glassy physics, thanks to its analytical solution achievable via the replica method. Unfortunately, the replica method has some drawbacks: it is very hard to apply to diluted models and the assumptions beyond it are not immediately clear. Both drawbacks can be overcome by the use of the cavity method; however, this needs to be applied with care to spherical models. Here, we show how to write the cavity equations for spherical p-spin models, both in the replica symmetric (RS) ansatz (corresponding to belief propagation) and in the one-step replica-symmetry-breaking (1RSB) ansatz (corresponding to survey propagation). The cavity equations can be solved by a Gaussian RS and multivariate Gaussian 1RSB ansatz for the distribution of the cavity fields. We compute the free energy in both ansatzes and check that the results are identical to the replica computation, predicting a phase transition to a 1RSB phase at low temperatures. The advantages of solving the model with the cavity method are many. The physical meaning of the ansatz for the cavity marginals is very clear. The cavity method works directly with the distribution of local quantities, which allows us to generalize the method to diluted graphs. What we are presenting here is the first step towards the solution of the diluted version of the spherical p-spin model, which is a fundamental model in the theory of random lasers and interesting per se as an easier-to-simulate version of the classical fully connected p-spin model.

Laser cooling limits in fields with a polarisation gradient of atoms with different recoil energies

Based on the numerical solution of the quantum kinetic equation for the atomic density matrix, which makes it possible to accurately take into account the recoil effects in the interaction of atoms with field photons, we have studied the limits of laser cooling of atoms using closed optical transitions characterised by different recoil parameters (the ratio of the recoil energy to the natural linewidth). It is shown that for optical transitions with an insufficiently small recoil parameter, the polarisation effects, which lead to the possibility of sub-Doppler laser cooling, lose their efficiency and do not ensure an attainment of the temperature below the Doppler limit. The analysis performed allows one to outline the boundaries of the sub-Doppler theory of laser cooling of atoms.

A theory for terahertz lasers based on a graphene hyperbolic metamaterial

We present a theory for terahertz (THz) wave emission in a cavity containing a graphene-based asymmetric hyperbolic metamaterial (AHMM), and the results of the calculation of the output power of the THz laser. An asymmetry appears in the spatial spectrum of the eigenmodes due to a tilt of an anisotropy axis with respect to the interfaces of the AHMM that results in a difference in the wave vector components for waves propagating in opposite directions. This AHMM is an active medium in the THz region due to an inverted population of charge carriers in graphene. The theory is based on the analysis of eigenwaves in the cavity containing the graphene-based AHMM and the use of the transfer matrix method. The components of the Poynting vector, normal to the interface of the AHMM, have been calculated for different eigenmodes, taking into account the gain saturation. The gain saturation manifests itself as a decrease of the negative imaginary part of the effective permittivity of the metamaterial. This gain saturation results from the dependence of the chemical potential in graphene on the component of the electric field transverse to the graphene sheets. Here, the balance of gain versus loss predicts the intensity of the AHMM THz laser emission.

Nonlinear theory of a Cherenkov free-electron laser

Plasma fluid and Maxwell equations are used to derive an analytical expression for the high-gain of a Cherenkov Free-Electron Laser (FEL). The evolution of the gain from the linear to the non-linear regime is investigated. The gain expression is then generalized to predict the linear and non-linear effects at any stage of interaction, permitting us to identify the criteria of optimal design of Cherenkov FELs. In this study, the spatial dependence of the characterizing parameters of electron beam velocity and density, such as the average value and modulation amplitude, are considered. Also, the phase shift between the bunched electrons and the radiation field is taken into account. In the linear limit, it is shown that our gain expression agrees satisfactorily with those obtained in previous studies.

Social Laser Model for the Bandwagon Effect: Generation of Coherent Information Waves.

During recent years our society has often been exposed to coherent information waves of high amplitudes. These are waves of huge social energy. Often they are of destructive character, a kind of information tsunami. However, they can also carry positive improvements in human society, as waves of decision-making matching rational recommendations of societal institutes. The main distinguishing features of these waves are their high amplitude, coherence (homogeneous character of social actions generated by them), and short time needed for their generation and relaxation. Such waves can be treated as large-scale exhibitions of the bandwagon effect. We show that this socio-psychic phenomenon can be modeled based on the recently developed social laser theory. This theory can be used to model stimulated amplification of coherent social actions. “Actions” are treated very generally, from mass protests to votes and other collective decisions, such as, e.g., acceptance (often unconscious) of some societal recommendations. In this paper, we concentrate on the theory of laser resonators, physical vs. social. For the latter, we analyze in detail the functioning of Internet-based echo chambers. Their main purpose is increasing of the power of the quantum information field as well as its coherence. Of course, the bandwagon effect is well known and well studied in social psychology. However, social laser theory gives the possibility to model it by using general formalism of quantum field theory. The paper contains the minimum of mathematics and it can be read by researchers working in psychological, cognitive, social, and political sciences; it might also be interesting for experts in information theory and artificial intelligence.

Study on micro-nano structures’ wettability transformation mechanism of femtosecond laser on aluminium alloy

In this work, superhydrophilic surfaces are obtained on LY12 aluminium alloy by irradiation of femtosecond (Fs). First, based on the theory of laser induced surface plasmon polariton, the formation mechanism of micro-nano structures is analyzed. Then the periodic change of the micro-nano structures with different wavelength and incident angle is calculated. According to the theoretical results, the structure period is 600 nm at the vertical incident and incident wavelength with 800 nm, and the experimental results agree with the theoretical results. Furthermore, two different kinds of postprocessing methods are employed to change the superhydrophilic surfaces to superhydrophobic surfaces. All the micro-nano structures are tested by scanning electron microscope (SEM), contact angle (CA) and Fourier transform infrared spectra (FTIR). By analyzing the function of the functional groups of C–H, C–F and C–O, the transition mechanism of the surface from superhydrophilicity to superhydrophobicity is also explored.

The development of laser drilling: A review

Laser drilling is of great significance for metal parts production, aerospace parts processing, precision chip manufacturing, etc. In recent years, drilling technology based on new laser devices has become an international research hotspot for its advantages of high speed, high accuracy and low thermal damage. In this paper, the laser drilling is compared with other drilling methods, and the influencing factors and application trends are introduced. At the same time paper introduced the main research on the model and theory of laser drilling at home and abroad. The important effects of thermal effect and molten material on drilling and the latest research progress are introduced.

Multimode description of self-mode locking in a single-section quantum-dot laser.

This paper describes a theory for mode locking and frequency comb generation by four-wave mixing in a semiconductor quantum-dot active medium. The derivation uses a multimode semiclassical laser theory that accounts for fast carrier collisions within an inhomogeneous distribution of quantum dots. Numerical simulations are presented to illustrate the role of active medium nonlinearities in mode competition, gain saturation, carrier-induced refractive index and creation of combination tones that lead to locking of beat frequencies among lasing modes in the presence of cavity material dispersion.

Quantum and semiclassical exceptional points of a linear system of coupled cavities with losses and gain within the Scully-Lamb laser theory

In the past few decades, many works have been devoted to the study of exceptional points (EPs), i.e., exotic degeneracies of non-Hermitian systems. The usual approach in those studies involves the introduction of a phenomenological effective non-Hermitian Hamiltonian (NHH), where the gain and losses are incorporated as the imaginary frequencies of fields and from which the Hamiltonian EPs (HEPs) are derived. Although this approach can provide valid equations of motion for the fields in the classical limit, its application in the derivation of EPs in the quantum regime is questionable. Recently, a framework [Minganti {\it et al.}, \href{https://doi.org/10.1103/PhysRevA.100.062131}{Phys. Rev. A {\bf 100}, 062131 (2019)}], which allows one to determine quantum EPs from a Liouvillian EP (LEP), rather than from an NHH, has been proposed. Compared to the NHHs, a Liouvillian naturally includes quantum noise effects via quantum-jump terms, thus allowing one to consistently determine its EPs purely in the quantum regime. In this work we study a non-Hermitian system consisting of coupled cavities with unbalanced gain and losses, where the gain is far from saturation, i.e, the system is assumed to be linear. We apply both formalisms, based on an NHH and a Liouvillian within the Scully-Lamb laser theory, to determine and compare the corresponding HEPs and LEPs in the semiclassical and quantum regimes. {Our results indicate that, although the overall spectral properties of the NHH and the corresponding Liouvillian for a given system can differ substantially, their LEPs and HEPs occur for the same combination of system parameters.}

Statistical theory of a self-seeded free electron laser with noise pedestal growth

We study the impact of a spectral noise pedestal on the statistical properties of a self-seeded free electron laser (FEL). The broad pedestal is assumed to come from self amplified spontaneous emission (SASE) in the second stage of the self-seeding system and is uncorrelated with the narrowband amplified seed. An analytic description is developed based on the statistical theory of a one-dimensional FEL in the high gain linear regime. The theory shows good agreement with experimental data, which show that the presence of a SASE background can have a strong impact on the statistical behavior of self-seeding systems.

Intelligent Modeling for Transfer Function Control of DBR Semiconductor Laser at Near-Working Point

In this paper, a novel intelligent modeling method for transfer function control of DBR semiconductor lasers at near-working point is proposed, which is based on the quantum particle swarm optimization (QPSO) algorithm. This modeling method effectively solves the difficulty of application in the traditional laser theory and equivalent circuit models in practical engineering. First, we analyze the input and output characteristics of DBR semiconductor laser at near-working point, where the laser system can be equivalent to a transfer function model including two main modules: the laser power control module and the laser wavelength control module, and determine the structure of this equivalent model based on the analysis results. Then, we use the QPSO intelligent algorithm to identify the model parameters and finally obtain the equivalent transfer function model that can be easily applied to practical engineering. The standard deviations of the steady-state errors of these two modules are 3.4×10 -3 and 1.2×10 -5 , respectively. Experiments verify the effectiveness and convenience of the proposed (intelligent modeling) method, which can be used for on-line modeling of the DBR semiconductor laser at near-working point.

随机激光研究综述（特邀）

Due to its unique structure and low coherence, random lasers are widely used in fields such as speckle-free imaging, sensing, and light therapy. The feedback mechanism of random lasers is light scattering introduced by disordered media. High threshold and non-directionality are its main disadvantages. In order to solve these problems, researchers used the one-dimensional confinement of optical fibers to obtain random fiber lasers with a low threshold and a certain directionality. In the past ten years, the development of random lasers has experienced a process from incoherent feedback to coherent feedback, from complete disorder to controllable output parameters. A large number of studies have tried to explain the physical nature of random lasers using quantum theory, chaotic laser theory, and numerical analysis. The origin and development history of random lasers and random fiber lasers were reviewed, the classification and related principles of random lasers were introduced, the methods of controlling random laser output parameters were summarized, the recent typical applications of random lasers were demonstrated, fiber random lasers feedback types and gain mechanisms were analyzed, and finally the future of random lasers development was prospected.

On the theory of a pulse laser with microwave pumping

The construction of a laser with a microwave pumping based on the light-pumping cell that uses radiation of sulfur vapor exposed to electromagnetic waves of the microwave band is discussed. The advantages of this device are high efficiency of the optical pumping cell, simple structure, the possibility of cooling the pumping element, and easiness of modifying the emission spectrum of the optical pumping by changing the admixtures in the light-emitting cell. A mathematical tool for the simulation of the short-pulse operation of this laser is proposed basing on the solving a generalized wave equation. The microwave waveguide with the sulfur vapors is interpreted as a regular transmission line with essentially nonlinear dispersion characteristic and substantial dissipation. Possible techniques for the numerical solving the generalized wave equation are described. The continuous approximation of the regular dispersive line is used. In the Fourier approach, the electric field in the system is calculated as a series in the longitudinal wavenumber. As an alternative, the D’Alembert approach may be used, with evaluating the electric field as a series in the frequency. Three-layer explicit and implicit second-order approximation schemes for the solving generalized wave equation in the Fourier approach are constructed. The modeling of radio pulse propagation in the “cold” regular electrodynamic system was performed. Stability of the algorithm and qualitative agreement between the numerical and analytic results confirm the correctness of the base equations as well as their solutions based at the finite-difference schemes.

Multimode lasing in wave-chaotic semiconductor microlasers

We investigate experimentally and theoretically the lasing behavior of dielectric microcavity lasers with chaotic ray dynamics. Experiments show multimode lasing for both D-shaped and stadium-shaped wave-chaotic cavities. Theoretical calculations also find multimode lasing for different shapes, sizes and refractive indices. While there are quantitative differences between the theoretical lasing spectra of the stadium and D-cavity, due to the presence of scarred modes with anomalously high quality factors, these differences decrease as the system size increases, and are also substantially reduced when the effects of surface roughness are taken into account. Lasing spectra calculations are based on Steady-State Ab Initio Laser Theory, and indicate that gain competition is not sufficient to result in single-mode lasing in these systems.

Laser induced arc dynamics destabilization in laser-arc hybrid welding

The interaction between laser and arc plasma is a central issue in laser-arc hybrid welding. We report a new interaction phenomenon called laser destabilizing arc dynamics in kilowatt fiber laser-TIG hybrid welding of 316L stainless steel. We found the laser action significantly oscillates the arc tail with a 1–3 kHz high frequency. Direct numerical simulation demonstrates that the destabilization mechanism is due to the high-speed oscillated metal vapor ejecting from the mesoscopic keyhole. More interestingly, the high-speed metal vapor could contrict the arc plasma by physical shielding. This provides a fundamentally different explanation from the generally adopted metal vapor ionization theory for laser constrict arc plasma phenomenon. Also, the results substantiate that the arc plasma cannot easily enter into the keyhole because of the violent metal vapor.

Perovskite quantum dot lasers

AbstractOwing to the excellent properties of perovskite quantum dots (QDs), such as an easy synthesis process, high photoluminescence quantum yields, high defect tolerance, and tunable bandgap with different elements, laser actions have been widely conducted. Over the past few years, several approaches have been used for successfully creating perovskite QD lasers. In this review, we summarize the progress of perovskite QD lasers from the aspects of laser theory, characteristics and applications of QD lasers, advantages of perovskite materials for lasers, factors influencing the QD laser threshold, two‐photon pumped QD lasers, and perovskite QD laser stability. At the same time, aiming at existing problems, possible solutions and prospects are presented. image

The influence various sources of fluctuation on the statistical parameters of the vertical cavity surface emitting lasers output radiation

Based on statistical modeling, a numerical analysis of the effects exerted by different factors (fluctuations of the spontaneous emission intensity, nonequilibrium carrier concentration, injection current density) on the statistical characteristics of radiation at the output of surface emitting semiconductor lasers in the region of polarization instability has been performed. In this region the effect of fluctuations is maximal, offering the possibility for substantiated conclusions about relative effects of the parameters. In a theory of semiconductor lasers it is thought that the intensity fluctuations of spontaneous emission represent the dominant source of fluctuations, whereas all other sources may be neglected. As demonstra ted by the results of conducted statistical modeling, this statement is too rigorous; moreover, such a source is not dominant. Taking into consideration fluctuations of the carrier concentration, which result in fluctuations of the amplification factor, we can derive a complete set of the relationships observed experimentally. This result cannot be associated with features our model because in our theory spontaneous emission is a significant factor. If the influence of spontaneous emission would be the dominant factor, it would affect the simulation results. The obtained data make it possible to doubt the key role of the spontaneous emission intensity fluctuations in the process of statistical characteristics formation for the output radiation and to take into account fluctuations of the nonequilibrium carrier concentration.

Semiconductor Fano Lasers

In this paper, a new type of semiconductor laser based on Fano interference is described. One of the laser mirrors relies on the interference between the continuum of waveguide modes and a side-coupled nanocavity, leading to a narrow-band mirror that provides the Fano laser with unique characteristics. In addition to being truly single-mode, the laser can be modulated through the mirror at frequencies far exceeding the relaxation oscillation resonance. Furthermore, nonlinearities in the nanocavity can be used to implement a saturable mirror, leading to passive pulse generation with repetition frequencies in the gigahertz range. This paper reviews the theory of Fano lasers and the current experimental status. Experimentally, the Fano laser concept is demonstrated using a photonic crystal platform with quantum dot active material. Both continuous wave operation and self-pulsing is observed for optically pumped lasers operating at room temperature.