Laser Theory Research Articles

Congeners of Pyrromethene-567 Dye: Perspectives from Synthesis, Photophysics, Photostability, Laser, and TD-DFT Theory.

In an attempt to develop photostable and efficient BODIPY (PM) dyes for use in liquid dye lasers, three new congeners of widely used laser dye, PM567, were synthesized and their photophysical properties in various organic solvents, laser performances, and photostabilities in a selected solvent, 1,4-dioxane, have been investigated using a frequency doubled Q-switched (10 Hz) Nd:YAG laser at 532 nm. The results of photostability study in nonpolar 1,4-dioxane revealed the remarkable enhancement in stability of the novel dyes compared to that of PM567 as well as improved laser performances. Cyclic voltammetry study strongly supports the observed enhancement in photostability of the novel dyes compared to that of PM567. The observed properties of the novel dyes in relation to those of PM567 have been rationalized by extensive use of DFT and TD-DFT using the B3LYP/6-31G(d) method of theory.

광음향 분광을 이용한 고체레이저의 방사양자효율 측정

The radiative quantum efficiency of a solid-state laser was measured by photoacoustic spectroscopy with a PZT as the detector. The radiative quantum efficiency was about 58.3 % for a laser-diode pumped Nd:S-VAP laser under lasing conditions. The measurement of radiative quantum efficiency was presented as an effective method for the optimization of a laser resonator.Keywords: Photoacoustic spectroscopy, Radiative quantum efficiency, Diode pumped solid-state laserOCIS codes: (110.5125) Photoacoustics; (260.2160) Energy transfer; (270.3430) Laser theory; (140.3480) Laser, diode-pumped

Electron acceleration in underdense plasmas described with a classical effective theory

AbstractAn effective theory of laser–plasma-based particle acceleration is presented. Here we treated the plasma as a continuous medium with an index of refraction nm in which a single electron propagates. Because of the simplicity of this model, we did not perform particle-in-cell (PIC) simulations in order to study the properties of the electron acceleration. We studied the properties of the electron motion due to the Lorentz force and the relativistic equations of motion were numerically solved and analyzed. We compared our results with PIC simulations and experimental data.

Steady-state ab initio laser theory for complex gain media.

We derive and test a generalization of the steady-state ab initio laser theory (SALT) to treat complex gain media. The generalized theory (C-SALT) is able to treat atomic and molecular gain media with diffusion and multiple lasing transitions, and semiconductor gain media in the free carrier approximation including fully the effect of Pauli blocking. The key assumption of the theory is stationarity of the level populations, which leads to coupled self-consistent equations for the populations and the lasing modes that fully include the effects of openness and non-linear spatial hole-burning. These equations can be solved efficiently for the steady-state lasing properties by a similar iteration procedure as in SALT, where a static gain medium with a single transition is assumed. The theory is tested by comparison to much less efficient finite difference time domain (FDTD) methods and excellent agreement is found. Using C-SALT to analyze the effects of varying gain diffusion constant we demonstrate a cross-over between the regime of strong spatial hole burning with multimode lasing to a regime of negligible spatial hole burning, leading to gain-clamping, and single mode lasing. The effect of spatially inhomogeneous pumping combined with diffusion is also studied and a relevant length scale for spatial inhomogeneity to persist under these conditions is determined. For the semiconductor gain model, we demonstrate the frequency shift due to Pauli blocking as the pumping strength changes.

Optomechanical laser cooling with mechanical modulations

We theoretically study the laser cooling of optomechanical cavities when the mechanical resonance frequency and damping depend on time. In the regime of weak optomechanical coupling we extend the theory of laser cooling using an adiabatic approximation. We discuss the modifications of the cooling dynamics and compare it with numerical simulations in a wide range of modulation frequencies.

Dispersion relation and growth rate of a relativistic electron beam propagating through a Langmuir wave wiggler

In this study, a theory of free-electron laser (FEL) with a Langmuir wave wiggler in the presence of an axial magnetic field has been presented. The small wavelength of the plasma wave (in the sub-mm range) allows obtaining higher frequency than conventional wiggler FELs. Electron trajectories have been obtained by solving the equations of motion for a single electron. In addition, a fourth-order Runge–Kutta method has been used to simulate the electron trajectories. Employing a perturbation analysis, the dispersion relation for an electromagnetic and space-charge waves has been derived by solving the momentum transfer, continuity, and wave equations. Numerical calculations show that the growth rate increases with increasing the e-beam energy and e-beam density, while it decreases with increasing the strength of the axial guide magnetic field.

Numerically exact solution of the many emitter–cavity laser problem: Application to the fully quantized spaser emission

A numerically exact solution to the many emitter -- cavity problem as an open many body system is presented. The solution gives access to the full, nonperturbative density matrix and thus the full quantum statistics and quantum correlations. The numerical effort scales with the third power in the number of emitters. Notably the solution requires none of the common approximations like good/bad cavity limit. As a first application the recently discussed concept of coherent surface plasmon amplification -- spaser -- is addressed: A spaser consists of a plasmonic nanostructure that is driven by a set of quantum emitters. In the context of laser theory it is a laser in the (very) bad cavity limit with an extremely high light matter interaction strength. The method allows us to answer the question of spasing with a fully quantized theory.

Light Amplification and Scattering by Clusters Made of Small Active Particles: The Local Perturbation Approach

The light amplification by finite active media is used extensively in modern optics applications. In this paper the light amplification and scattering by cluster of small particles is studied analytically and numerically with the help of the local perturbation method and phenomenological laser theory. It is shown that light amplification is possible even for one small particle, and that the amplification is more profound when the light frequency nears the frequency of the cluster's morphological resonance. Theoretical discussions are supplemented by numerical results for scattering by clusters which particles positioned at ordered and at slightly disordered positions.

激光原理与技术理论与实践教学的统一

激光原理与技术包括理论教学与实践教学两方面。然而在具体的教学过程中，理论教学与实践教学之间会出现一些不可避免的矛盾。通过在课堂上增加一些演示实验或演示实验视频，以及在实验室进行操作前进行理论指导，可以有效地解决理论教学与实践教学的不同步性以及内容脱节等问题，巩固学习效果。因此，理论教学与实践教学的高度统一有利于大学生的创新性学习。 Laser theory and technology is composed of two respects of theory teaching and practice teaching. However, some unavoidable contradiction can be appeared between theory teaching and practice teaching in the concrete teaching procedure. The problems such as asynchronism and disconnec-tion of contents between theory teaching and practice teaching can be effectively solved through adding some demonstrative experiments or some videos of them and introducing some theoretical direction before operation in laboratory which consolidates learning effect. Therefore, the highly unification of theory teaching and practice teaching is good for innovative learning of un-dergraduate students.

Dispersion relation and growth rate in two-stream thermal plasma-loaded free-electron laser with helical wiggler

Linear theory of the two-stream free-electron laser consisting of a relativistic electron beam transported along the axis of thermal plasma-loaded helical wiggler is proposed and investigated. The dispersion relation is derived employing linear fluid theory. The characteristics of the dispersion relation are analyzed by numerical solutions. The results show in that in the special values of the plasma temperature the growth rate is considerably enhanced. It is also shown that the growth rate after critical plasma density gradually decreases. Moreover, in the presence of the two-electron beam the growth rate of electrostatic mode is two times greater than that for electromagnetic mode.

Nonlinear Theory of Plasma-Beam Superheterodyne Free Electron Laser of Dopplertron Type with Non-Axial Injection of Electron Beam

We have developed a quadratic nonlinear theory of a plasma-beam superheterodyne free electron laser of dopplertron type with the non-axial injection of an electron beam with respect to the guiding magnetic eld. It is found out that the studied device can work in four di erent modes of operation. We determine the modes that have maximum gain coe cients of electromagnetic signals. It is shown that for all the operating modes the ampli cation factor of an electromagnetic signal is enhanced when the electron beam injection angle is increased.

Three-dimensional laser cooling at the Doppler limit

Predictions of the Doppler theory of laser cooling are quantitatively verified for the first time using helium atoms, for which the special atomic properties, particularly its small mass and narrow transition linewidth, inhibit sub-Doppler cooling mechanisms.

Low-threshold stimulated emission of surface plasmons polaritons

The stimulated emission threshold of surface plasmon polaritons (SPPs) propagating at the interface of silver and dye-doped polymer can be reduced by an order of magnitude if the operation range is changed from visible to near-infrared. The stimulated emission is observed at the polymer thickness, which is much below the cut-off for a regular photonic waveguide, proving SPP origin of the stimulated emission mode. With reduction of the polymer film thickness, the intensity of the stimulated emission decreases, its threshold increases, and the emission line experiences a blue shift—all in agreement with the laser theory. The dependence of the stimulated emission threshold on the polymer film thickness suggests quenching of dye molecule emission in vicinity of the silver surface.

Enhancement of laser power-efficiency by control of spatial hole burning interactions

The laser is an out-of-equilibrium nonlinear wave system where the interplay of the cavity geometry and nonlinear wave interactions mediated by the gain medium determines the self-organized oscillation frequencies and the associated spatial field patterns. In the steady state, a constant energy flux flows through the laser from the pump to the far field, with the ratio of the total output power to the input power determining the power-efficiency. Although nonlinear wave interactions have been modelled and well understood since the early days of laser theory, their impact on the power-efficiency of a laser system is poorly understood. Here, we show that spatial hole burning interactions generally decrease the power-efficiency. We then demonstrate how spatial hole burning interactions can be controlled by a spatially tailored pump profile, thereby boosting the power-efficiency, in some cases by orders of magnitude.

Loss-induced suppression and revival of lasing.

Controlling and reversing the effects of loss are major challenges in optical systems. For lasers, losses need to be overcome by a sufficient amount of gain to reach the lasing threshold. In this work, we show how to turn losses into gain by steering the parameters of a system to the vicinity of an exceptional point (EP), which occurs when the eigenvalues and the corresponding eigenstates of a system coalesce. In our system of coupled microresonators, EPs are manifested as the loss-induced suppression and revival of lasing. Below a critical value, adding loss annihilates an existing Raman laser. Beyond this critical threshold, lasing recovers despite the increasing loss, in stark contrast to what would be expected from conventional laser theory. Our results exemplify the counterintuitive features of EPs and present an innovative method for reversing the effect of loss.

Two-stream whistler-pumped free-electron laser

The theory for the two-stream free-electron laser with the whistler wave as a slow electromagnetic wave wiggler is studied theoretically. The results show that for small values of plasma density, there are seven groups of orbits and the plasma density enhancement leads to increase in the number of trajectories. Also, increasing the plasma density causes growth in the axial velocity for groups (1–4) of orbits and a reduction for groups (6–7). It is shown that for groups (6–7), the effect of plasma is to increase the gain, and for groups (1–4) the effect of plasma is to decrease the gain. For group (1) the effect of plasma is to decrease the maximum frequency and for group (3) it is to increase the maximum frequency.

Three-dimensional theory of Smith-Purcell free-electron laser with dielectric loaded grating

A dielectric loaded rectangular grating for Smith-Purcell devices is proposed in this paper. Regarding the electron beam as a moving plasma dielectric, a three dimensional (3D) linear theory of beam-wave interaction is developed. The first and second order growth rates are calculated, which are obtained by expanding hot dispersion equation at synchronous point. The results show that the cutoff frequency is affected by grating width. The dispersion curve becomes flatter and shifts towards lower frequency by loading dielectric in grooves. The simulation results, which are obtained by a 3D particle-in-cell code, are in good agreement with theoretical calculations. Compared the first and second order growth rate, it shows that the discrepancy is large when beam parameters are selected with high values. In this case, it is necessary to apply the second order growth rate, which can accurately describe the process of beam-wave interaction.

Quantum Langevin model for nonequilibrium condensation

We develop a quantum model for nonequilibrium Bose-Einstein condensation of photons and polaritons in planar microcavity devices. The model builds on laser theory and includes the spatial dynamics of the cavity field, a saturation mechanism, and some frequency dependence of the gain: quantum Langevin equations are written for a cavity field coupled to a continuous distribution of externally pumped two-level emitters with a well-defined frequency. As an example of application, the method is used to study the linearized quantum fluctuations around a steady-state condensed state. In the good-cavity regime, an effective equation for the cavity field only is proposed in terms of a stochastic Gross-Pitaevskii equation. Perspectives in view of a full quantum simulation of the nonequilibrium condensation process are finally sketched.

Scalable numerical approach for the steady-stateab initiolaser theory

We present an efficient and flexible method for solving the non-linear lasing equations of the steady-state ab initio laser theory. Our strategy is to solve the underlying system of partial differential equations directly, without the need of setting up a parametrized basis of constant flux states. We validate this approach in one-dimensional as well as in cylindrical systems, and demonstrate its scalability to full-vector three-dimensional calculations in photonic-crystal slabs. Our method paves the way for efficient and accurate simulations of lasing structures which were previously inaccessible.

A nonperturbative quantum electrodynamic approach to the theory of laser induced high harmonic generation

Using nonperturbative quantum electrodynamics, we develop a scattering theory for high harmonic generation (HHG). A transition rate formula for HHG is obtained. Applying this formula, we calculate the spectra of high harmonics generated from different noble gases shined by strong laser light. We study the cutoff property of the spectra. The data show that the cutoff orders of high harmonics are greater than that predicted by the “3.17” cutoff law. As a numerical experiment, the data obtained from our repeated calculations support the newly derived theoretical expression of the cutoff law. The cutoff energy of high harmonics described by the new cutoff law, in terms of the ponderomotive energy Up and the ionization potential energy Ip, is 3.34Up + 1.83Ip. The higher cutoff orders predicted by this theory are due to the absorption of the extra photons, which participate only the photon-mode up-conversion and do nothing in the photoionization process.