Theory Of Spontaneous Emission Research Articles

Unveiling the Enhancement of Spontaneous Emission at Exceptional Points

Exceptional points (EPs), singularities of non-Hermitian physics where complex spectral resonances degenerate, are one of the most exotic features of nonequilibrium open systems with unique properties. For instance, the emission rate of quantum emitters placed near resonators with EPs is enhanced (compared to the free-space emission rate) by a factor that scales quadratically with the resonance quality factor. Here, we verify the theory of spontaneous emission at EPs by measuring photoluminescence from photonic-crystal slabs that are embedded with a high-quantum-yield active material. While our experimental results verify the theoretically predicted enhancement, they also highlight the practical limitations on the enhancement due to material loss. Our designed structures can be used in applications that require enhanced and controlled emission, such as quantum sensing and imaging.

Collective spontaneous emission and kinetic equations for one-photon light in random media

We consider the theory of spontaneous emission for a random medium of stationary two-level atoms. We investigate the dynamics of the field and atomic probability amplitudes for a one-photon state of the system. At long times and large distances, we show that the corresponding average probability densities can be determined from the solutions to a pair of kinetic equations.

“ hv-Energy Packet floating in Front of the Wave Beam” Hybrid Structure of Photon and its Self-Interference in the Single Photon Double-Slits Experiment

In the single photon double slits experiment, what mechanism makes the interference? Whether it is owing to the external factor or the photon has a special structure it can interfere photon itself to make the interference pattern? Because the photon is a quantum of EM radiation as Einstein proposed, so we start the study from an EM wave beam (an EM radiation). Under the demand of the symmetry and quantization, we found the wave beam is certainly circular polarized and covered by a side membrane. There is a pair of ± charges ±q and the circular tension distributes double helically along the side membrane. Quantization of charges requires ( k=2.3,...). Mechanical equilibrium among the helical distributed or, tension and the circular polarized EM field inside construct a steady structure to keep the quantized EM beam integrity, shape and size. Its energy hv concentrates in a cylindrical packet of radius R max and length , named -(energy) packet. With the aid of Einstein theory of spontaneous emission, we proved that the photon is consisted of the energy packet and accompany with a conical - (EM) wave beam; -packet floats in front of the -wave. It is such hybrid structure that makes photon self interference in the double slits experiment.

Realizing quantum free-electron lasers: a critical analysis of experimental challenges and theoretical limits

We examine the experimental requirements for realizing a high-gain quantum free-electron laser (QFEL). Beyond fundamental constraints on electron beam and undulator, we discuss optimized interaction geometries, include coherence properties along with the impact of diffraction, space-charge and spontaneous emission. Based on desired QFEL properties, as well as current experimental capabilities, we provide a procedure for determining a corresponding set of experimental parameters. Even for an idealized situation, the combined constraints on space-charge and spontaneous emission put strong limits on sustaining the quantum regime over several gain lengths. Guided by these results we propose to shift the focus towards seeded QFELs instead of continuing to aim for self-amplified spontaneous emission. Moreover, we point out the necessity of a rigorous quantum theory for spontaneous emission as well as for space-charge in order to identify possible loopholes in our line of argument.

The Theory of Spontaneous Emission and Manifestation of Fermis Golden Rule

The Theory of Spontaneous Emission and Manifestation of Fermis Golden Rule

Effects of Thermal Fluctuations on Temperature Anisotropy Instabilities in the Solar Wind

AbstractThe proton temperature anisotropy in the solar wind, when plotted as a two‐dimensional histogram in (β∥,T⊥/T∥) phase space, shows that the bulk of the data are broadly distributed in quasi‐isotropic state, while the fringe of data distribution is apparently regulated by various marginal stability conditions. The physical origin of magnetic field fluctuations in the stable regime is an open question. Among plausible mechanisms is the spontaneous thermal emission process. Discrete particle effects are central to the theory of spontaneous emission and reabsorption of electromagnetic fluctuations in plasmas. The present paper formulates the quasi‐linear theory of temperature anisotropy‐driven instabilities in which discrete particle effects are taken into account. The hybrid simulation is also carried out with varying number of superparticles per cell in order to unveil the underlying relationship between the theoretical and simulated discrete particle effects. The present paper demonstrates that the inherent numerical noise in the particle simulation is not always undesirable but may actually be exploited in order to control the degree of particle discreteness. Such a finding may have a significant implication for simulating the solar wind plasmas.

High‐Frequency Thermal Fluctuations and Instabilities in the Radiation Belt Environment

AbstractThis paper overviews the electrostatic and electromagnetic theories of spontaneous emission in magnetized plasma as they relate to measured electric and magnetic field fluctuations in quiet time radiation belt and ring current region. The pervasively detected high‐frequency fluctuations in the upper‐hybrid frequency range as well as the background low‐frequency range spectral profile in the whistler mode range are explained within the context of the spontaneous emission theory. The quasilinear calculation of loss cone instability is also carried out in order to validate the assumption of spontaneous emission model. It is shown that the saturated wave amplitudes associated with the upper‐hybrid and multiple‐harmonic cyclotron instability are quite low, indicating that the theoretical explanation based upon the assumption of spontaneous emission theory may be adequate for understanding the observed background fluctuations during quiet times.

Simulation of electromagnetic fluctuations in thermal magnetized plasma

The present paper carries out a particle-in-cell (PIC) simulation in order to validate the recently formulated theory of electromagnetic fluctuations emitted spontaneously in thermal magnetized plasmas (Yoon and López 2017 Phys. Plasmas 24 022117). Numerical plots of theoretically constructed fluctuation spectra and computer simulated fluctuation spectra are compared. While the two results produce an overall favorable agreement for subluminous regime in angular frequency versus wave number space, namely, the domain characterized by phase speed less than the speed of light in vacuo, the present PIC simulation also shows that fluctuation spectra are highly enhanced in the close vicinity of linear eigenmodes, which includes superluminal range that does not satisfy the linear cyclotron wave-particle resonance condition. Since the theory of electromagnetic spontaneous emission, which is based upon linear plasma response and linear wave-particle resonant interactions, strictly forbids emissions in such a regime, the PIC code simulation can only be understood in terms of nonlinear wave-particle interaction. This calls for nonlinear generalization of the spontaneous emission theory.

Kinetic Scale Structure of Low-frequency Waves and Fluctuations

Abstract The dissipation of solar wind turbulence at kinetic scales is believed to be important for the heating of the corona and for accelerating the wind. The linear Vlasov kinetic theory is a useful tool for identifying various wave modes, including kinetic Alfvén, fast magnetosonic/whistler, and ion-acoustic (or kinetic slow), and their possible roles in the dissipation. However, the kinetic mode structure in the vicinity of ion-cyclotron modes is not clearly understood. The present paper aims to further elucidate the structure of these low-frequency waves by introducing discrete particle effects through hybrid simulations and Klimontovich formalism of spontaneous emission theory. The theory and simulation of spontaneously emitted low-frequency fluctuations are employed to identify and distinguish the detailed mode structures associated with ion-Bernstein modes versus quasi-modes. The spontaneous emission theory and simulation also confirm the findings of the Vlasov theory in that the kinetic Alfvén waves can be defined over a wide range of frequencies, including the proton cyclotron frequency and its harmonics, especially for high-beta plasmas. This implies that these low-frequency modes may play predominant roles even in the fully kinetic description of kinetic scale turbulence and dissipation despite the fact that cyclotron harmonic and Bernstein modes may also play important roles in wave–particle interactions.

General theory of spontaneous emission near exceptional points.

We present a general theory of spontaneous emission at exceptional points (EPs)-exotic degeneracies in non-Hermitian systems. Our theory extends beyond spontaneous emission to any light-matter interaction described by the local density of states (e.g., absorption, thermal emission, and nonlinear frequency conversion). Whereas traditional spontaneous-emission theories imply infinite enhancement factors at EPs, we derive finite bounds on the enhancement, proving maximum enhancement of 4 in passive systems with second-order EPs and significantly larger enhancements (exceeding 400×) in gain-aided and higher-order EP systems. In contrast to non-degenerate resonances, which are typically associated with Lorentzian emission curves in systems with low losses, EPs are associated with non-Lorentzian lineshapes, leading to enhancements that scale nonlinearly with the resonance quality factor. Our theory can be applied to dispersive media, with proper normalization of the resonant modes.

Weak Value Amplification in Resonance Fluorescence

The concept of weak measurement and associated weak value amplification has sharpened our understanding of the measurement process in quantum mechanics. Recent experiments show that elastic scattering events in resonance fluorescence experiments can exhibit weak value amplification effect, by post-selecting a particular measurement outcome. In this article, we theoretically analyse the physics behind this amplification process. We show that, in general, weak interaction and the associated weak value amplification in resonance fluorescence can be derived from the well-Known theory of spontaneous emission put forth by Wigner and Weiskopff. Using this theory we show that in the elastic scattering regime of resonance fluorescence, weak value amplification helps in segregating rare events. To our knowledge, weak value amplification has not been applied earlier in the time domain as a potential tool to study rare events.

Excited-to-excited-state scattering using weak measurements

Weak measurements are a subset of measurement processes in quantum mechanics wherein the system, which is being measured, interacts very weakly with the measuring apparatus. Measurement values of observables undergoing a weak interaction and their amplification are concepts that have sharpened our understanding of interaction processes in quantum mechanics. Recent experiments show that naturally occurring processes such as resonance fluorescence from excited states of an atom can exhibit weak value amplification effect. In this paper we theoretically analyze the process of elastic resonance fluorescence from a $\mathsf{V}$-type three-level atomic system, using the well-known Weiskopff-Wigner (WW) theory of spontaneous emission. Within this theory we show that a weak interaction regime can be identified and for suitable choices of initial and final excited states the mean scattering time between these states show an amplification effect during interaction with the vacuum bath modes of the electromagnetic field. We thus show that a system-bath interaction can show weak value amplification. Using our theory we reproduce the published experimental results carried out in such a system. More importantly, our theory can calculate scattering time scales in elastic resonance scattering between multiple excited states of a single atom or between common excited state configurations of interacting multiatom systems.

The influence of chiral spherical particles on the radiation of optically active molecules

Within the framework of nonrelativistic quantum electrodynamics, a theory of spontaneous emission of a chiral molecule located near a chiral (bi-isotropic) spherical particle is developed. It is shown that the structure of photons in the presence of chiral spherical particles differs significantly from the structure of usual transverse electric and transverse magnetic photons. Exact analytical expressions for a spontaneous emission decay rate of a chiral molecule with arbitrary electric and magnetic dipole moments of transition located near a chiral spherical particle with arbitrary parameters, are obtained and analyzed in detail. Simple asymptotes for the case of a nanoparticle are obtained. Substantial influence of even small chirality on a sphere made from dielectric or double negative metamaterial is found. It is shown that by using chiral spherical particles it is possible to effectively control the radiation of a given enantiomer of optically active molecules.

Enhanced Light Extraction in AlInGaN UV Light-Emitting Diodes by an Embedded AlN/AlGaN Distributed Bragg Reflector

A novel kind of AlInGaN ultraviolet (UV) light-emitting diode (LED) with an embedded AlN/Al0.3Ga0.7N distributed Bragg reflector (DBR) is proposed to enhance light extraction efficiency (LEE). The simulation technique we adopt to calculate the LEE of LEDs is based on the theory of spontaneous emission in a layered medium, the well-known mode-matching technique and the scattering matrix approach. The AlN/Al0.3Ga0.7N DBR was intentionally designed to have peak reflectivity at the LED emission wavelength and the optical properties of the DBR were simulated by using the transfer matrix method. A high LEE of 45.7% at 370 nm wavelength was predicted for a proposed AlInGaN UV LED consisting of 24 periods of the AlN/Al0.3Ga0.7N DBR, which is 1.5 times of that of the conventional AlInGaN UV LED. The investigation shows that the AlN/Al0.3Ga0.7N DBR grown on GaN templates with sapphire as a substrate by MOCVD can enhance the LEE effectively and would be very promising for the fabrication of high performance GaN-based UV LEDs.

The theory of spontaneous emission by electrons during their motion along the quantizing magnetic field of a star

The quantum motion of non-relativistic and relativistic electrons in the presence of constant magnetic fields at the surfaces of magnetic stars, magnetic white dwarfs, and pulsars is considered. The quantizing magnetic-field strengths for charged particles with specified energies are determined. The quantum motion of these particles in a plane perpendicular to the magnetic field is accompanied by spontaneous radiation due to electron transitions from higher to lower discrete energy levels, right down to the ground state. In the non-relativistic case, this emission is monochromatic. In the non-relativistic case, various frequencies are emitted, but lie within an order of magnitude of each other. The electron kinetic energy along the magnetic field varies from zero to a maximum value, due to the one-dimensional character of the motion along the field, between each pair of potential barriers corresponding to the discrete energy levels. The results may be relevant to describing gamma-ray flares of pulsars.

Spectrum in spontaneous emission: Beyond the Weisskopf-Wigner approximation

The theory of spontaneous emission presented by Weisskopf and Wigner [V. Weisskopf and E. Wigner, Z. Phys. 63, 54 (1930)] provides an excellent approximation of the actual decay that atoms undergo on optically allowed transitions. However, the theory cannot be rigorously correct since it leads to a Lorentzian spectrum that extends to negative frequencies. Within the rotating-wave approximation, we derive a closed-form expression for the spectrum that is valid for all frequencies.

Quantum theory of spontaneous emission in multilayer dielectric structures

A full quantum-mechanical formalism has been developed in order to evaluate the spontaneous emission rate of dipoles embedded in multilayer dielectric structures. A realistic example of an antisymmetric Silicon waveguide is shown and discussed.

Stimulated emission from two-dimensional nested rectangle photonic crystal fibers

In this paper, we use the theory of spontaneous emission of two-level atoms embedded in photonic crystals. The unavoidable existence of a localized field with a defect mode and its basic properties are confirmed, which enables one to study the inherent laws of spontaneous emission from doped media. We combine the theory of spontaneous emission with numerical simulation for studying stimulated emission in a doped localized field and conditions for increasing the emission. The inherent laws for the cases where transmissivity is larger than unity and the negative imaginary part of the complex refractive index in doped compound lattices appears are also studied. The analysis of two-dimensional nested rectangle photonic crystal fibers (PCFs) with and without active impurities was performed using the finite-difference time-domain (FDTD) method. We find that, when active impurities are introduced into photonic crystal fibers, the negative imaginary part of the complex refractive index appears and the doped mode with high quality factor and great mode density occurs in the photonic forbidden band, thus amplifying substantially the stimulated emission.

Photoluminescence Properties of Tensile-Strained GaAsP/GaInP Single Quantum Wells Grown By Metal Organic Chemical Vapor Deposition

Tensile-strained GaAsP/GaInP single quantum well (QW) laser diode (LD) structures have been grown by low-pressure metal organic chemical vapor deposition (LP-MOCVD) and related photoluminescence (PL) properties have been investigated in detail. The samples have the same well thickness of 16 nm but different P compositions in a GaAsP QW. Two peaks in room-temperature (RT) PL spectra are observed for samples with a P composition larger than 0.10. Temperature- and excitation-power-dependent PL spectra have been measured for a sample with a P composition of 0.15. It is found that the two peaks have a 35 meV energy separation independent of temperature and only the low-energy peak exists below 85 K. Additionally, both peak intensities exhibit a monotonous increase as excitation power increases. Analyses indicate that the two peaks arise from the intrinsic-exciton recombination mechanisms of electron–heavy hole (e–hh) and electron–light hole (e–lh). A theoretical calculation based on model-solid theory, taking into account the spin–orbit splitting energy, shows good agreement with our experimental results. The temperature dependence of PL intensity ratio is well explained using the spontaneous emission theory for e–lh and e–hh transitions, from which the ratio can be characterized mainly by the energy separation between the hh and lh states.

Theory of spontaneous emission of quantum dots in the linear regime

We develop a fully quantum-mechanical theory for the interaction of light andelectron–hole excitations in semiconductor quantum dots. Our theoretical analysis resultsin an expression for the photoluminescence intensity of quantum dots in the linear regime.Taking into account the single-particle Hamiltonian, the free-photon Hamiltonian, theelectron–hole interaction Hamiltonian, and the interaction of carriers with light, andapplying the Heisenberg equation of motion to the photon number expectation values, tothe carrier distribution functions and to the correlation term between the photongeneration (destruction) and electron–hole pair, we obtain a set of luminescence equations.Under quasi-equilibrium conditions, these equations become a closed-set of equations. Wesolve them analytically, in the linear regime, and we find an approximate solution of theincoherent photoluminescence intensity. The validity of the theoretical analysisis tested by investigating the emission spectra in the high-temperature regime,interpreting the experimental findings for the emission spectra of a lens-shapedIn0.5Ga0.5As self-assembled quantum dot. Our theoretical predictions for the interlevel spacing as well asfor the dephasing time caused by electron–longitudinal optical phonon interactions are ingood agreement with the experimental results.