Effect Of Spontaneous Emission Research Articles

Coherent superposition of states in N-pod systems by hyperbolic-tangent coincident pulses

We present an analytically exactly solvable N-pod quantum model, in which the coupling has a hyperbolic-tangent temporal shape and the frequency detuning is constant. The solution is derived by using the Morris–Shore transformation to reduce the N-pod system to a -like one and then to a two state system and a set of decoupled states in two steps. In this work, we use the propagator for a reduced two state system and the inverse transformation to achieve the reduced three state propagator. It is shown that any preselected coherent superposition of all N ground states can be obtained from an initial superposition of M ground states in the presence of detuning. We use a train of tanh coincident pulses and we show that the effect of spontaneous emission is efficiently suppressed by increasing the number of pulses. Besides, we explore that a coherent superposition of ground states from the initial excited state can be implemented using tanh pulses.

Theoretical investigation of the regenerative amplification of ultrashort pulses in a high-pressure transverse electric CO2 laser.

We have developed a theoretical model for the regenerative amplification of ultrashort mid-infrared radiation in high-pressure transverse electric discharge CO2 lasers. This model is based on counter-propagating wave equations that are combined with Boltzmann equation, six-temperature, and circuit-coupled kinetic models. For the first time to our knowledge, the effect of spontaneous emission is taken into account. The model describes noise behavior, contrast ratio, and self-lasing of amplified spontaneous emission. The proposed scheme allows the attainment of a few-picosecond pulse with GW-level peak power and contrast ratio on the order of 106 in the 9μm range.

Plasmon-Induced Quantum Interference near Carbon Nanostructures

We investigate the spontaneous emission properties of a V-type quantum emitter when interacting with either a graphene monolayer or a single-wall carbon nanotube. Quantum interference (QI) effects in spontaneous emission arise due to the anisotropic Purcell effect in both cases. For a graphene monolayer the degree of QI takes values close to 0.333. For the carbon nanotube the confinement of light in one dimension enhances significantly the degree of QI. The high degree of QI is obtained for different orientations of the respective transition dipoles and for a wide range of the emission energy of the quantum emitter, the chemical potential, and the nanotube radius.

Planar Double-Epsilon-Near-Zero Cavities for Spontaneous Emission and Purcell Effect Enhancement.

The enhancement of the photophysical response of fluorophores is a crucial factor for photonic and optoelectronic technologies that involve fluorophores as gain media. Recent advances in the development of an extreme light propagation regime, called epsilon-near-zero (ENZ), provide a promising approach in this respect. In this work, we design metal/dielectric nanocavities to be resonant with the absorption and emission bands of the employed fluorophores. Using CsPbBr3 perovskite nanocrystal films as light emitters, we study the spontaneous emission and decay rate enhancement induced by a specifically tailored double-epsilon-near-zero (double ENZ) structure. We experimentally demonstrate the existence of two ENZ wavelengths, by directly measuring their dielectric permittivity via ellipsometric analysis. The double ENZ nature of this plasmonic nanocavity has been exploited to achieve both surface plasmon enhanced absorption (SPEA) and surface plasmon coupled emission (SPCE), inducing a significant enhancement of both the spontaneous emission and the decay rate of the perovskite nanocrystal film that is placed on top of the nanocavity. Finally, we discuss the possibility of tailoring the two ENZ wavelengths of this structure within the visible spectrum simply by finely designing the thickness of the two dielectric layers, which enables resonance matching with a broad variety of dyes. Our device design is appealing for many practical applications, ranging from sensing to low threshold amplified spontaneous emission, since we achieve a strong PL enhancement with structures that allow for straightforward fluorophore deposition on a planar surface that keeps the fluorophores exposed and accessible.

Dynamics of interacting fermions under spin–orbit coupling in an optical lattice clock

Quantum statistics and symmetrization dictate that identical fermions do not interact via s-wave collisions. However, in the presence of spin-orbit coupling (SOC), fermions prepared in identical internal states with distinct momenta become distinguishable. The resulting strongly interacting system can exhibit exotic topological and pairing behaviors, many of which are yet to be observed in condensed matter systems. Ultracold atomic gases offer a promising pathway for simulating these rich phenomena. Two recent experiments reported the observation of single atom SOC in optical lattice clocks (OLCs) based on alkaline-earth atoms. In these works encoding the effective spin degree of freedom in the long-lived electronic clock states significantly reduced the detrimental effects of spontaneous emission and heating that have thus far hindered the study of interacting SOC with alkali atoms. Beyond first studies of interacting SOC with alkali atoms in a bulk gas and with two particles in a lattice, here we enter a new regime of many-body interacting SOC in an OLC. Using clock spectroscopy, we observe the precession of the collective magnetization and the emergence of spin locking effects arising from an interplay between p-wave and SOC-induced exchange interactions. The many-body dynamics are well captured by a collective XXZ spin model, which describes a broad class of condensed matter systems ranging from superconductors to quantum magnets. Furthermore, our work will aid in the design of next-generation OLCs by offering a route for avoiding the observed large density shifts caused by SOC-induced exchange interactions.

High-power wideband radio-frequency intensity modulated continuous wave laser

A high-power wideband radio-frequency (RF) intensity modulated continuous wave light source is demonstrated. The high-power dual-frequency light source is obtained via a dual-frequency laser signal seeding fiber power amplifier. A diode laser pumped dual-frequency laser is built as the seed and a diode laser pumped three-stage Yb3 + doped large mode area fiber power amplifier is used to enhance the output power to 50 W. In the dual-frequency seed laser, a coupled cavity composed of the Nd:YAG gain crystal and output coupler is used as the mode selector and enforces single longitude mode to oscillate. Two quarter wave plates are inserted in the laser cavity to lift the frequency degeneration of the two orthogonally polarized modes. By changing the angle between the fast axes of the two quarter wave plates, the frequency difference between the two orthogonally polarized modes can be tuned from 30 MHz to 1.5 GHz. The standard difference of beat frequency is 1.6144 MHz and stability is 1.52% when a frequency difference of output dual-frequency laser is 250 MHz. This stable dual-frequency seed signal is amplified via a diode pumped Yb3 +-doped fiber power amplifier. In order to suppress amplified spontaneous emission and other nonlinear effects, a three-stage fiber amplification system is used. The first stage is a diode pumped fiber (5 m, 6/125 m, NA = 0.13) power amplifier. The pump power is fixed at 600 mW. The input dual frequency signal is 3.2 mW, and it is amplified to several hundred mW by the first fiber power amplifier. The second fiber amplifier is a diode laser pumped fiber (5 m, 10/125 m, NA = 0.075/0.46) amplifier. The pump power is fixed at 10 W, and the dual frequency signal is amplified to sub watts after the second fiber amplifier. A 5 m large mode area fiber (25/250 m, NA=0.065/0.46) is used in the final amplification. A maximum amplified power of 50.2 W is obtained when the pump power is 70 W in the experiment. The signal-to-noise ratio of the beat note increases from 25 dB to 40 dB via amplification. The output power fluctuation of the amplified signal at 50 W is smaller than 0.1 W during 30 min. The RF frequency stability is well maintained during the amplification, and the beat-note frequency instability is 1.777 MHz. This high-power dual-frequency light source with wide beat note frequency bandwidth has potential applications in dual-frequency coherent lidar system for long distance ranging and imaging or underwater detections after the frequency has been doubled to 532 nm.

Microscopic Description of Spontaneous Emission in Stark Chirped Rapid Adiabatic Passages

A microscopic approach describing the effect of spontaneous emission in the stark-chirped rapid adiabatic passages (SCRAPs) for quantum computation is presented. Apart from the phenomenological model, this microscopic one can investigate the dependence of the population dynamics both on the temperature of the environment and the decay rate γ. With flux-biased Josephson qubits as a specifical example, we study the efficiency of the SCRAP for realizing the basic Pauli-X and iSWAP gates. Our results show clearly that the behavior of the population transfer described by the microscopic model is similar with the phenomenological one at zero temperature. In the limit of very high temperature, the population probabilities of the qubit states exhibit strong stability properties. High efficiency for the quantum gate manipulations in SCRAPs is available against the weak decay rate γ ≪ 1 at low temperature.

Demonstration of an all-optical feed-forward delay line buffer using the quadratic Stark effect and two-photon absorption in an SOA.

We have inserted into an unbiased semiconductor optical amplifier (SOA) a powerful control beam, with photon energy slightly smaller than that of the band-gap of its active region, for exciting two-photon absorption and the quadratic Stark effect. For the available SOA, we estimated these phenomena generated a nonlinear absorption coefficient β=-865 cm/GW and induced an appreciable birefringence inside the amplifier waveguide, which significantly modified the polarization-state of a probe beam. Based on these effects, we have experimentally demonstrated the operation of an all-optical buffer, using an 80 Gb/s optical pulse comb, as well as an unbiased SOA, which was therefore, devoid of amplified spontaneous emission and pattern effects.

Analytical model of amplified spontaneous emission with different thickness anti-ASE caps for thin disk lasers.

The amplified spontaneous emission (ASE) effect is a major factor affecting large, high gain Yb:YAG thin disk lasers. In this paper, the analytical model is built to estimate the ASE photon flux density of thin disk crystals with different thickness anti-ASE caps. Theoretical analysis shows that it is unnecessary to use a critical thickness anti-ASE cap to suppress the ASE effect. At the same time, adjusting the coupler reflectivity of the resonator is an effective way to decrease the ASE effect and decrease the thickness of the anti-ASE cap to effectively suppress the ASE effect. This paper will better reveal how the anti-ASE cap suppresses the ASE effect.

Amplified spontaneous emission of an end-pumped cesium vapor laser

Diode pumped alkali lasers (DPALs) provide a significant potential for construction of high-powered lasers. A series of models have been established to analyze the DPAL’s kinetic process and most of them are based on the algorithms in which the amplified spontaneous emission (ASE) effect has not been considered. However, ASE is harmful in realization of a high-powered DPAL since the gain is very high. Usually, ASE becomes serious when the volume of the gain medium is large and the pump power is high. Basically, the conclusions we obtained in this study can be extended to other kinds of laser configurations.

Effects of spontaneous emission excited state lifetime on the output performance of quantum well lasers

In this paper, we have investigated the effect of spontaneous emission’s excited state lifetime on the output performance of the quantum well laser using quantum well transmission line modelling technique. It has been found that as the spontaneous emission lifetime increases the turn-on transient response of the quantum well laser is delayed, exhibiting the larger peak and steady state arrival times and the laser steady output has larger noise. Meanwhile, the increase in spontaneous emission lifetime leads to the larger peak and steady state arrival times of the carrier density variation during the turn-on process. Also, due to the increase of the spontaneous emission lifetime, the bandwidth and magnitude of the laser steady output spectra increase while the side lobe suppression ratio decreases and there exists a blue shift in the peak wavelength. These studies confirm that the quantum well laser with less spontaneous emission lifetime has more superior output performance (fast transient response, less noise and narrow bandwidth); they also provide an effective method to compare and predict the spontaneous emission lifetimes for different semiconductor optical materials.

Theoretical analysis of uniform longitudinal gain for a double-end-pumped Yb:YAG multi-slab pulsed amplifier

For high average power lasers with double-end-pumped multi-slab (DEPMS) configuration, a non-uniform gain distribution within each slab caused by the step-doped multi-slab will lead to the amplified spontaneous emission (ASE) effect and then reduce the laser performance. As an example, longitudinal gradient doping is used in a DEPMS Yb:YAG pulsed amplifier to create uniform longitudinal gain (ULG) and then the ASE effect is effectively reduced. The ULG is theoretically derived and we show that the energy storage can be increased by 9% with ULG compared with the step-doped DEPMS. We found that the optimal pump efficiency, longitudinal gain-length product and columnar doping density are determined by the launched pump intensity. Numerical simulation also demonstrated that optimization of the amplifier with uniform gain and high pump efficiency can be achieved by placing additional gradient slabs to the original gradient doping slabs. Our investigations might provide an efficient way to carefully optimize a DEPMS pulsed amplifier with higher efficiency.

Amplified spontaneous emission model of thin disk laser with nonuniform temperature distribution

The amplified spontaneous emission (ASE) effect is a major factor affecting large size, high gain Yb:YAG thin disk lasers. In this paper, a model combining the rate equations including an ASE effect term and nonuniform temperature distribution in a thin disk crystal is presented. A numerical iterative method is used to analyze the dynamic behavior, the space distribution of ASE effect, and the optimum coupler reflectivity of the thin disk laser. It is shown that the temperature distribution and the size of the pump region are the main factors to influence the ASE effect and ultimately affect the output power and threshold of the laser system. In order to decrease the ASE effect, the coupler reflectivity can be adjusted to control the gain coefficient to increase output power.

Decoherence by spontaneous emission: A single-atom analog of superradiance

We show that the stationary decoherence rate of an open quantum system can be decomposed as a sum of local and nonlocal contributions, respectively related to the strength of the coupling between system and environment, and to the quality of the information about the system leaking into the environment. Both terms arise naturally in the framework of the influence functional, with the nonlocal term arising from the coupling between the backward and forward histories describing the open quantum system time evolution. While the local contribution always yields a positive decoherence rate, the nonlocal one may lead to recoherence when only partial information about the system is obtained from the disturbed environment. We illustrate these concepts in the framework of interferometry with trapped atoms, by analyzing the effect of spontaneous emission on the coherence of the center-of-mass.

Nuclear-state population transfer by a train of coincident pulses

Population transfer of three-level $\mathrm{\ensuremath{\Lambda}}$-like nuclei interacting with x-ray free electron lasers (XFEL) via a train of coincident pulses has been investigated theoretically. This study uses copropagating beams in which the frequency of the pump laser is different from that of the Stokes laser. We calculate the required laser intensities for each step which satisfy the condition of coincident-pulse technique in different nuclei. By employing the master equation and considering the effect of spontaneous emission, we show that complete nuclear population transfer occurs by choosing the appropriate number of pulse pairs. It is shown that the effect of laser intensity fluctuation is suppressed by increasing the number of pulse pairs. In this scheme, after each step, the populations of system are in stable states and the time delays between the neighboring pulses do not affect the transmission efficiency.

Effect of bremsstrahlung radiation emission on fast electrons in plasmas

Bremsstrahlung radiation emission is an important energy loss mechanism for energetic electrons in plasmas. In this paper we investigate the effect of spontaneous bremsstrahlung emission on the momentum−space structure of the electron distribution, fully accounting for the emission of finite−energy photons by modeling the bremsstrahlung interactions with a Boltzmann collision operator. We find that electrons accelerated by electric fields can reach significantly higher energies than predicted by the commonly used radiative stopping−power model. Furthermore, we show that the emission of soft photons can contribute significantly to the dynamics of electrons with an anisotropic distribution by causing pitch−angle scattering at a rate that increases with energy.

Enhancement of Ultracold Molecule Formation Using Shaped Nanosecond Frequency Chirps.

We demonstrate that judicious shaping of a nanosecond-time-scale frequency chirp can dramatically enhance the formation rate of ultracold (87)Rb(2) molecules. Starting with ultracold (87)Rb atoms, we apply pulses of frequency-chirped light to first photoassociate the atoms into excited molecules and then, later in the chirp, deexcite these molecules into a high vibrational level of the lowest triplet state a (3)Σ(u)(+). The enhancing chirp shape passes through the absorption and stimulated emission transitions relatively slowly, thus increasing their adiabaticity, but jumps quickly between them to minimize the effects of spontaneous emission. Comparisons with quantum simulations for various chirp shapes support this enhancement mechanism.

Electromagnetic fluctuations in magnetized plasmas. I. The rigorous relativistic kinetic theory

Using the system of the Klimontovich and Maxwell equations, the general linear fluctuation theory for magnetized plasmas is developed. General expressions for the electromagnetic fluctuation spectra (electric and magnetic fields) from uncorrelated plasma particles in plasmas with a uniform magnetic field are derived, which are covariantly correct within the theory of special relativity. The general fluctuation spectra hold for plasmas of arbitrary composition, arbitrary momentum dependences of the plasma particle distribution functions, and arbitrary orientations of the wave vector with respect to the uniform magnetic field. Moreover, no restrictions on the values of the real and the imaginary parts of the frequency are made. The derived fluctuation spectra apply to both non-collective fluctuations and collective plasma eigenmodes in magnetized plasmas. In the latter case, kinetic equations for the components of fluctuating electric and magnetic fields in magnetized plasmas are derived that include the effect of spontaneous emission and absorption. In the limiting case of an unmagnetized plasmas, the general fluctuation spectra correctly reduce to the unmagnetized fluctuation spectra derived before.

Analysis and simulation of nonlinearity and effects of spontaneous emission in Schottky-junction-based plasmonic amplifiers.

An amplifier that operates on surface plasmon polaritons has been analyzed and simulated. Nonlinearity behavior and the spontaneous emission effects of the plasmonic amplifier are investigated in this paper. A rate equations approach has been used in which parameters are derived from simulation results of the plasmonic amplifier (Silvaco/ATLAS). Details on the method of this derivation are included, which were not previously reported. Rate equations are solved numerically by MATLAB codes. These codes verify the Silvaco results. The plasmonic amplifier operates on surface plasmons with a free-space wavelength of 1550 nm. Results show that, even without the effect of spontaneous emission, gain of the plasmonic amplifier saturates in high input levels. Saturation power, which can be used for comparing nonlinearity of different amplifiers, is 2.1 dBm for this amplifier. Amplified spontaneous emission reduces the gain of the amplifiers, which is long. There is an optimum value for the length of the amplifier. For the amplifier of this work, the optimum length for the small signal condition is 265 μm.

Laser cooling without spontaneous emission.

This Letter reports the demonstration of laser cooling without spontaneous emission, and thereby addresses a significant controversy. It works by restricting the atom-light interaction to a time short compared to a cycle of absorption followed by natural decay. It is achieved by using the bichromatic force on an atomic transition with a relatively long excited state lifetime and a relatively short cooling time so that spontaneous emission effects are minimized. The observed width of the one-dimensional velocity distribution is reduced by ×2 thereby reducing the "temperature" by ×4. Moreover, our results comprise a compression in phase space because the spatial expansion of the atomic sample is limited. This accomplishment is of interest to direct laser cooling of molecules or in experiments where working space or time is limited.