Showing papers on "Spontaneous emission published in 1968"

Spontaneous Emission Probabilities and Quantum Efficiencies for Excited States of Pr3+ in LaF3

Abstract: Spontaneous emission probabilities for electric‐ and magnetic‐dipole transitions from several electronic states of Pr3+ in LaF3 are calculated using the Judd–Ofelt theory and experimental intensity parameters. The calculated radiative decay probabilities are combined with observed fluorescence lifetimes to determine the quantum efficiencies of the 1D2 and 3P0 states. Although the lifetimes were measured at low temperatures and small Pr concentrations where competing nonradiative decay by multiphonon emission and ion‐pair relaxation should be negligible, the predicted quantum efficiencies are less than unity. The radiative lifetimes obtained via the Einstein relations and measured integrated absorption and relative fluorescence intensities agree, however, with the observed lifetimes. The discrepancies appear to arise from approximations and possible limitations inherent in the approach used to calculate the radiative transition probabilities. Matrix elements of U(λ) for states of 4f2 in intermediate coupling are included in an appendix.

The quantum fluctuations of the optical parametric oscillator. i.

Abstract: Our treatment is based on a microscopically correct Hamiltonian which contains the Bose-operators of the light modes and the Fermi-operators of the optically active electrons in the medium. The coupling between modes and atoms is taken from quantum-electrodynamics. Besides that, the light modes may interact with external “heat baths” like the mirrors, scattering centers etc., while the atoms interact with lattice vibrations, incoherent light fields etc. Using recently developed methods the effect of these heatbaths is taken into account in a quantum mechanically consistent fashion. In the present paper we apply quantum mechanical Langevin equations for the field and electron operators which contain dissipation and fluctuation terms. The elimination of the electron operators by an iteration procedure finally leaves us with a set of coupled nonlinear field equations which are shown to be quantum mechanically consistent. They are solved in the Heisenberg picture below threshold by linearization and well above threshold by quantum mechanical quasi-linearization. The solutions show that the line width of the signal mode below threshold is due to the vacuum fluctuations in the idler and vice versa, whereas the thermal noise of the resonator and the spontaneous emission noise of the medium may be neglected. Above threshold the linewidth is caused by the undamped diffusion of the phase difference between signal and idler, to which the vacuum fluctuations of both modes contribute in equal parts. The phase sum of both modes adiabatically follows the slow phase diffusion of the external pump light, produced by a laser, and therefore contributes to the linewidth too. Well above threshold the amplitudes are stable. Correlation and cross-correlation functions of their small residual fluctuations are calculated.

Temperature dependence of emission efficiency and lasing threshold in laser diodes

Abstract: It is found that in GaAs 1-x P x injection lasers both the spontaneous emission efficiency η and the laser threshold j vary exponentially with the temperature T: \eta = \eta_{0} \exp(-T/\theta_{1}) and j = j_{0} \exp(T/\theta_{2}) . θ 1 and θ 2 are usually nearly equal and range between 50 and 110°K. The behavior of the external efficiency is correlated with the temperature dependence of the absorption along the propagation path of the radiation: as the temperature increases, the exponential absorption edge shifts to lower energies faster than the emission peak. The difference between the two shifts is nearly linear with temperature. The effect of self-absorption on external efficiency was tested by measuring the light emitted transversely to the p-n junction through a layer of constant thickness. With such a geometry, it is shown that the efficiency should vary as \exp [-A \exp (T/\theta)] . The experiment agrees with this prediction.

The emission and absorption of waves by charged particles in magnetized plasmas

Abstract: A semiclassical theory describing the emission and absorption of waves is applied to the interaction of charged particles with waves in magnetized plasmas. Spontaneous emission of all cold plasma wave modes is calculated in detail. The method gives the absorption coefficient for the waves and a diffusion equation in momentum space for the particles describing the effects of the induced processes.

Selective Excitation Spectroscopy and Some Possible Applications

Abstract: If an atomic transition is optically pumped by a beam of laser radiation, having the appropriate frequency, the population in the upper state of the transition can be considerably enhanced along the path of the beam. This will give rise to an intensification of the spontaneous emission originating from this upper state in the region localized by the beam. Such intensified emission contains information about the conditions within the pumped region and can, under certain conditions, be identified from the background radiation. It is shown that this process called ``selective excitation spectroscopy'' can be used in a variety of ways, one of the most important being the determination of the local values of electron temperature and density in low‐temperature plasmas. This technique is illustrated for both potassium‐ and barium‐seeded plasmas. Furthermore, because this technique involves a pulsed laser, temporal as well as spatial resolution is possible.

Spontaneous emission in the presence of a prescribed classical field.

Abstract: : The effect on the spectrum of spontaneous emission from a simple physical system of the application of a classical driving field is studied. The physical system consists of an ensemble of two level atoms interacting with a relaxation mechanism. The single Lorentzian line shape characteristic of the power spectrum of spontaneous emission for the undriven case, is split into components by the driving field. This splitting is associated with the establishment of definite phase relations between the corresponding components of the field spectrum. (Author)

Polarization of the fluorescence light of ne atoms interacting with a laser beam: Measurement of several relaxation times

Abstract: The fluorescent light from two atomic levels is studied assuming that the atoms are subjected to a gas discharge, to a static magnetic field, and, simultaneously, to a laser beam having the transition frequency between the atomic levels. Calculations are developed in the formalism of irreducible tensor operators. One takes into account the existence of isotropic relaxations and cascade effects due to spontaneous emission between the upper and the lower levels. Following Lamb, the laser beam is described classically by its electric field, and its effects are calculated by perturbation theory. It is predicted that the σ component of fluorescent light exhibits the Hanle effect, which allows one to measure the relaxation rates \Gamma(0) and \Gamma(2) of the population and of the alignment of the emitting level, respectively. Experimental agreement with the theoretical analysis has been found for the 3s_{2}, 2s_{2} , and 2P_{4} levels of Ne. Among the experimental results obtained by this method are the natural widths and the collision cross sections for population (quenching collision) and for alignment. Several transition probabilities are also derived. Furthermore, the theory of coherent imprisonment of the fluorescent line has been found valid even if laser pumping light is used.

Evolution of a q‐switched laser pulse from noise

Abstract: Q‐switched laser emission is calculated using a fluctuating dipole model of spontaneous emission. The emission pattern for a broadband noise source is a complicated quasi‐periodic train of short pulses.

New self-terminating laser transitions in calcium and strontium

Abstract: Sr I1 1.03305 + 0,00005 1.033014 5p2P8jz0 - 4d2Da/z 460 460 Sr I1 1.09175 =t 0.00005 1.091797 5p2pl/z0 - 4d2D3!z 460 Helium or Neon at a pressure of 3 torr was used as a carrier gas, carrying t,he discharge between the cold electrodes into the vapor region within a furnace. The effective length of active medium was 9 em. The single pass gain for all these transitions has been measured and is typically 300 dB/m. At such high levels of amplification, snperradiant emission could be observed by removing the optical cavity, this radiation being distinguishable from spontaneous emission by its relatively small angular divergence.

Stimulated emission from bulk field-ionized GaAs

Abstract: High-field domains passing through n -type gallium arsenide pulsed above a critical field produce electron-hole pairs by impact ionization, resulting in emission of recombination radiation of band gap energy. Specimens have been made in a configuration that allows intense ionization to be produced at 78°K in material having initial electron densities of the order of 5 \times 10^{17} cm-3Above a certain threshold ionization density, stimulated emission occurs; the linewidth is of the order of 30 A and emission is mainly within an angle of 7 degrees to the normal to the plane parallel polished faces of the specimen. The threshold current varies little with temperature up to 170°K and above this increases linearly. At currents above the threshold, the spectrum broadens somewhat; the output power is measured to be about 3 watts. The near-field pattern shows that lasing occurs over small separated areas that vary in location from pulse to pulse. Overall power efficiency of the device is low; if, however, only the lasing areas are considered, the efficiency comes much nearer to the theoretical maximum which, it is argued, is about 5 percent. Time dependence of the spectral distribution is complicated and evidence is given to show that carrier heating is an important phenomenon. The lasing process is similar to that occurring in a junction diode laser; some points of difference are discussed. The output frequently shows an unexplained strong polarization.

Characteristics of GaAs lasers near room temperature

Abstract: The efficiency and threshold of GaAs lasers can be affected by a delay occurring between the application of a current pulse and the laser output. The delays may be some tenths of a microsecond in certain diffused junction lasers when operated near room temperature, e.g., when the donor substrate dopant is selenium with a concentration of the order of 1018atoms per cm3. No such delays are observed in diodes in which the substrate dopant is silicon. In the case of selenium, the delay depends on the doping level, decreasing as the concentration increases. The delay is also very sensitive to injection current, decreasing rapidly with increasing current from its maximum at threshold. During the pulse, after lasing commences, the output continues to increase. A similar effect has also been observed with spontaneous emission. Operation of these devices below room temperature shows that the delay is also dependent on temperature. The delay decreases over a fairly narrow temperature range and in all cases is no longer observed ( < 20 n/s) below -70°C. These observations are explained by considering the effect of impurity trapping levels which may be associated with the

Solution by means of lie-series to the laser rate equations for a giant pulse including spontaneous emission

Abstract: An exact solution of the rate equations for the time evolution of a giant pulse including spontaneous emission, but neglecting pumping, is derived by the technique of Lieseries. It turns out that for normal optically pumped solid-state lasers the influence of spontaneous emission on the properties of the pulse is negligible; however, for materials with very short life time for the excited state involved in the laser transition, the pulse will be reduced or even suppressed by the effect of spontaneous emission.

Some characteristics of electron-beam pumped GaAs lasers

Abstract: The threshold current density of an electron-beam pumped GaAs laser is sensitively dependent upon the profile of the electron beam. Minimum power threshold current density is obtained with the excitation far above threshold value at the center of the Fabry-Perot cavity and with vanishingly small values at the ends. Threshold current density is also dependent upon beam voltage. For voltages of the order of 30 kV, the peak of the distribution of minority carriers (including the effects of diffusion and surface recombination) is of the order of one diffusion length from the surface, suggesting that nonradiative surface recombination plays an important role. This has been confirmed by measurements on samples with a Schottky barrier. Threshold current density of n -type GaAs at 77°K (40-mil Fabry-Perot cavity, 30-kV beam voltage) decreases from 0.69 A/cm2at 4 \times 10^{17} /cm3to 0.44 A/cm2at 4 \times 10^{18} /cm3. Power output over the same doping range peaks at about 2 \times 10^{18} /cm3with 20 watts obtained at an overall efficiency of 2.5 percent. Peak differential external quantum efficiency of the order of 30 percent occurs at about the same doping density. Using Hunsperger and Ballantyne's values of α, we deduce that the internal quantum efficiency may be as high as 90 percent.

Observations concerning self-damage in GaAs injection lasers

Abstract: A study has been made of the gradual degradation in the output of GaAs lasers at 300°K. The process of catastrophic degradation, which involves mechanical facet damage, has been previously described. The lasers investigated were fabricated by Nelson using the solution regrowth method. Typically, the lasers were 10 mils long and 5 mils wide and were not provided with reflective coatings. We have found that the degradation process is a bulk, rather than a surface, effect during which the near-field emission patterns decrease in uniformity. Except in rare cases, there is no external evidence of mechanical damage to the devices. This has been confirmed by scanning electron microscope observations of the facets before and after gradual degradation. Softening of the I-V characteristics occurs in addition to increases in the threshold current density and decreases in both the stimulated and spontaneous exterior differential efficiency. No evidence was found for significant changes in either the junction impurity profile or the optical emission spectra. From a detailed analysis of the change in the threshold currents and the efficiency, it is concluded that the internal quantum efficiency is gradually reduced during laser operation. This decrease may be the result of the formation of recombination centers in the recombination region. These centers may be similar to those introduced by high-energy (1 MeV) electron irradiation. We find that an irradiation flux of 1.5 \times 10^{16} electrons/cm2(resulting in the formation of an estimated density on the order of 1015defects/cm3) has a severe effect on laser performance, similar in many respects to that observed in gradual degradation: the emission pattern may become nonuniform, while the threshold current increases and the efficiency decreases. All this also occurs without any changes in the spectral energy distribution in the range investigated (1 to 1.5 eV) An evaluation of the factors affecting the gradual degradation rate indicates that it is primarily affected by the current density. The degradation rate is also influenced by the initial junction quality-lasers that initially exhibit highly nonuniform emission degrade faster than those that are relatively uniform. By limiting the device operation of a typical solution-grown laser to a current level between two and three times the threshold current, long-term (hundreds of hours) stable operation has been obtained at 300°K at repetition rates up to several kilohertz using a 100-ns pulse width.

Internal Q-switching in GaAs junction lasers

Abstract: Internally Q -switched light pulses have been obtained from junction lasers. It is believed that this is a completely new observation for semiconductor lasers. Using specially fabricated diodes, narrow bursts of light were detected immediately after the termination of the injection current pulse. The effect persists for a wide variation in the length of the current pulse, from less than 2 ns to several μs. The width of the Q -switched light pulse itself is less than 0.4 ns, this value being the resolution of our detection system. Its energy increases rapidly with the amplitude of the injection current. The occurrence of stimulated emission after the end of the injection pulse indicates Q -switching due to a reduction of the internal absorption. This reduction allows those injected carriers that have not yet spontaneously recombined to produce the narrow burst of stimulated light. The Q -switching is observed over a current amplitude range that is a very strong function of temperature. This range can be relatively large. One diode at 150°K showed Q -switching after current pulses from 1.2 to 5.0 amperes; for amplitudes greater than 5.0 amperes, normal stimulated emission occurred during the current pulse. As the temperature is increased, the Q -switched pulse is first observed near T t , the so-called "transition temperature" where trapping effects first start to cause long delays between the application of the current pulse and the onset of stimulated emission. Therefore, it is believed that the same traps are involved in both the long delays and the Q -switching. The latter is observed only in diodes with low T t values where the absorption due to traps accounts for a large proportion of the total losses of the laser. A model explaining these effects will be presented.

Thermal Noise in ir Quantum Counters

Abstract: Because an ion's strength of interaction with the lattice vibrations is not appreciably greater than with the radiation field, for energy gaps > lim ∼103 cm−1, the 300°K background radiation field will play an important role in determining the equilibrium population of the near‐infrared electronic levels. As a consequence, the Infrared Quantum Counter fluorescence produced by thermal population of the device's infrared level (a generalized form of spontaneous emission noise) cannot be reduced indefinitely by choosing arbitrarily cold cryogenic baths. It is shown that ``soft'' lattices with weak ion‐lattice interactions produce the most noise of this type, yet yield the best detector sensitivity. Qualitative experimental confirmation is provided, and the IRQC is shown to be potentially useful, if laser pumping can be achieved, despite the noise limitation imposed by the thermal background.

Electroluminescence from Ge-Doped GaP p—n Junctions

Abstract: The electroluminescence from GaP containing Ge has been studied. p—n junctions were formed by the liquid epitaxy method. The peak of the dominant emission from these diodes is near 1.96 eV at 77°K. The maximum external quantum efficiency at 77°K was 8%. The emission appears to involve Zn and Ge pairs even though these impurities were doped on opposite sides of the junction. It was not possible to ascribe any emission lines to recombination involving Te and Ge. Current, voltage, and intensity dependences of the diodes have been measured. The results indicate that radiative recombination between Zn and Ge at 77°K is predominantly in the space‐charge region. At low currents, photon‐assisting tunneling becomes important.

Änderung der effektiven Übergangswahrscheinlichkeit der induzierten Emission durch Störung des unteren Niveaus

Abstract: The perturbation of the process of stimulated transition, in case the lower level is coupled to a third level by a) a spontaneous emission or b) a second stimulated transition. is investigated theoretically. For strong perturbation a quantum mechanical treatment of the problem — in contrast to a calculation using rate equations — yields a reduction of the effective transition rate for the stimulated transition which may be interpreted in case a) as a broadening and in case b) as a splitting of the level, thus giving some insight into the disappearance of the laser effect due to sufficiently strong pumping discussed in a preceding paper.

Band-tailing effects and the temperature dependence of radiative recombination in compensated epitaxial GaAs laser junctions

Abstract: The temperature dependence of the spectrum, threshold current, and power output of compensated vapor-grown epitaxial GaAs lasers and incoherent emitters near room temperature is presented. It was found that an increase in temperature produced a narrowing of the spontaneous emission spectrum. This narrowing has been attributed to a movement of the Fermi level through an exponential density-of-states tail.

Cosmology and electrodynamics.

Abstract: THE approach of Hoyle and Narlikar1 to quantum electrodynamics provides a good explanation of spontaneous emission, and leads to an expression for the emission rate which is in agreement with the observed rate S, provided that the zero-point oscillations of the electrodynamic field are ignored. Previously it was considered2 that spontaneous emission was induced by these oscillations but, because there is only an energy of ½hν per state, the predicted rate in this case is only ½S.

Infrared emission as studied through the temperature dependence of integrated intensities of a molecular crystal.

Abstract: The expression for the integrated intensity of an infrared absorption band is normally obtained by neglecting spontaneous emission. At high temperatures, the emission term cannot be ignored, and in fact gives rise to a decrease in apparent absorption intensity with increasing temperature. We have studied this effect experimentally with the molecular crystal copper phthalocyanine at temperatures up to 600°C. Expressions based on the assumption of a harmonic potential function, and retaining only linear terms in the dipole‐moment operator have been derived for the temperature dependence of the fundamental absorptions. These expressions compare favorably with the experimental data.

Analysis of Population Inversion for Lasing Threshold in Semiconductors

Abstract: The rate of recombination due to spontaneous emission of light at lasing threshold is calculated for a direct band gap semiconductor. The recombination rate for doped semiconductor, where the donor state is assumed to be discrete, is numerically estimated in the temperature range from 50°K to 400°K and compared with the undoped case. It is shown that the threshold of excitation rate for lasing action is decreased by introducing donors (or acceptors) to a host semiconductor.