Showing papers on "Spontaneous emission published in 1976"

Concentration‐dependent absorption and spontaneous emission of heavily doped GaAs

Abstract: A model for the calculation of the absorption and emission spectra for GaAs at carrier concentrations in excess of 1×1018 cm−3 is described. This model utilizes a Gaussian fit to Halperin‐Lax band tails for the concentration‐dependent density of states and also includes an energy‐dependent matrix element. The calculated absorption and emission spectra are compared to previous experimental results. All results are for 297 K. For p‐type GaAs, the agreement is very good. The concentration dependence of the effective energy gap is obtained and can be expressed as Eg (eV) =1.424−1.6×10−8 [p (cm−3)]1/3. The concentration‐dependent thermal equilibrium electron‐hole density product n0p0 and the radiative lifetime τr are calculated for p‐type GaAs. The value of n0p0 increases from the low‐concentration value of 3.2×1012 cm−6 to 1.2×1013 cm−6 at p=1.6×1019 cm−3. This value of n0p0, together with the thermal generation rate obtained from the experimental absorption coefficient, gives τr as 0.37 nsec at p=1.6×1019 cm−3.

Theory of superradiance in an extended, optically thick medium

Abstract: This paper presents a semiclassical treatment of the evolution of an initially inverted system into a superradiant state in an extended, optically thick medium. In this process spontaneous emission and background thermal radiation initiate the collective radiative decay and produce a superradiant output pulse of intensity proportional to the square of the number of radiators. The treatment is based on the coupled Maxwell-Schr\"odinger equations, modified to include a fluctuating polarization source properly constructed to account for the effects of spontaneous emission. Computer results show that for a high-gain system only two parameters significantly influence the evolution process: ${T}_{R}$, the characteristic radiation damping time of the collective system; and ${\ensuremath{\theta}}_{0}$, a function of the conditions which initiate the superradiant process. In this limit one obtains a normalized emission curve and simple analytical expressions for the time delay, pulse width, and peak intensity of the output radiation. These results are in good agreement with experiments. A comparison of our model with previous treatments of superradiance is given.

Radiation from a hot, thin plasma from 1 to 250 A

Abstract: A calculation of emission spectrum of a hot, low-density plasma in the region 1--250 A is presented. The mechanisms considered are electron collision-induced line emission, bremsstrahlung, and radiative recombination; and the temperature range studied is 10/sup 5/--10/sup 7/ K. 795 lines are included. The elemental abundances of the ions of He, C, N, O, Ne, Mg, Si, S, Ca, Fe, and Ni were taken to be as in the solar corona. The line emission of Fe ions produces a maximum in the curve of an emission power between 1 and 250 A versus temperature around 10/sup 6/ K. The emission rate around 10/sup 6/ K is larger than the results calculated by Cox and Tucker and Tucker and Koren. (auth)

Saturation of the junction voltage in stripe‐geometry (AlGa)As double‐heterostructure junction lasers

Abstract: Saturation of the junction voltage has been observed to occur at the onset of lasing in cw stripe‐geometry (AlGa)As double‐heterostructure junction lasers. Simultaneous measurements of the nonlasing but amplified spontaneous emission confirm that saturation of the optical gain and spontaneous emission accompany the voltage saturation as expected. With well‐behaved lasers the observed saturation is maintained over currents to at least 50% above threshold. In other devices, a loss of saturation is commonly observed to occur simultaneously with a nonlinearity in the current dependence of the lasing emission.

Stimulated Emission Effects in Particle Creation Near Black Holes

Abstract: It has recently been shown that if a black hole is formed by gravitational collapse, spontaneous particle creation will occur and a thermal spectrum of all species of particles will be emitted to infinity if the quantum matter was initially in the vacuum state. In this paper we investigate the stimulated-emission effects which occur if particles are present initially. We show in general that for a Hermitian scalar field in an external potential or in curved, asymptotically flat spacetime, stimulated-emission effects can occur precisely in those modes for which there is spontaneous particle creation from the vacuum. For the case of a Schwarzschild black hole, this result appears paradoxical, since spontaneous emission occurs at late times but there is no classical analog of stimulated emission at late times. The resolution of this paradox is that in order to induce emission of particles which emerge at late times one must send in particles at early times, so that they reach the black hole very near the instant of its formation. However, enormous energy is required of these incoming particles in order to stimulate emission of particles which emerge at late times. Thus, for a Schwarzschild black hole, even if particles aremore » initially present (with limited energy) they will induce emission only at early times; at late times one will see only the spontaneously emitted blackbody thermal radiation. For the case of a Kerr black hole stimulated emission can be induced by particles sent in at late times with the appropriate frequencies and angular dependence. If the number of incoming particles is large, this quantum stimulated emission just gives the classical superradiant scattering. (AIP)« less

Lasing characteristics of distributed-feedback GaAs-GaAlAs diode lasers with separate optical and carrier confinement

Abstract: The lasing characteristics of separate-confinement-heterostructure (SCH-structure) distributed-feedback (DFB) diode lasers are examined theoretically and experimentally. Wave propagation in five-layer SCH waveguides is analyzed to estimate such parameters as the lasing wavelength, coupling constant, and external quantum efficiency. Spectral and modal behavior are studied in the experiment and compared with the theoretical predictions. Diodes are shown to lase in a single longitudinal mode with a definite polarization. Spectral width is about 300 MHz just above the threshold, and becomes wider with increased excitation level. An output power of 40 mW with an external quantum efficiency of 5 percent is obtained under CW operation.

Luminescence Spectra Due to Exciton-Exciton Collisions in Semiconductors. I. Spontaneous Emission Spectra

Abstract: Spontaneous emission spectra are calculated for typical radiative exciton-exciton collision processes at various temperatures A suitably weighted δ-function of the inter-exciton separation is employed as the effective interaction potential, and numerical evaluations are performed with the band parameters of GaAs The results describe well the band shape of the emission due to exciton-exciton collisions in GaAs which dominates the luminescence under intense nitrogen-laser excitation at low temperatures

The power spectrum of injection lasers: The theory and experiment on a nonlinear model of lasing

Abstract: A comprehensive treatment of the theory and experiment on a nonlinear model of injection lasers develops a general way to find the order underlying the complexities of the emission spectrum of lasers operated far above threshold by describing the emission in terms of a few measurable parameters of the cavity and material. The theory is largely unpublished, while the experimental work covers publications fortified with more recent studies and considerable new work. The laser is described by rate equations in which the gains and losses are approximated as prorated averages over one round trip. The gain contains an explicit nonlinearity described by a critical power at which the gain begins to saturate. The steady state solution at low level is the linear theory. At high level, the excited power in each cavity mode is an explicit function of its particular gain and loss coefficients. To express the characteristics of the radiation in terms of observables, the dependence of gain and loss coefficients on mode number are deduced by modeling. The unsaturated gain is taken as parabolic in frequency, while the loss coefficients are deduced by decomposing each cavity mode into three independent sets of waveguide modes. For a cavity with sawed side walls, this gives a scattering loss proportional to the square of the lateral mode number, which determines the shape of the lateral profile of the beam. The summation of the modal powers over all cavity modes expresses the lateral profile, the frequency spectrum, and the total power in terms of an overdrive parameter X associated with the dynamical state of the laser, which is an implicit function of the population inversion. Relating X to the spontaneous emission at short wavelength gives a set of equations, involving only observables, which determines all the parameters in the summation. The equations reveal previously unknown relations between power, spontaneous emission, beam width, spectral width, and polarization. The critical power is expressed from first principles in terms of phenomenological optical constants. The experiments examine the relation by pulsed studies at room temperature of wide lasers with sawed sides or stripe contacts. It is shown that many of the lasers have the dispersion of box modes assumed in the summation of the distribution function. At high enough power that a statistical average over the excited modes is meaningful, the general predictions are verified. The lateral profile has the correct shape to about 10 dB below the peak, its width grows as (X) 1 2 , and it's peak as (X) 3 2 . The power dependence of the polarization ratio follows quantitatively the predicted shape, and the total power is quadratic in X. Extensive records of 21 lasers with a wide variety of internal structures are intercompared, showing that all have a similar coefficient of side wall loss, that the curvature of gain against wavelength varies widely, and that the critical power P ∗ seems to be a general constant of the predicted magnitude, independent of internal structure, cavity geometry, and facet area. The same constant is also found for wide cavities with Hermite-Gaussian modes and one narrow stripe cavity. There is a discussion of geometries not yet covered, including lasers with many transverse modes and narrow stripe and cw lasers.

Room-temperature operation of GaAs Bragg-mirror lasers

Abstract: Room-temperature operation of GaAs distributed Bragg reflector lasers is reported. The diodes are fabricated from conventional double heterostructures involving only a single step of liquid-phase epitaxy. For gratings with a period of 3700 A, the diodes lased at 8770 A, which corresponds to the high-absorption side of the spontaneous emission spectrum. Thresholds as low as 6 kA/cm^2 have been realized.

Tunable superradiant emission from a planar dye laser

Abstract: A new, simple scheme to tune a double-pass, superradiant dye laser is reported. A planar cell with a thickness below 100 μm filled with a highly concentrated (4–5 × 10−3-M/liter) solution of Rhodamine 6G in ethanol is used. An unsaturated gain value of 1.7 cm−1 has been measured with flashlamp pumping, and the amplifier has been found to operate at a well saturated regime. Smooth laser linewidths ranging from broadband, gain narrowed values of 10 nm to narrowband values of 8 GHz have been achieved by using appropriate frequency dispersive feedback elements and suitable cell thickness. A 5-GHz output has been observed with a 25-μm gap cell. The absence of a resonant cavity avoids modal structure and ensures improved frequency and intensity stability. The reported configuration should be scalable up as far as output energy is concerned. Simultaneous two-wavelength operation is possible. Pumping with a frequency-doubled Nd-laser should allow a further reduction of the usable cell thickness with narrower output bandwidths.

Nitric oxide gamma band emission in an aurora

Abstract: Emission of the NO gamma (1,0) band at 2150 A has been observed by a rocket-borne spectrophotometer in an IBC II(+) aurora. The nu-prime progression of the gamma-system does not appear in the spectrum. The observed emission rate of the 2150 A feature increases relative to N2(+) first negative band emission with increasing altitude. We suggest radiative recombination of NO(+) ions with electrons as a possible excitation mechanism compatible with the data.

Observation of stimulated emission in KrCl

Abstract: Stimulated emission at 222.9±0.1 nm in KrCl has been observed in electron‐beam‐excited Ar/Kr/Cl2 mixtures. Approximately 50 kW of laser output in a 30‐ns pulse with ∼14% output coupling has been obtained. Both the peak spontaneous emission wavelength and vibrational spacing (ωe) of KrCl* are in agreement with the alkali‐halide analog (RbCl) model. The kinetics of this new rare‐gas–halide laser molecule are briefly discussed.

Radiative transfer across expanding laser-produced plasmas. II. Line intensities and stimulated emission

Abstract: For pt.I see ibid., vol.8, no.18, p.3044 (1975). The total intensity of an optically thick line can be greater from a differentially expanding plasma than from a non-expanding plasma of similar geometry, and can approach the optically thin value at sufficiently high streaming velocities. A population inversion, localized to one region of a differentially expanding plasma, can give rise to nett amplified spontaneous emission even though it would not do so in the absence of expansion. This enhances the prospects of a laser-produced plasma as a source of amplified spontaneous emission. Within the one line profile from a differentially expanding plasma, maximum amplified spontaneous emission and peak gain to an external signal need not occur at the line centre and need not occur at the same wavelength as one another.

A coefficients for spontaneous emission in CO

Abstract: Simultaneous end light emission measurements of the first‐ and second‐overtone bands of CO have been obtained in a flowing CO‐N2‐He laser plasma. Interpretation of the band structure using A coefficients from Young and Eachus show consistent agreement with observation up to v=26. This agreement supports the use of these A coefficients in laser and kinetic applications.

Intensities of optical transitions in absorption and luminescence spectra of neodymium in glasses

Abstract: Measurements are reported of the oscillator strengths of the absorption bands of the main glassy matrices and several industrial neodymium-doped laser glasses. The results are analyzed from the point of view of the Judd theory (1962) and represented by three parameters Ω2, Ω4, and Ω6. These parameters are used to calculate the spontaneous emission probabilities and stimulated emission cross sections of the 4F3/2→4I11/2 and 4F3/2→4I11/2 transitions. A comparison is made between the values of these quantities and the published data for Nd3+ -activated crystals, glasses, and solutions.

Some basic properties of stimulated and spontaneous emission: A semiclassical approach

Abstract: The coherence and directional properties of stimulated and spontaneous emission can be understood without a knowledge of quantum electrodynamics. This paper presents a semiclassical description of the interaction between a free atom and a weak optical field under conditions of either a stimulated emission or a stimulated absorption resonance. It is shown that, under certain conditions, the stimulated or spontaneous radiation fields emitted from a sample of atoms will propagate in the same direction as the incident beam, will be coherent with the incident beam, and will have the correct phase to provide amplification or attenuation.

Spontaneous Emission of Electromagnetic Waves by a Semi-bounded Non-equilibrium Plasma

Abstract: The boundary value problem of an anisotropic plasma half-space excitation by the given space-time distribution of sources is solved. The energy relations are determined for the spontaneous emission from a semi-bounded non-equilibrium plasma. The generalization of the well-known results of classical radiation theory is established (in particular, Kirchhoff's law) for the case of non-equilibrium emission from a half-space of anisotropic plasma. The quasistatic surface excitation spectra are investigated in a one-component plasma with a low-density electron beam and in a non-isothermal electron-ion plasma with electron drift relative to the ions. The possible instability of such excitations and the anomal effective temperature growth for the quasistationary part of the spontaneous field are shown. The threshold values of the electron beam velocities for various branches of the surface excitations are calculated.

Stimulated emission and photon correlations

Abstract: It is pointed out that photoelectric counting correlations in an electromagnetic field that are associated with induced and spontaneous emission at the source, and the so-called Hanbury Brown-Twiss effect, are indistinguishable effects. It is shown by examples that interference effects involving the stimulated emission may be regarded as a source of the counting correlations. The results reported in a recent experiment of Scarl and Smith are therefore readily understandable.

Transition from incoherence to coherence in the spontaneous emission of extended systems

Abstract: Using the results of a previous paper, we study the evolution of a completely excited system of N two-level atoms, distributed in a circular cylinder with dimensions much larger than the wavelength, interacting with all modes of radiation field. We emphasize the distinction between r-conserving (rC) and r-nonconserving (rNC) processes (r is the so-called cooperation number). Considering the number of photons emitted as the discrete independent variable, the evolution is described by a partial difference equation. Numerical solutions of this equation show the transition from rNC dominance at the beginning to rC dominance later. This is also a transition from incoherent to coherent emission of radiation.

Controlled energy extraction and sharp nanosecond pulses from an HF amplifier

Abstract: We report complete spatial control of laser energy extracted from an electron-beam-initiated HF amplifier. Control was achieved by application of a weak oscillator beam, which, by extraction of all of the available energy, overcame amplified spontaneous emission. In a separate experiment, temporal control was also demonstrated. Short (4-ns) sharp (contrast ∼20) amplified pulses were obtained by rapid polarization rotation and selective reflection in the amplified beam.

Photoluminescence Studies of As-Grown, Heat-Treated and Ion-Implanted CdS Single Crystals

Abstract: Photoluminescence in CdS has been studied for as-grown, heat-treated and ion-implanted crystals. 1) In the as-grown crystals, the green emission intensity increases with the carrier density of CdS up to 5×1016 cm-3 where the intensity is maximum, and decreases with the dislocation density. 2) Heat treatment decreases green emission and produces red emission. The production of lattice defects due to heat treatment is suppressed by coating the sample with SiO2. film. 3) Green emission is decreased and red emission is enhanced by ion implantation. 4) The depth distribution of the defects which are introduced by heat treatment and ion bombardment can be explained by the vacancy diffusion mechanism. 5) The red band is considered to originate from an associated center of a cadmium vacancy and an impurity.

Amplified spontaneous radiation emitted from disk laser amplifiers

Abstract: An analysis is made of the threshold conditions for the generation of stray oscillation modes in disk laser amplifiers and of amplified spontaneous radiation when the threshold is not reached. A comparison is made of the various methods for reducing the reflectivity of the disk surface. A calculation is made of the increase in the rate of decay of the population inversion in the prescence of amplified spontaneous radiation, which has a considerable influence on the energy characteristics of large-aperture disk amplifiers. The nature of the angular distribution of the amplified spontaneous radiation emerging for a disk amplifier is determined.

Radial profiles of upper‐laser‐level emission in an oscillating Ar II laser

Abstract: Radial profiles of the emission from upper‐laser‐level ions have been investigated in an oscillating Ar II laser and a significant dip in the tube center is observed.