Showing papers on "Spontaneous emission published in 1970"

Theory of a three-level gas laser amplifier

Abstract: A laser amplifier to be treated in this work consists of an ensemble of atoms three energy levels of which form two coupled transitions of arbitrary frequencies. Two classical monochromatic travelling light waves are to be close to resonance with the transitions. The gain profile (or spontaneous emission) on the transition corresponding to the weak “probe” wave, modified by the perturbing field on the other transition, is calculated via a susceptibility. Within this framework, the atoms are described by an ensemble-averaged density matrix with full account of level degeneracies, light polarizations, and inelastic and dephasing collisions; an extension to elastic collisions and disorientation is straightforward. An integration over the thermal velocity distribution gives results applicable to gas discharges: directionally anisotropic narrow structures superimposed on the Doppler-broadened probe-gain profile due to non-linear interference effects in addition to saturation. At alower probe frequency, a peculiar non-Lorentzian signal appears even with transparency on the perturbing transition. At low intensities a distinction is reasonable of frequency correlations due to generalized two-quantum processes, and of a dynamic Stark splitting. These effects permit an information on the linewidth of the third forbidden transition. The connection with numerous related approaches is pointed out.

Coherent Spontaneous Emission

Abstract: Coherent spontaneous emission from a system of $N$ two-level atoms interacting with a quantized radiation field is treated for the case in which Dicke's "cooperation number" $r$ is macroscopically large. It is shown how to modify the quasiclassical approach to this problem to incorporate quantum effects that are lost in the self-consistent-field approximation. The statistics of the emitted radiation are found to vary markedly with the initial state of the system of atoms. The photon statistics tend to that typical of blackbody radiation when the initial state of the atomic system is that which would result from incoherent pumping ($m\ensuremath{\sim}r$). When, on the other hand, the atoms are initially in a superradiant state ($m\ensuremath{\ll}r$), the emitted radiation may be represented approximately by a coherent state.

Master-equation approach to spontaneous emission.

Abstract: Spontaneous emission from a system of $N$ identical two-level atoms is considered using a master equation recently derived by the author. The master equation describing the time evolution of the phase-space distribution function associated with the reduced density operator of the atomic system is obtained. This master equation, which is of the type of a Fokker-Planck equation, is used to derive the equation of motion for the mean values of various atomic operators characterizing the physical properties of the system. This leads to a hierarchy of equations, which is decoupled by making a suitable approximation. The intensity of the spontaneously emitted radiation is then calculated. Next, the spontaneous emission from geometrically small systems is considered. For this case, the master equation is solved exactly, and an exact expression for the radiation rate is obtained. The exact solution of the master equation is also used to calculate the normally ordered correlation functions for the electric field. Section V deals with the spontaneous emission from a system of harmonic oscillators, the size of the system being small compared to a wavelength. The master equation for this problem is also solved exactly, and it is shown that this system also leads to superradiant emission in some cases, e.g., if all the oscillstors are excited initially to some coherent state $|{z}_{0}〉$.

Ultrashort-Pulse Generation by Q -Switched Lasers

Abstract: We have undertaken the complete temporal description of pulsed emission by a homogeneously broadened laser, including the effects of spontaneous emission, the detailed geometry of the laser cavity, and the variation of atomic polarization and level populations over wavelength distances. The model is based on traveling-wave equations which are derived from Maxwell's equations and solved in conjunction with boundary conditions imposed at the cavity mirrors. Thus, any direct assumptions concerning the nature of the laser's longitudinal mode structure is avoided. Variations in polarization and population over wavelength distances are treated by means of expansions in spatial Fourier series, having as fundamental a half optical wavelength. The Fourier series are truncated after the first harmonic. The treatment differs from earlier work in that the dephasing of the dipole moment is treated exactly without a rate-equation approximation. Spontaneous emission is simulated both as to spectrum and Gaussian character by including in the dipole equations stochastic shot-noise sources. The model equations are solved numerically, and results include the details of $Q$-switched pulse evolution from noise for both passive and active switching. In the case of an actively switched laser, the two-photon fluorescence intensity pattern has been calculated. It reveals a well-defined structure of subsidiary intensity maxima, even though subcavities are not assumed in the calculation. The pattern can be correlated directly with the emission pulse structure, and should vary from shot to shot. No single point in the pattern is suitable for a peak-to-background ratio determination. However, if the background is averaged over a distance in the fluorescing medium equal to twice the separation between cavity mirrors, the peak-to-background ratio would be \ensuremath{\approx} 1.6, indicating a highly uncorrelated spectrum.

Single pass gain of exciplex 4‐mu and rhodamine 6g dye laser amplifiers

Abstract: Single pass gain has been measured in N2 laser pumped exciplex 4‐Methylumbelliferone (4‐MU) and rhodamine 6G dye lasers using a technique developed by Silfvast and Deech for high‐gain pulsed metal vapor lasers. The gain is measured by comparing the intensities of amplified spontaneous emission in single and double cell lengths with the maximum unsaturated single‐pass gain being limited by the saturation of the spontaneous emission. Maximum unsaturated gains on the order of 20 dB/cm were measured for both dyes. The gain was found to vary linearly with pump power. The technique was also used to measure the variation of gain with wavelength for a given pump power. The maximum gain for exciplex 4‐MU occurred at the fluorescence peak, whereas for rhodamine 6G it was shifted to a longer wavelength due to ground‐state reabsorption. By fitting the measured gain versus wavelength to the theory, it was possible to deduce the fractional excited singlet population in the rhodamine 6G solution.

Formation and Radiative Recombination of Free Excitonic Molecule in CuCl by Ruby Laser Excitation

Abstract: New emission bands called the M , N 1 , N 2 and N 3 bands have been found near the band edge in CuCl single crystal with excitation by the two-photon absorption of a Q -switched ruby laser. All emission intensities show non-linear dependences upon the excitation intensity. One of them, the M band located at λ3919 A at 4.2°K, is concluded as the radiative decay of a free excitonic molecule. The shape variation and peak shift to the low energy side of the M emission band with respect to the increase of the laser power are similar to those due to the raising of temperature. The line shape is an inverted replica of the Maxwell distribution. The system of the excitonic molecule has been concluded to have the effective temperature which is higher than the lattice temperature in the high density state of excitonic molecule. The mechanism of the radiative decomposition of the excitonic molecule has been discussed.

Long-Term Solutions in Semiclassical Radiation Theory

Abstract: An improved form of semiclassical radiation theory is developed which includes the effect of the atom's radiation field back on the atom. This formalism is applied to the problem of a single "two-level atom" interacting with a monochromatic field. The resulting equations are solved without resorting to time-dependent perturbation theory, and are found to predict the behavior of the system over times long compared with the lifetime for spontaneous transitions. Not only stimulated emission and absorption, but also spontaneous emission with the proper Einstein $A$ coefficient, and a frequency shift which agrees at least semiquantitatively with the Lamb shift are described. In addition, several nonlinear effects involving the interference between spontaneous and stimulated radiation are described, and new experiments which might detect such effects are suggested.

Superradiant Intensity Fluctuations

Abstract: We describe a new approach to the problem of superradiant spontaneous emission, which is applicable whether the number of atoms is large or small. We use this approach to formulate intensity correlations of arbitrary order, and then explicitly evaluate the second-order normally ordered relative intensity fluctuation. Large fluctuations at the onset of emission indicate the discrete nature of the spontaneous emission process; and the very small fluctuations found later at the emission peak show that the emitting process has by then become essentially classical.

ERRATA: EFFICIENT ELECTROLUMINESCENCE FROM InP DIODES GROWN BY LIQUID‐PHASE EPITAXY

Abstract: Light‐emitting InP diodes were made by liquid‐phase epitaxy. The most efficient diodes result when the n‐type layers are grown from a Sn‐doped melt, while the p‐type layers are grown from the same melt, but overcompensated with Zn. External quantum efficiencies up to 0.75% at room temperature and 11.7% at 77°K were observed in uncoated diodes. Threshold current densities for stimulated emission were as low as 750 A/cm2 at 77°K. Spontaneous emission can be observed normal to the p‐n junction.

Characteristics of stimulated emission from CdS by electron-beam pumping

Abstract: The results of electron-beam experiments on undoped single-crystal CdS are presented. A description of the beam-injection apparatus is given. The relevance of various indications for lasing in this type of structure is discussed, and an operational definition of lasing threshold current is described. In this paper intensity versus pumping carves, stimulated-emission moding curves, and measurements of the frequency- and time-dependent characteristics of the stimulated emission from undoped CdS will be presented and discussed. New data show that there is a 1.1 A/ns shift of the stimulated peak emission to longer wavelengths which is independent of the pumping current density. This effect is not understood but is crucial to the understanding of the CdS electron-beam-pumped semiconductor laser.

Electroluminescence in Amphoteric Silicon‐Doped GaAs Diodes. I. Steady‐State Response

Abstract: Steady‐state measurements were made of the optical emission and electrical properties of amphoteric Si‐doped GaAs diodes. The width of the emitting region of these diodes is explained in terms of a theory involving the drift of electrons across a semi‐insulating p‐region. At low currents radiative recombination occurs in the p‐region within a diffusion length of the junction. However, at high currents a considerable number of injected electrons drift across the p‐region and light emanates from the entire p‐region. The theory is consistent with the observed I‐V characteristics and also with the spreading of the emission into the p‐region.

A Simple High Temperature System for cw Metal Vapor Lasers.

Abstract: The recent discovery of cw laser action in metal vapors has provided a new class of gas lasers having low current thresholds and in some instances high power and efficiency. To date, cw laser action has been observed in the ionic spectra of Cd, 1 4 Sn, Zn, and Pb 6 in mixtures of a few Torr of He and a few microns of the metal vapor. This letter describes a simple high temperature system that can be used interchangeably for any of these metal vapor lasers. In order to be able to use a large variety of metals, each with a different vapor pressure vs temperature curve, it is necessary to have a system that is capable of a wide range of temperatures. We have used quartz successfully for laser discharge tubes at temperatures as high as 1150°C. We therefore used a long cylindrical furnace capable of reaching 1200°C with a quartz discharge tube mounted inside, as shown in Fig. 1. A furnace with a flat axial temperature profile is necessary to provide a reasonable length for the active bore region. A cylindrical furnace designed to reach 1200°C will generally have a small inside diameter. We therefore used tapered ground joints (24/40) external to the furnace, so that the electrode and Brewster window enclosures could be constructed of Pyrex and fitted to the quartz bore with a silicone high vacuum grease seal. The vacuum grease was kept approximately 5 mm from the end of the taper on the inside so that the discharge would not come into direct contact with the grease. Although the greased joints were not essential, they were found to be a convenient way to assemble and disassemble the system, and were especially convenient for changing from one metal to another. The greased joints were cooled by dripping water onto cheesecloth wrapped around the joint region. This laser system has been operated for as much as 50 h with no deterioration in the laser output when using greased joints in the discharge region. The cataphoresis pumping scheme used in this system was developed independently by Sosnowski, Goldsborough, and Fendley el al. Sosnowski showed that when a single metal source was used in the bore region, a uniform distribution of Cd was obtained between the source and the cathode when the discharge current was higher than a certain critical value (depending on the bore size). This uniformity is maintained only as long as the

About unidentified ionized Xe laser lines

Abstract: Observation of eleven unknown spontaneous spectral lines explains the appearance of corresponding laser lines.

Photoluminescence of EuSe and the magnetic polaron model

Abstract: We report on extensive new measurements relating to the near-infrared photoluminescence of EuSe first reported by Busch and Wachter in 1966. The emission and excitation spectra are obtained as functions of temperature and external magnetic field. The total red shift is 0.13 eV in the excitation spectrum (in good agreement with the absorption-edge shift), and 0.25 eV in the emission spectrum. At 4.3°K an external magnetic field of 11 kOe quenches the yield to about 30% of its field-free value. The experimental results are discussed in relation to photoconductivity and optical absorption measurements. An intrinsic and localized model for the luminescence is proposed, in which the emission takes place by radiative recombination of a localized magnetic polaron with a hole in the europium 4f-shell.

Time‐resolved temperature measurements in the pulsed argon ion laser

Abstract: Time‐resolved measurements of the ion and electron temperature in a pulsed ion laser discharge are reported The data were obtained from spontaneous emission linewidths observed from the end and from the side of the discharge Significant heating of both ions and electrons during the current pulse was observed This heating and the resultant decrease in radiation trapping provide a likely explanation for the time behavior of the laser output

Temperature dependence of the power output of the spontaneous emission from GaAs laser diodes

Abstract: Modulation of photoluminescence in p–type GaAs by a field effect indicated that the radiative recombination occurred through recombination centres. This suggested that the recombination rate in the depletion region of a luminescent p–n junction would be governed by Shockley—Read statistics. The resulting model showed that, with suitable diode parameters, an increase in temperature would produce an increase in the radiative efficiency of the diode. Such an effect was observed experimentally with vapour-grown GaAs p–n junctions, the p–type sides of which were heavily compensated.

Optical amplification of 1.06-µ InAs 1-x P x injection-laser emission

Abstract: A system gain of 40 dB was realized with a semiconductor injection-laser oscillator operating into a neodymium-glass-fiber laser amplifier. InAsP ingots were grown by a modified Czochralski technique. The injection laser was fabricated from a mixed crystal of InAs 0.17 P 0.83 . This composition was chosen because laser emission occurs at 1.063 μ at liquid nitrogen temperatures. The p-n junction was obtained by zinc diffusion under phosphorus pressure. The method of fabricating contacts and the Fabry-Perot cavity is described, and optical and electrical characteristics of the devices are given. Coarse tuning of the lasing wavelength is based on x . Fine tuning can be accomplished by temperature variation of the laser junction in accordance with d\lambda/dT = 1.6 A/°K between 80- 140°K. The injection laser was optically mated to the samarium-clad neodymium-glass-fiber amplifier by microscope optics and a gimbal-mounted reflector. Design details of the optical amplifier are given in context with optimum low-noise-level operation. The peak optical amplifier wavelength can be varied over a 100- A interval by appropriate choice of the host-glass composition. The injection-laser pulse was delayed until the spontaneous emission had reached its peak for a maximum observed gain of approximately 5 \times 10^{4} .

Recombination Kinetics in GaP Red-Emitting Diodes Determined by Photocurrent and Decay Characteristics

Abstract: The photocurrent characteristic of GaP red light-emitting diodes excited in the band centered at 5900 A below the band edge at room temperature was studied in a temperature range of 170–500°K. The characteristic band is explained in accordance with the thermal release of electrons from optically excited excitons bound to the Zn–O nearest neighbor pairs to the conduction band. The Zn–O pair concentration in the light-emitting region is determined accurately by analysing the results, The time-decay characteristics of the red electroluminescence were also measured to obtain the transition rates of electrons trapped at the Zn–O centers. At room temperature, the thermal release rate of electrons WII is 1.8±0.2×106 sec-1 and the electron capture cross section of the Zn–O center is deduced to be 5.9±1.5×10-16 cm2 by using the observed binding energy of the center 300±10 meV. The radiative recombination rate WI and the non-radiative recombination rate WIII are estimated to be 1.6×106 and 2.4–3.4×106 sec-1, respectively. WIII seems to be determined by Auger process and depends largely on the free hole concentration in the active p region.

STUDY OF FLUORESCENT LIFETIME OF THE ν3 VIBRATIONAL MODE IN SF6

Abstract: Spontaneous emission from the ν3 fundamental vibration in SF6 gas after excitation by a pulse from a Q‐switched CO2 laser is observed. The pressure dependence of the flourescence decay time indicates that, at low pressure, diffusion of excited molecules followed by deactivation at vessel walls is the dominant relaxation mechanism. At higher pressures, the time for collisional deactivation within the bulk of the gas is measured. Evidence which suggests that a longitudinal‐acoustic‐shock wave was present in the gas at higher pressures is given.

Time Decay and Temperature Dependence of Radiative Recombination in (Zn, O)‐Doped GaP

Abstract: To understand the kinetics of the radiative emission in (Zn, O)‐doped GaP, primarily at room temperature, we have measured luminescence lifetimes using pulsed photoexcitation from argon and He–Ne lasers. Measurements of time decay and temperature dependences of the radiative emission have enabled us to identify the emission in the near infrared as resulting from transitions involving a bound electron at an oxygen site to a free hole in the valence band. Measurements of the temperature dependence of the decay times and the luminescence intensities of the exciton (red) and bound‐electron to free‐hole (infrared) emissions also substantiate the conclusions that nonradiative recombination paths exist out of the Zn–O complex and provide evidence for the existence of a nonradiative ``shunt path'' out of the conduction band. The bound electron to free‐hole recombination out of oxygen is believed to be primarily radiative. For an annealed sample, the decay times of the exciton and bound‐to‐free transitions are mea...