Showing papers on "Spontaneous emission published in 1973"

cw degradation at 300°K of GaAs double-heterostructure junction lasers. II. Electronic gain

Abstract: The rapid degradation at 300°K in the cw regenerative output of stripe‐geometry GaAs double‐heterostructure junction lasers is shown to be a result of the formation of a local optical absorber in the laser cavity. Gain measurements performed on diodes before and after degradation show that the optical loss within the cavity increases during degradation. By observing the (predominantly) spontaneous emission from the active region directly through the n‐GaAs substrate, it is confirmed that the increased loss is localized in a region where little or no spontaneous emission takes place at lasing energies. In such diodes, the internal radiative efficiency of the undegraded portion of the optical cavity shows a relatively small decrease compared to the external differential quantum efficiency. When the local absorber extends over a sufficient length of the cavity the electronic gain in the undegraded section is insufficient to overcome the loss and the device ceases to act as a regenerative optical oscillator. Net gain measurements on DH laser devices in which the active region is lightly (≈ 1017) n‐doped indicate that the optical gain increases linearly with current prior to degradation. At lasing threshold the medium exhibits net gain over a wavelength range of 100 A. After degradation the gain dependence on current can become superlinear due to the saturation of the optical absorber. Estimates on the attenuation constant in the local absorber at low currents give a value of ≈ 60 cm−1 at 8800 A. For pulsed currents close to lasing threshold the attenuation constant increases to nearly 160 cm−1 at 8760 A.

Carrier and gain spatial profiles in GaAs stripe geometry lasers

Abstract: The spontaneous emission generated in the active region of stripe geometry GaAs double‐heterostructure (DH) laser diodes is observed through the n‐GaAs substrate. The spatial variation of the spontaneous intensity along the junction plane is measured in a direction normal to the stripe axis at current levels below and above lasing threshold. It is observed that the carrier concentration profile is consistent with a model in which carriers out diffuse along the junction plane away from the active region. No evidence has been found of a local depletion of carrier concentration due to regenerative optical oscillations. Analytical expressions for the concentration profiles are obtained from a diffusion model. A comparison of the analytical expressions with experiment yields the carrier diffusion lengths inside and outside of the active region. The computed profiles are in excellent agreement with measurement. From the computed carrier concentration profiles and measurements of mode gain dependence on current ...

Structure in the nonlinear saturation spectrum of parametric instabilities

Abstract: The nonlinear saturation spectrum of the decay instability is obtained in the limit of small spontaneous emission, for comparable ion and electron temperatures, from numerical solutions of a kinetic equation based on an accurate expression for the nonlinearity. The spectral energy occupies several pairs of isolated saucer‐shaped regions in wave‐vector space. The regions increase in thickness, angular diameter, and number as the pump power is increased. The theory thus predicts the generation of waves propagating in directions which can differ substantially from the direction of the pump field. Ionospheric observations confirm this prediction; they were difficult to reconcile with the predictions of previous theories based on an approximate expression for the nonlinearity. The present work also corrects the results of previous one‐dimensional theories that used an accurate expression for the nonlinearity and predicted “spectral lines” in the limit of vanishing spontaneous emission. Excitation of the purely growing instability is predicted for pump powers greater than about 2.5 times the threshold of the decay instability.

Zinc oxide light emitting diode

Abstract: A light emitting diode comprising a single crystal of high efficiency violet fluorescing zinc oxide together with an ohmic cathode and a rectifying anode secured thereto. The diode emits near band-gap radiation at room temperature when energized with low D.C. voltage. The produced light is believed to be associated with radiative recombination of free excitons.

Excitation mechanisms in He–Cd and He–Zn ion lasers

Abstract: Experimental evidence from pulsed afterglow studies is presented which demonstrates that thermal energy charge transfer is the primary source of excitation for the high‐lying Zn II and Cd II laser levels, with configurations of the type 3d 10nx and 4d 10nx, respectively, whereas the main source of excitation of the Zn II and Cd II laser levels, with configurations of the type 3d 94s 2 and 4d 95s 2, is Penning ionization. The total velocity‐averaged cross sections for thermal energy charge exchange and for Penning ionization are measured to be of order 10−15 cm2. These large cross sections account for the relatively high efficiency and low threshold currents characteristic of He–Zn and He–Cd ion lasers. The intensity of spontaneous emission from laser levels excited in a flowing afterglow by charge‐transfer reactions is broken down into the component contributions: direct charge‐transfer excitation, radiative cascade, and collisional cascade excitation. The results are used to estimate the values of the in...

A continued fraction solution to the problem of a single atom interacting with a single radiation mode in the electric dipole approximation

Abstract: Difference equations are derived for the Laplace transforms of the time dependent probability amplitudes of a system consisting of a single photon interacting with a single two-level atom in the electric dipole approximation. Formally exact continued fraction solutions of these difference equations are obtained and applied to the problems of stimulated and spontaneous emission and absorption of radiation.

cw degradation at 300°K of GaAs double‐heterostructure junction lasers. I. Emission spectra

Abstract: Spectral changes in the spontaneous emission from stripe‐geometry GaAs double‐heterostructure junction lasers have been observed to occur during their noncatastrophic degradation under continuous operation at room temperature. As a result of prolonged continuous operation, the optical wavelength for maximum external spontaneous emission increases significantly as the degradation progresses. In some lasers the wavelength shift has been observed to occur only in the emission from one mirror of the laser with the spectral emission from the other mirror remaining unchanged. The observed changes in rapidly degrading lasers are interpreted in terms of the formation of a localized region of high optical absorption within the active region. The absorbing region is identified by visual observation through the n substrate of a degraded laser as a volume in which little or no spontaneous emission occurs near the lasing wavelength. In addition, the absorption is shown to be dispersive, exhibiting a sharply increasing absorption edge with increasing energy.

Saturation behavior of the spontaneous emission from double-heterostructure junction lasers operating high above threshold

Abstract: Saturation of the spontaneous emission intensity from stripe-geometry double-heterostructure junction lasers has been observed for currents from threshold up to three times threshold. The present observation of saturated luminescence depends upon the spatial uniformity of the saturated gain obtained in a laser with a thin active region ( \sim0.2-0.3 \mu m) and weak optical confinement. In lasers with thicker active regions ( \sim1 \mu m) and stronger optical confinement, the spontaneous emission does not saturate at threshold but continues to increase with current at a reduced rate. The increasing emission is shown to originate in spatial regions where the field intensity is low and consequently the gain is not saturated. At pumping levels high above threshold, the nonuniform saturation of the gain (transverse spatial hole burning) leads to increased gain and finally oscillation for other transverse modes.

Spontaneous and stimulated recombination in p + -n-n + (AlGa)As-GaAs heterojunction laser diodes

Abstract: A detailed study has been made of the lasing properties of p+-n-n+double heterojunction lasers where the n-type active region was lightly doped n type ( \sim 2 \times 10^{16} cm-3) and only weakly compensated. The structure of the diodes allowed a study of the spontaneous emission (below and above lasing threshold) through the nonabsorbing higher band-gap (AlGa)As surface of the diode. Only a single major spontaneous band is seen which peaks near the band-gap energy of the n-type active region. (However, when viewed only through the diode edge, a second spurious lower energy "ghost peak" is seen due to selective internal absorption of the spontaneous radiation in the GaAs substrate.) With increasing current density, the spontaneous emission band broadens with a slight upward shift in its peak energy. The lasing peak eventually emerges from the low-energy tail of the band at an energy where relatively little spontaneous radiation is observed at low current densities and where the absorption coefficient is relatively small. Observations made by cathodoluminescence of GaAs with similar doping shows an analogous behavior, with the energy separation between the spontaneous and lasing peaks increasing with increasing temperature between 77 and 300 K. The observed behavior is consistent with the hypothesis that the lasing transitions involve conduction and valence band tail states due to the screening of the crystal field (and carrier interaction) by the high density of injected carriers in the active region of the laser.

Rotating-Wave Approximation in Coherent Interactions

Abstract: Some consequences of the rotating-wave approximation (RWA) for spontaneous emission are explored. The Winger---Weisskopf theory of the spontaneous exponential decay of a single excited atom is shown to contain an implicit form of the RWA. However, to describe the cooperative superradiant level shift, the RWA should not be used, and care should be taken over the choice of interaction Hamiltonians.

Amplified Spontaneous Emission

Abstract: In 1954 Dicke [1] discussed the possibility of a coherent cooperative process between an ensemble of two-level atoms. His paper has become a classic, and discusses the process now known as super-radiance. At the time the paper attracted relatively little attention except that of a few who worked in r.f. spectroscopy.

Variation of spontaneous emission with current in GaAs homostructure and double-heterostructure injection lasers

Abstract: Partial saturation and change of the slope of the spontaneous emission are observed in CW homostructure GaAs lasers and pulsed homostructure and DH GaAs lasers at the onset of the laser modes. An important new effect that we have observed is a reduction of the spontaneous emission above threshold for homostructure lasers. The results are discussed in comparison with the previously published work and a possible explanation for the difference of behavior between homostructure and DH lasers is proposed.

Fluoreszenz der magnetisch ordnenden Halbleiter EuS, EuSe und EuTe

Abstract: The photoluminescence properties of the europium-chalcogenides EuS, EuSe and EuTe are described. Below approximately 150°K these compounds show a characteristic near-infrared fluorescence band with a half-width of about 0.25 eV. This radiative recombination is ascribed to an intrinsic transition 4f 6 5d→4f 7 of the Eu-ions. Its important frequency shift with respect to the corresponding optical absorption in the vicinity of the absorption edge is due to a Franck-Condon-type relaxation process. The spectral band position and the quantum efficiency of the emission are found to be very sensitive to magnetic ordering. Within the series of substances the different kind of magnetic order is clearly illustrated by the dissimilar spontaneous fluorescence behaviour: On cooling, the ferromagnet EuS displays a red shift and a quenching of the emission nearT c, whereas the antiferromagnet EuTe shows a blue shift of the band and an increase of its intensity nearT N. With respect to this behaviour in zero field an applied magnetic field causes the fluorescence to be quenched and shifted towards longer wavelengths. The largest response to the field is observed in the vicinity of the ordering temperatures and in the metamagnetic temperature range of EuSe. The results are discussed in connection with measurements of the magnetization, the optical absorption and the photoconductivity and compared with fluorescence measurements on Eu(II)-silicates. The behaviour of the excitation spectra is closely related to the absorption edge shift. On the other hand the shift of the emission is not similarly discribed by the mean ion spin correlation. We propose to ascribe this to a magnetic relaxation in the vicinity of the excited state. The unusual intensity behaviour is also discussed.

Electroluminescent shifting-peak spectra in GaAs with uniform excitation

Abstract: Steady‐state and time‐resolved spectra have been measured at room temperature for spontaneous emission from the GaAs active layer of a double‐heterostructure laser. The closely compensated p‐type active layer was heavily doped with Si. The double heterostructure provides carrier confinement and permits observation of spontaneous emission for conditions of uniform excitation. The emission at low levels of excitation was stationary in energy while increasing in intensity. At high levels of excitation, the emission shifts to higher energy while increasing in intensity. These spectra result from radiative recombination in which the injected electrons fill the empty conduction band tails and have been designated here as emission due to the band‐filling mechanism. Equivalence between steady‐state and time‐resolved spectra demonstrated that quasiequilibrium exists within the conduction and valence bands. Carrier distributions and emission spectra were calculated as a function of excitation for valence and conduction band density of states represented by Kane's Gaussian band tails. These calculations showed that the emission for the band‐filling mechanism was stationary in energy as the injected electrons filled the conduction band tails. As the tail states were filled, the low‐energy emission began to saturate and the emission shifted to higher energy. This emission for band filling is compared with the properties of radiative tunneling.

Waveguide and luminescent properties of thin‐film Pb‐salt injection lasers

Abstract: We describe the waveguide properties of the asymmetric dielectric slab formed by a very‐high‐index film on a low‐index substrate. The analysis is appropriate for Pb‐salt films grown on fluorite structure substrates. The mode reflectivities, surface scattering losses, and gain enhancements for the low‐order TE and TM modes are considered. The stronger confinement of the TM modes leads to larger gain enhancements than for the TE modes. For the film thicknesses of interest, 2–4 μm, the TE and TM mode reflectivities are comparable. Using the theory of Tien, the TE modes show substantially lower surface scattering losses than TM modes of the same order. The magnitude of this loss, however, depends on an unknown parameter σ, the variance of the surface height. Experimental results are presented for thin‐film diode lasers made with Pb Schottky barriers on p‐type PbTe (index 6.4) grown epitaxially on BaF2 substrates (index 1.42). The spontaneous emission at 77°K is shown to agree with the band‐band recombination ...

Advances in light‐emitting diodes

Abstract: The bright future of semiconductor lamps, especially “light‐emitting diodes,” is by now widely recognized. This recognition is fairly recent even though the existence of light emission from semiconductor junctions has been known for almost twenty years. The transformation of LED's from laboratory devices to commercial products constitutes a good example of the effective interaction between solid‐statematerials technology and the quantum theory of condensed matter. Moreover, this interaction continues to generate changes in the materials,fabrication techniques and cost of LED's. In this article we will examine the influence of the fundamental principles of the quantum theory of solids on the design of semiconductor light emitters. Emphasis is placed on some of the more recently recognized consequences of these principles for tunable solid‐state light sources. In particular, we describe how breakdowns of momentum conservation caused by the presence of neutral impurities in semiconductors has exerted a profound influence on the materials used to construct solid‐state lamps. Short‐range impurity states in ternary III‐V compounds enhance radiative recombination rates, thereby aiding the color tunability of semiconductor lamps and lasers.

Effect of saturable absorption on the behavior of spontaneous emission in semiconductor lasers

Abstract: A substantial reduction of the spontaneous emission intensity is observed at the onset of stimulated emission in some injection lasers. It is shown that this effect is caused by the saturation of optically absorbing traps present in the active region. It is suggested that saturable optical absorption is also responsible for previously unexplained similar reduction observed by Nicoll in bulk semiconductor lasers.

Experimental properties of injection lasers. III. Current dependence of polarization

Abstract: The power distribution function which describes the shape and current dependence of the lateral beam is applied to a phenomenological model of weakly polarized injection lasers It predicts that the polarization of coherent power should drop rapidly from an infinite value at threshold to near unity at high drive Careful measurement of polarization of a laser representative of the model, with especial attention to rejection of the spontaneous emission, supports this conclusion The form of the power distribution function used in the analysis implies that lasers with strong polarization at high drive have large threshold differences of the two polarizations, suggesting that present theories of the source of polarization are inadequate The difficulty of interpreting the present results in terms of spatial hole burning or inhomogeneities is pointed out

Continuum emission from recombining oxygen and nitrogen plasmas

Abstract: The quantum defect method is used to obtain photoionization cross-sections for some 40(55) levels of OI (NI) over a range of electron energies from threshold to one Rydberg. These cross-sections are then incorporated into a model that predicts the total radiative emission resulting from radiative recombination and bremsstrahlung in the presence of singly ionized oxygen (nitrogen) atoms. Modified emission coefficients are presented for the wavelength region extending from the visible to the long wave infrared for a variety of temperatures.

A Quantum Treatment of Spontaneous Emission Without Photons

Abstract: The question whether it is possible to account for spontaneous emission of electromagnetic radiation from atoms without quantum electrodynamics has lately been the subject of further discussion.[1–5] In a recent article Nesbet[5] has considered an approach to the problem that differs substantially from the neoclassical approach of Jaynes and his co-workers.[1–3] In this theory the electromagnetic field is expressed explicitly in terms of its sources, which are quantized, and it obeys the algebra of the sources, while the concept of the free boson field is discarded altogether. Although he has referred to it as ‘semi-quantized radiation theory’, the theory is actually a fully quantized one, in the sense that no c-number currents or fields appear. When the rate of energy flow into the far electromagnetic field written in normal order is equated to the rate of energy loss of a two-level atom, Nesbet’s theory apparently leads to exponential decay of the atomic energy.[5,6]

A Test of Jaynes’ Neoclassical Theory: Incoherent Resonance Fluorescence from a Coherently Excited State

Abstract: There were two motivations for observing the incoherent[1] resonance fluorescence from coherently excited Rb atoms. The first was to demonstrate the quantum-electro-dynamic (QED) coherent-optical effect that the fluorescence should have maxima when the atoms are left in a state of maximum excitation and minima when the excitation is minimized[2,3]. This effect in an optically thin sample is analogous to the precession of a permanent magnetic moment driven by an external magnetic field rotating at the Larmor frequency. The second motivation was to test the semiclassical or neoclassical theory (NCT) of Jaynes, Crisp, Stroud, and co-workers[4,5]. NCT assumes that the expectation value of the dipole moment operator is an actual dipole moment which radiates according to classical electrodynamics. Thus NCT predicts a maximum fluorescence for equal admixtures of the ground and excited states and minima when the atom is closest to a pure state whether it is the excited or ground state. Whereas QED predicts maximum fluorescence for a pure excited state, NCT predicts no fluorescence. NCT’s electromagnetic field is not quantized so no zero-point fluctuations exist to give rise to spontaneous emission from a pure excited state.