Showing papers on "Spontaneous emission published in 1990"

X-ray lasers

Abstract: The first in its field, this book is both an introduction to x-ray lasers and a how-to guide for specialists. It provides new entrants and others interested in the field with a comprehensive overview and describes useful examples of analysis and experiments as background and guidance for researchers

Spontaneous emission and absorption properties of a driven three-level system

Abstract: We investigate the steady-state spontaneous emission spectrum of a three-level atom driven by two coherent fields and the absorption spectrum of a weak probe passing through a collection of such driven atoms. We find that the fluorescence spectrum is strongly affected by the decay rates of all the levels involved in the atomic evolution and not just by the decay parameters of the specific transition whose emission spectrum is being monitored. In particular, the spectral components can acquire very different widths and peak heights relative to the case of the standard resonance fluorescence in which a two-level system is driven by a single near-resonant field. An external probe signal passing through the gas of three-level atoms may be absorbed or amplified, as in the standard two-level case, but under specific operating conditions, amplification (or absorption) occurs over the entire range where the atomic response is appreciable. In this case the existence of amplification or absorption is controlled solely by the population difference between two dressed states of the system. We provide numerical results in support of our arguments for arbitrary values of the atomic and field parameters and also develop an analytic description in the limit of strong driving fields that leads to explicit line-shape and linewidth formulas.

Enhanced spontaneous emission from GaAs quantum wells in monolithic microcavities

Abstract: Enhanced spontaneous emission has been observed with wavelength‐sized monolithic Fabry–Perot cavities containing GaAs quantum wells. With an on‐resonance cavity structure, the photoluminescence intensity increases in the cavity axis direction, and the spontaneous emission lifetime is experimentally found to decrease.

Observation of inhibited spontaneous emission in a periodic dielectric structure.

Abstract: Inhibition of the radiative decay of dye molecules embedded in an ordered aqueous suspension of polystyrene spheres has been measured using picosecond pump-probe techniques. The inhibition is interpreted in terms of excluded vacuum modes and compared with results based on the Wigner-Weisskopf formalism and a three-dimensional scattering model for the fcc colloidal structures.

Spontaneous emission in the optical microscopic cavity

Abstract: The quantum theory of the spontaneous emission (SpE) in the optical microscopic cavity (microcavity) is reported we believe for the first time.1,2 The forms of the modes propagating orthogonally to the mirrors and previously studied by Ley and Loudon3 are here generalized to cover all spatial directions. Microcavity field quantization leads to the expressions of the SpE probabilities Γ||, Γ⊥ for dipoles parallel and orthogonal to the mirrors, respectively. These are computer calculated for some experimentally significant cases involving both metal and dielectric multilayer-mirror coatings.

Spontaneous emission rate alteration in optical waveguide structures

Abstract: Optical microcavities hold technological promise for constructing efficient, high-speed, semiconductor lasers. Achieving the desired effects depends on the degree to which spontaneous emission may be altered by the presence of the cavity. The radiating modes of an oscillating electric dipole placed between two planar metallic mirrors (one-dimensional confinement) and placed in an optical-wire structure (two-dimensional confinement) are discussed. The analysis is carried out using a mode-counting method. The authors show that this method is simpler and more intuitive than traditional classical and quantum electrodynamical calculations. Using the results of the analysis, it is found that an optical wire provides much larger spontaneous-emission-rate alteration than a planar mirror structure. >

Signal-to-noise considerations in fiber links with periodic or distributed optical amplification

Abstract: The detected signal-to-noise power ratio is evaluated for fiber links with periodic amplification. It is shown that the highest ratio is achieved in the limit of a distributed amplifier (g = α), but that, alternatively, periodic amplification at intervals of α^-1 entails a penalty of less than 2 dB compared with the (ideal) distributed case.

Effect of an AlAs/GaAs mirror on the spontaneous emission of an InGaAs‐GaAs quantum well

Abstract: Data are presented demonstrating a strong influence of a closely spaced AlAs/GaAs distributed Bragg reflector on the spontaneous emission characteristics of an InGaAs‐GaAs quantum well. The mirror to quantum well spacings on different crystal samples correspond to optical path lengths of either 1/4, 1/2, or 3/4 of the emission wavelength. The samples are characterized using photoluminescence, electroluminescence, and reflectivity measurements. Spontaneous emission is found to be greatly enhanced for a 1/2 wavelength spacing, while 1/4 and 3/4 spacings suppress the spontaneous emission by a factor of ≳1000.

Analysis of the phase-amplitude coupling factor and spectral linewidth of distributed feedback and composite-cavity semiconductor lasers

Abstract: A Green's function approach to the analysis of semiconductor lasers is formulated in a form suitable for complex cavity structures. Both the spontaneous emission rate and the effective phase-amplitude coupling factor can be accurately evaluated. For distributed-feedback (DFB) lasers, the spontaneous emission rate is strongly dependent on both the facet reflectivities and the grating coupling coefficients. The effective phase-amplitude coupling factor depends on the wavelength detuning from the gain maximum. The calculated linewidth of DFB lasers differs considerably from previous calculated results and gives better agreement with experimental results. For composite-cavity lasers, the frequency dependence of the equivalent reflectivity has a strong impact on the phase-amplitude coupling factor and the spontaneous emission rate. Distributed Bragg reflector (DBR) lasers are investigated as an example of a composite-cavity structure. >

Optically-coupled mirror-quantum well InGaAs-GaAs light emitting diode

Abstract: Data are presented showing that it is possible to optically couple a semiconductor quantum well to a mirror, and thus influence its spontaneous emission lifetime, for mirror to quantum well spacings of less than an optical wavelength. Light emitting diodes with various mirror to quantum well spacings corresponding to quantum well placement at either interference nodes or antinodes are characterised in terms of light output efficiency and frequency response.

Lifetime broadening in GaAs-AlGaAs quantum well lasers

Abstract: Experimental observations of spontaneous emission spectra from GaAs-AlGaAs quantum well lasers shown that spectral broadening should be included in any realistic model of laser performance. A model of the lifetime broadening due to intraband Auger processes of the Landsberg type is described and developed for the case of electron-electron scattering in a 2-D system. The model is applied to the calculation of gain and spontaneous emission spectra and gain-current relationships in short-wavelength GaAs-AlGaAs quantum well lasers, and the results are compared with those obtained using both a fixed intraband scattering time and one that varies as n/sup -1/2/, where n is the volume injected carrier density. >

Nonlinear optical processes near the sodium 4D two-photon resonance.

Abstract: The nonlinear response of sodium vapor with a laser tuned near the 4D two-photon resonance is investigated over a large range of sodium concentrations and pump intensities with emphasis on the influence of coherent cancellation effects on the induced polarizations and the primary mechanism for the generation of the new frequencies. Over a wide range of circumstances, the response of the medium is dominated by parametric four-wave mixing as compared to amplified spontaneous emission or stimulated electronic hyper-Raman scattering. Secondary four-wave-mixing fields are generated by stimulated Raman scattering of one parametric four-wave-mixing field and a coupling of these fields with the driving laser field. A sharp onset of saturation of the parametric four-wave-mixing signals with sodium number density and a transition from a quadratic to a linear power dependence is observed. Both effects are explained in terms of the two-photon cancellation effect.

Wide-bandwidth receiver photodetector frequency response measurements using amplified spontaneous emission from a semiconductor optical amplifier

Abstract: The white optical noise (spontaneous-spontaneous beat noise) generated by amplified spontaneous emission from a semiconductor-optical amplifier is used to measure the frequency response of over-wide-bandwidth photodetectors and optical receivers. This technique can be used to characterize optoelectronic components of arbitrarily wide bandwidths. >

Population transfer by stimulated Raman scattering with delayed pulses and by the stimulated-emission pumping method: a comparative study

Abstract: The characteristics of the stimulated-emission pumping (SEP) method and the technique of stimulated Raman scattering by delayed pulses (STIRAP) are compared, based on numerical results from density matrix calculations. The latter technique requires the use of nearly transform-limited pulses, whereas the former method is successfully implemented with broadband lasers. Damping coefficients γ are introduced to model the consequences of phase fluctuations of the radiation. It is shown that the various techniques can be classified in terms of the delay D of the Stokes laser relative to the pump laser and the damping rate γ. The delay D/ΔτL ≈ −1 and γ = 0 corresponds to the STIRAP mode of operation, whereas the SEP method requires that D/ΔτL ≥ 0 and γΔτL ≫ 1, where ΔτL is the width of the laser pulse.

Characteristics of single- and two-dimensional phase coupled arrays of vertical cavity surface emitting GaAs-AlGaAs lasers

Abstract: Two-dimensional arrays of 3*3 vertical cavity surface emitting GaAs-AlGaAs lasers with 7- mu m center spacing are described. The lasers were grown by molecular beam epitaxy and contain one grown GaAs-AlAs mirror under the active layer and a second, thermally deposited, high-reflectivity, SiO/sub 2/-Si mirror on top of the epitaxial layer. The array has a 295-K threshold of 90 mA, corresponding to 10 mA per laser. The individual lasers in the array have Gaussian beam profiles both spatially in the near field and angularly in the far field. When all the lasers in the array are operating in a coupled manner, the emission exhibits a lobed far-field pattern with a 5 degrees separation in the direction parallel to the contact edge and a 2.5 degrees separation perpendicular to the contact edge. >

Linewidth enhancement factor for InGaAs/InP strained quantum well lasers

Abstract: The linewidth enhancement factor α in an InGaAs/InP strained‐layer multiple quantum well (MQW) laser emitting near 1.55 μm has been determined from the spontaneous emission spectra below threshold. The active layers in the MQW structure in this device are under 0.7% compressive strain. The measured α at the lasing wavelength is 2.0. The calculation of α using interpolated bandstructure parameters shows that it varies rapidly with injected carrier density and the calculated value for our device is close to the measured value. The small α for strained MQW InGaAs lasers should result in performance improvement that are advantageous for lightwave system application.

Quantum theory of spontaneous emission in a one-dimensional optical cavity with two-side output coupling.

Abstract: A quantum theory of spontaneous emission from an initially excited two-level atom in a one-dimensional optical cavity with output coupling from both sides is developed. Orthonormal mode functions with a continuous spectrum are employed, which are derived by imposing a periodic boundary condition on the whole space with a period much larger than the cavity length. The delay differential equation of the atomic state of Cook and Milonni [Phys. Rev. A 35, 5081 (1987)] is re-derived in a strict manner, where the reflectivity of the cavity mirrors is included naturally in the mode functions. An approximate solution at a single-resonant-mode limit shows the results of ``vacuum'' Rabi oscillation in an underdamped cavity and enhanced spontaneous emission rate in an overdamped cavity. For the latter case, it is found that in the optical range the spontaneous emission rate is enhanced by a factor F (finesse of the cavity).

Modeling of quantum-well lasers for computer-aided analysis of optoelectronic integrated circuits

Abstract: A two-port circuit model for quantum-well (QW) lasers has been developed from rate equations. With emphasis on the physical principles, the phenomena of the recombination process of electron-hole pairs and the light wave resonance in the active region have been incorporated into this new model. The model has been implemented in a circuit simulator and validated with measured DC and transient laser characteristics. The frequency effects on the modulation properties of QW lasers have been studied and analyzed using a small-signal model. Simulation results show excellent agreement with experimental data. >

Carrier decay in GaAs quantum wells

Abstract: Carrier decays following pulsed excitation in GaAs‐AlGaAs quantum wells have in the past been attributed to an excitonic recombination path. We show here that such an interpretation is inconsistent with the experimental evidence, and that the decays are controlled by recombination through electronic states at the GaAs‐AlGaAs interfaces. We discuss the expected magnitude of the decay times and the influence of carrier trapping on the decay process.

Effect of atomic-state coherence and spontaneous emission on three-level dynamics

Abstract: For a three-level atom in the {ital ssV} configuration (i.e., having two excited states each dipole-coupled to a common ground state), we have found a particular linear combination of bare-atom states in which Rabi oscillations and their associated collapses and revivals do not occur. Moving to a dressed-state picture, we discover that this particular linear combination state is just that dressed state which is decoupled from all the field modes. It is a dressed state for which the transition dipole moments with the other dressed states are zero. The existence of this decoupled dressed state depends on the tuning of the dressing laser field, which in turn depends on the bare-atom excited-state dipole moments and energy-level separation. When we include spontaneous emission, the population decays from the other dressed states into this decoupled state and remains coherently trapped there, producing a system that experiences no dynamical behavior. This is exact for {delta}-function photon statistics (i.e., if there is no intensity uncertainty). The trapping becomes less perfect as the photon statistics are allowed to have a greater bandwidth. Also, if the applied field is tuned incorrectly, the spontaneous realignment of the atomic state amplitudes does not result in a totally decoupledmore » dressed state, and the dynamics proceed normally.« less

Verification of a generalised Planck law by investigation of the emission from GaAs luminescent diodes

Abstract: A generalisation of Planck's law for the spontaneous emission from light emitting diodes with direct band gaps allows a calculation of the absolute spectral emission intensity to be performed when the applied voltage, temperature, and absorption coefficient of the light emitting region are known. For given values of these parameters the emission intensity is independent of the diode current and the quantum efficiency of the recombination process. GaAs LED spectra calculated on this basis are compared with measurements at 82 K and at 296 K, and are found to agree quite well. Differences are attributed to the uncertain knowledge of the absorption coefficient of the diode material.

Spectral characteristics of high-power 1.5 mu m broad-band superluminescent fiber sources

Abstract: The spectral variation of spontaneous emission from erbium-doped single-mode fibers is studied with the aim of producing high-power (more than 5 mW), broadband (in excess of 10 nm) amplified spontaneous emission sources for fiber gyroscope applications. In particular, the evolution of spectral shape and center wavelength with fiber length and output power in the previously unstudied high-power regime where saturation effects dominate is demonstrated. A visibility curve for a potential twin-peaked nonresonant erbium-doped fiber gyroscope source with a short (210 mu m) coherence length is also presented. >

Amplified spontaneous emission in Tm3+‐doped monomode optical fibers in the visible region

Abstract: We report the observation of amplified spontaneous emission in silica‐based single‐mode optical fibers doped with Tm3+. Frequency upconverted emission was observed in a number of visible and near‐infrared lines after pumping by 1064 nm radiation. The upconversion process is induced via the nonlinear absorption of the infrared beam and enhanced by stimulated Raman scattering in the fiber, leading to amplified spontaneous emission in the blue (∼473 nm) region.

Steady states in cavity QED due to incoherent pumping.

Abstract: It is shown that strong competition between the absorption and emission of the cavity photons and the incoherent processes like atomic spontaneous emission and cavity decay effects result in new nonequilibrium states in cavity quantum electrodynamics. Such states can exhibit saturation, nonclassical character, and strong atom-cavity correlation.

Wave-optics description of laboratory soft-x-ray lasers

Abstract: We describe a time-dependent wave-optics propagation model applicable to laboratory x-ray lasers and amplified spontaneous emission devices described in the literature. The model embodies a stochastic treatment of spontaneous emission, counterpropagating beams, diffraction, gain saturation, transverse variation of gain, and refractive effects due to electron density gradients. The model has been used to describe the output beam characteristics and spatial coherence properties of some plausible lasers.

Linewidth enhancement factor in strained quantum well lasers

Abstract: The linewidth enhancement factor α in an In0.2Ga0.8As/GaAs strained‐layer multiple quantum well (MQW) laser has been determined from the spontaneous emission spectra below threshold. The measured α at the lasing wavelength is found to be 1.0 compared to a value of 5 typically observed for InGaAsP/InP double‐heterostructure lasers. The smaller α shows that single wavelength strained MQW lasers may have smaller chirp width under modulation and also smaller cw linewidth.

1.4-eV photoluminescence and thermally stimulated conductivity in cadmium telluride

Abstract: Photoluminescence (PL) and thermally stimulated conductivity (TSC) data on high‐resistivity, p‐type CdTe single crystals are presented. The PL emission in these samples consists of two closely overlapping components peaking at approximately 1.47 and 1.49 eV. Thermal quenching of these signals reveals activation energies of ∼0.02 and ∼0.13 eV for the former component, and ∼0.11 eV for the latter. TSC signals at temperatures corresponding to those over which thermal quenching occurs are observed. The TSC peaks are due to hole release with activation energies which agree with those obtained from the thermal quenching studies. Etching of the samples removes surface damage caused by mechanical polishing. The surface damage produces nonradiative pathways by which electron‐hole recombination can take place without luminescence. A model based on free‐electron to trapped‐hole recombination is presented to account for the data. It is shown, from numerical solutions of the rate equations describing the model, that by explicitly incorporating into the model more than one hole state at which radiative recombination can occur, shifts in the emission energy during time‐resolved and intensity‐dependence studies can be expected.

Lateral quantization induced emission energy shift of buried GaAs/AlGaAs quantum wires

Abstract: Buried GaAs/AlGaAs quantum wires were prepared by Al0.2Ga0.8As overgrowth of deep etched wires defined by high‐resolution electron beam lithography and dry etching. The overgrown wires show a dramatic decrease of the optically inactive sidewall layer compared to open wires. For wires with a lateral dimension Lx=50 nm we observe a spectral shift to higher energies of the excitonic emission, in good agreement with calculations of the two‐dimensional confinement effects.

Mechanisms for the emission of visible light from GaAs field‐effect transistors

Abstract: The emission of visible light from GaAs metal‐semiconductor field‐effect transistors under high electric field conditions is studied in detail in order to learn more about the luminescence mechanism. The electroluminescence spectrum is examined for energies both greater and less than the energy gap. It is seen that the form of the subgap spectrum can be explained by a Bremsstrahlung mechanism. However, the prominence of a voltage‐dependent band‐gap peak indicates that recombination processes also contribute to visible photon emission. A strongly polarized component of electroluminescence further substantiates the existence of radiative elastic collisions (Bremsstrahlung) and may provide insights into the scattering behavior of electrons in the channel.