Showing papers on "Spontaneous emission published in 1992"

Monte Carlo simulation of the atomic master equation for spontaneous emission.

Abstract: A Monte Carlo simulation of the atomic master equation for spontaneous emission in terms of atomic wave functions is developed. Realizations of the time evolution of atomic wave functions are constructed that correspond to an ensemble of atoms driven by laser light undergoing a sequence of spontaneous emissions. The atomic decay times are drawn according to the photon count distribution of the driven atom. Each quantum jump of the atomic electron projects the atomic wave function to the ground state of the atom. Our theory is based on a stochastic interpretation and generalization of Mollow's pure-state analysis of resonant light scattering, and the Srinivas-Davies theory of continuous measurements in photodetection. An extension of the theory to include mechanical light effects and a generalization to atomic systems with Zeeman substructure are given. We illustrate the method by simulating the solutions of the optical Bloch equations for two-level systems, and laser cooling of a two-level atom in an ion trap where the center-of-mass motion of the atom is described quantum mechanically.

Physics and Device Applications of Optical Microcavities

Abstract: Optical microcavities are resonators that have at least one dimension on the order of a single optical wavelength. These structures enable one to control the optical emission properties of materials placed inside them. They can, for example, modify the spatial distribution of radiation power, change the spectral width of the emitted light, and enhance or suppress the spontaneous emission rate. In addition to being attractive for studying the fundamental physics of the interaction between materials and vacuum field fluctuations, optical microcavities hold technological promise for constructing novel kinds of light-emitting devices. One of their most dramatic potential features is thresholdless lasing. In this way and others, controlled spontaneous emission is expected to play a key role in a new generation of optical devices.

Luminescence and structural study of porous silicon films

Abstract: The luminescence properties of 3 μm thick, strongly emitting, and highly porous silicon films were studied using a combination of photoluminescence, transmission electron microscopy, and Fourier transform infrared spectroscopy. Transmission electron micrographs indicate that these samples have structures of predominantly 6–7 nm size clusters (instead of the postulated columns). In the as‐prepared films, there is a significant concentration of Si—H bonds which is gradually replaced by Si—O bonds during prolonged aging in air. Upon optical excitation these films exhibit strong visible emission peaking at ≊690 nm. The excitation edge is shown to be emission wavelength dependent, revealing the inhomogeneous nature of both the initially photoexcited and luminescing species. The photoluminescence decay profiles observed are highly nonexponential and decrease with increasing emission energy. The 1/e times observed typically range from 1 to 50 μs. The correlation of the spectral and structural information suggest...

Luminescence polarization of CdSe microcrystals.

Abstract: We have considered theoretically the polarization of the luminescence of small CdSe microcrystals due to the hexagonal structure of the lattice. We derive the energy spectrum and wave functions of holes in spherical microcrystals. It is shown that the value of the splitting between the A and B hole states in microcrystals could be five times smaller than the corresponding value of crystal-field splitting \ensuremath{\Delta} in bulk hexagonal semiconductors. The times of radiative recombination and the polarization of light connected with transitions between the electron and hole quantum size levels (QSL's) were calculated. The time dependence of the luminescence polarization has been found. At the first instant after short impulse excitation the degree of polarization should be equal to 13/51. It is shown that nonequilibrium electron-hole pairs, with long lifetimes, are formed in microcrystals as a result of the hole thermalization to the A state after excitation into the B state. The recombination of these states requires the participation of phonons, resulting in a strong dependence of the recombination rate on the temperature. The degree of luminescence polarization of these states depends on the type of phonons involved in optical transitions.

Effect of State Superpositions Created by Spontaneous Emission on Laser-Driven Transitions

Abstract: Spontaneous decay of an excited atomic state may give rise to a coherent superposition of two recipient states. Experiments utilizing laser-driven three-level Λ systems are devised whose outcomes differ qualitatively depending on the presence or absence of such a spontaneous generation of coherence.

One-atom lasers.

Abstract: One-atom lasers are important because their governing equations can be solved exactly, even with a quantized field. We present a fully quantum-mechanical treatment of one-atom lasers modeled by quantum-optical master equations. These are solved numerically without any significant approximations. We show that laser action is possible with one atom, and that it might be achievable experimentally. Laser action is characterized by the dominance of stimulated emission over spontaneous emission. We use the one-atom laser model to investigate, without approximation, some interesting generic laser phenomena. Under certain conditions lasers produce intensity squeezed light, and then the laser linewidth increases with the pumping rate, in contrast with standard lasers. We also report ``self-quenching'' behavior: lasers with incoherent pumping out of the lower laser level turn off when the pumping is sufficiently fast because the coherence between the laser levels is destroyed.

Spontaneous emission in absorbing dielectric media

Abstract: We have calculated the effect of material absorption on the rate of spontaneous emission by an embedded atom using a macroscopic Green-function approach and a microscopic Hamiltonian method that includes reservoir damping. When local field effects are neglected, the free-space emission rate is modified by the real part of the refractive index at the transition frequency of the embedded atom. Local field effects are introduced via a local field correction factor. This modification of the spontaneous emission rate generalizes a well-known result for transparent media and is of crucial importance in assessing the degree of inhibition of spontaneous emission for frequencies close to the resonances of the dielectric.

Antiphase dynamics and polarization effects in the Nd-doped fiber laser.

Abstract: The response of an Nd-doped optical fiber laser to modulation of the pump power reveals that the polarization of the laser light plays an important role in the linear and nonlinear dynamics of this laser. Experiments have been carried out under pulsed or sinusoidal modulation of the pump power as well as in the continuous-wave regime. Most of the observed phenomena may be interpreted in the framework of a theory of the two-mode laser in which each mode is associated with one polarization eigenstate of the laser. Effects such as the appearance of slow modes and antiphase behavior have also been observed. A model of a two-mode laser including spontaneous emission is theoretically and numerically analyzed. It reproduces well most of the experimental findings.

Photon echoes and related four-wave-mixing spectroscopies using phase-locked pulses

Abstract: The use of phase-locked pulses in various spectroscopic techniques related to the third-order polarization P (3) is analyzed. Using correlation function expressions for the nonlinear response function, we clarify the interrelationship among several photon echo, pump-probe, and spontaneous light emission techniques, without alluding to any specific model for the material system. By combining phase-locked pulses and heterodyne detection it becomes possible to probe separately the real and imaginary parts of the nonlinear response function. Combining two phase-locked pulse excitation with time-resolved detection of the spontaneous light emission allows direct separation of the Raman and fluorescence contributions.

Large-signal dynamic model of the DFB laser

Abstract: A computer model is proposed to analyze the characteristics of distributed feedback (DFB) lasers. The model is based on time-dependent coupled wave equations, with spontaneous emission taken into account. In order to avoid uncertain phase factors in spontaneous emission, a method of converting field equations to power equations in a matrix format before computation is introduced. The steady-state LI curve and transient response to the pulse excitation are calculated in the lambda /4 phase-shifted DFB lasers. The longitudinal variations of the carrier and photon densities as well as of the refractive index are considered in the model. >

Optimizing the efficiency and stored energy in quasi-three-level lasers

Abstract: A model is developed for calculating the pumping efficiency of quasi-three-level lasers operated in the energy storage mode. The model is specifically applied to room temperature 1.03- mu m Yb:YAG lasers and predicts that diode-pumped Yb:YAG lasers can be at least as efficient as diode-pumped Nd:YAG lasers provided that the pump intensity is sufficiently high. The effect of amplified spontaneous emission (ASE) is also investigated to find the relation between activator ion concentration and the ASE-limited stored energy in active mirror gain elements. >

Femtosecond spontaneous-emission studies of reaction centers from photosynthetic bacteria.

Abstract: Spontaneous emission from reaction centers of photosynthetic bacteria has been recorded with a time resolution of 50 fs. Excitation was made directly into both the special-pair band (850 nm) and the Qx band of bacteriochlorophylls (608 nm). Rhodobacter sphaeroides R26, Rhodobacter capsulatus wild type, and four mutants of Rb. capsulatus were studied. In all cases the fluorescence decay was not single exponential and was well fit as a sum of two exponential decay components. The short components are in excellent agreement with the single component detected by measurements of stimulated emission. The origin of the nonexponential decay is discussed in terms of heterogeneity, the kinetic scheme, and the possibility of slow vibrational relaxation.

Giant enhancement of luminescence intensity in Er‐doped Si/SiO2 resonant cavities

Abstract: Si/SiO2 Fabry–Perot microcavities with rare‐earth‐doped SiO2 active regions are realized for the first time. Cavity‐quality factors exceeding Q=300 are achieved with structures consisting of two Si/SiO2 distributed Bragg reflectors and an Er‐implanted (λ/2) SiO2 active region. The room‐temperature photoluminescence intensity of the on‐axis emission is 1–2 orders of magnitude higher for resonant cavity structures as compared to structures without a cavity.

Enhanced spectral power density and reduced linewidth at 1.3 μm in an InGaAsP quantum well resonant‐cavity light‐emitting diode

Abstract: The active region of an InGaAsP single‐quantum well light‐emitting diode (LED) emitting at 1.3 μm has been placed in the antinode of a resonant cavity consisting of a 32‐period distributed Bragg reflector (DBR) and a top silver mirror, with reflectivities of 92% and 95%, respectively. The dominant feature of the 300 K electroluminescence emission at all current levels is a 3 nm (2.8 meV) wide spontaneous emission peak centered on the cavity resonance wavelength. The spectral power density of the structure is more than one order of magnitude higher as compared to a structure without cavity. The resonant‐cavity LED operates without gain yet the extremely narrow spectrum indicates that the structure is suitable for wavelength division multiplexing applications.

Controlling spontaneous emission and threshold-less laser oscillation with optical microcavities

Abstract: We describe the alteration of spontaneous emission of materials in optical microcavities having dimensions on the order of the emitted wavelength. Particular attention is paid to one-dimensional optical confinement structures with pairs of planar reflectors (planar microcavities). The presence of the cavity causes great modifications in the emission spectrum and spatial emission intensity distribution accompanied by changes in the spontaneous emission lifetime. Experimental results are shown for planar microcavities containing GaAs quantum wells or organic dye-embedded Langmuir-Brodgett films as light emitting layers. Also discussed are the laser oscillation properties of microcavities. A remarkable increase in the spontaneous emission coupling into the laser oscillation mode is expected in microcavity lasers. A rate equation analysis shows that increasing the coupling of spontaneous emission into the cavity mode causes the disappearance of the lasing threshold in the input-output curve. Experimentally verification is presented using planar optical microcavities confining an organic dye solution. The coupling ratio of spontaneous emission into a laser mode increases to be as large as 0.2 for a cavity having a half wavelength distance between a pair of mirrors. At this point, the threshold becomes quite fuzzy. Differences between the spontaneous emission dominant regime and the stimulated emission dominant regime are examined with emission spectra and emission lifetime analyses.

On the linewidth of microcavity lasers

Abstract: In a recent letter by G. P. Agrawal and G. R. Gray [Appl. Phys. Lett. 59, 399 (1991)] the frequency and phase noise of vertical‐cavity surface‐emitting microlasers is calculated using rate equations and Langevin noise sources. In this letter we re‐examine their calculations and get different results. Specifically, we predict that an increase of the spontaneous emission coupling coefficient will not necessarily lead to an increase in linewidth. It is also found that the reported ‘‘anomalous linewidth enhancement’’ is present in all lasers with a nonzero linewidth enhancement factor.

Micro-cavity semiconductor lasers with controlled spontaneous emission

Abstract: The principle and applications of quantum electrodynamics in microcavity semi-conductor lasers are reviewed. The coupling efficiency of spontaneous emission into a lasing mode and the spontaneous lifetime are modified by various microcavity structures. As a consequence of the increased coupling efficiency, those microcavity semi-conductor lasers are expected to feature a low threshold current, high quantum efficiency and broad modulation bandwidth. One remarkable result of the increased coupling efficiency is ‘lasing without inversion’. The other is ‘intensity squeezing at any pump rate’.

Analysis of current injection efficiency of separate-confinement-heterostructure quantum-film lasers

Abstract: Current injection efficiency, i.e. the proportion of current into the active region to total current, is analyzed for separate confinement heterostructure (SCH) quantum film lasers. It is shown that the current injection efficiency changes stepwise with the active layer thickness. and is larger for lower injected carrier density and deeper quantum well. Comparison is made between step-, parabolic-GRIN-, and linear-GRIN-SCH structures. The efficiency of linear-GRIN-SCH is the highest for the same well depth. Threshold current density is discussed for these SCH structures, taking into account the current injection efficiency and the optical loss due to the carrier leakage to the optical confinement layers. >

Theory of timing jitter in actively mode-locked lasers

Abstract: An analysis of the pulse-to-pulse timing jitter in an actively mode-locked laser is presented. The model includes spontaneous emission noise, mode-locker driver phase noise, and cavity length detuning. Analytical expressions for the laser pulse train phase noise spectrum, the intensity power spectrum, and the RMS timing jitter are given. The timing fluctuations are characterized by a time constant proportional to the cavity round-trip time times the number of locked modes squared divided by the modulation depth. The contribution from the mode-locker driver phase noise will dominate unless high-stability RF sources are used. The residual timing jitter due to spontaneous emission noise is very sensitive to cavity detuning. >

Two-photon resonance induced by the dipole-dipole interaction

Abstract: The two-photon absorption by a pair of two-level atoms interacting with an incident field is examined. If the laser field is weak, then the existence of the dipole-dipole interaction-induced two-photon resonance follows from the higher-order Fermi golden rule. The effects of spontaneous emission and intense fields on the dipole-dipole interaction-induced two-photon resonance is studied using density-matrix methods. The presence of an intense field is shown to result in a significant enhancement of the two-photon resonance. Numerical results for the probability of simultaneous excitation of two atoms, for the atomic correlations, and for the radiated field intensity are presented.

Basic analysis of AR-coated, partly gain-coupled DFB lasers: the standing wave effect

Abstract: A theoretical analysis of distributed feedback (DFB) lasers with mixed gain and index coupling (partly gain-coupled DFB) is given for perfect antireflection (AR) coatings. Analytical expressions for the threshold gain, facet loss, and the relative depth of the standing wave pattern are derived. At the same time the importance of the standing wave effect and its consideration by coupled mode equations is shown. For purely gain-coupled DFB lasers, simple expressions for the effective linewidth enhancement factor and the longitudinal spontaneous emission factor are derived. In addition, various approximations describing the performance of purely gain-coupled DFB lasers are given. >

Spontaneous emission source having high spectral density at a desired wavelength

Abstract: A spontaneous emission source has a high spectral density at a desired wavelength. An integrated guided wave optical path is formed in either a semiconductor or silica substrate. An active medium and grating are provided within the optical path. The optical path is pumped to pass energy across the grating and active medium. The pump energy excites the active medium without lasing to output spontaneous emissions at a wavelength established by the grating. The source is particularly useful for reducing nonlinear effects in an optical fiber used for communicating AM or other information signals.

Theory of timing jitter in actively mode-locked lasers

Abstract: An analysis of the pulse-to-pulse timing jitter in an actively mode-locked laser is presented. The model includes spontaneous emission noise, mode-locker driver phase noise, and cavity length detuning. Analytical expressions for the laser pulse train phase noise spectrum, the intensity power spectrum, and the RMS timing jitter are given. The timing fluctuations are characterized by a time constant proportional to the cavity round-trip time times the number of locked modes squared divided by the modulation depth. The contribution from the mode-locker driver phase noise will dominate unless high-stability RF sources are used. The residual timing jitter due to spontaneous emission noise is very sensitive to cavity detuning. >

Novel technique for determining internal loss of individual semiconductor lasers

Abstract: A novel and accurate technique for measuring the internal losses of semiconductor lasers is demonstrated. The technique is based on two measurements: the injection current required for reaching material transparency and the corresponding ripple in the spontaneous emission spectrum. This method, which is independent of the internal quantum efficiency, allows the measurement of individual laser chips rather than an ensemble of chips, and can also be used to determine the loss dependence on wavelength and temperature.

Homogeneous linewidths of Rhodamine 6G at room temperature from cavity-enhanced spontaneous emission rates

Abstract: Fluorescence lifetimes of Rhodamine 6G in levitated micron‐sized droplets have been measured using a time‐correlated photon counting technique. The coupling of emission into spherical cavity modes of the droplet results in significant emission rate enhancements which allow estimation of the homogeneous linewidth at room temperature.

Phonon-absorption recombination luminescence of room-temperature excitons in Cu2O.

Abstract: We observe two luminescence lines in the range 70\char21{}340 K due to exciton recombination assisted by phonon emission and phonon absorption. The luminescence line shape fits a double Maxwell-Boltzmann distribution at all temperatures, with a temperature that agrees with that of the phonons implied by the relative heights of the two lines. From the fits to the luminescence, we also deduce the variation of the band gap of Cu${}_{2}$O and the exciton-phonon lifetime broadening up to room temperature. The lifetime-broadening data, when compared with time-resolved spectra and data from Ge and GaAs, indicate the existence of a strong two-phonon emission process.

Effect of phase-conjugate feedback on the noise characteristics of semiconductor lasers.

Abstract: This paper considers the effect of phase-conjugate feedback (PCF) on the noise characteristics of semiconductor lasers. By using the single-mode, rate-equation formalism, it is shown that semiconductor lasers can achieve a steady state as long as the amount of PCF is below a critical value. For an ideal phase-conjugate mirror, the average value of the steady-state phase of the semiconductor laser is found to be locked to a fixed value, determined by the linewidth-enhancement factor of the laser. The noise characteristics in the presence of PCF are studied by adding the Langevin-noise terms representing the effect of spontaneous emission to the rate equations and solving them approximately. Both the intensity noise and the frequency noise are reduced at low frequencies (below 100 MHz). In particular, the frequency noise nearly vanishes at zero frequency because of the phase-locked nature of the steady-state solution. The spectral line shape does not remain Lorentzian in the presence of PCF. The satellite peaks occurring at the relaxation-oscillation frequency are considerably enhanced because of a reduction in the damping rate of such oscillations.