Spontaneous Lifetime Research Articles

Inhibition of fluorescence emission in a donor molecule upon interaction with an excited acceptor

We present results of a strong laser field interacting with a two-center system. The two centers consist of a donor and acceptor pair which are coupled by Fœrster–Dexter resonance energy transfer. The donor molecule is the indole chromophore in indolylethylenediaminetetraacetic acid (indolyl-EDTA) which chelates the lanthanide ion acceptor holmium (Ho3+). The changes in the donor lifetime consequent upon excitation of the acceptor were examined. Excitation of the lanthanide acceptor with a second laser field resulted in the appearance of a substantially increased lifetime component. This additional (new) lifetime component is on the order of a second, compared with the original nanosecond lifetime of the donor in its complex with the acceptor. This new component is not attributable to the spontaneous lifetime of the acceptor. Rather, we propose that the large lifetime is due to the interaction of the donor with the excited state acceptor.

Many-body effects in near-resonant Kapitza-Dirac diffraction: The nonlinear Pendello-umlsung.

We investigate the modifications of the Pendello$iuml---sung of near-resonant Kapitza-Dirac diffraction due to many-body effects. For the case of ``two-beam'' doppleron (velocity-tuned) resonances, we find that in the absence of dissipation, the population oscillations between diffraction orders, which are complete in the linear case, can either be decreased in frequency, or become partial, or even be completely suppressed, in which case the system evolves to a quasistationary state where the atoms are in a coherent superposition of their upper and lower excited states. The influence of spontaneous emission on these predictions is then studied by numerically solving the effective single-atom Hartree-Fock master equation of nonlinear atom optics. We find that spontaneous emission is not detrimental in weakly allowed transitions, in which case the predicted effects survive for several spontaneous lifetimes.

Assessment of spontaneous lifetimes in CO/sub 2/ using an HF laser as radiation source

We have experimentally determined the absorption coefficient of CO/sub 2/ using the 3P(2) line emitted by an HF laser, as a function of the gas temperature up to 700/spl deg/K, while keeping its pressure as a parameter. The measurements were compared with a theoretical model for the absorption which takes into account the high monochromaticity of the source and the detuning of the HF laser lines with respect to the CO/sub 2/ absorption lines central wavelengths. A set of values was thus obtained for the A Einstein coefficient of the transitions given by (O O/sup 0/0)/spl rarr/(O 2/sup 0/1; 1 1/sup 1/ 1)/sub I,II/, (10/sup 0/ O; O 2/sup 0/ O)/sub I,II//spl rarr/(O 4/sup 0/1; 1 2/sup 0/1; 2 O/sup 0/1)/sub I,II,III/ and (01/sup 1/ O)/spl rarr/(1 1/sup 1/ 1; O 3/sup 1/ 1)/sub I,II/, in such a way that said values allowed, the coincidence between the experimental results and those predicted by the theoretical model. >

Calculation of lifetime dependence of Er3+ on cavity length in dielectric half-wave and full-wave microcavities

Angular spontaneous emission rates and lifetimes of Er3+ in Si/SiO2 microcavities are calculated for cavity lengths close to full and half-wavelengths. For a half-wavelength cavity, the calculations show that the spontaneous lifetime is sensitive to the variation of cavity length, and agrees very well with the experimental data measured by Vredenberg, Hunt, Schubert, Jacobson, Poate, and Zydzik [Phys. Rev. Lett. 71, 517 (1993)]. This variation comes from a coupling rate change into the cavity normal. For a full wavelength cavity, the lifetime is less sensitive to cavity length due to strong coupling of spontaneous emission to a fundamental waveguide mode.

Four-wave mixing and optical modulation in a semiconductor laser

There is a direct connection between nearly degenerate four-wave mixing in a semiconductor laser and optical modulation in the laser field. It can be understood using a model of an unlocked, optically injected laser, which emphasizes the effect of the laser resonator on the optical interactions. This model correctly describes the observed spectral characteristics and their dependence on the intrinsic parameters of the semiconductor laser. This is used to develop a simple and accurate technique using a single experimental setup for the parasitic-free characterization of the intrinsic laser parameters, including the relaxation resonance frequency, the total relaxation rate, the nonlinear relaxation rate, and the linewidth enhancement factor. Other parameters, such as the spontaneous carrier lifetime, the photon lifetime, the differential and nonlinear gain parameters, and the K factor, are determined from the power dependencies of these parameters. This technique requires only two CW lasers closely matched in wavelength and is applicable to semiconductor lasers of any wavelength and any dynamic bandwidth.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Spontaneous Emission from Planar Microstructures

Abstract The alteration of spontaneous emission characteristics in terms of the spontaneous lifetime and spectral emission characteristics are discussed for dipoles in the presence of nearby planar reflecting interfaces and cavities, specifically for the case of semiconductors. For dipoles closely spaced to absorbing metal mirrors, significant lifetime change is possible. Analysis and experimental data are presented for light emitting diodes. For dielectric Fabry-Perot microcavities, the expected lifetime change is small, but significant modification in the radiation pattern of the emitted light occurs. It is shown that the spectral characteristics of emission have a sensitive dependence on the dipole location in the cavity. Comparison is made between a classical against a quantum treatment of the spontaneous emission modification due to the cavity.

Role of waveguide light emission in planar microcavities

The role of light emission into waveguide modes of planar microcavities is elucidated through the calculation of the emission characteristics in various structures. Discrepancies which exist in the literature are discussed in some detail for the most popular forms of half-wavelength and full-wavelength planar microcavities which make use of dielectric Bragg reflectors. Particular emphasis is paid to the semiconductor AlGaAs/GaAs microcavities. Recent experiments on spontaneous lifetime changes in thin film semiconductor structures are also discussed. In addition, a SiO/sub 2//Si structure containing active atomic emitters is considered.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On the spontaneous lifetime change in an ideal planar microcavity-transition from a mode continuum to quantized modes

The spontaneous emission lifetime of an atom is known to be influenced by its environment. In general, the lifetime will be different if the atom is suspended in free space than if it is placed inside a cavity. A simple cavity geometry in which to explore this effect is the planar cavity. In a paper by Brorson, Yokoyama, and Ippen, the authors derived the minimum lifetime in a planar cavity to be one-third of the free space (no cavity) lifetime, and the appropriate mirror spacing was found to be one-half wavelength. In a paper by Yamamoto, Machida, and Bjork, it was concluded that the lifetime for a half-wavelength-long cavity was only two-thirds of the free space lifetime. In this paper, the discrepancy between these results is clarified and it is shown that the shortest lifetime is indeed one-third of the free space lifetime; but to achieve this, the cavity has to be slightly longer than one-half wavelength. This will influence the emission mode pattern in a way that, in general, is undesirable for a surface emitting laser. It is also shown that a strictly half-wavelength-long cavity has a lifetime of two-thirds of the free space lifetime. The model employed has the attractive feature that if allows a smooth transition from a fully open system with the atom interacting with a mode continuum, to the fully closed system with quantized photon modes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Transit-induced optical multistability

One effect of atomic transit on absorptive optical bistability, when the transit time is less than a spontaneous lifetime, is shown to be multistability due to coherent Rabi cycling. This multistability may be degraded by spontaneous emission, transverse variation of the cavity mode, and velocity distribution of the atomic beam. Numerical results are given showing the effects of each of these three factors. It is shown that, even for experimentally realizable conditions, multistability can survive in spite of these effects.

Controlled atomic spontaneous emission from Er3+ in a transparent Si/SiO2 microcavity.

The spontaneous emission lifetime and intensity of Er implanted into ${\mathrm{SiO}}_{2}$ is modified by a Si/${\mathrm{SiO}}_{2}$ planar microcavity. The cavity strongly affects the coupling of the optically excited ${\mathrm{Er}}^{3+}$ to the radiative and waveguiding modes of the system, resulting in a spontaneous lifetime of 14.8 ms for a cavity off-resonance with the ${\mathrm{Er}}^{3+}$ versus a lifetime of 9.1 ms for an on-resonance cavity. The observed spontaneous emission lifetime changes agree well with calculated values based on a computation method which is also presented.

Time resolved spectroscopy of the emission of fluorescent light upon UV laser assisted CVD of W from WF 6

Time resolved spectroscopy of ArF excimer laser excited WF 6/H 2/Ar and WF 6/Ar gas mixtures during LCVD of W has been performed with a time resolution of 20 and 5 ns, respectively. Using a high laser energy density (∼J/cm 2 and a WF 6/Ar gas mixture, the recorded spectra have both line and band structures which originate from W atoms and W ions. The spectral intensity distribution followed the shape of the laser pulse (FWHM≈20 ns). This means that the lifetime of the excited states is much shorter than that of the laser pulse duration. A broad band emission spectrum was obtained from a WF 6/H 2/Ar gas mixture at a laser energy density of ∼100 mJ/cm 2. This originates from excited W clusters produced in the gas phase. Time resolved measurements of this radiation showed that the spectral intensity distribution did not change with time. This indicates that the emission process cannot be black body radiation. The time dependence of the intensity of the fluorescent light could be separated into two parts: a fast decay (life time ∼400 ns) and a slow decay (∼5.7μs). The fast decay was explained in terms of vibrational quenching. The slow decay was assumed to be due to the spontaneous lifetime of excited states of the W clusters.

Spontaneous lifetime and quantum efficiency in light emitting diodes affected by a close metal mirror

The spontaneous lifetime and quantum efficiencies of dipoles placed close to a metal mirror are calculated. The importance of these effects is demonstrated in the recent experimental work on InGaAs-GaAs quantum-well light-emitting diodes (QW LEDs) in which the modulations rates were varied from 0.8 GHz to 1.4 GHz through the placement of a QW next to a Ag mirror (D.G. Deppe et al., 1990). Classical energy transfer theory is used to treat the spontaneous radiation of a QW in a semiconductor system. The spontaneous radiative lifetime and internal quantum efficiency are strongly affected by the location of the QW, which confines the dipoles, by the orientation of the dipole in the well, by the metal reflector material, and by the metal mirror thickness. In general, the internal quantum efficiency decreases when the dipole is closer to the metal, because of the nonradiative energy transfer from the dipole to the absorptive metal. The overall effect of closely spaced metal mirrors on the measurable external quantum efficiency, which is important for fast light emitting diode design, is presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Radiative lifetimes of the excited electronic states of CuCl

Spontaneous radiative lifetimes of the observed electronic states of CuCl have been calculated using a model including spin–orbit interaction mixings within the Cu+(3d94s)Cl−(3s23p6) structure. The required wave functions have been determined semiempirically and locations of the as-yet unobserved components have been estimated. For the A 3Σ1+, C 3Π0e, D 1Π1, E 1Σ0+, and F 3Δ1 components, the lifetimes calculated in the simple single-configuration pure-precession approximation with a slight modification in the Ω=0e block are in good agreement with earlier experimental measurements. On the contrary, for the B 3Π1 component, there is a marked discrepancy that cannot be reduced except by assigning very unlikely values to the off-diagonal spin–orbit parameters.

Spontaneous radiative properties of the excited electronic states of TiO

Ab initio electronic wavefunctions have been used to calculate and interpret absolute transition probabilities and oscillator strengths for all of the transitions involving the observed valence states of TiO. Spontaneous radiative lifetimes have been derived from the calculated probabilities and compared to related experimental data. The transitions are shown to be primarily atomically-forbidden transitions allowed in the molecule by orbital polarization.

Spectral signature of relaxation oscillations in semiconductor lasers

A novel and relatively simple expression is given for the optical spectrum of a single-mode semiconductor laser which, due to the presence of relaxation oscillations, consists of a strong central line with a broad weak sideband at each side. The coupling between phase and amplitude fluctuations is included in this derivation and is shown to result in an asymmetry between the relaxation oscillation sidebands. This asymmetry can be used to determine the linewidth enhancement factor. Using optical heterodyne detection, the spectrum of a Fabry-Perot-type AlGaAs laser has been measured as a function of output power. Information on the dynamics of the relaxation oscillations was thus obtained. The power dependence of the frequency and damping of the relaxation oscillations allowed the spontaneous lifetime and the dependence of the gain on both carrier density (differential gain) and intensity (gain saturation) to be separately determined.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Relaxation oscillations in an AlGaAs laser studied via the visibility

We have accurately measured the visibility of a Fabry-Pérot type AlGaAs laser as a function of its output power. A damped oscillation in the visibility indicates the presence of relaxation oscillations in the laser. From the data we obtained the output-power dependence of both the relaxation oscillation frequency and damping rate. This allowed us to separately determine the spontaneous lifetime and the dependence of the gain on both carrier density (differential gain) and intensity (nonlinear gain).

Imaging and focusing of an atomic beam with a large period standing light wave

A novel atomic lens scheme is reported. A cylindrical lens potential was created by a large period (⋍ 45 μm) standing light wave perpendicular to a beam of metastable He atoms. The lens aperture (25 μm) was centered in one antinode of the standing wave; the laser frequency was nearly resonant with the atomic transition 23S1−23P2 (λ=1.083 μm) and the interaction time was significantly shorter than the spontaneous lifetime (100 ns) of the excited state. The thickness of the lens was given by the laser beam waist (40 μm) in the direction of the atomic beam. Preliminary results are presented, where an atomic beam is focused down to a spot size of 4 μm. Also, a microfabricated grating with a period of 8 μm was imaged. We discuss the principle limitations of the spatial resolution of the lens given by spherical and chromatic aberrations as well as by diffraction. The fact that this lens is very thin offers new perspectives for deep focusing into the nm range.

Transient velocity-selective coherent population trapping in one dimension

We show theoretically that velocity-selective coherent population trapping in one dimension may be realized in atomic transitions other than Jg = 1 ↔ Je = 1. The atomic momentum distribution resulting from irradiation by counterpropagating σ+–σ− waves on the Jg = 3/2 ↔ Je = 1/2 and the Jg = 2 ↔ Je = 1 atomic transitions is investigated through solution of the optical Bloch equations and determination of the effective loss rates for atomic eigenstates. An inverted-W atomic level configuration is also used to investigate the features of velocity-selective coherent population trapping. The momentum distribution exhibits peaks at the ±ħk or the ±ħ2k and the 0 momenta, depending on the atomic transitions and the laser intensity. These structures, generated by atomic states that do not interact with the laser radiation, are stable when associated with eigenstates of the kinetic energy, or metastables; i.e., the structures last several hundred spontaneous lifetimes when generated by nonexact kinetic-energy eigenstates.

Radiative lifetimes, collisional mixing, and quenching of the cesium 5DJ levels.

We report the results of a series of pulsed and cw laser experiments which measure spontaneous and effective lifetimes of the Cs(5DJ) levels and investigate excitation-transfer collisions involving Cs(5DJ) atoms and ground-state-cesium perturbers. With cw excitation of the dipole-forbidden but electric-quadrupole-allowed 6S1/2\ensuremath{\rightarrow}5D5/2 transition, we monitor the ratio of sensitized to direct fluorescence, i.e., I5D3/2\ensuremath{\rightarrow}6P1/2/I5D5/2\ensuremath{\rightarrow}6P3/2. A rate-equation analysis of these data yields values for the Cs(5D5/2)+Cs(6S)\ensuremath{\rightarrow}Cs(5D3/2)+Cs(6S) excitation-transfer rate, and for the rate of quenching of Cs(5D) by ground-state perturbers. The role of out-of-multiplet quenching is discussed at length, and we have demonstrated that quenching by ground-state atoms dominates over that by cesium dimers under these conditions. In the pulsed-laser experiments, the temporal evolution of the 5DJ\ensuremath{\rightarrow}6S and cascade 6PJ\ensuremath{'}\ensuremath{\rightarrow}6S fluorescence was observed following either direct forbidden-line pumping of one fine-structure level, or indirect excitation of both fine-structure levels (i.e., molecular excitation followed by predissociation). Analysis of the buildup and decay rates of the various levels as a function of cesium density yields values for the natural lifetimes of the Cs(5DJ) levels, as well as for the excitation transfer and quenching rates. Best values for the lifetimes and cross sections for Cs-Cs collisions obtained from these combined experiments are \ensuremath{\tau}5D=1250\ifmmode\pm\else\textpm\fi{}115 ns, \ensuremath{\sigma}5/2\ensuremath{\rightarrow}3/2=36\ifmmode\pm\else\textpm\fi{}8 \AA{}2, and \ensuremath{\sigma}5D=30\ifmmode\pm\else\textpm\fi{}3 \AA{}2. In addition, numbers for the 6P level-quenching cross sections due to collisions with Cs atoms and Cs2 molecules, respectively, were obtained: \ensuremath{\sigma}6P(Cs)=6.6\ifmmode\pm\else\textpm\fi{}3.0 \AA{}2 and \ensuremath{\sigma}6P(Cs2)=863\ifmmode\pm\else\textpm\fi{}260 \AA{}2. In the discussion, we show how these new values can be used to reconcile the seemingly discrepant results of several previous studies.

Micro-cavity semiconductor lasers with controlled spontaneous emission

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’.