Spontaneous Emission Lifetime Research Articles

Enhancement of spontaneous emission from the resonant modes of a photonic crystal slab single-defect cavity.

Modification of the spontaneous-emission lifetime in photonic crystal single-defect resonant modes is studied with the finite-difference time domain method. We investigate spontaneous-emission enhancement from the monopole and the dipole modes of a hexagonal lattice cavity, considering the effects of the finite emitter linewidth and spectral detuning. Large spontaneous-emission enhancement of > 50 is achieved numerically from the high-quality-factor monopole mode when the emitter linewidth is comparable with the resonant-mode linewidth. However, if broad-linewidth material is used and a detuning effect is included, the dipole mode with a low quality factor and a smaller mode volume could be more advantageous for spontaneous-emission enhancement.

Microcavity effects in the high modulation response of thevertical cavity surface emitting laser

Based on physics of micro-cavity and quantum well, the spontaneous emission spectra and spontaneous emission lifetime in the vertical cavity surface emitting laser have been calculated. It is found that the minimum spontaneous emission lifetime is the main reason that the vertical cavity surface emitting laser can realize high\|modulation response after the transfer function of the semiconductor laser is studied.

Proposal to Use GaAs(114) Substrates for Improvement of the Optical Transition Probability in Nitride Semiconductor Quantum Wells

ABSTRACTThe dependence of the spontaneous emission lifetime of excitons in InGaN/GaN quantum disks (QDs) on the crystalline orientation is calculated. For 1-nm-thick QDs, it is found that the lifetime in the conventional c-oriented QDs is ten times as long as that in QDs tilted by 30° and 90°, and that the difference is pronounced by increasing the QDs thickness. This is totally due to the presence of the electric field in strained InGaN. Taking into account our preceding study, in which it was revealed that GaN on GaAs(114) was titled by 30°, we propose the use of GaAs(114) as a substrate for nitride light emitting devices to improve the optical transition probability.

Modification of spontaneous emission from CdSe/CdS quantum dots in the presence of a semiconductor interface

The spontaneous-emission lifetime of CdSe/CdS core-shell quantum dots was studied as a function of the distance between the dots and a polished Si surface. The experimental results reveal a significant modification of the spontaneous-emission rate of the quantum dots by the Si surface.

Molecular fluorescence above metallic gratings

We present measurements of the fluorescence of emitters located in close proximity $(d<\ensuremath{\lambda})$ to metallic grating surfaces. By measuring both the spontaneous emission lifetime and angle-dependent radiation pattern of a monolayer of dye molecules as a function of their separation from planar and periodically corrugated mirrors of increasing modulation depth, we are able to examine the effect of varying the surface profile on the emission process. Both the distance dependence of the lifetime and the spatial distribution of the emitted light are significantly changed upon the introduction of a corrugation, quite apart from the appearance of the familiar Bragg-scattered bound-mode features. It is postulated that these perturbations arise from the interference of the grating scattered dipole fields with the usual upward propagating and reflected fields. In addition, the measurement of nonexponential decay transients for the deepest gratings examined provide evidence for the existence of optically dissimilar dipole positions above the grating surface.

Spectroscopy of Pr 3+ ions in lithium borate and lithium fluoroborate glasses

Optical absorption and emission spectra of 1 mol% Pr 3+ doped lithium borate and lithium fluoroborate glasses have been recorded. The intensities of f–f transitions are parameterized in terms of Judd–Ofelt (JO) intensity parameters Ω λ ( λ=2, 4 and 6). The JO parameters obtained with modified JO theory have been used to predict radiative properties such as spontaneous emission probabilities ( A), lifetimes ( τ R) and branching ratios ( β R) for all the 12 excited states of these Pr 3+ doped glasses along with some of the reported Pr 3+ : systems. The predicted β R are compared with the experimental values for the emission from 3P 0 and 3P 1 levels. The stimulated emission cross-sections are also evaluated for all the observed emission transitions. The β R values for the potential laser transitions including the 1G 4→ 3H 5 (1.3 μm) transition useful for fibre amplifier have been compared for 17 Pr 3+ : systems. The effect of compositional changes of the glasses on the optical properties of Pr 3+ ions have been discussed.

Single-mode spontaneous emission from a single quantum dot in a three-dimensional microcavity.

The spontaneous emission from an isolated semiconductor quantum dot state has been coupled with high efficiency to a single, polarization-degenerate cavity mode. The InAs quantum dot is epitaxially formed and embedded in a planar epitaxial microcavity, which is processed into a post of submicron diameter. The single quantum dot spontaneous emission lifetime is reduced from the noncavity value of 1.3 ns to 280 ps, resulting in a single-mode spontaneous emission coupling efficiency of 78%.

Dynamic analysis of multiple wavelength DFB fiber lasers

The behavior of multi- and single-wavelength distributed feedback fiber lasers is simulated in a comprehensive dynamic model. The evolution of the spatial distribution of gain, saturation-induced gain gratings, and spontaneous emission is taken into account. Stability and relative intensity noise (RIN) of the different laser types and laser modes are compared, and the effect of varying degree of pump RIN and quenching of spontaneous emission lifetimes is analyzed.

Modified spontaneous emission from a two-dimensional photonic bandgap crystal slab

A two-dimensional photonic crystal patterned into a thin dielectric slab waveguide is shown to alter drastically the lifetime of spontaneous emission as well as the radiation pattern. This means that although the light extraction efficiency can be greatly enhanced, inhibited spontaneous emission within the photonic bandgap can result in low power output from such a structure. Strongly inhibited emission is found within the photonic bandgap as well as enhanced emission into the conduction band modes for certain geometries. Coupled with enhanced extraction efficiency in the photonic conduction band, this results in the possibility of a structure with increased total power efficiency.

Optical spectroscopy of Eu 3+ ions in lithium borate and lithium fluoroborate glasses

Optical absorption, excitation and emission spectra of Eu 3+ ions in lithium borate and lithium fluoroborate glasses have been investigated. Judd–Ofelt intensity parameters are derived from the integrated absorption spectra and also from the emission spectra under various constraints. The relative merits of thermal correction to the oscillator strengths of the transitions originating from the ground state ( 7 F 0 ) of Eu 3+ ion observed in the absorption spectra have been discussed. The Judd–Ofelt parameters obtained from the emission spectra have been used to calculate the total spontaneous emission probabilities ( A), lifetimes ( τ R) and branching ratios ( β R) for eight excited levels that include, 5 K 6, 3 P 0 , 5 H 3 , 5 L 7 , 5 D 3 , 5 D 2 , 5 D 1 and 5 D 0 . The predicted values of τ R and β R are compared with the measured values for 5 D 0 level. The stimulated emission cross-sections ( σ ( λ p)) are also evaluated for the 5 D 0→ 7 F J and 5 D 1→ 7 F J transitions. The variation of optical properties with compositional changes of lithium oxide and lithium fluoride contents in the glasses are discussed and compared with similar results.

AC Operation of GaN:Er Thin Film Electroluminescent Display Devices

ABSTRACTThin-film electroluminescence has been obtained from GaN:Er deposited directly on amorphous dielectric layers. Electroluminescent device (ELD) structures consisting of a dielectric/GaN/dielectric were formed on p+-Si substrates. In contrast to previous GaN:Er ELDs which used epitaxial growth conditions on crystalline substrates and were operated under DC bias, these ELDs were operated under AC bias. A maximum luminance value of 300, 60, and 15 cd/m2 has been achieved from GaN:Er and AlGaN:Er AC-ELDs biased at 180 V and 100, 10, and 1 kHz, respectively. The emission spectra, which originate from Er3+ 4f-4f transitions, consist of dominant visible emission at ∼537/558 nm and infra-red (IR) emission at 1.5 μm. A violet emission peak at 415 nm indicates that hot carriers can gain up to ∼3 eV energy for an applied voltage corresponding to 1.5 MV/cm applied field. The emitted intensity initially increases linearly with frequency, followed by a trend towards saturation. The frequency for 3 dB reduction from the linear relation is at ∼65 kHz for visible emission and ∼8 kHz for infrared emission. The saturation trends can be explained in terms of the spontaneous emission lifetimes of the visible (∼10 μs) and IR (∼1ms) Er3+ emissions.

Effect of indium doping on the transient optical properties of GaN films

We have investigated the effects of In doping on the optical properties of GaN films grown by gas-source molecular-beam epitaxy. Time-resolved photoluminescence was carried out to study the transient optical properties of the epitaxial films. In comparison to the undoped GaN film, the spontaneous emission lifetime was prolonged from below 20 to 70 ps by doping with In. Under high-excitation density, stimulated emission was observed from both samples. The threshold excitation density was found to be reduced in the In-doped sample. These significant improvements of the optical properties are attributed to the effective suppression of the formation of the nonradiative recombination centers caused by a change of the growth kinetics induced by a small amount of In supplied during growth of the GaN films.

Spontaneous emission alteration in InGaAs/GaAs vertical cavity surface emitting laser (VCSEL) structures

The spontaneous emission from vertical-cavity surface-emitting laser (VCSEL) structures consisting of an In0.2Ga0.8As/GaAs quantum well and AlAs/GaAs distributed Bragg reflectors (DBRs) was systematically investigated as a function of reflectivity of DBRs, energy difference between the excitonic energy and the cavity mode and emission wavelength. Significant lifetime alteration, which was much more than that of the previous experimental results and the theoretical estimation based on the atomic dipole model, was observed when the reflectivity was changed. This was attributed to the cooperation between the microcavity effect and the carrier-carrier scattering effect which is not considered in the atomic dipole model. The spontaneous emission lifetime of the VCSEL structure whose gain peak is not equal to the cavity mode was found to be changed depending on the energy difference between the excitonic energy and the cavity mode and the lifetime was the shortest when the excitonic energy was equal to the cavity mode. It was also observed that the spontaneous emission lifetime depends on the emission wavelength and was the shortest at the cavity mode, the reason for which has not been fully clarified yet.

Microscopic theory of optical nonlinearities and spontaneous emission lifetime in group-III nitride quantum wells

Microscopic calculations of the absorption/gain and luminescence spectra are presented for wide bandgap Ga{sub 1{minus}x}In{sub x}N/GaN quantum well systems. Whereas structures with narrow well widths exhibit the usual excitation dependent bleaching of the exciton resonance without shifting spectral position, a significant blue shift of the exciton peak is obtained for wider quantum wells. This blue shift, which is also present in the excitation dependent luminescence spectra, is attributed to the interplay between the screening of a strain induced piezoelectric field and the density dependence of many-body Coulomb effects. The calculations also show an over two orders of magnitude increase in the spontaneous electron-hole-pair lifetime with well width: due to the reduction of the electron-hole wavefunction overlap in the wider wells. The resulting decrease in spontaneous emission loss is predicted to lead to improved threshold properties in wide quantum well lasers.

Spontaneous emission lifetime of carriers in a semiconductor microcavity measured by photoluminescence without distortion by reabsorption

A theory for carrier decay rates and a technique for measuring them are reported. Modification of the spontaneous emission rate of carriers by a semiconductor microcavity is investigated with 100-nm-thick bulk GaAs. Reabsorption makes the cavity-mode photoluminescence (PL) decay much faster than the square of the carrier density. Here reabsorption distortion is avoided by collecting PL that escapes the microcavity directly without multiple reflections: a ZnSe prism glued to the top mirror allows PL to escape at angles beyond the cutoff angle for total internal reflection without the prism. At these steep angles, the stop band of the top mirror has shifted to higher energy, so that it does not impede PL emission. Removal of most of the bottom mirror decreases the true carrier decay rate by only 25%, showing that the large enhancements deduced from cavity-mode PL are incorrect. Fully quantum mechanical computation including guided modes corroborates this conclusion. The prism technique could be used to study carrier dynamics and competition between guided and cavity modes in microcavities below and near threshold.

Modification of spontaneous emission lifetimes in the presence of corrugated metallic surfaces

We report the results of measurements of the spontaneous emission lifetime of a ${\mathrm{Eu}}^{3+}$ complex in the presence of a nearby metal surface. The excited-state lifetime of the ions are measured as a function of their separation from both planar and periodically corrugated metallic surfaces. Several corrugated silver surfaces were used, each having a different combination of pitch and amplitude. No significant difference is found between data obtained for ions situated above a planar silver surface and those above corrugated surfaces that have pitches greater than the emission wavelength. However, for surfaces with a pitch less than the emission wavelength, differences are seen, and are shown to increase with increasing corrugation amplitude. Possible explanations for the observed differences are given, including a discussion of the role of surface plasmon polariton modes on corrugated metallic surfaces.

Finite-difference time-domain calculation of spontaneous emission lifetime in a microcavity

We developed a general numerical method to calculate the spontaneous emission lifetime in an arbitrary microcavity, using a finite-difference time-domain algorithm. For structures with rotational symmetry we also developed a more efficient but less general algorithm. To simulate an open radiation problem, we use absorbing boundaries to truncate the computational domain. The accuracy of this method is limited only by numerical error and finite reflection at the absorbing boundaries. We compare our result with cases that can be solved analytically and find excellent agreement. Finally, we apply the method to calculate the spontaneous emission lifetime in a slab waveguide and in a dielectric microdisk, respectively.

Exciton lifetimes due to spontaneous emission in free space

The concept of fractional dimensional space is used to study the spontaneous emission lifetimes of excitons in quantum confined systems. We focus on a parameter α, a measure of dimensionality of the confined system and which is related to the exciton coherence volume. An analytical expression for the exciton spontaneous emission lifetime is derived and is used to calculate free exciton lifetimes in GaAs/Al x Ga 1− x As quantum wells for known values of α. Our results confirm that a minimum in the exciton lifetime occurs at a critical size of the well width (quantum well) or lateral width (quantum wire).

Novel spontaneous emission control using 3-dimensional photonic bandgap crystal cavity

A novel surface-emitting-laser diode consisting of a 3-dimensional (3D) photonic-bandgap-crystal cavity (PBGC) is proposed. Spontaneous emission is controlled so as to radiate into the lasing direction with a narrow radiation angle by introducing a plane phase shift region into the crystal. We analyze the radiation mode density in the PBGC-cavity with a plane-wave method by using a 3D-diamond photonic crystal cavity and demonstrate the resonant property of PBGC-cavity. It is shown that the radiation angle of spontaneous emission is as narrow as that of the stimulated emission of conventional verticalcavity surface-emitting-laser (VC-SEL). We show also that the spontaneous emission lifetime is two to three orders of magnitude larger in comparison with that of conventional laser.

Spontaneous-emission-lifetime alteration inInxGa1−xAs/GaAs vertical-cavity surface-emitting laser structures

The spontaneous-emission lifetime of ${\mathrm{In}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{As}$/GaAs vertical-cavity surface-emitting laser structures was investigated at room temperature as a function of reflectivity of AlAs/GaAs distributed Bragg reflectors (DBR's). As the reflectivity of DBR's became higher, the spontaneous-emission lifetime measured at the resonance wavelength of the cavity became drastically shorter in the cavity axis direction. This spontaneous-emission-lifetime alteration was much more than that of the previous experimental results for quantum wells and the theoretical estimation for atomic dipoles in the planar microcavity structures.