Spontaneous Emission Decay Rate Research Articles

Frequency up-conversion in Lasing Without Inversion

AbstractIt is well known that in a three-level lambda system, spontaneous emission decay rate on the lasing transition must be smaller than or equal to the decay rate on the driving transition in order to observe LWI. Since spontaneous emission rate is proportional to cube of the corresponding transition frequency, satisfying the above condition requires the frequency of lasing transition to be smaller than the driving transitions frequency. In other words, according to this condition, frequency upconversion is not possible with LWI. The main theme of this article is to suggest the possibilty of frequency upconversion in LWI by overcoming the condition described above.

Efficient Coupling of a Single Diamond Color Center to Propagating Plasmonic Gap Modes

We report on coupling of a single nitrogen-vacancy (NV) center in a nanodiamond to the propagating gap mode of two parallel placed chemically grown silver nanowires. The coupled NV-center nanowire system is made by manipulating nanodiamonds and nanowires with the tip of an atomic force microscope cantilever. An efficient coupling of an NV-center to an easily accessible gap plasmon mode is demonstrated and we measure an enhancement of the spontaneous emission decay rate by a factor of 8.3.

The influence of chiral spherical particles on the radiation of optically active molecules

Within the framework of nonrelativistic quantum electrodynamics, a theory of spontaneous emission of a chiral molecule located near a chiral (bi-isotropic) spherical particle is developed. It is shown that the structure of photons in the presence of chiral spherical particles differs significantly from the structure of usual transverse electric and transverse magnetic photons. Exact analytical expressions for a spontaneous emission decay rate of a chiral molecule with arbitrary electric and magnetic dipole moments of transition located near a chiral spherical particle with arbitrary parameters, are obtained and analyzed in detail. Simple asymptotes for the case of a nanoparticle are obtained. Substantial influence of even small chirality on a sphere made from dielectric or double negative metamaterial is found. It is shown that by using chiral spherical particles it is possible to effectively control the radiation of a given enantiomer of optically active molecules.

Strongly modified spontaneous emission decay rate of silicon nanocrystals near semicontinuous gold films

We investigate the spontaneous emission decay rate of Si nanocrystals modified by thin semicontinuous gold films. It has been shown that the mean and standard deviation values of decay rate distribution obtained from the photo-emission decay curve analysis increase due to the deposition of semicontinuous gold films. These values are dependent on gold film thickness and emission wavelength. The observed results are well explained using a point-dipole decay rate model considering the effective dielectric functions of the gold films that exhibit peculiar structures in the localized surface plasmon resonance region.

Improving efficiency roll-off in phosphorescent OLEDs by modifying the exciton lifetime

Studies on phosphorescent organic light emitting devices (PhOLEDs) with phosphorescent emitter, fac-tris (2-phenylpyridine) iridium (Ir(ppy)(3)), show that the lifetime of triplet exciton is modified by surface plasmon coupling of Au nanoparticles (NPs). Interactions between the triplet exciton and gold (Au) nanoparticles (NPs) lead to a decrease in the exciton lifetime and result in the spontaneous emission decay rate of triplet exciton faster as the distance between the phosphorescent material and the Au NPs becomes smaller. This interaction reduces the efficiency roll-off of Au NPs containing device. These results provide new guides for device design to improve efficiency performance.

Constructive and destructive quantum interference sensitive to quantum vacuum mode structure in a metallic waveguide

Quantum vacuum mode structure can be changed due to length scale fluctuation of the cross section of a metallic waveguide. Such a structure change in vacuum modes (particularly in cutoff vacuum modes) would lead to dramatic enhancement or inhibition of spontaneous emission decay of atoms and, if the waveguide is filled with a dilute atomic vapor consisting of quantum-coherent atoms of a four-level tripod-configuration system, an optical wave propagating inside the waveguide can be coherently manipulated by tunable constructive and destructive quantum interference between two control transitions (driven by two control fields) in a quite unusual way (e.g., the optical response, in which a three-level dark state is involved, is sensitive to the waveguide dimension variations at certain positions of resonance of the atomic spontaneous emission decay rate). Therefore, an intriguing effect that can be employed to designs of new photonic and quantum optical devices could be achieved based on the present mechanisms of quantum-vacuum manipulation and quantum coherence control.

Modification of Exciton Lifetime by the Metal Cathode in Phosphorescent OLEDs, and Implications on Device Efficiency and Efficiency Roll‐off Behavior

AbstractTime resolved photoluminescence and electroluminescence measurements are used to study changes in the emission characteristics of materials typically used in phosphorescent organic light emitting devices (PhOLEDs). Studies on archetypical PhOLEDs with phosphorescent material, fac‐tris(2‐phenylpyridine) iridium (Ir(ppy)3), show that the lifetime of triplet exciton is modified when in close proximity to a metal layer. Interactions with a metal layer ∼30–100 nm away, as is typically the case in PhOLEDs, result in an increase in the spontaneous emission decay rate of triplet excitons, and causes the exciton lifetime to become shorter as the distance between the phosphorescent material and the metal becomes smaller. The phenomenon, possibly the result of the confined radiation field by the metal, affects device efficiency and efficiency roll‐off behavior. The results shed the light on phenomena affecting the efficiency behavior of PhOLEDs, and provide new insights for device design that can help enhance efficiency performance.

Large quantum dots with small oscillator strength

We have measured the oscillator strength and quantum efficiency of excitons confined in large InGaAs quantum dots by recording the spontaneous emission decay rate while systematically varying the distance between the quantum dots and a semiconductor-air interface. The size of the quantum dots is measured by in-plane transmission electron microscopy and we find average in-plane diameters of 40 nm. We have calculated the oscillator strength of excitons of that size assuming a quantum-dot confinement given by a parabolic in-plane potential and a hard-wall vertical potential and predict a very large oscillator strength due to Coulomb effects. This is in stark contrast to the measured oscillator strength, which turns out to be so small that it can be described by excitons in the strong confinement regime. We attribute these findings to exciton localization in local potential minima arising from alloy intermixing inside the quantum dots.

Influence of water on non-radiative decay of Yb 3+– 2F 5/2 level in phosphate glass

The total decay rate and hydroxyl concentration of glass compositions of 60PO 5/2–15AlO 3/2–5BO 3/2–15KO 1/2–5NbO 5/2– xYbO 3/2 ( x = 0.1, 0.2, 0.5, 1, 2, 5) with very low hydroxyl content were measured. The concentration of hydroxyl groups ranged from 2.4 × 10 19 ions/cm 3 to 2.9 × 10 20 ions/cm 3. A linear relationship was obtained between the total decay rate and the product of hydroxyl concentration and concentration of Yb 3+ ions. Thus, it was estimated that the interaction constant of the probability of hydroxyl quenching is 8.0 × 10 −52 m 6/s. We compared the interaction constants of Yb 3+ in phosphate glasses. These were different but did not change in order of magnitude. The quantum efficiency can be estimated using the value for the known hydroxyl content. In our glass of x = 2, if the hydroxyl concentration is below 2.4 × 10 19 ions/cm 3, the probability of hydroxyl quenching can be ignored. The total decay rate was found to be consistent with the spontaneous emission decay rate.

Plasmon Nanoparticle Array Waveguides for Single Photon and Single Plasmon Sources

This Letter discusses how linear coupled plasmon particle arrays inspired by radio frequency Yagi-Uda antennas can be used to construct both efficient unidirectional single photon sources and efficient directional single plasmon sources. Calculations using an exact multipole expansion method are presented of the spontaneous emission directivity, efficiency, and spontaneous emission decay rates, taking into account material loss in real noble metals. An analysis of the emission properties in terms of the dispersion relation of infinite arrays reveals how one can use guided mode dispersion to achieve desirable figures of merit. The key ingredient is to couple the source to array eigenmodes that are just beyond the light line but still wave vector matched to propagating modes to within the momentum uncertainty set by the inverse antenna length. Finally, this Letter shows that the emission decay rates can be controlled independently of the directionality and without penalty in quantum efficiency.

Molecular architectures based on π-conjugated block copolymers for global quantum computation

We propose a molecular setup for the physical implementation of a barrier global quantum computation scheme based on the electron-doped π-conjugated copolymer architecture of nine blocks PPP-PDA-PPP-PA-(CCH-acene)-PA-PPP-PDA-PPP (where each block is an oligomer). The physical carriers of information are electrons coupled through the Coulomb interaction, and the building block of the computing architecture is composed by three adjacent qubit systems in a quasi-linear arrangement, each of them allowing qubit storage, but with the central qubit exhibiting a third accessible state of electronic energy far away from that of the qubits' transition energy. The third state is reached from one of the computational states by means of an on-resonance coherent laser field, and acts as a barrier mechanism for the direct control of qubit entanglement. Initial estimations of the spontaneous emission decay rates associated to the energy level structure allow us to compute a damping rate of order 10-7 s, which suggest a not so strong coupling to the environment. Our results offer an all-optical, scalable, proposal for global quantum computing based on semiconducting π-conjugated polymers.

Controlling molecular orientation through radiative rotational transitions in strong static electric fields

The effects of a static, homogeneous, and strong electric field on the radiative and steric properties of the LiCs molecule in its $^{1}\ensuremath{\Sigma}^{+}$ electronic ground state are investigated. Combining discretization and basis-set methods, the rovibrational Schr\odinger equation is solved and dipole transition rates are calculated. Spontaneous emission decay rates and radiative lifetimes of rovibrationally excited states have been studied extensively and particularly in the presence of the external homogeneous electric field. The intriguing possibility to control the alignment and orientation by applying a sufficiently strong field while switching between different configurations via absorption and emission processes is demonstrated.

Decay rate and level shift in a circular dielectric waveguide

Using a Green tensor formalism, we perform a comprehensive investigation into the spontaneous emission decay rate and energy-level shift in a circular dielectric waveguide. The two-dimensional (2D) Green tensor for an embedded line source and 3D Green tensor for a point source are obtained by a combination of Fourier integral and multipole methods. We fully characterize resonant states, which include whispering gallery modes, and examine the associated cavitylike effects. The quality factor of resonant states increases with frequency, resulting in a strongly enhanced decay rate and a large anomalous level shift in both the 2D and 3D cases.

Spontaneous Emission of Broadened Molecules on the Surface of a Microdroplet

The spontaneous emission of a broadened molecule on the surface of a microsphere is studied. Through detailed analysis, we show that only weak coupling between a broadened atom (molecule) and the electromagnetic fields exists in a spherical dielectric cavity whether the sphere is small or large. From these results, we find an explicit expression for the spontaneous emission decay rate for a broadened surfactant molecule on the surface of a microdroplet with radius a, in which only 1/a and 1/a 2 components are exhibited. Then we apply this expression to a real experiment and obtain a consistent result with the experiment.

Atom-field interactions with a frequency-dependent reservoir

Owing to the frequency dependence of the vacuum density of states, the spontaneous emission decay rate varies as the cube of a frequency. The actual frequency that enters into the decay rate depends to some extent on the physical system under investigation. We show that, in the weak-field limit and the rotating-wave approximation, the decay parameters associated with Ralyleigh scattering are evaluated at the field frequency, while the transient response following the turn-on of the field has a decay rate evaluated at the atomic frequency. The optical Bloch equations are modified to include additional coupling between populations and coherences, both in bare and dressed-state pictures.

Spectral tuning of the coupling between isolated InAs quantum dots and the fundamental micropost cavity mode

The emission from two InAs quantum dots is coupled to the fundamental cavity mode of a micropost cavity. The emission from each quantum dot is spectrally separated by approximately 2 nm. By adjusting the sample temperature the single exciton emission from each quantum dot is spectrally tuned in and out of resonance with the cavity mode. The ratio of the spontaneous emission decay rate at on and off resonance conditions is nearly 4 for both quantum dots.

Large spontaneous emission enhancement in InAs quantum dots coupled to microdisk whispering gallery modes

AbstractMeasuring the enhancement of spontaneous emission decay rates of quantum dot emission coupled to microcavity modes is typically hampered by variable coupling of the quantum dot emission. This is particularly evident in the microdisk cavity since the whispering gallery modes are localized near the disk edge, while quantum dot emitters are typically uniformily distributed throughout the disk. The distribution of spontaneous emission decay rates under these circumstances can be determined using a distribution function for the various spontaneous decay rates, and demonstrate that large decay rate enhancement are present. To remove the spatial coupling variation, quantum dots are selectively placed near the microdisk edge. Initial photoluminescence measurements indicate that recombination processes in these quantum dots are not dominated by surface recombination.

Tuning the single optical mode spontaneous emission coupling of a quantum dot in a micropost cavity

Using a micropost cavity, the emission from a quantum dot (QD) is coupled to the discrete fundamental optical mode of the cavity. By adjusting the temperature of the device between 4 and 70 K, the coupling between the QD and the cavity can be tuned. The ratio of the spontaneous emission decay rate between on and off resonance is approximately 4.

Control of atomic and molecular motion via the Stark effect in Rydberg states

A novel theoretical study of the control of translational motion of atomic and molecular Rydberg states in inhomogeneous static electric fields is presented. Simulations have been carried out demonstrating that, under realistic conditions, the deflection and focusing of Rydberg atoms and molecules should be achievable. Advantage is taken of the high susceptibility of the Rydberg states to external electric fields, allowing the use of much smaller fields than would be necessary for ground state neutrals. The simulations presented are for trajectories of Rydberg states with n = 18-20 in the fields of an electric dipole, quadrupole and hexapole. A deflection of 7 mm is predicted for n = 18 Rydberg states travelling parallel to the dipole after 100 µs time of flight. In the hexapole n = 20 Rydberg states are refocused to a spot size of the order of the laser beam waist (10 µm) after 20 µs. It is demonstrated that the hexapole can also act as a cylindrical lens if its axis is perpendicular to the Rydberg beam direction. Spontaneous emission and black-body decay rates are also calculated, and their variation with the applied field is investigated. The potential applications of this work might include the use of focusing and deflection for controlled low-energy collisions of Rydberg molecules with surfaces.

Modification of the spontaneous emission rate of Eu^{3+} ions embedded within a dielectric layer above a silver mirror

We investigate the spontaneous emission rate of ${\mathrm{Eu}}^{3+}$ ions in an asymmetric microcavity. The cavity consists of a thin dielectric layer bounded on one side by silver and on the other by air. Samples were constructed using the Langmuir-Blodgett technique, the emitters being incorporated as a single monolayer within the microcavity. This technique allowed both the cavity thickness and the position of the emitters within it to be controlled to one monolayer (\ensuremath{\approx}2.6 nm). Measurements are presented of the spontaneous emission decay rate as a function of both the dielectric layer thickness and the position of the emitters within the layer. We find good agreement between our data and a classical model. The model allows us to investigate the role of the electromagnetic modes of the microcavity in controlling the spontaneous emission decay rate of the emitters. In particular, we are able to evaluate the contribution each of the modes makes to the decay. We discuss the implication these results may have in increasing the photoluminescence efficiency from microcavity based emissive devices.