Coherence Lifetime Research Articles

Comparison of the biexciton and exciton coherent polarization lifetimes in ZnSe/(Zn,Cd)Se multiple quantum wells

We report the relative coherence lifetimes for the biexciton polarization and the exciton polarization following ultrafast resonant excitation in ZnSe/(Zn,Cd)Se multiple quantum wells. The excitonic and biexcitonic nature of the observed third-order polarization is demonstrated by selection rules and comparison with a theoretical treatment. We show the polarization coherence lifetimes to be related by a factor of two and discuss the origin of this relationship.

Coherent control of high- n Rydberg lifetimes using wavepacket technology

We describe a simple, experimentally feasible scheme for enhancing lifetimes of high- n Rydberg states. We propose a new control scheme, based upon a ‘wavepacket technology’ which uses time-dependent, crossed electric and magnetic fields. Model (hydrogenic) calculations illustrating the control over l and m quantum numbers are presented and experimental parameters are suggested. The effects of core multipole moments on the decay of the target state (high l, high m) are discussed. Coherent lifetime enhancement should be applicable to small signal zero-kinetic-energy (ZEKE) photoelectron spectroscopy experiments and to Rydberg photofragment translational energy spectroscopy.

Temperature dependence of phase breaking in ballistic quantum dots.

Ballistic conductance fluctuations are measured in a zero-dimensional (0D) GaAs quantum dot and are used to measure the phase coherence lifetime ${\mathrm{\ensuremath{\tau}}}_{\mathit{c}\mathit{p}\mathit{h}\mathit{i}}$(T) based on a semiclassical ``extra lead'' model of phase breaking. Above 300 mK, the measured ${\mathrm{\ensuremath{\tau}}}_{\mathit{c}\mathit{p}\mathit{h}\mathit{i}}$ decreases as a power law, ${\mathrm{\ensuremath{\tau}}}_{\mathit{c}\mathit{p}\mathit{h}\mathit{i}}$(T)=a(T [K]${)}^{\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\zeta}}}$, with \ensuremath{\zeta}\ensuremath{\sim}1.2 and a\ensuremath{\sim}3.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ s, and appears to saturate below this temperature. Variance of fluctuations increases from zero with dot conductance for few conducting channels in the leads, then saturates once several channels are open. Above 150 mK, variance decreases with temperature as a power law with an exponent between -1.2 and -1.6 depending on 〈g〉.

Primary state diffusion

Primary quantum state diffusion (PSD) theory is an alternative quantum theory from which classical dynamics, quantum dynamics and localization dynamics are derived. It is based on four principles, that a system is represented by an operator, its state by a normalized state vector, the state vector satisfies a Langevin-Itô state diffusion equation, and the resultant density operator for an ensemble must satisfy an equation of elementary Lindblad form. There are three conditions. The ז 0 first determines the operator, to within an undetermined universal time constant ז 0 . The second and third conditions put opposing bounds on ז 0 . Dissipation of coherence is distinguished from destruction of coherence. The state diffusion destroys coherence and produces the localization or reduction that makes classical dynamics possible. PSD theory is a development of the environmental quantum state diffusion theory of Gisin and Percival and particularly resembles earlier proposals by Gisin and by Milburn. It is also related to the spontaneous localization theories of Ghirardi, Rimini and Weber, of Diósi and of Pearle. The non-relativistic PSD theory is of value only for systems which occupy small regions of space. Special relativity is needed for more extended systems even when they contain only slowly moving massive particles. Experiments on coherence lifetimes and matter interferometry are proposed which either measure ז 0 or put bounds on it, and which might distinguish between PSD and ordinary quantum mechanics.

Dynamics of an erbium-doped fiber laser.

The temporal dynamics of an erbium-doped fiber laser shows various interesting modes of behavior. In the stationary regime, a behavior going from a cw working mode to deterministic chaos through different self-pulsed working modes has been observed. In the transient regime, effects such as antiphase phenomena, between two orthogonal states of polarization, can be observed. To describe the dynamics of the fiber laser we have studied a theoretical model based on two coupled lasers coherently pumped. A linear stability analysis is given which demonstrates the existence of a Hopf bifurcation for low values of the pump parameter. Numerical calculation results allow a good description of the experimental results. The value of the newly introduced coherence lifetime parameter is discussed.

Pair excitations in solids.

The pair-excitation probability in the weak-field regime is calculated using a fully quantized interaction Hamiltonian, for both the rotating- and fixed-atom cases. The peak of the absorption occurs at the sum of the final-state energies of the two atoms and the transition probability is shown to be proportional to the inverse square of the laser detuning and to the inverse cube of the nearest-neighbor distance. In addition, retardation effects result in a system size dependence for the transition rate not previously predicted. The significance of retardation effects is shown to depend on the excited-state coherence lifetime, the distance of closest approach, and the wavelength of the transition.

Dephasing-induced damping of superradiant emission in J-aggregates

Results of temperature-dependent accumulated photon echo, fluorescence lifetime and fluorescence quantum yield measurements are reported on aggregates of the dye pseudoisocyanine bromide in an ethylene glycol/water glass. The results show that the lowest excited state of the J-aggregate can be described as a superradiative exciton, whose radiative and optical coherence lifetime are determined by dephasing processes. The results further indicate that the phonon-induced fluctuations of the site energies must be highly correlated. The observed discrepancy between the low-temperature photon echo decay time of 16 ps and fluorescence lifetime of 70 ps is ascribed to observation of dephasing of an excitonic wave packet in the echo and incoherent relaxation of the same k-states in the fluorescence.

Quantum theory of the full pressure dependence of collision induced intersystem crossing

The quantum theory of the pressure dependence of collision induced intersystem crossing is developed in the impact limit where collision times are short compared to time scales for molecular decay. A basic model is introduced which contains singlet–triplet intramolecular couplings and collisional processes that correspond in a pure spin basis to dephasing of singlet–triplet and triplet–triplet coherences and to triplet state population decay. Expressions are derived for the ratio of the emission yield at zero pressure divided by that at pressure p in the separate limits of low triplet level density and of short triplet–triplet coherence lifetimes. The first order corrections to the latter case are found to be negligible over a wide range of pressure and molecular parameters. The theory shows that pressure saturation of collision induced intersystem crossing emerges from a competition between dephasing and population decay processes induced by the collider. A strong sensitivity is exhibited to the minimum zeroth order singlet–triplet splitting.

Resonant wave mixing inI2with nonsynchronous pulses

A technique of third-harmonic generation (THG) with nonsynchronous pulses is applied to study the dynamics of resonant wave mixing. A circularly polarized laser pulse is focused into a cell containing ${\mathrm{I}}_{2}$ vapor to create the coherent dipole moments. An oppositely circularly polarized pulse is then focused into the same region to generate the third harmonic. Since no THG is produced by any individual pulse, correlated THG can be easily measured without using any fast detection scheme. A theory is presented to explain the measured coherent lifetime of approximately 1 ns. The technique of wave mixing with two linearly polarized pulse is applied to study saturation in resonant THG and to measure the homogeneous relaxation rate of the molecular states. This technique is also applied to study the photon echo generating process.

A simple model for the difference between coherence time and radiative lifetime in NO 2

An intramolecular mechanism is proposed to reconcile the different lifetimes obtained from direct radiative decay measurements and from Hanle effect measurements on NO 2. This mechanism predicts a dependence of the coherence lifetime on the excited state rotational quantum number which is in good agreement with the previous experiments.

Anomalous magnetic depolarization of fluorescence from the NO22B2 state

Hanle-effect studies were made of 13 identified lines belonging to the 5933-, 6117-, 6125-, and 6126-Å bands of the NO 2 2 B 2 state. The fluorescence was excited using a tunable cw dye laser operated single mode, using NO 2 pressures as low as 0.2 mTorr. From the variation of the fluorescence intensity with magnetic field the product gτ C is deduced, where g and τ C are the excited state g value and coherence lifetime, respectively. Using previously determined radiative lifetimes, g is obtained and compared to the values of g expected from Hund's case ( b) coupling. The observed g values do not display the expected N′ dependence and are about an order of magnitude smaller than predicted. An alternative explanation is that the g values are behaved, but the coherence lifetime is shorter than the radiative lifetime, caused by radiationless dephasing processes which may be collisional or intrinsic.

Lifetime-limited photochemical hole-burning in s-tetrazine-benzene mixed crystals at 2 K

We show that the technique of photochemical hole-burning can be utilized to measure sub-nanosecond fluorescence lifetimes of molecular excited states. s-tetrazine is taken as an example and from the width of the hole (0.7 ± 0.1 GHz), burned in the origin of the lowest 1B 3u ← 1A 1g transition at 2 K in a benzene mixed crystal, a fluorescnece lifetime of 455 +75 −55 ps is obtained. The coherence lifetime of the 16 a 2 0 vibronic transition is found to be 25 ps.

Photon-Echo Spectroscopy of Organic Mixed Crystals

We report the first observation of photon echoes in an organic mixed crystal. The echo was observed after excitation with a nitrogen-laser-pumped dye laser in the $^{1}B_{2\mathrm{u}}\ensuremath{\leftarrow}^{1}A_{g}$ electronic transition at 3739.9 \AA{} of pyrene in biphenyl at 2 K. The photon-echo excitation spectrum of the origin (zero-phonon line) region shows that drastic variations occur in the coherence lifetime of sites which, qua incoherent (fluorescent) decay, are indistinguishable.