Coherence Decay Time Research Articles

Determination of N2-N2 and N2-O2 S-branch Raman linewidths using time-resolved picosecond pure rotational coherent anti-Stokes Raman scattering.

N2-N2 and N2-O2 S-branch Raman linewidths have been determined using picosecond dual-broadband pure rotational coherent anti-Stokes Raman scattering (CARS). Time-resolved rotational CARS measurements were performed in gas-phase N2 and air for temperatures up to 1900 K in order to determine the time constants of the coherence decay to subsequently calculate the S-branch Raman linewidths. Coherence decay time traces and the resulting S-branch Raman linewidths are presented for N2-N2 and N2-O2 collisions. Therewith, we reduce the gap of widely missing S-branch linewidth data in the temperature regime of many combustion processes. Further, we demonstrate that the standard monoexponential fitting of the coherence decay, as it is commonly done for nitrogen, is not applicable to oxygen.

Spinning faster: protein NMR at MAS frequencies up to 126\xa0kHz

We report linewidth and proton T1, T1ρ and T2′ relaxation data of the model protein ubiquitin acquired at MAS frequencies up to 126 kHz. We find a predominantly linear improvement in linewidths and coherence decay times of protons with increasing spinning frequency in the range from 93 to 126 kHz. We further attempt to gain insight into the different contributions to the linewidth at fast MAS using site-specific analysis of proton relaxation parameters and present bulk relaxation times as a function of the MAS frequency. For microcrystalline fully-protonated ubiquitin, inhomogeneous contributions are only a minor part of the proton linewidth, and at 126 kHz MAS coherent effects are still dominating. We furthermore present site-specific proton relaxation rate constants during a spinlock at 126 kHz MAS, as well as MAS-dependent bulk T1ρ (1HN).

Spatiotemporal dynamics of Raman coherence in hollow-core fibers for a pump-probe setup

We present an experimental and theoretical study of the stimulated Raman emission in hollow-core kagome waveguides in a pump-probe arrangement. We perform an experimental investigation of the power of the Stokes signal from the probe, which is below the stimulated Raman scattering threshold, as a function the pump-probe delay time. The results show the Stokes power to increase with pump-probe delay, reaching a maximum at 10 ns, and to decrease afterward. In view of a coherence decay time of only 0.25 ns, we demonstrate a surprisingly slow reduction of Stokes signal with the characteristic time much longer than the coherence decay time by a factor of up to 40. The numerical investigations explain the observed phenomenon as a result of the spatiotemporal dynamics of the probe pulse and Raman coherence. We show that the increase of the characteristic time can be related to the spatial position of the intense sideband generation event and its dependence on the pump-probe delay.

Simultaneous observation of free and defect-bound excitons in CH3NH3PbI3 using four-wave mixing spectroscopy.

Solar cells incorporating organic-inorganic perovskite, which may be fabricated using low-cost solution-based processing, have witnessed a dramatic rise in efficiencies yet their fundamental photophysical properties are not well understood. The exciton binding energy, central to the charge collection process, has been the subject of considerable controversy due to subtleties in extracting it from conventional linear spectroscopy techniques due to strong broadening tied to disorder. Here we report the simultaneous observation of free and defect-bound excitons in CH3NH3PbI3 films using four-wave mixing (FWM) spectroscopy. Due to the high sensitivity of FWM to excitons, tied to their longer coherence decay times than unbound electron- hole pairs, we show that the exciton resonance energies can be directly observed from the nonlinear optical spectra. Our results indicate low-temperature binding energies of 13 meV (29 meV) for the free (defect-bound) exciton, with the 16 meV localization energy for excitons attributed to binding to point defects. Our findings shed light on the wide range of binding energies (2–55 meV) reported in recent years.

Two-photon interference from remote quantum dots with inhomogeneously broadened linewidths

In this paper, we study the influence of quasiresonant and nonresonant excitation on the interference properties of single photons emitted from quantum dots (QDs). The quasiresonant excitation scheme leads to an increase of interference visibility of photons emitted from the same QD to 69% compared to 12% for nonresonant excitation. Furthermore, we demonstrate quantum interference of photons emitted from separate QDs which are simultaneously excited into their $p$ shell. We can readily extract a two-photon interference visibility as high as (39 \ifmmode\pm\else\textpm\fi{} 2)% for nonpostselected coincidences exceeding the predicted value based on coherence and radiative decay times of the quantum dot emission ($\ensuremath{\sim}$25%). We account for this observation by treating the emission of both quantum dots as inhomogeneously broadened ensembles of Fourier-limited photons and observe good congruence between experiment and model.

Coherence decay measurement of v = 2 vibrons in solid parahydrogen

The coherence decay of the v = 2 vibrational state (vibrons) of solid parahydrogen was measured via time-resolved coherent anti-Stokes Raman spectroscopy. We found that the decay curve has a non-exponential time profile in the time scale of 200 ns at a low temperature below 5 K and a low orthohydrogen impurity concentration (~0.01%). This behavior, as also observed in the case of the v = 1 vibrons, represents a signature of band structure of the v = 2 state in the solid phase. The maximum coherence decay time of 50 ns in an exponential part was achieved, which shows excellence of the v = 2 state for coherent processes. We also found that finite temperatures, orthohydrogen impurities, and other structural inhomogeneity accelerate the decay, hiding the non-exponential feature of the vibron band.

Femtosecond coherence and quantum control of single molecules at room temperature

Quantum mechanical phenomena, such as electronic coherence and entanglement, play a key role in achieving the unrivalled efficiencies of light-energy conversion in natural photosynthetic light-harvesting complexes, and triggered the growing interest in the possibility of organic quantum computing. Since biological systems are intrinsically heterogeneous, clear relations between structural and quantum-mechanical properties can only be obtained by investigating individual assemblies. However, single-molecule techniques to access ultrafast coherences at physiological conditions were not available so far. Here we show by employing femtosecond pulse-shaping techniques that quantum coherences in single organic molecules can be created, probed, and manipulated at ambient conditions even in highly disordered solid environments. We find broadly distributed coherence decay times for different individual molecules giving direct insight into the structural heterogeneity of the local surroundings. Most importantly, we induce Rabi-oscillations and control the coherent superposition state in a single molecule, thus performing a basic femtosecond single-qubit operation at room temperature.

Environmental Broadening of the CTTS Bands: The Hexaammineruthenium(II) Complex in Aqueous Solution

Cluster ab initio quantum chemistry approach is developed to simulate the charge-transfer-to-solvent (CTTS) absorption band and satellite ligand field bands of hexaammineruthenium(II) ion in aqueous solution. Several cluster models, including 16, 21, and 38 water molecules, are explored for this purpose. TDDFT method with long-range corrected BLYP (LC-BLYP) functional is used to obtain the vertical transition characteristics, and DFT B3LYP is used for calculation of the ground state geometry and vibrational frequencies of the solvated complex. A simple harmonic bath model is employed to estimate the absorption bandwidths and coherence decay times with the parameters taken from the quantum chemistry calculations. The present approach provides rather reasonable estimates for the CTTS band position and shape, also giving an additional insight for the mechanism of the CTTS band broadening.

Broadband light generation using a relatively weak Raman mode in lead tungstate crystal

We obtain broadband generation in a Raman-active crystal lead tungstate (PbWO4) by selectively exciting a relatively weak Raman mode using two femtosecond laser pulses. A third probe beam is used to identify the Raman mode being excited and to measure the coherence decay time of the two simultaneously excited strong Raman modes in the crystal.

Dynamics of a stored Zeeman coherence grating in an external magnetic field

We investigate the evolution of a Zeeman coherence grating induced in a cold atomic cesium sample in the presence of an external magnetic field. The gratings are created in a three-beam light storage configuration using two quasi-collinear writing and reading laser pulses with a counterpropagating pulse after a variable time delay. The phase-conjugated pulse arising from the atomic sample is monitored. Collapses and revivals of the retrieved pulse are observed for different polarizations of laser beams and for different directions of the applied magnetic field. While magnetic field inhomogeneities are responsible for the decay of the coherent atomic response, a five-fold increase in the coherence decay time, with respect to no applied magnetic field, is obtained for an appropriate choice of the direction of the applied magnetic field. A simplified theoretical model illustrates the role of the magnetic field mean and its inhomogeneity on the collective atomic response.

Ultrafast H2 and D2 rotational Raman responses in near critical CO2: An experimental and theoretical study of anisotropic solvation dynamics

The optical heterodyne detected anisotropic rotational Raman responses of H(2) and D(2) (22 mol %) in a near critical CO(2) (rho(*) = rho/rho(c) = 0.8, T = 308 K) solution are reported. J-specific rotational Raman correlation functions (RCFs) for the S(J) transitions of H(2) (J = 0,1,2) and D(2) (J = 0,1,2,3) in this CO(2) solution are determined from these measurements. A mixed classical-quantum simulation methodology results in RCFs that are in excellent agreement with the experimentally derived J-specific responses. The observed S(J) coherence decay time scales, J-dependence, rotor mass dependence, and solvent-induced transition frequency shifts are well captured by these simulations. Pure dephasing of these rotational Raman transitions is shown to be close to the homogeneous limit of the standard Kubo line shape analysis and attributable to the rotor center-of-mass translation in an anisotropic solvent cage. Rotor translational motion in the vicinity of a single CO(2) appears to dominate this dephasing mechanism. Mixed classical-quantum simulations, incorporating the effects of solution fluctuation driven nonadiabatic coupling of instantaneous adiabatic states, including full J-mixing, are required for the agreement between theory and experiment obtained here. Simulations of the classically excited angular kinetic energy of D(2) rotors are used as an estimate of T(1) relaxation rates and are found to be negligible compared to the D(2) rotational Raman coherence time scale. These results are discussed in the context of previous mixed classical-quantum and rotational friction calculations of the dephasing and energy relaxation contributions to H(2) rotational Raman coherence decays. Advantages of time domain acquisition of these rotational Raman responses as compared to spontaneous Raman measurements are illustrated here.

Decoherence and fidelity of single-electron spin states in quantum dots: effects of nuclearhyperfine coupling and double occupancy

The decoherence and fidelity of spin states in a localized single-electron quantum dot in thepresence of a dc magnetic field, arising either from the nuclear hyperfine interaction withinthe dot or due to its coupling with another localized quantum dot, are examined in detail.A general framework for determining the time evolution of the reduced density matrixρ for a single dot is presented, which is exact up to the second order in interaction with anyreservoir. In particular, it is applied to the problem of nuclear hyperfine coupling, andapproximate estimates of coherence decay time are made when the nuclear spins are eitherpolarized or unpolarized and the internal dynamics of nuclear spins is determinedmainly by the nuclear magnetic dipole–dipole interaction. The fidelity of a pureunperturbed electronic one-qubit spin state is obtained as a function of time,which is exact even on a very short timescale of logic gate operations. The timevariation of the fidelity of the same one-qubit state on the localized dot as a part ofthe direct product with another one-qubit state on another localized dot arisingbecause of coupling between these quantum dots is also calculated in this paper.In this case, we include both the single-particle tunnelling between the dots aswell as the direct and exchange Coulomb interactions, including on-site Coulombrepulsion. This allows for the double occupation of a single dot. It is found that theloss of fidelity of such two-qubit states due to double occupancy and additionalphase errors in the presence of appreciable dot–dot coupling can become a moresevere limiting factor than that due to the hyperfine interaction in individual dots.

Influence of phase coherence decay on generation of solitary waves in stimulated Raman scattering

The influence of the phase coherence decay process on the characteristics of both the single solitary pulses in stimulated Raman scattering and their train has been theoretically studied with the aim of their further application in practice. The strong correlation between the phase coherence decay time and solitary pulse position in the depletion region of a pumping pulse, as well as the solitary pulse duration has been found. The new method of the short pulse train generation has been proposed.

Time-resolved CARS measurements of the vibrational decoherence of I2 isolated in an Ar matrix

Time-resolved coherent anti-Stokes Raman scattering is applied to prepare and interrogate vibrational coherences on the ground electronic surface of molecular iodine isolated in Ar matrices. The coherence decay time shows a linear dependence on vibrational quantum numbers for v=3–15. The temperature dependence of decoherence rates is negligible for v<7 in the experimental range T=18–32 K. For a v=13, 14 superposition, the temperature dependence indicates dephasing by a 66 cm−1 pseudo-local phonon, just outside the Debye edge of the solid. The accuracy of the data is limited due to two-photon induced dissociation of the molecule, a process which is characterized using polarized fields. The T→0 limit of dephasing is discussed.

Excitonic polarons in semiconductor quantum dots.

The discretization of the electronic spectrum in semiconductor quantum dots implies a strong coupling behavior between the optical phonons and the electron-hole pairs, despite the fact that a pair is electrically neutral. The excitonic polarons strongly modify the optical spectra. In particular, the ground excitonic polaron contains one or two phonon components, which leads to the existence of phonon replicas in the luminescence. The population and coherence decay times of the optical transition associated with the ground excitonic polaron are calculated.

Coherent external and internal phonons in quasi-one-dimensional organic molecular crystals.

We have directly time resolved coherent phonon oscillations in quasi-one-dimensional organic crystals of MePTCDI ( N-N'-dimethylperylene-3,4,9,10-dicarboximide), using femtosecond pump-probe experiments. We observe both higher-energy oscillations caused by intramolecular vibrations (internal phonons) and, for the first time in a quasi-one-dimensional organic system, lower-energy modulations which are related to coherent lattice phonons (external phonons). For internal A(g) vibrations, the coherence decay time of about 2 ps is almost independent of the mode. In contrast, the damping time of the external phonons increases strongly with decreasing energy.

Dark state in ruby: Analysis of the feasibility

Bichromatic excitation of the R1 line in ruby at liquid-helium temperature is considered. The frequency difference of the spectral components of the driving field is resonant with the energy gap between the anticrossed spin sublevels 11/2 and 13/2 in the ground state 4 A2. The condition of the spin coherence trapping in the nonabsorbing state is analyzed. Population trapping is revealed in the reduced spontaneous emission from the excited state and in large spin magnetization of the ground state Cr 31 precessing with the frequency difference of the spectral components of the driving field. This magnetization can be detected by a pickup coil. The lifetime of the induced magnetization is strongly dependent on the ratio of the lifetime of the excited state Eand the spin coherence decay time (T2) s pin .

Measuring coherence while observing noise.

We present a new approach to the measurement of coherence. By monitoring the quantum interference fluctuations in the population excited by a pair of time-delayed, randomly phased pulses, it is possible to extract information on internal dynamics, energy level splittings, and characteristic coherence decay times of the medium. As a proof of concept, we demonstrate the measurement of phase relaxation and doublet separation in atomic potassium. The principle of coherence observation by interference noise is very general, is shown to be robust and with inherent time resolution of a few optical cycles, and is proposed as an alternative to many interferometric applications.

Dynamic Stark effect of exciton and continuum states in CdS.

The first observation of the optical Stark effect of electron-hole continuum states in addition to the bound-exciton states is reported under femtosecond excitation conditions chosen to minimize the generation of real carriers. The experimental results agree well with calculations using the generalized semiconductor Bloch equations. The theory shows that the commonly used adiabatic approximation is correct only for pulses which are longer than the coherence decay time. For shorter pulses coherent dynamic effects strongly influence the spectral changes.

Theory of the Optical Stark Effect in Semiconductors under Ultrashort‐Pulse Excitation

AbstractThe optical Stark effect is computed for femtosecond excitation of the electron–hole‐pair states in semiconductors. The theory is based on the effective Bloch equations for semiconductors and is evaluated for the large detuning, low intensity limit. For the example of CdS, good agreement with recent femtosecond experiments is found. It is shown that the coherent interaction of the pulses and the medium polarization dominates the semiconductor response as long as the pulse duration is of the order of, or less than, the coherence decay time of the medium.