Fast Modulation Regime Research Articles

Spectrally reconfigurable quantum emitters enabled by optimized fast modulation

The ability to shape photon emission facilitates strong photon-mediated interactions between disparate physical systems, thereby enabling applications in quantum information processing, simulation and communication. Spectral control in solid state platforms such as color centers, rare earth ions, and quantum dots is particularly attractive for realizing such applications on-chip. Here we propose the use of frequency-modulated optical transitions for spectral engineering of single photon emission. Using a scattering-matrix formalism, we find that a two-level system, when modulated faster than its optical lifetime, can be treated as a single-photon source with a widely reconfigurable photon spectrum that is amenable to standard numerical optimization techniques. To enable the experimental demonstration of this spectral control scheme, we investigate the Stark tuning properties of the silicon vacancy in silicon carbide, a color center with promise for optical quantum information processing technologies. We find that the silicon vacancy possesses excellent spectral stability and tuning characteristics, allowing us to probe its fast modulation regime, observe the theoretically-predicted two-photon correlations, and demonstrate spectral engineering. Our results suggest that frequency modulation is a powerful technique for the generation of new light states with unprecedented control over the spectral and temporal properties of single photons.

RENEWAL AGING IN NON-HOMOGENEOUS POISSON PROCESSES WITH PERIODIC RATE MODULATION

The statistical properties of sequences of Waiting Times generated by Non-Homogeneous Poisson Processes are investigated by means of Renewal Aging, i.e. a statistical analysis being able to detect the presence of genuine renewal events with non-Poisson statistics. The Renewal Aging features of two non-homogeneous Poisson models with different prescriptions of the rate modulation are compared. Both prescriptions are essentially periodic. In the first model, the rate is a totally deterministic and smooth harmonic function. In the second model, the rate modulation is deterministic and smooth almost everywhere, except in some singular points occurring with a periodicity affected by weak random fluctuations. The main parameter of both models is the modulation speed, defined as the ratio between the modulation period and the average Waiting Time. This allows to distinguish between slow, intermediate and fast modulation of the rate. As expected, in the very slow modulation case both models have a strongly reduced Renewal Aging, practically zero, because the analysis is affected by local clusters of Waiting Times with the same Poisson rate. In the intermediate range of modulation speeds, both models show non-zero Renewal Aging and the behavior of the two models tends to be more different as the modulation speed decreases. Going towards the fast modulation regime, the behavior of the two models becomes completely different from one another. Model I does not show a definite global Renewal Aging. On the time scale of the modulation period, local oscillations arise in the Probability Density Function and an apparent oscillating aging is also observed. However, this form of aging is not the manifestation of genuine renewal non-Poisson events, but it is related to the local oscillations. On the contrary, Model II displays Renewal Aging features in agreement with a homogeneous Renewal non-Poisson Process. It is argued that this is related to the singular points in the rate modulation of Model II, which, due to their random occurrence, could be identified with genuine critical non-Poisson events. The approach presented here could have interesting applications in problems involving dichotomous noise and, in particular, in the statistical characterization of ON–OFF fluorescence intermittency experimentally observed in complex systems, such as nano-crystals and single bio-molecules stimulated by laser fields.

The Quantum Solvation, Adiabatic versus Nonadiabatic, and Markovian versus Non-Markovian Nature of Electron-Transfer Rate Processes

In this work, we revisit the electron-transfer rate theory, with particular interests in the distinct quantum solvation effect and the characterizations of adiabatic/nonadiabatic and Markovian/non-Markovian rate processes. We first present a full account for the quantum solvation effect on the electron transfer in Debye solvents, addressed previously in J. Theor. Comput. Chem. 2006, 5, 685. Distinct reaction mechanisms, including the quantum solvation-induced transitions from barrier crossing to tunneling and from barrierless to quantum barrier crossing rate processes, are shown in the fast modulation or low viscosity regime. This regime is also found in favor of nonadiabatic rate processes. We further propose to use Kubo's motional narrowing line shape function to describe the Markovian character of the reaction. It is found that a non-Markovian rate process is most likely to occur in a symmetric system in the fast modulation regime, where the electron transfer is dominant by tunneling due to the Fermi resonance.

Electron Transfer Theory Revisit: Motional Narrowing Induced Non-Markovian Rate Processes

While the exact theory of chemical reaction rate processes is always non-Markovian, experimental rates do often show practically Markovian that supports kinetics rate constant description. In this work, we propose to use the Kubo's motional narrowing line shape function to characterize the Markovian character of the simplest two-state electron transfer reaction system. On the basis of analytical results, we demonstrate the related Markovianicity parameter as an interplay between the fluctuating solvent environment and the coherent transfer coupling. It is found that a non-Markovian rate process is most likely to occur in a symmetric system in the fast solvent modulation regime, where the resonant tunneling enhancement plays the important role. The effect of quantum solvation on electron transfer, which is dominant in the fast modulation regime, will also be highlighted.

IR Study on the Relaxation of the Phosphate Group of 5‘-dCMP in2H2O and H2O Solutions

The analysis of the IR νs(PO32-) band shape of disodium deoxycytidine 5‘-monophosphate, 5‘-dCMP, in 2H2O and H2O solutions at different concentrations, 0.005−0.90 mol dm-3, and temperatures, 7−70 °C, provides information on the relaxation and dynamics of the phosphate group. Self-association of this mononucleotide is detected at concentrations higher than ∼0.28 mol dm-3. The relaxation, dominantly vibrational, is ∼0.31 ps faster in H2O than in 2H2O and ∼70 fs faster than for the mononucleotide 5‘-CMP. Kubo−Rothschild's and Oxtoby's models for vibrational dephasing describe the IR time-correlation functions up to ∼3 ps. The stretched exponential model takes values for the dispersion parameter, α, close to 1, i.e., approaches the Markovian Kubo−Rothschild's model, and random fluctuations prevail at concentrations ≤0.28 mol dm-3. The modulation parameters resulting from these models indicate that the νs(PO32-) oscillator is under a fast modulation regime at low concentration but is slowing to an intermediate modulation regime as concentration approaches ∼0.3 mol dm-3.

Calculation of the vibrational linewidth and line shape of Raman spectra using the relaxation function. I. Method and application to nitrogen

The spectral line shape of the fundamental vibration of nitrogen is calculated from molecular dynamics simulations by determining the Fourier transform of the relaxation function. It has been applied to the fluid phase at various pressures and temperatures, and to solid δ-N2. The validity of the assumption that the spectrum at relatively high temperatures and pressures can be calculated by assuming that these systems are in the fast modulation regime (Δτc≪1), has been verified. A deviation of the vibrational line shape from the motional narrowing limit has been found for fluid nitrogen at low pressure, with a Kubo parameter, Δτc, equal to 0.23, and for the vibrational line of the molecules on the a sites in δ-N2, with Δτc equal to 0.075. It is concluded that the value of the Kubo parameter is not an unambiguous criterion for the fast modulation regime. Moreover, a detailed comparison reveals a difference in the dynamical behavior of the molecules on the a and c sites. It is shown that this procedure can also be used if one does not know whether or not closely spaced lines are to be expected. The present procedure is suited to calculate line shapes in the intermediate Kubo regime, e.g., in concentrated mixtures, where no simple relations are available. Finally it is shown that in nitrogen at low density and 126 K the ratio of the correlation time of the frequency autocorrelation function and the dephasing time is smaller than in CH3I, where the n dependence of the vibrational overtone is subquadratic.

Raman spectroscopic study of the single particle dynamics of CS2in liquid binary mixtures with CCl4

Using the polarized and depolarized scattering geometry we studied the concentration dependence of the isotropic and anisotropic Raman spectra of the symmetric stretching mode v 1 of CS2 in liquid mixtures with CCl4 at 25°C. The vibrational relaxation of the v 1 mode can be characterized as a pure dephasing process in the fast modulation regime over the whole concentration range. We discuss orientational correlation functions and correlation times τor indicating a diffusional tumbling motion of the symmetry axis. Testing the most relevant hydrodynamic models we found a strong deviation from linearity of τor against η/T. The orientational diffusion behaviour may be characterized by the transition from the hydrodynamic to the non-hydrodynamic or the so-called sub slip boundary condition.

Vibrational and reorientationl relaxation in liquid tert-butyl bromide and tert-butyl bromide- d9

The vibrational and reorientational correlation functions of the ν s(CBr) mode in liquid tert-butyl bromide (TBBr) and in its deuterated analogue (TBBr- d 9) were determined from Raman bandshapes. The Kubo product δτ m obtained by fitting the Rothschild equation to experimental data amounts to 0.36–0.45 for the studied bands, which indicates a fairly fast modulation regime in these systems. The approximation of experimental rotational correlation functions with the help of the J-diffusion model yields values of 0.11–0.12ps for the angular momentum correlation time. The second-order rotational correlation times (1.7–2.0ps) agree well with the NMR results of D.W. Aksnes, K. Ramstad and O.P. Björlykke, Magn. Reson. Chem., 25 (1987) 1063. Selected bandshape parameters for δ(CCC) in both compounds, ϱ(CH 3) in TBBr and ν s(CC) in TBBR- d 9 were also quoted.

Dynamical inhomogeneous broadening and lattice-assisted intramolecular energy transfer in the fundamental A1 vibrations of liquid chloroform Cl-35

This report gives extensive isotropic Raman spectral data and their numerical Fourier transformation into the time domain (∼0–20 ps) for the three A1 vibrational fundamentals of Cl-35 isotopically pure chloroform. It was found that the vibrational amplitude correlation decay of the ν1 mode (C–H stretch) at 300 K follows a rapid vibrational dephasing process (inhomogeneous broadening with motional narrowing) that is 1.7 times faster than the concomitant intramolecular energy transfer to the overtone of the C–H bending mode. From the temperature dependence (213 K, 300 K) of the amplitude correlation decay of the ν3 mode (C–Cl deformation) we deduce that it relaxes essentially only by vibrational dephasing and in such a fast modulation regime that the ν3 oscillator is effectively decoupled from the local lattice anisotropies. From the temperature dependence of the amplitude correlation decay of the ν2 mode (C–Cl stretch) we infer a simultaneous vibrational dephasing and lattice-assisted intramolecular energy transfer process, the latter possibly to the overtone of ν3. This paper concludes with some general remarks on the predictability of relative contributions of various types of vibrational relaxation processes to vibrational amplitude correlation decay in molecular liquids and, in an Appendix, gives estimates on the rates of excess phonon depopulation into the lattice as governed by a permanent dipole–transition dipole coupling process for modes ν6, ν3, ν2, and ν5 of liquid chloroform.

Cation dependence of the vibrational and rotational relaxation of OH− ion in molten MOH(M=Li, Na, K, Rb, and Cs) by Raman scattering measurements

Polarized and depolarized Raman scattering measurements have been performed for the stretching vibrational mode of OH− ion in molten MOH (M= Li, Na, K, Rb, and Cs). Vibrational and rotational autocorrelation functions, Cv(t) and Cr(t), have been evaluated from Fourier transformation of the spectra. The relaxation rate of Cv(t) becomes more rapid and the asymmetry of the isotropic band becomes larger as the cation size becomes smaller. These results can be interpreted in terms of the coulombic interaction. Applying the two-parameters model based on the Kubo line shape theory to the observed spectra, it is suggested that the OH− ion apparently follows the fast modulation regime due to the libration of OH− ion. For all melts, the Cr(t) has oscillatory character caused by libration of the OH− ion at small t <0.1 ps. The function shows ordinary exponential decay at large t≳0.1 ps. The cation dependence of the libration at t< 0.1 ps and the rotational diffusion process at t≳0.1 ps were investigated separately. The larger the cation size becomes, the longer and larger the period and amplitude of the libration become, respectively. This indicates that in the libration process the OH− ion seems to have more extended space in the melt as the cation size becomes larger. With respect to the rotational diffusion process, the rate shows rather small cation dependence.