Negative Time Delay Research Articles

Photodissociation dynamics of CF3I investigated by two-color femtosecond laser pulses

We report an experimental study of the multiphoton dissociation dynamics of CF3I performed on a home-built femtosecond laser pump-probe system; with time-of-flight mass spectrometer. The first repulsive A band and the 5p pi(3)7s sigma v(2) = 1 Rydberg state of CF3I were accessed by one- and two-photon transitions at 267 nm, respectively; with the latter two-photon absorption followed by a further two-photon probe transition at 401 nm to the (B) over tilde state of the parent ion. The observed signals from the CF3+ and I+ fragments show similar multi-component exponential decay patterns but the former is 4 times stronger than the latter. However; the parent CF3I+ signal was observed to evolve in a very different manner; decreasing sharply when probed in the first 289 fs following excitation and subsequently rising again after 860 fs to a constant level below that measured at negative pump-probe delay times when the pump and probe pulses exchange roles. This dip observed in the parent ion profile; is very different from that previously reported at shorter pump wavelengths of 264 nm or 265 ran, and is interpreted as the competition between two different ionization channels. One from the vibrationally excited v(2) = 1 of the irradiated Rydberg state and the other from the dissociative vibrational origin of the same electronic state which is populated by internal vibrational relaxation.

Apparent causality paradox in frustrated Gires–Tournois interferometers

Negative group delay times upon total reflection from frustrated Gires-Tournois interferometers seem to violate causality. Contrary to what has been recently argued in the literature, I show with the example of total light reflection upon an ideal nonabsorbing plasma that adding the time associated with the Goos-Hänchen shift does not solve the paradox, which remains open.

Biexcitons and their dephasing processes in ZnO

We have investigated free excitons and biexcitons of bulk ZnO by time-integrated and spectrally-resolved four-wave mixing. The polarization- and wave-vector-dependent FWM signals for A and B excitons demonstrate clearly that the valence-band ordering is A–Γ9, B–Γ7, and C–Γ7. Next, we gave qualitative explanation of the FWM signals induced by excitons and biexcitons in ZnO. For negative delay times, bound and unbound AB-biexcitons (A exciton-B exciton correlation), which have been considered to play a minor role in GaAs heterostructures, was found to play an important role and give a significant qualitative impact to the optical response of ZnO. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Detection of light-induced band gaps by ultrafast femtosecond pump and probe spectroscopy

We use the semiconductor Bloch equations formulated for full bands with a semiempirical description of the density- and energy-dependent dephasing and relaxation in order to analyze the proposed resonant 5 fs pump-probe experiments. For slightly negative delay times a modulation of the probe beam absorption or the differential transmission spectrum is obtained due to the band Mollow triplet caused by coherent band mixing and the resulting light-induced gaps both in the valence and conduction bands. This ultrafast femtosecond experiment with presently available 5 fs pulses would provide a rather direct observation of a basic prediction for the interaction of a strong coherent light beam with the band electrons in semiconductors and would complement recent observations of light-induced gaps in nonlinear spectroscopy.

Hartman effect and nonlocality in quantum networks

We study the phase time for various quantum mechanical networks having potential barriers in their arms to find the generic presence of Hartman effect. In such systems it is possible to control the ‘super arrival’ time in one of the arms by changing parameters on another, spatially separated from it. This is yet another quantum nonlocal effect. Negative time delays (time advancement) and ‘ultra Hartman effect’ with negative saturation times have been observed in some parameter regimes.

Dependence of line shapes in femtosecond broadband stimulated Raman spectroscopy on pump-probe time delay

The effect of the time delay between the picosecond Raman pump and the femtosecond Stokes probe pulse on the Raman gain line shape in femtosecond broadband stimulated Raman spectroscopy (FSRS) is presented. Experimental data are obtained for cyclohexane to investigate the dependence of the FSRS line shape on this time delay. Theoretical simulations of the line shapes as a function of the time delay using the coupled wave theory agree well with experimental data, recovering broad line shapes at positive time delays and narrower bands with small Raman loss side wings at negative time delays. The analysis yields the lower bounds of the vibrational dephasing times of 2.0 ps and 0.65 ps for the 802 and 1027 cm(-1) modes for cyclohexane, respectively. The theoretical description and simulation using the coupled wave theory are also consistent with the observed Raman gain intensity profile over time delay, reaching the maximum at a slightly negative time delay (approximately -1 ps), and show that the coupled wave theory is a good model for describing FSRS.

INTER-LANDAU-LEVEL TRANSITIONS AND ULTRAFAST DYNAMICS OF 2D MAGNETOEXCITONS

We study the role of many-body correlations in the ultrafast dynamics of quantum well magnetoexcitons. In linear response, inter-Landau level transitions are suppressed if magnetic field is stronger than magnetoexciton binding energy. However, in nonlinear response, two-electron Auger processes remain unsuppressed even at very strong field. We show that Auger scattering plays a dominant role in the coherent exciton dynamics in strong magnetic field. We perform numerical calculations of four-wave-mixing (FWM) polarization which incorporate Auger processes nonperturbatively, and find a strong amplitude enhancement of the FWM signal accompanied with polarization oscillations at negative time delays. We identify these oscillations as quantum beats between dark two-exciton states related to each other via Auger processes.

Time-delay matrix analysis of resonances: application to the positronium negative ion

A procedure to analyse resonances in scattering and photoionization calculations is discussed. It is theoretically connected to a general relation between the trace of the time-delay matrix and the energy derivative of the eigenphase sum. This procedure is especially useful in analysing resonances with strong background contributions, such as shape resonances lying just above the threshold of a new channel with an overcritical dipole field. In this case the background eigenphase sum diverges towards the threshold, and it can easily change the time delay due to an S-matrix pole into a negative time delay, or time advance. As an application, we analyse results of hyperspherical close-coupling calculations of S-, P-, D-, F-, G- and H-wave resonances in the positronium negative ion Ps− near the Ps(n = 2, 3) thresholds. We find examples of a strong background leading to an overall time advance and the dipole oscillation in the cross section above and near a threshold.

Polarization and wave-vector-dependent measurements by four-wave mixing in ZnO: valence-band ordering and biexcitons

We have investigated free excitons and biexcitons of bulk ZnO by time-integrated and spectrally resolved four-wave mixing (FWM). The polarization- and wave-vector-dependent FWM signals for A and B excitons demonstrate clearly that the valence-band ordering is A– Γ 9 , B– Γ 7 , and C– Γ 7 . Also, we gave qualitative explanation of the FWM signals induced by excitons and biexcitons in ZnO. For negative delay times, bound and unbound AB-biexcitons (A exciton–B exciton correlation), which have been considered to play a minor role in GaAs heterostructures, was found to play an important role and give a significant qualitative impact to the optical response of ZnO.

Ultrafast Auger Spectroscopy of Quantum Well Excitons in Strong Magnetic Fields

We study theoretically the ultrafast nonlinear optical response of quantum well excitons in a perpendicular magnetic field. We address the role of many-body correlations originating from the electron scattering between Landau levels (LL). In the linear optical response, the processes involving inter-LL transitions are suppressed provided that the magnetic field is sufficiently strong. However, in the nonlinear response, the Auger processes involving inter-LL scattering of two photoexcited electrons remain unsuppressed. We show that Auger scattering plays a dominant role in the coherent exciton dynamics in strong magnetic fields. We perform numerical calculations for the third-order four-wave-mixing (FWM) polarization, which incorporate the Auger processes nonperturbatively. We find that inter-LL scattering leads to a strong enhancement of FWM signal and to oscillations at negative time delays. These oscillations represent quantum beats between optically inactive two-exciton states related to each other via Auger processes.

Synchrotron Self‐Compton Model for Rapid Nonthermal Flares in Blazars with Frequency‐dependent Time Lags

We model rapid variability of multifrequency emission from blazars occurring across the electromagnetic spectrum (from radio to gamma-rays). Lower energy emission is produced by the synchrotron mechanism, whereas higher energy emission is due to inverse Compton scattering of the synchrotron emission. We take into account energy stratification established by particle acceleration at shock fronts and energy losses due to synchrotron emission. We also consider the effect of light travel delays for the synchrotron emission that supplies the seed photons for inverse Compton scattering. The production of a flare is caused by the collision between a relativistic shock wave and a stationary feature in the jet (e.g., a Mach disk). The collision leads to the formation of forward and reverse shocks, which confine two contiguous emission regions resulting in complex profiles of simulated flares. Simulations of multifrequency flares indicate that relative delays between the inverse Compton flares and their synchrotron counterparts are dominated by energy stratification and geometry of the emitting regions, resulting in both negative and positive time delays depending on the frequency of observation. Light travel effects of the seed photons may lead to a noticeable delay of the inverse Compton emission with respect to synchrotron variability if the line of sight is almost perfectly aligned with the jet. We apply the model to a flare in 3C 273 and derive the properties of shocked plasma responsible for the flare. We show that the pronounced negative time delay between the X-ray and IR light curves (X-rays peak after the maximum in the synchrotron emission) can be accounted for if both forward and reverse shocks are considered.

Nonlinear optical properties of 75TeO 2 –20Nb 2 O 5 –5ZnO glasses doped with CeO 2

Nonlinear optical properties of 75TeO2–20Nb2O5–5ZnO glasses doped with CeO2 have been investigated with a self-diffracted time-resolved degenerate four-wave mixing (DFWM) technique at different excitation intensities and lattice temperatures. The DFWM signal exhibits three peaks at higher excitation intensities, where a main peak appears at zero delay time and two rather weak side peaks are located symmetrically at the negative and positive time delay. Due to destructive interferences between the fifth- and third-order polarizations, the line-shape of the main peak around the zero time delay evolves from single peak into a double-peak structure with increasing excitation intensity. Two side peaks emerge at the positive and negative time delay and gradually intensify with increasing excitation intensity or lattice temperature and their positions are independent of the pulse duration, temperature and excitation intensity, which are attributed to the many-body Coulomb interaction.

On the relation between retarded and advanced arrival of a pulse at the output of an optical system

It is shown that the possibility of negative delay time of a pulse at the output of an optical system (compared to the pulse at its input) is a necessary consequence of the possibility of positive delay time of the pulse and the superposition principle. In particular, two optical systems with different positive delay times are sufficient for creating an optical system with a negative delay time. The negative delay time of a pulse can be considered as a spline extrapolation of its time dependence resulting from the interference of several copies of the pulse with different delays. It is shown that a negative delay time of a pulse arises necessarily in any interferometer in an area of destructive interference of two copies of the initial pulse that are similar in amplitude and undergo different delay times.

Spectrally filtered time domain study of coherent phonons in semimetals

We have investigated the spectrally resolved coherent phononoscillation in semimetals of Sb and Bi. The oscillation initial phase isπ-shifted for the high- and low-frequency components of the probe pulse, indicating that notonly the integrated intensity, but also the spectral content of the probe, are a function oftime delay. This means that the coherent phonon, corresponding to nuclear motion, is awavepacket localized in different regions of the phase space. Furthermore, the spectralfiltering results in the appearance of the signal for negative delay time with thesimultaneous disappearance of the coherent artefact. We conjecture that all these findingsare the indication of the non-classical nature of coherent phonons that has beenearlier inferred from squeezing and revivals of the lattice mode in semimetals.

On the Negative Delay Time of a Narrow-Band Signal as It Passes Through the Resonant Filter of Absorption

It is shown that a narrow-band signal with smoothly varying envelope can have a negative delay time upon filtering by certain physically realizable filters, i.e., the smooth-signal envelope shifts forward rather than backward. We show that this does not contradict the casuality principle, since the advanced appearance of the signal has the nature of prediction of the future signal level from the time dependence of its level in the past. It is shown that in the case of a negative delay time, the time dependence of a switched-off signal can be recovered, i.e., reception of the untransmitted part of the signal is possible. We propose to use these phenomena for predicting signals of natural and artificial origin.

Characterization of a supercontinuum generated from a photonic crystal fiber and its application to coherent Raman spectroscopy.

The temporal and spectral profiles of supercontinuum radiation generated from a photonic crystal fiber are evaluated with a polarization-gate frequency-resolved optical gating technique. The supercontinuum is then applied to coherent inverse Raman spectroscopy. A stimulated Raman signal of cyclohexane is observed as an induced absorption signal with an instantaneous response. The Raman signal has a peak at a slight negative delay time, which is explained by perturbed Raman-induced coherence.

Exceptionally slow rise in differential reflectivity spectra of excitons in GaN: Effect of excitation-induced dephasing

Femtosecond pump-probe (PP) differential reflectivity spectroscopy (DRS) and four-wave mixing (FWM) experiments were performed simultaneously to study the initial temporal dynamics of the exciton line-shapes in GaN epilayers. Beats between the A-B excitons were found \textit{only for positive time delay} in both PP and FWM experiments. The rise time at negative time delay for the differential reflection spectra was much slower than the FWM signal or PP differential transmission spectroscopy (DTS) at the exciton resonance. A numerical solution of a six band semiconductor Bloch equation model including nonlinearities at the Hartree-Fock level shows that this slow rise in the DRS results from excitation induced dephasing (EID), that is, the strong density dependence of the dephasing time which changes with the laser excitation energy.

Polarization dependence of excitonic and biexcitonic contributions to FWM signals

The time-averaged polarization angles of excitonic and biexcitonic four-wave-mixing signals are investigated in spectrally resolved measurements for different angles between the linear excitation polarizations, delay times and pulse energies. For an angle close to 90° between the two incident polarizations the signal emitted at the spectral position of the excitonic resonance clearly shows different polarization angles as the delay is changed from negative to positive. Additionally, the polarization angles of the signals exhibit oscillations which are differently pronounced for negative and positive delay times. The experimental results are compared with numerical calculations based on a phenomenologically extended five-level model of the exciton-biexciton system.

Biexcitonic effects in the time integrated four-wave mixing with picosecond pulses

We propose a simple method to estimate the biexcitonic contribution to the excitonic non-linearity. The method is based on the time integrated four-wave mixing (TI FWM) with picosecond pulses. The TI FWM signal, which is measured as a function of the delay between pump and test pulses, shows shift towards positive and negative time delay when the laser is tuned at the exciton and biexciton resonance, respectively. We show theoretically that the magnitude of the shift allows us to estimate the biexcitonic contribution to the third-order non-linearity at the fundamental band edge.

Bounds and enhancements for negative scattering time delays

The time of passage of the transmitted wave packet in a tunneling collision of a quantum particle with a square potential barrier becomes independent of the barrier width in a range of barrier thickness. This is the Hartman effect, which has been frequently associated with ``superluminality.'' A fundamental limitation on the effect is set by nonrelativistic ``causality conditions.'' We demonstrate first that the causality conditions impose more restrictive bounds on the negative time delays (time advancements) when no bound states are present. These restrictive bounds are in agreement with a naive, and generally false, causality argument based on the positivity of the ``extrapolated phase time,'' one of the quantities proposed to characterize the duration of the barrier's traversal. Nevertheless, square wells may in fact lead to much larger advancements than square barriers. We point out that close to the thresholds of new bound states, the time advancement increases considerably, while, at the same time, the transmission probability is large, which facilitates the possible observation of the enhanced time advancement.