Coherent Spike Research Articles

Signal-to-noise ratio gain of a noisy neuron that transmits subthreshold periodic spike trains

The transmission properties of an integrate-and-fire neuron model that transmits coherent subthreshold spike trains in a shot noise environment are investigated by numerical simulation. For very weak coherent couplings, it is shown that the input-output signal-to-noise ratio (SNR) gain is easier to exceed unity; while for stronger coherent couplings it is difficult to observe the SNR gain larger than unity at the optimal noise intensity. These observations are different from those acquired in the case of continuous noise. Our analysis further suggests that the larger SNR gain in the very weak coherent coupling case should be due to the noise-induced resonance. It is also shown that there is more possibility of the SNR gain above unity for slower periodic spike trains transmitted by the model. The results may be useful in understanding the performance of real noisy neurons acting as signal-processing elements.

Coherent and sequential contributions to femtosecond transient absorption spectra of a rhodamine dye in solution

A unified description is presented of sequential and coherent contributions to optical transient absorption measured by femtosecond pump-supercontinuum probe spectroscopy. All inherent transient terms are taken into account. The "coherence spike" seen during pump-probe overlap is thereby decomposed into its components. The method is demonstrated with rhodamine 110 in methanol. Pure homogeneous dephasing times are obtained from a simultaneous fit of all pertinent measurements. Vibronic structure in the coherence spectrum is assigned to stimulated Raman scattering between vibrational levels in the first excited electronic state. The time-zero spectrum for stimulated emission and the solvation relaxation function are also obtained.

Rayleigh-enhanced attosecond sum-frequency polarization beats via twin color-locking noisy lights

Based on color-locking noisy field correlation, a time-delayed method is proposed to suppress the thermal effect, and the ultrafast longitudinal relaxation time can be measured even in an absorbing medium. One interesting feature in field-correlation effects is that Rayleigh-enhanced four-wave mixing (RFWM) with color-locking noisy light exhibits spectral symmetry and temporal asymmetry with no coherence spike at $\ensuremath{\tau}=0$. Due to the interference between the Rayleigh-resonant signal and the nonresonant background, RFWM exhibits hybrid radiation-matter detuning with terahertz damping oscillations. The subtle Markovian high-order correlation effects have been investigated in the homodyne- or heterodyne-detected Rayleigh-enhanced attosecond sum-frequency polarization beats (RASPBs). Analytic closed forms of fourth-order Markovian stochastic correlations are characterized for homodyne (quadratic) and heterodyne (linear) detection, respectively. Based on the polarization interference between two four-wave mixing processes, the phase-sensitive detection of RASPBs has also been used to obtain the real and imaginary parts of the Rayleigh resonance.

Coherent interactions in femtosecond transient grating.

Transient grating of a dye in liquid has been measured as a function of the electronic coherence period. A diffractive beam splitter and a pair of wedge prisms are implemented to achieve precise spatial phase overlap and interferometrically accurate control of the time delay between the pump pulses. As the electronic coherence period is varied, coherent interactions lead to an enhancement or loss of the sharp feature in the transient grating signal near time zero, which is usually called coherent spike. Sensitivity of the transient grating signal to the solvation process also changes by the coherence time delay. All the features can be accounted for by invoking third-order nonlinear response functions. Numerical simulations have been performed to corroborate our description. This work identifies a major source of the coherent spike in the transient grating and transient absorption experiments. In addition, it allows us to propose a method that measures the solvation function more efficiently than conventional transient grating technique does.

Scheme for time-resolved experiments based on the use of statistical properties of the third harmonic of the SASE FEL radiation

A closer inspection of the statistical properties of the third-harmonic radiation from the SASE FEL reveals that it is possible to select single, temporary coherent radiation spikes by using a simple intensity trigger. A carefully designed optical system for splitting, delaying, filtering, and recombining the radiation would then allow time-resolved measurements with resolution down to the coherence time of the FEL, i.e. a few femtoseconds in the case of the TTF FEL.

Influence of intramolecular vibrations in third-order, time-domain resonant spectroscopies. I. Experiments

This is the first in a two-paper series that investigates the influence of intramolecular vibrational modes on nonlinear, time-domain, electronically resonant signals. Both Transient Grating (TG) and Three Pulse Photon Echo Peak Shift (3PEPS) signals were collected from several probe molecules: Nile Blue, N,N-bis-dimethylphenyl-2,4,6,8-perylenetetracarbonyl diamide, and Rhodamine 6G dissolved in different solvents: benzene, dimethylsulfoxide, and acetonitrile. The effects of excitation of different vibronic transitions on the electronically resonant signals were identified by comparing signals collected with laser pulses at different excitation wavelengths. In the 3PEPS profiles, we find that excitation on the blue edge of the absorption spectrum causes a decreased initial peak shift values and more rapid initial decays, whilst in the TG signals, the magnitude of the “coherent spike” is strongly wavelength dependent. Additional thermally activated vibronic effects were studied via temperature dependent 3PEPS profiles. Our results reveal the sensitivity of the nonlinear signals to the excitation wavelengths and to the distinct vibronic structure of the different chromophores studied. Pronounced modulations in both the 3PEPS and TG signals originating from coherently excited vibrational modes were directly observed. Additional oscillations were observed that are attributed to difference frequencies and higher harmonics of the fundamental modes. In paper II we demonstrate that detailed account of the vibronic nature of the chromophore is required to describe the wavelength dependent signals.

Opposition Effect from Clementine Data and Mechanisms of Backscatter

An analysis of Clementine data obtained from a UVVIS camera and simulating laboratory photometric and polarimetric measurements is presented with the use of a new photometric three-parameter function combining the shadow-hiding and coherent backscatter mechanisms. The fit of calculated curves to the average brightness phase function of the Moon derived from Clementine data indicates that the coherent backscatter component is nonzero. The average amplitude of the opposition surge of the Moon in the range of phase angles 0°–1° is approximately 10%. The Clementine data also show a flattening of phase-dependent brightness at angles less than 0.25° that is caused by the angular size of the solar disk. The lunar brightness phase curves at small phase angles are nearly the same in different wavelengths even though at larger phase angles (5°–50°) the lunar surface becomes distinctly redder with increasing phase angle. According to the model, the lack of wavelength-dependent brightness variations at small phase angles can be due to quasifractal properties of the lunar surface. Results of related laboratory measurements suggest that: (1) besides the narrow coherent backscatter opposition spike there is a broad component which can contribute to phase angles up to 10° and (2) a component of coherent backscatter can be important even for low albedo surfaces. The latter testifies the opposition effect of the lunar surface to be substantially formed by the coherent backscatter mechanism.

Mechanisms of ion beam induced atomic mixing in solids

Abstract In the present paper, selected typical studies on the ion-beam mixing of bi- and multi-layer systems are reviewed. It is shown that by proper variation of the materials (atomic number, chemical affinity between top and bottom layer) and of the irradiation conditions (ion species, energy, fluence, target temperature), significant conclusions can be drawn concerning the relevant mixing mechanisms. It is found that low temperature ion-beam mixing of light systems ( Z ≤18) is of pure ballistic character, without any influence of chemical driving forces. For higher atomic numbers and low or medium mass ions, mixing occurs by chemically biased diffusion in spatially separated local thermal-spikes. For very heavy ions mixing effects, which are non-linear with respect to the deposited energy density, point to the formation of coherent global spikes along the ion path by the overlapping of local spikes. Very heavy ions might also be able to initiate thermal-spikes in otherwise ballistic systems ( Z ≤18) at their end of range (end-of-range spikes) by a high density of subcascades. Chemically guided motion of residual defects from the collision cascade seems to play a role for ion beam induced mixing only at elevated temperatures.

Enzymatic reactions in small spatial volumes: comment on a model of Hess and Mikhailov

Recently Hess and Mikhailov pointed out that in small subcellular compartments diffusion is so fast that mixing is instantaneous on the time scale of many enzymatic reactions. This opens the possibility for synchronizing individual reaction events. To illustrate this fact they discuss as example an irreversible enzymatic reaction with allosteric product activation. Under appropriate conditions their model shows coherent spiking in the number of product molecules, caused by the strong correlation between reaction events. In this model only substrate binding is an indeterministic process, all other subsequent transitions between different enzyme states being deterministic, contrary to real processes. The purpose of the present paper was to investigate this interesting phenomenon by means of a more realistic modification of the original model, with only probabilistic transitions. In an attempt to obtain spiking, which was not observed under these conditions, the model was extended to make a clear distinction between allosteric high and low affinity substrate binding, in contrast to the original model using a product dependent mean binding probability. However no periodic signal was detectable in the indeterministic version of the Hess Mikhailov model or the extended version, either by means of direct visualization or on autocorrelation or Fourier analysis. Reasons why spiking is not observed in indeterministic enzyme models are discussed.

Transition from molecular chaos to coherent spiking of enzymic reactions in small spatial volumes

Theoretical study of an irreversible enzymic reaction with allosteric product activation reveals that, in small spatial volumes, it can undergo a transition to coherent spiking regime characterized by the presence of strong correlations between reaction events and the states of individual enzyme molecules.

A TEST FOR A BIOLOGICAL SIGNAL ENCODED BY NOISE

We consider here a simple example of stimulated sensory neurons operating under the influence of their own internal noise: the hair mechanoreceptor of the crayfish stimulated by a weak, periodic, hydrodynamic signal. Action potential spike trains from the sensory neuron are recorded and assembled into two objects for analysis: the interspike interval histogram (ISIH) and the cycle histogram of the spike density. A time transformation is carried out on the ISIH’s in order to test the hypothesis that the spike train is basically random and that the probability of coherent spike generation is related to the instantaneous stimulus amplitude. Moreover it is shown that the physiological spike train data can be qualitatively mimicked by an electronic Fitzhugh-Nagumo model, operated in the subcritical mode, driven by noise and a weak periodic signal. A discussion of how the Fitzhugh-Nagumo model is properly operated to mimic noisy data from sensory neurons is included.

Time-delayed Raman-enhanced nondegenerate four-wave mixing with a broadband laser source.

We report an experiment of the time-delayed Raman-enhanced nondegenerate four-wave mixing (RENFWM) with a broadband laser in carbon disulfide (${\mathrm{CS}}_{2}$). We studied the RENFWM signal intensity as a function of the relative time delay \ensuremath{\tau} between two beams which originate from a single broadband laser source. Our results indicate that the temporal behavior of the RENFWM is asymmetric with the maximum of the signal shifted from \ensuremath{\tau}=0. Furthermore, unlike the corresponding coherent Stokes Raman scattering (CSRS) no coherent spike appears at \ensuremath{\tau}=0. From our experimental results the relaxation time of the Raman mode can be extracted directly. We present a second-order coherence function theory to elucidate the physics of the basic features of the time-delayed RENFWM. We also discuss the difference between the RENFWM and the CSRS from a physical viewpoint.

Background-free autocorrelators by counter-propagating surface plasmons

Taking advantage of the nonlinear interaction between two counter-propagating surface plasmons, we have devised a background-free autocorrelator for short laser pulses diagnostics. Although it has been tested only to measure the coherence spike width of a Q-switched Nd:YAG laser, it can work in the near ir, visible and ultraviolet ranges as well.

Relationship between coherent acoustic wave generation and a coherence spike in an impulsive stimulated Brillouin scattering experiment

Impulsive stimulated Brillouin scattering has been measured for liquid CCl4 and CS2. A spike and an oscillation that are due to the generated acoustic wave are observed. The spike in CCl4 changes its sign when the period of the acoustic vibration is varied, implying interference between these processes. The spike in CS2 is accompanied by a broad component. The peak ratio of the spike intensity to the broad component intensity reaches nearly 5:1 and varies with the pulse duration. When the time interval between the two pump pulses is increased, the spike splits in two, and the acoustic vibration disappears. These results are well described by a third-order nonlinear polarization model that takes account of light-induced acoustic wave generation.

EEG coherence as a predictor of spike propagation

The relationship between resting EEG coherence and the propagation of spike activity from an experimental cortical focus was investigated in 6 rats. EEG was collected from an array of 6 epicortical electrodes positioned over the posterior hind limb sensorimotor (HL) and frontal (Fr1, Fr2) cortices. Coherence decreased non-linearly for all frequency bands with increasing interelectrode distance. The greatest decrement in coherence occured in the region corresponding to the junction between hind limb (HL) and frontal (Fr1) cortices. The decrease in coherence was greatest for the highest frequency band for all interelectrode pairs. A spike focus was induced in all 6 animals following the application of bicuculline at the most posterior electrode of the cortical array. In 5 of the 6 animals spike propagation was delayed at the junction of HL and Fr1 cortices, where the greatest decrease in coherence was observed. In one animal spike activity never advanced across this region. These results demonstrate an association between intracortical coherence and the spatial propagation of spike activity. The function of short and long cortico-cortical pathways as mediators of both EEG coherence and cortical spike propagation are discussed.

Characteristics of picosecond coherent anti-Stokes Raman spectroscopy with mode-locked dye lasers in comparison with stimulated Raman gain

Time-resolved coherent anti-Stokes Raman scattering (CARS) has been performed with a double synchronously pumped mode-locked dye-laser system that is the same as had been used earlier for stimulated Raman gain (SRG) experiments; the CARS signal is also spectrally resolved. We present an extensive experimental and theoretical study of the CARS technique and compare the results with the characteristics of SRG. Experimental tests have been performed on the 656- and 991-cm−1 modes in liquid CS2 and C6H6, respectively. In particular, the influence of the pulse chirps on the time resolutions in CARS and SRG has been examined. SRG profits from oppositely chirped dye-layer pulses. In these circumstances, the observed time resolution in SRG is typically a factor of 3 better than in CARS. Also, when measured under off-resonance conditions, CARS decay curves exhibit a narrow and pronounced coherence spike that is symmetrical near zero delay: its appearance is discussed in detail. In addition, we establish the conditions for which the phenomena, of critical frequency mistuning and spectral narrowing in time-delayed CARS are observable.

Pulse distortion in GaAs quantum wells studied by a light-gating technique.

The pulse distortion in GaAs quantum wells was studied by means of a light-gating technique. In the experiment, we used the incoherent picosecond pulse to observe accurately the changes of the coherent spike and its surrounding envelope. The experimental results clearly showed that the pulse is not anomalously delayed but is strongly distorted around the exciton resonance. We also compared the results with a computational simulation based on the absorption saturation mechanism. The simulated results agreed well with experimental results. The simulation showed that the recovery time of the exciton saturation is 90 ps around the tail part of the exciton absorption. We also observed a reduction of the coherent spike around the exciton resonance. Theoretical calculations indicate that the coherent-propagation effect in the linear absorber causes the reduction of the coherent spike.

High-resolution optical probing by means of long laser pulses

The purpose of this article is to discuss techniques and present data related to the performance of optical probing of the thickness of thin transparent materials and of the dimensions of mechanical components to a few-microns resolution. The experimental apparatus for high-resolution optical probing is based on nonlinear autocorrelation of partially mode-locked laser pulses. Using the presence of the coherent spike in the autocorrelation trace as a marker, a resolution of the order of a few microns is obtained independently of the length of the pulse. The experimental apparatus consists of a rhodamine 6G dye laser synchronously pumped by the second harmonic of a cw mode-locked Nd:YAG laser. The picosecond pulse train is sent to an optical autocorrelator similar to that used to measure pulse duration. To show the precision and versatility of the technique, applications are presented to the measurement of the thickness of transparent materials (plastic and liquid films), and to the measurement of dimensions of mechanical components.

High resolution measurement of jet stream thickness by optical ranging

A non-contact, high resolution (2 μm) measurement of jet stream thickness has been performed using an optical ranging technique. By measuring the coherent spike displacement of the non-linear autocorrelation function of partially mode-locked picosecond light pulses, a complete characterization of jet stream profiles has been achieved.

Coherent propagation effect of incoherent light

The coherent propagation phenomenon of incoherent light through a resonant medium was experimentally demonstrated in the subpicosecond time region, using Na atoms and the light from an imperfectly mode-locked cw dye laser. The coherence spike with a 0.35-psec width in the cross-correlation profile between the propagated incoherent light and the initial light develops the same specific oscillatory structure as seen in the propagation of a weak coherent subpicosecond pulse. A theoretical analysis was also performed, and the experimental results are in considerably good agreement with the calculated ones.