Counter-propagating Pulses Research Articles

INTERFERENCE INTERACTION OF COUNTER-PROPAGATING PULSES ON A MAGNETO-DIELECTRIC SLAB

Dynamicsofinterferenceinteractionofcounter-propagating electromagnetic pulses on a magneto-dielectric slab is studied in time domain. Energy redistribution in the counter-propagating pulses with arbitrary waveforms is considered. The maximal energy redistribu- tion in the difiracted fleld takes place under certain conditions. The conditions are found and their physical explanation is supplied. The problem of transient electromagnetic wave difiraction on homogeneous magneto-dielectric slab is solved analytically by means of Laplace transform. The analytical solution is in agreement with numerical simulation based on flnite difierence time domain approach.

Spatial and Spectral Coherent Control over Direct Frequency Comb Excitation

We demonstrate a method for spatial control over two-photon excitation, by using counter-propagating frequency comb pulse trains. This type of control is shown to enable significant improvement for direct frequency comb spectroscopy.

Spatial and spectral coherent control with frequency combs

Quantum coherent control (1-3) is a powerful tool for steering the outcome of quantum processes towards a desired final state, by accurate manipulation of quantum interference between multiple pathways. Although coherent control techniques have found applications in many fields of science (4-9), the possibilities for spatial and high-resolution frequency control have remained limited. Here, we show that the use of counter-propagating broadband pulses enables the generation of fully controlled spatial excitation patterns. This spatial control approach also provides decoherence reduction, which allows the use of the high frequency resolution of an optical frequency comb (10,11). We exploit the counter-propagating geometry to perform spatially selective excitation of individual species in a multi-component gas mixture, as well as frequency determination of hyperfine constants of atomic rubidium with unprecedented accuracy. The combination of spectral and spatial coherent control adds a new dimension to coherent control with applications in e.g nonlinear spectroscopy, microscopy and high-precision frequency metrology.

Photon–photon scattering in collisions of intense laser pulses

A scenario for measuring the predicted processes of vacuum elastic photon–photon scattering and four-wave mixing with intense modern lasers is investigated. The numbers of measurable scattered photons are calculated for the collision of two, Gaussian-focused, pulsed lasers. It is shown that a single intense 10 PW optical laser beam split into two counter-propagating pulses is sufficient for measuring the elastic process. Moreover, when these pulses are sub-cycle, by also considering the collision of two sech pulses, the results suggest that the frequency-shifting, four-wave mixing process should be measurable too.

Quasi-phase-matching high harmonic generation using trains of pulses produced using an array of birefringent plates

Quasi-phase-matched high harmonic generation using trains of up to 8 counter-propagating pulses is explored. For trains of up to 4 pulses the measured enhancement of the harmonic signal scales with the number of pulses N as (N + 1)², as expected. However, for trains with N > 4, no further enhancement of the harmonic signal is observed. This effect is ascribed to changes of the coherence length Lc within the generating medium. Techniques for overcoming the variation of Lc are discussed. The pressure dependence of quasi-phase-matching is investigated and the switch from true-phase-matching to quasi-phase-matching is observed.

Interaction of counterpropagating few cycle light pulses in nonlinear dielectric media and radiation generation at combination frequencies in this process

The regularities of the interaction of counterpropagating optical pulses of only a few oscillations of the light field in nonlinear dielectric media are studied theoretically. The temporal structure of the pulse field, due to its interaction with the counterpropagating pulse, is shown to be varied such that the field zeros are the most shifted in time and its extreme values are not shifted. The center of mass of the spectral density is shifted to the short-wavelength region. The efficiency of these phenomena is determined by the energy of the counterpropagating pulse and does not depend on its duration and spectral composition. The possibility of wave generation at tripled and combination frequencies is demonstrated for the media whose size is less than the interaction region. The efficiency of this generation with respect to the phase mutual modulation is shown to be increasing with a decrease in the number of oscillations in colliding pulses.

Femtosecond pulses in a dense two-level medium: Spectral transformations, transient processes, and collisional dynamics

Propagation of ultrashort optical pulses in a dense resonant medium is considered in the semiclassical limit. In our analysis, we place emphasis on several main points. First, we study transformations of spectra in the process of pulse propagation and interactions with another pulse. The second point involves the transient processes (including pulse compression) connected with self-induced transparency soliton formation inside the medium. Finally, the third aspect is the study of collisions of co- and counter-propagating pulses in the medium. In the last case, the investigation of symmetric and asymmetric collisions shows the possibility of effectively controlling the parameters of transmitted radiation.

All-optical processing of RZ-DPSK signals using counter-propagating pulses in a saturated SOA

We demonstrate three novel all-optical signal processing methods for RZ-DPSK signals using a saturated SOA. The two logically opposite pulse streams from a demodulated noisy RZ-DPSK signal counter-propagate in the SOA. This configuration decreases the fluctuations in the demodulated marks while preventing the spaces from getting amplified. Photonic balancing provides functionality analogous to electrical balanced detection by reducing the required OSNR, and saturated asymmetric filtering removes the in-band ASE noise before detection. These methods are demonstrated experimentally by observing eye diagrams and BER measurements at 10.7 Gb/s and 22.3 Gb/s.

High-power diode-directly-pumped tenth-order harmonic mode-locked TEM00 Nd:YVO4 laser with 1 GHz repetition rate

A high-efficiency high-power diode-directly-pumped tenth-order harmonic mode-locked TEM00 Nd:YVO4 laser with 1 GHz repetition rate was first demonstrated. The maximum output power was 10.4 W with optical-optical efficiency of 41.8% and slope efficiency of 78.1%, respectively, the pulse width was about 30 ps at the output power of 9.6 W. Based on the large third-order nonlinearity of Nd:YVO4, the tenth-order harmonic mode-locked pulses were induced by the intensity-dependent Kerr effect and the cooperative action of counter-propagating pulses colliding in the laser crystal for a colliding-pulse-modelocking-like cavity. The pulses were further modulated by a semiconductor saturable absorber mirror.

Swarm of ultra-high intensity attosecond pulses from laser-plasma interaction

We report on the realistic scheme of intense X-rays and γ-radiation generation in a laser interaction with thin foils. It is based on the relativistic mirror concept, i.e., a flying thin plasma slab interacts with a counterpropagating laser pulse, reflecting part of it in the form of an intense ultra-short electromagnetic pulse having an up-shifted frequency. A series of relativistic mirrors is generated in the interaction of the intense laser with a thin foil target as the pulse tears off and accelerates thin electron layers. A counterpropagating pulse is reflected by these flying layers in the form of a swarm of ultra-short pulses resulting in a significant energy gain of the reflected radiation due to the momentum transfer from flying layers.

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.

Relaxed damage threshold intensity conditions and nonlinear increase in the conversion efficiency of an optical parametric oscillator using a bi-directional pump geometry

A novel bi-directional pump geometry that nonlinearly increases the nonlinear optical conversion efficiency of a synchronously pumped optical parametric oscillator (OPO) is reported. This bi-directional pumping method synchronizes the circulating signal pulse with two counter-propagating pump pulses within a linear OPO resonator. Through this pump scheme, an increase in nonlinear optical conversion efficiency of 22% was achieved at the signal wavelength, corresponding to a 95% overall increase in average power. Given an almost unchanged measured pulse duration of 260 fs under optimal performance conditions, this related to a signal wavelength peak power output of 18.8 kW, compared with 10 kW using the traditional single-pass geometry. In this study, a total effective peak intensity pump-field of 7.11 GW/cm(2) (corresponding to 3.55 GW/cm(2) from each pump beam) was applied to a 3 mm long periodically poled lithium niobate crystal, which had a damage threshold intensity of 4 GW/cm(2), without impairing crystal integrity. We therefore prove the application of this novel pump geometry provides opportunities for power-scaling of synchronously pumped OPO systems together with enhanced nonlinear conversion efficiency through relaxed damage threshold intensity conditions.

Quantum entanglement via superradiance of a Bose-Einstein condensate

We adopt the coherence and built-in swap mechanism in sequential superradiance as a tool for obtaining continuous-variable (electric/magnetic fields) quantum entanglement of two counter-propagating pulses emitted from the two end-fire modes. In the first-sequence, end-fire modes are entangled with the side modes. In the second sequence, this entanglement is swapped to in between the two opposite end-fire modes. Additionally, we also examine the photon number correlations. No quantum correlations is observed in this variable.

Generation of attosecond X-ray pulses via Thomson scattering of counter-propagating laser pulses

AbstractIt is proposed that single attosecond pulses be generated via electron's Thomson scattering of two counter-propagating laser pulses. In the case of linear polarization, the generation of a single attosecond pulse is highly sensitive to the carrier envelope phase (CEP). However, in the case of circular polarization, it is completely independent on the CEP, which will make circular polarization favorable to generate a stable attosecond X-ray pulse. For either linear or circular polarization, the radiation obtained by using two counter-propagating pulses can be much more intense than that obtained by only using one of these two pulses.

Transition between Raman and Compton regimes in laser pulse amplification

AbstractAmplification of a short laser pulse by means of its interaction with a counter-propagating long pulse in a plasma has been suggested as a way of reaching high intensities. Two regimes have been discussed in the literature. The first is the Raman regime, where the process is backward Raman scattering, described by the standard equations for resonant three-wave coupling, while the second is the Compton regime, in which electron dynamics is dominated by the ponderomotive force generated by the high-frequency wave, and electrons behave as single particles rather than producing a Langmuir wave. Our aim here is to use a simple model of electron dynamics to investigate the transition between these regimes.

Strong monochromatic harmonics generated by the interaction of two counter-propagating pulses with a foil target

This paper studies the strong harmonic generation in the interaction of two conter-propagating pulses with a foil target. When the thin target is irradiated by high-contrast circularly polarized laser pulse, the light pressure push high-density plasma forward.At the same time, because of the role of charge separation field, the ion beam and electron beam bunch well in longitudinal direction, and generate the plasma mirror with a forward movement at relativistic speed. When incidenting on a probe laser pulse on the plasma mirror in the opposite direction, they produce strong monochromatic N-order harmonics by Doppler shift, with the new duration of pulses compressed to 1/N of the original ones. This paper also discusses the effect of the laser and plasma parameters on plasma mirror movement which influences the order of harmonics, and also the effect of the stability of the relativistic plasma mirror on the harmonics.

Ensemble of ultra-high intensity attosecond pulses from laser–plasma interaction

The efficient generation of intense X-rays and γ-radiation is studied. The scheme is based on the relativistic mirror concept, i.e., a flying thin plasma slab interacts with a counterpropagating laser pulse, reflecting part of it in the form of an intense ultra-short electromagnetic pulse having an up-shifted frequency. In the proposed scheme a series of relativistic mirrors is generated in the interaction of the intense laser with a thin foil target as the pulse tears off and accelerates thin electron layers. A counterpropagating pulse is reflected by these flying layers in the form of an ensemble of ultra-short pulses resulting in a significant energy gain of the reflected radiation due to the momentum transfer from flying layers.

Coherent x-ray production via pulse reflection from laser-driven dense electron sheets

A scheme to obtain brilliant x-ray sources by coherent reflection of a counter-propagating pulse from laser-driven dense electron sheets is theoretically and numerically investigated in a self-consistent manner. A radiation pressure acceleration model for the dynamics of the electron sheets blown out from laser-irradiated ultrathin foils is developed and verified by PIC simulations. The first multidimensional and integral demonstration of the scheme by 2D PIC simulations is presented. It is found that the reflected pulse undergoes Doppler-upshift by a factor 4γz2, where γz=(1- vz2/c2)-1/2 is the effective Lorentz factor of the electron sheet along its normal direction. Meanwhile the pulse electric field is intensified by a factor depending on the electron density of the sheet in its moving frame ne/γ, where γ is the full Lorentz factor.

Mechanisms of electron injection into laser wakefields by a weak counter-propagating pulse

Numerical studies are conducted on the electron injection into the first acceleration bucket of a laser wakefield by a weak counter-propagating laser pulse. It is shown that there are two injection mechanisms involved during the colliding laser interaction, the collective injection and stochastic injection. They are caused by the time-averaged ponderomotive force push and stochastic acceleration in the interfering fields, respectively. The threshold amplitude of the injection laser pulse is estimated for the occurrence of electron injection, which is close to that for stochastic acceleration and depends weakly upon the plasma density. The trapping of a large number of injection electrons can result in significant decay of the laser wakefield behind the first wave bucket.

Femtosecond pulse shaping using counter-propagating pulses in a semiconductor optical amplifier

An efficient pulse shaping method using counter-propagating pulses in the femtosecond regime is proposed and investigated. The effects of pump pulse power and pulsewidth on probe pulse are studied in counter-propagation scheme. It is shown that, with the proposed method, output probe pulse temporal and spectral peak shift due to femtosecond nonlinearities can be compensated, while the output pulse is amplified sufficiently. Furthermore, in relatively high power regime, the probe pulsewidth and time-bandwidth product are improved using counter-propagating pump pulse.