Femtosecond pulse shaping

A detailed analysis of the relationship between the duration of the chirped probe pulse and the bipolar terahertz (THz) pulse length in the spectral encoding technique is carried out. We prove that there is an optimal chirped probe pulse length (or an optimal chirp rate of the chirped probe pulse) matched to the input THz pulse length and derive a rigorous relationship between them. We find that only under this restricted condition the THz signal can be correctly retrieved.

We propose and demonstrate a robust control scheme by ultrafast nonadiabatic chirped laser pulse, designed for targeting coherent superpositions of two-level systems. Robustness against power fluctuation is proved by our numerical study and a proof-of-principle experiment performed with femtosecond laser interaction on cold atoms. They exhibit for the final driven dynamics a cusp on the Bloch sphere, corresponding to a zero curvature of fidelity. This solution is particularly simple and thus applicable to a wide range of potential applications.

Graphene is at the center of a significant research effort for ultrafast photonics due to its unique optical properties. Here, we demonstrated the generation of stretched and soliton femtosecond mode-locking pulses in an erbium doped fiber laser (EDFLs) by using graphene saturable absorber and managing the net cavity dispersion. The novelty of this work arises due to the simple fabrication of the graphene SA and the realization of two types of modelocking pulse by manipulating the cavity dispersion. At total cavity dispersion of -0.028 ps 2 , stretched pulses train was successfully obtained. The laser has a pulse width of 750 fs at repetition rate of 35.1 MHz and pulse energy of 0.054 nJ at maximum output power of 1.9 mW. By varying the net cavity dispersion so that the anomalous dispersion has been achieved with total dispersion of -0.3 ps 2 , soliton mode locked pulse train was successfully obtained. The laser has a pulse width of 820 fs at repetition rate of 11.5 MHz and pulse energy of 0.42 nJ at output power of 4.85 mW. These results make the proposed EDFLs suitable for applications in optical communications, metrology, environmental sensing, and biomedical diagnostics.

We propose a scheme to generate broadband high harmonic supercontinuum and single attosec ond pulse emission in time domain by employing a chirped polarization gating laser field. It is shown that this scheme can significantly extend the high harmonic cutoff to higher energy region, and the produced isolated attosecond pulse is shorter than 100 attoseconds after proper dispersion compensation, when a 7 fs driving laser pulse is used.

We propose and demonstrate a new approach, to the best of our knowledge, for avoiding nonlinear effects in the amplification of ultrashort optical pulses. The initial pulse is divided longitudinally into a sequence of lower-energy pulses that are otherwise identical to the original, except for the polarization. The lowintensity pulses are amplified and then recombined to create a final intense pulse. This divided-pulse amplification complements techniques based on dispersion management.

We report on a simple and robust technique to temporally shape ultrashort pulses. A number of birefringent crystals with appropriate crystal length and orientation form a crystal set. When a short pulse propagates through the crystal set, the pulse is divided into numerous pulses, producing a desired temporal shape. Flexibility in the final pulse shape is achieved through varying initial pulse duration, divided-pulse number, the polarization-mode delay, and energy distribution of the divided pulses. The energy efficiency of the technique is near 100% for a pulse train of alternating polarizations, and 50% for a linearly polarized pulse train.

The generation of flat top pulse with 10 ps duration at 266 nm, is of extreme importance to achieve low emittance electron beam from a photoinjector. The production of this pulse shape is carried out using manipulation in the frequency domain with the aid of new programmable pulse shapers. In this paper we present the experimental measurements we performed to characterize two pulse shapers: the acousto-optics filter, namely the DAZZLER, and the liquid crystal mask spatial light modulator. The measurement have been performed with the amplified Ti:Sa laser system used to drive the SPARC photoinjector at LNF-INFN. The results obtained, the operation and the main limitations of the two techniques are also presented.

Natively, atomic and molecular processes develop in a sub-femtosecond time scale. In order to, for instance, track and capture the electron motion in that scale we need suitable 'probes'. Attosecond pulses configure the most appropriate tools for such a purpose. These ultrashort bursts of light are generated when a strong laser field interacts with matter and high-order harmonics of the driving source are produced. In this work, we propose a way to twist attosecond pulse trains. In our scheme, each of the attosecond pulses in the train has a well-defined linear polarization, but with a different polarization angle between them. To achieve this goal, we consider an infrared pulse with a particular polarization state, called amplitude polarization. This kind of pulse was experimentally synthesized in previous works. Our twisted attosecond pulse train is then obtained by nonlinear driving an atomic system with that laser source, through the high-order harmonics generation phenomenon. We reach a high degree of control in the polarization of the ultrashort coherent XUV-generated radiation. Through quantum mechanical simulations, supplemented with signal processing tools, we are able to dissect the underlying physics of the generation process. We are confident these polarized-sculpted XUV sources will play an instrumental role in future pump-probe-based experiments. Classical electrodynamics and quantum mechanics are the fundamental building blocks for the description of many natural phenomena. By measuring the wavelength and speed of light, electromagnetism provides us with tools to extract the velocity of the light field oscillations. Likewise, quantum mechanics links the rapidity of electronic motion with the energy distribution of the populated quantum states. By adequately tuning the light sources, these states can be accessed by photon absorption and emission. The native scale of both the light oscillations and electron dynamics falls within the attosecond range. These elementary processes comprise the constitutional steps of any change in the physical, chemical, and biological properties of materials and soft matter. The capability of capturing and manipulating them in real-time is therefore relevant for the development of novel materials and technologies, as well as the understanding of fundamental atomic and molecular phenomena initiated by light fields [1, 2]. Since the first measurement of an attosecond pulse train (APT) [3], attosecond science has grown enormously, from the obtaining of an isolate attosecond pulse (IAP) [4] to the

A femtosecond pulse characterization technique is developed using frequency-resolved optical gating based on the two-photon absorption in an InP crystal. The technique provides direct visual monitoring of pulse frequency chirping in the time-spectral domain for femtosecond pulses with a pulse energy as low as 3.8 pJ. Pulse chirping and pedestal wings of 10-GHz optical fiber solitons are characterized experimentally for the first time by this technique. 2000 Optical Society of America OCIS codes: (320.7100) Ultrafast measurements; (190.4180) Multiphoton processes

A comprehensive model of processes involved in femtosecond laser inscription and the subsequent structural material modification is developed. Different time scales of the pulse-plasma dynamics and thermo-mechanical relaxation allow for separate numerical treatments of these processes, while linking them by an energy transfer equation. The model is illustrated and analysed on examples of inscription in fused silica and the results are used to explain previous experimental observations.

Impact of such terms as third order dispersion, self-steepening and stimulated Raman scattering on evolution of ultrashort pulses is considered in detail. Under influence of these effects, pulse did not maintain its initial shape. Pulse splits into constituents, its spectrum also evolving into several bands which are known as optical shock and self-frequency shift phenomena. We concluded that when the input peak power is large enough, dynamics of pulse splitting will be complicated. Our numerical simulations were in good agreement with experimental results.

A technique is proposed for generating sub-10 fs deep ultraviolet (DUV) pulses with Gaussian temporal and spectral profiles. In this approach, a broadband DUV pulse with a negative frequency chirp is generated and compressed by normal group-velocity dispersion in a transparent medium. The pulse shape is not significantly distorted by high-order spectral phase distortion. In principle, nearly transform-limited sub-6 fs UV pulses with pulse energies of several tens of microjoules are expected to be generated. Numerical simulations indicate that a Ti:sapphire chirped-pulse amplifier with a pulse energy of several millijoules and a pulse duration of 35 fs can be used to generate high-energy sub-6 fs UV pulses. The technique is partially demonstrated by an experiment using a low-power Ti:sapphire chirped-pulse amplifier to generate low-energy sub-10 fs single DUV pulses with waveforms that quantitatively agree with those predicted by the numerical simulations. This technique can be used to generate high-energy sub-10 fs DUV pulses with Gaussian temporal and spectral profiles for ultrafast spectroscopy in the gas phase as well as the liquid and solid phases.

Our recent experimental results of three methods related to and useful for the generation of attosecond pulses are summarized. The pulses obtained by all of them have high qualities in terms of phase, temporal, spectral and spatial properties which are based on the physical principles associated with the parametric processes. First, carrier-envelope phase (CEP) stable sub-5 fs and sub-3 fs pulses by non-collinear optical parametric amplification (NOPA) in the near-infrared and visible spectral range will be described. The mechanism of the passive CEP stabilization is described. Passively stabilized idler and its second harmonic (SH) pulses from NOPAs are compressed to sub-5fs and sub-3fs, respectively. Compression of the idler output from a NOPA and its SH is attained with a specially designed characterization method during the compression. Second, generation of multicolour pulses by the cascaded four-wave mixing process in bulk media is discussed. As short as 15-fs multicoloured femtosecond pulses are obtained with two ∼40 fs pulses incident to a fused-silica glass plate by this method. These broadband multicolour sidebands are expected to provide single-cycle or sub-fs pulses after the Fourier synthesis. Third, a new technique based on self-diffraction in the Kerr medium is used to clean and shorten the femtosecond laser pulse. The cleaned pulse with high temporal contrast is expected to be used as a seed for a background-free petawatt laser system and then used as the laser source for high-energy attosecond pulse generation in a solid target. The mechanisms of CEP stabilization, pulse spectral smoothening and pulse contrast enhancement are comparatively discussed.

An all-reflective, simple noncollinear second harmonic (SH) autocorrelator is described for monitoring the shot-to-shot behavior of ultrashort high-power laser pulses. Two mirrors are used for the dispersion-free splitting of a pulse into two halves. One of the mirrors is able to adjust the delay time and angle between two halves of the laser pulse in a nonlinear crystal. We present the possibility of real-time measurement of the pulse duration, peak intensity (or energy), and the pointing jitters of a laser pulse, by analyzing the spatial profile of the SH autocorrelation signal measured by a CCD camera. The measurement of the shot-to-shot variation of those parameters will be important for the detailed characterization of laser accelerated electrons or protons.

I, Janelle Claire Shane, hereby certify that this thesis, which is approximately 24,000 words in length, has been written by me, that it is the record of work carried out by me and that it has not been submitted in any previous application for a higher degree. Date 9.5.09 Signature of Candidate I was admitted as a research student in August 2007 and as a candidate for the degree of Master of Philosophy in August 2007; the higher study for which this is a record was carried out in the University of St Andrews between 2007 and 2008. Date 9.5.09 Signature of Candidate I hereby certify that the candidate has fulfilled the conditions of the Resolution and Regulations appropriate for the degree of Master of Philosophy in the University of St Andrews and that the candidate is qualified to submit this thesis in application for that degree. Date 19.5.09 Signature of Supervisor In submitting this thesis to the University of St Andrews we understand that we are giving permission for it to be made available for use in accordance with the regulations of the University Library for the time being in force, subject to any copyright vested in the work not being affected thereby. We also understand that the title and the abstract will be published, and that a copy of the work may be made and supplied to any bona fide library or research worker, that my thesis will be electronically accessible for personal or research use unless exempt by award of an embargo as requested below, and that the library has the right to migrate my thesis into new electronic forms as required to ensure continued access to the thesis. We have obtained any third-party copyright permissions that may be required in order to allow such access and migration, or have requested the appropriate embargo below. The following is an agreed request by candidate and supervisor regarding the electronic publication of this thesis: Access to Printed copy and electronic publication of thesis through the University of St Andrews.

Single 9.6 fs deep ultraviolet pulses with a spectral range of 255-290 nm are generated by a chirpedpulse four-wave mixing technique for use as pump and probe pulses. The electronic excited state and vibrational dynamics are simultaneously observed for an aqueous solution of uracil and thymine over the full spectral range using a 128-channel lock-in amplifier detector. Two probe photon energy-dependent lifetimes gradually increasing with the probe photon energy are obtained from the decay dynamics data. Ultrafast decay dynamics through the conical intersection is assigned from the first excited pp* to the final ground state involving the np* states. Vibrational modes of the electronic ground state and excited states can be observed, which are strongly coupled to the decay dynamics of the electronic excited state.

Four-wave mixing (FWM) was used to generate and optimize ultrashort pulses with wavelengths from ultraviolet (UV) to near-infrared in bulk media and gases. Wavelength-tunable multicolored pulses were simultaneously generated by cascaded FWM in glass plates. By using incident chirped pulses, 15-fs selfcompressed multicolored pulses were obtained with excellent properties. Self-compression was also used to generate ultrashort deep-UV (DUV) pulses. The positive frequency chirp in a self-phasemodulated pulseissuitable for this objective. Sub-10-fs DUV pulses were generated without using any additional pulse compressors. Self-diffraction was used to clean the pulse, broaden the spectrum, and enhance the spatial beam quality. These multicolored pulses can be simultaneously amplified and compressed by a four-wave optical parametric amplifier. The generated multicolored pulses are useful for multicolored ultrafast spectroscopy, microscopy experiments, and as seeds for petawatt lasers with high temporal contrasts.

The interferometric autocorrelation technique for the measurement of ultrashort pulse durations is studied in detail. Effects of group velocity mismatch, group velocity dispersion, fundamental depletion, and pulse shape are carefully examined. A simple semianalytical calculation is developed that takes group velocity mismatch into account that can be used to predict the validity of this technique with real experimental parameters. A more complete calculation is also presented to analyze the effects of fundamental depletion or phase mismatch. Finally, the influence of the pulse shape is considered and a simple experimental procedure is proposed to determine whether a pulse is transform limited.

Using an acousto-optically mode-locked chromium-doped forsterite laser, operated at 77 K and coupled to a nonlinear resonator containing a single-mode fiber, we have produced femtosecond pulses of 150-fs duration at 1.23 gm with useful output powers of approximately 60 mW This represents what is to our knowledge the first demonstration of femtosecond pulse generation from this laser system using the coupled-cavity mode-locking scheme.

A detailed numerical analysis of heavily chirped pulses in the positive-dispersion regime (PDR) is presented on the basis of the distributed cubic-quintic generalized complex nonlinear Ginzburg-Landau equation. It is demonstrated that there are three main types of pulse spectra: truncated parabolictop, -and M-shaped profiles. The strong chirp broadens the pulse spectrum up to 100 nm for a Ti : Sa oscillator, which provides compressibility of the picosecond pulse down to sub-30 fs. Since the picosecond pulse has a peak power lower than the self-focusing power inside a Ti : Sa crystal, the microjoule energies become directly available from a femtosecond oscillator. The influence of the third-and fourth-order dispersions on the pulse spectrum and stability is analysed. It is demonstrated that the dynamic gain saturation plays an important role in pulse stabilization. The common action of dynamic gain saturation, self-amplified modulation (SAM) and saturation of the SAM provides pulse stabilization inside the limited range of the positive group-delay dispersions (GDDs). Since the stabilizing action of the SAM cannot be essentially enhanced for a pure Kerrlens mode-locking regime, a semiconductor saturable absorber is required for

Previous investigations on "double-pulse" nanosecond (ns) laser drilling reported in the literature typically utilize double pulses of equal or similar pulse energies. In this paper, "double-pulse" ns laser drilling using double pulses with energies differing by more than ten times has been studied, where both postprocess workpiece characterizations and in situ time-resolved shadowgraph imaging observations have been performed. A very interesting physical phenomenon has been discovered under the studied conditions: the "double-pulse" ns laser ablation process, where the low-energy pulse precedes the highenergy pulse (called "low-high double-pulse" laser ablation) by a suitable amount of time, can produce significantly higher ablation rates than "high-low double-pulse" or "single-pulse" laser ablation under a similar laser energy input. In particular, "low-high double-pulse" laser ablation at a suitable interpulse separation time can drill through a $0.93 mm thick aluminum 7075 workpiece in less than 200 pulse pairs, while "high-low double-pulse" or "single-pulse" laser ablation cannot drill through the workpiece even using 1000 pulse pairs or pulses, respectively. This indicates that "low-high doublepulse" laser ablation has led to a significantly enhanced average ablation rate that is more than five times those for "single-pulse" or "high-low double-pulse" laser ablation. The fundamental physical mechanism for the ablation rate enhancement has been discussed, and a hypothesized explanation has been given.

Propagation of laser pulses of femtosecond time duration (focused through a focusing lens inside the crystalline lens) has been investigated in this paper. Transverse beam diffraction, group velocity dispersion, graded refractive index structure of the crystalline lens, self-focusing, and photodisruption in which plasma is formed due to the high intensity of laser pulses through multiphoton ionization have been taken into account. The model equations are the modified nonlinear Schrödinger equation along with a rate equation that takes care of plasma generation. A close analysis of model equations suggests that the femtosecond laser pulse duration is critical to the breakdown in the lens. Our numerical simulations reveal that the combined effect of self-focusing and multiphoton ionization provides the breakdown threshold. During the focusing of femtosecond laser pulses, additional spatial pulse splitting arises along with temporal splitting. This splitting of laser pulses arises on a...

Pulse stacking is an effective method to generate a long-shaped pulse from short pulses. In this paper, we study all-fiber coherent pulse stacking systematically; we show that the time delays and phase differences between the short pulses are the key parameters of the stacked pulse. The permitted variation of the time delay and phase difference are obtained. The ability of the stacker to produce arbitrary pulse shapes is discussed.

The effect of temporal profile of an ultra-intense, ultrashort few-cycle laser pulse incident normally on a solid surface on harmonics generation (HHG) and attosecond pulse trains has numerically been studied. The results of transformation of initially narrow spectrum of the laser pulse into a broad harmonic spectrum are obtained. The reflected signal has the form of a train of ultrashort pulses of duration ~100 attoseconds for the case of a Ti:sapphire laser. The effect of collisions on electron dynamics has been taken into account.

The effect of temporal profile of an ultra-intense, ultrashort few-cycle laser pulse incident normally on a solid surface on harmonics generation (HHG) and attosecond pulse trains has numerically been studied. The results of transformation of initially narrow spectrum of the laser pulse into a broad harmonic spectrum are obtained. The reflected signal has the form of a train of ultrashort pulses of duration ~100 attoseconds for the case of a Ti:sapphire laser. The effect of collisions on electron dynamics has been taken into account.

Ultrashort-laser-pulse retrieval in frequency-resolved optical gating has previously required an iterative algorithm. Here, however, we show that a computational neural network can directly and rapidly recover the intensity and phase of a pulse.

We present a numerical model for the interaction of a femtosecond probe pulse with a Raman medium excited by two nanosecond laser pulses in solid hydrogen. The model consists of a solution of a system of Bloch equations ͑for the medium state͒ and of the two-dimensional wave equation ͑for the field propagation͒. It is shown that by optimizing the tilt angle between the two driving pulses and the interaction length, the power of the probe pulse can be converted either into a quasicontinuous broadband spectrum or into a selected high-order sideband. The first situation can be used for generation of subfemtosecond pulses. The second situation can be used for the wavelength conversion of a femtosecond probe pulse from the infrared to the ultraviolet region. Due to the transfer of energy from the driving pulses, the energy conversion efficiency of these processes ͑with respect to the probe pulse͒ can be larger than unity.

A novel interferometry technique is presented by which, in one shot, one can measure phase changes with a resolution of tens of femtoseconds while extending the measurement over picoseconds or even longer. The method is based on spectral (frequency-domain) interferometry with a pair of linearly chirped pules as probes. With this technique we obtained single-shot measurements of the rapid phase changes induced by optical f ield ionization of air. This allowed us to calculate the time prof ile of the electron density created by an intense short laser pulse.

A new technique to measure the frequency-resolved temporal image of a single ultrashort light pulse is presented. This technique is based on a single-shot self-diffraction autocorrelator in which the pulse duration is related to the spatial width of a diffracted beam, the pulse spectrum is related to its divergence, and the pulse chirp appears as a curvature of the diffracted-beam wave front. An analysis of wave-front curvature by means of a slit diaphragm and a cylindrical lens results in a time -frequency image of the initial pulse when a spectrometer is not used. This image gives a simple intuitive picture of the chirp and the phase modulation of a pulse.

We have experimentally improved the temporal contrast of the petawatt J-KAREN-P laser facility. We have investigated how the generation of pre-pulses by post-pulses changes due to the temporal overlap between the stretched pulse and the post-pulse in a chirped-pulse amplification system. We have shown that the time at which the pre-pulse is generated by the post-pulse and its shape are related to the time difference between the stretched main pulse and the post-pulse. With this investigation, we have found and identified the origins of the pre-pulses and have demonstrated the removal of most pre-pulses by eliminating the post-pulse with wedged optics. We have also demonstrated the impact of stretcher optics on the picosecond pedestal. We have realized orders of magnitude enhancement of the pedestal by improving the optical quality of a key component in the stretcher.

Diode-pumped femtosecond chirped pulse regenerative amplifiers based on Yb3+-materials are of practical importance for wide range of scientific, industrial and biomedical applications. The aim of this work was to study the amplification of broadband chirped femtosecond pulses in regenerative amplifier based on Yb3+:CaYAlO4crystal.Such systems use femtosecond mode-locked lasers as seed pulse sources and amplify nJ-seed pulses to sub-mJ energy range. Most chirped pulse regenerative amplifier systems described in the literature use seed lasers with typical pulse spectral width at the level of 10–15 nm full width at half maximum (FWHM) that limit the seed pulse duration of about 90 fs and amplified pulse duration at the level of 200 fs due to strong influence of gain narrowing effect on the amplified pulse parameters. Yb3+-doped crystals with wide and smooth gain bandwidth as an active medium of chirped femtosecond pulse regenerative amplification systems allow to reduce negative contri...

A number of factors that influence spectral position of ultrashort pulses in mode-locked lasers have been identified: high-order dispersion, gain saturation, reabsorption from the ground state, and stimulated Raman scattering. Using the one-dimensional numerical model for the simulation of the laser cavity, we analyze the relative contributions of different factors to the spectral position of the mode-locked pulses using the example of the Cr:LiSGaF laser. In this case the Raman effect provides the largest self-frequency shift from the gain peak (up to 60 nm), followed by the gain saturation (ϳ25 nm), whereas the high-order dispersion contribution is insignificant (ϳ5 nm). The results of the simulation are in good agreement with experimental data, confirming that stimulated Raman scattering is the dominant mechanism that causes the pulse self-frequency shift.

A detailed numerical analysis of heavily chirped pulses in the positive-dispersion regime (PDR) is presented on the basis of the distributed cubic-quintic generalized complex nonlinear Ginzburg-Landau equation. It is demonstrated that there are three main types of pulse spectra: truncated parabolictop,-and M-shaped profiles. The strong chirp broadens the pulse spectrum up to 100 nm for a Ti : Sa oscillator, which provides compressibility of the picosecond pulse down to sub-30 fs. Since the picosecond pulse has a peak power lower than the self-focusing power inside a Ti : Sa crystal, the microjoule energies become directly available from a femtosecond oscillator. The influence of the third-and fourth-order dispersions on the pulse spectrum and stability is analysed. It is demonstrated that the dynamic gain saturation plays an important role in pulse stabilization. The common action of dynamic gain saturation, self-amplified modulation (SAM) and saturation of the SAM provides pulse stabilization inside the limited range of the positive group-delay dispersions (GDDs). Since the stabilizing action of the SAM cannot be essentially enhanced for a pure Kerrlens mode-locking regime, a semiconductor saturable absorber is required for

We track the dynamic response of GaAs quantum-well semiconductor diode lasers after injection of a femtosecond pulse, using ultrafast upconversion. The continuous-wave emission shows gain dynamics and relaxation oscillations on timescales of 1–100 ps. The recirculating femtosecond pulse evolves into an ultrafast “dark pulse” in the wake of subpicosecond oscillations.

We introduce a non-interferometric single beam method for automated spectral phase characterization and adaptive pulse compression of amplified ultrashort femtosecond pulses taking advantage of third order harmonic generation in air. This new method, air-MIIPS, compensates high-order phase distortions based on multiphoton intrapulse interference phase scan (MIIPS).

We investigate the effects of pulse duration on optical trapping with high repetition rate ultrashort pulsed lasers, through Lorentz-Mie theory, numerical simulation, and experiment. Optical trapping experiments use a 12 femtosecond duration infrared pulsed laser, with the trapping microscope's temporal dispersive effects measured and corrected using the Multiphoton Intrapulse Interference Phase Scan method. We apply pulse shaping to reproducibly stretch pulse duration by 1.5 orders of magnitude and find no material-independent effects of pulse temporal profile on optical trapping of 780nm silica particles, in agreement with our theory and simulation. Using pulse shaping, we control two-photon fluorescence in trapped fluorescent particles, opening the door to other coherent control applications with trapped particles.

We have developed efficient multipass Ti:sapphire amplifiers for femtosecond chirped-pulse amplification. With only two of these devices we get an amplification factor of 108, which corresponds to a peak power of-0.5 TW after compression. We present a detailed analysis of such a system and its advantages in terms of its high quantum yield (0.3), flexibility, optical quality, and potential for tunability.

The space-time Wigner function is a powerful tool for the analysis of femtosecond optical devices that act on both the spatial and the temporal features of ultrashort pulses. The advantages of this approach and the properties of the space-time Wigner function are reviewed. The matrix formalism that can be used to describe the action of dispersive first-order devices on any femtosecond pulse is presented. The method is used to describe and understand the behavior of an ultrashort light pulse propagating through a pulse shaper. The consequences of the limited resolution in the spectral-to-spatial conversion for the output pulse are fully analyzed.

Ultrafast optical diagnostics play a vital role in probing the dynamics in laser-matter interactions, including those observed in the high intensity ultrashort laser pulse regime. We developed a new femtosecond optical diagnostic, single-shot supercontinuum spectral interferometry (SSSI), which measures ultra-rapid transients in the complex index of refraction induced by an intense laser pulse. This measurement provides a direct view on how the laserproduced perturbations evolve in time and space. To date, SSSI has been successfully used to diagnose femtosecond dynamics in the interaction of intense laser pulses with gases, nanometersized atomic or molecular clusters, and plasmas, including plasma waveguides.

Trains of ultrashort laser pulses separated by the time of rotational revival (typically, tens of picoseconds) have been exploited for creating ensembles of aligned molecules. In this work we introduce a chiral pulse train-a sequence of linearly polarized pulses with the polarization direction rotating from pulse to pulse by a controllable angle. The chirality of such a train, expressed through the period and direction of its polarization rotation, is used as a new control parameter for achieving selectivity and directionality of laser-induced rotational excitation. The method employs chiral trains with a large number of pulses separated on the time scale much shorter than the rotational revival (a few hundred femtosecond), enabling the use of conventional pulse shapers.

We propose a new method of compressing laser pulses to ultrahigh powers based on spatially varying dispersion of an inhomogeneous plasma. Here, compression is achieved when a long, negatively frequency-chirped laser pulse reflects off the density ramp of an over-dense plasma slab. As the density increases longitudinally, high-frequency photons at the leading part of the laser pulse penetrate more deeply into the plasma region than lower-frequency photons, resulting in pulse compression in a similar way to that by a chirped mirror. Proof-of-principle simulations performed using particle-in-cell simulation codes predict compression of a 2.35 ps laser pulse to 10.3 fs—a ratio of 225. As plasma is robust and resistant to damage at high intensities—unlike solid-state gratings commonly used in chirped-pulse amplification—the method could be used as a compressor to reach exawatt or zettawatt peak powers.