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We present a study of optically pumped laser emission from a microcavity containing an oriented layer of the liquid crystalline conjugated polymer poly(9,9-dioctylfluorene). The birefringence of the oriented polymer results in the definition of two cavity modes with electric vectors polarized perpendicular and parallel to the alignment direction. At high excitation density, the emission intensity from the cavity mode polarized parallel to the orientation direction increases rapidly compared to the orthogonal polarized mode and its linewidth narrows from 8 to 2 nm. We identify the parallel-polarized emission as laser action with a threshold excitation density of 7.8×1019 cm−3.

The interaction of a few-optical-cycle pulse with matter have shown novel phenomena [1]. Moreover, the high intensity which can be realized for a few semi-cycles of the laser optical-period may induce the tunnel ionization or even ionization due to barrier suppression. The resultant photoelectrons are driven in a coherent motion which influences both the above-threshold-ionization electron spectrum as well as the photon spectrum emitted due to recombination. In particular, in this work we analyze the condition of enhanced generation efficiency and brightness of extreme ultraviolet (XUV) radiation in the case of a 7 fs pump pulse on a jet of different gases.

Production of high-order harmonics excited by high-intensity laser pulses is a promising tool for the generation of coherent, extreme ultraviolet radiation. Properties of such radiation source are strongly affected by temporal and spatial behavior of the driving laser pulses. In this work we describe the generation of high-order harmonics using multi-cycle and few-cycle laser pulses; the nonlinear process is characterized as a function of focusing geometry and driving pulse duration; numerical simulations of harmonic generation are compared to experimental results, allowing to single out the unique properties that arise using driving pulses in the few-optical-cycle regime.

The dynamics of the ring-closure reaction of poly-1,2-bis(2-methylthien-3-yl)perfluorocyclopentene in chloroform solution is investigated. Ultrafast transient absorption spectra are measured using 260 nm pump pulses. It is shown that the optical switching occurs on a sub-200 fs timescale. The presence of a transient band around 450 nm, which disappears in ∼ 200 ps , is ascribed to excited-state absorption of the dithiophene moiety.

Low-divergence, high-brightness harmonic emission has been generated by using a fundamental beam with a truncated Bessel intensity profile. Such a beam is directly obtained by using the hollow-fiber compression technique, which indeed allows one to optimize both temporal and spatial characteristics of the high-order harmonic generation process. This is particularly important for the applications of radiation, where extreme temporal resolution and high brightness are required.

We present applications of high energy few optical cycle pulses to intense-field photoionization, pointing out the role of the absolute phase, and to the generation of high brightness high-order harmonics.

We present the design and characterization of a double-configuration grating monochromator for the spectral selection of extreme-ultraviolet ultrafast pulses. Two grating geometries are joined in an instrument with two interchangeable diffracting stages, both used at grazing incidence: one with the gratings in the off-plane mount (OPM), the other in the classical diffraction mount (CDM). The use of two stages gives great flexibility: the OPM stage is used for sub-50 fs time response and low spectral resolution, while the CDM stage is for 100-200 fs time response and high spectral resolution. The monochromator spectral and temporal performances have been experimentally demonstrated on a high-order laser-harmonics beam line.

Currently, the shortest laser pulses1 that can be generated in the visible spectrum consist of fewer than two optical cycles (measured at the full-width at half-maximum of the pulse's envelope). The time variation of the electric field in such a pulse depends on the phase of the carrier frequency with respect to the envelope—the absolute phase. Because intense laser–matter interactions generally depend on the electric field of the pulse, the absolute phase is important for a number of nonlinear processes2,3,4,5,6,7,8. But clear evidence of absolute-phase effects has yet to be detected experimentally, largely because of the difficulty of stabilizing the absolute phase in powerful laser pulses. Here we use a technique that does not require phase stabilization to demonstrate experimentally the influence of the absolute phase of a short laser pulse on the emission of photoelectrons. Atoms are ionized by a short laser pulse, and the photoelectrons are recorded with two opposing detectors in a plane perpendicular to the laser beam. We detect an anticorrelation in the shot-to-shot analysis of the electron yield.

The ultrafast relaxation dynamics of photoexcited carriers in Ge nanocrystals is here investigated using a pump-and-probe technique, Three different excitation energies (1.59, 3.18, 4.77 eV) are used, and the spectral interval between 1.7 and 2.8 eV is probed. The electron dynamics has been studied and the scattering times have been evaluated, together with their dependence on the pump energy. The transient signal has been interpreted in terms of two main contributions: (i) conduction band filling and (ii) band gap renormalization. A model based on the bulk band structure allows to explain the overall response.

Femtosecond pump-probe spectroscopy was used to directly monitor, in the time domain, energy transfer from the blue-emitting poly(9,9-dioctylfluorene) conjugated polymer to the red-emitting tetraphenylporphyrin molecule. This system is under investigation for use as saturated red light-emitting component in an organic full-color display. The differential transmission spectrum of the system evolves, within a few picoseconds after photoexcitation, from that of the host polymer to that of the guest molecule. Experiments performed at different acceptor concentrations provide clear evidence for a Forster energy transfer mechanism and allow a determination of the interaction radius. A value of 4.2±0.2 nm was found.

The potentiality of using sub-10 fs pulses for investigating molecular spectroscopy are discussed in relation to the fundamental issue still open in the field of optical properties and dynamics of linear conjugated chains.

We discuss the time-dependent photoinduced transmission spectra of a ladder-type oligophenyl, giving an estimation of its gain cross section. The amplified spontaneous emission observed in this material is also characterised.

Summary form only given. The recent availability of high-peak-power few-optical-cycle light pulses has opened the way to the investigation of light-matter interaction in the strong-field regime with unprecedented temporal resolution. In particular high-order harmonic generation in the few-optical-cycle regime is attractive for the generation of soft-x-ray pulses with duration down to the sub-femtosecond range. In this work we present both an experimental and a theoretical analysis of harmonic generation with sub-10-fs light pulses obtained using the hollow-fiber compression technique. To observe the harmonic emission we used a high-resolution x-ray spectrometer, based on a chirped grating and a high-resolution CCD detector.

The high-order harmonic generation process was studied using laser pump pulses of different duration, down to 7 fs. The time profile of this kind of pulse corresponds to a few optical cycles at the fundamental frequency, and therefore a cycle-to-cycle variation of the maximum amplitude occurs. These facts violate the adiabatic approximation of the generation process, giving rise to new aspects of the physical process. The comparison of results with pulses in the two regimes are reported and discussed. The modeling of the complete generation and propagation process in three dimensions is also presented.

A novel scheme of interaction exploiting a truncated Bessel beam have demonstrated to induce high brightness emission of high order harmonics up to soft-X-rays generated using a few-optical-cycle pump pulse focused on a gas jet.

Recent advances have dramatically shortened the minimum pulse duration obtainable from optical parametric amplifiers. In this work we report on two different powerful schemes for the generation of few-optical-cycle pulses in the visible and in the near-infrared respectively; both systems use β-barium borate (BBO) as amplifier crystal, but different phase matching conditions. In the visible, with non-collinear type I phase matching and long (≈ 150 fs) pump pulses, we obtain ultra-broadband sub-10-fs pulses; in the near-infrared, using collinear type II phase matching and short (≈18 fs) pump pulses, we achieve pulses tunable from 1.1 to 2.6 µm and with duration as short as 14.5 fs.

The spectral properties of harmonic radiation generated in neon by laser pulses with duration from 30 fs down to 5 fs have been investigated. In the 30-fs temporal regime we have achieved continuously tunable sub-20-nm radiation by changing the gas jet position relative to the laser beam waist. We show that the results of a one-dimensional propagation code partially including three-dimensional focusing effects are in good agreement with the experimental results. Finally, we have performed a spectral characterization of harmonic radiation in the sub-10-fs temporal regime.

Many fundamental processes of radiation–matter interaction, which take place on the ultra-short time scale, can now be directly investigated by using few-optical-cycles light pulses with duration

We have investigated the effect of free electrons on the spectral properties of high-order harmonics generated in a neon gas jet by a 30 fs Titanium:Sapphire pumping laser with intensities in the range 5–10 × 1014 W/cm2. The main feature of our observations concerns the possibility of continuously tuning the harmonic wavelength in the spectral region 20–7 nm, by taking advantage of the blue shift of harmonic wavelengths induced by the presence of free electrons to cover the entire spectral region between two consecutive harmonics of the unshifted spectrum. Different amounts of blue shift, which can be as large as 0.3–0.4 nm, are imparted to the given harmonic by simply changing the gas-jet-laser-beam-waist relative position. We have also interpreted our experimental results with a simple model for the generation process based on the “barrier suppression” ionization of an atom exposed to an ultraintense laser field.

Spectra of high-order harmonics, generated by 30 fs and sub-10 fs laser pulses in neon and krypton, have been obtained. The main characteristics of the measured spectra are analysed and discussed. For the 30 fs regime experimental results are compared with spectra calculated by means of a 1D propagation code which partially accounts for 3D focusing effects.

Harmonic radiation generated in a neon gas jet by sub-10-fs laser pulses was investigated both experimentally and theoretically. The spectral profile of the harmonics with respect to the order, their intensity and relative spectral shifts were measured as a function of the position of the gas jet. The results point out spectral features typical of the quasi-single-cycle excitation regime. A nonadiabatic three-dimensional numerical model was developed, which provides harmonic spectra in remarkable agreement with the experiments.

Recently there has been considerable progress in the generation of ultrashort light pulses in the visible. Using wavelength-independent phase-matching in p-barium borate (BBO) [1] non-collinear optical parametric amplifiers (OPAs) in the visible with amplified bandwidths in excess of 100 THz and pulse durations well below 10 fs were demonstrated [2-4].

Results from femtosecond/picosecond transient absorption spectra measurements in the near-infrared/visible spectral region of the red form of a soluble polycarbazolyldiacetylene in a polyethylene matrix are reported. Intrachain singlet and triplet excitons are identified by the different dynamics of the photoinduced bands. The results are consistent with the ultrafast generation (

We performed field-assisted time-resolved femtosecond pump-probe experiments in light emitting diodes of laddertype poly-(para-phenylene). Field-induced charge carriers generation is directly observed monitoring the characteristic polaron absorption band at 1.91 eV. Charge carriers are directly formed by singlet exciton dissociation in the picosecond time domain.

We report on traveling-wave parametric conversion of

We report on two different optical parametric amplifier (OPA) schemes for the generation of few-optical-cycles pulses. Pulses with 180-THz bandwidth and 2-µJ energy are generated by a noncollinear OPA in the visible; a portion of the amplified pulse spectrum is compressed down to 7.2 fs using a thin prism sequence. IR pulses tunable from 1.1 to 2.6 µm with duration as short as 14.5 fs are achieved in a type-II OPA pumped by 780-nm, 18-fs pulses generated by a hollow-fiber compressor.

Powerful techniques for spectral broadening and ultrabroad-band dispersion control, which allow compression of high energy femtosecond pulses to duration of a few optical cycles, are presented. Spectral broadening by propagation along hollow fiber filled with noble gases is studied under two excitation regimes with high energy input pulses of 140-fs and 20-fs duration respectively. With 20-fs input pulses and under optimum compression conditions we show a pulse shortening down to 4.5 fs with output energy up to 70 μJ using a high throughput prism-chirped mirror delay line.

The development of highly luminescent conjugated polymers is of great importance for promising applications in LEDs and "plastic" lasers. It was recently found that photoluminescence (PL) from polymer films under high optical excitation shows a strong line narrowing effect [1], Amplified spontaneous emission (ASE), superfluorescence, and exciton condensation are amongst the proposed mechanisms.

Transient and quasi-steady state photomodulation spectroscopy has been applied to films of substituted sexithiophene, namely α, ωdihexylsexithiophene (α, ωDHT 6 ), which is prototype two dimensional organic semiconductor. Two photoexcited states are seen in the ultrafast time domain characterized by photoinduced absorption bands at 1.7 eV and 1.5 eV respectively and different dynamics. We assign them to Frenkel excitons and inter-layer charge transfer excitons. The photoinduced absorption spectrum in the ms time domain shows a very narrow line at 0.7 eV and a broad structure around 1 eV which are assigned to doubly occupied states, π-dimers and bipolarons respectively.

We studied the sub-picosecond dynamics of photoexcitations in methyl-substituted poly(para--phenylene)-type ladder polymer (m-LPPP) films in a wide excitation density range up to values typical for the regime of narrow band emission by pump-probe spectroscopy. The singlet excitons relaxation dynamics, monitored at the stimulated emission (SE) peak at 2.53 eV, showed an intensity dependent ultrafast decay component, which occurs on a sub-picosecond time scale, ascribed to the onset of amplified spontaneous emission (ASE). In addition we identified photoinduced absorption (PA) bands of singlet excitons at 1.48 eV and polarons at 1.9 eV respectively. At high excitation density an additional absorption band becomes evident at 2.63 eV.

We report on the photoinduced absorption spectra of polycarbazolydiacetylenes polyDCHD and its soluble analogous polyDCHD-S in a broad time domain ranging from femtosecond to millisecond. Both the blue and red forms of polyDCHD-S are investigated and the results are compared with those of blue polyDCHD. In the blue form of the two polymers charged states are detected in the ms domain in addition to triplet excitons. In the red form of polyDCHD-S only triplets are detected. Femtosecond pump and probe measurements allow us to interpret the generation mechanism of the triplet states as due to singlet excitons fission.

The dynamics of excited streets in a special class of linear carbon chains, e.g. dinaphthyl polyynes, has been investigated by ultrafast transient absorption spectroscopy and DFT//TDDFT calculations. The ultrafast formation of a narrow photo-induced absorption band has been observed and assigned to an inter-system crossing event bringing to the formation of triplet excited streets.

We study the generation of high order harmonics produced by the interaction of three intense and ultrafast noncollinear laser pulses. Harmonic radiation, not overlapped to the direction of the incident pulses, is observed.

A novel pulse compression technique, based on spectral broadening in an hollow quartz waveguide filled with noble gases at high pressure, provides pulses of 10 fs duration and 240 μJ energy.

High resolution x-ray spectra of plasmas produced by the interaction of Ti:Sa laser pulses (duration from 60 fs to 1 ps, and energy from 15 to 128 mJ) with a solid iron target are investigated. Interestingly, both L-shell and K-shell Fe spectra were simultaneously measured using a focusing spectrometer with spatial resolution. In particular, the L-shell Fe radiation from F-, Ne-, and Na-like ions between similar to 14-16.3 angstrom were observed from the first-order reflection with a mica crystal. Simultaneously measured alongside the L-shell radiation was the Fe cold K alpha lines (similar to 1.94 angstrom), from eighth-order reflection. Two non-LTE Fe kinetic models have been developed to account for the L-shell radiation from the Fe ions and for the K-shell radiation from low ionization stages emitting the Fe cold K alpha lines. Preliminary analysis of the simultaneously measured Fe L-and K-shell radiations from changing the time-duration, laser contrast, and focus of the experiments shows varying relative line intensities from different ionization stages. Resulting plasma parameters and their dependence on changing the time-duration, laser contrast, and laser focus will be discussed.

We implemented a new experimental scheme for the generation of single-shot extreme-UV continua that exploits a combination of transform-limited 15 fs, 800 nm pulses and chirped 35 fs, 800 nm pulses with orthogonal polarizations. Continua are interpreted as the formation of a single attosecond pulse and attributed to the interplay between polarization, ionization gating, and trajectory selection operated by suitable phase-matching conditions.

High resolution X-ray spectra of plasmas produced by the interaction of Ti:Sa laser pulses (duration from 60 fs to 1 ps, and energy from 15 mJ to 128 mJ) with a solid Fe target are investigated experimentally and theoretically. Interestingly, both L-shell and K-shell Fe spectra were simultaneously measured while changing the time-duration of the laser experiments. In particular, the L-shell Fe radiation from F-, Ne-, and Na-like ions between ~14?16.3 ? were observed from the first order reflection with a mica crystal. Simultaneously measured alongside the L-shell radiation was the Fe cold K? lines (~1.94 ?), from 8th order reflection. Two non-LTE Fe kinetic models have been developed to account for the L-shell radiation from the Fe ions and for the K-shell radiation from the low ionization stages emitting the Fe cold K? lines. Preliminary analysis of the simultaneously measured Fe L- and K-shell radiations from changing the time-duration of the experiments shows varying relative line intensities from different ionization stages. Resulting plasma parameters as well as the opacity of the strongest resonance line, 3C, and their dependence on changing the time-duration will be discussed.

We applied femtosecond pump-probe spectroscopy to a guest-host system of conjugated polymers, the blue-emitting poly(paraphenylene) (m-LPPP) and the red-emitting poly(perylene-co-diethynylbenzene) (PPDB). Excitation energy transfer between m-LPPP and PPDB, monitored by the formation of the PPDB photobleaching band at 2.15 eV, takes place on a picosecond time scale. The nonexponential kinetics of the excitation energy transfer and its dependence on acceptor (PPDB) concentration provide clear evidence for a Forster-type energy transfer.

We present a broadband self-phase-stabilized near-IR source based on difference frequency generation (DFG) of a filament broadened supercontinuum. followed by a two-stage optical parametric amplifier. We demonstrate pulses with energy up to 1.2 mJ and duration down to 17 fs. We theoretically study the process of DFG and investigate the carrier-envelope phase (CEP) dependence on the driving pulse parameters. We find that robust CEP stability is possible even with fluctuations in the phase and intensity of the DFG seed pulse. These characteristics make this parametric source suitable as a driver for high-order harmonic generation and isolated attosecond pulse production. (C) 2008 Optical Society of America.

Extreme-ultraviolet pulses, produced by high-order harmonic generation, have been spectrally selected by a time-delay-compensated grating monochromator. Temporal characterization of the harmonic pulses has been obtained using cross-correlation method: pulses as short as 8 fs, with high photon flux, have been measured at the output of the monochromator. (c) 2007 Optical Society of America.

High-order harmonic generation in argon driven by 25-fs-light pulses is investigated from the gaseous to the cluster regime. The harmonic cutoff observed in presence of clusters shows a considerable extension with respect to the gaseous phase. Harmonic spectra are investigated as a function of cluster size, showing the existence of an optimal cluster dimension, which maximizes the harmonic photon yield. (C) 2005 American Institute of Physics.

We show, both experimentally and theoretically, that the strong-field processes involved in high-order harmonic generation are significantly affected by the pulse carrier-envelope phase even in the multi-optical-cycle regime.

For the first time single-shot harmonic spectra generated by few-optical-cycle pulses have been measured. Clear carrier-envelope phase effects have been observed in the cutoff harmonic spectral structure. Results have been interpreted in terms of the nonadiabatic single-atom response of the nonlinear medium excited by few-optical-cycle pulses.

We experimentally demonstrate a new technique for the temporal gating of high-order harmonic generation, based on the mixing of two mid-infrared laser pulses coming from op- tical parametric amplifiers, at wavelengths of 1.35 and 1.75 μm, respectively. Both parallel and perpendicular configurations of the polarization directions were investigated. Results obtained in xenon and argon show continuous spectra with a significant cutoff extension, up to 160 eV. These outcomes demonstrate the possibility of extending the two-color gating technique for the generation of isolated attosecond bursts to a mid-infrared laser source and are, to our knowledge, the first experimental demonstration of the gating on a single half-cycle of high-order harmonics driven by laser pulses longer than 30 fs. Delay, fs

Summary form only given. CITIUS is a multi-partner and multi-disciplinary project lead by University of Nova Gorica (Slovenia) to develop a facility based on a high repetition rate and ultra-short tunable laser source that is used to produce ultrafast extreme-ultraviolet (XUV) pulses through high-order harmonic (HH) generation. The XUV radiation is monochromatized and can be focused on different end-stations for materials and surface science and atomic and molecular physics and chemistry. We present here the characterization of the monochromatic XUV source. The HHs are generated in a gas cell using a Ti:Sa laser operated at 5-Khz repetition rate with an energy of 2 mJ/pulse. The XUV beam is monochromatized and focused in the experimental chamber. Since the beam is handled by a grating monochromator, a temporal broadening has to be accepted at the output due to the pulse front-tilt given by the diffraction [2]. In the case of the CITIUS beamline, an innovative single-grating configuration has been adopted, that combines in a single instrument two different grating geometries: the classical-diffraction mount (CDM) and the off-plane mount (OPM), as shown in Fig. 1. It has been demonstrated that the two geometries are complementary [3]: the CDM is used for relatively long instrumental response in the 100-200 fs range, and high spectral resolution, i.e. λ/Δλ > 200; the OPM is used for ultrashort responses in the 10-50 fs range and low spectral resolution, i.e. λ/Δλ

It is shown that the ultrafast carrier dynamics in Ge nanoparticles involving the electronic density of states well above the gap can be separated in two regimes: a faster one (∼1 ps) characterized by bleaching of the absorption around 490 nm and a slower one (up to a few hundred picoseconds) governed by band gap renormalization. There is also a clear correspondence between the spectral response in both regimes and a bulk-like band structure for sizes below the exciton Bohr radius. Quantum confinement is manifested through the measurable blueshift of E1+Δ1 spectral structure in the ultrafast optical response.

Laser driven electron recollision provides a unique tool for measuring the structure and dynamics of matter. We illustrate this with experiments that use HHG to measure molecular structure with sub-Angstrom spatial and sub-femtosecond temporal resolution.