Spatial phase dislocations in femtosecond laser pulses

We show that spatial phase dislocations associated with optical vortices can be embedded in femtosecond laser beams by computer-generated holograms, provided that they are built in a setup compensating for the introduced spatial dispersion of the broad spectrum. We present analytical results describing two possible arrangements: a dispersionless 4 setup and a double-pass grating compressor. Experimental results on the generation of optical vortices in the output beam of a 20 fs Ti:sapphire laser and the proof-of-principle measurements with a broadband-tunable cw Ti:sapphire laser confirm our theoretical predictions.This research was partially supported by the National Science Fund (Bulgaria), under contract F-1303/2003, and the Australian Research Council

Momentum distributions of sequential ionization generated by an intense laser pulse

Journals published by the American Physical Society can be found at http://publish.aps.org/The relative yield and momentum distributions of all multiply charged atomic ions generated by a short (30 fs) intense (10(14)-5 x 10(18) W/cm(2)) laser pulse are investigated using a Monte Carlo simulation. We predict a substantial shift in the maximum (centroid) of the ion-momentum distribution along the laser polarization as a function of the absolute phase. This effect should be experimentally detectable with currently available laser systems even for relatively long pulses, such as 25-30 fs. In addition to the numerical results, we present semianalytical scaling for the position of the maximum

Intensity-resolved measurement of above-threshold ionization of Ar-H2_2O

Above-treshold ionization (ATI) by femtosecond laser pulses centered at 515\,nm is studied for a gas mixture containing the Van-der-Waals complex Ar-H$_2$O. By detecting photoions and -electrons in coincidence, the ATI spectra for Ar, Ar$_2$, \HHO, and Ar-\HHO are discerned and measured simultaneously. Using an intensity-scanning technique, we observe the red-shift of the ATI spectra as a function of the laser intensity. The intensity-dependent shift of the ATI peak positions observed for Ar-H$_2$O and H$_2$O match but significantly differ from the ones measured for Ar and Ar$_2$. This indicates that the photoelectron is emitted from the \HHO site of the complex and the vertical ionization potential of Ar-H$_2$O is determined as $(12.4 \pm 0.1)$\,eV. For resacttered electrons, however, an enhancement of high-order ATI is observed for Ar-H$_2$O, as compared to H$_2$O, suggesting that the relatively large Ar atom acts as a scattering center, which influences the ionization dynamics.Comment: 8 pages, 7 figure

Ellipticity dependence of 400 nm-driven high harmonic generation

We studied the dependence of high harmonic generation efficiency on the ellipticity of 400 nm driving laser pulses at 7.7 x 10(14) W/cm(2) and compared it with the 800 nm driving laser under the same conditions. The measured decrease of high harmonic yield with the ellipticity of the 400 nm laser is similar to 1.5 times slower that of the 800 nm, which agrees well with theoretical predictions based on a semi-classical model. The results indicate that it is feasible to use the generalized double optical gating with 400 nm lasers for extracting single attosecond pulses with high efficiency

Dependence of high-order-harmonic-generation yield on driving-laser ellipticity

High-order-harmonic-generation yield is remarkably sensitive to driving laser ellipticity, which is interesting from a fundamental point of view as well as for applications. The most well-known example is the generation of isolated attosecond pulses via polarization gating. We develop an intuitive semiclassical model that makes use of the recently measured initial transverse momentum of tunneling ionization. The model is able to predict the dependence of the high-order-harmonic yield on driving laser ellipticity and is in good agreement with experimental results and predictions from a numerically solved time-dependent Schrodinger equation

In situ tomography of femtosecond optical beams with a holographic knife-edge

We present an in situ beam characterization technique to analyze femtosecond optical beams in a folded version of a 2f-2f setup. This technique makes use of a two-dimensional spatial light modulator (SLM) to holographically redirect radiation between different diffraction orders. This manipulation of light between diffraction orders is carried out locally within the beam. Because SLMs can withstand intensities of up to I~10^11 W/cm, this makes them suitable for amplified femtosecond radiation. The flexibility of the SLM was demonstrated by producing a diverse assortment of "soft apertures" that are mechanically difficult or impossible to reproduce. We test our method by holographically knife-edging and tomographically reconstructing both continuous wave and broadband radiation in transverse optical modes.This work was partially supported by the Robert A. Welch Foundation (grant No. A1546), the National Science Foundation (NSF) (grants Nos. 0722800 and 0555568), the Qatar National Research Fund (grant NPRP30-6-7-35), and the United States Air Force Office of Scientific Research (USAFOSR) (grant FA9550-07-1-0069)

A large aperture reflective wave-plate for high-intensity short-pulse laser experiments

We report on a reflective wave-plate system utilizing phase-shifting mirrors (PSM) for a continuous variation of elliptical polarization without changing the beam position and direction. The scalability of multilayer optics to large apertures and the suitability for high-intensity broad-bandwidth laser beams make reflective wave-plates an ideal tool for experiments on relativistic laser-plasma interaction. Our measurements confirm the preservation of the pulse duration and spectrum when a 30-fs Ti:Sapphire laser beam passes the system

Characterization of encapsulated graphene layers using extreme ultraviolet coherence tomography

Many applications of two-dimensional materials such as graphene require the encapsulation in bulk material. While a variety of methods exist for the structural and functional characterization of uncovered 2D materials, there is a need for methods that image encapsulated 2D materials as well as the surrounding matter. In this work, we use extreme ultraviolet coherence tomography to image graphene flakes buried beneath 200 nm of silicon. We show that we can identify mono-, bi-, and trilayers of graphene and quantify the thickness of the silicon bulk on top by measuring the depth-resolved reflectivity. Furthermore, we estimate the quality of the graphene interface by incorporating a model that includes the interface roughness. These results are verified by atomic force microscopy and prove that extreme ultraviolet coherence tomography is a suitable tool for imaging 2D materials embedded in bulk materials