S. de Rossi

Optimization toward a high-average-brightness soft-x-ray laser pumped at grazing incidence

We report the near-field imaging characterization of a 10 Hz Ni-like 18.9 nm molybdenum soft-x-ray laser pumped in a grazing incidence pumping (GRIP) geometry with a table-top laser driver. We investigate the effect of varying the GRIP angle on the spatial behavior of the soft-x-ray laser source. After multiparameter optimization, we were able to find conditions to generate routinely a high-repetition-rate soft-x-ray laser with an energy level of up to 3 microJ/pulse and to 6x10(17) photons/s/mm2/mrad2/(0.1% bandwidth) average brightness and 1x10(28) photons/s/mm2/mrad2/(0.1% bandwidth) peak brightness.

Minimization of out-of-plane losses in planar photonic crystals by optimizing the vertical waveguide

A two-dimensional phenomenological approach previously developed for the modeling of out-of-plane losses in low refractive index contrast planar photonic crystals (PPhCs) is used to study the dependence of the different loss terms on the planar waveguide parameters. It is demonstrated that: (i) Losses can be minimized by designing vertical heterostructures optimized for a given technological process and/or for a given hole shape; and (ii) any small reduction of the loss value has a strong impact on the optical performances of PPhC structures.

Control of the attosecond synchronization of XUV radiation with phase-optimized mirrors

We report on the advanced amplitude and phase control of attosecond radiation allowed by specifically-designed multilayer XUV mirrors. We first demonstrate that such mirrors can compensate for the intrinsic chirp of the attosecond emission over a large bandwidth of more than 20 eV. We then show that their combination with metallic foils introduces a third-order dispersion that is adjustable through the mirrors incidence angle. This results in a controllable beating allowing the radiation to be shaped from a single to a series of sub-100 as pulses.

Temporal coherence and spectral linewidth of an injection-seeded transient collisional soft x-ray laser

The temporal coherence of an injection-seeded transient 18.9 nm molybdenum soft x-ray laser was measured using a wavefront division interferometer and compared to model simulations. The seeded laser is found to have a coherence time similar to that of the unseeded amplifier, ~1 ps, but a significantly larger degree of temporal coherence. The measured coherence time for the unseeded amplifier is only a small fraction of the pulsewidth, while in the case of the seeded laser it approaches full temporal coherence. The measurements confirm that the bandwidth of the solid target amplifiers is significantly wider than that of soft x-ray lasers that use gaseous targets, an advantage for the development of sub-picosecond soft x-ray lasers.

Fourier-limited seeded soft x-ray laser pulse

We present what we believe to be the first measurement of the spectral properties of a soft x-ray laser seeded by a high-order harmonic beam. Using an interferometric method, the spectral profile of a seeded Ni-like krypton soft x-ray laser (32.8 nm) generated by optical field ionization has been experimentally determined, and the shortest possible pulse duration has been deduced. The source exhibits a Voigt spectral profile with an FWHM of 3.1+/-0.3 mA, leading to a Fourier-transform pulse duration of 4.7 ps. This value is comparable with the upper limit of the soft x-ray pulse duration determined by experimentally investigating the gain dynamics, from which we conclude that the source has reached the Fourier limit. The measured bandwidth is in good agreement with the predictions of a radiative transfer code, including gain line narrowing and saturation rebroadening.

EUV reflectivity and stability of tri-component Al- based multilayers

We report on further development of three-material multilayer coatings made with a use of aluminum for the extreme ultra-violet (EUV) applications such as solar physics, high-order harmonic generation or synchrotron radiation. It was found that an introduction of refractory metal in Al-based periodic stack helps to reduce significantly an interfacial roughness and provides for a higher theoretical reflectance in the spectral range from 17 to 40 nm. The normal incidence reflectivity as high as 55 % at 17 nm, 50 % at 21 nm and 42 % at 30 nm was achieved with the new Al/Mo/SiC and Al/Mo/B4C multilayer mirrors, which have been optimized, fabricated and characterized with x-rays and synchrotron radiation. A good temporal and thermal stability of the tri-component Al-based multilayers has been observed over 3 years.

Duration of ultrashort pulses in the presence of spatio-temporal coupling

We report on a simple method allowing one to decompose the duration of arbitrary ultrashort light pulses, potentially distorted by space-time coupling, into four elementary durations. Such a decomposition shows that, in linear optics, a spatio-temporal pulse can be stretched with respect to its Fourier limit by only three independent phenomena: nonlinear frequency dependence of the spectral phase over the whole spatial extent of the pulse, spectral amplitude inhomogeneities in space, and spectral phase inhomogeneities in space. We illustrate such a decomposition using numerical simulations of complex spatio-temporal femtosecond and attosecond pulses. Finally we show that the contribution of two of these three effects to the pulse duration is measurable without any spectral phase characterization.

How to focus an attosecond pulse

Attosecond experiments involving focusing of attosecond light pulses can suffer from a spread of the attosecond radiation both in space and time due to optical aberrations. We present a detailed numerical study of the distortions induced in the most common focusing geometries that make use of parabolic, spherical, toroidal and ellipsoidal mirrors. We deduce the consequences on the pulse duration and possible issues that could arise in applications of attosecond pulses. This should serve as a guideline for setting up attosecond focusing optics.

Quantitative study of 10 Hz operation of a soft x-ray laser-energy stability and target considerations

A soft x-ray laser from Ni-like Mo, pumped in grazing incidence (GRIP), is analyzed with regard to high repetition rate operation. Reliable lasing is obtained, but with significant energy fluctuations attributed mainly to beam pointing jitter from the pump laser. Two modes of operation are compared: continuously moving target and stationary target. With a moving target the soft X-ray output is constant on average, whereas the repeated use of the same target position leads to a pulse energy which increases for several tens of shots. This effect might be caused by improved guiding of the pump laser in the formed groove and the removal, through laser ablation, of the oxide layer on the target surface.

Photorefractive vibrometer for the detection of high-amplitude vibrations on rough surfaces

We present and characterize theoretically and experimentally a photorefractive velocimeter. This device, based on two-wave mixing in a rapid photorefractive crystal, measures the instantaneous velocity of a vibrating target. It is particularly adapted to the measurement of high-amplitude (up to several mm) low-frequency (up to several kHz) vibrations. Instantaneous velocities as high as 25 mm s-1 are expected to be measured with common photorefractive semiconductors and CW lasers.