Spectral features and modeling of high-order harmonics generated by sub-10-fs pulses

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.

Grazing incidence spectrograph-monochromator for xuv spectroscopy in the 5-900 A region.

A combination grazing incidence spectrograph for photographic recording and scanning monochromator has been realized. The instrument is of a modular concept and all the internal components like the grating, the plateholder, and the scanning exit slit are easily removable. The radius of curvature of the Rowland cylinder can also be easily changed. The entrance beam is always kept fixed in position and direction, allowing the instrument to be permanently aligned with fixed light sources. The vacuum case is mechanically independent from the optical components.

Grazing-incidence telescope-spectrograph for space solar-imaging spectroscopy.

The design of a stigmatic grazing-incidence instrument for space applications to solar-imaging spectroscopy is presented. It consists of a double telescope and a spectrograph: Telescope I consists of a single cylindrical mirror with parabolic section, focusing the radiation on the entrance slit of the spectrograph in the spectral dispersion plane; telescope II consists of two cylindrical mirrors with aspherical section in a Wolter configuration, focusing the radiation on the spectrograph focal plane in the direction perpendicular to the spectral dispersion plane. The spectrograph consists of a grazing-incidence spherical variable-line-spaced grating with flat-field properties. Telescope II is crossed with respect to the grating and telescope I; i.e., it is mounted with its tangential planes coincident with the grating equatorial plane. The spectrum is acquired by a detector mounted at near-normal incidence with respect to the direction of the exit beam. The spectral resolution is also preserved for off-axis angles. The effective collecting area of the instrument can be preserved by adoption of a nested configuration for telescope II without degradation of the spectral resolution.

Grazing incidence Czerny-Turner monochromator utilizing sagittal focusing mirrors.

A new soft x-ray monochromator design, especially conceived for high brightness undulator radiation, is discussed. Its aim is to achieve a resolution limited only by grating figuring slope errors, together with a simple grating rotation scanning; thereby operating with fixed entrance and exit slits. Three cylindrical optical surfaces are employed between the slits, including two mirrors focusing in their sagittal planes, and a resolution lambda/Deltalambda up to approximately 14000 at the carbon K-edge is predicted. Its implementation for the planned Trieste synchrotron radiation facility is described in detail.

Design of a high-throughput grazing-incidence flat-field spectrometer.

The optical design of a high-throughput grazing-incidence flat-field spectrometer is presented. The spectral focal curve is almost a straight line because of the flat-field focusing properties of spherical variable-line-spaced gratings. The angular acceptance in the direction perpendicular to the plane of dispersion is maximized by means of a focusing spherical mirror mounted with its tangential plane coincident with the sagittal plane of the grating. Analytical calculations for the determination of the optimum mirror parameters are presented. A spectrometer for high-throughput experiments in the 800-60-eV region is designed with an extreme-ultraviolet-enhanced CCD detector: when the available flux is compared with that of a spectrometer with the same kinds of grating and detector but without a focusing mirror, the increase is as much as a factor 3.

Spherical-grating monochromator with a variable-line-spaced grating for synchrotron radiation.

An optical design for spherical-grating monochromators for application to synchrotron radiation is presented. High spectral and spatial performance is obtained with a spherical variable-line-spaced grating coupled to a spherical mirror with its tangential plane coincident with the grating's equatorial plane. The monochromator works without an entrance slit in an off-Rowland configuration with a fixed entrance arm and demagnification on the exit slit. The law for groove-space variation of the grating compensates for the main spectral aberrations; spectral focusing in an extended energy range is ensured by a slight change in the exit arm with translations of the order of a few tens of millimeters. The inclusion of a spherical mirror ensures focusing on a plane perpendicular to the plane of spectral dispersion. The ultimate resolution is limited by the slope errors of a single spherical surface. The layout is applied to the design of a high-resolution monochromator for the 1000-250-eV region.

Grazing incidence toroidal mirror pairs in imaging and spectroscopic applications.

The optical performance of pairs of toroidal mirrors in grazing incidence has been studied analytically and numerically. Two types of toroidal surface are possible: football and bicycle tire. In grazing incidence and for configurations that compensate up to second-order aberrations, there are significant differences in performance between the two types. For football-type tori the best configuration appears to be Z-shaped with tangential and sagittal foci at the middle point between the mirrors. For bicycle tire-type tori the best configuration is U-shaped with the tangential focus at the middle point and the sagittal at infinity.

Continuum source and a focusing technique for the 80-500-a spectral range: improvements.

The performance of a triggered vacuum spark source, emitting continuum radiation in the vacuum uv down to 80 A, has been improved by adding a capillary made of insulating material and increasing the capacity of the condenser. A focusing system consisting of a toroidal mirror has been used in conjunction with the source and a grazing incidence spectrograph in the 500-80-A spectral range. The results obtained indicate considerable gains (of the order of a factor of 200) with respect to previously used sources, allowing photographic recording of spectra with only one flash while maintaining good spectral resolution.

Stigmatic observations of laser-produced plasmas with a grazingincidence spectrograph.

A stigmatic spectrograph composed of a toroidal mirror and a concave grating both working at grazing incidence has been built and applied to the observation of laser-produced plasmas. Spatial resolution of 20-30 microm over a region of 1-mm extension and spectral resolution of approximately 1500 have been obtained with a considerable increase in speed of the spectrograph. By varying the relative positions of the optical elements, it is possible to change the wavelength of stigmatic condition in the range 10-300 A. Examples of observations of a beryllium plasma are shown.

Stigmatic performance of an EUV spectrograph with a single toroidal grating.

The stigmatic performance of conventionally ruled toroidal gratings used near normal incidence is explored in detail. A single-grating spectrograph, particularly suitable for observing the dynamics of the solar corona in the 520-630-A wavelength range, is described. The arrangement uses a 3600-line/mm grating with a horizontal radius of curvature of 2 m that generates images with a blur of less than 20 microm over an area 2.6 mm high x 80 mm wide, while over an area 6 mm x 80 mm the blur does not exceed 40 microm. In terms of wavelength intervals these blurs correspond to 28 mA and 56 mA, and the width of the area covers a spectral range of 110 A. If the spectrograph is equipped with a 20-microm wide entrance slit, and if this slit is placed in the focal plane of a telescope with 4-m focal length, spatial resolution elements of 1 x 1 (sec of arc)(2) and 1 x 2 (sec of arc)(2) over a slit height of 2 min of arc and 5 min of arc, respectively, result.

Characterization of a charge-coupled-device detector in the 1100-0.14-nm (1-eV to 9-keV) spectral region.

The performances of a CCD detector have been evaluated in a wide spectral region, which ranges from the near IR to the soft x ray. Four different experimental setups have been used: a Czerny-Turner monochromator for the 1100-250-nm region, a normal-incidence Johnson-Onaka monochromator for the 250-30-nm region, a grazing-incidence Rowland monochromator for the 30-0.27-nm region, and a test facility with broadband filters for the 0.27-0.14-nm region. The CCD is thinned and backilluminated, with a 512 x 512 format and 24 microm x 24 microm pixels. The quantum efficiency was measured in the 1100-0.14-nm (1-eV to 9-keV) region, and the uniformity of response was in the 1100-58-nm (1-21-eV) region. Contamination effects in the vacuum UV range are also discussed.

Grazing incidence spectrograph-monochromator with a focusing toroidal mirror.

The use of a toroidal mirror in front of a spherical grating spectrograph working at grazing incidence provides a system that is stigmatic in one dimension in the extreme ultraviolet (XUV) region and allows spectroscopy with space resolution. The system is optimized for spectral and spatial resolution and for efficiency. By using three toroidal mirrors mounted on a suitable turret and a new model of grazing incidence spectrometer, the system is capable of covering the whole XUV region. All the necessary movements are internal to the system and do not involve the source. Two examples are treated, one for a diagnostic study of laser-produced pellets and another for observation of large plasma machines.

Toroidal grating obtained on an elastic substrate.

The performance of a grating that is elastically deformed from a sphere into a toroid is that expected from ray traces assuming an ideal toroidal surface. Thus, it has become possible to select the wavelength range, in which a toroidal grating spectrograph is near stigmatic, by adjusting the forces acting on the elastic grating substrate. The possibility of producing toroidal gratings by replication of an elastically deformed blank is pointed out.

Grazing-incidence high-resolution stigmatic spectrograph with two optical elements.

Using two optical toroidal elements, a mirror and a grating, both working at grazing incidence, a spectrometer can be built that is stigmatic in the XUV region at one wavelength. Good compensation of the aberrations is achieved when the intermediate sagittal image is nearly at infinity. By varying the angle of incidence on the grating with simple movements, a given couple of optical elements could cover stigmatically a rather extended spectral range. If coupled with bidimensional array detectors, such a spectrograph could find applications in planned solar XUV telescopes.

Imaging extreme ultraviolet spectrometer employing a single toroidal diffraction grating: the initial evaluation.

A high-efficiency extreme ultraviolet (EUV) imaging spectrometer has been constructed and tested. The spectrometer employs a concave toroidal grating illuminated at normal incidence in a Rowland circle mounting and has only one reflecting surface. The toroidal grating has been fabricated by a new technique employing an elastically deformable submaster grating which is replicated in a spherical form and then mechanically distorted to produce the desired aspect ratio of the toroidal surface for stigmatic imaging over the selected wavelength range. The fixed toroidal grating used in the spectrometer is then replicated from this surface. Photographic tests and initial photoelectric tests with a 2-D pulse-counting detector system have verified the image quality of the toroidal grating at wavelengths near 600 A. The results of these initial tests are described in detail, and the basic designs of two instruments which could employ the imaging spectrometer for astrophysical investigations in space are briefly described, namely, a high-resolution EUV spectroheliometer for studies of the solar chromosphere, transition region, and corona and an EUV spectroscopic telescope for studies of nonsolar objects.

Optical performances of the Ultraviolet Coronagraph Spectrometer of the Solar Heliospheric Observatory.

The optical performances of the spectrometer assembly for the Ultraviolet Coronagraph Spectrometer of the Solar and Heliospheric Observatory mission have been tested. The flight unit of the spectrometer assembly, consisting of the structure equipped with the entrance slits assembly, the grating drive mechanisms mounting two toroidal gratings, and the photon-counting detectors, has been integrated and aligned; also the flight unit of the White Light Channel has been integrated and aligned in the spectrometer assembly. Tests with both visible and UV radiation have been performed. Aberration and stray-light measurements have shown that the instrument performs satisfactorily, almost in compliance with the scientific requirements; also some measurements of the polarimeter modulation curve and the relative error have shown performances within the specified requirements.

Effects of carrier-envelope phase differences of few-optical-cycle light pulses in single-shot high-order-harmonic spectra.

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.

Observation of carrier-envelope phase phenomena in the multi-optical-cycle regime.

So far the role of the carrier-envelope phase of a light pulse has been clearly experimentally evidenced only in the sub-6-fs temporal regime. Here we show, both experimentally and theoretically, the influence of the carrier-envelope phase of a multi-optical-cycle light pulse on high-order harmonic generation. For the first time, we demonstrate that the short and long electron quantum paths contributing to harmonic generation are influenced in a different way by the pulse carrier-envelope phase.

High-brightness high-order harmonic generation by truncated bessel beams in the sub-10-fs regime.

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.