Broadband normal-incidence aperiodic multilayer mirrors for soft X-ray dispersion spectroscopy: theory and implementation

摘要

Aperiodic multilayer structures (MSs) show promise as broadband soft X-ray mirrors for stigmatic diffraction spectroscopic instruments. We have derived aperiodic MSs optimized for maximum uniform normal-incidence reflectivity throughout a prescribed wavelength range, e.g., 130 ― 190 A (R=24 %), 125 ― 250 A (16 %), etc. Each of these aperiodic MSs exhibits an unparalleled integral reflectivity which substantially exceeds that of any periodic multilayer mirror pertaining to the corresponding wavelength range. Several Mo/Si aperiodicMSs were synthesized by magnetron sputtering on concave substrates to exhibit a relatively uniform reflectivity of (15 ― 11)% in the 125 ― 250 A range, close to the theoretical predictions. These multilayers served as the focusing elements of high-transmission stigmatic transmission grating spectrograph with a plate scale of 20 or 4 A/mm. The spectrograph was employed to investigate the laser plasma ― gas jet interaction and characterize the Xe gas-jet laser-plasma XUV radiation source simultaneously with spatial and spectral resolution in the wavelength range above 124 A. We have theoretically explored into the reflection of extremely short X-ray pulses by MSs. Special-design aperiodic MSs were shown to possess the unrivaled capacity to reflect few-cycle X-ray pulses ranging into the attoseconds. Aperiodic MSs have been calculated that are capable of reflecting pulses which comprise only three wave periods of an X-ray electromagnetic wave.