Ultrashort Laser Pulse Phenomena, 2nd Edition

Generation of Ultrafast X-Ray Pulses 565

12.2. GENERATION OF ULTRAFAST X-RAY PULSES

12.2.1. Incoherent Bursts of X-Rays

When a powerful light pulse is focused onto a solid, a high-temperature plasma

is produced, which can emit X-rays. Stimulated by laser fusion research, the

physics of laser induced X-ray generation has been extensively investigated. The

availability of high energy (milliJoule) tabletop lasers has made laser-produced

plasmas a unique and practical source of ultrashort X-ray pulses [15–21].

A diagram of optical pulse induced X-ray production is sketched in Figure 12.8(a).

In the absorption process, which takes place in a thin surface layer, the opti-

cal energy is initially transferred to the electronic system. Depending on the

target and the laser pulse intensity multiphoton processes ionize the material,

and the plasma is subsequently heated. The cooling of the laser-produced plasma

is accompanied by the emission of a burst of X-rays. Figure 12.8(b) shows the

time integrated spectral distribution from a Si target as was obtained by Murnane

et al. [22].

Several processes contribute to the X-ray emission spectrum:

• the emission of distinct lines resulting from transitions between inner

subshells of the ions,

• broadband emission from the recombination of unbound (free) electrons and

ions, and

• broadband emission consisting of bremsstrahlung radiation.

428

Wavelength (nm)

X-ray detector

Relative intensity

6101412

(a) (b)

Figure 12.8 (a) Sketch of laser-generated X-ray plasmas. (b) Time integrated soft X-ray emission.

Pulses of 160-fs duration and 5 mJ of energy were focused onto an Si target at an intensity of

10

16

W/cm

2

(Adapted from Murnane et al. [22]).