X-ray lasers are lasers that generate bright laser beams at discrete wavelengths ranging from 3.56 nm to 46.9 nm. A traditional gain medium cannot be used in x-ray lasers. They use a highly ionized plasma region created in a capillary discharge as the active medium and the plasma is created with a laser beam or with an electrical discharge. Mirrors that can reflect x-rays with normal incidence are not yet available. Therefore, an x-ray laser is a mirrorless laser, which means there is no optical feedback with a resonator. The laser radiation in the x-ray laser is generated with amplified spontaneous emission (ASE). When a light beam propagates through the plasma, the spontaneously generated radiation gets simplified by stimulated emission of radiation.

Working of an X-ray laser

In X-ray lasers, when a light pulse strikes a target made from a thin foil, fiber, or solid material, the atoms or electrons get stripped off from them forming ions by excitation or amplification. When each of the excited ions gets decayed from a higher energy state to a lower energy state, a photon is emitted. When millions of these photons having the same wavelength get amplified at the same time, the x-ray laser beam is created. When high-intense (⩾10¹⁵ W cm⁻²) ultrashort laser pulses of low or moderate energy (from several to ten or a hundred millijoules) are focused on a solid target, compact short-pulse high-power x-ray sources can be produced. For example, X-ray pulses of high power and short duration (in the picosecond range) can be produced by sub-picosecond laser pulses. 

Figure 1: De-excitation of electrons in an atom

X-rays are produced by the de-excitation of electrons from the outer shell to the inner shell of an atom, as shown in Figure 1. So, for population inversion to exist in a gain medium to produce X-ray laser output, the inner shell electrons need to be excited to a higher energy level. For that purpose, the lasing atom is made to undergo a repeated ionization process which sequentially removes most of the electrons in it to attain an appropriate electronic configuration. Also, the lasing species will be in a plasma state with a sufficient temperature that maintains it. For e.g., the Argon-based X-ray laser requires a plasma temperature of 100 eV, which can produce and maintain ions of Neon-like Ar⁸⁺ electronic configuration. This electronic configuration is achieved by removing 8 electrons from the neutral Argon atom that has an atomic number of 18. Higher plasma temperatures are needed to maintain X-ray-producing ionized states of heavier atoms as more electrons must be removed from them. For eg., a Selenium atom with an atomic number of 34 requires a plasma temperature of 1 keV to produce and maintain a Neon-like Se²⁴⁺ electronic configuration after removing 24 electrons from its neutral state. Excitation to the upper laser level occurs when the plasma electrons collide with the ions in the ionic ground state. X-ray lasers use laser excitation, electric discharge excitation, and optical field ionization mechanisms to produce population inversion.

Pumping Mechanism for X-ray laser

X-ray lasers can be pumped by using a laser beam, electrical discharge, or optical field ionization to form plasma.

    Laser Excitation

Figure 2: Illustration of laser excitation to form plasma

A high-power laser beam can be focused using a cylindrical lens on a solid metal target of atomic species which is used as the gain material. The atoms in the focused region will undergo multiple ionization processes and as a result, plasma is formed with highly ionized laser species along the focused line above the solid metal target. This act as the gain medium for the x-ray laser. Lasers with a pulse energy of a few hundred joules with nanosecond range pulse duration are used as the pump laser source. Figure 2 shows the laser excitation process in an x-ray laser.

Laser excitation is a two-step process to form highly ionized plasma. In the first step, a laser pulse with a pulse duration of 500 ps and pulse energy of 1-3 J is line-focused using a cylindrical lens on a solid target to produce hot dense plasma.

Figure 3: Two-step laser excitation process

In the second step, another pulse with a pulse duration of 10ps and pulse energy of 3-7 J is focused on this already-formed plasma after a time delay of 2 ns. This causes a rapid excitation of laser species to the upper laser level and helps in achieving population inversion. Hence x-ray laser output is obtained in the direction of plasma. Figure 3 shows the two-step laser excitation process.

    Electric Discharge Excitation

Figure 4: Illustration of electric discharge excitation

In electric discharge excitation, a high current or voltage is applied across the elongated capillary tube that has a very small diameter. The laser species is introduced into the capillary in gaseous form or as a coating on the capillary wall. By the application of discharge current, the laser species undergo rapid ionization, and plasma is produced with a suitable ionization state. The plasma electrons collide with each other and cause population inversion and subsequently, the x-ray laser output is produced. Figure 4 shows the pumping method using electric discharge.

    Optical Field Ionization

In the optical field ionization technique, an ultrashort laser pulse of sub-picosecond pulse duration is used to produce non-equilibrium plasmas for the excitation of laser species. Non-equilibrium plasmas are plasma produced without providing the required temperature to maintain it in that state. Since the pulse duration is very short, the population inversion is achieved quickly which leads to laser emission. But the energy of the laser output obtained is in the nanojoule range, which is not very useful for applications.

Parameters of X-ray Laser

Applications of X-ray Laser

X-ray lasers are used for medical purposes to detect fractures in human bones. In airports, train stations, and other locations, they are utilized as luggage scanners. These lasers are widely used to detect defects in the welds and are also used for restoring old paintings. X-rays are emitted by celestial objects and are studied to understand the environment.

High brightness of X-ray lasers is used for many imaging applications like imaging of biological objects and imaging of rapid processes in laser fusion plasmas. Synchrotron sources use x-ray lasers for the imaging of biological microstructures. 

The X-ray laser source is used for applications like material ablation, photolithography, micro probing, holography of living biological materials, crystallography, dense plasma interferometry, x-ray microscopy, etc.