X-ray laser
Encyclopedia
An X-ray laser is a device that uses stimulated emission
to generate or amplify electromagnetic radiation
in the near X-ray
or extreme ultraviolet
region of the spectrum, that is, usually on the order of several of tens of nanometers (nm) wavelength
.
Because of high gain in the lasing medium, short upper-state lifetimes (1–100 ps
), and problems associated with construction of X-ray mirrors, X-ray lasers usually operate without any resonator
. The emitted radiation, based on amplified spontaneous emission
, has relatively low spatial coherence. The line is mostly Doppler broadened
, which depends on the ions' temperature.
As the common visible-light
laser transitions between electronic
or vibrational states correspond to energies up to only about 10 eV
, different active media
are needed for X-ray lasers.
plasmas
, created in a capillary discharge or when a linearly focused optical pulse hits a solid target. In accordance with the Saha ionization equation
, the most stable electron configurations are neon
-like with 10 electrons remaining and nickel
-like with 28 electrons remaining. The electron transitions in highly ionized plasmas usually correspond to energies on the order of hundreds of electron volts (eV).
An alternative amplifying medium is the relativistic electron beam in a free electron laser
, which, strictly speaking, uses stimulated Compton scattering
instead of stimulated emission.
A different approach to optically induced coherent X-ray generation high-harmonic generation
, stimulated Thomson scattering
or radiation produced by oscillating electrons during laser wakefield acceleration.
medical imaging, and material
surface research.
Stimulated emission
In optics, stimulated emission is the process by which an atomic electron interacting with an electromagnetic wave of a certain frequency may drop to a lower energy level, transferring its energy to that field. A photon created in this manner has the same phase, frequency, polarization, and...
to generate or amplify electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
in the near X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
or extreme ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
region of the spectrum, that is, usually on the order of several of tens of nanometers (nm) wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
.
Because of high gain in the lasing medium, short upper-state lifetimes (1–100 ps
Picosecond
A picosecond is 10−12 of a second. That is one trillionth, or one millionth of one millionth of a second, or 0.000 000 000 001 seconds. A picosecond is to one second as one second is to 31,700 years....
), and problems associated with construction of X-ray mirrors, X-ray lasers usually operate without any resonator
Resonator
A resonator is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonant frequencies, with greater amplitude than at others. The oscillations in a resonator can be either electromagnetic or mechanical...
. The emitted radiation, based on amplified spontaneous emission
Amplified spontaneous emission
Amplified spontaneous emission or superluminescence is light, produced by spontaneous emission, that has been optically amplified by the process of stimulated emission in a gain medium. It is inherent in the field of random lasers....
, has relatively low spatial coherence. The line is mostly Doppler broadened
Doppler broadening
In atomic physics, Doppler broadening is the broadening of spectral lines due to the Doppler effect caused by a distribution of velocities of atoms or molecules. Different velocities of the emitting particles result in different shifts, the cumulative effect of which is the line broadening.The...
, which depends on the ions' temperature.
As the common visible-light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
laser transitions between electronic
Molecular electronic transition
Molecular electronic transitions take place when electrons in a molecule are excited from one energy level to a higher energy level. The energy change associated with this transition provides information on the structure of a molecule and determines many molecular properties such as color...
or vibrational states correspond to energies up to only about 10 eV
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
, different active media
Active laser medium
The active laser medium is the source of optical gain within a laser. The gain results from the stimulated emission of electronic or molecular transitions to a lower energy state from a higher energy state...
are needed for X-ray lasers.
X-ray laser active media
The most often used media include highly ionizedIonization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
plasmas
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
, created in a capillary discharge or when a linearly focused optical pulse hits a solid target. In accordance with the Saha ionization equation
Saha ionization equation
The Saha ionization equation, also known as the Saha–Langmuir equation, was developed by the Indian astrophysicist Meghnad Saha in 1920, and later by Irving Langmuir. One of the important applications of the equation was in explaining the spectral classification of stars...
, the most stable electron configurations are neon
Neon
Neon is the chemical element that has the symbol Ne and an atomic number of 10. Although a very common element in the universe, it is rare on Earth. A colorless, inert noble gas under standard conditions, neon gives a distinct reddish-orange glow when used in either low-voltage neon glow lamps or...
-like with 10 electrons remaining and nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
-like with 28 electrons remaining. The electron transitions in highly ionized plasmas usually correspond to energies on the order of hundreds of electron volts (eV).
- Capillary plasma-discharge media: In this setup, a several centimeters long capillary made of resistant material (e.g., alumina) confines a high-current, submicrosecond electrical pulse in a low-pressure gas. The Lorentz forceLorentz forceIn physics, the Lorentz force is the force on a point charge due to electromagnetic fields. It is given by the following equation in terms of the electric and magnetic fields:...
causes further compression of the plasma discharge (see pinchPinch (plasma physics)A pinch is the compression of an electrically conducting filament by magnetic forces. The conductor is usually a plasma, but could also be a solid or liquid metal...
). In addition, a pre-ionization electric or optical pulse is often used. An example is the capillary neon-like Ar8+ laser (generating radiation at 47 nm).
- Solid-slab target media: After being hit by an optical pulse, the target emits highly excited plasma. Again, a longer "pre-pulse" is often used for plasma creation and a second, shorter and more energetic pulse is used for further excitation in the plasma volume. For short lifetimes, a sheared excitation pulse may be needed (GRIP - grazing incidence pump). The gradientGradientIn vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....
in the refractive indexRefractive indexIn optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
of the plasma causes the amplified pulse to bend from the target surface, because at the frequencies above resonance the refractive index decreases with matter density. This can be compensated for by using curved targets or multiple targets in series.
- Plasma excited by optical field: At optical densities high enough to cause effective electron tunnellingQuantum tunnellingQuantum tunnelling refers to the quantum mechanical phenomenon where a particle tunnels through a barrier that it classically could not surmount. This plays an essential role in several physical phenomena, such as the nuclear fusion that occurs in main sequence stars like the sun, and has important...
, or even to suppress the potential barrier (> 1016 W/cm2), it is possible to highly ionize gas without contact with any capillary or target. A collinear setup is usually used, enabling the synchronization of pump and signal pulses.
An alternative amplifying medium is the relativistic electron beam in a free electron laser
Free electron laser
A free-electron laser, or FEL, is a laser that shares the same optical properties as conventional lasers such as emitting a beam consisting of coherent electromagnetic radiation which can reach high power, but which uses some very different operating principles to form the beam...
, which, strictly speaking, uses stimulated Compton scattering
Compton scattering
In physics, Compton scattering is a type of scattering that X-rays and gamma rays undergo in matter. The inelastic scattering of photons in matter results in a decrease in energy of an X-ray or gamma ray photon, called the Compton effect...
instead of stimulated emission.
A different approach to optically induced coherent X-ray generation high-harmonic generation
High Harmonic Generation
- Perturbative Harmonic Generation :Perturbative Harmonic Generation is a process whereby laser light of frequency ω and photon energy ħω can be used to generate new frequencies of light. The newly generated frequencies are integer multiples nħω of the original light's frequency...
, stimulated Thomson scattering
Thomson scattering
Thomson scattering is the elastic scattering of electromagnetic radiation by a free charged particle, as described by classical electromagnetism. It is just the low-energy limit of Compton scattering: the particle kinetic energy and photon frequency are the same before and after the scattering...
or radiation produced by oscillating electrons during laser wakefield acceleration.
Applications
Applications of coherent X-ray radiation include research into dense plasmas (not transparent to visible radiation), X-ray microscopy, phase-resolvedPhase contrast microscopy
Phase contrast microscopy is an optical microscopy illumination technique of great importance to biologists in which small phase shifts in the light passing through a transparent specimen are converted into amplitude or contrast changes in the image.A phase contrast microscope does not require...
medical imaging, and material
Materials science
Materials science is an interdisciplinary field applying the properties of matter to various areas of science and engineering. This scientific field investigates the relationship between the structure of materials at atomic or molecular scales and their macroscopic properties. It incorporates...
surface research.
See also
- Strategic Defense Initiative X-ray laser and Project ExcaliburProject ExcaliburProject Excalibur was a United States government nuclear weapons research program to develop a nuclear pumped x-ray laser for ballistic missile defence. It became part of the Strategic Defense Initiative . Conceived by nuclear scientist Edward Teller, the concept involved packing large numbers...
- European x-ray free electron laserEuropean x-ray free electron laserThe European x-ray free electron laser is an international project with 14 participating countries that is located in the German federal states of Hamburg and Schleswig-Holstein. A free electron laser generates high-intensity electromagnetic radiation by accelerating electrons to relativistic...
- Industrial CT scanningIndustrial CT ScanningIndustrial CT scanning is a process which uses X-ray equipment to produce three-dimensional representations of components both externally and internally. Industrial CT scanning has been used in many areas of industry for internal inspection of components...