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142-591: The European X-Ray Free-Electron Laser Facility ( European XFEL ) is an X-ray research laser facility commissioned during 2017. The first laser pulses were produced in May 2017 and the facility started user operation in September 2017. The international project with twelve participating countries; nine shareholders at the time of commissioning (Denmark, France, Germany, Hungary, Poland, Russia, Slovakia, Sweden and Switzerland), later joined by three other partners (Italy, Spain and

284-409: A diffraction grating . At a particular distance this pattern resembles exactly the structure of the grating and is recorded by a detector. The position of the interference pattern can be altered by bringing an object in the beam, that induces a phase shift. This displacement of the interference pattern is measured with the help of a second grating, and by certain reconstruction methods, information about

426-519: A gain medium , a mechanism to energize it, and something to provide optical feedback . The gain medium is a material with properties that allow it to amplify light by way of stimulated emission. Light of a specific wavelength that passes through the gain medium is amplified (power increases). Feedback enables stimulated emission to amplify predominantly the optical frequency at the peak of the gain-frequency curve. As stimulated emission grows, eventually one frequency dominates over all others, meaning that

568-471: A lens system, as is always included, for instance, in a laser pointer whose light originates from a laser diode . That is possible due to the light being of a single spatial mode. This unique property of laser light, spatial coherence , cannot be replicated using standard light sources (except by discarding most of the light) as can be appreciated by comparing the beam from a flashlight (torch) or spotlight to that of almost any laser. A laser beam profiler

710-458: A phase moiré effect by Wen and colleagues. It led to interferometry beyond the Talbot self-imaging range, using only phase gratings and conventional sources and detectors. X-ray phase gratings can be made with very fine periods, thereby allowing imaging at low radiation doses to achieve high sensitivity. Conventional X-ray imaging uses the drop in intensity through attenuation caused by an object in

852-583: A Bragg crystal as angular filter to reflect only a small part of the beam fulfilling the Bragg condition onto a detector. Important contributions to the progress of this method have been made by a US collaboration of the research teams of Dean Chapman, Zhong Zhong and William Thomlinson, for example the extracting of an additional signal caused by ultra-small angle scattering and the first CT image made with analyzer-based imaging. An alternative to analyzer-based imaging, which provides equivalent results without requiring

994-456: A broad range of techniques and scientific fields, from classical phase-contrast X-ray imaging to coherent X-ray diffraction imaging ( CXDI ) and with applications, e.g. in strain imaging inside nanostructured materials to bio-imaging of whole cells. In many cases the aim is to obtain a 3D representation of the investigated structure. By phase retrieval methods it is possible to pass from the measured diffraction patterns in reciprocal space to

1136-464: A broad spectrum of light or emit different wavelengths of light simultaneously. Certain lasers are not single spatial mode and have light beams that diverge more than is required by the diffraction limit . All such devices are classified as "lasers" based on the method of producing light by stimulated emission. Lasers are employed where light of the required spatial or temporal coherence can not be produced using simpler technologies. A laser consists of

1278-436: A coherent beam has been formed. The process of stimulated emission is analogous to that of an audio oscillator with positive feedback which can occur, for example, when the speaker in a public-address system is placed in proximity to the microphone. The screech one hears is audio oscillation at the peak of the gain-frequency curve for the amplifier. For the gain medium to amplify light, it needs to be supplied with energy in

1420-461: A cost of €850 million, under the provision that it should be financed as a European project. The European XFEL GmbH that built and operates the facility was founded in 2009. Civil construction of the facility began on 8 January 2009. Construction of the tunnels was completed in summer 2012, and all underground construction was completed the following year. The first beams were accelerated in April 2017, and

1562-632: A crystal interferometer , made from a large and highly perfect single crystal . Not less than 30 years later the Japanese scientists Atsushi Momose , Tohoru Takeda and co-workers adopted this idea and refined it for application in biological imaging, for instance by increasing the field of view with the assistance of new setup configurations and phase retrieval techniques. The Bonse–Hart interferometer provides several orders of magnitude higher sensitivity in biological samples than other phase-contrast techniques, but it cannot use conventional X-ray tubes because

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1704-419: A device lacks the spatial and temporal coherence achievable with lasers. Such a device cannot be described as an oscillator but rather as a high-gain optical amplifier that amplifies its spontaneous emission. The same mechanism describes so-called astrophysical masers /lasers. The optical resonator is sometimes referred to as an "optical cavity", but this is a misnomer: lasers use open resonators as opposed to

1846-508: A gain medium must have a gain bandwidth sufficiently broad to amplify those frequencies. An example of a suitable material is titanium -doped, artificially grown sapphire ( Ti:sapphire ), which has a very wide gain bandwidth and can thus produce pulses of only a few femtoseconds duration. Such mode-locked lasers are a most versatile tool for researching processes occurring on extremely short time scales (known as femtosecond physics, femtosecond chemistry and ultrafast science ), for maximizing

1988-480: A given pulse energy, this requires creating pulses of the shortest possible duration utilizing techniques such as Q-switching . The optical bandwidth of a pulse cannot be narrower than the reciprocal of the pulse width. In the case of extremely short pulses, that implies lasing over a considerable bandwidth, quite contrary to the very narrow bandwidths typical of CW lasers. The lasing medium in some dye lasers and vibronic solid-state lasers produces optical gain over

2130-674: A grating Bonse–Hart interferometer. At the same time, two further approaches to phase-contrast imaging emerged with the aim to overcome the problems of crystal interferometry. The propagation-based imaging technique was primarily introduced by the group of Anatoly Snigirev  [ de ] at the ESRF (European Synchrotron Radiation Facility) in Grenoble, France, and was based on the detection of "Fresnel fringes" that arise under certain circumstances in free-space propagation. The experimental setup consisted of an inline configuration of an X-ray source,

2272-399: A higher energy level with energy difference ΔE, it will not stay that way forever. Eventually, a photon will be spontaneously created from the vacuum having energy ΔE. Conserving energy, the electron transitions to a lower energy level that is not occupied, with transitions to different levels having different time constants. This process is called spontaneous emission . Spontaneous emission is

2414-471: A laser beam, it is highly collimated : the wavefronts are planar, normal to the direction of propagation, with no beam divergence at that point. However, due to diffraction , that can only remain true well within the Rayleigh range . The beam of a single transverse mode (gaussian beam) laser eventually diverges at an angle that varies inversely with the beam diameter, as required by diffraction theory. Thus,

2556-471: A laser is normally a material of controlled purity, size, concentration, and shape, which amplifies the beam by the process of stimulated emission described above. This material can be of any state : gas, liquid, solid, or plasma . The gain medium absorbs pump energy, which raises some electrons into higher energy (" excited ") quantum states . Particles can interact with light by either absorbing or emitting photons. Emission can be spontaneous or stimulated. In

2698-401: A laser to be focused to a tight spot, enabling applications such as optical communication, laser cutting , and lithography . It also allows a laser beam to stay narrow over great distances ( collimation ), a feature used in applications such as laser pointers , lidar , and free-space optical communication . Lasers can also have high temporal coherence , which permits them to emit light with

2840-427: A lightbulb or a star, the energy is emitted from many different levels giving photons with a broad range of energies. This process is called thermal radiation . The underlying physical process creating photons in a laser is the same as in thermal radiation, but the actual emission is not the result of random thermal processes. Instead, the release of a photon is triggered by the nearby passage of another photon. This

2982-433: A more intense and versatile source of X-rays than X-ray tubes ; this, combined with progress in the development of X-rays optics, was fundamental for the further advancement of X-ray physics. The pioneer work to the implementation of the phase-contrast method to X-ray physics was presented in 1965 by Ulrich Bonse and Michael Hart, Department of Materials Science and Engineering of Cornell University, New York. They presented

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3124-418: A multi-level system as a method for obtaining the population inversion, later a main method of laser pumping. Townes reports that several eminent physicists—among them Niels Bohr , John von Neumann , and Llewellyn Thomas —argued the maser violated Heisenberg's uncertainty principle and hence could not work. Others such as Isidor Rabi and Polykarp Kusch expected that it would be impractical and not worth

3266-448: A pair of diffracted beams that co-propagate from the third grating to the camera. They interfere with each other to produce intensity fringes if the gratings are slightly misaligned with each other. The central pair of diffraction paths are always equal in length regardless of the x-ray energy or the angle of the incident beam. The interference patterns from different photon energies and incident angles are locked in phase. The imaged object

3408-434: A phase shift image of the object. This technique achieved substantially higher sensitivity than other techniques with the exception of the crystal interferometer. A basic limitation of the technique is the chromatic dispersion of grating diffraction, which limits its spatial resolution. A tabletop system with a tungsten-target x-ray tube running at 60 kVp will have a limiting resolution of 60 μm. Another constraint

3550-441: A process called pumping . The energy is typically supplied as an electric current or as light at a different wavelength. Pump light may be provided by a flash lamp or by another laser. The most common type of laser uses feedback from an optical cavity —a pair of mirrors on either end of the gain medium. Light bounces back and forth between the mirrors, passing through the gain medium and being amplified each time. Typically one of

3692-462: A quantum-mechanical effect and a direct physical manifestation of the Heisenberg uncertainty principle . The emitted photon has a random direction, but its wavelength matches the absorption wavelength of the transition. This is the mechanism of fluorescence and thermal emission . A photon with the correct wavelength to be absorbed by a transition can also cause an electron to drop from the higher to

3834-707: A real space visualization of the scattering object. Complex nanoscale dynamics is an ubiquitous phenomenon of fundamental interest at the forefront of condensed matter science, and comprises a multitude of processes from visco-elastic flow or dissipation in liquids and glasses to polymer dynamics, protein folding, crystalline phase transitions, ultrafast spin transitions, domain wall dynamics, magnetic domain switching and many more. The extremely brilliant and highly coherent X-ray beams will open up unseen possibilities to study dynamics in disordered systems down to atomic length scales, with timescales ranging from femtoseconds to seconds using techniques such as XPCS . The experiments in

3976-418: A relatively long time. In laser physics , such a material is called an active laser medium . Combined with an energy source that continues to "pump" energy into the material, it is possible to have enough atoms or molecules in an excited state for a chain reaction to develop. Lasers are distinguished from other light sources by their coherence . Spatial (or transverse) coherence is typically expressed through

4118-746: A sample and a detector and did not require any optical elements. It was conceptually identical to the setup of Dennis Gabor's revolutionary work on holography in 1948. An alternative approach called analyzer-based imaging was first explored in 1995 by Viktor Ingal and Elena Beliaevskaya at the X-ray laboratory in Saint Petersburg, Russia, and by Tim Davis and colleagues at the CSIRO (Commonwealth Scientific and Industrial Research Organisation) Division of Material Science and Technology in Clayton, Australia. This method uses

4260-418: A sample. Then, one can obtain a tomogram which maps the value f ." In other words, in phase-contrast imaging a map of the real part of the refraction index δ(x,y,z) can be reconstructed with standard techniques like filtered back projection which is analog to conventional X-ray computed tomography where a map of the imaginary part of the refraction index can be retrieved. To get information about

4402-432: A seminar on this idea, and Charles H. Townes asked him for a copy of the paper. In 1953, Charles H. Townes and graduate students James P. Gordon and Herbert J. Zeiger produced the first microwave amplifier, a device operating on similar principles to the laser, but amplifying microwave radiation rather than infrared or visible radiation. Townes's maser was incapable of continuous output. Meanwhile, in

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4544-431: A small volume of material at the surface of a workpiece can be evaporated if it is heated in a very short time, while supplying the energy gradually would allow for the heat to be absorbed into the bulk of the piece, never attaining a sufficiently high temperature at a particular point. Other applications rely on the peak pulse power (rather than the energy in the pulse), especially to obtain nonlinear optical effects. For

4686-566: A synchronized optical laser allow for capturing ultrafast nuclear dynamics with very high resolution. The scope of the MID instrument are material science experiments using the unprecedented coherent properties of the X-ray laser beams of the European XFEL. The scientific applications reach from condensed matter physics, studying for example glass formation and magnetism, to soft and biological material, such as colloids, cells and viruses. Imaging covers

4828-430: A tool to safely determine the inner structures of different objects, although the information was for a long time obtained by measuring the transmitted intensity of the waves only, and the phase information was not accessible. The principle of phase-contrast imaging was first developed by Frits Zernike during his work with diffraction gratings and visible light. The application of his knowledge to microscopy won him

4970-423: A total of ten experimental stations. The experimental beamlines enable unique scientific experiments using the high intensity, coherence and time structure of the new source to be conducted in a variety of disciplines spanning physics , chemistry , materials science , biology and nanotechnology . The German Federal Ministry of Education and Research granted permission to build the facility on 5 June 2007 at

5112-431: A very high energy and temporal resolution. The FFT and CHEM chambers can be both coupled to a forward scattering DEPMOS Sensor with Signal Compression (DSSC) detector. SCS offers a variety of different optical sources to be used as a pump to induce transient states or photoactivated reactions in the samples. All the end-stations are equipped with an optical laser in-coupling which allows for spatial and temporal overlap of

5254-646: A very narrow frequency spectrum . Temporal coherence can also be used to produce ultrashort pulses of light with a broad spectrum but durations as short as an attosecond . Lasers are used in optical disc drives , laser printers , barcode scanners , DNA sequencing instruments , fiber-optic and free-space optical communications, semiconductor chip manufacturing ( photolithography , etching ), laser surgery and skin treatments, cutting and welding materials, military and law enforcement devices for marking targets and measuring range and speed, and in laser lighting displays for entertainment. Semiconductor lasers in

5396-430: A wide bandwidth, making a laser possible that can thus generate pulses of light as short as a few femtoseconds (10 s). In a Q-switched laser, the population inversion is allowed to build up by introducing loss inside the resonator which exceeds the gain of the medium; this can also be described as a reduction of the quality factor or 'Q' of the cavity. Then, after the pump energy stored in the laser medium has approached

5538-492: A wide range of technologies addressing many different motivations. Some lasers are pulsed simply because they cannot be run in continuous mode. In other cases, the application requires the production of pulses having as large an energy as possible. Since the pulse energy is equal to the average power divided by the repetition rate, this goal can sometimes be satisfied by lowering the rate of pulses so that more energy can be built up between pulses. In laser ablation , for example,

5680-479: Is a general term for different technical methods that use information concerning changes in the phase of an X-ray beam that passes through an object in order to create its images. Standard X-ray imaging techniques like radiography or computed tomography (CT) rely on a decrease of the X-ray beam's intensity ( attenuation ) when traversing the sample , which can be measured directly with the assistance of an X-ray detector . However, in phase contrast X-ray imaging,

5822-404: Is a transition between energy levels that match the energy carried by the photon or phonon. For light, this means that any given transition will only absorb one particular wavelength of light. Photons with the correct wavelength can cause an electron to jump from the lower to the higher energy level. The photon is consumed in this process. When an electron is excited from one state to that at

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5964-477: Is also required for three-level lasers in which the lower energy level rapidly becomes highly populated, preventing further lasing until those atoms relax to the ground state. These lasers, such as the excimer laser and the copper vapor laser, can never be operated in CW mode. In 1917, Albert Einstein established the theoretical foundations for the laser and the maser in the paper " Zur Quantentheorie der Strahlung " ("On

6106-406: Is an anacronym that originated as an acronym for light amplification by stimulated emission of radiation . The first laser was built in 1960 by Theodore Maiman at Hughes Research Laboratories , based on theoretical work by Charles H. Townes and Arthur Leonard Schawlow . A laser differs from other sources of light in that it emits light that is coherent . Spatial coherence allows

6248-441: Is called stimulated emission . For this process to work, the passing photon must be similar in energy, and thus wavelength, to the one that could be released by the atom or molecule, and the atom or molecule must be in the suitable excited state. The photon that is emitted by stimulated emission is identical to the photon that triggered its emission, and both photons can go on to trigger stimulated emission in other atoms, creating

6390-413: Is called an optical amplifier . When an optical amplifier is placed inside a resonant optical cavity, one obtains a laser. For lasing media with extremely high gain, so-called superluminescence , light can be sufficiently amplified in a single pass through the gain medium without requiring a resonator. Although often referred to as a laser (see, for example, nitrogen laser ), the light output from such

6532-409: Is considered to be the most sensitive to the phase shift, of the 4 methods, consequently providing the highest density resolution in range of mg/cm . But due to its high sensitivity, the fringes created by a strongly phase-shifting sample may become unresolvable; to overcome this problem a new approach called "coherence-contrast X-ray imaging" has been developed recently, where instead of the phase shift

6674-407: Is diffracted by a first grating of period 2P into two beams. These are further diffracted by a second grating of period P into four beams. Two of the four merge at a third grating of period 2P. Each is further diffracted by the third grating. The multiple diffracted beams are allowed to propagate for sufficient distance such that the different diffraction orders are separated at the camera. There exists

6816-421: Is formed by single-frequency quantum photon states distributed according to a Poisson distribution . As a result, the arrival rate of photons in a laser beam is described by Poisson statistics. Many lasers produce a beam that can be approximated as a Gaussian beam ; such beams have the minimum divergence possible for a given beam diameter. Some lasers, particularly high-power ones, produce multimode beams, with

6958-443: Is frequently used in the field, meaning "to give off coherent light," especially about the gain medium of a laser; when a laser is operating, it is said to be " lasing ". The terms laser and maser are also used for naturally occurring coherent emissions, as in astrophysical maser and atom laser . A laser that produces light by itself is technically an optical oscillator rather than an optical amplifier as suggested by

7100-450: Is from a few microradians to tens of microradians and is related to the full width at half maximum (FWHM) of the rocking curve of the crystal. When the analyzer is perfectly aligned with the monochromator and thus positioned to the peak of the rocking curve, a standard X-ray radiograph with enhanced contrast is obtained because there is no blurring by scattered photons. Sometimes this is referred to as "extinction contrast". If, otherwise,

7242-421: Is impossible. In some other lasers, it would require pumping the laser at a very high continuous power level, which would be impractical, or destroying the laser by producing excessive heat. Such lasers cannot be run in CW mode. The pulsed operation of lasers refers to any laser not classified as a continuous wave so that the optical power appears in pulses of some duration at some repetition rate. This encompasses

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7384-406: Is in fact less than c ." The impact of the index of refraction on the behavior of the wave can be demonstrated with a wave propagating in an arbitrary medium with a fixed refractive index n . For reason of simplicity, a monochromatic plane wave with no polarization is assumed here. The wave propagates in direction normal to the surface of the medium, named z in this example (see figure on

7526-580: Is more sensitive to density variations in the sample than conventional transmission-based X-ray imaging . This leads to images with improved soft tissue contrast. In the last several years, a variety of phase-contrast X-ray imaging techniques have been developed, all of which are based on the observation of interference patterns between diffracted and undiffracted waves. The most common techniques are crystal interferometry, propagation-based imaging, analyzer-based imaging, edge-illumination and grating-based imaging (see below). The first to discover X-rays

7668-401: Is not intrinsically linked to the absorption of X-rays in the sample, the absorbed dose can potentially be reduced by using higher X-ray energies. As mentioned above, concerning visible light, the real part of the refractive index n can deviate strongly from unity (n of glass in visible light ranges from 1.5 to 1.8) while the deviation from unity for X-rays in different media is generally of

7810-444: Is placed near the central grating. Absolute phase images are obtained if the object intersects one of a pair of coherent paths. If the two paths both pass through the object at two locations which are separated by a lateral distance d, then a phase difference image of Φ(r) - Φ(r-d) is detected. Phase stepping one of the gratings is performed to retrieve the phase images. The phase difference image Φ(r) - Φ(r-d) can be integrated to obtain

7952-411: Is split at the first crystal (S) by Laue diffraction into two coherent beams, a reference beam which remains undisturbed and a beam passing through the sample. The second crystal (T) acts as a transmission mirror and causes the beams to converge one towards another. The two beams meet at the plane of the third crystal (A), which is sometimes called, the analyzer crystal, and create an interference pattern

8094-406: Is that the x-ray beam is slitted down to only tens of micrometers wide. A potential solution has been proposed in the form of parallel imaging with multiple slits. Analyzer-based imaging (ABI) is also known as diffraction-enhanced imaging , phase-dispersion Introscopy and multiple-image radiography Its setup consists of a monochromator (usually a single or double crystal that also collimates

8236-423: Is the atomic number , k the length of the wave vector , and r 0 the classical electron radius . This results in the following expressions for the two parts of the complex index of refraction: Inserting typical values of human tissue in the formulas given above shows that δ is generally three orders of magnitude larger than β within the diagnostic X-ray range. This implies that

8378-481: Is the length of the wave vector of the incident radiation and the second term on the right hand side is the first derivative of the phase in the diffraction direction. Since not the phase itself, but the first derivative of the phase front is measured, analyzer-based imaging is less sensitive to low spatial frequencies than crystal interferometry but more sensitive than PBI. Contrary to the former methods analyzer-based imaging usually provides phase information only in

8520-537: Is the requirement of a very high stability of the setup; the alignment of the crystals must be very precise and the path length difference between the beams should be smaller than the wavelength of the X-rays; to achieve this the interferometer is usually made out of a highly perfect single block of silicon by cutting out two grooves. By the monolithic production the very important spatial lattice coherence between all three crystals can be maintained relatively well but it limits

8662-504: Is to pump the laser material with a source that is itself pulsed, either through electronic charging in the case of flash lamps, or another laser that is already pulsed. Pulsed pumping was historically used with dye lasers where the inverted population lifetime of a dye molecule was so short that a high-energy, fast pump was needed. The way to overcome this problem was to charge up large capacitors which are then switched to discharge through flashlamps, producing an intense flash. Pulsed pumping

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8804-430: Is used to measure the intensity profile, width, and divergence of laser beams. Diffuse reflection of a laser beam from a matte surface produces a speckle pattern with interesting properties. The mechanism of producing radiation in a laser relies on stimulated emission , where energy is extracted from a transition in an atom or molecule. This is a quantum phenomenon that was predicted by Albert Einstein , who derived

8946-619: The Nobel Prize in Physics in 1953. Ever since, phase-contrast microscopy has been an important field of optical microscopy . The transfer of phase-contrast imaging from visible light to X-rays took a long time, due to slow progress in improving the quality of X-ray beams and the inaccessibility of X-ray lenses. In the 1970s, it was realized that the synchrotron radiation , emitted from charged particles circulating in storage rings constructed for high-energy nuclear physics experiments, may have been

9088-419: The magnetic fields of special arrays of magnets called undulators , where they follow slalom like trajectories resulting in the emission of X-rays whose wavelength is in the range of 0.05 to 4.7  nm . The X-rays are generated by self-amplified spontaneous emission (SASE), where electrons interact with the radiation that they or their neighbours emit. Since it is not possible to build mirrors to reflect

9230-410: The phase of the emitted light is 90 degrees in lead of the stimulating light. This, combined with the filtering effect of the optical resonator gives laser light its characteristic coherence, and may give it uniform polarization and monochromaticity, depending on the resonator's design. The fundamental laser linewidth of light emitted from the lasing resonator can be orders of magnitude narrower than

9372-421: The transverse modes often approximated using Hermite – Gaussian or Laguerre -Gaussian functions. Some high-power lasers use a flat-topped profile known as a " tophat beam ". Unstable laser resonators (not used in most lasers) produce fractal-shaped beams. Specialized optical systems can produce more complex beam geometries, such as Bessel beams and optical vortexes . Near the "waist" (or focal region ) of

9514-410: The velocity of light c . This leads to a different behavior of X-rays in a medium compared to visible light (e.g. refractive angles have negative values) but does not contradict the law of relativity , "which requires that only signals carrying information do not travel faster than c . Such signals move with the group velocity , not with the phase velocity, and it can be shown that the group velocity

9656-497: The "pencil beam" directly generated by a common helium–neon laser would spread out to a size of perhaps 500 kilometers when shone on the Moon (from the distance of the Earth). On the other hand, the light from a semiconductor laser typically exits the tiny crystal with a large divergence: up to 50°. However even such a divergent beam can be transformed into a similarly collimated beam employing

9798-670: The Quantum Theory of Radiation") via a re-derivation of Max Planck 's law of radiation, conceptually based upon probability coefficients ( Einstein coefficients ) for the absorption, spontaneous emission, and stimulated emission of electromagnetic radiation. In 1928, Rudolf W. Ladenburg confirmed the existence of the phenomena of stimulated emission and negative absorption. In 1939, Valentin A. Fabrikant predicted using stimulated emission to amplify "short" waves. In 1947, Willis E. Lamb and R.   C.   Retherford found apparent stimulated emission in hydrogen spectra and effected

9940-503: The Soviet Union, Nikolay Basov and Aleksandr Prokhorov were independently working on the quantum oscillator and solved the problem of continuous-output systems by using more than two energy levels. These gain media could release stimulated emissions between an excited state and a lower excited state, not the ground state, facilitating the maintenance of a population inversion . In 1955, Prokhorov and Basov suggested optical pumping of

10082-534: The United Kingdom), 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 speeds and directing them through special magnetic structures. The European XFEL is constructed such that the electrons produce X-ray light in synchronisation, resulting in high-intensity X-ray pulses with

10224-482: The University of Tokyo. In 2005, independently from each other, both David's and Momose's group incorporated computed tomography into grating interferometry, which can be seen as the next milestone in the development of grating-based imaging. In 2006, another great advancement was the transfer of the grating-based technique to conventional laboratory X-ray tubes by Franz Pfeiffer and co-workers, which fairly enlarged

10366-423: The X-ray beam and the radiation is treated as rays like in geometrical optics . But when X-rays pass through an object, not only their amplitude but their phase is altered as well. Instead of simple rays , X-rays can also be treated as electromagnetic waves . An object then can be described by its complex refractive index (cf. ): The term δ is the decrement of the real part of the refractive index, and

10508-580: The X-rays and optical laser pulses at the interaction point. The SQS instrument is developed to investigate fundamental processes of light-matter interaction in the soft X-ray wavelength radiation. Typical objects of investigation are in the range from isolated atoms to large bio-molecules, and typical methods are variety of spectroscopic techniques. The SQS instrument provides three experimental stations: Photon energy range between 260 eV and 3000 eV (4.8 nm to 0.4 nm). The ultrashort FEL pulses of less than 50 fs duration in combination with

10650-461: The X-rays for multiple passes through the electron beam gain medium, as with light lasers, the X-rays are generated in a single pass through the beam. The result is spontaneous emission of X-ray photons which are coherent (in phase) like laser light, unlike X-rays emitted by ordinary sources like X-ray machines , which are incoherent. The peak brilliance of the European XFEL is billions of times higher than that of conventional X-ray light sources, while

10792-398: The absorption cross section is approximately stated by where 0.02 is a constant given in barn , the typical unit of particle interaction cross section area, k the length of the wave vector , k 0 the length of a wave vector with wavelength of 1 Angstrom and Z the atomic number . The valid formula under these conditions for the phase shift cross section is: where Z

10934-440: The absorption edges (peaks in the absorption cross-section due to the enhanced probability for the absorption of a photon that has a frequency close to the resonance frequency of the medium), dispersion effects can be neglected; this is the case for light elements ( atomic number Z <40) that are the components of human tissue and X-ray energies above 20 keV, which are typically used in medical imaging. Assuming these conditions,

11076-423: The acronym. It has been humorously noted that the acronym LOSER, for "light oscillation by stimulated emission of radiation", would have been more correct. With the widespread use of the original acronym as a common noun, optical amplifiers have come to be referred to as laser amplifiers . Photons , the quanta of electromagnetic radiation are released and absorbed from energy levels in atoms and molecules. In

11218-413: The analyzer at a specific angle and rotating the sample through 360° while the projection data are acquired. Several sets of projections are acquired from the same sample with different detuning angles and then a tomographic image can be reconstructed. Assuming that the crystals are normally aligned such that the derivative of the refractive index is measured in the direction parallel to the tomographic axis,

11360-400: The analyzer is oriented at a small angle (detuning angle) with respect to the monochromator then X-rays refracted in the sample by a smaller angle will be reflected less, and X-rays refracted by a larger angle will be reflected more. Thus the contrast of the image is based on different refraction angles in the sample. For small phase gradients the refraction angle can be expressed as where k

11502-411: The average brilliance is 10,000 times higher. The higher electron energy allows the production of shorter wavelengths. The duration of the light pulses can be less than 100 femtoseconds . There are seven instruments at European XFEL, run by scientists from all over the world. SCS is the soft X-rays spectroscopy and scattering instrument of the European XFEL. The scientific interest of SCS is focused on

11644-514: The beam's phase shift caused by the sample is not measured directly, but is transformed into variations in intensity, which then can be recorded by the detector. In addition to producing projection images , phase contrast X-ray imaging, like conventional transmission, can be combined with tomographic techniques to obtain the 3D distribution of the real part of the refractive index of the sample. When applied to samples that consist of atoms with low atomic number Z , phase contrast X-ray imaging

11786-521: The beam) in front of the sample and an analyzer crystal positioned in Bragg geometry between the sample and the detector. (See figure to the right) This analyzer crystal acts as an angular filter for the radiation coming from the sample. When these X-rays hit the analyzer crystal the condition of Bragg diffraction is satisfied only for a very narrow range of incident angles. When the scattered or refracted X-rays have incident angles outside this range they will not be reflected at all and don't contribute to

11928-476: The beam. A beam produced by a thermal or other incoherent light source has an instantaneous amplitude and phase that vary randomly with respect to time and position, thus having a short coherence length. Lasers are characterized according to their wavelength in a vacuum . Most "single wavelength" lasers produce radiation in several modes with slightly different wavelengths. Although temporal coherence implies some degree of monochromaticity , some lasers emit

12070-425: The blue to near-UV have also been used in place of light-emitting diodes (LEDs) to excite fluorescence as a white light source; this permits a much smaller emitting area due to the much greater radiance of a laser and avoids the droop suffered by LEDs; such devices are already used in some car headlamps . The first device using amplification by stimulated emission operated at microwave frequencies, and

12212-413: The change of the degree of coherence caused by the sample is relevant for the contrast of the image. A general limitation to the spatial resolution of this method is given by the blurring in the analyzer crystal which arises from dynamical refraction, i.e. the angular deviation of the beam due to the refraction in the sample is amplified about ten thousand times in the crystal, because the beam path within

12354-436: The compounding of a sample, basically the density distribution of the sample, one has to relate the measured values for the refractive index to intrinsic parameters of the sample, such a relation is given by the following formulas: where ρ a is the atomic number density, σ a the absorption cross section , k the length of the wave vector and where p the phase shift cross section. Far from

12496-446: The crystal depends strongly on its incident angle. This effect can be reduced by thinning down the analyzer crystal, e.g. with an analyzer thickness of 40 μ m a resolution of about 6 μ m was calculated. Alternatively the Laue crystals can be replaced by Bragg crystals , so the beam doesn't pass through the crystal but is reflected on the surface. Another constraint of the method

12638-476: The crystal interferometer is that the Laue crystals filter most of the incoming radiation, thus requiring a high beam intensity or very long exposure times. That limits the use of the method to highly brilliant X-ray sources like synchrotrons. According to the constraints on the setup the crystal interferometer works best for high-resolution imaging of small samples which cause small or smooth phase gradients . To have

12780-403: The crystals only accept a very narrow energy band of X-rays (Δ E / E ~ 10 ). In 2012, Han Wen and co-workers took a step forward by replacing the crystals with nanometric phase gratings. The gratings split and direct X-rays over a broad spectrum, thus lifting the restriction on the bandwidth of the X-ray source. They detected sub nano radian refractive bending of X-rays in biological samples with

12922-517: The crystals. Because only phase gratings are used, grating fabrication is less challenging than techniques that use absorption gratings. The first grating Bonse-Hart interferometer (gBH) operated at 22.5 keV photon energy and 1.5% spectral bandwidth. The incoming beam is shaped by slits of a few tens of micrometers such that the transverse coherence length is greater than the grating period. The interferometer consists of three parallel and equally spaced phase gratings, and an x-ray camera. The incident beam

13064-499: The diffraction direction, but is not sensitive to angular deviations on the plane perpendicular to the diffraction plane. This sensitivity to only one component of the phase gradient can lead to ambiguities in phase estimation. By recording several images at different detuning angles, meaning at different positions on the rocking curve, a data set is gained which allows the retrieval of quantitative differential phase information. There are several algorithms to reconstruct information from

13206-561: The effect of nonlinearity in optical materials (e.g. in second-harmonic generation , parametric down-conversion , optical parametric oscillators and the like). Unlike the giant pulse of a Q-switched laser, consecutive pulses from a mode-locked laser are phase-coherent; that is, the pulses (and not just their envelopes ) are identical and perfectly periodic. For this reason, and the extremely large peak powers attained by such short pulses, such lasers are invaluable in certain areas of research. Another method of achieving pulsed laser operation

13348-521: The effort. In 1964, Charles H. Townes, Nikolay Basov, and Aleksandr Prokhorov shared the Nobel Prize in Physics , "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser–laser principle". In April 1957, Japanese engineer Jun-ichi Nishizawa proposed the concept of a " semiconductor optical maser " in a patent application. Phase-contrast X-ray imaging Phase-contrast X-ray imaging or phase-sensitive X-ray imaging

13490-468: The experimental stations, laboratories and administrative buildings are located. Electrons are accelerated to an energy of up to 17.5  GeV by a 2.1 km (1.3 mi) long linear accelerator with superconducting RF-cavities . The use of superconducting acceleration elements developed at DESY allows up to 27,000 repetitions per second, significantly more than other X-ray lasers in the U.S. and Japan can achieve. The electrons are then introduced into

13632-459: The exploration of light-induced transient phenomena in quantum materials as well as in molecules. The beamline hosts a soft X-rays grating monochromator for monochromatic operations. The instrument is equipped with three main end-stations that can be coupled to different experimental probes: The CHEM and XRD chambers can be couple with a high-resolution resonant inelastic X-ray scattering spectrometer to perform pump and probe RIXS experiments with

13774-575: The facility are controlled via the in-house developed control system named Karabo . It is a distributed SCADA system written in C++ and python . The short laser pulses make it possible to measure chemical reactions that are too rapid to be captured by other methods. The wavelength of the X-ray laser may be varied from 0.05 to 4.7 nm, enabling measurements at the atomic length scale. Today, three photon beamlines with seven instruments can be used. Later this will be upgraded to five photon beamlines and

13916-439: The field of view to a small size,(e.g. 5 cm x 5 cm for a 6-inch ingot) and because the sample is normally placed in one of the beam paths the size of the sample itself is also constrained by the size of the silicon block. Recently developed configurations, using two crystals instead of one, enlarge the field of view considerably, but are even more sensitive to mechanical instabilities. Another additional difficulty of

14058-415: The first X-ray beams were produced in May 2017. XFEL was inaugurated in September 2017. The overall cost for the construction and commissioning of the facility is as of 2017 estimated at €1.22 billion (price levels of 2005). Laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation . The word laser

14200-595: The first demonstration of stimulated emission. In 1950, Alfred Kastler (Nobel Prize for Physics 1966) proposed the method of optical pumping , which was experimentally demonstrated two years later by Brossel, Kastler, and Winter. In 1951, Joseph Weber submitted a paper on using stimulated emissions to make a microwave amplifier to the June 1952 Institute of Radio Engineers Vacuum Tube Research Conference in Ottawa , Ontario, Canada. After this presentation, RCA asked Weber to give

14342-413: The first pre-clinical studies using grating-based phase-contrast X-ray imaging were published. Marco Stampanoni and his group examined native breast tissue with "differential phase-contrast mammography", and a team led by Dan Stutman investigated how to use grating-based imaging for the small joints of the hand. Most recently, a significant advance in grating-based imaging occurred due to the discovery of

14484-409: The form of which depends on the optical path difference between the two beams caused by the sample. This interference pattern is detected with an X-ray detector behind the analyzer crystal. By putting the sample on a rotation stage and recording projections from different angles, the 3D-distribution of the refractive index and thus tomographic images of the sample can be retrieved. In contrast to

14626-413: The imaginary part β describes the absorption index or extinction coefficient. Note that in contrast to optical light, the real part of the refractive index is less than but close to unity, this is "due to the fact that the X-ray spectrum generally lies to the high-frequency side of various resonances associated with the binding of electrons". The phase velocity inside of the object is larger than

14768-430: The integral where λ is the wavelength of the incident X-ray beam. This formula means that the phase shift is the projection of the decrement of the real part of the refractive index in imaging direction. This fulfills the requirement of the tomographic principle , which states that "the input data to the reconstruction algorithm should be a projection of a quantity f that conveys structural information inside

14910-448: The intensity of the dark-field signal depends on the orientation of the grid and this is due to the anisotropy of the bone structure. They made significant progress towards biomedical applications by replacing mechanical scanning of the gratings with electronic scanning of the X-ray source. The grating-based phase-contrast CT field was extended by tomographic images of the dark-field signal and time-resolved phase-contrast CT. Furthermore,

15052-422: The laser power inside the cavity; this equilibrium determines the operating point of the laser. If the applied pump power is too small, the gain will never be sufficient to overcome the cavity losses, and laser light will not be produced. The minimum pump power needed to begin laser action is called the lasing threshold . The gain medium will amplify any photons passing through it, regardless of direction; but only

15194-501: The lasing medium or pumping mechanism, then it is still classified as a "modulated" or "pulsed" continuous wave laser. Most laser diodes used in communication systems fall into that category. Some applications of lasers depend on a beam whose output power is constant over time. Such a laser is known as a continuous-wave ( CW ) laser. Many types of lasers can be made to operate in continuous-wave mode to satisfy such an application. Many of these lasers lase in several longitudinal modes at

15336-414: The latter case, the photon is emitted in the same direction as the light that is passing by. When the number of particles in one excited state exceeds the number of particles in some lower-energy state, population inversion is achieved. In this state, the rate of stimulated emission is larger than the rate of absorption of light in the medium, and therefore the light is amplified. A system with this property

15478-450: The linewidth of light emitted from the passive resonator. Some lasers use a separate injection seeder to start the process off with a beam that is already highly coherent. This can produce beams with a narrower spectrum than would otherwise be possible. In 1963, Roy J. Glauber showed that coherent states are formed from combinations of photon number states, for which he was awarded the Nobel Prize in physics . A coherent beam of light

15620-402: The literal cavity that would be employed at microwave frequencies in a maser . The resonator typically consists of two mirrors between which a coherent beam of light travels in both directions, reflecting on itself so that an average photon will pass through the gain medium repeatedly before it is emitted from the output aperture or lost to diffraction or absorption. If the gain (amplification) in

15762-522: The lower level, emitting a new photon. The emitted photon exactly matches the original photon in wavelength, phase, and direction. This process is called stimulated emission. The gain medium is put into an excited state by an external source of energy. In most lasers, this medium consists of a population of atoms that have been excited into such a state using an outside light source, or an electrical field that supplies energy for atoms to absorb and be transformed into their excited states. The gain medium of

15904-412: The maximum possible level, the introduced loss mechanism (often an electro- or acousto-optical element) is rapidly removed (or that occurs by itself in a passive device), allowing lasing to begin which rapidly obtains the stored energy in the gain medium. This results in a short pulse incorporating that energy, and thus a high peak power. A mode-locked laser is capable of emitting extremely short pulses on

16046-498: The medium is larger than the resonator losses, then the power of the recirculating light can rise exponentially . But each stimulated emission event returns an atom from its excited state to the ground state, reducing the gain of the medium. With increasing beam power, the net gain (gain minus loss) reduces to unity and the gain medium is said to be saturated. In a continuous wave (CW) laser, the balance of pump power against gain saturation and cavity losses produces an equilibrium value of

16188-437: The methods below, with the crystal interferometer the phase itself is measured and not any spatial alternation of it. To retrieve the phase shift out of the interference patterns; a technique called phase-stepping or fringe scanning is used: a phase shifter (with the shape of a wedge) is introduced in the reference beam. The phase shifter creates straight interference fringes with regular intervals; so called carrier fringes. When

16330-429: The order of 10 . Thus, the refraction angles caused at the boundary between two isotropic media calculated with Snell's formula are also very small. The consequence of this is that refraction angles of X-rays passing through a tissue sample cannot be detected directly and are usually determined indirectly by "observation of the interference pattern between diffracted and undiffracted waves produced by spatial variations of

16472-451: The order of tens of picoseconds down to less than 10  femtoseconds . These pulses repeat at the round-trip time, that is, the time that it takes light to complete one round trip between the mirrors comprising the resonator. Due to the Fourier limit (also known as energy–time uncertainty ), a pulse of such short temporal length has a spectrum spread over a considerable bandwidth. Thus such

16614-410: The output being a narrow beam, which is diffraction-limited . Laser beams can be focused to very tiny spots, achieving a very high irradiance , or they can have a very low divergence to concentrate their power at a great distance. Temporal (or longitudinal) coherence implies a polarized wave at a single frequency, whose phase is correlated over a relatively great distance (the coherence length ) along

16756-417: The phase-shift of an X-ray beam propagating through tissue may be much larger than the loss in intensity thus making phase contrast X-ray imaging more sensitive to density variations in the tissue than absorption imaging. Due to the proportionalities the advantage of phase contrast over conventional absorption contrast even grows with increasing energy. Furthermore, because the phase contrast image formation

16898-418: The photons in a spatial mode supported by the resonator will pass more than once through the medium and receive substantial amplification. In most lasers, lasing begins with spontaneous emission into the lasing mode. This initial light is then amplified by stimulated emission in the gain medium. Stimulated emission produces light that matches the input signal in direction, wavelength, and polarization, whereas

17040-406: The possibility of a chain reaction . For this to happen, many of the atoms or molecules must be in the proper excited state so that the photons can trigger them. In most materials, atoms or molecules drop out of excited states fairly rapidly, making it difficult or impossible to produce a chain reaction. The materials chosen for lasers are the ones that have metastable states , which stay excited for

17182-409: The power output is essentially continuous over time or whether its output takes the form of pulses of light on one or another time scale. Of course, even a laser whose output is normally continuous can be intentionally turned on and off at some rate to create pulses of light. When the modulation rate is on time scales much slower than the cavity lifetime and the period over which energy can be stored in

17324-593: The properties of laser light and at intensities much brighter than those produced by conventional synchrotron light sources. The 3.4-kilometre (2.1 mi) long tunnel for the European XFEL housing the superconducting linear accelerator and photon beamlines runs 6 to 38 m (20 to 125 ft) underground from the site of the DESY research center in Hamburg to the town of Schenefeld in Schleswig-Holstein, where

17466-662: The properties of the emitted light, such as the polarization, wavelength, and shape of the beam. Electrons and how they interact with electromagnetic fields are important in our understanding of chemistry and physics . In the classical view , the energy of an electron orbiting an atomic nucleus is larger for orbits further from the nucleus of an atom . However, quantum mechanical effects force electrons to take on discrete positions in orbitals . Thus, electrons are found in specific energy levels of an atom, two of which are shown below: An electron in an atom can absorb energy from light ( photons ) or heat ( phonons ) only if there

17608-531: The real part of the refractive index is gained. The so-called Talbot–Lau interferometer was initially used in atom interferometry , for instance by John F. Clauser and Shifang Li in 1994. The first X-ray grating interferometers using synchrotron sources were developed by Christian David and colleagues from the Paul Scherrer Institute (PSI) in Villingen, Switzerland and the group of Atsushi Momose from

17750-412: The real part of the refractive index." Crystal interferometry , sometimes also called X-ray interferometry , is the oldest but also the most complex method used for experimental realization. It consists of three beam splitters in Laue geometry aligned parallel to each other. (See figure to the right) The incident beam, which usually is collimated and filtered by a monochromator (Bragg crystal) before,

17892-457: The relationship between the A coefficient , describing spontaneous emission, and the B coefficient which applies to absorption and stimulated emission. In the case of the free electron laser , atomic energy levels are not involved; it appears that the operation of this rather exotic device can be explained without reference to quantum mechanics . A laser can be classified as operating in either continuous or pulsed mode, depending on whether

18034-417: The resulting "refraction CT image" shows the pure image of the out-of-plane gradient. For analyzer-based imaging, the stability requirements of the crystals is less strict than for crystal interferometry but the setup still requires a perfect analyzer crystal that needs to be very precisely controlled in angle and the size of the analyzer crystal and the constraint that the beam needs to be parallel also limits

18176-410: The right). The scalar wave function in vacuum is Within the medium, the angular wavenumber changes from k to nk . Now the wave can be described as: where δkz is the phase shift and e is an exponential decay factor decreasing the amplitude E 0 of the wave. In more general terms, the total phase shift of the beam propagating a distance z can be calculated by using

18318-415: The rocking curves, some of them provide an additional signal. This signal comes from Ultra-small-angle scattering by sub-pixel sample structures and causes angular broadening of the beam and hence a broadening of the shape of the rocking curve. Based on this scattering contrast a new kind of image called Dark-field image can be produced. Tomographic imaging with analyzer-based imaging can be done by fixing

18460-495: The same time, Han Wen and co-workers at the US National Institutes of Health arrived at a much simplified grating technique to obtain the scattering (“dark-field”) image. They used a single projection of a grid and a new approach for signal extraction named "single-shot Fourier analysis". Recently, a lot of research was done to improve the grating-based technique: Han Wen and his team analyzed animal bones and found out that

18602-410: The same time, and beats between the slightly different optical frequencies of those oscillations will produce amplitude variations on time scales shorter than the round-trip time (the reciprocal of the frequency spacing between modes), typically a few nanoseconds or less. In most cases, these lasers are still termed "continuous-wave" as their output power is steady when averaged over longer periods, with

18744-500: The sample is placed in the other beam, the carrier fringes are displaced. The phase shift caused by the sample corresponds to the displacement of the carrier fringes. Several interference patterns are recorded for different shifts of the reference beam and by analyzing them the phase information modulo 2 π can be extracted. This ambiguity of the phase is called the phase wrapping effect and can be removed by so-called "phase unwrapping techniques". These techniques can be used when

18886-482: The signal-to-noise ratio of the image is sufficiently high and phase variation is not too abrupt. As an alternative to the fringe scanning method, the Fourier-transform method can be used to extract the phase shift information with only one interferogram, thus shortening the exposure time, but this has the disadvantage of limiting the spatial resolution by the spacing of the carrier fringes. X-ray interferometry

19028-467: The signal. Refracted X-rays within this range will be reflected depending on the incident angle. The dependency of the reflected intensity on the incident angle is called a rocking curve and is an intrinsic property of the imaging system, i.e. it represents the intensity measured at each pixel of the detector when the analyzer crystal is "rocked" (slightly rotated in angle θ) with no object present and thus can be easily measured. The typical angular acceptance

19170-445: The superior sensitivity of crystal Bonse-Hart interferometry without some of the basic limitations, the monolithic crystals have been replaced with nanometric x-ray phase-shift gratings. The first such gratings have periods of 200 to 400 nanometers. They can split x-ray beams over the broad energy spectra of common x-ray tubes. The main advantage of this technique is that it uses most of the incoming x-rays that would have been filtered by

19312-427: The technique's potential for clinical use. About two years later the group of Franz Pfeiffer also accomplished to extract a supplementary signal from their experiments; the so-called "dark-field signal" was caused by scattering due to the porous microstructure of the sample and provided "complementary and otherwise inaccessible structural information about the specimen at the micrometer and submicrometer length scale". At

19454-425: The two mirrors, the output coupler , is partially transparent. Some of the light escapes through this mirror. Depending on the design of the cavity (whether the mirrors are flat or curved ), the light coming out of the laser may spread out or form a narrow beam . In analogy to electronic oscillators , this device is sometimes called a laser oscillator . Most practical lasers contain additional elements that affect

19596-537: The use of a crystal, was developed by Alessandro Olivo and co-workers at the Elettra synchrotron in Trieste, Italy. This method, called “edge-illumination”, operates a fine selection on the X-ray direction by using the physical edge of the detector pixels themselves, hence the name. Later on Olivo, in collaboration with Robert Speller at University College London, adapted the method for use with conventional X-ray sources, opening

19738-410: The very high-frequency power variations having little or no impact on the intended application. (However, the term is not applied to mode-locked lasers, where the intention is to create very short pulses at the rate of the round-trip time.) For continuous-wave operation, the population inversion of the gain medium needs to be continually replenished by a steady pump source. In some lasing media, this

19880-576: The way to translation into clinical and other applications. Peter Munro (also from UCL) substantially contributed to the development of the lab-based approach, by demonstrating that it imposes practically no coherence requirements and that, this notwithstanding, it still is fully quantitative. The latest approach discussed here is the so-called grating-based imaging, which makes use of the Talbot effect , discovered by Henry Fox Talbot in 1836. This self-imaging effect creates an interference pattern downstream of

20022-407: Was Wilhelm Conrad Röntgen in 1895, where he found that they had the ability to penetrate opaque materials. He recorded the first X-ray image, displaying the hand of his wife. He was awarded the first Nobel Prize in Physics in 1901 "in recognition of the extraordinary services he has rendered by the discovery of the remarkable rays subsequently named after him". Since then, X-rays have been used as

20164-598: Was called a maser , for "microwave amplification by stimulated emission of radiation". When similar optical devices were developed they were first called optical masers , until "microwave" was replaced by "light" in the acronym, to become laser . Today, all such devices operating at frequencies higher than microwaves (approximately above 300 GHz ) are called lasers (e.g. infrared lasers , ultraviolet lasers , X-ray lasers , gamma-ray lasers ), whereas devices operating at microwave or lower radio frequencies are called masers. The back-formed verb " to lase "

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