1. Stimulated Emission

    The theoretical foundations of Laser science were laid by Albert Einstein in his 1917 paper on the quantum theory of radiation: Quantentheorie der Strahlung (Physikalische Zeitschrift, 18, 121-128).

    In atoms, electrons normally occupy the lowest energy configuration – the ground state. However, electrons can be can be excited to a higher energy level by gaining the exact energy difference between the two levels. However, this excited state is not stable and the electron will spontaneously return, after some time, to its ground state whilst emitting the excess energy.

    This adsorbed/emitted energy is emitted as photon, a quantised unit of electromagnetic radiations, which light is a small subsection.

    There is another way for an excited species to emit a photons. Einstein predicted that the emission of a photon from an excited state can be stimulated from the interaction of another photon of the same energy with the excited state. The two resulting photons travel in the same direction with identical energy and phase.

    Albert Einstein
    A photon interacts with an electronically excited species
    Stimulated emission of a photon
  2. Optical Pumping

    In any normal system, the majority of particles are in their electronic ground state and short-lived excited particles quickly spontaneously relax to the ground state. In order for stimulated emission of photons to become a useful process, there needs to be a way to convert the majority of ground-state particles into an excited electronic state. This is known a population inversion.

    Nobel prize winner (1966) Alfred Kastler proved a solution to this problem by optical pumping with a suitable light-source such as an arc lamp, flash tube and later, a laser.

    The key process in maintaining the population inversion is a metastable energy level transition which has a very slow spontaneous emission rate, due to quantum mechanical restrictions. An optical pulse overexcites the particle which can then quickly relax to the slow emitting state. A population inversion occurs when this state is populated at a great rate than the slow spontaneous emission. A spontaneous emission from the metastable stable produces one photon which can then stimulate the emission of another photon which results in a cascade of identical photon emissions

    Optical Pumping schematic
    Alfred Kastler
  3. MASER – The Laser Prototype

    The theories of using stimulated emission of microwave radiation in conjunction with a population inversion were simultaneously developed by Joseph Weber (US) and Nikolay Basov and Alexander Prokhorov (USSR). Their research, which resulted in the Physics Nobel prize in 1964 led the way for a practical device. Microwaves technology had been heavily developed during World War II and subsequently academic research in the 1950s focused upon the improvement of the resolution of microwaves spectroscopic devices

    The initial devices used Einsten’s stimulated emissions theory to generate a coherent microwave radiation by probing well-known molecular microwaves transitions within a microwave resonator cavity. This process is known as Microwave Amplification by Stimulation Emission of Radiation (MASER). The first maser was made using a beam of excited ammonia (NH3) but the maser technique was expanded to use other gases and more recently solid-state devices. Masers are still in use as high quality, low-noise microwave amplifiers and as highly precise frequency references.

    The ammonia maser at Columbia University in 1953 with Charles H. Townes. The brass box is the vacuum chamber through which the ammonia ions travel. The ammonia gas nozzle is at left. The four rods at center are the quadrupole filter which filters out the lower state ammonia molecules, leaving a population inversion. In the resonant cavity at right, stimulated emission of microwaves by the molecules excites standing waves in the cavity, which pass out through the vertical output waveguide. The devices at bottom are vacuum pumps which evacuated the box
  4. The First Demonstration of a Laser

    The first working laser was fired by Theodore Harold Maiman (July 11, 1927 – May 5, 2007) on the 16th May 1960 and the Hughes Aircraft Company demonstrated the laser to the world on 7th July, 1960
    . T. H. Maiman started to work for the Hughes Aircraft Company in 1956. He was the leader of the ruby maser redesign project for the U. S. Army Signal Corps and following the success of this project, the company agreed on a $50,000 fund for Maiman’s laser project, starting in mid-1959. Maiman developed the laser using a synthetic ruby crystal and used a pulsed high-power quartz flashlight bulb to create the population inversion in the ruby crystal.
    A continuous He-Ne gas laser emitting IR radiation at 1.15 μm was demonstrated shortly afterwards.

    Theodore Harold Maiman
    The Ruby laser
    The internal workings of a Ruby laser
  5. Optical Fibers

    Elias Snitzer, whilst working for American Optical, developed the scientific theories and then built a single-mode fiber laser. Thin glass fibers had been demonstrated to guide light by total internal refraction in the mid-19th Century and had undergone numerous technical refinements and applications in the 20th Century. Snitzer had developed doped glass-based lasers and combining these two technologies led to the fiber laser. Three years later, he also demonstrated amplification in 1 m fiber laser - Koester and Snitzer, Applied Optics 3, 1182 (1964).

    These developments have led to high-power, high performance lasers which are currently used in a wide range of applications e.g., https://doi.org/10.1364/OL.12.000888, https://dx.doi.org/10.1049/ip-j:19880071, https://dx.doi.org/10.1049/ip-j:19880071, https://dx.doi.org/10.1038/nphoton.2013.280

    Fiber optic transmitting a laser
  6. Nonlinear Optical Phenomena

    In 1961, Peter. A. Franken and colleagues observed the first nonlinear optical (NLO) effect by firing laser light, from the recently invented ruby laser, through a quartz crystal - Phys. Rev. Lett. 7, 118 (1961).

    Low intensity light usually shows linear optical effects in reflection, refraction and diffraction and the exiting light has the same wavelength as the initial input. However, the high-intensity of the coherent laser light interacts with quartz resulting in two beams exiting the crystal, one with the original wavelength (694.2 nm) and a new coherent laser beam in the ultraviolet (347.1 nm). One year later Terhune et al. discovered Third Harmonic Generations (231.3 nm) in Calcite - R. W. Terhune, P. D. Maker, and C. M. Savage, Phys. Rev. Lett. 8, 404 (1962).

    Peter Franken

  7. The Semiconductor Laser

    The first successful lasing demonstration started the race to make a semiconducting laser and in 1962, the race was won (and patented) by Dr Robert N. Hall et al. (24th September 1962) and closely followed by M. I. Nathan et al. (http://dx.doi.org/10.1063/1.1777371); Nick Holonyak Jr and S. F. Bevacqua (17th October 1962) and T. M. Quist et al. (23rd October 1962).

    These original devices were made from Gallium Arsenide (GaAs) that had to be cooled to 77 K by liquid nitrogen and emitted lasing Infrared radiation whilst Holonyak Jr. achieved visible light emission with the addition of trace amounts of Phosphorous.

    Einstein only consider spontaneous emission from a single atomic energy level. In a solid material, there are lots of these energy levels, close to each other and these form bands. These bands can be altered by the addition of trace elements which either give an extra electron into the band (n-type doping) or have one less electron and effectively generate a positive “hole” in the band structure (p-type). Light is generated when a negatively charged electron interacts with a positively charged hole. This is known as recombination. Silicon, the most common semiconductor is an indirect semiconductor which means that a quick direct light-emitting recombination is not possible. GaAs, however, is a direct semiconductor and hence is used for light emitting devices.

    The population inversion required for lasing is generated at the junction between n- and p- type materials. The original patent figure shows how this results in an excess of electrons in the upper state and an “excess of holes” – effectively a reduction in the number of lower state electrons.

    Energy levels within a laser diode from the original patent

    Modern devices separate the n- and p- doped materials with a layer of undoped material for improved performance. The laser cavity is created within the device by the careful polishing of the ends of the diode. These devices eventually developed; became operational at room temperature and can produce the whole spectrums of visible light. The technology resulted in light emitting diodes (LED) and LED lasers which are used extensive in modern life e.g., CD/DVD/BlueRay devices

    The R. N. Hall patented laser diode from the original patent

  8. The First Hungarian Laser

    Coming Soon

  9. The 1964 Nobel Prize for Physics

    The 1964 Nobel prize for Physics was awarded to Charles H. Townes, Nicolay G. Basov, Aleksandr M. Prokhorov "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". This award showed that laser/maser technology was accepted into mainstream physics.

    Charles H. Townes from www.nobelprize.org
    Nicolay G. Basov from www.nobelprize.org
    Aleksandr M. Prokhorov from www.nobelprize.org

    This is the presentation speech

    Your Majesties, Your Royal Highnesses, Ladies and Gentlemen.
    The Nobel Prize for physics is in this year given for the invention of the maser and the laser. "Maser" stands for "microwave amplification by stimulated emission of radiation", and the word "laser" is obtained by replacing "microwave" by "light".
    The key to the invention is the concept of stimulated emission which was introduced by Einstein already in 1917. By a theoretical analysis of the Planck radiation formula he found that the well-known process of absorption must be accompanied by a complementary process implying that received radiation can stimulate the atoms to emit the same kind of radiation. In this process lies a potential means for amplification. However, the stimulated emission was long regarded as a purely theoretical concept which never could be put to work or even be observed, because the absorption would be the completely dominating process under all normal conditions. An amplification can occur only if the stimulated emission is larger than the absorption, and this in turn requires that there should be more atoms in a high energy state than in a lower one. Such an unstable energy condition in matter is called an inverted population. An essential moment in the invention of the maser and the laser was, therefore, to create an inverted population under such circumstances that the stimulated emission could be used for amplification.
    The first papers about the maser were published 10 years ago as a result of investigations carried out simultaneously and independently by Townes and co-workers at Columbia University in New York and by Basov and Prochorov at the Lebedev Institute in Moscow. In the following years there were designed a number of masers of widely different types, and many people made important contributions to this development. In the type that is now being mostly used the maser effect is obtained by means of the ions of certain metals imbedded in a suitable crystal. These masers work as extremely sensitive receivers for short radiowaves. They are of great importance in radio astronomy and are being used in space research for recording the radio signals from satellites.
    The optical maser, that is, the laser, dates from 1958, when the possibilities of applying the maser principle in the optical region were analysed by Schawlowand Townes as well as in the Lebedev Institute. Two years later the first laser was operating.
    The step from the microwaves to visible light means a 100000-fold increase in frequency and causes such changes in the operation conditions that the laser may be regarded as an essentially new invention. In order to achieve the high radiation density required for the stimulated emission to become dominating, the radiating matter is enclosed between two mirrors that force the light to traverse the matter many times. During this process the stimulated radiation grows like an avalanche until all the atoms have given up their energy to the radiation. The fact that the stimulated and stimulating radiation have exactly the same phase and frequency is essential for the result of the process. By virtue of resonance all parts of the active medium combine their forces to give one strong wave. The laser emits what is called coherent light, and this is the decisive difference between the laser and an ordinary light source where the atoms radiate quite independent of each other.
    Lasers have now been made in many different shapes. The first, and still most frequently used, type consists of a ruby rod, a few inches long, with the polished and silvered end faces serving as mirrors. The radiation leaves eventually the crystal through one of the end faces which is made slightly transparent. The ruby consists of aluminium oxide with a small admixture of chromium. The chromium ions give to the ruby its red colour, and they are also responsible for the laser effect. The inverted population is produced by the light from a xenon flash lamp. This is absorbed by the ions, putting them in such a condition that they can be stimulated to emit a red light with a welldefined wavelength.
    Normally, a large number of successive pulses of laser light is emitted during the time of one flash from the lamp, but by retarding the release until the stored energy has reached a maximum all the energy can be put into one big pulse. The power of the emitted light can then reach more than a hundred million watts. Since, moreover, the emerging ray bundle is strictly parallel, the whole energy can be concentrated by means of a lens on a very small area, producing an enormous power per unit area. From a scientific point of view it is especially interesting that the electrical field strength produced in the light wave may amount to some hundred million volts/cm and thus surpass the forces that keep the electron shells of the atoms together. The high photon density opens up quite new possibilities for studying the interaction of radiation and matter.
    Another type of laser, in which the light is emitted from a gas excited by an electric discharge, produces continuously a radiation with a very sharply defined wavelength. This radiation can be used for measurements of lengths and velocities with a previously unattainable precision.
    The invention of the laser has provided us with a powerful new tool for research in many fields, the exploitation of which has only just started. Its potential technical applications have been much publicised and are therefore well known. Regarding, especially, the extreme power concentration obtainable with a laser, it should be noted that this effect is limited to short time intervals and very small volumes and therefore attains its main importance for micro-scale operations. It should be emphasized, finally, that the use of a laser beam for destructive purposes over large distances is wholly unrealistic. The "death ray" is and remains a myth. Dr. Townes, Dr. Basov and Dr. Prochorov. By your ingenious studies of fundamental aspects of the interaction between matter and radiation you have made the atoms work for us in a new and most remarkable way. These magic devices called maser and laser have opened up vast new fields for research and applications which are being exploited with increasing intensity in many laboratories all over the world. On behalf of the Royal Swedish Academy of Sciences I extend to you our warm congratulations and now ask you to receive the Nobel prize from the hands of His Majesty the King.

  10. Dye Lasers

    Scientific applications of lasers were limited to a narrow selection of light frequencies, fundamentally linked to the mechanism by which the laser light was generated.
    Some complex large organic molecules can demonstrate Phosphorescence - a long-lasting emission of light. This light emission originates from a slow electronic transition which is “forbidden” by quantum mechanics. There are several energy levels which are close in energy and this results in a wide (λ ≈ 50 nm) range of emitted light.

    Dye lasers energy levels from here

    The figure belows shows how an optical cavity with a cuvette filled with a dilute solution of these molecules can produce a laser output. A spontaneous “forbidden” photon emission from one of the excited states is only trapped (and thus generating a cascade stimulated effect) if it matches the resonance frequency of the optical cavity. This frequency is determined by the angle of the diffraction grating and thus rotating the grating changes the laser frequency. The output of these lasers is tunable and enables the study of the rotational, vibrational and electronic configuration of molecular species via laser spectroscopy

    Dye Laser generation from here
  11. Erbium Doped Fiber Lasers

    Erbium (Er) is a lanthanide metallic element. The electronic structure of the pink Er3+ ion in glass has a slow laser transition window (1520 – 1570 nm) which corresponds to the third transmission window in Silicon optical fibers. These levels can be populated by pumping a Er doped fiber with 980 nm or 1480 nm light and these form the basis of a purely optical fiber laser amplifier, as shown in the second figure. Input light (from the left) is mixed with the pumping light and coupled into the Er-doped fiber. The input light stimulates emission from the 4I13/2 states and results in an amplified output. An isolator at the end of the fiber removes any remaining pump light without any signal degradation; prevents back-reflections and stops any possible undesired induced lasing effects.

    Er3+ ions energy levels

    The vastly different pumping and lasing frequencies and Er3+ electronic structures minimizes amplified spontaneous emission and results in an efficient high-gain optical amplifier which is extensively used throughout modern fiber optical implementations.

    An experimental Erbium fiber implementation

    The first demonstration of an Erbium-based fiber laser was by Mears et al. in Electron Lett. 22, 159 (1986). These lasers only became feasible after the development of double-clad fibers enabled the generation of powers in excess of 1 W. Double-clad fibers have a doped core in which the laser propagates and the pump light is propagated through a surrounding cladding, enabling the pump light to be absorbed by the Er ions whilst also supporting a large number of propagation modes.

  12. The 1971 Nobel Prize for Physics

    The Nobel Prize in Physics 1971 was awarded to Dennis Gabor (5th June 1900 - 5th February 1979) "for his invention and development of the holographic method" - The theory of imaging with the illusion of depth
    . Photographic imaging was developed during the 19th century, producing better and better two-dimensional images. Gábor Dénes developed a method based upon the interference of coherent light waves. Light falling on an object is captured on photographic film along with a coherent reference wave that has not interacted with the object. A three-dimensional reproduction of the imaged object is produced when the developed film has the reference wave shone on it at the correct angle

    Dennis Gabor from www.nobelprize.org
    The principle of holography from here
    Holographific Reconstruction from here

    Gabor published his theories of holography in a series of papers between 1946 and 1951 – see here for a review of the process.
    During the 50s, the lack of a coherent light source prevented the testing of this theory but the rapid development of lasers, the first coherent light source, meant that the earliest hologram was made in 1964 and holography became commercially realised soon after.
    Holography has led to various scientific and technical development and can be used to enriched optical measurement techniques. His Nobel prize lecture can be found here.

  13. The 1981 Nobel Prize for Physics

    In 1981, Arthur Schawlow and Nicolaas Bloembergen were awarded half the Nobel Prize in physics "for their contribution to the development of laser spectroscopy". Their joint work enable lasers to become tools for probing and understanding atomic and molecular species.

    Arthur Schawlow was a laser pioneer and initially worked with Charles Townes. He authored a book on microwaves and calculated the fundamental quantum limit for laser linewidth. Schawlow built a failed ruby laser prototype but after the first laser demonstration, he built a working version and characterised the laser light. At Stanford University, he led a research group, which developed Doppler-free spectroscopy using laser saturation, two-photon absorption, polarization labelling, optogalvanic spectroscopy and precise wavelength measurement.

    Arthur Schawlow from here
    Nicolaas Bloembergen from here

    Nicolaas Bloembergen's background was in nuclear magnetic resonance (also known as MRI) and his research group developed a solid-state MASER in the late 1950s. His attention then shifted to the recently discovered field of Non Linear Optics (NLO). He wrote the seminal book “Non Linear Optics” a few years after the initial discovery of Second Harmonic Generation. Within this tome, he laid the fundamentals and theoretical background for nonlinear optical processes including Second and Third Harmonic Generation and Rectification; the optical Kerr effect; Quasi-phase-matching; Cross-Phase Modulation and Optical Phase Conjugation.

    These multi-beam techniques have expanded both the lower and upper frequencies range that laser light can be generated and has thus considerable expanded the application of lasers within scientific investigations. NLO process are fundamental in modern laser science for generating high-power laser light at different frequencies, from Infrared to Ultraviolet, by the nonlinear combination of source laser photons.

  14. The First Sub 100 Femotosecond Laser

    In 1981, R. L. Fork et al. reported a new method for generating sub pico second laser pulses by using a continuous wave Argon ion laser, a ring laser configuration and an ultrathin dye absorber. They generated a 140 fs pulse (corresponding to a 90 fs pulse assuming sech2 geometry) using colliding pulse modelocking.

    Schematic diagram of ring laser

    The laser works by combining two laser pulses travelling in opposite directions, in a saturable absorber. The pulse train originates from noise fluctuations in the pump laser and the cavity is designed so that pulses are individually amplified in the laser gain material but two pulses coincide within in the adsorbed media. The generation of short pulse is a balance between the saturation of the gain material (steepening the trailing edge of the pulse), the absorber saturation (steepening the leading edge), the self phase modulation and group velocity dispersion.

    Autocorrelation function of the optical pulse from Appl. Phys. Lett. 38, 671 (1981)
  15. Observations of High Harmonic Generation in a Plasma

    In 1981, R. L Carman et al. reported the generation of a virtually constant visible/UV harmonic spectrum from a CO2 laser and a plasma. The plasma is generated by nanosecond laser pulses interacting with thin carbon containing wires.
    These studies recorded harmonics up to the 46th order, greatly improving the previously observed record of 11 th order harmonics. The generated spectra is dominated by odd and even harmonics and in some cases, much weaker half integer harmonics are observed. The resulting harmonics are short (< 200 ps). This work showed that the steep density gradient arising from radiation pressure and self-focusing, effectively enhances the optical field tenfold and plays a critical role in the generation of harmonics.

    Raw High Harmonic data from Phys. Rev. A, 24, 2649 (1981)
  16. Chirped Pulse Amplification

    Intense short laser pulses have a high peak power output and these high levels of power cause numerous technical and physical problems in pulse generation due to nonlinear processes. Power is a measure of rate of energy and thus as a laser pulse length (in time) gets shorter, the power of the pulse increases.
    Donna Strickland and Gerard Mourou modified a simple idea developed for amplifying radar signals and successfully applied it to optical laser amplifiers opening a new regime of high-power short laser pulses. The new amplifiers work by stretching the initial pulse in time and frequency. This is done by separating the laser frequencies in the pulse by making different frequencies travel different distances using diffraction devices, like gratings or a prism. A pulse which increase in pitch (frequency) with time is known as a chirp after the bird call.
    The “chirped” pulse is now longer, has lower power and can be amplified using standard conventional methods. The original but now amplified pulse is then reconstructed using a compressor, effectively a device which is the opposite of the pulse stretcher.

    Schematic of chirped pulse amplication

  17. High Harmonic Generation in Gases

    The generation of non-fundamental laser frequencies of light had been well established by the 1980s using non-linear optics. These approaches only generate the second and/or third harmonic and are usually accompanied by a large decrease in harmonic intensity and power.
    In 1987, A. McPherson et al. focused picosecond laser pulses from an excimer KrF laser (λ = 248 nm, Intensity ~1015-1016 Wcm‑2) into a pulsed jet of noble gases and discovered that a high number of high harmonics of the input laser wavelength could be detected. The generated harmonic order correlated only to odd integers and the intensity was much higher than previously expected.

    Gas Max Harmonics order observed wavelength
    He 13 19.1 nm
    Ne 17 14.6 nm
    Ar 7 35.5 nm
    Kr 7 35.5 nm
    Xe 9 27.9 nm
    The number of Harmonic orders observed in Noble gas by A. McPherson et al..

    One year later, M. Ferray et al. used an Infrared laser (Nd:Yag. 1064 nm) and found that Harmonic light as high as the 33rd harmonic in the XUV range (32.2 nm) could be generated in argon. Their work also showed that intensity of the first few harmonics decreases with order but harmonic intensity remains fairly constant from the fifth order up to the high harmonic cut-off, a sharp feature which is the last measured harmonic. This work shows that XUV/soft X-rays could be generated under laboratory conditions using relatively cheap reliable equipment – a high-power IR laser and Noble gases.

    Harmonics spectrum of Xe shows harmonics 9 to 21 from M. Ferray et al.

  18. Femtosecond Titanium Sapphire Laser

    In 1991, the Sibbett group reported the generation of a 60 fs laser pulse from a Titanium:Sapphire (Ti:Sa) laser within an optical cavity. This pulse could be further reduced to 45 fs using compression. An interesting feature of this experiment was that pulse generation required the cavity to be temporary disturbed. These conditions were later known as Kerr-Lens Mode Locking and features two unique optical properties of Ti:Sa lasers.

    Ti:Sa lasers were developed and characterised in the early 1980s by P. F. Moulton who overcame the inherent scattering and loss processes. Ti:Sa lasers quickly became established as the tuneable solid-state laser source due to the wide tuning range (660 to 1180 nm); the excellent thermal, physical and optical properties of sapphire; efficient power conversion; a rather simple experimental configuration and the lasers can be pumped by visible light from numerous sources including lasers.

    These short pulses were generated by a technique known as mode-locking. In an optical cavity (present in lasers), there are numerous longitudinal modes in which wavelengths of light perfectly fit the cavity and resonant. Under standard conditions, the phase difference between these modes is random and there is no overall gain. However when there is a constant phase between the modes, interference occurs resulting in the formation of a single pulse.

    A large number of frequency modes results in shorter generated pulses and the broad lasing bandwidth of Ti:Sa lasers is ideal for mode locking. The active laser component is an isolated Ti3+ ion with one d electron. The surrounding ions split the 5d orbitals into two separate groups with different energy (Crystal Field Theory). The ion absorbs green light and is electronically excited into the E state (A➡B). This is not at an energy minimum and the ion quickly relaxes to the lower energy state by coupling the excess energy into the sapphire structure as vibrations. The broad lasing transition (C➡D) occurs from the Zero point vibrational level to a vibrationally excited electronic ground state. This quickly vibrationally relaxes and helps maintain the population inversion.

    Titanium:sapphire lasing transitions from here

    Ti:Sa lasers crystals also demonstrate the non-linear Kerr lensing effect where the refractive index of the material depends upon the field strength of the incident laser pulse. This, under correct experimental configuration, can lead to the removal of weaker components from the laser cavity and causes the laser to mode lock and generate short-pulses.

  19. A Proposed Method for Attosecond Pulse Generation

    Technological advances have resulted in lasers which could produce few cycle laser pulse which were in the order of a few femtosecond pulses. However, new techniques are required in order to break the femtosecond barrier. Fourier Synthesis offers a way in which attosecond pulses could be generated from an evenly spaced frequency comb, like a mode-locked laser. This frequency comb can be generated from the nonlinear optical combination of two extremely well matched ultrashort laser pulse or from cascading stimulated Raman scattering.

    In 1992, Gy. Farkas and Cs. Toth proposed using a femtosecond laser pulse to generate high harmonics (HHG) from atoms to create the required frequency comb. This relatively simple method uses the HHG plateau. This results in all Fourier components being emitted from the same focal point, propagating collinearly, have an equidistant and fixed phase and have good directional matching. The pulse duration can be estimated from the number of odd harmonics in the plateau and base frequency of the driving laser. Table 1 shows potential pulse durations for Noble gases and Figure 1&2 shows the electric field strength E(t) (1) and the square of the electric field strength E2(t) (2) of the light beam, in which high harmonic modes are coherently summarised from the 5th up to oscillation periods (2T) of the basic harmonic component ω0 of the Nd laser beam (T = 3.53 fs).


    Number of harmonics

    Pulse duration

    Pulse duration
    from E2(t)/as
















    Table 1 - Fourier Synthesis of HHG components for various noble gases

  20. The Semiclassical Interpretation of High-Order Harmonic Generation

    High harmonic generation from IR laser light in Noble gases had been experimentally observed for many years. In 1993, this process was first accurately described by a semi classical three-step model, simultaneously but independently developed by Corkum and Kulander.
    The theory looks at the interaction of the intense electric field of the laser with an electron within an atom. The atom is ionised when an electron quantum mechanically tunnels through the ionization potential barrier, which is reduced by the incident laser pulse. The escaped electron has no velocity and is subsequently accelerated away from the atom by the electric field of the laser. The electron is subsequently returned to the atom by the reverse electric field of the laser pulse where the recombination of the electron into the atom results in the emission of ultrashort bremsstrahlung-like radiation. The motion of the free electron is well described by classical mechanics. Linearly polarized light improves the probability of the returning electron recombining with the parent ion.

    Schematic of the semiclassical interpretation of high harmonic generation from here

  21. Thin Disc Lasers

    Solid-state lasers had remained relatively similar to the initial ruby laser design with the active material being a rod.
    In 1993 Adolf Giesen and co-workers made a working Yb:YAG laser using a thin laser disc on a heat sink. This thin disc, also known as an active mirror, is thinner than the laser diameter and the backside of the crystal is dielectrically coated to be reflective for both the pump and output light. The disc is pumped at an angle by the pump light (in this example a diode laser) and the resulting lasing light is emitted parallel to the surface of the lasing crystal. The optical setup results in the majority of heat being extremely efficiently dissipated into the heatsink. This, combined with the large surface to volume ratio, enables operation at extremely high volume power densities whilst minimising thermal lensing effects. The technology can easily and effectively scale with power. However, the thin disc size can results in incomplete pump absorption and may require multiple optical pass for full power abstraction. The thin geometry also can generate and trap amplified spontaneous emissions (ASE).

    The principle of Thin Disc Laser Technology

  22. Chirped Mirrors

    The propagation of light through a medium alters its optical properties. This effect, known as dispersion, is highly dependent of the properties of the material and the wavelength of the incident light. Dispersion can be described using power series with the first order group delay (GD); the second order group delay dispersion (GDD), and higher orders. GDD has a key role for most laser related application.

    Ultrashort pulses are usually generated in the near IR regime where most of optical materials have a positive GDD, resulting in pulse lengthening. Robert Szipöcs et al. developed and patented a novel dielectric mirror which compensates for this positive GDD by providing a approximately constant negative dispersion i.e., group delay increases with wavelength, for a bandwidth of 710 - 900 nm. This mirror is made from 42 thin alternative layers of SiO 2 and TiO2 and is made by standard electron-beam evaporation. These mirrors have a reflectivity greater that 99.9% at the central wavelength and only drops to 99.5% for the outer wavelength region.

    The high performance of these mirrors means that these mirrors can be integrated into ultrafast optical systems to compensate for normal optical GDD without a power reduction. The customable design means that the mirror can be easy tuned and extended into different wavelengths by the use of different materials and layer thicknesses.

  23. The 1999 Nobel Prize for Chemistry

    Ahmed Zewail was awarded the Nobel prize for Chemistry on the 10th December 1999 for his groundbreaking work in Femtochemistry. Chemical processes happen by the intra and inter molecular movement of atoms, which occur on the femtosecond time scale (10 −15 s). Most chemical processes pass through a transition state, a chemical tipping point from which the molecule can return to the starting arrangement or progress towards the products. The identification and characterization of this state yields important chemical information. Progress along the reaction coordinate can be monitored using pump-probe experiments where an initial pulse excites a molecule and subsequent delayed pulses measure the relative changes in the molecular state as a function of time. For this level of detail, laser pulses, used as the snapshot of the system, must be high-quality, short (>10 fs) and highly reproducible.

    Ahmed Zewail from www.nobelprize.org

    Zewail is the “founding father” of femtochemistry and his work has looked at the dissociation of ICN; the change from covalent to ionic systems for Alkali Halides dissociations; probed the separated nuclear, vibrational, rotational motions of the Iodine molecule and bimolecular chemical reactions between HI and CO2. His work has also looked at organic chemical reaction pathways, like Isomerization, pericyclic addition and cleave and Diels-Alder reaction; electron and proton transfer process, with a strong biological focus; inorganic and atmospheric chemistry and the behavior of condensed matter. More details are available here.

    Potential energy surface for NaI dissociation from here

    The Holy Grail for femtochemistry is the complete control of chemical reactions.

  24. Carrier Envelope Phase (CEP) Stablization

    The phase offset between the carrier wave and the intensity envelope (CEP) is an important experimental measure for ultrashort pulses like those produce by mode locked Titanium:Sapphire lasers which approach the two optical cycle range. For most lasing processes, there is no locking process between the Carrier and Envelope which results in a shot by shot variation in the CEP. This can have a large effect on the experimental properties of these pulses, particularly in the generation of High Harmonics. The two figures show how a different phase offset dramatically changes the electric field in an identical envelope/

    CEP monitoring was first demonstrated by Harald R. Telle by using nonlinear optical processes using broad (650-1100 nm) pulses from a Kerr-lens mode locked Titanium:sapphire laser and with 658 nm and 812 nm external reference cw diode lasers. CEP detection works by measuring the interference between two light pulses with a similar frequency. The interference caused from frequency doubled light from the red end of the pulse with light from the blue end of the pulse at a similar frequency directly correlates to the Phase offset and can be measured.

    This work was expanded by David J. Jones which used the same f to 2f interferometry method but used a supercontinuum generated for the frequency comb rather than external laser oscillators. This broadband pulse is generated from the ultrashort laser pulse interacting with air or silica fibre. The resulting beats were electronically processed and feedback into the laser system resulting in the ability to lock in the laser into a relative carrier phase.
    These techniques provide the bedrock for the frequency comb method which resulted in the 2005 Nobel prize in Physics.

  25. Petawatt (PW) Lasers

    During the 1980s and 90s, the development of individual laser techniques including Chirp Pulse Amplification and Titanium sapphire lasers, led to Peak laser output power breaking the PW barrier in 1999. One of the first lasers to do this was built by M. D. Perry et al. at the Lawrence Livermore National Laboratory. They generated pulses using a Ti:Sapphire Mode locked laser at 1054 nm and subsequent pulses are amplified by Ti:Sapphire regenerative; Nd:phosphate glass rod and thin disk amplifiers. The system generated 660 J photons in a compressed 440 ± 20 fs pulse resulting in peak power greater than 1.5 PW.

    Optical Parametric Chirped Pulse Amplication (OPCPA) also has become a new way to access these high power regimes. The broadband high gain observed in these systems occurs via parametric coupling of a low-energy chirped broadband pulse with a high energy monochromatic pulse. This non-linear optical coupling occurs within a crystal, typically Beta Barium borate (BBO) and results in an amplified beam and an idler beam. The advantages of these systems include large ultrabroadband amplification, low levels of amplified spontaneous emissions and a simpler experimental setup. These OPCPA devices can be daisy chained together.

    OPCPA schematic from Appl Opt. 1999 36, 7486-93.

  26. Experimental Observations of Attosecond Pulses

    The temporal characteristics of attosecond pulse trains can only be obtained indirectly. In 2001 Paul et. al. reported an experimental method which is known as the Reconstruction of Attosecond Beating By Interference of Two-photon transitions or RABBIT for short.

    The measurement of the amplitude of XUV pulses is relatively easy using a Time of Flight (TOF) electron spectrometer, where gas atoms are ionised and the time for the resulting electron to travel a known distance is measured, from which the initial photon energy can be determined. High Harmonic Generation (HHG) in noble gases results in odd numbered harmonics but if the ionisation occurs in the presence of weak IR radiation even harmonics appear in the spectra (as shown in the figure). Varying the temporal delay of the IR pulse with respect to the XUV pulse results in oscillation in the amplitude of these sidebars. This variation can be convoluted into the phase data and the attosecond pulses trains can be reconstructed by using this phase and amplitude information.

    Experimental RABBIT setup - insert shows the formation of sidebands from IR and XUV - from Science 292, 1689-1692 (2001)

    The realisation of this principle experimentally requires careful design so that resulting XUV pulses overlap with a remnant of the generating IR pulse. This reported method uses spatial filters to enable IR transmission throughout the experiment. The central core of an ultrashort Titanium Sapphire laser pulse is controllably delayed and the outer remaining IR torus is focussed into the gas-jet, generating high harmonics. A small component (and the delayed IR component) is focused into the TOF spectrometer, using an aperture and mirror, from which the attosecond pulse can eventually be reconstructed.

    Reconstructed RABBIT Temporal Trace from Science 292, 1689-1692 (2001)

    In 2001, Hentschel et al. developed Attosecond streaking as a way of probing the electric field of an attosecond X-ray pulse by using a second ultrastrong laser pulse. The incident X-Ray pulse ionises a target atom and the resulting photoelectron has a characteristic kinetic energy profile. The strong electric field of the secondary pulse accelerates the electron, modifying the electron’s kinetic energy. The electric field of the ionising pulse can be probed by changing the delay between the two pulses. This technique was used by M Schultze et al. to profile the ionisation of Neon by a 120 eV by a XuV pulse. The incident pulse is sufficiently energetic to be able to release an electron from different bound electronic states and subtle timing difference between the states can be measured.

    Attosecond streaking measurement of the ionisation of Neon
  27. The 2005 Nobel Prize for Physics

    The bedrock of the scientific method is reliable and accurate measuring systems. Improvements in measurement are often correlated to new discoveries. In 2005, John L. Hall and Theodor Hänsch were each awarded a quarter of the Nobel prize for Physics for their work on frequency measurements. Their work has improved the measurements of atomic Hydrogen electronic transitions and their greatest contribution is the frequency comb – a method in which an unknown frequency is compared with a sequence of precisely separated frequencies.

    John L. Hall from www.nobelprize.org
    Theodor W. Hänsch from www.nobelprize.org

    A train of ultrashort femtosecond pulses (Figure 1 below) can be expressed as a comb of different frequencies (Figure 2). The individual frequencies of the comb can then be described by a function of the pulse repetition rate and the carrier envelope offset (CEO) frequency, the measurable offset of the electric field within the pulse envelope when comparing two pulses. The CEO frequency can be calculated using a nonlinear f-to-2f interferometry method which measures the interference between the high frequency and a frequency doubled low frequency components of the pulse. These two variables can be phase-locked to an external clock. An unknown frequency can then be determined by measuring the beat interference between the unknown light source and the closest mode frequency.

    Femtosecond pulse train
    Resulting Frequency Comb

    The advent of the broadband mode-locked titanium sapphire lasers ultimately enabled the mapping of optical frequencies to a cesium clock. Following this work, the precision of optical laser spectroscopy has reached the level of microwave spectroscopy and is set to eclipse it in the near-future.

  28. Proposal for a European ELI

    The Extreme Light Infrastructure, ELI, was officially proposed in 2005. Gérard Mourou and Ferenc Krausz were two visionaries who saw the potential for a laser research infrastructure which investigated two frontiers of modern physics – Attosecond timescale and the ultrarelavistic regime. They also saw the need to makes these laser resources to explores these parameters available for all scientific users.

    Ferenc Krausz
    Gérard Mourou

    The ESFRI proposal in 2005 stated that ELI was (and still is) the first infrastructure dedicated to the fundamental study of laser-matter interaction in a new and unsurpassed regime of laser intensity – the ultra-relativistic regime (IL >1023 W/cm2). At its centre will be an exawatt class laser ~1000 times more powerful than either the Laser Mégajoule in France or the National Ignition Facility (NIF) in the US. In contrast to these projects, ELI will attain its extreme power from the shortness of its pulses (femtosecond and attosecond). The infrastructure will serve to investigate a new generation of compact accelerators delivering energetic article and radiation beams of femtosecond (10-15 s) to attosecond (10-18 s) duration. Relativistic compression offers the potential of intensities exceeding I>10-25 W/cm2, which will challenge the vacuum critical field as well as provide a new avenue to ultrafast attosecond to zeptosecond (10-21 s) studies of laser-matter interactions. ELI will afford wide benefits to society ranging from improvement of oncology treatment, medical imaging, fast electronics and the understanding of aging nuclear reactor materials to development of new methods of nuclear waste processing.

    This proposal was followed by the ELI ESFRI roadmap and the scientific support for this bold statement was substantiated in the ELI white book. This 536 tome edited by Gérard A. Mourou, Georg Korn, Wolfgang Sandner and John L. Collier rigorously laid the detailed and complex required scientific foundations for ELI. This proposal was successful and on the 1st of November, 2007, the Extreme Light Infrastructure Preparatory Phase started with a budget around 6 M€.

  29. A Distributed ELI With Three Sites

    ELI was announced to be the first large-scale infrastructure to be built in the former socialist block. The adoption of a Pan-Europe multi-site strategy enabled the broadening of the laser community throughout the region and the creation of a distributed facility with one mission, coordinated goals, one governing body and one user community. The pros, according to Gérald Mourou, include the solid commitment from the three countries, a larger workforce and more total lasers and more opportunities for students.

    The three sites were designated to study an unique aspect of Laser physics with ELI-NP, the Romanian pillar, focusing on Nuclear Physics; ELI-beamlines, the Czech pillar, probing Secondary beams radiation and High Energy particles and ELI-ALPS, Szeged Hungary, investigating attosecond physics.

    Construction of the individual sites costs about €280 Million and annual running will be in the order of €30 million per annum. However, the site separation enables the individual governments to access to EU structural funds for new members and each host government has already committed €250 - €300 Million.

  30. High Flux Attosecond Pulses from Surface Plasma Harmonics

    The superposition of phase-locked High harmonics generated from atoms results in trains of attosecond pulses. However, the gas-based generation process is very inefficient and has spurred research into alternative generation methods. One promising process involves dense plasmas as plasmas have no restriction upon the incident laser intensity.

    Two distinct mechanism exist for Surface High Harmonic Generation. Coherent Wake emission (CWE) dominates the sub / moderate relativistic interactions. This originates from electrons being pulled out from the surface plasma by the incident laser’s electric field. The subsequent return of the electrons to the plasma by the next half of the electric cycle results in the generation of plasma oscillations. These subsequently decay into high harmonic electromagnetic radiation of the incident laser frequency. At higher laser intensities, high harmonics are generated from Doppler effects occurring from reflections from a plasma acting as a relativistic oscillating mirror (ROM) which moves close to the speed of light.

    Y. Nomura et al. were the first group to report the successful generation of an attosecond pulse from a plasma surface using XUV autocorrelation. The temporal characteristics of the harmonics were initially measured using a coarse scan (3.3 fs scan step size) followed by a high resolution scan (Fig 1 - 0.13 fs scan step size) in which the quasiperiodic structure correlates to a pulse duration ≈ 0.9±0.4 fs.

    Experimental setup for measure surface generated attosecond pulses from Nature Physics 5, 124 - 128 (2009)
    Fine scan showing experimental data and subsequent guassian fit from Nature Physics 5, 124 - 128 (2009)

  31. The 2018 Nobel Prize in Physics

    A 2018-as fizikai Nobel-díjat

    The 2018 Nobel prize for Physics (presented 8th December 2018) was awarded "for ground-breaking inventions in the field of laser physics" with one half to Arthur Ashkin "for the optical tweezers and their application to biological systems", the other half jointly to Gérard Mourou and Donna Strickland "for their method of generating high-intensity, ultra-short optical pulses."

    A 2018-as fizikai Nobel-díjat, amelyet 2018. december 8-án adnak át „a lézerfizikában elért áttörésekért” megosztva kapta Arthur Ashkin „az optikai csipesz megalkotásáért és azok biológiai rendszerekben való alkalmazásáért”, valamint Gérard Mourou és Donna Strickland együtt, “a nagy intenzitású, ultrarövid lézerimpulzusok létrehozásának kidolgozásáért."

    The work by Prof. Mourou, the initiator of the Extreme Light Infrastructure (ELI) project, and Dr. Strickland laid the experiment foundations for Chirped Pulsed Amplification of a laser. Their work was inspired by studies which showed that the propagation of light pulse in an optical fibre become stretched by self-modulation and group velocity dispersion (GVD) – The pulse is effectively chirped. These effect can be reversed (compressed) by pair of gratings which have a well matched negative GVD. The initial paper, D. Strickland, G. Mourou, Opt. Commun. 56, 219(1985), worked on the millijoule level.

    Prof. Mourou, az Extreme Light Infrastructure (ELI) projekt megálmodója, megalapozta a lézerek fázismodulált impulzuserősítését (chirped pulse amplification, CPA). Stricklanddal közös munkájukat azok a tanulmányok ihlették, amelyek szerint a fényimpulzusok terjedése optikai szálban az önmoduláció és a csoportsebesség-diszperzió miatt megnyúlik, azaz az impulzus fázismodulált lesz. Ez a hatás visszafordítható, tehát az impulzus összenyomható egy negatív csoportsebesség-diszperziójú optikai rácspár segítségével. Az első tanulmányukban (D. Strickland, G. Mourou, Opt. Commun. 56, 219 (1985)) milli-Joule-os tartományban végzett vizsgálatot írnak le.

    The next step in CPA development, inspired by the telecommunication industry, was the incorporation of the Martinez compressor (O. E. Martinez, IEEE J. Quantum Electron. 23, 1385 (1987)) with a Treacy grating. This could stretch a 80 fs pulse to picosecond length and then subsequently recompress the elongated pulse to the original form. This led to the “Maine Event” where a 1 ps pulse was generated with one joule of energy in a Nd:glass amplifier. This was the first terawatt pulse on a table and was over a thousand time greater than standard amplification techniques – P. Maine, D. Strickland, P. Bado, M. Pessot, G. Mourou, IEEE J. Quantum Electron. 24, 398 (1988). This power level was quickly superseded by developments in glass laser technologies and the use of Titanium Sapphire lasers.

    A CPA kifejlesztésének következő lépése, amelyet a telekommunikációs ipar ösztönzött, a Martinez-kompresszor (O. E. Martinez, IEEE J. Quantum Electron. 23, 1385 (1987)) és a Treacy optikai rács egyesítése volt. Ezzel egy 80 femtoszekundumos impulzus pikoszekundumosra nyújtható, majd újra összenyomható az eredetivel megegyezőre. Ez vezetett el a Maine-eseményhez, amikor 1 ps-os impulzus jött létre 1 J energiával Nd:üveg erősítőben. Asztalon ez volt az első terawattos impulzus, amely több mint ezerszer nagyobb a standard erősítő módszereknél – P. Maine, D. Strickland, P. Bado, M. Pessot, G. Mourou, IEEE J. Quantum Electron. 24, 398 (1988). Később ezt a teljesítményszintet gyorsan túlhaladták az üveglézer-technológiai fejlesztések, valamint a titán-zafír lézerek.

    The graph shows how CPA technology radically altered the power development of laser systems and effectively kick started laser development towards and beyond the Petawatt power levels. The genius of CPA technology is that there have been no major modifications to the experimental configurations in the thirty years since its invention but it is used in every high-power laser system in various fields from medical procedures to fundamental physics research.

    Az ábrán látható, hogy a CPA-technológia milyen radikálisan változtatta meg a lézerrendszerek teljesítményfejlődését és hogyan indította el a lézerek fejlődését a petawattos – és azon túli – lépték felé. A technológia zsenialitása, hogy bár a feltalálása óta eltelt 30 évben nem történt nagyobb változás a kísérleti összeállításban, ma is számtalan területen használják az orvosi eljárásoktól az alapvető fizikai kutatásokig.

    Laser power timeline
    Lézerek intenzitásának fejlődése