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A photoionization mode is a mode of interaction between a laser beam and matter involving photoionization. [1]
Laser light affects materials of all types through fundamental processes such as excitation, ionization, and dissociation of atoms and molecules. These processes depend on the properties of the light, as well as on the properties of the material. Using lasers for material processing requires understanding and being able to control these fundamental effects. A better understanding can be achieved by defining distinct interaction regimes, hence the definition of four photoionization modes.
This new way of looking at the laser interaction with matter was first proposed by Tiberius Brastaviceanu in 2006, after his description of the "filamentary ionization mode" (Sherbrooke University, 2005). In his Master's work he provided the empirical proof of the formation of filamentary distributions of solvated electrons in water, induced by high-power fs (femtosecond, one trillionth of a second) laser pulses in the self-focusing propagation regime, and described the theoretical context in which this phenomenon can be explained and controlled. Refer to main article on filament propagation.
The SP mode is obtained at small wavelengths (UV, X-ray), or high energy per photon, and at low intensity levels.[ citation needed ] The only photoionization process involved in this case is the single-photon ionization.
The OB mode is observed when a material is subjected to powerful laser pulses.[ citation needed ] It manifests a power threshold in the range of MW for the majority of dielectric materials, which depends on the duration and on the wavelength of the laser pulse. Optical breakdown is related to the dielectric breakdown phenomenon which was studied and modeled successfully towards the end of the 1950s. One describes the effect as a strong local ionization of the medium, where the plasma reaches densities beyond the critical value (between 1020 and 1022 electrons/cm³). Once the plasma critical density is achieved, energy is very efficiently absorbed from the light pulse, and the local plasma temperature increases dramatically. An explosive Coulombian expansion follows, and forms a very powerful and damaging shockwave through the material that develops on ns timescale. In liquids, it produces cavitation bubbles. If the rate of plasma formation is relatively slow, in the nanosecond time regime (for nanosecond excitation laser pulses), energy is transferred from the plasma to the lattice, and thermal damages can occur. In the femtosecond time regime (for femtosecond excitation laser pulses) the plasma expansion happens on a timescale smaller than the rate of energy transfer to the lattice, and thermal damages are reduced or eliminated. This is the basis of cold laser machining using high-power sub-ps laser sources.
The optical breakdown is a very "violent" phenomenon and changes drastically the structure of the surrounding medium. To the naked eye, optical breakdown looks like a spark and if the event happens in air or some other fluid, it is even possible to hear a short noise (burst) caused by the explosive plasma expansion.
There are several photoionization processes involved in optical breakdown, which depend on the wavelength, local intensity, and pulse duration, as well as on the electronic structure of the material. First, we should mention that optical breakdown is only observed at very high intensities. For pulse durations greater than a few tens of fs avalanche ionization plays a role. The longer pulse duration, the greater the avalanche ionization's contribution. Multi-photon ionization processes are important in the fs time regime, and their role increases as the pulse duration decreases. The type of multi-photon ionization processes involved is also wavelength dependent.
The theory needed to understand the most important features of optical breakdown are:
B/OB mode is an intermediary between the optical breakdown mode (OB mode) and the filamentary mode (F mode).[ citation needed ] The plasma density generated in this mode can go from 0 to the critical value i.e. optical breakdown threshold. Intensities reached inside the B/OB zone can range from multi-photon ionization threshold to the optical breakdown threshold. In the visible-IR domain, B/OB mode is obtained under very tight external focusing (high numerical aperture), to avoid self-focusing, and for intensities below optical breakdown threshold. In the UV regime, where optical breakdown intensity threshold is below self-focusing intensity threshold, tight focusing is not necessary. The shape of the ionization area is similar to that of the focal area of the beam, and can be very small (only a few micrometres). B/OB mode is possible only at short pulse durations, where AI's contribution to the total free electron population is very small. As the pulse duration becomes even shorter, the intensity domain where B/OB is possible becomes even wider.
The principles governing this mode of ionization are very simple. Localized plasma must be generated in predictable fashion, under the optical breakdown threshold. Optical breakdown intensity threshold is strongly correlated to the input intensity only at short pulse durations. Therefore, one important requirement, in order to systematically avoid the optical breakdown, is to operate at short pulse durations. In order for the ionization to take place, multi-photon ionization (MPI) intensity threshold must be reached. The idea is to adjust the duration of the laser pulse so that multi-photon ionization, and perhaps to a lesser extent avalanche ionization, have no time to raise the plasma's density above the critical value.
In the UV, the distinction between single-photon mode (SP) and B/OB is that for the latter multi-photon ionization, single-photon ionization, and perhaps to a lesser extent avalanche ionization, are operating, whereas for the former, only single-photon ionization is operating.
B/OB relies mostly on MPI processes. Therefore, it is more selective than OB in terms of which type of atom or molecule is ionized or dissociated. The theory needed to understand the most important features of B/OB are:
The B/OB mode was described by A. Vogel et al. [ref 2].
In the F mode, filamentary or linear ionization patterns are formed.[ citation needed ] The plasma density within these filaments is below the critical value.
The self-focusing effect is responsible for the most important characteristics of the dose distribution. The diameter of these filamentary ionization traces is the same within 20% (in the order of a few micrometres). Their length, their number, and their relative position are controllable parameters. The plasma density and the yield of photolytic species are believed to be homogeneously distributed along these filaments. The local intensity reached by the laser light during propagation is also practically constant along their length. The power range of operation of the F mode is above self-focusing threshold and below optical breakdown threshold. Consequently, a necessary condition for it to exist is that the self-focusing threshold must be smaller than the optical breakdown threshold.
The F mode exhibits very important characteristics, which in combination with the other three photoionization modes makes possible the generation of a wide range of dose distributions, expanding the application range of lasers in the domain of material processing. The F mode is the only mode capable of generating linear ionization traces.
The theory needed to understand the most important features of the F mode are:
The first concrete connection between non-linear optical effects, such as the supercontinuum generation, and photoionization was established by A. Brodeur and S.L. Chin [ref 4] in 1999, based on optical experimental data and modeling. In 2002 T. Brastaviceanu published the first direct measurement of the spatial distribution of photoionization induced in the self-focusing regime, in water [ref 5].
It is possible to control the spatial distribution of the dose induced by laser pulses, and the relative yields of primary photolytic species, by controlling the properties of the laser beam. The dose distribution can be conveniently shaped by inducing a superposition of the four modes of photoionization. The mixed ionization modes are: SP-OB, SP-B/OB, and F-OB.
Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typically observed only at very high light intensities (when the electric field of the light is >108 V/m and thus comparable to the atomic electric field of ~1011 V/m) such as those provided by lasers. Above the Schwinger limit, the vacuum itself is expected to become nonlinear. In nonlinear optics, the superposition principle no longer holds.
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule is called an ion. Ionization can result from the loss of an electron after collisions with subatomic particles, collisions with other atoms, molecules and ions, or through the interaction with electromagnetic radiation. Heterolytic bond cleavage and heterolytic substitution reactions can result in the formation of ion pairs. Ionization can occur through radioactive decay by the internal conversion process, in which an excited nucleus transfers its energy to one of the inner-shell electrons causing it to be ejected.
Mode locking is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds (10−12 s) or femtoseconds (10−15 s). A laser operated in this way is sometimes referred to as a femtosecond laser, for example, in modern refractive surgery. The basis of the technique is to induce a fixed phase relationship between the longitudinal modes of the laser's resonant cavity. Constructive interference between these modes can cause the laser light to be produced as a train of pulses. The laser is then said to be "phase-locked" or "mode-locked".
Photoionization is the physical process in which an ion is formed from the interaction of a photon with an atom or molecule.
Polaritonics is an intermediate regime between photonics and sub-microwave electronics. In this regime, signals are carried by an admixture of electromagnetic and lattice vibrational waves known as phonon-polaritons, rather than currents or photons. Since phonon-polaritons propagate with frequencies in the range of hundreds of gigahertz to several terahertz, polaritonics bridges the gap between electronics and photonics. A compelling motivation for polaritonics is the demand for high speed signal processing and linear and nonlinear terahertz spectroscopy. Polaritonics has distinct advantages over electronics, photonics, and traditional terahertz spectroscopy in that it offers the potential for a fully integrated platform that supports terahertz wave generation, guidance, manipulation, and readout in a single patterned material.
Laser-induced breakdown spectroscopy (LIBS) is a type of atomic emission spectroscopy which uses a highly energetic laser pulse as the excitation source. The laser is focused to form a plasma, which atomizes and excites samples. The formation of the plasma only begins when the focused laser achieves a certain threshold for optical breakdown, which generally depends on the environment and the target material.
The Madison Symmetric Torus (MST) is a reversed field pinch (RFP) physics experiment with applications to both fusion energy research and astrophysical plasmas.
In nonlinear optics, filament propagation is propagation of a beam of light through a medium without diffraction. This is possible because the Kerr effect causes an index of refraction change in the medium, resulting in self-focusing of the beam.
Rydberg ionization spectroscopy is a spectroscopy technique in which multiple photons are absorbed by an atom causing the removal of an electron to form an ion.
Ultrafast laser spectroscopy is a spectroscopic technique that uses ultrashort pulse lasers for the study of dynamics on extremely short time scales. Different methods are used to examine the dynamics of charge carriers, atoms, and molecules. Many different procedures have been developed spanning different time scales and photon energy ranges; some common methods are listed below.
Resonance-enhanced multiphoton ionization (REMPI) is a technique applied to the spectroscopy of atoms and small molecules. In practice, a tunable laser can be used to access an excited intermediate state. The selection rules associated with a two-photon or other multiphoton photoabsorption are different from the selection rules for a single photon transition. The REMPI technique typically involves a resonant single or multiple photon absorption to an electronically excited intermediate state followed by another photon which ionizes the atom or molecule. The light intensity to achieve a typical multiphoton transition is generally significantly larger than the light intensity to achieve a single photon photoabsorption. Because of this, a subsequent photoabsorption is often very likely. An ion and a free electron will result if the photons have imparted enough energy to exceed the ionization threshold energy of the system. In many cases, REMPI provides spectroscopic information that can be unavailable to single photon spectroscopic methods, for example rotational structure in molecules is easily seen with this technique.
Argus was a two-beam high power infrared neodymium doped silica glass laser with a 20 cm (7.9 in) output aperture built at Lawrence Livermore National Laboratory in 1976 for the study of inertial confinement fusion. Argus advanced the study of laser-target interaction and paved the way for the construction of its successor, the 20 beam Shiva laser.
Gurgen Ashotovich Askaryan was a prominent Soviet - Armenian physicist, famous for his discovery of the self-focusing of light, pioneering studies of light-matter interactions, and the discovery and investigation of the interaction of high-energy particles with condensed matter.
An axicon is a specialized type of lens which has a conical surface. An axicon transforms a laser beam into a ring shaped distribution. They can be convex or concave and be made of any optical material. The combination with other axicons or lenses allows a wide variety of beam patterns to be generated. It can be used to turn a Gaussian beam into a non-diffractive Bessel-like beam. Axicons were first proposed in 1954 by John McLeod.
High harmonic generation (HHG) is a non-linear process during which a target is illuminated by an intense laser pulse. Under such conditions, the sample will emit the high harmonics of the generation beam. Due to the coherent nature of the process, high harmonics generation is a prerequisite of attosecond physics.
Self-focusing is a non-linear optical process induced by the change in refractive index of materials exposed to intense electromagnetic radiation. A medium whose refractive index increases with the electric field intensity acts as a focusing lens for an electromagnetic wave characterized by an initial transverse intensity gradient, as in a laser beam. The peak intensity of the self-focused region keeps increasing as the wave travels through the medium, until defocusing effects or medium damage interrupt this process. Self-focusing of light was discovered by Gurgen Askaryan.
Photoelectrochemical processes are processes in photoelectrochemistry; they usually involve transforming light into other forms of energy. These processes apply to photochemistry, optically pumped lasers, sensitized solar cells, luminescence, and photochromism.
A streamer discharge, also known as filamentary discharge, is a type of transient electrical discharge which forms at the surface of a conductive electrode carrying a high voltage in an insulating medium such as air. Streamers are luminous writhing branching sparks, plasma channels composed of ionized air molecules, which repeatedly strike out from the electrode into the air.
The laser damage threshold (LDT) or laser induced damage threshold (LIDT) is the limit at which an optic or material will be damaged by a laser given the fluence, intensity, and wavelength. LDT values are relevant to both transmissive and reflective optical elements and in applications where the laser induced modification or destruction of a material is the intended outcome.
A plasma mirror is an optical mechanism which can be used to specularly reflect high intensity ultrafast laser beams where nonlinear optical effects prevent the usage of conventional mirrors and to improve laser temporal contrast. If a sufficient intensity is reached, a laser beam incident on a substrate will cause the substrate to ionize and the resulting plasma will reflect the incoming beam with the qualities of an ordinary mirror. A single plasma mirror can be used only one time, as during the interaction the beam ionizes the subtrate and destroys it.
