none

When an intense laser beam interacts with matter, the Kerr nonlinearity results in self-focusing. Above the critical intensity, self-focusing dominates pulse spreading through diffraction leading to continuous pulse narrowing and thus an increase of the laser peak intensity. Collapse is prevented through the fact that peak intensities ultimately reach a level where ionization occurs. The profile of ionized electrons represents a negative lens which balances Kerr nonlinear self-focusing and causes the formation of stable filaments. From filaments radiation is emitted in a cone around the filament which has been termed conical emission. Filament formation happens at non-perturbative intensities where the formalism of perturbative nonlinear optics loses its validity. This opens the question of how the Kerr nonlinearity behaves in the non-perturbative limit and how large the Kerr nonlinear coefficient is. The expression for the Kerr nonlinearity is derived by perturbation theory; the validity of this expression in the non-perturbative limit is questionable. Further, experimentally the Kerr nonlinear index is extracted from measurements of the self-focusing distance as a function of beam intensity which is called the Z-scan method. This method fails at non-perturbative intensities due to the presence of the negative lens coming from the ionized electrons. The effects of the positive focusing and negative self-defocusing lens cannot be separated by the Z-scan method. As a result, not much is known about the Kerr nonlinearity in the regime of non-perturbative nonlinear optics. The purpose of this thesis is twofold. First, recently it has been discovered that conical emission can be utilized as a broadband and very efficient amplification mechanism in the far infrared. The process has been dubbed Kerr instability amplification. The difference between conical emission and Kerr instability amplification is that they take place in two different regimes of the nonlinear interaction. Whereas conical emission grows out of noise and therewith only takes place once the pump pulse has been substantially restructured due to filamentation, Kerr instability amplification is seeded with a second pulse and therewith occurs long before filamentation happens. The theory developed for Kerr instability amplification has been developed based on a stability analysis of the scalar wave equation. This analysis has shown that with pump lasers in the 1-2 μm range amplification of infrared radiation up to the 10’s of μm can be achieved. For amplification over such a wide range it is not adhoc clear to which exent vectorial wave effects can be neglected. The first part of the thesis closes this gap by developing the vectorial theory of vector instability amplification. The second part uses the results derived for Kerr instability amplification to answer the question of how to measure the Kerr nonlinear index in the nonperturbative laser intensity limit. The idea rests on the fact that Kerr instability amplification is maximum for a specific angle between pump and seed beam which varies as a function of laser pump intensity. A relation is derived that connects this angle with the Kerr nonlinear refractive index. As a result, from the maximum angle measured as a function of pump intensity, both magnitude and functional form of the Kerr nonlinear index as a function of laser intensity can be determined.

In the disertation, the self-focusing phenomenon of intense femtosecond light pulses in media with cubic nonlinearity is investigated and the origin of formation of light filaments is revealed. In this work, new measurement techniques are introduced and applied experimentally, which allowed high resolution temporal, spatial and spectral mapping of light wave-packet dynamics during the nonlinear propagation in transparent media, and enabled to observe and make accurate quantitative evaluation of the ultrafast change of medium properties. By means of high temporal (20 fs) and spatial (1 μm) resolution laser 3D mapping technique, it was shown that the initial Gaussian wave packet during self-action in Kerr media redistributes its energy in a way that in spatio-temporal domain the wave packet takes a complex X-type intensity distribution. This transformation is universal and is determined by temporal and spatial spectral broadening (as a consequence of self-focusing and self phase modulation), conical emission (as a consequence of four-wave mixing) and nonlinear losses caused by multiphoton absorption. By means of imaging spectrometer technique it was shown that in the medium with normal group velocity dispersion the far-field angular spectrum of the wave packet takes a characteristic X shape, whereas in anomalous group velocity dispersion regime – a characteristic O shape. In both cases the localization of the wave packet is observed, however the quantitative differences of the... [to full text] / Disertacijoje nagrinėjamas intensyvių femtosekundinių šviesos impulsų fokusavimosi reiškinys kubinio netiesiškumo terpėse bei atskleidžiama šviesos gijų formavimosi prigimtis. Darbe yra pasiūlytos ir eksperimentiškai realizuotos šviesos bangų paketų registravimo metodikos, kurios leidžia su didele skyra fiksuoti laikinę, erdvinę bei spektrinę šviesos bangų paketų dinamiką netiesinio sklidimo metu, taip pat stebėti bei itin tiksliai kiekybiškai įvertinti terpės savybių sparčius pokyčius. Pasitelkus didelės laikinės (20 fs) ir erdvinės (1 μm) skyros lazerinės tomografijos metodiką, parodyta, kad pradinio Gauso formos bangų paketo energija saviveikos Kero terpėje metu persiskirsto taip, kad erdvėlaikyje jis įgauna sudėtingą X tipo intensyvumo skirstinį. Ši transformacija yra universali, ir ją nulemia laikinis ir erdvinis spektro plitimas (dėl fokusavimosi ir fazės moduliavimosi), kūginė spinduliuotė (dėl keturbangio maišymosi) bei nuostoliai dėl daugiafotonės sugerties. Remiantis atvaizduojančio spektrometro metodika parodyta, kad formuojantis šviesos gijai normalios grupinių greičių dispersijos terpėje bangų paketo tolimojo lauko kampinis spektrinis skirstinys yra X formos, o anomalios grupinių greičių dispersijos terpės atveju – O formos. Abiem atvejais stebima bangų paketo lokalizacija, tačiau kiekybinius kampinių spektrinių skirstinių skirtumus nulemia keturbangio dažnių maišymo fazinio sinchronizmo sąlygos. Ir galiausiai, naudojant didelės laikinės (23 fs) ir erdvinės (1,5... [toliau žr. visą tekstą]

Šviesos netiesinio fokusavimosi reiškinys tyrinėjamas jau nuo pat lazerio sukūrimo. Ypatingai aktyviai nagrinėjamas šviesos gijų susidarymas bei jų dinamika skirtingos fazinės būsenos medžiagose. Šviesos gijos įdomios tuo, kad šviesa jose sklinda beveik nepatirdama difrakcijos ir dispersijos. Mokslininkai pasiūlė nemažai šviesos gijos taikymų, kaip pvz. atmosferos sluoksnių spektrinė analizė, atosekundinių impulsų generacija inertinėse dujose ar bangolaidžių užrašymas kietose skaidriose terpėse. Šviesos saviveikos metu pradinė bangų paketų forma erdvėje bei laike gali stipriai pakisti. Standartiniai impulsų charakterizavimo metodai, kurie remiasi erdvėje integruotų autokoreliacinių ir kryžminių koreliacinių funkcijų matavimu, įgalina gauti tik dalinę informaciją apie vykstančias šviesos impulso bei terpės savybių transformacijas ir jų fizikinę prigimtį. Todėl, norint gerai įvaldyti minėtus taikymus bei suprasti vykstančius fizikinius reiškinius, visiškam šviesos impulsų sąveikos su medžiaga ir šviesos gijų formavimo proceso charakterizavimui reikalingi nauji matavimo metodai, kuriuos pasitelkus būtų galima išmatuoti bangų paketo intensyvumo pasiskirstymą trijų matmenų erdvėje bet kuriuo laiko momentu bei registruoti momentinius terpės struktūros pokyčius. Antra vertus, šviesos ir medžiagos sąveikos procesai yra labai spartūs, tad jų tyrimui reikalinga labai didelė tiek erdvinė (μm eilės), tiek laikinė (fs eilės) skyra. Šiame darbe pasiūlėme didelės laikinės (20 fs) ir... [toliau žr. visą tekstą] / In the disertation, the self-focusing phenomenon of intense femtosecond light pulses in media with cubic nonlinearity is investigated and the origin of formation of light filaments is revealed. In this work, new measurement techniques are introduced and applied experimentally, which allowed high resolution temporal, spatial and spectral mapping of light wave-packet dynamics during the nonlinear propagation in transparent media, and enabled to observe and make accurate quantitative evaluation of the ultrafast change of medium properties. By means of high temporal (20 fs) and spatial (1 μm) resolution laser 3D mapping technique, it was shown that the initial Gaussian wave packet during self-action in Kerr media redistributes its energy in a way that in spatio-temporal domain the wave packet takes a complex X-type intensity distribution. This transformation is universal and is determined by temporal and spatial spectral broadening (as a consequence of self-focusing and self phase modulation), conical emission (as a consequence of four-wave mixing) and nonlinear losses caused by multiphoton absorption. By means of imaging spectrometer technique it was shown that in the medium with normal group velocity dispersion the far-field angular spectrum of the wave packet takes a characteristic X shape, whereas in anomalous group velocity dispersion regime – a characteristic O shape. In both cases the localization of the wave packet is observed, however the quantitative differences of the... [to full text]

This thesis descibes a theoretical study of nonlinear self-focusing as applied to the metrology of the nonlinear optical parameters of a medium. It also studies the phe- nomenon of optical power limiting which utilizes self-focusing e ects. As an analytical tool, a mode decomposition method which uses an orthogonal and complete set of Gaussian-Laguerre modes as a basis set is used to treat these problems. Nonlinear media both in the thin and thick limits are investigated. For thin media, a closed form expression is derived which describes the optical eld of an initally Gaussian beam that is perturbed by a thin nonlinear material which exhibits nonlinear absorption as well as nonlinear refraction. This result is valid for any regime of nonlinearity in the thin medium approximation. Thick media are treated using a numerical extension of the Gaussian-Laguerre Mode Decomposition technique. Spatial scanning techniques such as the Z-scan that rely on self-focusing e ects and that are used to measure the nonlinear optical parameters of a material are studied in detail. Optical limiting in both thick and thin media is also investigated.

This thesis descibes a theoretical study of nonlinear self-focusing as applied to the metrology of the nonlinear optical parameters of a medium. It also studies the phe- nomenon of optical power limiting which utilizes self-focusing e ects. As an analytical tool, a mode decomposition method which uses an orthogonal and complete set of Gaussian-Laguerre modes as a basis set is used to treat these problems. Nonlinear media both in the thin and thick limits are investigated. For thin media, a closed form expression is derived which describes the optical eld of an initally Gaussian beam that is perturbed by a thin nonlinear material which exhibits nonlinear absorption as well as nonlinear refraction. This result is valid for any regime of nonlinearity in the thin medium approximation. Thick media are treated using a numerical extension of the Gaussian-Laguerre Mode Decomposition technique. Spatial scanning techniques such as the Z-scan that rely on self-focusing e ects and that are used to measure the nonlinear optical parameters of a material are studied in detail. Optical limiting in both thick and thin media is also investigated.

There is a strong push worldwide to develop multi-Joule femtosecond duration laser pulses at wavelengths around 3.5-4 and 9-11 mu m within important atmospheric transmission windows. We have shown that pulses with a 4 mu m central wavelength are capable of delivering multi-TW powers at km range. This is in stark contrast to pulses at near-IR wavelengths which break up into hundreds of filaments with each carrying around 5 GW of power per filament over meter distances. We will show that nonlinear envelope propagators fail to capture the true physics. Instead a new optical carrier shock singularity emerges that can act to limit peak intensities below the ionization threshold leading to low loss long range propagation. At LWIR wavelengths many-body correlations of weakly-ionized electrons further suppress the Kerr focusing nonlinearity around 10 mu m and enable whole beam self-trapping without filaments.

No description available.

We investigate the spatial evolution of an intense laser pulse as it generates high-order harmonics in a long gas cell, filled with 80 torr of helium. A thin foil separates the gas-filled region of the cell from a subsequent evacuated region. The exit plane of the gas cell can be scanned along the laser axis so that the evolution of the laser throughout the focus can be observed (full scanning range of 9 cm). We constructed an apparatus that images the laser radial energy profile as it exits the cell. The high harmonics, odd orders ranging from 45 to 91, are observed at the same time that the laser spot is characterized. Re-absorption of the harmonics within the gas cell restricts the region of harmonic emission to the final centimeter (or less) of the cell. We present the first direct evidence (to our knowledge) of laser filamentation under conditions ideal for high-order harmonic generation. The 30 fs, 4 mJ, laser pulses were observed to undergo double focusing within the gas cell, with about 4 cm separating the two foci. The region with best harmonic emission occurs midway between the two foci. The radial profile of the laser focus, 150-200 microns in diameter, evolves from a Gaussian-like profile to a more square-top profile as it propagates over several centimeters. The filamentation phenomenon as well as the brightness of the harmonics improves when an aperture is partially closed on the laser beam before reaching the focusing mirror. A spectral sampling of the imaged laser focus revealed a 4 nm blue-shift associated with the generation of plasma in the gas cell. The blue-shifting occurs primarily in the center of the laser beam and less at the wider radii. The initial laser pulse had a spectrum centered at 800 nm with a 35 nm bandwidth. The energy associated with each of the observed 26 harmonic beams was found to be approximately 1 nJ, yielding a conversion efficiency of approximately 2e−7.

No description available.