Superluminally propagating localized optical pulses
Date
2010-07-29
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Abstract
Käesolev uurimistöö kuulub füüsikalise optika valdkonda ja tegeleb ülilühikeste optiliste ehk valgusimpulsside levi uurimisega.
1983. aastal avaldas ameerika teadlane James Neill Brittingham uurimistöö, mille tihedasse ja keerukasse valemirägasse oli sügavale peidetud info, et elektromagnetlained võivad moodustada ülilühikesi impulsse (ehk ‘lainepakette’), mis on võimelised läbima pikki vahemaid ilma kuju muutmata ehk levi-invariantselt ning mille intensiivsusmaksimumi ruumilised mõõtmed on samas suurusjärgus lainepikkusega, ehk kümme korda väiksemad juuksekarva ristlõikest. See on märkimisväärne eriti seetõttu, et üldreeglina kõik lainepaketid, nagu ka näiteks laseri fokuseeritud kiirtekimp, valguvad levides laiali seda rohkem, mida väiksemad olid nende algsed mõõtmed.
Levi-invariantsed laineväljad ehk lokaliseeritud lained leiavad rakendust näiteks mikroosakestega ning -organismidega manipuleerimise metoodikais, fluorestsentsmikroskoopias, mittelineaarses ja ülikiires optikas, plasmakanali juhtimiseks mööda kindlat trajektoori aga ka aatomite optiliseks lõksustamiseks ning nii optilise kui ultraheli kujutise edasikandmiseks.
Märkimisväärne ja kirjanduses diskussiooni põhjustanud on ka asjaolu, et teatavat tüüpi lokaliseeritud lainete rühmakiirus on suurem valguse kiirusest (samas keskkonnas) ehk superluminaalne. Nii nagu levi-invariantsus, ei ole superluminaalne rühmakiirus vastuolus fundamentaalsete füüsikaseadustega, vaid vastupidi – sügavamal analüüsil ikkagi kooskõlas nendega.
Doktoritöös on uuritud ülevalgusekiirusega levivaid lokaliseeritud laineid, välja on töötatud meetod nende eksperimentaalseks moodustamiseks ning läbi viidud nn Bessel-X impulsi ja selle kiirenevalt ja aeglustuvalt levivate analoogide kõrgrlahutusega ajalis-ruumilised mõõtmised. Muuhulgas on otseste katsete ja mõõtmistega demonstreeritud, et ülilühikese valgusimpulsi difrageerumisel ümmarguselt avalt, kettalt või pilult moodustub tõkke taga superluminaalselt, aeglustuvalt liikuv Besseli-tüüpi lainepakett, mis on klassikalises optikas tuntud Arago-Poissoni täpi impulss-versioon.
The thesis belongs to the field of physical optics. In the thesis the propagation of ultrashort optical pulses has been studied. In year 1983 an American scientist James Neill Brittingham published a very theoretical paper announcing three-dimensional classical electromagnetic pulses, which propagate invariantly without any distortion or spread over long distances. The intensity maximum of the novel pulses was tightly localized to a “bulletlike” core which dimensions are in order of a wavelength, hence remain smaller than a cross-section of a human hair. The result was startling because in general all wave packets, among those also a focused laser beam, spread out in the course of propagation. The smaller the initial spatial dimensions of the wave packet, the larger the spread will be. The propagation-invariant pulses or the localized waves have applications in optical trapping and particle micromanipulation, in fluorescence microscopy, in nonlinear and ultrafast optics, for guiding plasma channel, as optical atom guide and for transfer of optical or ultrasound images. The fact that the group velocity of localized waves of certain type is superluminal, has started debates among scientific community. Similar to the propagation invariance, the superluminal group velocity is not in contradiction with fundamental laws of physics. Instead, the deeper analysis proves otherwise. In the thesis the superluminally propagating localized waves have been studied. A method for their experimental implementation has been proposed and analyzed. A localized wave called Bessel-X pulse and accelerating and decelerating Bessel-type waves have been generated experimentally and measured spatiotemporally with high resolution. Moreover, formation of superluminally propagating and decelerating Bessel-type pulses in the diffraction process was explicitly demonstrated by propagating ultrashort pulses through circular apertures, disks and slits. These Bessel-type pulses are in that case pulsed versions of the well-known Arago-Poisson spot.
The thesis belongs to the field of physical optics. In the thesis the propagation of ultrashort optical pulses has been studied. In year 1983 an American scientist James Neill Brittingham published a very theoretical paper announcing three-dimensional classical electromagnetic pulses, which propagate invariantly without any distortion or spread over long distances. The intensity maximum of the novel pulses was tightly localized to a “bulletlike” core which dimensions are in order of a wavelength, hence remain smaller than a cross-section of a human hair. The result was startling because in general all wave packets, among those also a focused laser beam, spread out in the course of propagation. The smaller the initial spatial dimensions of the wave packet, the larger the spread will be. The propagation-invariant pulses or the localized waves have applications in optical trapping and particle micromanipulation, in fluorescence microscopy, in nonlinear and ultrafast optics, for guiding plasma channel, as optical atom guide and for transfer of optical or ultrasound images. The fact that the group velocity of localized waves of certain type is superluminal, has started debates among scientific community. Similar to the propagation invariance, the superluminal group velocity is not in contradiction with fundamental laws of physics. Instead, the deeper analysis proves otherwise. In the thesis the superluminally propagating localized waves have been studied. A method for their experimental implementation has been proposed and analyzed. A localized wave called Bessel-X pulse and accelerating and decelerating Bessel-type waves have been generated experimentally and measured spatiotemporally with high resolution. Moreover, formation of superluminally propagating and decelerating Bessel-type pulses in the diffraction process was explicitly demonstrated by propagating ultrashort pulses through circular apertures, disks and slits. These Bessel-type pulses are in that case pulsed versions of the well-known Arago-Poisson spot.
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Keywords
doktoritööd, valgusimpulsid, füüsikaline optika