Nelinearni efekti interakcije lasera i atomskih para

doctoral thesis

• Title (english): Nonlinear effects in the interaction between the laser and the alkai vapor
• Author: Silvije Vdović
• Mentor: Goran Pichler (mentor)
• Committee member: Damir Veža (predsjednik povjerenstva)
• Committee member: Goran Pichler (član povjerenstva)
• Committee member: Hrvoje Buljan (član povjerenstva)
• Granter: University of Zagreb; Faculty of Science; (Department of Physics); Zagreb
• Defense date and country: 2010-07-15, Croatia
• Scientific / art field, discipline and subdiscipline: NATURAL SCIENCES; Physics
• Universal decimal classification (UDC): 53 - Physics

Abstract

Istraživana je nelinearna interakcija atomskih para alkalija i laserskih izvora u različitim uvjetima. Elektromagnetski inducirana transparencija nastaje zbog interakcije izmedu kontinuiranih lasera i atomskih para rubidija u V shemi vezanja stanja. Transparencija je ponajviše rezultat smanjenja populacije osnovnog nivoa zbog saturacije prijelaza vezanog jakim veznim laserom. Usporedbom s teorijskim modelom zaključeno je da, osim saturacije, i stvorena koherencija u atomima takoder doprinosi povećanju transparencije. Istražena je interakcija femtosekundnih pulseva s rezonantnim parama rubidija različitih koncentracija korištenjem tehnike frekventno razlučivog optičkog uzorkovanja (FROG). Efekti prostiranja femtosekundnih laserskih pulseva optički tankim sredstvom mogu se opisati stvaranjem 0-π pulseva kod kojih dolazi do značajnih vremenskih modulacija električnog polja pulsa. Dobiveni rezultati usporedeni su s teorijskim simulacijama prostiranja pulsa rezonantnim medijem u okviru linearne disperzijske teorije, čime su odredene granice primjene FROG tehnike na karakterizaciju kompleksnih pulseva. FROG tehnikom mjereni su i pulsevi nakon rezonantne nelinearne interakcije s gustim parama rubidija. Rezultat takve interakcije je konusna emisija. U okviru aproksimacije adijabatske slijednosti zbog različitog predznaka razlike izmedu centralne valne duljine pulsa i valne duljine najbliže rezonantne linije dolazi do prostornog samofokusiranja odnosno samodefokusiranja. U frekventnoj domeni dolazi do samomodulacije faze pulsa koja takoder ovisi o predznaku razlike valnih duljina pulsa i rezonantne linije. Mjerenja faze pulsa potvrdila su gore navedene teze kroz različito ponašanje spektralne i vremenske faze pulsa ovisno o položaju spektra pulsa u odnosu na rezonanciju. Degenerirano četverovalno miješanje (DFWM) femtosekundnih pulseva u natrijevim parama mjereno je u režimu tranzijentne rešetke u BOXCARS geometriji. Proučavan je vremenski razvoj UV emisije uzrokovane višefotonskom apsorpcijom u mediju kao i vremenski razvoj generiranog DFWM signala. Modulacije intenziteta tranzijenata objašnjavaju se kvantnim udarima zbog koherentnog pobudivanja stanja fine strukture. Period modulacije odreden je razlikom energija nivoa različitog spina. Modulacije eksponencijalno trnu u vremenu odredenom karakterističnim vremenom relaksacija koherencija.

Abstract (english)

This thesis examines nonlinear interaction between the alkali vapor and different laser sources in various conditions. Interaction between cw lasers and rubidium atomic vapor in the V coupling scheme produces electromagnetically induced transparency, where the main contibution to the induced transparency comes from nonlinear saturation effects on the strongly coupled transition. By comparing the theoretical model and experimental results it is shown that part of the induced transparency comes from coherences induced between the uncoupled transitions. Interaction between the femtosecond (fs) pulses and resonant rubidium atomic vapor is investigated using frequency-resolved optical gating (FROG) technique. Propagation of the fs pulses in the optically thin medium results in formation of 0-π pulses which have strong modulation of the electric field amplitude on a time scale of several ps. Since it is impossible to cover the whole time evolution of the 0-π pulse, the FROG reconstruction process can never be accurate enough. Propagation of nearly-resonant pulses through the optically thick rubidium vapor results in conical emission due to the self-focusing of the pulse in time domain and self-phase modulation in frequency domain. Using the adiabatic following approximation, it is shown that the physical processes depend on the sign of detuning of pulse wavelength from the rubidium resonance lines. The pulse electric field phase in both domains, retrieved from the FROG measurements, shows different behavior with respect to the sign of the pulse detuning. Degenerate four-wave mixing (DFWM) of femtosecond pulses in sodium vapor was performed in BOXCARS geometry. When on three-photon atomic resonance, transient grating signal appeared on the frequency of the third harmonic of fundamental frequency at negative delay times with modulations whose period corresponds to the energy difference between the coherently excited upper state fine stucture levels. In positive time delays signal appears at the fundamental frequency and there is no signal at third harmonic frequency. This is explained by destructive interference between the fundamental field and the field at the frequency of third harmonic. The amplitudes of modulations at negative time delays have exponential time dependence determined by the coherence relaxation time.

• Keywords: elektromagnetski inducirana transparencija; frekventno razlučivo optičko uzorkovanje; femtosekundni puls; degenerirano četverovalno miješanje; konusna emisija; samofazno moduliranje; samofokusiranje; kvantni udari; stvaranje trećeg harmonika
• Keywords (english): electromagnetically induced transparency; frequency-resolved optical gating; femtosecond pulse; degenerate four-wave mixing; conical emission; self-phase modulation; self-focusing; quantum beats; third-harmonic generation
• Language: croatian
• URN:NBN: urn:nbn:hr:217:610261
• Study programme: Title: Physics; Study programme type: university; Study level: postgraduate; Academic / professional title: doktor/doktorica znanosti, područje prirodnih znanosti, polje fizika (doktor/doktorica znanosti, područje prirodnih znanosti, polje fizika)
• Type of resource: Text
• Extent: 153 str.
• File origin: Born digital
• Access conditions: Open access
• Created on: 2017-08-24 11:50:11