Aspects of femtosecond pulse shape transfer via difference frequency mixing

Botha, Gerda Nicolene (2015-12)

Thesis (DSc)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: We investigate the generation of shaped femtosecond pulses in the infrared spectral regime via a nonlinear process called di erence frequency mixing. First we develop a detailed model of the process, incorporating pulse propagation during di erence frequency mixing, in the slowly varying envelope approximation. Di erence frequency mixing (DFM) is numerically simulated for several wavelengths, nonlinear crystals and Type I and Type II frequency mixing. The di erent factors in uencing the shape transfer e ciency of a shaped pulse to a di erent wavelength regime, as well as the conversion e ciency, is identi ed and investigated thoroughly by doing a parametric study. The numerical modeling demonstrates that the e ciency, with which a shaped pulse in the near-infrared is transferred to another wavelength regime, depends strongly on the refractive index of the nonlinear medium for the interacting pulses and is optimal when the velocity of the generated pulse equals that of the shaped input pulse. We show that it is possible to control the temporal pulse duration of the generated pulse by using speci c input angles and so manipulating the e ective refractive index of the nonlinear material for the input and generated pulses. It was found that it is possible to temporally broaden or narrow the generated pulse relative to the input pulses. We compare the developed numerical model to experimental measurements. A liquid crystal spatial light modulator (SLM), inserted in a 4f setup, is used to generate the shaped pulses. Experimentally we demonstrate high- delity shape transfer by mixing 795 nm and 398 nm femtosecond pulses in a BBO crystal. The temporal broadening and narrowing of the generated pulse is also shown and compared to the numerical simulations showing excellent agreement with measured results.

AFRIKAANSE OPSOMMING: Ons ondersoek die generasie van gevormde femtosekonde pulse in die infrarooi spektrale regime via 'n nie-liniêre proses genaamd verskil frekwensie vermenging. Ons ontwikkel eerste a gedetaleerde model van die proses wat the pulse se beweging gedurende verskil frekwensie vermenging in die stadige wisselende envelope benadering insluit. Verskil frekwensie vermenging is numeries gesimuleer vir 'n paar gol engtes, nie-liniêre kristalle en Tipe I en Tipe II frekwensie vermenging. Die verskillende faktore wat die vorm oordrag e ektiwiteit van 'n gevormde puls van een gol engte na 'n ander gol engte beinvloed; sowel as die doeltre endheid; is geïdenti seer en deeglik ondersoek deur 'n parametriese studie te doen. Die numeriese model toon dat die oordragse ektiwieteit waarmee die gevormde puls in die naby-infrarooi na 'n ander gol engte oorgedra word, afhanklik is op die brekingsindekse van die nie-liniêre meduim vir die interaktiewe pulse en is optimaal wanneer die snelhed van die gegenereerde puls gelyk is aan die snelheid van die gevormde inkomende puls. Ons wys dat dit moontlik is om die pulsduur van die gegenereerde puls te beheer deur spesi eke invalshoeke te gebruik en so die e ektiewe brekingsindeks van die nie-liniêre materiaal vir die inkomende en gegenereerde pulse te manipuleer. Daar is gevind dat dit moontlik is om die gegenereerde pulse langer of korter to maak relatief tot die inkomende pulse. 'n Vloeibare kristal ruimtelike lig moduleerder (SLM), geplaas in 'n 4f opstel, word gebruik om die vorm van die pulse te genereer. Eksperimenteel demonstreer ons hoë-trou vorm oordrag deur 795 nm en 398 nm femtosekonde pulse in 'n BBO kristal te meng. Die rek en krinp van die duur van die gegenereerde pulse word ook gewys en vergelyk met die numeriese simulasies wat uitstekend vergelyk met die gemete resultate.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/97804
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