Single-beam coherent anti-Stokes Raman scattering combining polarization shaping with spectral focusing

Viljoen, Ruan (2021-03)

Thesis (PhD)--Stellenbosch University, 2021.

Thesis

ENGLISH ABSTRACT: Single-beam coherent anti-Stokes Raman scattering (SB-CARS is a compact approach to CARS which employs a single broadband laser source to probe molecular vibrations. A variety of light sources can be used, one of which is a photonic crystal fibre (PCF pumped by a femtosecond laser. The SB-CARS setup presented herein utilises a polarization main-taining all-normal dispersion PCF (PM-ANDi-PCF pumped by a femtosecond oscillator. The oscillator produces 80 fs pulses with a low average power of 1W at a repetition rate of 80 MHz. When these pulses are used to pump the PM-ANDi-PCF, supercontinuum pulses are produced with a much broader bandwidth (150 nm at the same repetition rate. The supercontinuum pulses are pulse-to-pulse phase stable with high peak intensi-ties, perfect for fast acquisition of spectra in applications of non-linear spectroscopy and microscopy. As a result of using a broadband laser source, the produced CARS spectrum is also broad and dominated by a non-resonant background which is generated by a four-wave mixing non-linear process. The SB-CARS setup utilises an active shaping approach which allows for a multitude of approaches, that can reduce the non-resonant background, to be implemented within the same setup. A spatial light modulator in a 4-f shaper affords the ability to shape the supercontinuum pulses at will and also enables the integration of pulse characterisation techniques into the same SB-CARS setup. Implementation of a phase-only temporal ptychography technique, i2PIE, in a SB-CARS setup is reported on for the first time. Pulse characterisation is a vitally important component of this setup, as produced supercontinuum pulses are initially temporally broad, because of dispersion in the optical fibre, and need to be compressed for applications in spectroscopy. Comparison of i2PIE to another phase-only technique, multiphoton intrapulse interference phase scan (MIIPS, shows remarkable improvements in compression. Second harmonic spectra generated in a beta-barium borate crystal when the supercontinuum pulses are compressed using phase reconstructed with i2PIE and MIIPS, showed an increase of integrated spectral intensity by a factor of 4. When applied to SB-CARS measurements, the integrated intensity of SB-CARS spectra increased by a factor 6.5 when i2PIE was used. The improvement was not limited to spectral intensity yield, but also showed an improvement of signal-to-background in extracted CARS spectra by a factor of 4 when i2PIE was used. Different SB-CARS strategies were implemented in the same setup and evaluated through simulation and experimentation to illustrate the versatility of the SB-CARS apparatus. A new spectral focusing SB-CARS approach that utilises the polarization shaping and phase shaping capabilities of an SLM, is introduced and compared to other SF techniques. Non-resonant background is reduced in this technique by using a narrow probe decoupled from the pump and Stokes fields. Spectral focusing is achieved with phase shaping of the pump which focuses the bandwidth to mostly target a single Raman resonance. Results from spectral measurements demonstrate that the new technique (PP-QPSF) yields spectra with an overall increased S:B. This new technique is evaluated spectroscopically and shows promise for an application in SB-CARS microscopy.

AFRIKAANSE OPSOMMING: Enkelstraal koherente anti-Stokes Raman verstrooiing (SB-CARS is 'n kompakte benadering tot CARS wat 'n enkele breëband laserbron gebruik om molekulêre vibrasies te ondersoek. 'n Fotoniese-kristalvesel (PCF gepomp deur 'n femtosekonde ossilator is een van verskeie ligbronne wat so gebruik kan word as 'n breëband ligbron. Die SB-CARS opstelling wat in hierdie skrif beskryf word gebruik 'n ossilator wat 80 fs pulse met 'n lae drywing van 1 W teen 80 MHz produseer. Wanneer hierdie pulse gebruik word om die PCF te pomp, word die pulse se bandwydte vergroot tot 150 nm terwyl die herhalings tempo behoue bly. Hierdie pulse is ook puls-tot-puls fasestabiel met hoë piek intensiteits waardes, wat dit geskik maak vir vinnige meting van spektra in toepassings van nie-lineêre spektroskopie en mikroskopie. As gevolg van die toepassing van 'n breëband ligbrong vir CARS, word 'n breë CARS spektrum wat oorheers is deur 'n nie-resonante bydra geproduseer. Die SB-CARS opstelling maak gebruik van 'n aktiewe pulsvorming benadering wat toelaat vir 'n verskeidenheid van maniere om die nie-resonante bydra te verminder of uit die metings verwyder. 'n Lig-modulator (SLM in 'n 4f-geometrie stel mens in staat om die verskillende pulsvormings benaderings toe te pas en laat 'n mense toe om puls karakteriserings tegnieke met dieselfde instrument uit te voer. Die implementering van 'n spesiale karakteriserings tegniek, i2PIE, is vir die eerste keer in 'n SB-CARS toestel geïmplementeer en in hierdie skrif verslag gelewer. Vergelyking van metings van tweede harmoniek, opgewek met pulse gekarakteriseer deur i2PIE, met die van pulse gekarakteriseer deur 'n ander tegniek 'intrapulse interference phase scan' (MIIPS, wys 'n verbetering van die geïntegreerde spektrale intensiteite met 'n faktor van 4. Soortgelyke metings van die SB-CARS spektrum wys 'n 6.5 keer verhoging in intensiteitswaardes wanneer i2PIE gebruik is. Hierdie verbetering was nie beperk tot slegs spektrale intensiteit nie, maar het ook tot verbeterings in sein-tot-agtergrond in gemete CARS spektra gelei. Verskillende SB-CARS benaderings is met die opstelling uitgevoer en deur middel van simulasie en eksperimente ondersoek. 'n Nuwe spektrale fokus SB-CARS benadering wat polariserings pulsvorming gebruik is in hierdie skrif voorgestel, ondersoek, en vergelyk met ander bestaande tegnieke. Resultate geproduseer deur hierdie tegniek wys 'n verbetering in sein-tot-agtergrond in vergelyking met ander tegnieke.

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