Ackermann, Patric (2019)
Efficient frequency conversion towards ultrashort VUV pulses using coherent control and multi-photon resonances.
Technische Universität Darmstadt
Ph.D. Thesis, Primary publication
Abstract
In the present work the frequency up-conversion of (ps) laser pulses towards the EUV spectral regime was investigated. We addressed the problem of limited conversion efficiency in prevalent gaseous nonlinear media by three approaches. On the atomic scale, the source term for the harmonic generation in form of the nonlinear polarizability was strongly enhanced by tuning the laser frequency in the vicinity of a five-photon resonance in argon. By monitoring the relative conversion efficiency for fifth harmonic generation versus the multi-photon detuning for several laser intensities of the visible driving laser, we identified pronounced AC Stark shifts of the transition frequency in excess of 50 THz, exceeding the pulse bandwidth by more than one order of magnitude already at an intensity of the order of 10 TW/cm². We concluded, that for a given peak intensity, due to the strong level shifts a transient enhancement is achieved. This enhancement occurs when the laser is tuned to a wavelength such, that the dynamically shifted level comes in resonance at intensities slightly lower than the peak intensity of the pulse. By tuning the laser accordingly, we achieved an enhancement of about one order of magnitude for the fifth harmonic and also strong enhancements of the seventh and ninth harmonic generated simultaneously. This form of resonance enhancement of multiple harmonics emphasizes the importance of intermediate resonances also in higher harmonic generation. In a second approach, we focused on coherent control of frequency up-conversion towards the EUV regime by actively controlling the nonlinear polarization in a bi-color laser field. A four photon transition to the 5p^5 6p²[5/2] level in xenon is driven by intense laser pulses in the visible regime around 512 nm and can interfere with a two-photon transition, driven by the second field at a wavelength of 256 nm. By tuning the relative phase between the two fields, we achieve control of the excitation probability as confirmed by a pronounced modulation of the laser-induced fluorescence from the excited level. A further photon at 512 nm serves to drive fifth harmonic generation and simultaneous four-wave mixing with the UV photons, both yielding radiation at 102 nm wavelength. In systematic measurements, we examined the several preconditions to achieve maximum control in an experiment involving high nonlinear orders and ultra-short laser pulses at intensities around 1 TW/cm². As a result, we gained a visibility of 90 % in the interference of the two conversion pathways - to our knowledge the highest achieved visibility in phase control of harmonic generation so far. While coherent control can only gain a limited enhancement of less than a factor of 4, the data exhibit a convincing demonstration of the feasibility of coherent control also with ultra-short pulses at TW/cm² intensity, gaining a factor of 18 in modulation between destructive and constructive interference. Furthermore, we examined the dependence of absolute signal and control strength (visibility) concerning the detuning from the resonance and showed a change in the temporal modulation period with respect to the detuning. In a simultaneous measurement of the excited state population and frequency conversion, both processes show similar interference with equal modulation period and large modulation depth, but phase lag of 0.03 π between the two interferograms. We attributed this phase lag to the contribution of further atomic levels (in this case especially the Rydberg levels and the ionization continuum) to the nonlinear polarization, possibly introducing additional phase compared to the excitation channel. We finally enhanced the product of number density N and interaction length L by about a factor of 1000 by confining the gas medium inside a hollow core waveguide and balancing the gas dispersion with the waveguide dispersion. We showed that the combination of resonance enhancement and phase-matched harmonic generation at high N*L is possible in argon in the vicinity of the strong 3p^6 -> 3p^5 4s'² [1/2]° transition. Because of the effect of the AC Stark shift of the transition frequency on the refractive index of argon at the fifth harmonic frequency, the Stark shift could be determined directly by analyzing the shift in phase matching pressure at a constant fundamental wavelength. The obtained (averaged) energy shift of 0.85 Φ, is close to the ponderomotive energy Φ and in good agreement with recent publications. In comparing the relative experimental conversion efficiency versus gas pressure and resonance detuning with an extensive numerical simulation, we unveiled the important contribution of a quasi-phase matching scheme resulting from the mode beating at the fundamental frequency, even with only less than 3% of power guided in modes higher than EH11. Furthermore, we reproduced the resonance enhancement, which is significantly detuned even from the shifted resonance and explained the detuning and enhancement lineshape by the phase matching precondition and the required phase matching bandwidth to convert the full spectrum of the pulses. Within these constraints, we rate the investigated coupling scheme capable of resonantly enhanced frequency up-conversion of pulses as short as 100 fs. At optimal conditions, we achieved more than a factor of 800 higher conversion efficiency compared with an atomic jet operated at 1 bar of stagnation pressure and could even enhance this efficiency by another factor of 1.5 in admixing a positively dispersive buffer gas.
Item Type: | Ph.D. Thesis | ||||
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Erschienen: | 2019 | ||||
Creators: | Ackermann, Patric | ||||
Type of entry: | Primary publication | ||||
Title: | Efficient frequency conversion towards ultrashort VUV pulses using coherent control and multi-photon resonances | ||||
Language: | English | ||||
Referees: | Halfmann, Prof. Dr. Thomas ; Walther, Prof. Dr. Thomas | ||||
Date: | 2019 | ||||
Place of Publication: | Darmstadt | ||||
Refereed: | 18 February 2019 | ||||
URL / URN: | https://tuprints.ulb.tu-darmstadt.de/8569 | ||||
Abstract: | In the present work the frequency up-conversion of (ps) laser pulses towards the EUV spectral regime was investigated. We addressed the problem of limited conversion efficiency in prevalent gaseous nonlinear media by three approaches. On the atomic scale, the source term for the harmonic generation in form of the nonlinear polarizability was strongly enhanced by tuning the laser frequency in the vicinity of a five-photon resonance in argon. By monitoring the relative conversion efficiency for fifth harmonic generation versus the multi-photon detuning for several laser intensities of the visible driving laser, we identified pronounced AC Stark shifts of the transition frequency in excess of 50 THz, exceeding the pulse bandwidth by more than one order of magnitude already at an intensity of the order of 10 TW/cm². We concluded, that for a given peak intensity, due to the strong level shifts a transient enhancement is achieved. This enhancement occurs when the laser is tuned to a wavelength such, that the dynamically shifted level comes in resonance at intensities slightly lower than the peak intensity of the pulse. By tuning the laser accordingly, we achieved an enhancement of about one order of magnitude for the fifth harmonic and also strong enhancements of the seventh and ninth harmonic generated simultaneously. This form of resonance enhancement of multiple harmonics emphasizes the importance of intermediate resonances also in higher harmonic generation. In a second approach, we focused on coherent control of frequency up-conversion towards the EUV regime by actively controlling the nonlinear polarization in a bi-color laser field. A four photon transition to the 5p^5 6p²[5/2] level in xenon is driven by intense laser pulses in the visible regime around 512 nm and can interfere with a two-photon transition, driven by the second field at a wavelength of 256 nm. By tuning the relative phase between the two fields, we achieve control of the excitation probability as confirmed by a pronounced modulation of the laser-induced fluorescence from the excited level. A further photon at 512 nm serves to drive fifth harmonic generation and simultaneous four-wave mixing with the UV photons, both yielding radiation at 102 nm wavelength. In systematic measurements, we examined the several preconditions to achieve maximum control in an experiment involving high nonlinear orders and ultra-short laser pulses at intensities around 1 TW/cm². As a result, we gained a visibility of 90 % in the interference of the two conversion pathways - to our knowledge the highest achieved visibility in phase control of harmonic generation so far. While coherent control can only gain a limited enhancement of less than a factor of 4, the data exhibit a convincing demonstration of the feasibility of coherent control also with ultra-short pulses at TW/cm² intensity, gaining a factor of 18 in modulation between destructive and constructive interference. Furthermore, we examined the dependence of absolute signal and control strength (visibility) concerning the detuning from the resonance and showed a change in the temporal modulation period with respect to the detuning. In a simultaneous measurement of the excited state population and frequency conversion, both processes show similar interference with equal modulation period and large modulation depth, but phase lag of 0.03 π between the two interferograms. We attributed this phase lag to the contribution of further atomic levels (in this case especially the Rydberg levels and the ionization continuum) to the nonlinear polarization, possibly introducing additional phase compared to the excitation channel. We finally enhanced the product of number density N and interaction length L by about a factor of 1000 by confining the gas medium inside a hollow core waveguide and balancing the gas dispersion with the waveguide dispersion. We showed that the combination of resonance enhancement and phase-matched harmonic generation at high N*L is possible in argon in the vicinity of the strong 3p^6 -> 3p^5 4s'² [1/2]° transition. Because of the effect of the AC Stark shift of the transition frequency on the refractive index of argon at the fifth harmonic frequency, the Stark shift could be determined directly by analyzing the shift in phase matching pressure at a constant fundamental wavelength. The obtained (averaged) energy shift of 0.85 Φ, is close to the ponderomotive energy Φ and in good agreement with recent publications. In comparing the relative experimental conversion efficiency versus gas pressure and resonance detuning with an extensive numerical simulation, we unveiled the important contribution of a quasi-phase matching scheme resulting from the mode beating at the fundamental frequency, even with only less than 3% of power guided in modes higher than EH11. Furthermore, we reproduced the resonance enhancement, which is significantly detuned even from the shifted resonance and explained the detuning and enhancement lineshape by the phase matching precondition and the required phase matching bandwidth to convert the full spectrum of the pulses. Within these constraints, we rate the investigated coupling scheme capable of resonantly enhanced frequency up-conversion of pulses as short as 100 fs. At optimal conditions, we achieved more than a factor of 800 higher conversion efficiency compared with an atomic jet operated at 1 bar of stagnation pressure and could even enhance this efficiency by another factor of 1.5 in admixing a positively dispersive buffer gas. |
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URN: | urn:nbn:de:tuda-tuprints-85692 | ||||
Classification DDC: | 500 Science and mathematics > 530 Physics | ||||
Divisions: | 05 Department of Physics 05 Department of Physics > Institute of Applied Physics 05 Department of Physics > Institute of Applied Physics > Nonlinear Optics/Quantum Optics |
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Date Deposited: | 31 Mar 2019 19:55 | ||||
Last Modified: | 31 Mar 2019 19:55 | ||||
PPN: | |||||
Referees: | Halfmann, Prof. Dr. Thomas ; Walther, Prof. Dr. Thomas | ||||
Refereed / Verteidigung / mdl. Prüfung: | 18 February 2019 | ||||
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