|
Publications
|
|
||
| |
||
|
||
| ||
|
|
"Optical parametric processes to the extreme:
From new insights in first principles to tunability over more than 4 octaves" Christian HomannDoktorarbeit am LS für BioMolekulare Optik, LMU München Abstract: Optical parametric amplification (OPA) is a nonlinear process that allows amplification of an electromagnetic wave at the expense of a wave with shorter wavelength in a suitable medium. In combination with spectral broadening of a narrow bandwidth laser pulse, this allows the generation of extremely broadband, and therefore potentially very short pulses, as well as widely tunable pulses. Therefore it is very popular in many areas of physics, as e.g. time-resolved spectroscopy or high field physics, which studies intense light-matter interaction. In this thesis, the efficiently accessible parameter range by optical parametric processes was substantially expanded. The new developments were thereby always initiated by specific requirements, that could not be fulfilled previously or only in very complex ways. This includes expanding the tuning range of ultrashort pulses in the deep ultraviolet and mid-infrared spectral region, adapting these concepts for MHz repetition rate, generating carrier-envelope phase stable sub-two-cycle pulses in the infrared with 100 kHz repetition rate and broadening the spectral range accessible with multi-mJ energies. The developed systems and concepts are now routinely used in such different fields as laser-excited photoemission electron microscopy, time-resolved photoelectron spectroscopy, investigations of polaron pairs in low-bandgap polymers for photovoltaics, and laser induced electron emission from nano-scale metal tips. In the course of these investigations, new fundamental insights about the parametric amplification process were gained, especially about the spontaneously generated parametric superfluorescence. The number of photons acting as seed source for this process was determined experimentally for the first time in absolute numbers. Additionally it was shown that this number is independent of the pump pulse energy, but proportional to the area of the pump pulse. The newly gained insight now provides design guidelines for parametric amplifier chains with a highly optimized ratio between amplified signal and unwanted superfluorescence noise. In more detail, a noncollinear OPA (NOPA) was developed with a more than octave-wide tuning range from 440 to 990 nm in a single amplification stage and with a repetition rate of up to 2 MHz. By careful and optimized use of the available pump energy it was furthermore possible to operate a second, independently tunable NOPA with a tuning range from 620 to 990 nm, which makes this unique system very useful for pump-probe spectroscopy. To access the nearand midinfrared region, an OPA was developed that delivers tunable pulses from 1 to 5 μm at 100 kHz repetition rate and with Fourier-limits below 50 fs over large parts of the tuning range. The broad applicability of the underlying concept was demonstrated for two different pump laser systems with differing central wavelength (800 nm and 1025 nm) and pulse duration (50 fs and 300 fs). In the region around 2 μm even carrier-envelope phase stable pulses with sub-two-cycle duration (~ 10 fs) could be generated at 100 kHz. First experiments with these pulses on tungsten tips already show promising results indicating new physics in the interplay between multi-photon ionization and the tunneling regime. By combining two amplification stages pumped by different harmonics of the pump laser, the output spectrum in optical parametric chirped pulse amplification was significantly broadened. This opens up the route to sub-5-fs pulses with energies in the multi millijoule regime. In these experiments an influence of the optical parametric phase was found, that is especially important in the spectral overlap region of the two amplifiers. That this influence is controllable and can be compensated for was shown in an additional experiment at 100 kHz repetition rate and with microjoule energies. By chirp-optimized sum-frequency mixing, tunable pulses from 190 to 220 nm with durations around 30 fs for time-resolved photoelectron spectroscopy were generated. As no routinely available and easy to handle method was established for the measurement of the pulse duration of UV pulses, an autocorrelator based on two-photon absorption was developed. It now allows measuring pulse durations down to below 20 fs and up to several hundreds fs in a spectral range from 150 nm up to the visible, and for energies down to a few nanojoules. BMO authors (in alphabetic order): Christian Homann  WWW-Version |
mailto: webmaster
Letzte Änderung: 2016-09-14 13:34
-------------------------------------------
nav.inc[abfrage], query:Delete from kurzzeitstatistik where TS<"1714072767"
-------------------------------------------
ERRNO: 0
-------------------------------------------
RESULT:
-------------------------------------------
ABORTED
-------------------------------------------
-------------------------------------------
nav.inc[abfrage], query:INSERT INTO kurzzeitstatistik (Datei, Adresse, id, TS) VALUES ("", "3.138.122.4" , "2958" , "1714073367 " )
-------------------------------------------
ERRNO: 0
-------------------------------------------
RESULT:
-------------------------------------------
ABORTED
-------------------------------------------
-------------------------------------------
nav.inc[abfrage], query:INSERT INTO statistik (Datei, Adresse, id, TS, Laufzeit) VALUES ("", "3.138.122.4" , "2958" , "1714073367 ","" )
-------------------------------------------
ERRNO: 0
-------------------------------------------
RESULT:
-------------------------------------------
ABORTED
-------------------------------------------