Final Report Summary - HARMONICS-PLASMA (Harmonic generation in extreme ultraviolet region through the interaction of short laser pulses with low-excited laser-produced plasma and gas jets)
High-order harmonic generation (HHG) is an increasingly used and promising technique for achieving the extreme ultraviolet (XUV) spectral range with highest brightness, short pulse duration, and coherence. Extensive studies of this phenomenon have been mostly carried out using jets of neutral atomic gas, which have resulted in novel coherent XUV sources. However, typically observed high-order harmonics presently have the disadvantage of low conversion efficiency (10-6). This is problematic for many potential applications of HHG radiation. Recent studies have shown that this weakness can be partially overcome by using the ablated plasma as a nonlinear medium.
The project is aimed at the enhancement of HHG efficiency from laser ablation produced on the surfaces of solid-state materials and comparison with HHG from gas jets. The milestones of the proposed investigations include:
(a) analysis and optimisation of harmonic generation from laser plasma produced on the surface of various targets using high pulse repetition rate lasers;
(b) search of resonance-induced enhancement of single harmonic in the XUV range;
(c) harmonic generation from the laser plumes containing nanoclusters; and
(d) HHG from gas jets and comparison with the HHG from laser plasma.
As a result of project, further improvements of the harmonic efficiency in the XUV range through the HHG from laser plasma and gas jets expected to be achieved.
The following work was performed since the beginning of the project:
(a) The experimental scheme for plasma HHG has been developed. We demonstrated the HHG in laser-produced plasma on the metal surfaces using the laser operating at high pulse repetition rate (1 kHz). The harmonics up to the 61st order (Ag plasma) were obtained. In these studies, variation of the divergences of low- and high-order harmonics was carried out by change of the divergence of probe radiation, which led to considerable re-distribution of converted energy from low-order harmonics toward the higher order ones. The application of bulk blocks as the targets allowed diminish the overheating-induced instability of harmonics from plasma plumes.
(b) We carried out the systematic studies of quantum path signatures (QPS) in the case of HHG in a laser-produced plasma. The high pulse repetition rate (1 kHz) of a Ti:sapphire laser allowed the observation of the influence of short and long electron trajectories on the harmonic pattern from the metal plasma. We presented the results of studies of the influence of focusing conditions, laser intensity, plasma concentration, and chirp of laser radiation on the spectral and spatial pattern of quantum paths. We discussed the difference between our results and those reported in the case of gas HHG, which was attributed to the involvement of ionised particles and free electrons in the process of harmonic generation. The importance of these studies is related to the fact that QPSs could be used to measure intramolecular dynamics.
(c) We have studied HHG from laser-produced plasmas using three recently introduced techniques to improve the plasma harmonic yield: resonance-induced and two-colour pump-induced enhancement, together with the application of a high-repetition rate laser source. Together with an increased HHG output, we observed an enhanced yield of even harmonics compared with odd ones, and resonance-induced enhancement in the intensity of some single even harmonics when the ratio between the 780 and 390 nm pulse energies was 25:1. During our experiments, several new resonance-enhanced processes were revealed, mostly due to the modification of electron trajectories in HHG by the presence of a weak second harmonic field. This led to the generation of enhanced odd and even harmonics in different regions of the plateau.
(d) We carried out the harmonic generation studies using ablation carbon plasma as a nonlinear medium. We systematically analysed plasma HHG using ultrashort (3.5 and 40 fs) laser pulses. Strong harmonics from carbon plasma were attributed to the specific properties of this medium, in particular, the presence of clusters in the plume and their involvement in the enhancement of harmonic yield. The low-order (9th - 15th) harmonics of a few-cycle laser generated in an ablation plasma from a graphite target were found to be two to five times stronger compared with those generated from argon gas in the case of ablation using the 10-ns pulses.
(e) We have studied the stable plasma ablation and HHG at a 1-kHz pulse repetition rate using a rotating target. The developed technique allowed us to achieve long-term stability (for > 106 shots) in the harmonics generated in laser-produced plasmas. Our approach opens the doors for more efficient and convenient use of in-plasma HHG and their advanced properties (i.e. resonance enhancement of harmonics, application of clustered plasmas, etc.) compared with in-gas HHG. This will be of great importance for applications such as harmonic spectroscopy where signal stability is paramount, harmonic pulse duration measurements, surface physics experiments, and HHG studies of clusters and organic molecules.
(f) We studied the harmonic spectra from manganese plasmas driven by 3.5-fs pulses, which are dominated by a single enhanced 31st harmonic at around 50 eV. Analysis of the harmonic energy tuning upon the variation of driving laser pulse duration controlled by change of gas pressure in the hollow fibre compression system show stabilisation of the enhanced harmonic's energy and wavelength, which we explain with resonance-induced enhancement at 51 eV. Our theoretical modelling suggests that the emission could constitute an isolated sub-femtosecond pulse. The isolation of a single harmonic order without any filtering could also be useful for various applications where the coherent, short pulse XUV radiation is required, without the losses induced by spectral dispersion or filtering.
(g) We presented the results of the experimental and theoretical studies of HHG in the plasmas containing fullerenes (up to the 29th and 41st orders for the 780 and 1 300 nm probe radiation) under different plasma conditions and laser parameters in the case of few-cycle and multicycle pulses. Our comparative studies using 3.5 and 40 fs pulses showed that, for few-cycle pulses, the harmonic cut-off is extended compared with multicycle pulses, which can be attributed to reduced fragmentation of C60 for the shorter pulse. The comparison of fullerene harmonic spectra generated in the case of 1 300 and 780 nm multicycle probe pulses showed the extension of generating harmonic orders in the former case.
All the objectives of the project were achieved.
The project is aimed at the enhancement of HHG efficiency from laser ablation produced on the surfaces of solid-state materials and comparison with HHG from gas jets. The milestones of the proposed investigations include:
(a) analysis and optimisation of harmonic generation from laser plasma produced on the surface of various targets using high pulse repetition rate lasers;
(b) search of resonance-induced enhancement of single harmonic in the XUV range;
(c) harmonic generation from the laser plumes containing nanoclusters; and
(d) HHG from gas jets and comparison with the HHG from laser plasma.
As a result of project, further improvements of the harmonic efficiency in the XUV range through the HHG from laser plasma and gas jets expected to be achieved.
The following work was performed since the beginning of the project:
(a) The experimental scheme for plasma HHG has been developed. We demonstrated the HHG in laser-produced plasma on the metal surfaces using the laser operating at high pulse repetition rate (1 kHz). The harmonics up to the 61st order (Ag plasma) were obtained. In these studies, variation of the divergences of low- and high-order harmonics was carried out by change of the divergence of probe radiation, which led to considerable re-distribution of converted energy from low-order harmonics toward the higher order ones. The application of bulk blocks as the targets allowed diminish the overheating-induced instability of harmonics from plasma plumes.
(b) We carried out the systematic studies of quantum path signatures (QPS) in the case of HHG in a laser-produced plasma. The high pulse repetition rate (1 kHz) of a Ti:sapphire laser allowed the observation of the influence of short and long electron trajectories on the harmonic pattern from the metal plasma. We presented the results of studies of the influence of focusing conditions, laser intensity, plasma concentration, and chirp of laser radiation on the spectral and spatial pattern of quantum paths. We discussed the difference between our results and those reported in the case of gas HHG, which was attributed to the involvement of ionised particles and free electrons in the process of harmonic generation. The importance of these studies is related to the fact that QPSs could be used to measure intramolecular dynamics.
(c) We have studied HHG from laser-produced plasmas using three recently introduced techniques to improve the plasma harmonic yield: resonance-induced and two-colour pump-induced enhancement, together with the application of a high-repetition rate laser source. Together with an increased HHG output, we observed an enhanced yield of even harmonics compared with odd ones, and resonance-induced enhancement in the intensity of some single even harmonics when the ratio between the 780 and 390 nm pulse energies was 25:1. During our experiments, several new resonance-enhanced processes were revealed, mostly due to the modification of electron trajectories in HHG by the presence of a weak second harmonic field. This led to the generation of enhanced odd and even harmonics in different regions of the plateau.
(d) We carried out the harmonic generation studies using ablation carbon plasma as a nonlinear medium. We systematically analysed plasma HHG using ultrashort (3.5 and 40 fs) laser pulses. Strong harmonics from carbon plasma were attributed to the specific properties of this medium, in particular, the presence of clusters in the plume and their involvement in the enhancement of harmonic yield. The low-order (9th - 15th) harmonics of a few-cycle laser generated in an ablation plasma from a graphite target were found to be two to five times stronger compared with those generated from argon gas in the case of ablation using the 10-ns pulses.
(e) We have studied the stable plasma ablation and HHG at a 1-kHz pulse repetition rate using a rotating target. The developed technique allowed us to achieve long-term stability (for > 106 shots) in the harmonics generated in laser-produced plasmas. Our approach opens the doors for more efficient and convenient use of in-plasma HHG and their advanced properties (i.e. resonance enhancement of harmonics, application of clustered plasmas, etc.) compared with in-gas HHG. This will be of great importance for applications such as harmonic spectroscopy where signal stability is paramount, harmonic pulse duration measurements, surface physics experiments, and HHG studies of clusters and organic molecules.
(f) We studied the harmonic spectra from manganese plasmas driven by 3.5-fs pulses, which are dominated by a single enhanced 31st harmonic at around 50 eV. Analysis of the harmonic energy tuning upon the variation of driving laser pulse duration controlled by change of gas pressure in the hollow fibre compression system show stabilisation of the enhanced harmonic's energy and wavelength, which we explain with resonance-induced enhancement at 51 eV. Our theoretical modelling suggests that the emission could constitute an isolated sub-femtosecond pulse. The isolation of a single harmonic order without any filtering could also be useful for various applications where the coherent, short pulse XUV radiation is required, without the losses induced by spectral dispersion or filtering.
(g) We presented the results of the experimental and theoretical studies of HHG in the plasmas containing fullerenes (up to the 29th and 41st orders for the 780 and 1 300 nm probe radiation) under different plasma conditions and laser parameters in the case of few-cycle and multicycle pulses. Our comparative studies using 3.5 and 40 fs pulses showed that, for few-cycle pulses, the harmonic cut-off is extended compared with multicycle pulses, which can be attributed to reduced fragmentation of C60 for the shorter pulse. The comparison of fullerene harmonic spectra generated in the case of 1 300 and 780 nm multicycle probe pulses showed the extension of generating harmonic orders in the former case.
All the objectives of the project were achieved.