HHG Spectroscopy

Introduction

We aim to use High Harmonic Generation to study the molecular structure and internal dynamics of a range of organic molecules [1]. From this we hope to understand new features in their sub- femtosecond electronic dynamics and structural dynamics.

Since the majority of these molecules are available in liquid phase it is essential to develop a high stability heating system for the generation of a vapour to back a continuous flow gas jet. It is also necessary to have control over the setting of the vapour pressure generated as this allows us to control the density in the interaction region which is of importance for comparing harmonic yields between samples [2]. The use of thin gas jets (100/200 μm) minimises any potential phase matching or harmonic propagation issues. A high stability heating system with reliable and reproducible control over the gas density in the interaction region has been developed. Initial tests have demonstrated a stable gas jet (sub 3% fluctuations in the harmonic signal over one hour of HHG spectra acquisition).

Experiment

Using the idler at 1800 nm, three categories of comparative spectra have been taken.

1) Using the PACER technique we are investigating the influence of hydrogen dynamics following ionisation on the spectra. From this we are hoping to extract some information about the rate of change of the nuclear autocorrelation function. For this we are using benzene which has had its hydrogen replaced by deuterium. This leaves all of the electronic properties essentially unhindered whilst allowing the dynamics of the CH bonds to change. Trends in the ratio of the harmonic spectra from C6D6 to C6H6 follow the expected trend with the ratio increasing as a function of harmonic order, indicating faster dynamics in the protonated than the deuterated species.

Even though the trend is going in the expected direction, one can see some discrepancies between runs on different days, mainly due to spatial overlap stability issues between the laser and the generating medium. 

2) We also compare the harmonic yields from benzene to toluene and xylenes. This is to join our experimental data with theoretical work presented here. Attaching one (toluene) or two (xylene) methyl groups to the benzene rings lifts some of the degeneracy present in the molecule. This leads to potential measurement of Jahn-Teller effects in the molecules. Figure 2 obviously displays a decrease in the ratio between the various methylated benzenes to benzene, indicating faster dynamics, may they be electronic or nuclear, in the substituted species.

3) The last family of large organic molecules undergoing investigation are benzene rings with one of their hydrogen substituted by a halogen. We have compared fluoro, chloro, bromo and iodo-benzene. Some noticeable differences in the harmonic spectra from the different species have been recorded and the data is undergoing further analysis.

Conclusions

We have demonstrated high stability reproducible spectra in a range of organic molecules from our apparatus and are working towards the first results in HHG spectroscopy on these systems. We next aim to utilise our short pulse long wavelength light source (sub two cycle pulses at 1800 nm) to extend HHG spectroscopy to these molecules with increased resolution and dynamical information from the spectra since we are exploiting the λ² cut off extension [3].

Current researchers

F.McGrath, E. Simpson, P. Hawkins, Z. Diveki, T. Siegel, M. Castillejo¹, J.W.G Tisch, A. Zair, J.P Marangos.

¹Instituto de Química Física Rocasolano, CSIC, Serrano 119, 28006 Madrid, Spain.

References and relevant links

[1] J. P. Marangos et.al, PCCP, 10, 35, (2008)
[2] N. Hay et.al, Phys. Rev Lett A, 61, 053810 (2000)
[3] Torres et.al, Optics Express,18, 3174, (2010)

Introduction

Laser driven electron recollision has been identified as a unique tool for imaging molecular structure and ultrafast internal dynamics. Via High Harmonic Generation (HHG), it has been employed in the molecular frame to identify specific ionisation channels and track subsequent hole dynamics [1]. This has not yet been achieved in inelastic recollision phenomena such as nonsequential double ionisation (NSDI), due to its competition with sequential pro- cesses when using 800 nm laser pulses.

Experiment

Here we use a long wavelength drive field to ensure that NSDI is the only route to CO₂++ (see fig.1). Employing a TOF spectrometer and impulsive molecular alignment [2], we then fully characterise the inelastic rescattering event by measuring the shape of the returning electron wavepacket [3] (fig.1) and the recollision cross section in the molecular frame (fig.2). This allows us to unambiguously identify the contribution of the ionic ground and first excited state to the NSDI mechanism due to their d istinct orbital symmetry. Our results also provide the first angularly resolved electron-ion collision measurement in CO₂, albeit over a finite energy range. The universal applicability of this method thus demonstrates the potential of using inelastic recollisions in the molecular frame of reference for extracting and controlling molecu lar dynamics.

Figure 1 CO₂ ++ yield versus laser ellipticity ɛ comparing 800 nm and 1350 nm at an i ntensity of 2 × 10¹⁴ Wcm⁻². θ denotes the angle between the molecular an d the major axis of the laser polarisation ellipse. The narrow width of the ellipticity curves at 1350 nm identify recollision as the exclusive double ionisation mechanism. The curves do not agree with theoretical simulations which only take the molecular ground state (HOMO) into account, suggesting the contribution of further molecular states.

Figure 2 Total CO₂+ and CO₂++ yields and their ratio versus the angle θ between the linear laser polarisation and molecular axis for 1350 nm at 2 × 10¹⁴Wcm⁻². The lines are obtained from polynomial fits. The ratio represents the recollision cross section, which differs significantly from the CO₂+ rate. This identifies the first ionic state contributing to NSDI in CO₂.

Current researchers

M. Opperman, S. J. Weber, L. J. Frasinski , M. Yu. Ivanov, and J. P. Marangos.

References and relevant links

[1] O. Smirnova et al., Nature 460, 9 72 (2009).
[2] M. Oppermann et al., PCCP 14, 9785 (2012).
[3] V.R. Bhardwaj et al., Phys. Rev. Lett. 87, 253003 (2001).

Introduction

The aim of these studies was to extend earlier work on the control of HHG in strong perpendicularly polarised fields [1, 2] by comparing the variation of electronic trajectories and intensity yields of subsequent odd and even harmonics with relative phase between the driving and dressing fields.

Experiment

High-order harmonics were generated from argon using a 1.2 × 10¹⁴ Wcm⁻², 800nm, 30fs laser field and a perpendicularly polarised 400nm dressing field at 3×10¹³ Wcm⁻², generated by a BBO crystal. A thin glass plate in the beam path was rotated to control the relative phase (or, equivalently, temporal delay) between the two colours. We observe a modulation harmonic intensity with temporal delay between the peaks of the driving and dressing fields (see Fig.1 (left pane)). This is visible in the short and long trajectory contributions to both the odd and even harmonic emission, these being identified by their distinct far-field divergences. The observed phase-offset between the peak short and long trajectory emission is found to be different for odd and even harmonics, with the long trajectory peak preceding the short by up to 0.25fs for the odd harmonics, but trailing by up to 0.15fs for the even harmonics.

Theory and analysis

These results were modelled by classical and quantum orbit calculations with which we were able to qualitatively explain the observed odd and even harmonic intensity modulations in relation to the modified electron recollision time-energy mappings and transverse recollision momentum due to the presence of the second colour. A strong-field approximation (SFA) model of HHG was also employed to calculate the single-atom harmonic emission using the experimental laser parameters. This emission was then propagated through to the spatially-dependent far-field and the harmonic intensity modulations as detailed above were accurately reproduced (see Fig.1), including quantitative agreement in phase offset between short and long trajectory contributions for both odd and even harmonics.

Current researchers

D. J. Hoffmann, C. Hutchison, S. Houver, N. Lin¹, F. McGrath, E. Simpson, M. Arnold, A. Zaır and J. P. Marangos.

¹CEA-Saclay, IRAMIS, Service des Photons, Atomes et Molecules, 91191 Gif-sur-Yvette, France.

References and relevant links

[1] L. Brugnera, F. Frank, D. J. Hoffmann et al., Opt. Lett. 35, pp. 3994-3996 (2010).
[2] L. Brugnera, D. J. Hoffmann, T. Siegel et al., Phys. Rev. Lett 107, p. 153902 (2011).

Introduction

Investigations into Auger-type dynamics of core- ionised atoms can be split into two approaches: traditional energy domain investigations where the high- resolution kinetic energy spectra of the Auger electrons are obtained, e.g. [1], and attosecond streaking pump-probe type investigations [2] from which time resolved information of the Auger decay can be obtained. Here we propose an alternative time resolved technique; high order harmonic generation (HHG) spectroscopy of Auger transitions.

Theory

The new scheme is described by (Fig. 1), first the atom or molecule is ionised by an extreme ultra- violet (XUV), sub-femtosecond pulse. After ioni sation the produced photoelectron is accelerated by an IR field back into the parent system where it can recombine. This process is known as XUV-initiated high harmonic generation (XIHHG). However if the ion has decayed to any other state before the photoelectron is rescattered then the electron cannot re- combine to form the same ground state. Consequently the longer the photoelectron spends in the continuum the less likely it is to contribute to the XI-HHG radiation. Altering any parameter that changes the time-energy mapping of the harmonics, e.g. the position of the XUV pulse in the IR period, the IR frequency or IR intensity, allows us to compare the damping experienced by different excursion times and therefore extract the Auger decay lifetime.

Contrary to attosecond streaking that requires emission of secondary electron [2], the present technique can resolve quasi-exponential hole decay dynamics of the bound-bound type, e.g. due to breakdown of molecular orbital picture in the molecular inner- valence subshell [3]. We theoretically demonstrate the applicability of our technique to inner-valence molecular processes of this nature [4] and Auger decay in krypton (as shown in the left side of the figure).

Current researchers

J. Leeuwenburgh, B. Cooper, V. Averbukh, M. Yu. Ivanov and J.P. Marangos

References and relevant links

[1] M. Jurvansuu, A. Kivimki, S. Aksela , PRA 64, 0125 02 (2001).
[2] M. Drescher et al. Nature 419, 803 (2002).
[3] L.S. Cederbaum, W. Domcke, J. Schirmer and W. von Niessen, Adv. Chem. Phys. 65, 115 (1986).
[4] J. Leeuwenburgh, B. Cooper, V. Averbukh, J.P. Marangos, M. Ivanov, PRL 111, 123002 (2013)
[5] J. Leeuwenburgh, B. Cooper, V. Averbukh, M. Ivanov, to be submitted (2014)

Introduction

Laser-induced nonlinear polarization responsible for high harmonic generation can be written as the coherent sum of allpossible quantum paths contributing t o the emissi on of a given high-order harmonic. The two main paths, called ’short’ and ’long’ trajectories, have a phase difference ∆φ_n~ −U_p∆τ_n≈ −∆α_nI, where n is the harmonic order, Upis the ponderomotive energy, ∆τnis the excursion time difference between the short and the long trajectories, and I is the laser intensity. The interference of these quantum paths (QPI) leads to oscillations in the har- monic intensity as a function of the laser intensity, with the oscillation period 2π/∆α_n [1]. Here we show how attosecond electron dynamics inside the molecular ion, between ionization and recombina- tion, affects this interference, and how the QPI measurements [2] can be used to detect this dynamics.

Theory

We perform full quantum mechanical analysis of the model H2-like di a tomic molecule with two dimensions per electron, at a fixed internuclear distance. To induce attosecond electron dynamics in the ion, we augment the driving intense IR laser pulse with a weaker, perpendicularly polarized UV field, resonant with the transition between the ground and the first excited state of the ion. To obtain the QPI pattern, we perform calculations for many IR laser intensities. In our ab initio analysis, we can separate the QPI patterns associated with a given ionic state [3]. For molecules aligned perpendicular to the IR field the ground ionic state dominates in the process.

Fig. 1 (a) shows the typical QP I pattern as a function of IR intensity for the harmonic H15 (the contribution of the ionic ground state channel). The QPI contrast is dramatically reduced for a range of UV intensities I₂. The QPI oscillations, present at zero UV field, almost disappear for I₂ = 8 × 10¹² Wcm^-2, reappearing again at I₂ = 3 × 10¹³ Wcm^-2. This oscillatory pattern persists at higher I₂ (not shown). The reduced QPI contrast (red solid line) is due to nearly complete σ_g → σ_u population transfer between ionization and recombination for the long trajectories. The population dynamics shown in Fig. 1 (b) confirms this analysis. Changing the UV frequency by 20% detunes the resonance, reduces depopulation of the ionic ground state and the QPI oscillations reappear (red dashed line). Increasing the UV intensity returns the population of σg for long trajectories and thus the QPI contrast back (black line).

Our results show that the QPI technique should enable observation of sub-femtosecond hole dynamics in molecules.

Current researchers

S. Sukiasyan, A. Zaır, J. P. Marangos, and M. Yu. Ivanov.

References and relevant links

[1] A. Zaır, et al, Phys. Rev. Lett. 100, 143902 (2008).
[2] A. Zaır, et al, Chem. Phys. 414, 184 (2013).
[3] S. Sukiasyan, et al, Phys. Rev. A. 82, 043414 (2010).

Introduction

High order harmonic generation (HHG) over the last two decades has proven to be a powerful tool for the generation of coherent short bursts of X-rays and for the studies of ultra-fast atomic and molecular dynamics. A recent development in the field o f HHG has been the use of laser ablation to promote the constituents of solid target materials into a weakly ionised plasma plume phase. The ai m of this work is to extend this technique to study differences between the non-linear response of uracil and thymine.

Experiment

Ablation targets consisted of pressed powder pellets made from uracil or thymine. The sur fa ce of the target was ablated with a heating pulse (780 nm 160 ps or 1064 nm 10 ns) to create a plasma plume. After a delay of ≈ 60 ns a probe pulse (780 nm, 30 fs or 1300 nm, 40 fs) was used as driving radiation for HHG. These were sorted and analysed using an XUV spectrometer and micro channel plate detector. The target was rotated to reduced surface damage and en- sure stable ablation. Time of fight mass spectrometry (TOFMS) was used to study the ion composition of the plumes.

Results

Harmonics with energies up to 55 eV where successfully generated from uracil plumes using both heating and probe pulses. No harmonic signal could be obtained from thymine and a target that was composed of 50% uracil and thymine 50% showed a reduced signal (figure 1, left image). TOFMS measurements showed in thymine a great number of atomic/small molecular fragments and a negligible signal peak from intact molecular ions. Uracil plumes contained larger fragments and a clear signature assigned to the parent ion (figure 1, right image).

Conclusion

We believe that in the ablation process, a high degree of fragmentation occurs in thymine, producing a large number of free electrons which destroy the phase matching of the HHG process. In the case of uracil the fragmentation processes are reduced, although at the present stage it is not possible to ascertain that uracil molecules are the only source of HHG.

Current researchers

C. Hutchison, R. A. Ganeev, I. López-Quintás¹, A. Zaır, S. J. Weber, F. McGrath, Z. Abdelrahman, M. Oppermann, M Martin¹, D. Y. Lei, S. A. Maier, M. Castillejo¹, J. W. G. Tisch, and J. P. Marangos.

¹Instituto de Qumica Fsica Rocasolano, CSIC, Serrano 119, 28006 Madrid, Spain.

Introduction

Few-cycle carrier-envelope phase (CEP) controlled laser pulses of millijoule energies are central to attoscience and strong-field physics, enabling isolated attosecond pulse production [1] and precisely controlled electron recollision [2]. They typically exist at optical wavelengths due to their origin in widely deployed Ti:Sapphire-based amplifiers. Moving to longer wavelengths offers several advantages salient to the goals of the project:

1. Increased electron recombination energy at a fixed intensity: This quantity determines the photon energy produced via high-order harmonic generation (HHG) as well as the spatial resolution of the electron when viewed as a diffractiveprobe. It cannot be arbitrarily increased with the laser intensity due several saturation effects in the single-atom and macroscopic response. An alternative is increasing the laser wavelength, which affords the continuum electron more time to be accelerated.
2. Longer time window in HHG spectroscopy: The cation which results from strong-field ionization may undergo ultrafast dynamics, which are then probed at the moment at recombination. In current experiments, the time between ionization and recombination is at most three-quarters of a laser cycle. Increasing the wavelength is therefore an immediate route to extending the time range of these studies.

Optical-parametric chirped pulse amplification [3] is a direct means of producing suitable pulses, but its complexity is such that compression of the multi-cycle pulses available from simpler setups is an attractive option. Filamentation in gases offers simplicity [4], but as yet has not achieved sub-two-cycle pulses. Spectral broadening in a gas-filled hollow core fibre (HCF) [5] has achieved 1.6 cycle, 240 µJ pulses at 1.8 µm and is an elegant solution because the required anomalous dispersion can be simply implemented through propagation in bulk glass.

Results

We have developed a source of few-cycle pulses at 1.75µm based on hollow fibre compression. Our system features:

• 350 µJ deliverable energy.
• Duration tunable from 9–40fs full width at half maximum (FWHM).
• Closed loop CEP stabilization and control, with shot-to-shot stability of 710 mrad.
• Temporal characterization using spectral shearing interferometry.
• Theoretical description using 3D simulations.

Figure 1 is an overview of the setup. A Ti:Sapphire chirped pulse amplifier (CPA) pumps a commercial OPA (HE- TOPAS) consisting of three stages of amplification in BBO seeded by white light generation (WLG). A piezo-mounted mirror (PM) in the beam path of the third pump is used for carrier-envelope phase control of the idler. The many cycle idler is separated, reshaped and focused into a 95cm long, 400µm diameter differentially pumped hollow core fibre with 1mm thick CaF 2 entrance and exit windows. The output beam is collimated by and directed to a SEA-F-SPIDER [6] for temporal characterisation or to an f-to-2f interferometer [7], which provides a feedback signal for closed loop phase stabilization and control. A pair of 4° fused silica wedges provide fine dispersion control.

Figure 2 shows some results at 1700 nm. The incident pulse duration is 26.5 fs and the output pulse energy 350 µJ. The three rows show the exit pressure increasing from 1 bar to 1.8 bar, with the wedges being continually adjusted to obtain optimal compression. There is a compromise between FWHM duration and pulse cleanliness, with 1.6 bar and 1.7 mm fused silica yielding a 9.7 fs pulse, corresponding to 1.8 optical cycles.

Figure 3 illustrates closed loop control of the carrier-envelope phase in a stepwise sawtooth pattern and a sinusoidal pattern. The f-to-2f fringes (Fig. 3(a)(a)) are obtained after passing the HCF output throu g h a 200 µm BBO cu t for type I second harmonic generation followed by a polarizer. No additional spectral broadening is necessary. The extracted phase (blue) is shown in Fig. 3(b), along with the requested target phase (red). The interferograms were exposed for three laser shots and the measured stability was 410 mrad, implying a shot-to-shot stability of 710 mrad.

High-order harmonic generation results

We are developing a new target and XUV spectrometer suitable for high-order harmonic generation (HHG) up to the oxygen K edge at 530 eV. Meanwhile, we have acquired harmonic spectra using our current apparatus designed for many- cycle 800 nm pulses. Figure 4 shows a set of harmonic spectra generated in argon for various pressures in the hollow fibre compressor. At 0 bar, the harmonics are driven by a many-cycle pulse and discrete harmonics are visible up to 50 eV, limited by the spectrometer resolution. As the pressure is increased, the pulse becomes shorter and the cut-off extends to beyond the reliable range of the spectrometer. The harmonic visibility at lower photon energies decreases, indicating emission from fewer laser half-cycles.

Current researchers

D. R. Austin, T. Witting, A. Johnson, T. Siegel, J.W.G. Tisch, J.P. Marangos.

References and relevant links

[1] Goulielmakis, E. et al., Science 320, 1614 (2008)
[2] Shiner, A. D. et al., Nature Phys. 7, 464 (2011)
[3] Ishii, N. et al., Opt. Lett. 37, 4182 (2012)
[4]  Driever, S. et al., Appl. Phys. Lett. 102, 191119 (2013)
[5] Schmidt, B.E. et al., Opt. Express 19, 6858 (2011)
[6] Witting, T. et al., Opt. Express 20, 27974 (2012)
[7] Kakehata, M., Opt. Lett. 26, 1436 (2001)

Introduction

In recent years several groups started pushing the central wavelength of the driving laser field for high harmonic generation further and further to the near IR. One immediate advantage is that the cut-off energy of the high harmonics increases with ∝ λ2. However the process gets less efficient the longer the wavelengths ( ∝ λ−5.5). Hence going to the few- cycle pulse regime in this waveleng th range is highly favorable. This was tackled theoretically and experimentally [1]-[8]. Several experimental approaches have been taken in order t o achieve this. Sophisticated optical parametric amplifiers (OPA) systems were used by [1] and [2] with subsequent filamentation post compression at 2 µm and 1.5 µm. The hollow core fibre compression technique was employed to get to the f ew-cycle regime at 1.8 µm by [4]. We propose to investigate filamentation with subsequent compression as a versatile tool employing a tunable source in the near IR.

Experiment

The experimental setup comprised a 800 nm Ti:sapphire laser which was used to pump a commercial OPA system, tunable from 1.6 to 2 µm. The out- put beam was focused loosely into a 4 bar statically filled Krypton cell. We managed to achieve tunable spectral broadening for these wavelengths. For 1.7 µm, 1.8 µm and 1.9 µm we obtained about 300 nm bandwidth. The compression was achieved by phase compensation via the negative GVD of bulk fused silica. The second order autocorrelation traces, see Fig. 1 show compression factor of 2-3 down to the few- cycle regime. The corresponding AC widths were 18.5 fs for 1.7, 26.6 fs for 1.8 and 19.5 fs for 1.9 µm. The input pulse energies were 480 to 520 µJ resulting in about 200 µJ in the filament.

Current researchers

S. Driever, D. Bigourd, N. Federov¹, M. Cornet, M. Arnold, F. Burgy¹, S. Montant¹, S. Petit¹, D. Descamp¹, E. Cormier¹, E. Constant¹ and A. Zaır.

¹Universit´ e de Bordeaux, CEA, CNRS UMR 5107, CELIA (Centre Lasers Intenses et Applications), FR-33400 Talence, France

References and relevant links

[1] Hauri et al. Opt. Lett. 32, 7, 868 (2007).
[2] Mücke et al. Opt. Lett. 34, 2498 (2009).
[3] Bergé. Opt. Exp. 16, 21529 (2008).
[4] Schmidt et al. App. Phys. Lett. 96, 121109 (2010).
[5] Prade et al. Opt. Lett. 31, 2604-2606 (2006).
[6] Skupin and Bergé. Opt. Com. 280, 173182 (2007).
[7] Aliauskas et al. Lith. J. Phys. 50, 111 (2010)
[8] Voronin et al. Phys. Rev. A 84, 023832 (2011).