Tracking primary radiation-induced processes in matter requires ultrafast sources and high precision timing. While compact laser-driven ion accelerators are seeding the development of novel high instantaneous flux applications, combining the ultrashort ion and laser pulse durations with their inherent synchronicity to trace the real-time evolution of initial damage events has yet to be realized. Here we report on the absolute measurement of proton bursts as short as 3.5±0.7 ps from laser solid target interactions for this purpose. Our results verify that laser-driven ion acceleration can deliver interaction times over a factor of hundred shorter than those of state-of-the-art accelerators optimized for high instantaneous flux. Furthermore, these observations draw ion interaction physics into the field of ultrafast science, opening the opportunity for quantitative comparison with both numerical modelling and the adjacent fields of ultrafast electron and photon interactions in matter.
To date the experimental investigation of exactly how matter recovers in response to ion damage, and the emerging pathways for the resultant reactive species generated during the interaction, has been limited by pulse length in conventional accelerators and probe timing jitter1. When a pulse of ions (with kinetic energy >1 MeV per nucleon) interacts with condensed matter the individual particles generate nanometre-wide tracks of ionization with correspondingly steep energy density gradients2,3,4,5,6. The generation of steep energy density gradients drives a rapid diffusion of the hot electron population that tends to homogenize the dose distribution3,4. This process leads to secondary ionization cascades and the formation of long-lived excited states/chemically reactive species, which equilibrate with the background material over picosecond timescales7,8. Studying the lifetime of these states is critical for understanding the relationship between the incident ion flux and time-dependent defect concentration in ion-irradiated matter9 and cell death/repair rate for radiobiology driven by ultrafast energy deposition10. This interaction, and subsequent evolution of dose, is unambiguously distinct from the near-homogeneous excitation that results from both X-ray and electron interactions due to significantly different stopping powers of the different ionizing species in matter.
While pump–probe measurements can in principle reveal these inherently multi-scale processes, they require that the ion pump pulse is significantly shorter than the mean lifetime of the species under investigation and that there is a comparatively high degree of synchronicity between the pump and probe sources. As mentioned above, the corresponding few picosecond bunch duration is not routinely available from conventional accelerators1,11 and so an alternative source is required. One method to overcome this problem is to capitalize on the ultrafast acceleration phase provided during the interaction of high-power lasers with thin, solid-density foils12,13,14. The high instantaneous flux is suitable for a host of high energy density applications15,16,17,18 but to date the excellent synchronicity between the pulse of protons (pump) and the driving laser (probe) has not been exploited to directly investigate ultrafast damage processes in matter8,10,19.
Here we demonstrate an absolute duration measurement of proton pulses generated during high-power laser solid target interactions that overcomes these limitations. Our technique relies on the observation of prompt ionization dynamics8,20 in high-purity SiO2 glass (a transparent wide bandgap dielectric) irradiated by laser-driven protons using highly synchronized optical laser pulses. As part of this measurement we are also able to place an upper limit on the mean free lifetime of the excited electron population in the conduction band of <0.2 ps for our experimental conditions. It is this ultrafast response of SiO2 that permits the single-shot temporal characterization (with <0.5 ps resolution for our entire detection system) of the proton pulse driving the ionization dynamics. In addition we demonstrate the general applicability of this technique by extending our measurements to perform jitter-free tracking of ion-induced dynamics in an alternative sample dielectric sample (borosilicate glass). This is made possible because the absolute proton pulse duration has been established. In future this technique can be combined with equally synchronized laser-driven coherent ultraviolet/X-ray probes21,22 to allow for metrology of ion interactions on timescales of the rising edge of the interaction and potentially detailed studies of the initial damage track structure in the medium.
To date the duration of laser-accelerated proton bunches has only been inferred from proton radiography17,18. For ultrafast applications, however, a more direct method of pulse metrology is desirable. In this section we show that the absolute measurement of laser-driven ion pulses is made possible by studying ultrafast ionization dynamics8,9,20 in ion-irradiated SiO2 (Fig. 1). We also demonstrate the application of this method to novel studies of the lifetime of excited conduction band electrons in proton-irradiated matter.
First we examine the temporal characteristics of protons accelerated via the target normal sheath acceleration (TNSA) mechanism (see Methods), although the technique described below is also applicable to ion bursts from alternative laser-driven acceleration schemes14,23. TNSA is an inherently broadband generation mechanism, producing a Maxwellian spectrum with a sharp, high-energy cutoff (Eco). For nonrelativistic proton energies (MeV scale) this large energy spread causes the initially short ion pulse to stretch rapidly due to velocity dispersion, as it drifts from the source. The initial short pulse duration is still, however, preserved in narrow energy slices (bandwidths) of the spectrum. One way to recover this bandwidth is by stopping24 the lower-energy protons in bulk SiO2 (Fig. 1). The pulse duration τ(D) can then simply be estimated from time-of-flight considerations for the fastest and slowest edges of the bandwidth remaining at a given depth in the SiO2 (Fig. 1b). See Methods for further details on bandwidth narrowing and the corresponding pulse duration.
To measure τ(D) experimentally we study the lifetime of electrons excited into the conduction band of SiO2 by proton stopping. The schematic in the inset of Fig. 1 (dashed box) shows how this can provide the basis for an ultrafast detector. Prompt (<10−15 s) ionization by protons (i) results in the excitation of electrons into the conduction band (ii), where the Roman numerals correspond to labels in the inset of Fig. 1. Previous experiments based on uniform photoexcitation in SiO2 demonstrate a decay time constant of ∼0.15 ps for moderate excitation densities (∼1019 cm−3)8,20,25, with recombination being driven by ultrafast defect formation (iii) in the material8,9,20. Before recombination, however, these excited electrons are available to take part in free-free absorption of optical radiation (∼1 eV). It is this rapid switching in the optical transmission properties of irradiated SiO2 (transient opacity) that provides the platform for ultrafast metrology of proton pulses.
Observation of the proton pulse duration
Figure 2 shows a schematic for the optical streaking26 technique used to temporally resolve the transient opacity in SiO2 induced by laser-driven ion pulses. To experimentally verify this technique the TARANIS laser facility at Queens University Belfast27 is used to accelerate protons via the TNSA mechanism to a spectral cutoff energy (Eco) of 10±0.5 MeV. This pulse then interacts with a high-purity SiO2 sample placed 5±0.5 mm behind the generation point. A typical optical streak obtained from this setup is shown in Fig. 3a. In agreement with full modelling for the optical streak (based on Monte Carlo simulations, Fig. 3b), the duration of the transient opacity gets successively shorter with respect to D due to bandwidth narrowing as a result of low-energy proton stopping. For D=530±10 μm the duration of the opacity is measured to be 3.5±0.7 ps (Fig. 4a). This is in excellent agreement with the expected proton pulse duration τ(D) of ∼3.3 ps from both calculations (green trace, Fig. 1b) and modelling (Fig. 3b).
In fact, using this modelling of the optical streak we can analyse the temporal profile of the measured proton pulse in some detail. The resolution of this measurement is 0.45±0.05 ps, which corresponds to a proton pulse bandwidth of <0.1 MeV for the conditions described in Fig. 4a. We find that best agreement between experiment and modelling is obtained for an assumed detector response time tr of 0.45 ps (green dashed trace, inset, Fig. 4a). This places an upper bound of 0.1 MeV on the decay constant of the TNSA spectral intensity above Eco and an upper limit of <0.2 ps on the lifetime of excited electrons in the conduction band of the irradiated sample SiO2 (from temporal profile fitting, inset, Fig. 4a). Furthermore, this analysis demonstrates that proton straggling does not lead to a significant increase in the observed pulse duration beyond what is expected due to slowing in SiO2. In all, this analysis confirms the efficacy of SiO2 as a sub-picosecond resolution detector for proton bursts. Finally, our ultrafast metrology verifies that there is negligible thermal spread imposed on the nascent bunch during the acceleration mechanism for the fastest protons (>6 MeV), that is, the protons are accelerated from an initially cold cathode for our experimental conditions. The observation of emission from a cold cathode is in agreement with previous experimental measurements of the transverse emittance of the TNSA beam on similar specification systems28.
It is worth noting that for proton bunches with an inherently narrow energy spread, stopping in bulk SiO2 is not required to achieve ultrafast temporal resolution. In this scenario the bulk sample can be replaced by a thin SiO2 pellicle and the proton pulse duration can be obtained using a near-collinear probing geometry20. For proton beams with sufficiently high kinetic energy this will have a negligible effect on beam quality and is a possible route to online temporal metrology. In addition, this near-collinear geometry can provide information on the two-dimensional beam spatial/angular distribution by imaging the opacity induced by the proton–SiO2 interaction directly onto a charge-coupled device as an ultrafast replacement for comparatively slow scintillation screens.
Ultrafast ion damage in matter
Next we demonstrate how this technique can be applied more generally to study ultrafast dynamics in proton-irradiated condensed matter. In Fig. 4b we show a direct comparison of the lifetime of conduction band electrons in SiO2 and borosilicate glass (BK7) for identical proton pulse conditions to those presented in Fig. 3a. BK7 is a multicomponent derivative of SiO2, representing a low-cost alternative for laboratories due to its similar thermal shock and optical properties. However, as can be seen, the proton-induced transient opacity is observed to have a >400-ps recovery time to full transmission (red dotted trace, Fig. 4b). Photoexcitation experiments in BK7 suggest that this long recovery can be interpreted as being due to occupied interstitial levels preventing rapid relaxation of the hot electron population29. However, those experiments also indicate that the lifetime of photoexcited electrons is ∼5,000 ps. Our measurement of 430±20 ps, while being ∼100 times longer than that of SiO2, is significantly shorter than 5,000 ps.
One hypothesis for this discrepancy lies in the nature of proton interactions in condensed matter. Monte Carlo modelling for damage tracks in SiO2 shows that an instantaneous proton flux (that is, time=0 fs) of 50 μm−2 produces a strongly inhomogeneous dose distribution with excited electron densities falling from >1021 cm−3 to equilibrium level over transverse widths <4 nm (Fig. 4c). From simple electronic diffusivity considerations this distribution will evolve rapidly over ps timescales in comparison with homogenous photoexcitation3. While a detailed study of this evolution is beyond the scope of the work presented here, the observation of significantly reduced recovery timescales in BK7 provides an example of the new insights into physical processes this metrology can provide for transient dynamics resulting from proton–matter interactions. Furthermore, the high degree of synchronicity between the proton and probe sources means that our technique can be readily extended to study the evolution of specific transient defects by changing the probe wavelength, either through frequency conversion in crystals or nonlinear conversion using either gas or relativistic plasma21,22 sources, to temporally resolve absorption bands for specific excited species.
Looking to the future, the prospect of generating narrow-bandwidth proton spectra with femtosecond duration driving lasers14 offers the possibility of extending this technique to timescales suitable for investigating primary radiation events during the rising edge of the interaction using femtosecond scale probing. Another possibility is the potential to extend work examining energy transfer in non-equilibrium warm dense matter generated through isochoric heating using laser-accelerated protons30. For the present TNSA source, schemes that require the maintenance of high proton beam quality can be realized using a simple magnetic spectrometer and slit arrangement, or more complicated chromatic focusing schemes using a laser-driven electrostatic lens31 to select the desired flux and energy bandwidth (or pulse duration) instead of via stopping in bulk material. This will permit ultrafast studies of proton implantation and resulting damage at well-specified depths in materials.
Target normal sheath acceleration
During intense laser–foil (μm scale thickness) interactions an accelerating potential at the rear surface of the foil is rapidly established by electrons driven through the target from the front surface by the intense laser field. As they exit the foil, these electrons ionize a thin hydrocarbon contamination layer on the rear surface leaving a net positive charge in their wake. Simulations show that this process leads to the formation of a charge separation sheath producing a strong electrostatic field exceeding 1 TV m−1, which provides a transient accelerating potential for the rear-surface ions14. However, since the fastest electrons only contribute to this sheath for a fraction of the driving laser duration, the peak in this accelerating potential is very short-lived. It is this ultrafast evolution of the accelerating potential that provides the basis for ultrashort ion pulse generation during TNSA.
Stopping of protons in SiO2 for TNSA bandwidth/pulse duration selection
As the broad bandwidth of TNSA protons transverses the SiO2 sample, they continuously lose kinetic energy, that is, a 10-MeV proton in vacuum has <<1 eV kinetic energy after ∼570 μm of SiO2. This implies that the bandwidth of the TNSA spectrum continually narrows and shifts towards lower energies with respect to D. At a given depth the remaining bandwidth is bounded on the leading edge by the fastest protons. These protons have a well-defined energy (Eco before entering the SiO2). Next the trailing/slowest edge is bounded by the lowest-energy protons that have yet to be stopped at that depth24. Together these edges define a sharply bounded, depth-dependent bandwidth that narrows as the fastest protons slow to thermal energies (which implies that all protons with lower initial kinetic energy are already stopped). This is illustrated in Fig. 1b where the pulse duration (obtained from the bandwidth) is shown to reduce with respect to D.
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We acknowledge support for this work from the EPSRC through the following grants EP/L02327X/1 and EP/K022415/1. R.S. acknowledges support from SFI. We acknowledge insightful discussions with Prof. Padraig Dunne (University College Dublin).
The authors declare no competing financial interests.
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