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Zeptoscience

Jonathan Wheeler

Jonathan A. Wheeler is currently a researcher with IZEST based at Ecole polytechnique and concentrating on the compression techniques for high energy laser pulses toward their fundamental limits.  He studied ultrafast femtosecond and attosecond physics for short XUV pulse creation from high harmonic generation in laser-gas interactions during his Ph.D. at the Ohio State University (OSU) in the Agostini-DiMauro Ultra-fast Atomic Physics Research Group and then researched laser-solid interactions driven by few-cycle laser pulses involving controlled spatio-temporal couplings in the Physique du Cycle Optique Group at the Laboratoire d’Optique Appliquée (LOA).  This was followed by post-doctoral work with the Physics at High Intensity group at CEA-Saclay and affiliated with IZEST that focused on the development of spatial-temporal diagnostics for ultrashort laser pulses.

The goal of this project is to foster the theoretical simulation of Zeptosecond/Exawatt physics in parallel with the experimental development and implementation of the laser pulse compression tools to gain access to high fields and photon energies required for studies of Strong Field QED and laser-driven nuclear physics.  Since its invention the laser has been ideally suited to focus on atomic physics, or eV physics of electronic interactions. With the advent of Chirped Pulsed Amplification (CPA) 1 and later Optical Parametric CPA (OPCPA) there has been a considerable leap in peak power and intensity of 6 to 8 orders of magnitude, peaking from 1015 W/cm2 to 1022 W/cm2 or 4 orders of magnitude above the level where the electron quiver energy equals the rest mass energy i.e. 1018 W/cm2 of the electron. Above this limit is the relativistic regime of the electron and the threshold for subatomic physics including nuclear and particle physics. The next intensity level leads to a quiver energy equal to the proton rest mass, or 1025 W/cm2. It is these enormous intensities that are attempted to be delivered by the various large-scale European laser infrastructures such as those being built at ELI, LMJ and Apollon, as well as others in Russia, China and Korea. These facilities rely on CPA/OPCPA technology housed in 100 m size buildings and have reached the affordable limits to modern techniques in terms of size and cost. To explore the regime up to the Schwinger intensity (1029 W/cm2 ) —defined as the intensity threshold for vacuum electron-positron pair production—requires an additional 3-4 orders of magnitude increase in pulse energy and is well beyond what is currently affordable.  A radical change in technology is required to truly make this dramatic leap.  Seeking to increase pulse peak intensity of existing technologies requires one of three actions: an increase in the energy within the pulse (Ep), already deemed cost prohibitive; a decrease in the focus radius size (R), often already near the diffraction limit; or a decrease in the pulse duration (τp) which promises to offer the greatest gains. 

 

Efficient compression of existing high energy, joule-level laser pulses to a single-cycle of the carrier wavelength (2.5 fs for 800 nm) offers the fundamental limit for pulse duration achievable for a laser system.  Single-cycle pulses are readily achieved in low-energy (mJ-level) laser systems but current methods are limited in their ability to be extended to modern state-of-the-art high energy laser facilities.  A Thin Film Compressor2 (TFC) was proposed in 2014 that offers a path to creating such high energy, single-cycle laser pulses and opens the door to a new frontier in laser physics.  The development of a Thin Film Compressor for use with PW-level pulses is of paramount importance for achieving this next extraordinary level of laser compression. 

Figure 1  Evolution of Pulse Compression in the single cycle regime.  Early work by Grischkowsky et al.3 compression technique which relies upon the nonlinear response of a material to a high intensity pulse within a single mode fiber. In their experiment a picosecond pulse with nJ energy was further compressed to a duration of tens of femtoseconds. This work triggered an enormous interest that culminated with the introduction by O. Svelto, F. Krausz et al 4 of a fused silica hollow-core capillary, filled with noble gases and demonstrated that a 20 fs pulse can be further compressed to 5 fs, or 2 cycles of light, at 800nm. Due to the energy losses associated with coupling the laser pulse spatial mode into a single-mode fiber or hollow-core capillary the resulting pulse energies from these schemes are typically limited to the sub mJ level.

The concept for the Thin Film Compressor is shown in Figure 2 and makes use of the excellent beam spatial uniformity and the high peak power of new high power laser systems to broaden the spectral bandwidth of the pulse by driving the nonlinear process of self-phase modulation in a thin film material with the unfocused, freely-propagating beam.  As the film will need to be large diameter to accommodate the beams while sub-millimeter to provide an appropriate medium length for the nonlinearity, it is expected that a plastic material will be the ideal candidate for such a film being both cost-effective and easy to produce.  The resulting chirped pulse is then recompressed by a series of negatively dispersive mirrors designed to compensate the positive dispersion introduced to the new pulse spectrum during the broadening process.  A reimaging telescope with pinhole then maintains the beam’s spatial mode, if required.  The energy loss through the system is expected to depend primarily on the reflectivity of the mirrors and so the total transfer efficiency is expected to be high and allow for several stages of compression to reach the desired pulse duration. 

With appropriate focusing to the diffraction limit, the energy of a pulse of this duration is contained within its minimum volume as defined by the laser wavelength (λ) and enters the so-called λ3 regime5

Figure 2 Compression Concept. The Thin Film Compression stage relies on the interplay between the spectral broadening produced by self-phase-modulation and the group velocity dispersion necessary to stretch the pulse in a sub-millimeter, large aperture material. The combination of both effects contributes to create a linearly frequency-chirped pulse with increased spectral content compared to the initial pulse that can be compressed using dispersive elements such as chirped mirrors.  The second stage of compression requires delivery of the single-cycle pulse with appropriate tight focusing quality to apply an ultra-relativistic intensity field to a solid target plasma a compressed pulse with up-conversion to X-rays.

A collaboration with the Extreme Light Infrastructure - Nuclear Physics facility (ELI-NP) to study the feasibility of further compressing their expected PW laser pulses was begun at the end of 2014 and is now in its first year.  In this time the Proof-of-Concept phase has been completed on a smaller-scale TW-level laser system which showed that spectral broadening is possible in thin plastic film material while maintaining a well-behaved relative spectral phase as in Figure 3.  Currently the work is focused on improved characterization and manipulation of the spatio-temporal properties of the laser beam to better control the laser-film interaction.

Figure 3 Intensity-dependence of Thin Film Spectral Broadening. Resultant spectral intensity (solid line) and relative spectral phase (dashed line) as a function of increasing peak intensity for a pulse of 37 fs after interaction with a film of cellulose acetate of thickness 0.5 mm.

This project’s ultimate goal is efficient pulse compression to transform 10 PW multi-cycle pulses toward fundamental, single-cycle limit 100 PW pulses that offer access to even more opportunities within existing and future high power laser infrastructures. This includes not only the application of these unprecedented pulse intensities but also the possibility to drive ultra-relativistic, plasma mirror processes that could up-convert these high intensity laser beams to keV photon level, coherent X-ray source with a attosecond-scale pulse durations.  A single joule contained within such a pulse structure makes this a coherent Exawatt photon source.  In considering the potential of such beam combinations many applications have already become apparent.6

Single-cycle Proton Acceleration

Prompted by the possibility to produce high energy, single-cycle laser pulses with tens of PW power, theoretical investigations of have already been carried out for laser-matter interactions in the few optical cycle and ultra relativistic intensity regime. A particularly interesting instability-free regime for ion production was revealed leading to the efficient coherent generation of coherent, femtosecond monoenergetic ion bunches with a peak energy greater than GeV. The interaction is absent of the instabilities and hole boring that plague previously studied techniques such as Target Normal Sheath Acceleration (TNSA), Break Out Afterburner acceleration (BOA) and Radiation Pressure Acceleration (RPA). As the laser pulse interacts with a thin, planar foil, an electron bunch is pushed forward by the laser ponderomotive force, and ions are effectively accelerated in the resultant stable longitudinal electrostatic field over a prolonged distance. Thus the present method is relatively simplistic and robust, yielding high quality, ultrashort and high energy proton bunches in a very compact fashion.  The dramatic improvement in the proton beam energy from a single-cycle pulse over a multiple-cycle pulse is highlighted in Figure 4.  By moving to the single-cycle regime, currently proposed laser systems are capable of meeting the conditions necessary for driving this proton acceleration process.  Such proton bunches should have broad and revolutionary applications ranging from high energy physics to medicine. The first including use as extremely compact injectors, and high fluence muon or neutron sources that might in turn serve as the driver for accelerator-driven subcritical reactors. In medicine, such accelerated bunches are readily applicable to proton oncology. Because of the femtosecond time resolution, time sensitive measurements and proton triggers may become available for the first time.

Figure 4 Proton acceleration cutoff energy. The resulting proton energies with varying σ/a0, the black line indicates laser pulse with a0=50, and pulse duration τ=16T, the blue and red lines indicate laser pulses with a0=100 (τ=4T), and a0=200 (τ=1T), respectively. The upper right inset identifies the location of the single-cycle regime within the laser ion acceleration map. Here σ is the normalized areal density of the target and a0 is the normalized vector potential of the laser field.

Coherent Attosecond X-ray Pulse Source

The application of these single-cycle, high-intensity laser pulses to laser-plasma interactions also promises access to the production of X-ray beams with attosecond-scale pulse durations.  Theoretical studies exploring the so-called λ3 regime and depicted in Figure 5 predict that the application of a single-cycle PW laser system to a drive a relativistic oscillating mirror (ROM) leads to further compression of the pulse down to sub-attosecond timescales (zeptosecond = 10-21 s) under the proper conditions.  This pulse duration corresponds to the upconversion of the pulse photons from near-infrared eV energies to coherent hard X-rays up to MeV-scale energies.  This unprecedented source opens the door to study a novel regime that is best described as Zeptoscience and includes nuclear processes, vacuum physics, and laser wakefield acceleration within solid density crystal plasmas (with TeV/cm acceleration gradients).5

Figure 5 Reflection from a Relativisitic Mirror. (a) A high intensity single-cycle pulse encounters a solid target plasma and (b) pushes the plasma electron critical density surface further into the plasma until the laser field reaches equilibrium with the electrostatic potential that has developed between the displaced electrons and stationary ions to pull the critical surface back so that (c) a portion of the incoming pulse is compressed as it is swept up by its encounter with this relativistically moving reflective surface. Deformations in the target surface due to the shape of the focus help to spatially isolate portions of the reflected beam.

References

[1] Strickland, D. & Mourou, G. Compression of amplified chirped optical pulses. Opt. Commun. 56, 219–221 (1985).

[2] Mourou, G., Mironov, S., Khazanov, E. & Sergeev,  a. Single cycle thin film compressor opening the door to Zeptosecond-Exawatt physics. Eur. Phys. J. Spec. Top. 223, 1181–1188 (2014).

[3] Grischkowsky, D. Optical pulse compression based on enhanced frequency chirping. Appl. Phys. Lett. 41, 1 (1982).

[4]Nisoli, M. et al. Invited paper A novel-high energy pulse compression system: generation of multigigawatt sub- 5 - fs pulses. Appl. Phys. B Lasers Opt. 65, 189–196 (1997).

[5]Naumova, N. M., Nees, J. a., Sokolov, I. V., Hou, B. & Mourou, G. a. Relativistic Generation of Isolated Attosecond Pulses in a λ^{3} Focal Volume. Phys. Rev. Lett. 92, 3–6 (2004).

[6] Tajima, T. Laser acceleration in novel media. Eur. Phys. J. Spec. Top. 223, 1037–1044 (2014).

Publications

- Quéré, F. et al. Applications of ultrafast wavefront rotation in highly nonlinear optics. J. Phys. B At. Mol. Opt. Phys. 47, 124004 (2014).
- Mourou, G., Mironov, S., Khazanov, E. & Sergeev, A. Single cycle thin film compressor opening the door to Zeptosecond-Exawatt physics. Eur. Phys. J. Spec. Top. 223, 1181–1188 (2014).
- Tajima, T. Laser acceleration in novel media. Eur. Phys. J. Spec. Top. 223, 1037–1044 (2014).
- Mourou, G., Wheeler, J. A. & Tajima, T. Extreme Light: An Intense Pursuit of Fundamental High Energy Physics. Europhys. News 46, 31–35 (2015).
- Mourou, G., Tajima, T. Compression of High Energy Pulses to the Sub-attosecond Regime: Route to Exawatt Laser Subatomic Physics. Journal of Optics special publication: Roadmap on Ultrafast Optics submitted.

Talks/Posters

- Workshop on Damageless Optics – Dusseldorf, Germany – Diagnostics of Ultrashort Pulses
- IZEST Meeting 2014 – Romanian Embassy, Paris, France – Poster: Thin Film Compression toward the Zeptosecond-scale
- SPIE – Prague 2015 – Extreme Light: First steps toward Zeptosecond and Zettawatt Science
- IZEST Meeting 2015 – CERN, Geneva, Switzerland – Single Cycle and Exawatt Lasers