Isolated soft x-ray pulse generation

Currently, one of my main interests lies in techniques for generating isolated bursts of soft x-ray radiation of sub-femtosecond (1 fs = 10-15 s) duration using high-order harmonic generation.

The need for isolated soft x-ray pulses

Isolated sub-femtosecond soft x-ray pulses are a key ingredient in advancing transient spectroscopy to enable to real-time observation of electronic motion in chemical reactions. Sub-fs pulses are required to produce “snapshots” of this motion, which occurs on timescales of 100 as (1 as = 10-18 as) to several fs. The soft x-ray spectral region is incredibly useful because if offers element specificity. For example, at photon energies around 400 eV, absorption of radiation in a material is dominated by nitrogen. Changes in the absorption spectrum around 400 eV therefore reflect changes in the electronic structure around the nitrogen atoms in the sample. Soft x-ray spectroscopy of static samples provides insight into the structure of materials indicating, for instance, which valence electronic orbitals are unoccupied [1]. Dynamic, or transient spectroscopy, brings the dimension of time to the problem. Some kind of process is triggered in the sample – for example, a burst of ultraviolet light can initiate a rapid change in the arrangement of a biomolecule. The soft x-ray probe pulse is subsequently applied after a certain time delay. Provided the probe pulse is shorter than the timescale of the process, it captures the state of the sample at that moment. Repeating the process with different time delays allows one to build up a dynamic picture – almost a “molecular movie” – of the evolution.

Setup for generating isolated attosecond bursts

Due to the lack of suitable gain media, coherent ultraviolet and x-ray radiation cannot be generated directly by a laser like visible light. Instead we start with an intense infrared laser pulse and use a highly nonlinear optical process to covert the radiation to the soft x-ray region.

Few-cycle CEP-stable 1.8 micron pulses

We have developed a state-of-the-art setup for producing 0.7 mJ, 9 fs carrier-envelope phase stable pulses at 1800 nm. It is based on the concept of hollow fiber pulse compression [2], with phase compensation provided by propagation in bulk glass [3].

idler_hcf_seafspider_overview_fs

Here is the setup in more detail.

topas_idler_hcf_setup

High-harmonic generation is incredibly sensitive to the spatio-temporal profile of the drive laser pulses and so it is important to fully characterize this. To this end we developed a form of spectral shearing interferometry, called SEA-F-SPIDER [4], capable of measuring the spatio-temporal profile of few-cycle pulses in the spectral region around 1.8 micron.

Evidence of isolated pulse: carrier-envelope phase scan

In the animation below, the left-hand plot shows the cutoff of an experimental spectrum (red) overlaid on a theoretical calculation (blue) as the carrier-envelope phase of the drive pulse is scanned. The color plot shows the spectrogram (time-frequency distribution) of the calculated radiation. The bottom plot shows the calculated temporal profile (blue) of the generated soft x-ray pulse, along with the envelope (red dashed) and electric field (solid red) of the driving laser pulse.

cep_scan

 

References

[1] [doi] A. S. Johnson, L. Miseikis, D. A. Wood, A. D. R. C. Brahms, S. Jarosch, C. S. Strüber, P. Ye, and J. P. Marangos, “Measurement of sulfur L2,3 and carbon K edge XANES in a polythiophene film using a high harmonic supercontinuum,” Structural Dynamics, vol. 3, iss. 6, 2016.
[Bibtex]
@Article{Johnson-2016-Measurement,
author = {A. S. Johnson and L. Miseikis and D. A. Wood and D. R. Austin C. Brahms and S. Jarosch and C. S. Str{\"u}ber and P. Ye and J. P. Marangos},
title = {{Measurement of sulfur L2,3 and carbon K edge XANES in a polythiophene film using a high harmonic supercontinuum}},
journal = {{Structural Dynamics}},
year = {2016},
volume = {3},
number = {6},
__markedentry = {[Dane:]},
doi = {http://dx.doi.org/10.1063/1.4964821},
eid = {062603},
url = {http://scitation.aip.org/content/aca/journal/sdy/3/6/10.1063/1.4964821},
}
[2] [doi] M. Nisoli, D. S. Silvestri, and O. Svelto, “Generation of high energy 10 fs pulses by a new pulse compression technique,” Appl. phys. lett., vol. 68, iss. 20, pp. 2793-2795, 1996.
[Bibtex]
@Article{Nisoli-1996-Generation,
author = {M. Nisoli and S. De Silvestri and O. Svelto},
title = {Generation of high energy 10 fs pulses by a new pulse compression technique},
journal = {Appl. Phys. Lett.},
year = {1996},
volume = {68},
number = {20},
pages = {2793-2795},
abstract = {First report of noble gas filled hollow fibre compression of intense pulses to my knowledge.},
date-added = {2007-08-24 15:55:14 +0100},
date-modified = {2007-08-24 15:55:14 +0100},
doi = {10.1063/1.116609},
file = {Nisoli-1996-Generation.pdf:N/Nisoli-1996-Generation.pdf:PDF},
keywords = {NONLINEAR OPTICS; PULSE GENERATION; COMPRESSION; WAVEGUIDES; PULSE PROPAGATION; DELAY LINES},
publisher = {AIP},
url = {http://link.aip.org/link/?APL/68/2793/1},
}
[3] [doi] B. E. Schmidt, P. Béjot, M. Giguère, A. D. Shiner, C. Trallero-Herrero, É. Bisson, J. Kasparian, J. Wolf, D. M. Villeneuve, J. Kieffer, P. B. Corkum, and F. Légaré, “Compression of 1.8 micron laser pulses to sub two optical cycles with bulk material,” Appl. Phys. Lett., vol. 96, iss. 12, p. 121109, 2010.
[Bibtex]
@Article{Schmidt-2010-Compression,
author = {Bruno E. Schmidt and Pierre B\'{e}jot and Mathieu Gigu\`{e}re and Andrew D. Shiner and Carlos Trallero-Herrero and \'{E}ric Bisson and J\'{e}r\^{o}me Kasparian and Jean-Pierre Wolf and David M. Villeneuve and Jean-Claude Kieffer and Paul B. Corkum and Fran\c{c}ois L\'{e}gar\'{e}},
title = {Compression of 1.8 micron laser pulses to sub two optical cycles with bulk material},
journal = {{Appl. Phys. Lett.}},
year = {2010},
volume = {96},
number = {12},
pages = {121109},
abstract = {We demonstrate a simple scheme to generate 0.4 mJ 11.5 fs laser pulses at 1.8??m. Optical parametrically amplified pulses are spectrally broadened by nonlinear propagation in an argon-filled hollow-core fiber and subsequently compressed to 1.9 optical cycles by linear propagation through bulk material in the anomalous dispersion regime. This pulse compression scheme is confirmed through numerical simulations.},
doi = {10.1063/1.3359458},
eid = {121109},
file = {Schmidt-2010-Compression.pdf:S/Schmidt-2010-Compression.pdf:PDF},
keywords = {fibre lasers; laser beams; optical fibre dispersion; optical materials; optical pulse compression; optical pulse generation},
numpages = {3},
publisher = {AIP},
timestamp = {2010.10.22},
url = {http://link.aip.org/link/?APL/96/121109/1},
}
[4] [doi] D. R. Austin, T. Witting, S. J. Weber, P. Ye, T. Siegel, P. Matía-Hernando, A. S. Johnson, J. W. G. Tisch, and J. P. Marangos, “Spatio-temporal characterization of intense few-cycle 2 micron pulses,” Opt. Express, vol. 24, iss. 21, pp. 24786-24798, 2016.
[Bibtex]
@Article{Austin-2016-Spatio,
author = {Dane R. Austin and Tobias Witting and S\'{e}bastien J. Weber and Peng Ye and Thomas Siegel and Paloma Mat\'{i}a-Hernando and Allan S. Johnson and John W.G. Tisch and Jonathan P. Marangos},
title = {{Spatio-temporal characterization of intense few-cycle 2 micron pulses}},
journal = {{Opt. Express}},
year = {2016},
volume = {24},
number = {21},
pages = {24786--24798},
month = {Oct},
__markedentry = {[Dane:]},
abstract = {We present a variant of spatially encoded spectral shearing interferometry for measuring two-dimensional spatio-temporal slices of few-cycle pulses centered around 2 \&\#x003BC;m. We demonstrate experimentally that the device accurately retrieves the pulse-front tilt caused by angular dispersion of two-cycle pulses. We then use the technique to characterize 500\&\#x02013;650 \&\#x003BC;J pulses from a hollow fiber pulse compressor, with durations as short as 7.1 fs (1.3 optical cycles).},
doi = {10.1364/OE.24.024786},
keywords = {Ultrafast optics; Pulse compression; Ultrafast measurements},
publisher = {OSA},
url = {http://www.opticsexpress.org/abstract.cfm?URI=oe-24-21-24786},
}