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Efficient High Harmonic Generation

Efficient High Harmonic Generation. Simon Hutchinson December 4 2012. 12:15 – Efficient HHG ... ... 5 – Classical Cello. Motivation. Experiments that require a large number of harmonic photons at a specific energy range. Specifically in my case:. XUV initiated high harmonic generation.

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Efficient High Harmonic Generation

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  1. Efficient High Harmonic Generation Simon Hutchinson December 4 2012

  2. 12:15 – Efficient HHG ... ... 5 – Classical Cello

  3. Motivation Experiments that require a large number of harmonic photons at a specific energy range. Specifically in my case: XUV initiated high harmonic generation

  4. How to get more harmonic signal

  5. Technique: Phase Matching Just like in standard nonlinear optics, we want a coherent build up of signal – ie we want all the emitters to add constructively! This requires matching the phase velocity of the driving and the harmonic fields. Image: Popmintchevet al.”Phase matching of high harmonic generation in the soft and hard X-ray regions of the spectrum”, Proceedings of the National Academy of Sciences of the United States of America, 106, 10516–10521 (2009)

  6. Factors in HHG Phase Matching 1. Dispersion from neutral gas and ionised gas Ionisation fraction Pressure How much of the gas is ionised. How much gas is put in. EQUATION: Paul et al. “Phase-Matching Techniques for Coherent Soft X-Ray Generation”, IEEE Journal of Quantum Electronics, 42, 14–26 (2006)

  7. Factors in HHG Phase Matching 2. Geometric phase This is due to how the laser is focused, or how it behaves in the interaction region. Images: Siegel T. “Imaging of molecular structure and dynamics using laser driven electron recollisions”, PhD Thesis, pg 43, (2010)

  8. Factors in HHG Phase Matching 2. Geometric phase This is due to how the laser is focused, or how it behaves in the interaction region.

  9. Factors in HHG Phase Matching 2. Geometric phase kgeo= Waveguiding kgeo= EQUATION: Rundquist et al. “Phase-Matched Generation of Coherent Soft X-rays”, Science, 280, 1412–1415 (1998)

  10. Factors in HHG Phase Matching 3. Atomic phase When is the high harmonic light emitted in relation to the driving field. Directly related to which trajectory is selected. α katom Trajectory Time Should stay constant for a specific trajectory, if the intensity remains unchanged. EQUATION: Lewenstein et al. “Phase of the atomic polarization in high-order harmonic generation”, Phys Rev A, 52, 6, 4747-4754 (1995)

  11. Coherence Length How far can the signal travel before falling π out of phase. Δkq=Δkdisp+kgeo+katom

  12. Absorption Length How far can the signal travel before being absorbed. gas density ionisation cross-section Low gas density is necessary

  13. Ideal Conditions EQUATION: Constant, E. et al. “Optimizing High Harmonic Generation in Absorbing Gases: Model and Experiment”, Physical Review Letters, 82, 1668–1671 (1999)

  14. Experimental Setup Pulse energy = 500 µJ Pulse duration = 25 fs Focal length = 75 cm Beam diameter = 10 mm Focal spot size = 130-180 µm Capillary diameter = 220 µm Intensity = 5 x 1013 W/cm2 Gas species: Argon Gas pressure: 0-200 mbar

  15. Experimental Results H13 H19 H15 H17 Exposure time: 10ms An order of magnitude shorter than the normal 100ms.

  16. Experimental Results Harmonic 13 in Argon, 10mm target Yellow open circle shows intensity from standard 1.5 mm target. (This point is arbitrarily placed on the x-axis and does not represent the harmonics from that backing pressure.)

  17. Experimental Results Harmonic 15 in Argon, 10mm target Yellow open circle shows intensity from standard 1.5 mm target. (This point is arbitrarily placed on the x-axis and does not represent the harmonics from that backing pressure.)

  18. Experimental Results Harmonic 17 in Argon, 10mm target Yellow open circle shows intensity from standard 1.5 mm target. (This point is arbitrarily placed on the x-axis and does not represent the harmonics from that backing pressure.)

  19. Experimental Results

  20. Conclusion and forward motion Result: About an order of magnitude increase in harmonic flux for a given harmonic energy To Improve: Try slightly different lengths, 10 mm – 20 mm. Try to improve coupling into fibre. Future application: Mid-IR, can “easily” approach the water window with a useful harmonic flux. Refer to the paper of Popmintchev (2009)

  21. Critical Ionisation Tables

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