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Two-Color-Interferometer and Coherent Focusing

This project explores the generation of high-order harmonics (HHG) using a two-color laser field (800 nm and 400 nm). By focusing an intense laser into an atomic gas, we investigate how introducing a second field impacts harmonic properties and the pulse train itself. The setup includes Ti:Sapphire lasers, a doubling crystal, and a two-color interferometer. Key procedures involve aligning beams, selecting delay plates, and mitigating dispersion effects from the Brewster window. Utilizing FRED modeling software enhances understanding of the optical components and their interactions.

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Two-Color-Interferometer and Coherent Focusing

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  1. Two-Color-Interferometerand Coherent Focusing FRED project course, LTH Kathrin Klünder

  2. red only red and blue Motivation: HHG in a Two-Color-Field • High-order harmonic generation (HHG) by focusing an intense laser field (800nm) into an atomic gas • Properties of the harmonics determined by fundamental driving field • Adding a second field to the generation process by frequency doubling a fraction of the fundamental laser frequency => now wavelengths of 800nm and 400nm • Ability of changing properties of harmonics and pulse train itself • Standard procedure in our lab Experiment Theory

  3. focusing mirror Ti:Sapphire, 800nm, 35fs, 4mJ doubling crystal interferometer Brewster window (55.47˚ for 800nm) HHG generation (gas inlet) probe beam delay plate Setup: HHG in a Two-Color-Field • Changing the relative phase by rotation of delay plate

  4. The FRED project • Procedure for aligning interferometer: • two beams overlap spatially and in time directly after passing interferometer • No further check after propagation though rest of the setup • => Works fine, BUT what are the possible problems or what should one keep in mind? • Use FRED to model influence of delay plate and investigate • standard procedure when aligning/working with the Two-Color-Interferometer. Our setup in FRED…

  5. Defining the Light Source desired properties: • coherent • polarized • pulsed (35fs) => in FRED not possible • define spectral bandwidth of ≈ 30nm

  6. new materials can added from a huge range of cataloged materials Properties of the Delay Plate

  7. Using FRED Scripting built-in functions to get ray informations and control optical components: create variables ray-tracing and drawing the rays define and open output file #1 access delay plate coordinates get infos about ray j for angle i if rays ended up on right detector: print infos in #1 close output file #1

  8. thickness of delay plate 510µm 340µm 170µm analysis surface delay of red arm as a function of angle delay plate red arm blue arm Performance of Interferometer Interferometer works fine.

  9. Influence of Brewster Window focusing mirror (f=75cm) • Brewster window introduces dispersion for both colors • relative delay of the two arms changes • temporal spread of the two pulses; bigger influence on blue pulse (approx. 10fs for spectral bandwidth) blue arm vacuum delay of red arm as a function of angle red arm Brewster window (1mm)

  10. Influence of Brewster Window beam in focal plane: foci positions red and blue beams: • 3µm offset in direction of beam propagation • 6µm offset in Y-direction no Brewster window: with Brewster window: offset 6µm

  11. spherical focusing mirror x before focus in focus Some Words about the Focusing decrease distance x: some other bad example… • focus size and astigmatism depends on incoming angle • for better focusing conditions spatial offset gets more important • perturbation of wave fronts due to focusing mirror

  12. In the End FRED is... • easy to get started with • convenient to model small questions • good to get an idea what happens in the setup

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