Two-Color-Interferometer and Coherent Focusing

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