Two-Color-Interferometerand Coherent Focusing

FRED project course, LTH

Kathrin Klünder

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

red only

red and blue

Experiment Theory

Setup: HHG in a Two-Color-Field

doubling crystal

Brewster window (55.47˚ for 800nm)

HHG generation (gas inlet)

probe beam

focusing mirror

interferometer

delay plate

Ti:Sapphire, 800nm, 35fs, 4mJ

•Changing the relative phase by rotation of delay plate

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 investigatestandard procedure when aligning/working with the Two-Color-Interferometer.

Our setup in FRED…

Defining the Light Source

desired properties: • coherent• polarized• pulsed (35fs)

=> in FRED not possible

• define spectral bandwidth of ≈ 30nm

Properties of the Delay Plate

new materials can added from a huge range of cataloged materials

Using FRED Scripting

create variables

ray-tracing and drawing the rays

define and open output file #1

access delay plate coordinates

close output file #1

get infos about ray j for angle i

if rays ended up on right detector: print infos in #1

built-in functions to get ray informations and control optical components:

Performance of Interferometer

delay plate

analysis surface

170µm

340µm

510µm

thickness of delay plate

delay of red arm as a function of angle

red arm

blue arm

Interferometer works fine.

Influence of Brewster Window

• 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)

focusing mirror (f=75cm)

Brewster window (1mm)

vacuum

red arm

blue arm

delay of red arm as a function of angle

Influence of Brewster Window

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

beam in focal plane:

Some Words about the Focusing

• 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

decrease distance x:

spherical focusing mirror x

in focusbefore focus

some other bad example…

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