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Femto-second Pulse
• Train of femto-second pulses generated by one of our titanium sapphire mode locked lasers.
• Cannot use conventional diode detectors as they are not fast enough (~0.1ns).
Time
Inte
nsit
y
10 ns
100 fs
Experimental Arrangement
PM tube
SampleUp conversion Crystal
Translational retro-reflector
Femto-second pulse
Beam splitter
Probe →
Pump → Response →
Pump Beam
• Pump pulse excites a response from the sample under study e.g. A semiconductor optical amplifier (SOA).
• Pump pulse does not necessarily have to be same wavelength as probe pulse.
Time (ps)
Inte
nsit
y
Response
Probe Beam
• Probe pulse much shorter in time than response.
• Using the delay stage to ‘scan’ along response.
• 150 ps in time requires a path change of 45 mm.
• 100 fs resolution requires a path change of 30 μm.
Time (ps)
Inte
nsit
y
Response
Probe
Up Conversion
• Non-linear crystal takes two identical photons and creates one larger photon.
• Acts as a gate as will only give a response when probe pulse coincides with response pulse.
ω,k
ω,k
2ω,2k
Experimental Set-up
• Probe pulse is focused onto sample.
• Pulse is injected through a window in the base of the sample.
• Response is then steered towards up-conversion crystal.