Pump Probe Measurements of Femto-second Pulses By David Baxter

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 Intensity 10 ns 100 fs

Experimental Arrangement PM tube Sample Up 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) Intensity 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) Intensity 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 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.

Data Analysis Rep-rate of pulses is 100 kHz (10 ns). At each point perform averaging over many pulses. Signal from PM tube is a convolution of response and probe pulse. Can fully characterise probe pulse and therefore deduce response.