Optical pump Terahertz probe spectroscopy is used to follow the dynamic evolution of the far infra-red absorption of a sample following excitation by a femtosecond laser pulse. The excitation pulse can be tuned to any wavelength in the range ~235-9000 nm using an optical parametric amplifier (TOPAS), seeded with a 1 kHz, 40 fs, 800 nm pulse (Ti:Sapphire Spitfire ACE) to create pulse powers of around 10 uJ / pulse. The probe pulse is generated by focussing 800 nm radiation onto an electro-optic crystal. Off-axis parabolic mirrors are then used to steer and focus the resulting THz radiation onto the sample being studied. The output THz beam is focused onto an electro-optic detection crystal along with a further 800 nm pulse. The resulting THz response is split in a Wollaston prism and detected using BPW-34 photodiodes in a balanced detection arrangement. A lock-in amplifier (Signal Recovery 7230) detects both the THz pulse and the photoinduced change in the THz pulse.. This technique allows us to study the dynamics of such properties as intramolecular electron mobility on a femtosecond timescale.
The stage that generates the delay between pump and probe pulses has a travel range of 500 mm with a resolution of 6 microns, reproducible to within 1.25 microns. This corresponds to a maximum temporal window of 3.3 nanoseconds at a resolution of 20 femtoseconds. The stage that scans the THz pulse has a travel of 150 mm witha resolution of 2 microns, giving a 1 nanosecond window and a 7 fs resolution. The THz pulse line is enclosed in a vacuum box which can achieve 10-4 mbar vacuum. A cryostat can be used to measure temperature dependant signals in samples allowing a range of accesible temperatures of 10-400 K. Several options are available for detection: (1) electro-optic sampling with a 200um-thick <110> GaP on a 3.0mm <100> GaP substrate (bandwidth up to 4.0THz). (2) a 2.0mm-thick ZnTe (improved signal, bandwidth up to 2.5THz), (3) a 0.5mm-thick <111> ZnTe for polarisation-resolved measurements of the orthogonal components of the THz electric field (E_x, E_y) (bandwidth up to 3.0THz) and ; (4) a 200um-thick <111> GaP on 500um <100> GaP also for polaristion-resolved studies (lower signal; bandwidth up to 4.0THz). The lock-in amplifier has a frequency range of 1 mHz to 250 kHz and an amplitude range of 1 microvolt to 5 V and can store data at a rate of up to 1MHz.