THz Spectrometer Schematic

Laser System

The experiments that are performed in the Korter Lab utilize a laser system that produces very short, high energy pulses of near-infrared radiation. This lasern system is composed of four parts: the oscillator, the amplifier, and two pump lasers.

The oscillator is a Kerr lens mode-locked Ti:Sapphire laser (Coherent Mira-SP).

The pump laser that supplies energy to the oscillator is a continuous-wave diode-pumped solid state laser (Coherent Verdi V5). The gain medium is Nd:YVO₄.

The output of the Ti:Sapphire oscillator is then amplified in a Ti:Sapphire regenerative amplifier [Coherent (formerly Positive Light) Legend-USPHE]. Typical pulse characteristics are a pulse duration of ~35 femtoseconds with an energy of ~2.5 mJ/pulse.

The pump laser that supplies energy to the amplifier is a Q-switched, diode-pumped solid-state laser [Coherent (formerly Positive Light) Evolution 30]. The gain medium is Nd:YLF.

Optical rectification is used to generate broadband THz pulses. Ultrashort (30 femtoseconds), near-IR pulses enter a nonlinear crystal (ZnTe or GaP) and difference frequency mixing occurs between the frequencies contained in the ultrafast pulses. The frequency components mix to form sub-picosecond THz pulses with bandwidths of ~3 THz in ZnTe or ~6 THz in GaP. The generated THz radiation is then collimated by off-axis parabolic mirrors, focused on the sample and the transmitted portion detected.

Free-space electro-optic sampling is used for THz detection. After transmitting through the sample, the THz pulses strike a second semiconductor crystal causing it to become birefringent (Pockels effect). This birefringence is directly related to the amplitude of the THz pulse and a linearly polarized 800 nm probe pulse with a variable delay is used to measure this magnitude. By scanning the time delay of the THz pulse versus the probe pulse, a time-domain trace of the THz electric field is observed.