Abstract

Far-infrared spectroscopy is a powerful tool in the study of low-energy excitations in solids, such as magnons, phonons, and electronic transitions in semiconductor quantum structures. With the recent development of picosecond pulsed FIR sources, such as those based on laser-driven photoconductive antennas,¹ a new class of nonequilibrium phenomena can be studied in the FIR. We focus on picosecond experiments^2,3 in which a near-visible laser "pump" pulse creates an excitation that is monitored by time-delayed FIR "probe" pulses. We present a new experimental scheme using an unamplified 790–890 nm tunable Ti:sapphire laser and advanced photoconductive antenna technology. An ultrafast photoconductive transmitter and receiver antenna form the THz beam system,⁴ which is used to generate and detect the FIR probe pulse. The pump pulses are derived from the laser. The setup allows us to measure the transient change of the complete complex amplitude transmission coefficient (Δt/t) in a range of 0.3–2.3 THz with a superior detection limit (|Δt/t| ≥ 5 × 10⁻⁴). The unique features of the presented approach will be demonstrated in our study of the FIR-optical properties and population relaxation dynamics of carriers and excitons in 100-Å GaAs/Al_0.33Ga_0.67As multiple quantum wells at a temperature of 15 K.

© 1994 IEEE