Bright terahertz sources

■ ■ What was terahertz science like in the early days? When I first became involved in terahertz (THz) research in the late 1980s at Columbia University, only three groups in the USA were actively investigating THz pulses produced from ultrashort laser pulses: the group at IBM led by Daniel Grischkowsky, that at Bell Laboratories led by Martin Nuss and that at Columbia University led by David Auston. As there was no established experimental technique for generating or detecting broadband THz waves, we had to fabricate optical components and develop a THz measurement system. We tried generating THz waves using a photoconductive antenna (sometimes referred to as an Auston switch) and also by employing electro-optic rectification. During my time at Auston’s laboratory, I was curious whether THz radiation has the ability to ‘see through’ optically opaque materials. I first observed pulsed THz emission from an unbiased semiconductor wafer using femtosecond laser excitation. In a search for new THz-emitting materials, I investigated many different materials, including dielectrics, metals, tissue paper, fabrics and wood — basically anything I could lay my hands on (even my fingers!). At that time, most of us used photoconductive dipole antennas to detect THz waves.