Near THz radiation from optically-induced plasma sources

Abstract

Summary form only given. Short pulse lasers can be used to generate two types of short-lived, radiating plasmas. When absorbed by a semiconductor, a sudden optical pulse creates a charge-neutral, electron-hole plasma that radiates as the carriers relax in local crystal, surface, or applied electric fields. When focused onto the surface of an insulator or metal, the electric fields in an intense optical pulse break down the solid surface and surrounding gas. The charged particles in this surface-breakdown plasma radiate as they are created at the solid-gas (air) interface. The sudden creation of either of these types of plasmas with a sub-picosecond optical pulse produces a non-equilibrium distribution of charges that radiates with an extremely wide bandwidth in the THz regime. This paper reports the radiated electric field intensities, spectra, and associated radiation patterns for electron-hole plasmas from gallium arsenide and silicon, and for surface-breakdown plasmas from aluminum, glass (BK-7), gallium arsenide, and silicon. The plasmas are created and diagnosed with a chirped-pulse regenerative amplifier laser that produces a 1 kHz train of 120 fs wide pulses with up to 2 mJ of energy per pulse at a wavelength of 800 nm. To explore the wide bandwidth of these plasma radiation sources, two electric field diagnostics were used. A conventional electric field probe (D-dot) with a 40 GHz bandwidth and a 70 GHz sampling oscilloscope measured the low frequency tail of the radiation spectrum. To measure the high frequency range of the spectrum, electro-optic crystals (zinc telluride) sampled the radiated electric field as it co-propagated with an optical probe beam that was split off the excitation (pump) beam. The bandwidth of the electro-optic sampling is limited by the thickness of the electro-optic crystals, so a range of thicknesses was employed to demonstrate source-limited measurements. The fundamental differences between electron-hole and surface-breakdown plasmas will be discussed. Similarities and contrasts in radiation patterns and efficiencies will be described for all of these plasma radiation sources. Simple models for the picosecond carrier motion in these sources will be presented and compared with measurements. The advantages and challenges of using a kilohertz, millijoule, short-pulse laser for plasma generation and electro-optic sampling will also be mentioned.