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Resonant semiconductor laser absorption of atomic Rb in a pulsed Nd:YAG laser-induced plasma

ABSTRACT Nanosecond laser pulses from a 1064 nm Nd:YAG laser were focused on the atomized spray of RbCl aqueous solution whose threshold energy of plasma ignition is around 550 mJ in atmosphere. A narrow-linewidth (100 MHz) vertical-cavity surface-emitting laser (VCSEL) has been used to probe the transient density of ground-state atomic Rb. The frequency of VCSEL probe beam was locked to the 87Rb transition |52S1/2, F = 2 → |52P1/2, F′ = 1 which corresponds to an absorption cross-section of up to 10−10 cm2. The atomic Rb number density was inferred from the extinction ratio of transmitted light intensity. The simple mathematical model developed to describe the density evolution n(t) conforms with the detected increase-peak-decline-vanish process well. It is a sensitive and real-time method used to diagnose the evolution process of generated plasma in air. The detection limit of RbCl concentration is lower than 1 gram per litre. Rb atoms have a mean lifetime of 250 µs in each pulsed plasma.