Ultrafast stress pulse detection by laser beam deflection

Abstract

We describe a direct method for the detection of optically excited ultrashort stress pulses in thin films using a time-resolved pump and probe scheme. Changes in the surface profile are monitored by the angular deflection of a probe beam. Application to the detection of interfacial layers is also demonstrated. The time-resolved detection of high frequency phonons excited by picosecond or femtosecond optical pulses can be achieved by a variety of methods. Terahertz optical phonons, for example, have been detected through the electrooptic effect,[ll whereas acoustic phonons up to the 100-GHz range have been detected through the photoelastic effect.[2] Here we describe a n ultrafast method for the detection of stress pulses associated with longitudinal acoustic phonons, which i s based on laser beam deflection from ultrafast surface vibrations.[3] Such a detection scheme has been widely used by the photoacoustics community in lower frequency photothermal deflection or photothermal displacement experiments.E4,51 Optical pump pulses of duration 3 ps (FWHM), repetition rate 76 MHz, wavelength A = 630 nm and energy 0.6 n J are used to excite the stress pulses. Light absorbed within the optical absorption depth (-10 nm for the metal samples used here) is converted to heat, and, through the resulting thermal expansion, longitudinal stress pulses in the 100-GHz region are generated. Changes in the surface slope of the opaque films, induced by the stress pulses bouncing back and forth inside the films, are interrogated by measuring the angular deflection (68-1 prad) of a probe laser beam derived from the same laser (see Fig. 1). This beam is focused to a 20-pm diam. spot size to partially overlap with the pump spot of similar size. The angular deflection is monitored with a dual-element photodiode. The displacement of the surface, typically -0.001 nm, can therefore be measured. By scanning the delay line, time-resolved detection on a picosecond time scale is achieved. This angular deflection scheme was previously used to measure the transient thermal expansion of bulk crystalline silicon a t lower frequencies of order 2 GHz, but not for the detection of stress pulses.[61 ( 1 ) Present address: Consiglio Nazionale delle Ricerche (CNR), Istituto di Acustica O.M. Corbino, Via Cassia 12 16,OO 1 X9 Roma, Italy Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19947165 JOURNAL DE PHYSIQUE IV