Picosecond acoustics have long been used to probe the response of material properties  and, more recently, structural excitations such as surface acoustic waves   . Optical excitation via pulsed lasers offers the ability to use picosecond acoustics to provide non-contact, non-destructive, and ultrafine temporal resolution of these phenomena. We demonstrate the generation and characterization of the two-dimensional Rayleigh surface acoustic wave (2D SAW) on a set of silicon line grating samples with varying line widths (400 nm, 800 nm, 1600 nm, and 3200 nm) fabricated via electron beam lithography. The structures were patterned with line spacing equal to the grating width, etched to a depth of 100 nm, and then conformally coated with an 80 nm aluminum film that acts as an optical transducer. The existence of this 2D SAW in patterned structures was previously studied by Li et al.  In this work, we use time-domain thermoreflectance (TDTR) to investigate the propagation of 2D SAWs (v ~ 0-20 GHz) in the silicon line gratings. Prior studies  have shown that most of the acoustic oscillations die out in a few hundreds of picoseconds. Our TDTR measurements show SAW oscillations up to ~5000 ps in the 800 and 1600 nm silicon line grating samples. The time periods of the acoustic oscillations are approximately 90 ps, 300 ps, and 600 ps for the silicon line grating widths of 400 nm, 800 nm, and 1600 nm, respectively. We employed two variations of TDTR, two color (400 nm pump/800 nm probe) and two tint (spectrally split 800 nm pump/800 nm probe) in order to study the variations in excitation based on pump and probe excitation and sensing. Fast Fourier transforms (FFT) of the resultant data were used to obtain the SAW frequencies. The two tint (800 nm pump) measurements show a typical simple single SAW frequency f0, which is caused by the periodicity of the silicon line grating. A fundamental SAW frequency f0 and a secondary SAW with a frequency 20.5f0 are excited in the two color (400 nm pump) measurements. This 20.5f0 SAW is attributed to the acoustic wave in the diagonal direction of the silicon line grating. This diagonal wave is known as a 2D SAW . In addition to measurement and quantification of the 2D SAWs, we discuss the effects of the silicon grating periodicity as it relates to excitation, or lack of excitation, as a function of the TDTR system used to probe the SAW.
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