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The mechanical resonance of laterally grown silicon nanowires measured by an optical interferometric technique is reported. The lengths and diameters of the nanowires ranged from L= 2 to 20 μ m and D= 39 to 400 nm⁠, respectively. The wires showed resonant frequencies in the f 0= 1–12 MHz range and resonant quality factors Q at low pressure ranging from Q= 5000 to Q= 25 000⁠. The dependence of resonant frequency on the ratio of diameter to length squared, D∕ L 2⁠, yielded a ratio of E∕ ρ= 9400±450 m∕ s⁠. Assuming a density of ρ= 2330 kg∕ m 3⁠, this experimental result yields an experimental Young modulus of E= 205±10 GPa⁠, consistent with that of a bulk silicon. As the wires were cooled from T= 270 K to T= 77 K⁠, a 0.35% increase of resonant frequency was observed. This increase of resonant frequency with cooling resulted from a change in Young’s modulus and from the thermal …