By manipulating the surface-plasmon-polariton (SPP) excitation properties of two nanogrooves, we demonstrate unidirectional launching of SPPs using a groove-doublet structure both numerically and experimentally, with the groove separation being downscaled to 1/4 and even 1/8 of the wavelength. Thus, the total lateral dimension of the SPP launcher is only about 1/3 and 1/6 of the wavelength, which is truly subwavelength. The measured extinction ratio at incident wavelength of 800 nm reaches as high as 130 and 18. Such subwavelength SPP unidirectional launchers may have important applications in highly integrated plasmonic circuits. (C) 2014 AIP Publishing LLC.