  For integrated optical devices, realistic utilization of the superior
wave-matter interaction offered by plasmonics is typically impeded by optical
losses, which increase rapidly with mode volume reduction. Although
coupled-mode plasmonic structures has demonstrated effective alleviation of the
loss-confinement trade-off, stringent symmetry requirements must be enforced
for such reduction to prevail. In this work, we report an asymmetric plasmonic
waveguide that is not only capable of guiding subwavelength optical mode with
long-range propagation, but is also unrestricted by structural, material, or
modal symmetry. In these composite hybrid plasmonic waveguides (CHPWs), the
versatility afforded by coupling dissimilar plasmonic structures, within the
same waveguide, allow better fabrication tolerance and provide more degrees of
design freedom to simultaneously optimize various device attributes. The CHPWs
used to demonstrated the concept in this work, exhibit propagation loss and
mode area of only 0.03 dB/{\mu}m and 0.002 {\mu}m^2 respectively, corresponding
to the smallest combination amongst experimentally demonstrated long-range
plasmonic structures to-date. Moreover, CHPW micro-rings were realized with
record in/out coupling excitation efficiency (71%), extinction ratio (29 dB),
as well as Purcell factor (1.5\times10^4).
