High-performance and low-power network-on-chips (NoCs) will be required to support the increasing number of cores in future chip multiprocessors. In this paper, we propose a scalable low-power 64-core NoC design called PROPEL that uses emerging nanophotonic technology. PROPEL strikes a balance between cheaper electronics and more expensive optics by facilitating nanophotonic interconnects for long distance interrouter communication and electrical switching for routing and flow control. In addition, PROPEL reduces the number of required components by facilitating communication in both the x- and y-directions. We also propose a 256-core scaled version of PROPEL called E-PROPEL that uses four separate PROPEL networks connected together by an optical crossbar. We also propose two different optical crossbar implementations using single and double microring resonators, where the single microring design has minimal optical losses (-4.32 dB) and the double microring design has minimal area overhead (0.0576 mm2). We have simulated both PROPEL and E-PROPEL using synthetic and SPLASH-2 traffic, where our results indicate that PROPEL and E-PROPEL significantly reduce power (tenfold) and increase performance (twofold) over other well-known electrical networks.