Transformation optics provides a powerful tool for controlling electromagnetic fields and designing novel optical devices. In practice, devices designed by this method often require material optical properties that cannot be achieved at visible or near IR light wavelengths. The conformal transformation technique can relax this requirement to isotropic dielectrics with gradient refractive indices. However, there are few effective methods for achieving large arbitrary refractive index gradients at large scales, so the limitation for building transformation optical devices is still in fabrication. Here we present a photoelectrochemical (PEC) silicon etching technique that provides a simple and effective way to fully control the macro scale profiles of refractive indices by structuring porous silicon on the nanoscale. This work is, to our knowledge, the first demonstration of using light to control porosity in p-type silicon. We demonstrate continuous index variation from n = 1.1 to 2, a range sufficient for many transformation optical devices. These patterned porous layers can then be lifted off of the bulk silicon substrate and transferred to other substrates, including patterned or curved substrates, which allows for the fabrication of three dimensional or other more complicated device designs. We use this technique to demonstrate a gradient index parabolic lens with dimensions on the order of millimeters, which derives its properties from the distribution of nanoscale pores in silicon.
Laser science has tackled physical limitations to achieve higher power, faster and smaller light
sources. The quest for ultra-compact laser that can directly generate coherent optical fields at
the nano-scale, far beyond the diffraction limit of light, remains a key fundamental challenge.
Microscopic lasers based on photonic crystals3, metal clad cavities4 and nanowires can now
reach the diffraction limit, which restricts both the optical mode size and physical device
dimension to be larger than half a wavelength. While surface plasmons are capable of tightly
localizing light, ohmic loss at optical frequencies has inhibited the realization of truly nano-scale
lasers. Recent theory has proposed a way to significantly reduce plasmonic loss while
maintaining ultra-small modes by using a hybrid plasmonic waveguide. Using this approach, we
report an experimental demonstration of nano-scale plasmonic lasers producing optical modes
100 times smaller than the diffraction limit, utilizing a high gain Cadmium Sulphide
semiconductor nanowire atop a Silver surface separated by a 5 nm thick insulating gap. Direct
measurements of emission lifetime reveal a broad-band enhancement of the nanowire's exciton
spontaneous emission rate up to 6 times due to the strong mode confinement and the signature
of apparently threshold-less lasing. Since plasmonic modes have no cut-off, we show downscaling
of the lateral dimensions of both device and optical mode. As these optical coherent
sources approach molecular and electronics length scales, plasmonic lasers offer the possibility to
explore extreme interactions between light and matter, opening new avenues in active photonic
circuits, bio-sensing and quantum information technology.