on silicon dioxide posts with dimensions such that the second harmonic was enhanced by the major axis of the ellipsoid the silicon dioxide posts were fabricated by starting with a silicon wafer 10 in fig1 having a 500 nanometer thick thermally grown oxide layer 11 this silicon dioxide layer 11 is coated with a layer 12 consisting of 300 u00e5 of chromium followed by a layer 13 consisting of 1000 u00e5 of photoresist the photoresist was then patterned by exposing it to an interference pattern having a 320 nanometer period that is formed by the interference from two 325 nanometer helium cadium laser beams two exposures were made with the sample rotated by 90 degrees between the exposures in order to create a crossed grating pattern after development an array of photoresist posts 13 is formed as shown in fig2 of the drawing this array of photoresist posts is then used as a mask for argon ion milling the chromium layer 12 thereby producing an array of photoresist and chromium posts as illustrated in fig3 the chromium makes an excellent mask for the final highly directional reactive plasma etching of the silicon dioxide layer 11 a chf 3 plasma erosion of the chromium mask produces the slightly conically shaped posts of silicon dioxide shown in fig4 the entire substrate is then chemically cleaned to remove all masking layers thereby producing a silicon substrate with silicon dioxide posts 14 as shown in fig5 the substrates are quite durable and because silver can be easily removed for example with aqua regia they can be reused many times the next step in the process is to evaporate silver onto the slightly conical silicon dioxide posts as illustrated in fig6 as shown in fig7 the silver is evaporated.