gas etching techniques, such as reactive ion etching, ion beam milling and chemically assisted ion beam etching, have been used to produce a range of optical and electronic components in gaas. large structures, typically 10-100 mu m deep, such as spherical microlenses, mesas, columns and fibre-location-channels, may be fabricated in gaas by a fast reactive ion etching process using freon-12 (ccl/sub 2/f/sub 2/). three-dimensional ir microlenses of spherical geometry have been fabricated in single crystal (111) and (100) gaas. microlens structures of this form were transferred from a hemispherical photoresist mask cap into the gaas substrate by ion beam milling and by ccl/sub 2/f/sub 2/ assisted ion beam etching etching rates for crystalline gaas by rie with ccl/sub 2/f/sub 2/ were fast: at a power density of 6 w cm/sup -2/ and gas pressure of 18 pa ( approximately 150 mtorr) an etch rate of 3.5 mu m min/sup -1/ was recorded. however, the selectivity between photoresist mask and gaas using rie was more difficult to control. by inert argon ion beam milling at a beam energy of 500 ev and current density 0.55 ma cm/sup -2/, etch rates for crystalline gaas of circa 700 aa min/sup -1/ were obtained. improving surface finish is also important if highly optically efficient lenses are to be produced with reduced losses (i.e. scattering, aberrations). low damage etching of gaas has been demonstrated with